Assessment of Enhanced Load-Bearing Capacity of Expanded Steel Pipe Piles Considering Optimal Configuration

Assessment of Enhanced Load-Bearing Capacity of Expanded Steel Pipe Piles Considering Optimal Configuration | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Assessment of Enhanced Load-Bearing Capacity of Expanded Steel Pipe Piles Considering Optimal Configuration Sangwoo Park, Uiseok Kim, Hyeontae Park, Hangseok Choi This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-3999368/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Recently, advanced types of micropiles have been developed to enhance their load-bearing capacity, responding to increased demands for micropile applications. However, these improved micropiles present challenges for rapid construction within confined spaces due to construction complexities. This study introduces an expanded steel pipe pile, which not only offers improved load-bearing capacity but also facilitates rapid construction in limited spaces. The expanded steel pipe pile is created by expanding a pre-installed small-diameter steel pipe at specific intervals to form shear keys along the length of the pile. In this study, the potential for enhancing the load-bearing capacity of the expanded steel pipe pile was initially verified through preliminary numerical analyses. Subsequently, comprehensive field experiments were carried out comparing two traditional micropiles with two expanded steel pipe piles, each installed using different methods. The results showed that the expanded steel pipe piles had up to 1.4 times the allowable bearing capacity of conventional micropiles. Finally, utilizing the numerical model validated by the field experiment results, the optimal configuration for the expanded steel pipe pile was provided. Considering both load-bearing efficiency and constructability, it was concluded that the expanded steel pipe pile should have a post-expansion diameter of 360 mm (with a deformation ratio of 13%) or less and incorporate at least three shear keys. Expanded steel pipe pile Micropile Small-diameter steel pipe pile Load-bearing capacity of micropiles Static load test Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 Figure 15 Figure 16 Figure 17 1. Introduction As urban areas become denser, there has been a growing emphasis on the development of underground spaces to promote sustainable urban development [ 1 , 2 ]. This trend is accompanied by a rising demand for foundational and seismic reinforcements in older buildings, particularly in large cities, leading to a greater need for pile construction in these areas [ 3 ]. However, conventional prestressed high-strength concrete (PHC) piles or cast-in-place piles face challenges when installed in urban areas with numerous adjacent buildings and limited access for large construction equipment. In particular, the use of small construction equipment, capable of operating inside buildings, has become essential for pile construction in the reinforcement or underground development of existing structures. Consequently, the use of micropiles in construction has seen steady growth recently [ 4 ]. Micropiles have a relatively smaller borehole diameter (typically around 100–300 mm) compared to conventional piles [ 5 ]. Their construction is possible with small casing penetration equipment, making them suitable for small construction spaces while minimizing noise and vibration during installation. Additionally, micropiles offer advantages in flexible pile placement, rapid construction, and minimal environmental impact on the ground [ 6 ]. Therefore, micropiles have been widely used in foundation construction within urban areas and for the reinforcement of existing buildings. Farhangi and Karakouzian [ 7 ] employed micropile as the foundation for a bridge in a construction site where the use of large equipment was restricted due to adjacent structures and challenging soil conditions. Gupta and Chawla [ 8 ] applied micropiles to improve the ground beneath a railway track without disrupting traffic or dismantling the track. Micropiles have also been employed for stabilizing soft ground or unstable slopes. An unstable slope with a 42-degree angle was reinforced by installing 139 micropiles and geocells, achieving a safety factor exceeding 1.0 [ 9 ]. Pandit et al. [ 10 ] analyzed the performance of micropiles in reinforcing slopes at risk of debris slides, considering factors such as diameter, spacing, and aspect ratio. The construction of 420 micropiles using jet grouting improved the safety of liquefaction-prone areas [ 11 ]. Recently, there has been a rise in construction projects employing the "floating and underground extension method," which involves expanding or excavating underground spaces beneath existing buildings without demolishing them. This method reinforces upper structures using micropiles while developing underground spaces beneath existing buildings [ 12 ]. One notable case is the excavation of the underground space beneath the Seoul City Hall building, where 137 double-tube steel pipe micropiles were installed to support the upper structures before bottom-up excavation. Bracings were also utilized to prevent micropile buckling in areas with high stresses [ 13 ]. Studies have shown that the floating and underground extension method using micropiles enhances economic efficiency compared to conventional top-down or bottom-up methods for developing underground spaces [ 14 ]. Moreover, micropiles have been used to reinforce existing foundations, supporting additional loads on existing buildings [ 15 , 16 ]. Sometimes, micropiles were employed to mitigate the tilt of raft foundations beneath existing structures [ 17 ]. Despite their advantages, micropiles have been criticized for their lower end-bearing capacity compared to conventional piles due to their small diameter [ 18 ]. Consequently, micropiles primarily rely on frictional resistance to support loads. However, increasing the length of micropiles to enhance frictional resistance can elevate the risk of buckling due to a higher slenderness ratio, ultimately reducing their horizontal bearing capacity [ 19 , 20 ]. Parametric studies have indicated that pile diameter enlargement contributes more to load-bearing capacity improvement than lengthening the piles [ 21 ]. Some projects have utilized groups of micropiles along with a raft foundation to distribute loads evenly [ 22 ]. However, the advantage of micropiles, which can be installed in small construction spaces, may diminish when installed in groups. On the other hand, to enhance horizontal bearing capacity, research has explored the installation of inclined micropiles, which improved seismic performance [ 20 , 23 , 24 ]. However, the installation of inclined micropiles is constrained by the presence of adjacent structures, limiting their use to urban areas or tight spaces for existing building reinforcement. Note that rather than installing piles inclined or in groups, one way to maximize economic feasibility is to enhance the load-bearing capacity of individual micropiles. In this regard, waveform micropiles were developed and constructed by applying jet-grouting to the surface of existing micropiles, increasing the diameter by approximately 1.5 times. Shear keys are configured along the pile length, improving both end-bearing capacity and frictional resistance. Loading tests have demonstrated that waveform micropiles can increase load-bearing capacity by up to 2.3 times compared to conventional micropiles [ 25 ]. The proximity and number of shear keys in waveform micropiles also contribute to further bearing capacity improvement [ 26 ]. When waveform micropiles are combined with conventional micropiles in a single raft foundation, waveform micropiles share a load more effectively, resulting in over twice of load-sharing ratio compared to conventional micropiles [ 27 ]. However, waveform micropiles require expensive and large equipment for high-pressure jet-grouting after drilling. Moreover, the long curing time of grout makes waveform micropiles unsuitable for construction methods requiring rapid construction. As a similar concept, nodular piles, featuring bamboo-shaped joints along the pile axis, have been introduced as bored micropiles. Their load-bearing capacity has been found to be 1.6 times greater than that of conventional micropiles [ 28 ]. However, nodular piles demand significant construction space and drilling costs. Moreover, pre-manufactured nodular piles come with additional costs for manufacturing them using advanced technology. Therefore, this study introduces the expanded steel pipe pile, an innovative micropile that can be rapidly constructed even in tight spaces, while also offering improved load-bearing capacity. Expanded steel pipe piles are created by expanding small-diameter steel pipes inserted underground at specific intervals to form shear keys along the pile axis. The diameter of the steel pipe increases in the expanded section, enhancing both load-bearing capacity and resistance to buckling. The bump effect exerted by shear keys can enhance both frictional and tip resistances significantly. Moreover, the applied horizontal pressure for expanding the inserted steel pipe can be transmitted to the ground, compacting and reinforcing the surrounding soil. This study performed a series of numerical analyses and field verification experiments to ascertain the effectiveness of expanded steel pipe piles in improving load-bearing capacity. Initially, expansion tests were carried out using a specially developed hydraulic expansion device, which served to expand the steel pipes after they were inserted into the ground. Based on these tests, the configuration for the expanded steel pipe piles was designed to perform preliminary numerical analyses before field experiments. In the field verification experiments, static pile load tests were conducted on two conventional micropiles and two expanded steel pipe piles, each installed using different construction techniques. One approach involved injecting grout after inserting the steel pipe into the borehole, while the other approach comprised pressing the steel pipe directly into the ground. The conclusive phase of the study utilized the numerical model, which had been validated by the results of the field experiments. Parametric studies were performed, focusing on the post-expansion diameter of the steel pipe and the quantity of shear keys. These studies aimed to determine the optimal configuration for the expanded steel pipe pile, taking into account both the improvement in load-bearing capacity and constructability. 2. Development of hydraulic expansion device A hydraulic expansion device was developed to efficiently expand steel pipes inserted into the ground, even in constrained spaces. Initially, a downsized hydraulic expansion device was manufactured to assess the potential for expanding steel pipes based on their thickness [ 29 ]. The first expansion test considered steel pipe thicknesses of 2.9, 4.0, and 6.0 mm, with the results summarized in Table 1 , where the deformation ratio was defined as the change in diameter divided by initial diameter (i.e., 114.3 mm). Table 1 Results of expansion test for downsized hydraulic expansion device [ 29 ] Thickness Initial diameter Post-expansion diameter Deformation ratio Expansion time 2.9 mm 114.3 mm 128.2 mm 12.2% 42.2 s 4.0 mm 114.3 mm 122.5 mm 7.2% 26.2 s 6.0 mm 114.3 mm 115.2 mm 0.8% 10.6 s The downsized hydraulic expansion device had a maximum hydraulic pressure capacity of 70 MPa, leading to minor deformation ratios in thicker steel pipes. Consequently, a steel pipe with a thickness of 6.0 mm experienced minimal expansion. In contrast, deformation ratios increased in thinner steel pipes, resulting in longer expansion times. Specifically, when compared to a steel pipe with a thickness of 4.0 mm, the deformation amount and expansion time increased by 1.56 and 1.61 times, respectively, for a steel pipe with a thickness of 2.9 mm. Upon confirming the feasibility of steel pipe expansion, a full-scale hydraulic expansion device was developed. This device is capable of expanding carbon steel tubes typically used for steel pipe piles in field applications [ 30 ], specifically those with a diameter exceeding 300 mm and a thickness over 6.0 mm. Additionally, this device offers the flexibility of adjusting the expansion amount, rather than just the hydraulic pressure, allowing for a more versatile formation of shear keys in steel pipes. The design drawing and configuration of the full-scale hydraulic expansion device are illustrated in Fig. 1. The device operates by hydraulically pulling the steel rod attached to Expander B, which in turn pushes Expander A radially to expand the steel pipe. Utilizing the full-scale hydraulic expansion device, a second expansion test was carried out on a steel pipe with a diameter of 318.5 mm and a thickness of 6.4 mm [ 31 ]. The results demonstrated that the steel pipe could be expanded up to 370 mm in diameter (representing a deformation ratio of 16.2%) within a remarkable 150 seconds. This expansion rate is notably efficient in terms of constructability and installation time, especially when compared to waveform micropiles or nodular piles. 3. Preliminary numerical analysis for designing expanded steel pipe pile 3.1 Development of numerical model A preliminary numerical analysis was conducted using FLAC3D, a commercial program utilizing the finite difference method, to validate the enhancement in load-bearing capacity of the expanded steel pipe pile prior to field experiments. The ground was modeled with sufficient dimensions, featuring a width 20 times the diameter of the steel pipe and a height 3 times the length of the steel pipe to minimize boundary effects. The steel pipe was modeled according to the same specifications as in the first expansion test using the downsized hydraulic expansion device, except for the 6.0 mm thick steel pipe, which was excluded due to the lack of expansion observed during the first test. The dimensions of the expanded section (i.e., shear key) of the steel pipes were determined based on the results from the first expansion test. The details of the expanded steel pipe piles applied in the preliminary numerical analysis are summarized in Table 2. Additionally, Fig. 2 displays the numerical modeling of the ground and steel pipe pile. Table 2 Specifications of expanded steel pipe piles adopted in preliminary numerical analysis Thickness Initial diameter Post-expansion diameter Height of expanded part Length of pile 2.9 mm 114.0 mm 128.0 mm 50.0 mm 1.0 m 4.0 mm 114.0 mm 123.0 mm 50.0 mm 1.0 m In the numerical analysis, the elasticity model was applied to the steel pipe, while the Mohr-Coulomb model was employed for the ground. Interface elements were utilized at the boundary between the steel pipe pile and the ground to simulate the interaction between these two materials. The values of the vertical stiffness coefficient (\({K}_{n}\)) and the shear stiffness coefficient (\({K}_{t}\)) assigned to the interface elements were estimated by Eqs. (1) and (2), respectively. $${K}_{n}=\frac{2R(1-{v}_{i})}{(1-2{v}_{i})}\times \frac{{E}_{soil}}{2(1+{v}_{soil})}$$ (1) $${K}_{t}=\frac{R\bullet {E}_{soil}}{2{t}_{v}(1+{v}_{soil})}$$ (2) where, \(R\) represents the strength reduction coefficient of the interface, \({v}_{i}\) is the Poisson’s ratio of the interface, \({E}_{soil}\) is the elastic modulus of the soil, \({v}_{soil}\) is the Poisson’s ratio of the soil, and \({t}_{v}\) is the virtual thickness of the interface, which decreases as the stiffness difference between materials increases. In this study, a value of 0.67 was applied for \(R\), and \({v}_{i}\) was assumed to be 0.45 to ensure numerical simulation convergence. The ground model was based on sandy soil conditions for the preliminary numerical analyses. The material properties adopted in the numerical model are summarized in Table 3. Table 3 Material properties adopted in preliminary numerical analysis Properties Steel pipe pile Sandy soil Elastic modulus 210,000 MPa 25 MPa Poisson’s ratio 0.30 0.30 Unit weight 78.5 kN/m 3 17.0 kN/m 3 Viscosity – 15 kPa Friction angle – 25\(^\circ\) The mesh for the steel pipe pile was constructed using uniform-sized hexahedron elements, while the ground mesh consisted of tetrahedral elements with gradually increasing sizes towards the boundaries, taking into account optimized simulation time considerations. During the simulations, the settlement changes were monitored while progressively increasing the load on the steel pipe pile. The load that caused a settlement of 25.0 mm, conforming to the allowable settlement in the design standard for structural foundations, was determined as the ultimate bearing capacity of the pile. Furthermore, various configurations of expanded steel pipe piles were numerically simulated to assess the impact of shear key location, expansion configuration, and expansion time on their ultimate bearing capacity. 3.2 Effect of shear key location To assess the impact of shear key location on the load-bearing capacity of expanded steel pipe piles, numerical simulations were carried out for three different pile configurations, each with a single shear key positioned at the tip, middle, or head of the pile. The ultimate bearing capacities of these three expanded steel pipe piles were then compared with that of a conventional micropile without shear keys. The pile specifications considered in this comparison are detailed in Table 4. Figure 3 illustrates the modeling results of the steel pipe piles according to the shear key locations, while Fig. 4 displays the ultimate bearing capacities of the steel pipe piles, categorized by shear key location and steel pipe thickness. It was observed that the ultimate bearing capacity increased when a shear key was present in both 2.9 mm and 4.0 mm thick steel pipe piles. Given the minor difference in steel pipe thickness (i.e., between 2.9 mm and 4.0 mm), the enhancement in ultimate bearing capacity due to increased steel pipe thickness was negligible in conventional micropiles. However, the enhancement ratio of ultimate bearing capacity was more pronounced with the incorporation of a shear key, especially as the steel pipe thickness increased. Additionally, the ultimate bearing capacity was more effectively improved when the shear key was positioned closer to the tip. For expanded steel pipe piles with a shear key at the tip, the ultimate bearing capacity increased by 25.0% for the 2.9 mm thick steel pipe piles and by 31.5% for the 4.0 mm thick steel pipe piles, compared to those with a shear key at the head. In summary, the ultimate bearing capacity of an expanded steel pipe pile with a single shear key at the pile tip was enhanced by up to 1.32 times compared to the conventional micropile. Given that only one shear key was formed and the deformation ratio of the shear key was less than 12.2%, further improvements in the ultimate bearing capacity of the expanded steel pipe pile could be expected by increasing both the number of shear keys and their deformation ratio. Table 4 Variables in numerical simulations according to shear key location Pile specification Variables Thickness of steel pipe 2.9 mm / 4.0 mm Shear key location No expansion / Tip / Middle / Head 3.3 Effect of expansion configuration Numerical simulations were conducted to assess the enhancement in ultimate bearing capacities of expanded steel pipe piles, specifically between two scenarios: one involving the configuration of a single shear key with an extended height through consecutive steel pipe expansions and the other involving multiple shear keys with shorter heights at fixed intervals. Accordingly, two configurations were modeled: one with a single 250 mm high shear key, and another with five shear keys, each 50 mm high and spaced at 200 mm intervals. Furthermore, the ultimate bearing capacity of the expanded steel pipe pile with a single shear key at the pile tip was also compared to analyze the influence of the number of shear keys on the ultimate bearing capacity. The relevant variables considered for pile specification and the modeling results to analyze the effect of the expansion configuration on the ultimate bearing capacity of expanded steel pipe piles are presented in Table 5 and Fig. 5, respectively. Figure 6 displays the ultimate bearing capacities of the steel pipe piles based on the expansion configuration and steel pipe thickness. As previously analyzed, thicker steel pipes exhibited a more pronounced enhancement in the ultimate bearing capacity of steel pipe piles when shear keys were introduced. Regarding the number of shear keys, the ultimate bearing capacity increased by an average of 29.0% when a single shear key was incorporated at the tip compared to conventional micropiles. In contrast, when five shear keys were arranged at 200 mm intervals, the ultimate bearing capacity increased by an average of 33.2%. Interestingly, the increase in ultimate bearing capacity of expanded steel pipe piles was not directly proportional to the number of shear keys. Compared to piles with consecutive expansion, those expanded at specific intervals showed a greater increase in ultimate bearing capacity. Since expanded steel pipe piles enhance load-bearing capacity by improving both frictional and tip resistances in shear keys, forming multiple shorter shear keys proved more effective. The ultimate bearing capacity of the expanded steel pipe pile equipped with five shear keys was up to 1.37 times greater than that of the conventional micropile. Given that the steel pipe length was limited to 1 meter in the preliminary numerical simulations, it is anticipated that the load-bearing capacity of expanded steel pipe piles could be further enhanced by increasing the number of shear keys on longer steel pipes at actual construction sites. Table 5 Variables in numerical simulations according to expansion configuration Pile specification Variables Thickness of steel pipe 2.9 mm / 4.0 mm Expansion method No expansion / One expansion at tip / Consecutive expansion / Expansion 5 times at intervals of 200 mm 3.4 Effect of expansion time In the first expansion test using the downsized hydraulic expansion device, the average time required to form one shear key by expanding a 2.9 mm thick steel pipe was 42.2 seconds, whereas for a 4.0 mm thick steel pipe, it was 26.2 seconds. In other words, a 2.9 mm thick steel pipe required approximately 1.6 times more expansion time than a 4.0 mm thick steel pipe to create the same number of shear keys. This difference in expansion times was attributed to varying deformation ratios, arising from the consistently maintained maximum pressure of the downsized hydraulic expansion device. Consequently, numerical simulations were conducted for a 2.9 mm thick steel pipe pile with one shear key and a 4.0 mm thick steel pipe pile with two shear keys to identify an efficient expansion method that balances both expansion time and load-bearing capacity. The ultimate bearing capacities of steel pipe piles obtained from numerical simulation, categorized by expansion time, are presented in Fig. 7. The results showed that the 4.0 mm thick expanded steel pipe pile with two shear keys, each having a relatively small deformation ratio, exhibited an ultimate bearing capacity approximately 7.6% higher than the 2.9 mm thick expanded steel pipe pile with one shear key at a relatively large deformation ratio. In essence, forming multiple shear keys, even with smaller deformation ratios, proved to be more effective in terms of both load-bearing capacity and construction time compared to forming a limited number of shear keys with larger deformation ratios. 4. Test bed field experiments The enhanced load-bearing capacity of expanded steel pipe piles was investigated through static load tests on full-scale steel pipe piles constructed in a test bed. Two conventional micropiles and two expanded steel pipe piles were built using different construction methods, depending on whether grout injection was utilized. In the grout injection method, grout was injected after the steel pipe was inserted into a pre-drilled borehole, resulting in a pile foundation diameter larger than that of the steel pipe itself. In contrast, the no-grout method (i.e., press-in method) involved directly pressing steel pipes into the ground using a screw auger, without pre-drilling. Two construction techniques for steel pipe piles implemented in the test bed are shown in Fig. 8. The test bed ground composition consists of a landfill layer with silty sand from the surface down to a depth of 6.5 meters, followed by weathered rock from 6.5 meters to 8.0 meters, and soft rock beneath that. Considering constructability, the length of the steel pipe piles constructed in the test bed was designed to be 6.0 m. In accordance with the specifications for carbon steel tubes intended for general structural purposes [ 32 , 33 ], steel pipes with a diameter of 318.5 mm and a thickness of 6.4 mm were utilized. The post-expansion diameter was set to 360 mm (approximately 13% of the deformation ratio), guided by the results of another expansion test using the full-scale hydraulic expansion device. The shear keys were designed to be 100 mm high. The steel pipe expansion was carried out at three different depths. Based on the preliminary numerical simulation results indicating that the ultimate bearing capacity of expanded steel pipe piles increased more effectively when the shear key was located closer to the pile tip, the expansion was executed at depths of 5.5 m, 5.0 m, and 4.5 m. Figure 9 illustrates the expansion process of steel pipes inserted into the ground. The configuration of the expanded steel pipe piles constructed in the test bed and the results of ground investigation are presented in Fig. 10. In addition, the specifications of the steel pipe piles constructed in the test bed are summarized in Table 6 . Table 6 Pile specifications constructed in the test bed No. Type Construction method Configuration of shear key (Diameter / Height / Number) 1 Conventional micropile Grout injection No expansion 2 Expanded steel pipe pile Grout injection 360 mm / 100 mm / 3 3 Conventional micropile Press-in No expansion 4 Expanded steel pipe pile Press-in 360 mm / 100 mm / 3 After constructing the four steel pipe piles, static load tests were conducted, incrementally applying a load of 49.05 kN. The resulting load-settlement curves are shown in Fig. 11. However, it’s worth noting that the ultimate bearing capacity was not definitively determined for all the constructed steel pipe piles as the tests were completed before a nonlinear relationship between settlement and applied load could be established as shown in Fig. 11. Therefore, the allowable bearing capacities of the constructed steel pipe piles were derived using Davisson’s method with a safety factor of 2.0. Table 7 summarizes the static load test results for the four steel pipe piles installed in the test bed. Table 7 also highlights the relative improvement in allowable bearing capacity of the expanded steel pipe pile compared to the conventional micropile, utilizing the same construction method. When using the grout injection method, the steel pipe piles exhibited increased allowable bearing capacities compared to pressing the steel pipe directly into the ground without grout (i.e., press-in method). This increase was attributed to the enlarged pile foundation volume (i.e., diameter), resulting from the injected grout. The amount of injected grout was greater in the conventional micropile than in the expanded steel pipe pile because the expanded steel pipe piles had shear keys created by expanding the steel pipe in advance before grout injection. Consequently, the allowable bearing capacity of the conventional micropile experienced a more significant enhancement than that of the expanded steel pipe pile when the grout injection method was applied. Specifically, when installing steel pipe piles using the grout injection method, the allowable bearing capacity increased by 1.26 times for the conventional micropile and by 1.16 times for the expanded steel pipe pile. Compared to the conventional micropiles, the allowable bearing capacity of the expanded steel pipe piles increased by 1.11 times when the grout injection method was applied and by 1.21 times when the press-in method was applied. For the steel pipe piles installed with grout injection, the diameter of the pile foundation was the same for both conventional micropiles and expanded steel pipe piles. However, when the press-in method was utilized, only the expanded steel pipe piles exhibited an increase in the diameter of the pile foundation (i.e., shear keys), leading to a more significant enhancement in allowable bearing capacity compared to the conventional micropile. Previous studies have shown that waveform micropiles have approximately 1.4 to 2.3 times higher load-bearing capacity compared to conventional micropiles [ 26 ], In contrast, the allowable bearing capacity of expanded steel pipe piles in the field experiments of this study increased by up to 1.21 times. It is important to note that the load-bearing capacity comparison for waveform micropiles was made under conditions where their diameter was increased by 1.5 times due to injected grout. Similarly, a significant difference in allowable bearing capacity (about 1.4 times) was observed when comparing the conventional micropiles installed without grout injection (252.5 kN) to the expanded steel pipe piles installed with grout injection (354.1 kN). Furthermore, the field tests limited the post-expansion diameter to 360 mm and the number of shear keys to three to avoid potential fractures or construction issues during the field experiments. Based on the preliminary numerical simulation results, it is conceivable that the allowable bearing capacity of expanded steel pipe piles could be further enhanced by increasing both the post-expansion diameter and the number of shear keys. Table 7 Results of static load tests for four steel pipe piles constructed in the test bed No. Type Installation method Allowable bearing capacity Improvement of allowable bearing capacity 1 Conventional micropile Grout injection 318.8 kN - 2 Expanded steel pipe pile Grout injection 354.1 kN 11.1% 3 Conventional micropile Press-in 252.5 kN - 4 Expanded steel pipe pile Press-in 305.2 kN 20.9% After the static load tests were completed, the interior of the expanded steel pipes was inspected using a camera, and subsequently, the pipes were extracted to examine the condition of their expansion. As illustrated in Fig. 12, the hydraulic expansion device effectively and reliably expanded the steel pipes inserted into the ground of the test bed. The field verification experiments demonstrated that the load-bearing capacity of expanded steel pipe piles could be enhanced compared to conventional micropiles. Notably, expanded steel pipe piles can offer several advantages, including ease of construction, suitability for tight spaces, and cost-effectiveness due to minimal equipment and technical requirements. Consequently, it can be concluded that expanded pipe piles exhibit significantly improved performance and practical applicability for field use. 5. Optimal configuration of expanded steel pipe pile 5.1 Verification of numerical model using field experiments Because of the relatively low applied loads during the static load tests, it was not possible to determine the yield load and ultimate bearing capacity of the steel pipe piles from the field experiment results. Consequently, the static load tests were numerically simulated using the FLAC3D program under the same conditions as those encountered in the field experiments. This allowed for a reanalysis of the load-bearing capacity of the expanded steel pipe pile. In the numerical analysis, the ground model was constructed to reflect the actual subsurface layers in the test bed: landfill layer, weathered rock, and soft rock, as determined by the ground investigation. The material properties for each ground layer applied in the numerical model were estimated through empirical formulas [ 34 ]. The four steel pipe piles were modeled to match the specifications of those installed in the test bed. The material properties for each ground layer, steel pipe, and grout applied in the numerical simulations are summarized in Table 8 . To minimize boundary effects, the ground was modeled with dimensions of 18 m in width and height. The design drawings for the four steel pipe pile models used in the numerical simulations are illustrated in Fig. 13, with the results of modeling and mesh formation presented in Fig. 14. Table 8 Material properties of ground, steel pipe, and grout applied in numerical simulations Properties Landfill layer Weathered rock Soft rock Steel pipe Grout Elastic modulus 20 MPa 200 MPa 1,000 MPa 205,000 MPa 200 MPa Poisson’s ratio 0.35 0.31 0.25 0.2 0.31 Unit weight 19 kN/m 3 21 kN/m 3 25 kN/m 3 78.5 kN/m 3 21 kN/m 3 Viscosity 20 kPa 31 kPa 100 kPa – 31 kPa Friction angle 30 \(^\circ\) 23 \(^\circ\) 35 \(^\circ\) – 33 \(^\circ\) During the numerical simulations, loads were applied to the steel pipe piles following the same loading increments as in the field experiments, and the resulting settlement was obtained accordingly. However, since the field experiments did not apply sufficient load, in the numerical simulations, the load was incrementally increased by 10% after reaching the final load step used in the field experiments. The results of the numerical simulations were then compared with those from the field experiments, as illustrated in Fig. 15. Similar to the field experiments, Davisson’s method was employed to estimate the allowable bearing capacity. The difference between the allowable bearing capacities estimated by the numerical simulations and those from field experiments averaged 11.1% for the conventional micropiles and 0.8% for the expanded steel pipe piles. This demonstrates that the developed numerical model can reliably estimate the allowable bearing capacity of steel pipe piles. Furthermore, in contrast to the field experiment results, the load-settlement curves derived from the numerical simulations exhibited nonlinear behavior, allowing for the estimation of ultimate bearing capacity. When estimating the ultimate bearing capacities of steel pipe piles based on the applied load corresponding to a settlement of 10% of the pile diameter, it was observed that the ultimate bearing capacity increased in the expanded steel pipe piles compared to the conventional micropiles at a similar ratio to the bearing capacity estimated using Davisson’s method. As a result, it was concluded that the developed numerical model is suitable for analyzing the load-bearing capacity of expanded steel pipe piles under various conditions. Consequently, this numerical model was used to determine the optimal configuration of expanded steel pipe piles by conducting a series of parametric studies described in the next section. 5.2 Parametric studies for optimal expanded steel pipe piles Parametric studies were conducted, taking into account different construction methods for the piles, post-expansion diameters, and the number of shear keys. The variables considered in these parametric studies are summarized in Table 9 . The numerical model employed for these parametric studies was identical to the one employed for simulating the field experiment, with modifications only made in accordance with the parameter variables listed in Table 9 . Settlements were estimated by applying the same load increments at each loading step as employed in the field experiments. In these parametric studies, the allowable bearing capacity of the steel pipe piles was determined by averaging the ultimate load with a safety factor of 3.0 and the yield load with a safety factor of 2.0. The ultimate loads were determined using the P (Pull-out load) -S (Settlement) analysis method, while the yield loads were derived using both the log P-log S analysis method and Davisson’s method. Figure 16 illustrates how the allowable bearing capacity of expanded steel pipe piles varied with different post-expansion diameters. As the post-expansion diameters increased, their allowable bearing capacities also increased. However, it is important to note that this increase was not directly proportional to the post-expansion diameters. Furthermore, there was no significant increase in allowable bearing capacity when the expansion in steel pipes exceeded a diameter of 360 mm. This tendency became more apparent with the addition of more shear keys. In essence, as the number of shear keys increased, the impact on improving the load-bearing capacity due to an increase in post-expansion diameter became less significant. Considering factors such as construction time and the risk of pipe fractures during the expansion process of steel pipes, the optimal post-expansion diameter was determined to be 360 mm, corresponding to a deformation ratio of 13%. The variation in the allowable bearing capacity of expanded steel pipe piles in relation to the number of shear keys is illustrated in Fig. 17. It was observed that the allowable bearing capacity increased proportionally, following a specific slope, as the number of shear keys increased. Specifically, when the number of shear keys was three or fewer, the allowable bearing capacity showed a relatively significant increase with respect to the number of shear keys. This rate of increase slightly declined when the number of shear keys exceeded four. Based on the results of the parametric studies, it can be concluded that maximizing the number of shear keys by expanding the steel pipe is advantageous for enhancing load-bearing capacity. Nevertheless, when taking constructability and construction time into consideration, if extensive steel pipe expansion is not feasible, configuring at least three shear keys is recommended for efficiently improving the load-bearing capacity. Table 9 Variables considered in parametric studies Considered parameters Variables Construction method of piles Grout injection method / Press-in method Post-expansion diameter 340 mm / 350 mm / 360 mm / 370 mm Number of shear keys 1 / 2 / 3 / 4 / 5 / 6 / 7 6. Conclusion This study investigated the enhancement of load-bearing capacity of expanded steel pipe piles compared to conventional micropiles through numerical simulations and field experiments. Additionally, optimal configurations for expanded steel pipe piles, balancing constructability and load-bearing capacity, were determined using parametric studies with the developed numerical model. The key findings of this study are as follows. A hydraulic expansion device was developed to construct expanded steel pipe piles, and its potential for expanding steel pipes in the ground was verified through pipe expansion tests. The formation of a shear key within 150 seconds indicated that expanded steel pipe piles have higher constructability compared to existing improved micropiles. The preliminary numerical analysis confirmed the enhanced load-bearing capacity of expanded steel pipe piles. The ultimate bearing capacity was effectively increased when shear keys were positioned closer to the pile tip, and more shear keys were formed. Moreover, it was found that configuring many shear keys with a small deformation ratio was more effective for increasing load-bearing capacity than forming fewer shear keys with a large deformation ratio within the same construction time. The field verification experiments showed that the expanded steel pipe piles had up to 1.4 times higher allowable bearing capacity than the conventional micropiles. Notably, the use of the press-in method for steel pipe installation resulted in more effective load-bearing capacity improvement in expanded steel pipe piles than the grout injection method. However, both expanded steel pipe piles and conventional micropiles exhibited higher allowable bearing capacity when installed using the grout injection method. Post-experiment inspections confirmed that steel pipes were stably expanded underground. In the parametric studies, expanding the pipe diameter to 360 mm or more did not significantly enhance the allowable bearing capacity of expanded steel pipes. Conversely, the allowable bearing capacity of expanded steel pipe piles increased proportionally with the number of shear keys, although the rate of increase declined when more than four shear keys were configured. Considering the efficiency of load-bearing capacity enhancement and stability during pipe expansion, expanded steel pipe piles should be constructed with at least three shear keys and a post-expansion diameter of 360 mm (13% deformation ratio) or less. Declarations Data availability Data will be made available on request to the corresponding author. CRediT authorship contribution statement Sangwoo Park: Data curation, Validation, Visualization, Writing-original draft. Uiseok Kim: Investigation, Experiment, Visualization. Hyeontae Park: Software, Data curation. Hangseok Choi: Project administration, Supervision, Writing review & editing, Funding acquisition. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This study was supported by National Research Foundation of Korea (NRF) grants funded by the Korean government (MSIT) (No. 2019R1A2C2086647 and No. 2020R1A6A1A03045059). References Sterling, R., Admiraal, H., Bobylev, N., Parker, H., Godard, J. P., Vähäaho, I., Rogers, C. D. F., Shi, X., Hanamura, T. Sustainability issues for underground space in urban areas. Urban Design and Planning. 2012, 165(4), 241-254. https://doi.org/10.1680/udap.10.00020 Cui, J., Broere, W., Lin, D. Underground space utilisation for urban renewal. Tunnelling and Underground Space Technology. 2021, 108, 103726. https://doi.org/10.1016/j.tust.2020.103726 Ministry of Land, Infrastructure, and Transport. A total of 7,126,526 buildings across the country / 3641 million m2. Press release of Department of Green Architecture. Available online: https://www.molit.go.kr/USR/NEWS/m_71/dtl.jsp?lcmspage=1&id=95080363 (accessed on 10 Apr. 2023). Liu, W., Mao, J., Zhao, H., Shao, G. (2022). 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Experimental and Numerical Study on Lateral Resistance of Micropile in Sand. KSCE Journal of Civil Engineering, 27(9), 3740-3752. https://doi.org/10.1007/s12205-023-0977-0 Moradi Moghaddam, H., Keramati, M., Ramesh, A., & Naderi, R. (2021). Experimental evaluation of the effects of structural parameters, installation methods and soil density on the micropile bearing capacity. International Journal of Civil Engineering, 19, 1313-1325. https://doi.org/10.1007/s40999-021-00629-5 Das, M. (2022). Effect of load transferring platform on micropile. Journal of Building Pathology and Rehabilitation, 7(1), 53. https://doi.org/10.1007/s41024-022-00193-y Capatti, M. C., Dezi, F., Carbonari, S., & Gara, F. (2020). Dynamic performance of a full-scale micropile group: Relevance of nonlinear behaviour of the soil adjacent to micropiles. Soil Dynamics and Earthquake Engineering, 128, 105858. https://doi.org/10.1016/j.soildyn.2019.105858 Malik, B. A., Shah, M. Y., & Sawant, V. A. (2021). Influence of micropile parameters on bearing capacity of footings. Environmental Science and Pollution Research, 28(35), 48274-48283. https://doi.org/10.1007/s11356-021-14062-7 Jang, Y. E., & Han, J. T. (2018). Field study on axial bearing capacity and load transfer characteristic of waveform micropile. Canadian Geotechnical Journal, 55(5), 653-665. https://doi.org/10.1139/cgj-2017-0155 Jang, Y. E., & Han, J. T. (2019). Analysis of the shape effect on the axial performance of a waveform micropile by centrifuge model tests. Acta Geotechnica, 14, 505-518. https://doi.org/10.1007/s11440-018-0657-2 Wang, C., Han, J. T., & Kim, S. (2022). A field study on the load sharing behavior of a micropiled raft underpinned by a waveform micropile. Canadian Geotechnical Journal, 59(7), 1175-1187. https://doi.org/10.1139/cgj-2020-0547 Jung, C–W.; Chae, H–S.; Kim, W–J. A Study on Vertical Bearing Capacity of Nodular Cylinder Pile. In Proceedings of Architectural Institute of Korea, Daegu, Korea, Oct. 2006; pp. 129–132. J. Kim, U. Kim, J. Kim, M. Kang, H. Choi, 2022, An Experimental Study on the Performance of Expandable Steel Pipe Pile, Journal of the Korean Geo-Environmental Society, 23(1), 39-49. https://doi.org/10.14481/jkges.2022.23.1.39 National Institute of Technology and Standards, Carbon steel tubes for general structural purposes, 2018, Korea Kim, J., Kim, U., Min, B., Choi, H., Park, S. (2022). Development of Expanded Steel Pipe Pile to Enhance Bearing Capacity. Sustainability, 14(5), 3077. https://doi.org/10.3390/su14053077 Ministry of Land, Infrastructure, and Transport, Guidelines of Design Practices for National Highway Construction Works, 2021. 02. 01. The Korea Expressway Corporation, Guidelines of Design for Road, 2021. 12. 16. Korean Geotechnical Society, Commentary on design standards for structure foundation, 2018 Additional Declarations No competing interests reported. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-3999368","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":276327841,"identity":"ab68f729-5a5e-4338-a254-f0998942d63d","order_by":0,"name":"Sangwoo Park","email":"","orcid":"","institution":"Korea Military Academy","correspondingAuthor":false,"prefix":"","firstName":"Sangwoo","middleName":"","lastName":"Park","suffix":""},{"id":276327842,"identity":"299ed1c6-0167-4e8b-9d4f-9589df1d9e36","order_by":1,"name":"Uiseok Kim","email":"","orcid":"","institution":"Samho engineering 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19:33:33","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":52466,"visible":true,"origin":"","legend":"\u003cp\u003eNumerical modeling of ground and steel pipe pile\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3999368/v1/266e7102cb59788f44f15e47.jpg"},{"id":52104641,"identity":"677ffcbc-39f7-47e8-93f4-cff4903d2fbb","added_by":"auto","created_at":"2024-03-06 19:25:29","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":34044,"visible":true,"origin":"","legend":"\u003cp\u003eModeling results of steel pipe pile according to shear key location\u003c/p\u003e","description":"","filename":"3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3999368/v1/8f8af79815a35a9c0bdcc4fc.jpg"},{"id":52104646,"identity":"784f091a-b281-4976-a5b1-794038daf898","added_by":"auto","created_at":"2024-03-06 19:25:30","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":84043,"visible":true,"origin":"","legend":"\u003cp\u003eUltimate bearing capacities of steel pipe piles estimated by numerical simulations according to shear key location\u003c/p\u003e","description":"","filename":"4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3999368/v1/cdc73199a0f41a1bc2326781.jpg"},{"id":52105880,"identity":"a8555603-f0ff-4eaf-9689-d246e00e72b7","added_by":"auto","created_at":"2024-03-06 19:33:29","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":35989,"visible":true,"origin":"","legend":"\u003cp\u003eModeling results of steel pipe pile according to expansion configuration\u003c/p\u003e","description":"","filename":"5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3999368/v1/b133ba8b37ab1769e5b9481c.jpg"},{"id":52104645,"identity":"af3520c7-00e0-4982-852c-4b7d7dbfc3ad","added_by":"auto","created_at":"2024-03-06 19:25:30","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":99719,"visible":true,"origin":"","legend":"\u003cp\u003eUltimate bearing capacities of steel pipe piles estimated by numerical simulations according to expansion configuration\u003c/p\u003e","description":"","filename":"6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3999368/v1/cc0922aa075ca734cc9f1601.jpg"},{"id":52104663,"identity":"d232b189-523c-4b62-abb7-c049e1d9f5eb","added_by":"auto","created_at":"2024-03-06 19:25:34","extension":"jpg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":42888,"visible":true,"origin":"","legend":"\u003cp\u003eUltimate bearing capacities of 2.9 mm thick expanded steel pipe pile with one shear key and 4.0 mm thick expanded steel pipe pile with two shear keys\u003c/p\u003e","description":"","filename":"7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3999368/v1/a1ad023c8e982d1a04eea6be.jpg"},{"id":52104655,"identity":"d1fd1789-97b9-4631-9c0e-0d7adf92e888","added_by":"auto","created_at":"2024-03-06 19:25:33","extension":"jpg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":102631,"visible":true,"origin":"","legend":"\u003cp\u003eConstruction methods of steel pipe piles in the test bed\u003c/p\u003e","description":"","filename":"8.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3999368/v1/3d68d3e94bb9cc402bc3de9f.jpg"},{"id":52104651,"identity":"35e23a5f-6012-467f-8548-301fdaa14a06","added_by":"auto","created_at":"2024-03-06 19:25:33","extension":"jpg","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":129587,"visible":true,"origin":"","legend":"\u003cp\u003eExpansion process of steel pipe inserted into the ground\u003c/p\u003e","description":"","filename":"9.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3999368/v1/a4b8a6c9330e85f10bc31c11.jpg"},{"id":52104659,"identity":"16b39d80-240c-4ddc-a9c0-bef17d3c7001","added_by":"auto","created_at":"2024-03-06 19:25:34","extension":"jpg","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":75929,"visible":true,"origin":"","legend":"\u003cp\u003eConfigurations of constructed expanded steel pipe piles with ground investigation results\u003c/p\u003e","description":"","filename":"10.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3999368/v1/bb64da5ff3e206187734faac.jpg"},{"id":52104649,"identity":"2f1d698a-87b1-436f-a3dd-3797a17746bd","added_by":"auto","created_at":"2024-03-06 19:25:31","extension":"jpg","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":117191,"visible":true,"origin":"","legend":"\u003cp\u003eResults of static load tests for conventional micropiles and expanded steel pipe piles\u003c/p\u003e","description":"","filename":"11.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3999368/v1/9fd5430dc67c47baaf07b1a2.jpg"},{"id":52104647,"identity":"a4c29e27-e34d-48ed-9745-175be7b29c5c","added_by":"auto","created_at":"2024-03-06 19:25:31","extension":"jpg","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":142924,"visible":true,"origin":"","legend":"\u003cp\u003eInspection of shear keys of expanded steel pipe piles\u003c/p\u003e","description":"","filename":"12.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3999368/v1/45c015ecec05f0d5423e493f.jpg"},{"id":52104656,"identity":"903e029a-567b-4f8f-8325-26750b31180d","added_by":"auto","created_at":"2024-03-06 19:25:34","extension":"jpg","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":132397,"visible":true,"origin":"","legend":"\u003cp\u003eDesign drawings of steel pipe pile models in numerical simulations\u003c/p\u003e","description":"","filename":"13.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3999368/v1/d660ebf9b753e7ce5c5cfbca.jpg"},{"id":52104661,"identity":"f65c2d34-e958-4b3e-991a-d2489d8d351f","added_by":"auto","created_at":"2024-03-06 19:25:34","extension":"jpg","order_by":14,"title":"Figure 14","display":"","copyAsset":false,"role":"figure","size":103782,"visible":true,"origin":"","legend":"\u003cp\u003eModeling and mesh configurations of steel pipe piles in numerical simulations\u003c/p\u003e","description":"","filename":"14.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3999368/v1/2b3a6b2701af256c7d9f045f.jpg"},{"id":52104650,"identity":"2b043c3f-8038-428e-bbe9-6a9ec34d499b","added_by":"auto","created_at":"2024-03-06 19:25:33","extension":"jpg","order_by":15,"title":"Figure 15","display":"","copyAsset":false,"role":"figure","size":123821,"visible":true,"origin":"","legend":"\u003cp\u003eComparison between results of numerical simulations and field experiments\u003c/p\u003e","description":"","filename":"15.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3999368/v1/1091939ce44a7ed78e02b3f6.jpg"},{"id":52104654,"identity":"d7493e90-228e-4b04-ac29-5bd0e8fe5fa5","added_by":"auto","created_at":"2024-03-06 19:25:33","extension":"jpg","order_by":16,"title":"Figure 16","display":"","copyAsset":false,"role":"figure","size":120668,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of post-expansion diameters on allowable bearing capacity\u003c/p\u003e","description":"","filename":"16.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3999368/v1/bb7b450022fc85854b714b7e.jpg"},{"id":52104660,"identity":"1c2f9a84-ac12-45c3-af1b-0a92c57bbe02","added_by":"auto","created_at":"2024-03-06 19:25:34","extension":"jpg","order_by":17,"title":"Figure 17","display":"","copyAsset":false,"role":"figure","size":131457,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of the number of shear keys on allowable bearing capacity\u003c/p\u003e","description":"","filename":"17.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3999368/v1/b1f9b04d6ef13467d2d2314a.jpg"},{"id":59327071,"identity":"d56b8aff-b911-4df1-86e0-523751a283d6","added_by":"auto","created_at":"2024-06-29 14:41:04","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2343296,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-3999368/v1/1d3fcabd-d6ba-4c6f-a922-7a6fe8dfa138.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"\u003cp\u003eAssessment of Enhanced Load-Bearing Capacity of Expanded Steel Pipe Piles Considering Optimal Configuration\u003c/p\u003e","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eAs urban areas become denser, there has been a growing emphasis on the development of underground spaces to promote sustainable urban development [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. This trend is accompanied by a rising demand for foundational and seismic reinforcements in older buildings, particularly in large cities, leading to a greater need for pile construction in these areas [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. However, conventional prestressed high-strength concrete (PHC) piles or cast-in-place piles face challenges when installed in urban areas with numerous adjacent buildings and limited access for large construction equipment. In particular, the use of small construction equipment, capable of operating inside buildings, has become essential for pile construction in the reinforcement or underground development of existing structures. Consequently, the use of micropiles in construction has seen steady growth recently [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eMicropiles have a relatively smaller borehole diameter (typically around 100\u0026ndash;300 mm) compared to conventional piles [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Their construction is possible with small casing penetration equipment, making them suitable for small construction spaces while minimizing noise and vibration during installation. Additionally, micropiles offer advantages in flexible pile placement, rapid construction, and minimal environmental impact on the ground [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. Therefore, micropiles have been widely used in foundation construction within urban areas and for the reinforcement of existing buildings. Farhangi and Karakouzian [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e] employed micropile as the foundation for a bridge in a construction site where the use of large equipment was restricted due to adjacent structures and challenging soil conditions. Gupta and Chawla [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e] applied micropiles to improve the ground beneath a railway track without disrupting traffic or dismantling the track. Micropiles have also been employed for stabilizing soft ground or unstable slopes. An unstable slope with a 42-degree angle was reinforced by installing 139 micropiles and geocells, achieving a safety factor exceeding 1.0 [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. Pandit et al. [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e] analyzed the performance of micropiles in reinforcing slopes at risk of debris slides, considering factors such as diameter, spacing, and aspect ratio. The construction of 420 micropiles using jet grouting improved the safety of liquefaction-prone areas [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eRecently, there has been a rise in construction projects employing the \"floating and underground extension method,\" which involves expanding or excavating underground spaces beneath existing buildings without demolishing them. This method reinforces upper structures using micropiles while developing underground spaces beneath existing buildings [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. One notable case is the excavation of the underground space beneath the Seoul City Hall building, where 137 double-tube steel pipe micropiles were installed to support the upper structures before bottom-up excavation. Bracings were also utilized to prevent micropile buckling in areas with high stresses [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Studies have shown that the floating and underground extension method using micropiles enhances economic efficiency compared to conventional top-down or bottom-up methods for developing underground spaces [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. Moreover, micropiles have been used to reinforce existing foundations, supporting additional loads on existing buildings [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. Sometimes, micropiles were employed to mitigate the tilt of raft foundations beneath existing structures [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eDespite their advantages, micropiles have been criticized for their lower end-bearing capacity compared to conventional piles due to their small diameter [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. Consequently, micropiles primarily rely on frictional resistance to support loads. However, increasing the length of micropiles to enhance frictional resistance can elevate the risk of buckling due to a higher slenderness ratio, ultimately reducing their horizontal bearing capacity [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. Parametric studies have indicated that pile diameter enlargement contributes more to load-bearing capacity improvement than lengthening the piles [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. Some projects have utilized groups of micropiles along with a raft foundation to distribute loads evenly [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. However, the advantage of micropiles, which can be installed in small construction spaces, may diminish when installed in groups. On the other hand, to enhance horizontal bearing capacity, research has explored the installation of inclined micropiles, which improved seismic performance [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e, \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. However, the installation of inclined micropiles is constrained by the presence of adjacent structures, limiting their use to urban areas or tight spaces for existing building reinforcement.\u003c/p\u003e \u003cp\u003eNote that rather than installing piles inclined or in groups, one way to maximize economic feasibility is to enhance the load-bearing capacity of individual micropiles. In this regard, waveform micropiles were developed and constructed by applying jet-grouting to the surface of existing micropiles, increasing the diameter by approximately 1.5 times. Shear keys are configured along the pile length, improving both end-bearing capacity and frictional resistance. Loading tests have demonstrated that waveform micropiles can increase load-bearing capacity by up to 2.3 times compared to conventional micropiles [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. The proximity and number of shear keys in waveform micropiles also contribute to further bearing capacity improvement [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. When waveform micropiles are combined with conventional micropiles in a single raft foundation, waveform micropiles share a load more effectively, resulting in over twice of load-sharing ratio compared to conventional micropiles [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. However, waveform micropiles require expensive and large equipment for high-pressure jet-grouting after drilling. Moreover, the long curing time of grout makes waveform micropiles unsuitable for construction methods requiring rapid construction. As a similar concept, nodular piles, featuring bamboo-shaped joints along the pile axis, have been introduced as bored micropiles. Their load-bearing capacity has been found to be 1.6 times greater than that of conventional micropiles [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. However, nodular piles demand significant construction space and drilling costs. Moreover, pre-manufactured nodular piles come with additional costs for manufacturing them using advanced technology.\u003c/p\u003e \u003cp\u003eTherefore, this study introduces the expanded steel pipe pile, an innovative micropile that can be rapidly constructed even in tight spaces, while also offering improved load-bearing capacity. Expanded steel pipe piles are created by expanding small-diameter steel pipes inserted underground at specific intervals to form shear keys along the pile axis. The diameter of the steel pipe increases in the expanded section, enhancing both load-bearing capacity and resistance to buckling. The bump effect exerted by shear keys can enhance both frictional and tip resistances significantly. Moreover, the applied horizontal pressure for expanding the inserted steel pipe can be transmitted to the ground, compacting and reinforcing the surrounding soil. This study performed a series of numerical analyses and field verification experiments to ascertain the effectiveness of expanded steel pipe piles in improving load-bearing capacity. Initially, expansion tests were carried out using a specially developed hydraulic expansion device, which served to expand the steel pipes after they were inserted into the ground. Based on these tests, the configuration for the expanded steel pipe piles was designed to perform preliminary numerical analyses before field experiments. In the field verification experiments, static pile load tests were conducted on two conventional micropiles and two expanded steel pipe piles, each installed using different construction techniques. One approach involved injecting grout after inserting the steel pipe into the borehole, while the other approach comprised pressing the steel pipe directly into the ground. The conclusive phase of the study utilized the numerical model, which had been validated by the results of the field experiments. Parametric studies were performed, focusing on the post-expansion diameter of the steel pipe and the quantity of shear keys. These studies aimed to determine the optimal configuration for the expanded steel pipe pile, taking into account both the improvement in load-bearing capacity and constructability.\u003c/p\u003e"},{"header":"2. Development of hydraulic expansion device","content":"\u003cp\u003eA hydraulic expansion device was developed to efficiently expand steel pipes inserted into the ground, even in constrained spaces. Initially, a downsized hydraulic expansion device was manufactured to assess the potential for expanding steel pipes based on their thickness [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. The first expansion test considered steel pipe thicknesses of 2.9, 4.0, and 6.0 mm, with the results summarized in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, where the deformation ratio was defined as the change in diameter divided by initial diameter (i.e., 114.3 mm).\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eResults of expansion test for downsized hydraulic expansion device [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eThickness\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eInitial diameter\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003ePost-expansion diameter\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eDeformation ratio\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eExpansion time\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2.9 mm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e114.3 mm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e128.2 mm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e12.2%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e42.2 s\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e4.0 mm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e114.3 mm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e122.5 mm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e7.2%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e26.2 s\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e6.0 mm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e114.3 mm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e115.2 mm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.8%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e10.6 s\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eThe downsized hydraulic expansion device had a maximum hydraulic pressure capacity of 70 MPa, leading to minor deformation ratios in thicker steel pipes. Consequently, a steel pipe with a thickness of 6.0 mm experienced minimal expansion. In contrast, deformation ratios increased in thinner steel pipes, resulting in longer expansion times. Specifically, when compared to a steel pipe with a thickness of 4.0 mm, the deformation amount and expansion time increased by 1.56 and 1.61 times, respectively, for a steel pipe with a thickness of 2.9 mm.\u003c/p\u003e \u003cp\u003eUpon confirming the feasibility of steel pipe expansion, a full-scale hydraulic expansion device was developed. This device is capable of expanding carbon steel tubes typically used for steel pipe piles in field applications [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e], specifically those with a diameter exceeding 300 mm and a thickness over 6.0 mm. Additionally, this device offers the flexibility of adjusting the expansion amount, rather than just the hydraulic pressure, allowing for a more versatile formation of shear keys in steel pipes. The design drawing and configuration of the full-scale hydraulic expansion device are illustrated in Fig.\u0026nbsp;1.\u003c/p\u003e \u003cp\u003eThe device operates by hydraulically pulling the steel rod attached to Expander B, which in turn pushes Expander A radially to expand the steel pipe. Utilizing the full-scale hydraulic expansion device, a second expansion test was carried out on a steel pipe with a diameter of 318.5 mm and a thickness of 6.4 mm [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. The results demonstrated that the steel pipe could be expanded up to 370 mm in diameter (representing a deformation ratio of 16.2%) within a remarkable 150 seconds. This expansion rate is notably efficient in terms of constructability and installation time, especially when compared to waveform micropiles or nodular piles.\u003c/p\u003e "},{"header":"3. Preliminary numerical analysis for designing expanded steel pipe pile","content":"\u003cdiv id=\"Sec4\"\u003e\n \u003ch2\u003e3.1 Development of numerical model\u003c/h2\u003e\n \u003cp\u003eA preliminary numerical analysis was conducted using FLAC3D, a commercial program utilizing the finite difference method, to validate the enhancement in load-bearing capacity of the expanded steel pipe pile prior to field experiments. The ground was modeled with sufficient dimensions, featuring a width 20 times the diameter of the steel pipe and a height 3 times the length of the steel pipe to minimize boundary effects. The steel pipe was modeled according to the same specifications as in the first expansion test using the downsized hydraulic expansion device, except for the 6.0 mm thick steel pipe, which was excluded due to the lack of expansion observed during the first test. The dimensions of the expanded section (i.e., shear key) of the steel pipes were determined based on the results from the first expansion test. The details of the expanded steel pipe piles applied in the preliminary numerical analysis are summarized in Table\u0026nbsp;2. Additionally, Fig.\u0026nbsp;2 displays the numerical modeling of the ground and steel pipe pile.\u003c/p\u003e\n \u003cdiv\u003e\n \u003ctable id=\"Tab2\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv\u003eTable 2\u003c/div\u003e\n \u003cdiv\u003e\n \u003cp\u003eSpecifications of expanded steel pipe piles adopted in preliminary numerical analysis\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eThickness\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eInitial diameter\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003ePost-expansion diameter\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eHeight of expanded part\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eLength of pile\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.9 mm\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e114.0 mm\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e128.0 mm\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e50.0 mm\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.0 m\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4.0 mm\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e114.0 mm\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e123.0 mm\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e50.0 mm\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.0 m\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n \u003cp\u003eIn the numerical analysis, the elasticity model was applied to the steel pipe, while the Mohr-Coulomb model was employed for the ground. Interface elements were utilized at the boundary between the steel pipe pile and the ground to simulate the interaction between these two materials. The values of the vertical stiffness coefficient (\\({K}_{n}\\)) and the shear stiffness coefficient (\\({K}_{t}\\)) assigned to the interface elements were estimated by Eqs.\u0026nbsp;(1) and (2), respectively.\u003c/p\u003e\n \u003cdiv id=\"Equa\"\u003e\n \u003cdiv id=\"FileID_Equa\" name=\"EquationSource\"\u003e$${K}_{n}=\\frac{2R(1-{v}_{i})}{(1-2{v}_{i})}\\times \\frac{{E}_{soil}}{2(1+{v}_{soil})}$$\u003c/div\u003e\n \u003c/div\u003e\n \u003cp\u003e(1)\u003c/p\u003e\n \u003cdiv id=\"Equb\"\u003e\n \u003cdiv id=\"FileID_Equb\" name=\"EquationSource\"\u003e$${K}_{t}=\\frac{R\\bullet {E}_{soil}}{2{t}_{v}(1+{v}_{soil})}$$\u003c/div\u003e\n \u003c/div\u003e\n \u003cp\u003e(2)\u003c/p\u003e\n \u003cp\u003ewhere, \\(R\\) represents the strength reduction coefficient of the interface, \\({v}_{i}\\) is the Poisson\u0026rsquo;s ratio of the interface, \\({E}_{soil}\\) is the elastic modulus of the soil, \\({v}_{soil}\\) is the Poisson\u0026rsquo;s ratio of the soil, and \\({t}_{v}\\) is the virtual thickness of the interface, which decreases as the stiffness difference between materials increases.\u003c/p\u003e\n \u003cp\u003eIn this study, a value of 0.67 was applied for \\(R\\), and \\({v}_{i}\\) was assumed to be 0.45 to ensure numerical simulation convergence. The ground model was based on sandy soil conditions for the preliminary numerical analyses. The material properties adopted in the numerical model are summarized in Table\u0026nbsp;3.\u003c/p\u003e\n \u003cdiv\u003e\n \u003ctable id=\"Tab3\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv\u003eTable 3\u003c/div\u003e\n \u003cdiv\u003e\n \u003cp\u003eMaterial properties adopted in preliminary numerical analysis\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eProperties\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eSteel pipe pile\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eSandy soil\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eElastic modulus\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e210,000 MPa\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e25 MPa\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ePoisson\u0026rsquo;s ratio\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.30\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.30\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eUnit weight\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e78.5 kN/m\u003csup\u003e3\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e17.0 kN/m\u003csup\u003e3\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eViscosity\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026ndash;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e15 kPa\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eFriction angle\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026ndash;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e25\\(^\\circ\\)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n \u003cp\u003eThe mesh for the steel pipe pile was constructed using uniform-sized hexahedron elements, while the ground mesh consisted of tetrahedral elements with gradually increasing sizes towards the boundaries, taking into account optimized simulation time considerations. During the simulations, the settlement changes were monitored while progressively increasing the load on the steel pipe pile. The load that caused a settlement of 25.0 mm, conforming to the allowable settlement in the design standard for structural foundations, was determined as the ultimate bearing capacity of the pile. Furthermore, various configurations of expanded steel pipe piles were numerically simulated to assess the impact of shear key location, expansion configuration, and expansion time on their ultimate bearing capacity.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec5\"\u003e\n \u003ch2\u003e3.2 Effect of shear key location\u003c/h2\u003e\n \u003cp\u003eTo assess the impact of shear key location on the load-bearing capacity of expanded steel pipe piles, numerical simulations were carried out for three different pile configurations, each with a single shear key positioned at the tip, middle, or head of the pile. The ultimate bearing capacities of these three expanded steel pipe piles were then compared with that of a conventional micropile without shear keys. The pile specifications considered in this comparison are detailed in Table\u0026nbsp;4. Figure\u0026nbsp;3 illustrates the modeling results of the steel pipe piles according to the shear key locations, while Fig.\u0026nbsp;4 displays the ultimate bearing capacities of the steel pipe piles, categorized by shear key location and steel pipe thickness.\u003c/p\u003e\n \u003cp\u003eIt was observed that the ultimate bearing capacity increased when a shear key was present in both 2.9 mm and 4.0 mm thick steel pipe piles. Given the minor difference in steel pipe thickness (i.e., between 2.9 mm and 4.0 mm), the enhancement in ultimate bearing capacity due to increased steel pipe thickness was negligible in conventional micropiles. However, the enhancement ratio of ultimate bearing capacity was more pronounced with the incorporation of a shear key, especially as the steel pipe thickness increased. Additionally, the ultimate bearing capacity was more effectively improved when the shear key was positioned closer to the tip. For expanded steel pipe piles with a shear key at the tip, the ultimate bearing capacity increased by 25.0% for the 2.9 mm thick steel pipe piles and by 31.5% for the 4.0 mm thick steel pipe piles, compared to those with a shear key at the head.\u003c/p\u003e\n \u003cp\u003eIn summary, the ultimate bearing capacity of an expanded steel pipe pile with a single shear key at the pile tip was enhanced by up to 1.32 times compared to the conventional micropile. Given that only one shear key was formed and the deformation ratio of the shear key was less than 12.2%, further improvements in the ultimate bearing capacity of the expanded steel pipe pile could be expected by increasing both the number of shear keys and their deformation ratio.\u003c/p\u003e\n \u003cdiv\u003e\n \u003ctable id=\"Tab4\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv\u003eTable 4\u003c/div\u003e\n \u003cdiv\u003e\n \u003cp\u003eVariables in numerical simulations according to shear key location\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003ePile specification\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eVariables\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eThickness of steel pipe\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.9 mm / 4.0 mm\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eShear key location\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eNo expansion / Tip / Middle / Head\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n \u003cp\u003e\u003cbr\u003e\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec6\"\u003e\n \u003ch2\u003e3.3 Effect of expansion configuration\u003c/h2\u003e\n \u003cp\u003eNumerical simulations were conducted to assess the enhancement in ultimate bearing capacities of expanded steel pipe piles, specifically between two scenarios: one involving the configuration of a single shear key with an extended height through consecutive steel pipe expansions and the other involving multiple shear keys with shorter heights at fixed intervals. Accordingly, two configurations were modeled: one with a single 250 mm high shear key, and another with five shear keys, each 50 mm high and spaced at 200 mm intervals. Furthermore, the ultimate bearing capacity of the expanded steel pipe pile with a single shear key at the pile tip was also compared to analyze the influence of the number of shear keys on the ultimate bearing capacity. The relevant variables considered for pile specification and the modeling results to analyze the effect of the expansion configuration on the ultimate bearing capacity of expanded steel pipe piles are presented in Table\u0026nbsp;5 and Fig.\u0026nbsp;5, respectively. Figure\u0026nbsp;6 displays the ultimate bearing capacities of the steel pipe piles based on the expansion configuration and steel pipe thickness.\u003c/p\u003e\n \u003cp\u003eAs previously analyzed, thicker steel pipes exhibited a more pronounced enhancement in the ultimate bearing capacity of steel pipe piles when shear keys were introduced. Regarding the number of shear keys, the ultimate bearing capacity increased by an average of 29.0% when a single shear key was incorporated at the tip compared to conventional micropiles. In contrast, when five shear keys were arranged at 200 mm intervals, the ultimate bearing capacity increased by an average of 33.2%. Interestingly, the increase in ultimate bearing capacity of expanded steel pipe piles was not directly proportional to the number of shear keys. Compared to piles with consecutive expansion, those expanded at specific intervals showed a greater increase in ultimate bearing capacity. Since expanded steel pipe piles enhance load-bearing capacity by improving both frictional and tip resistances in shear keys, forming multiple shorter shear keys proved more effective.\u003c/p\u003e\n \u003cp\u003eThe ultimate bearing capacity of the expanded steel pipe pile equipped with five shear keys was up to 1.37 times greater than that of the conventional micropile. Given that the steel pipe length was limited to 1 meter in the preliminary numerical simulations, it is anticipated that the load-bearing capacity of expanded steel pipe piles could be further enhanced by increasing the number of shear keys on longer steel pipes at actual construction sites.\u003c/p\u003e\n \u003cdiv\u003e\n \u003ctable id=\"Tab5\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv\u003eTable 5\u003c/div\u003e\n \u003cdiv\u003e\n \u003cp\u003eVariables in numerical simulations according to expansion configuration\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003ePile specification\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eVariables\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eThickness of steel pipe\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.9 mm / 4.0 mm\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eExpansion method\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eNo expansion / One expansion at tip / Consecutive expansion /\u003c/p\u003e\n \u003cp\u003eExpansion 5 times at intervals of 200 mm\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec7\"\u003e\n \u003ch2\u003e3.4 Effect of expansion time\u003c/h2\u003e\n \u003cp\u003eIn the first expansion test using the downsized hydraulic expansion device, the average time required to form one shear key by expanding a 2.9 mm thick steel pipe was 42.2 seconds, whereas for a 4.0 mm thick steel pipe, it was 26.2 seconds. In other words, a 2.9 mm thick steel pipe required approximately 1.6 times more expansion time than a 4.0 mm thick steel pipe to create the same number of shear keys. This difference in expansion times was attributed to varying deformation ratios, arising from the consistently maintained maximum pressure of the downsized hydraulic expansion device. Consequently, numerical simulations were conducted for a 2.9 mm thick steel pipe pile with one shear key and a 4.0 mm thick steel pipe pile with two shear keys to identify an efficient expansion method that balances both expansion time and load-bearing capacity. The ultimate bearing capacities of steel pipe piles obtained from numerical simulation, categorized by expansion time, are presented in Fig.\u0026nbsp;7.\u003c/p\u003e\n \u003cp\u003eThe results showed that the 4.0 mm thick expanded steel pipe pile with two shear keys, each having a relatively small deformation ratio, exhibited an ultimate bearing capacity approximately 7.6% higher than the 2.9 mm thick expanded steel pipe pile with one shear key at a relatively large deformation ratio. In essence, forming multiple shear keys, even with smaller deformation ratios, proved to be more effective in terms of both load-bearing capacity and construction time compared to forming a limited number of shear keys with larger deformation ratios.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"4. Test bed field experiments","content":"\u003cp\u003eThe enhanced load-bearing capacity of expanded steel pipe piles was investigated through static load tests on full-scale steel pipe piles constructed in a test bed. Two conventional micropiles and two expanded steel pipe piles were built using different construction methods, depending on whether grout injection was utilized. In the grout injection method, grout was injected after the steel pipe was inserted into a pre-drilled borehole, resulting in a pile foundation diameter larger than that of the steel pipe itself. In contrast, the no-grout method (i.e., press-in method) involved directly pressing steel pipes into the ground using a screw auger, without pre-drilling. Two construction techniques for steel pipe piles implemented in the test bed are shown in Fig.\u0026nbsp;8.\u003c/p\u003e\n\u003cp\u003eThe test bed ground composition consists of a landfill layer with silty sand from the surface down to a depth of 6.5 meters, followed by weathered rock from 6.5 meters to 8.0 meters, and soft rock beneath that. Considering constructability, the length of the steel pipe piles constructed in the test bed was designed to be 6.0 m. In accordance with the specifications for carbon steel tubes intended for general structural purposes [\u003cspan class=\"CitationRef\"\u003e32\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e33\u003c/span\u003e], steel pipes with a diameter of 318.5 mm and a thickness of 6.4 mm were utilized. The post-expansion diameter was set to 360 mm (approximately 13% of the deformation ratio), guided by the results of another expansion test using the full-scale hydraulic expansion device. The shear keys were designed to be 100 mm high. The steel pipe expansion was carried out at three different depths. Based on the preliminary numerical simulation results indicating that the ultimate bearing capacity of expanded steel pipe piles increased more effectively when the shear key was located closer to the pile tip, the expansion was executed at depths of 5.5 m, 5.0 m, and 4.5 m. Figure \u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003e illustrates the expansion process of steel pipes inserted into the ground.\u003c/p\u003e\n\u003cp\u003eThe configuration of the expanded steel pipe piles constructed in the test bed and the results of ground investigation are presented in Fig. 10. In addition, the specifications of the steel pipe piles constructed in the test bed are summarized in Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e.\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u003ctable id=\"Tab6\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 6\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003ePile specifications constructed in the test bed\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"4\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eNo.\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eType\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eConstruction method\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eConfiguration of shear key\u003c/p\u003e\n \u003cp\u003e(Diameter / Height / Number)\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eConventional micropile\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eGrout injection\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eNo expansion\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eExpanded steel pipe pile\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eGrout injection\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e360 mm / 100 mm / 3\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eConventional micropile\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ePress-in\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eNo expansion\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eExpanded steel pipe pile\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ePress-in\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e360 mm / 100 mm / 3\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003eAfter constructing the four steel pipe piles, static load tests were conducted, incrementally applying a load of 49.05 kN. The resulting load-settlement curves are shown in Fig. 11. However, it\u0026rsquo;s worth noting that the ultimate bearing capacity was not definitively determined for all the constructed steel pipe piles as the tests were completed before a nonlinear relationship between settlement and applied load could be established as shown in Fig. 11. Therefore, the allowable bearing capacities of the constructed steel pipe piles were derived using Davisson\u0026rsquo;s method with a safety factor of 2.0. Table \u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e summarizes the static load test results for the four steel pipe piles installed in the test bed. Table \u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e also highlights the relative improvement in allowable bearing capacity of the expanded steel pipe pile compared to the conventional micropile, utilizing the same construction method.\u003c/p\u003e\n\u003cp\u003eWhen using the grout injection method, the steel pipe piles exhibited increased allowable bearing capacities compared to pressing the steel pipe directly into the ground without grout (i.e., press-in method). This increase was attributed to the enlarged pile foundation volume (i.e., diameter), resulting from the injected grout. The amount of injected grout was greater in the conventional micropile than in the expanded steel pipe pile because the expanded steel pipe piles had shear keys created by expanding the steel pipe in advance before grout injection. Consequently, the allowable bearing capacity of the conventional micropile experienced a more significant enhancement than that of the expanded steel pipe pile when the grout injection method was applied. Specifically, when installing steel pipe piles using the grout injection method, the allowable bearing capacity increased by 1.26 times for the conventional micropile and by 1.16 times for the expanded steel pipe pile.\u003c/p\u003e\n\u003cp\u003eCompared to the conventional micropiles, the allowable bearing capacity of the expanded steel pipe piles increased by 1.11 times when the grout injection method was applied and by 1.21 times when the press-in method was applied. For the steel pipe piles installed with grout injection, the diameter of the pile foundation was the same for both conventional micropiles and expanded steel pipe piles. However, when the press-in method was utilized, only the expanded steel pipe piles exhibited an increase in the diameter of the pile foundation (i.e., shear keys), leading to a more significant enhancement in allowable bearing capacity compared to the conventional micropile.\u003c/p\u003e\n\u003cp\u003ePrevious studies have shown that waveform micropiles have approximately 1.4 to 2.3 times higher load-bearing capacity compared to conventional micropiles [\u003cspan class=\"CitationRef\"\u003e26\u003c/span\u003e], In contrast, the allowable bearing capacity of expanded steel pipe piles in the field experiments of this study increased by up to 1.21 times. It is important to note that the load-bearing capacity comparison for waveform micropiles was made under conditions where their diameter was increased by 1.5 times due to injected grout. Similarly, a significant difference in allowable bearing capacity (about 1.4 times) was observed when comparing the conventional micropiles installed without grout injection (252.5 kN) to the expanded steel pipe piles installed with grout injection (354.1 kN). Furthermore, the field tests limited the post-expansion diameter to 360 mm and the number of shear keys to three to avoid potential fractures or construction issues during the field experiments. Based on the preliminary numerical simulation results, it is conceivable that the allowable bearing capacity of expanded steel pipe piles could be further enhanced by increasing both the post-expansion diameter and the number of shear keys.\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u003ctable id=\"Tab7\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 7\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eResults of static load tests for four steel pipe piles constructed in the test bed\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"5\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eNo.\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eType\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eInstallation\u003c/p\u003e\n \u003cp\u003emethod\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eAllowable\u003c/p\u003e\n \u003cp\u003ebearing capacity\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eImprovement of allowable bearing capacity\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eConventional micropile\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eGrout injection\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e318.8 kN\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eExpanded steel pipe pile\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eGrout injection\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e354.1 kN\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e11.1%\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eConventional micropile\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ePress-in\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e252.5 kN\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eExpanded steel pipe pile\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ePress-in\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e305.2 kN\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e20.9%\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003e\u003cbr\u003e\u003c/p\u003e\n\u003cp\u003eAfter the static load tests were completed, the interior of the expanded steel pipes was inspected using a camera, and subsequently, the pipes were extracted to examine the condition of their expansion. As illustrated in Fig. 12, the hydraulic expansion device effectively and reliably expanded the steel pipes inserted into the ground of the test bed. The field verification experiments demonstrated that the load-bearing capacity of expanded steel pipe piles could be enhanced compared to conventional micropiles. Notably, expanded steel pipe piles can offer several advantages, including ease of construction, suitability for tight spaces, and cost-effectiveness due to minimal equipment and technical requirements. Consequently, it can be concluded that expanded pipe piles exhibit significantly improved performance and practical applicability for field use.\u003c/p\u003e"},{"header":"5. Optimal configuration of expanded steel pipe pile","content":"\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\n \u003ch2\u003e5.1 Verification of numerical model using field experiments\u003c/h2\u003e\n \u003cp\u003eBecause of the relatively low applied loads during the static load tests, it was not possible to determine the yield load and ultimate bearing capacity of the steel pipe piles from the field experiment results. Consequently, the static load tests were numerically simulated using the FLAC3D program under the same conditions as those encountered in the field experiments. This allowed for a reanalysis of the load-bearing capacity of the expanded steel pipe pile. In the numerical analysis, the ground model was constructed to reflect the actual subsurface layers in the test bed: landfill layer, weathered rock, and soft rock, as determined by the ground investigation. The material properties for each ground layer applied in the numerical model were estimated through empirical formulas [\u003cspan class=\"CitationRef\"\u003e34\u003c/span\u003e]. The four steel pipe piles were modeled to match the specifications of those installed in the test bed. The material properties for each ground layer, steel pipe, and grout applied in the numerical simulations are summarized in Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003e. To minimize boundary effects, the ground was modeled with dimensions of 18 m in width and height. The design drawings for the four steel pipe pile models used in the numerical simulations are illustrated in Fig. 13, with the results of modeling and mesh formation presented in Fig. 14.\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u003ctable id=\"Tab8\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 8\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eMaterial properties of ground, steel pipe, and grout applied in numerical simulations\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"6\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eProperties\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eLandfill layer\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eWeathered rock\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eSoft rock\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eSteel pipe\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eGrout\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eElastic modulus\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e20 MPa\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e200 MPa\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1,000 MPa\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e205,000 MPa\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e200 MPa\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ePoisson\u0026rsquo;s ratio\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.35\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.31\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.25\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.31\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eUnit weight\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e19 kN/m\u003csup\u003e3\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e21 kN/m\u003csup\u003e3\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e25 kN/m\u003csup\u003e3\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e78.5 kN/m\u003csup\u003e3\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e21 kN/m\u003csup\u003e3\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eViscosity\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e20 kPa\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e31 kPa\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e100 kPa\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026ndash;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e31 kPa\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eFriction angle\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e30\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(^\\circ\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e23\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(^\\circ\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e35\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(^\\circ\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026ndash;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e33\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(^\\circ\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n \u003cdiv class=\"gridtable\"\u003e\n \u003cdiv align=\"left\" class=\"colspec\"\u003e\u003cbr\u003e\u003c/div\u003e\n \u003c/div\u003e\n \u003cp\u003eDuring the numerical simulations, loads were applied to the steel pipe piles following the same loading increments as in the field experiments, and the resulting settlement was obtained accordingly. However, since the field experiments did not apply sufficient load, in the numerical simulations, the load was incrementally increased by 10% after reaching the final load step used in the field experiments. The results of the numerical simulations were then compared with those from the field experiments, as illustrated in Fig.\u0026nbsp;15.\u003c/p\u003e\n \u003cp\u003eSimilar to the field experiments, Davisson\u0026rsquo;s method was employed to estimate the allowable bearing capacity. The difference between the allowable bearing capacities estimated by the numerical simulations and those from field experiments averaged 11.1% for the conventional micropiles and 0.8% for the expanded steel pipe piles. This demonstrates that the developed numerical model can reliably estimate the allowable bearing capacity of steel pipe piles. Furthermore, in contrast to the field experiment results, the load-settlement curves derived from the numerical simulations exhibited nonlinear behavior, allowing for the estimation of ultimate bearing capacity. When estimating the ultimate bearing capacities of steel pipe piles based on the applied load corresponding to a settlement of 10% of the pile diameter, it was observed that the ultimate bearing capacity increased in the expanded steel pipe piles compared to the conventional micropiles at a similar ratio to the bearing capacity estimated using Davisson\u0026rsquo;s method.\u003c/p\u003e\n \u003cp\u003eAs a result, it was concluded that the developed numerical model is suitable for analyzing the load-bearing capacity of expanded steel pipe piles under various conditions. Consequently, this numerical model was used to determine the optimal configuration of expanded steel pipe piles by conducting a series of parametric studies described in the next section.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\n \u003ch2\u003e5.2 Parametric studies for optimal expanded steel pipe piles\u003c/h2\u003e\n \u003cp\u003eParametric studies were conducted, taking into account different construction methods for the piles, post-expansion diameters, and the number of shear keys. The variables considered in these parametric studies are summarized in Table \u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003e. The numerical model employed for these parametric studies was identical to the one employed for simulating the field experiment, with modifications only made in accordance with the parameter variables listed in Table \u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003e. Settlements were estimated by applying the same load increments at each loading step as employed in the field experiments.\u003c/p\u003e\n \u003cp\u003eIn these parametric studies, the allowable bearing capacity of the steel pipe piles was determined by averaging the ultimate load with a safety factor of 3.0 and the yield load with a safety factor of 2.0. The ultimate loads were determined using the P (Pull-out load) -S (Settlement) analysis method, while the yield loads were derived using both the log P-log S analysis method and Davisson\u0026rsquo;s method.\u003c/p\u003e\n \u003cp\u003eFigure\u0026nbsp;16 illustrates how the allowable bearing capacity of expanded steel pipe piles varied with different post-expansion diameters. As the post-expansion diameters increased, their allowable bearing capacities also increased. However, it is important to note that this increase was not directly proportional to the post-expansion diameters. Furthermore, there was no significant increase in allowable bearing capacity when the expansion in steel pipes exceeded a diameter of 360 mm. This tendency became more apparent with the addition of more shear keys. In essence, as the number of shear keys increased, the impact on improving the load-bearing capacity due to an increase in post-expansion diameter became less significant. Considering factors such as construction time and the risk of pipe fractures during the expansion process of steel pipes, the optimal post-expansion diameter was determined to be 360 mm, corresponding to a deformation ratio of 13%.\u003c/p\u003e\n \u003cp\u003eThe variation in the allowable bearing capacity of expanded steel pipe piles in relation to the number of shear keys is illustrated in Fig. 17. It was observed that the allowable bearing capacity increased proportionally, following a specific slope, as the number of shear keys increased. Specifically, when the number of shear keys was three or fewer, the allowable bearing capacity showed a relatively significant increase with respect to the number of shear keys. This rate of increase slightly declined when the number of shear keys exceeded four. Based on the results of the parametric studies, it can be concluded that maximizing the number of shear keys by expanding the steel pipe is advantageous for enhancing load-bearing capacity. Nevertheless, when taking constructability and construction time into consideration, if extensive steel pipe expansion is not feasible, configuring at least three shear keys is recommended for efficiently improving the load-bearing capacity.\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u003ctable id=\"Tab9\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 9\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eVariables considered in parametric studies\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"2\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eConsidered parameters\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eVariables\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eConstruction method of piles\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eGrout injection method / Press-in method\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ePost-expansion diameter\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e340 mm / 350 mm / 360 mm / 370 mm\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eNumber of shear keys\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1 / 2 / 3 / 4 / 5 / 6 / 7\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n\u003c/div\u003e"},{"header":"6. Conclusion","content":"\u003cp\u003eThis study investigated the enhancement of load-bearing capacity of expanded steel pipe piles compared to conventional micropiles through numerical simulations and field experiments. Additionally, optimal configurations for expanded steel pipe piles, balancing constructability and load-bearing capacity, were determined using parametric studies with the developed numerical model. The key findings of this study are as follows.\u003c/p\u003e \u003cp\u003e \u003col\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eA hydraulic expansion device was developed to construct expanded steel pipe piles, and its potential for expanding steel pipes in the ground was verified through pipe expansion tests. The formation of a shear key within 150 seconds indicated that expanded steel pipe piles have higher constructability compared to existing improved micropiles.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eThe preliminary numerical analysis confirmed the enhanced load-bearing capacity of expanded steel pipe piles. The ultimate bearing capacity was effectively increased when shear keys were positioned closer to the pile tip, and more shear keys were formed. Moreover, it was found that configuring many shear keys with a small deformation ratio was more effective for increasing load-bearing capacity than forming fewer shear keys with a large deformation ratio within the same construction time.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eThe field verification experiments showed that the expanded steel pipe piles had up to 1.4 times higher allowable bearing capacity than the conventional micropiles. Notably, the use of the press-in method for steel pipe installation resulted in more effective load-bearing capacity improvement in expanded steel pipe piles than the grout injection method. However, both expanded steel pipe piles and conventional micropiles exhibited higher allowable bearing capacity when installed using the grout injection method. Post-experiment inspections confirmed that steel pipes were stably expanded underground.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eIn the parametric studies, expanding the pipe diameter to 360 mm or more did not significantly enhance the allowable bearing capacity of expanded steel pipes. Conversely, the allowable bearing capacity of expanded steel pipe piles increased proportionally with the number of shear keys, although the rate of increase declined when more than four shear keys were configured. Considering the efficiency of load-bearing capacity enhancement and stability during pipe expansion, expanded steel pipe piles should be constructed with at least three shear keys and a post-expansion diameter of 360 mm (13% deformation ratio) or less.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003c/ol\u003e \u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eData will be made available on request to the corresponding author.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCRediT authorship contribution statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSangwoo Park:\u0026nbsp;\u003c/strong\u003eData curation, Validation, Visualization, Writing-original draft.\u003cstrong\u003e\u0026nbsp;Uiseok Kim:\u0026nbsp;\u003c/strong\u003eInvestigation, Experiment, Visualization. \u003cstrong\u003eHyeontae Park:\u003c/strong\u003e \u003cstrong\u003e\u0026nbsp;\u003c/strong\u003eSoftware, Data curation. \u0026nbsp;\u003cstrong\u003eHangseok Choi:\u003c/strong\u003e Project administration, Supervision, Writing review \u0026amp; editing, Funding acquisition.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDeclaration of competing interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis study was supported by National Research Foundation of Korea (NRF) grants funded by the Korean government (MSIT) (No. 2019R1A2C2086647 and No. 2020R1A6A1A03045059).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eSterling, R., Admiraal, H., Bobylev, N., Parker, H., Godard, J. 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G\u0026eacute;otechnique Letters, 9(3), 198-204. https://doi.org/10.1680/jgele.18.00209\u003c/li\u003e\n\u003cli\u003eLiu, N., Huang, Y. X., Wu, B., \u0026amp; Huang, W. (2021). Experimental Study on Lateral Bearing Mechanical Characteristics and Damage Numerical Simulation of Micropile. Advances in Civil Engineering, 2021. https://doi.org/10.1155/2021/9927922\u003c/li\u003e\n\u003cli\u003eHwang, T. H., Kim, K. H., Shin, J. H. (2023). Experimental and Numerical Study on Lateral Resistance of Micropile in Sand. KSCE Journal of Civil Engineering, 27(9), 3740-3752. https://doi.org/10.1007/s12205-023-0977-0\u003c/li\u003e\n\u003cli\u003eMoradi Moghaddam, H., Keramati, M., Ramesh, A., \u0026amp; Naderi, R. (2021). Experimental evaluation of the effects of structural parameters, installation methods and soil density on the micropile bearing capacity. International Journal of Civil Engineering, 19, 1313-1325. https://doi.org/10.1007/s40999-021-00629-5\u003c/li\u003e\n\u003cli\u003eDas, M. 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Choi, 2022, An Experimental Study on the Performance of Expandable Steel Pipe Pile, Journal of the Korean Geo-Environmental Society, 23(1), 39-49. https://doi.org/10.14481/jkges.2022.23.1.39\u003c/li\u003e\n\u003cli\u003eNational Institute of Technology and Standards, Carbon steel tubes for general structural purposes, 2018, Korea\u003c/li\u003e\n\u003cli\u003eKim, J., Kim, U., Min, B., Choi, H., Park, S. (2022). Development of Expanded Steel Pipe Pile to Enhance Bearing Capacity. Sustainability, 14(5), 3077. https://doi.org/10.3390/su14053077\u003c/li\u003e\n\u003cli\u003eMinistry of Land, Infrastructure, and Transport, Guidelines of Design Practices for National Highway Construction Works, 2021. 02. 01. \u003c/li\u003e\n\u003cli\u003eThe Korea Expressway Corporation, Guidelines of Design for Road, 2021. 12. 16.\u003c/li\u003e\n\u003cli\u003eKorean Geotechnical Society, Commentary on design standards for structure foundation, 2018\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Expanded steel pipe pile, Micropile, Small-diameter steel pipe pile, Load-bearing capacity of micropiles, Static load test","lastPublishedDoi":"10.21203/rs.3.rs-3999368/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-3999368/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eRecently, advanced types of micropiles have been developed to enhance their load-bearing capacity, responding to increased demands for micropile applications. However, these improved micropiles present challenges for rapid construction within confined spaces due to construction complexities. This study introduces an expanded steel pipe pile, which not only offers improved load-bearing capacity but also facilitates rapid construction in limited spaces. The expanded steel pipe pile is created by expanding a pre-installed small-diameter steel pipe at specific intervals to form shear keys along the length of the pile. In this study, the potential for enhancing the load-bearing capacity of the expanded steel pipe pile was initially verified through preliminary numerical analyses. Subsequently, comprehensive field experiments were carried out comparing two traditional micropiles with two expanded steel pipe piles, each installed using different methods. The results showed that the expanded steel pipe piles had up to 1.4 times the allowable bearing capacity of conventional micropiles. Finally, utilizing the numerical model validated by the field experiment results, the optimal configuration for the expanded steel pipe pile was provided. Considering both load-bearing efficiency and constructability, it was concluded that the expanded steel pipe pile should have a post-expansion diameter of 360 mm (with a deformation ratio of 13%) or less and incorporate at least three shear keys.\u003c/p\u003e","manuscriptTitle":"Assessment of Enhanced Load-Bearing Capacity of Expanded Steel Pipe Piles Considering Optimal Configuration","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-03-06 19:25:13","doi":"10.21203/rs.3.rs-3999368/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"d4024c66-050d-4424-9767-e954ebd51bd2","owner":[],"postedDate":"March 6th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2024-06-29T14:32:55+00:00","versionOfRecord":[],"versionCreatedAt":"2024-03-06 19:25:13","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-3999368","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-3999368","identity":"rs-3999368","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}