Study on Determining Safe Distance and Optimization of Supporting Methods for Adjacent Tunnels in Fault Fracture Zone

Study on Determining Safe Distance and Optimization of Supporting Methods for Adjacent Tunnels in Fault Fracture Zone | 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 Study on Determining Safe Distance and Optimization of Supporting Methods for Adjacent Tunnels in Fault Fracture Zone Pak Chol-Hyok This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8706282/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 The design and construction of tunnels and underground structures in deformed rock masses, such as fault fracture zones, pose significant challenges in geotechnical engineering. This study proposes an optimized excavation method for adjacent tunnels in fault fracture zones, integrating in-situ testing, numerical simulation, and field verification. First, the physical and mechanical properties of the surrounding rock mass were characterized using a Point Load Tester (PLT) and an elastic wave velocity measuring instrument (ZBL-U5200), and the rock mass strength weakening law was quantified. The rock mass strength degradation coefficient was calibrated based on field monitoring data to reflect the actual geological conditions. Subsequently, FLAC3D numerical simulation was employed to evaluate the stability of adjacent tunnels in the weak rock mass. A technical scheme combining sequential excavation and gradual cross-sectional enlargement was proposed for the weak rock sections, accompanied by a parallel simultaneous construction strategy. The effectiveness of the proposed method was verified through numerical simulation and field application. Advanced excavation and support technologies were successfully implemented, significantly improving the stability of adjacent tunnels. This study provides a reliable technical approach for tunnel excavation and underground structure construction in highly fractured and weak rock masses, particularly in fault fracture zones. Geology Fault fracture zone Adjacent tunnels Safety distance FLAC3D Excavation method Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 1. Introduction Faults are critical geological discontinuities that dominate the stability of underground structures during design and construction (Wei et al. 2025 ). Structural planes, including faults, joints, and bedding planes, are the primary factors controlling the stability of deep underground structures. Under high-stress conditions, these planes can trigger rock mass instability, fracturing, and large-scale failure, leading to severe safety hazards and engineering accidents (Xu et al. 2024 ; Yin et al. 2024 ; Huang et al. 2025 ). Consequently, major fault zones are generally avoided in underground construction to mitigate the risks of structural collapse and rock mass failure (Xu et al. 2018 ). However, complete avoidance of fault zones is often impractical due to spatial layout constraints, economic considerations, geological uncertainties (e.g., unforeseen fault zones), and complex interactions between faults and associated joints (Xu et al. 2024 ; Chen et al. 2025 ). Thus, the scientific analysis of fault characteristics and their impacts on geotechnical stability is a pivotal research topic in geological and geotechnical engineering (Fasching and Vanek 2011 ). Brittle fault zones typically exhibit strong anisotropy, resulting in significant differences in stress distribution and displacement between surrounding rocks and underground structures. Such geological conditions frequently induce critical issues, including rock mass instability, premature collapse, and structural failure during tunneling (Entfellner and Moritz 2025 ). Numerous studies have focused on the safety and stability of tunnels in fault zones. Li et al. ( 2022 ) used COMSOL Multiphysics to analyze the safety distance between adjacent tunnels under different excavation cross-sections and water pressure conditions. Their findings revealed that increased pressure in the fault fracture zone and reduced distance between the working face and the fault zone enhance groundwater connectivity with rock fractures, significantly compromising excavation face stability. Groundwater infiltration into fractured rock layers within fault zones is the primary driver of progressive rock stability deterioration post-excavation. Water erosion expands existing fractures, increases rock moisture content, and drastically weakens rock mass strength (Chen et al. 2023 ; Yao et al. 2025 ). Additionally, groundwater flow can trap large rock particles in fractures, reducing the bearing capacity of surrounding rocks and triggering collapse (Chen et al. 2023 ). Therefore, comprehensive geological investigations of fault zones and accurate characterization of tunnel fracture and deformation characteristics are indispensable for safe construction in such areas (Yao et al. 2025 ; Xie et al. 2025 ). Stability issues and blast stress wave effects between adjacent tunnels in fault fracture zones can be evaluated using computer simulations, field experiments, and theoretical calculations. To minimize errors, these evaluations should integrate numerical predictions with field-measured data (Feng et al. 2022 ). Xia et al. ( 2026 ) investigated the dynamic response of twin tunnels to blasting and collapse impact loads through a case study of a 24-story building demolition. Other studies have assessed the safe distance and dynamic adaptation state of adjacent tunnels considering geological structures and explosive stress wave effects (Zhang et al. 2025 ; Wang et al. 2025 ; Li et al. 2025 ). Many scholars have also proposed tunnel stability assessment methods and optimized blasting designs for fault zones, which have been successfully applied in engineering practice. However, tunnel collapse induced by faults may still require permanent repairs or bypass tunnel excavation, and research on systematic excavation methods and safety distance determination for adjacent tunnels in weak fault zones remains limited. In the study area, a 2500-meter-long concrete tunnel (6 m wide, 2.3 m high) constructed in the 1980s was severely damaged by fault activity. A 100-meter section from the collapse zone to the tunnel entrance direction was completely filled with collapsed rock. Therefore, in order to carry out tunnel excavation in such fault zones, it is necessary to clarify measure the physical and mechanical properties of rocks, while carefully considering the characteristics of the fault zones. To overcome this difficulty, can select two method. First, it is to repair the collapsed tunnel and second, it is to excavate a bypass tunnel through the fault fracture zone. In this paper, we have confirmed the physical and mechanical properties of the fault zone rock and its surrounding rocks by using PLT and ZBL-U5200, based on this, were derived the safe distance for the bypass tunnel accordingly, and then, were established the supporting method of tunnel. 2. Geotechnical investigation 2.1. Geological condition of study area The study area features complex terrain with significant undulations, well-developed fault lines, and a high average altitude of 1400–1800 m. Fault lines are randomly oriented relative to the rock mass axis and slope direction, with some areas experiencing intersecting faults. The dominant rock type in the study area is dolomite, classified into white dolomite and gray dolomite based on color. The fault zone is primarily composed of fractured rock masses, with clayey granular rocks dominating the fracture zone interior. Most faults in the study area extend to the surface and taper with increasing depth, with a typical thickness of 8–20 m. Blasting vibration and surface water infiltration triggered tunnel surface collapse, which progressed to large-scale collapse and surface subsidence. Fault slip caused severe tunnel damage, completely destroying facilities and structures within a 100-meter range. In this area, fragmented rocks (e.g., mud and soil) exhibit flow characteristics, and collapsed rock masses behave similarly to concrete mixtures. Therefore, accurate characterization of rock physical and mechanical properties and thorough consideration of fault zone characteristics are essential for safe tunnel excavation in this region. 2.2. Physical and mechanical properties of rocks 2.2.1. Determination of axial strength index by PLT The Point Load Test (PLT) was used to determine the physical and mechanical properties of the rock mass. PLT is widely applied for measuring rock compressive and tensile strength due to its simple experimental procedure, easy sample preparation, and suitability for field applications (Han et al. 2021 ; Paraskevopoulou and Diederichs 2018 ; ISRM 1985). Given the brittle nature of the rocks and limited on-site instrumentation, a point load tester was employed to measure uniaxial compressive strength. The testing equipment included a 5-ton hydraulic press, an SDB-410C tensometer, a deformation sensor, and a sliding caliper (Fig. 1 ). PLT results are shown in Table 1 . Table 1 Physical and mechanical properties of rock samples (PLT results) Rock type Uniaxial compressive strength, (σₙₘ), MPa Tensile strength (σₜ), MPa Internal friction angle (φ), ° Cohesion (c), MPa Density (ρ), kg/m³ Young’s modulus (E r ₘ), GPa Acoustic velocity (cₚ), m/s Poisson’s ratio (µ) White dolomite 84.02 8.58 65.7 23.6 2750 16.50 4329 0.22 Gray dolomite 85.12 8.89 66.5 25.3 2820 15.86 4520 0.21 Fractured zone 58.31 5.79 37.8 4.1 2450 15.12 2543 0.32 Table 1 shows no significant difference in compressive and tensile strength between dolomite and fault fracture zone rocks, indicating that the fracture zone rock is relatively hard in its intact state. However, field construction observations revealed that the fault fracture zone rock mass exhibited flow-like collapse similar to concrete mixtures, leading to tunnel collapse. This discrepancy indicates that although the fracture zone rock itself has high inherent strength, its mechanical properties are significantly degraded by the combined effects of fracturing, moisture infiltration, and blasting vibration. 2.2.2. Determination of rock mass strength weakening coefficient The ZBL-U5200 non-metallic ultrasonic detector (China) was used to measure elastic wave propagation velocity. This device transmits and receives ultrasonic waves through a pair of planar transducers, records the travel time through the rock mass, and calculates the propagation velocity by dividing the wave path length by the travel time (Fig. 2 ). Elastic wave propagation velocity is a well-recognized key parameter for quantifying rock mass fracturing degree (Wang et al. 2024 ). Measurements were conducted by mounting sensors on the rock mass surface and applying vibration. The elastic wave propagation velocity results are presented in Table 2 . Table 2 Elastic wave propagation velocity in rock masses Rock type density ρ, ㎏/㎥ acoustic velocity \(\:{c}_{p},\:\) ㎧ White dolomite Gray dolomite Fractured zone 2750 2820 2450 3772 3706 1068 Comparing Tables 1 and Table 2 , significant differences are observed between the elastic wave velocities of rock samples and in-situ rock masses. Based on these results, the rock mass strength weakening coefficient (kₛ) was calculated (Table 3 ), and the relationship between elastic wave velocities of rock samples and in-situ rock masses is illustrated in Fig. 3 . Table 3 Rock mass strength weakening coefficients Rock type Rock mass, ㎧ Rock sample, ㎧ Strength weakening Coefficient, \(\:{k}_{s}\) White dolomite Gray dolomite Fractured zone 3772 3706 1068 4329 4520 2543 0.89 0.82 0.42 Integrating Tables 1 and Table 3 , the corrected physical and mechanical properties of the in-situ rock masses were derived (Table 4 ), accounting for strength weakening effects. Table 4 Corrected physical and mechanical properties of in-situ rock masses Rock type Uniaxial compressive strength, (σₙₘ), MPa Tensile strength (σₜ), MPa Internal friction angle (φ), ° Cohesion (c), MPa Density (ρ), kg/m³ Young’s modulus (E r ₘ), GPa Acoustic velocity (cₚ), m/s Poisson’s ratio (µ) White dolomite 74.7 7.6 65.7 21 2750 14.7 3852 0.22 Gray dolomite 69.7 7.2 66.5 20 2820 13..0 3706 0.21 Fractured zone 24.4 2.4 37.8 1.7 2450 6.3 1068 0.32 As shown in Fig. 3 and Table 4 , the rock mass strength—particularly in the fault fracture zone—has been significantly weakened. This highlights the necessity of optimizing tunnel construction processes to accommodate the degraded rock mass properties. 3. Stability Analysis of Adjacent Tunnels 3.1. Displacement Analysis Between Adjacent Tunnels Located in Fault Fracture Zones FLAC3D numerical simulation software was used to evaluate the stability of adjacent tunnels with a cross-section of 6.1 m (width) × 5.2 m (height) (Han et al. 2021 ). The tunnel cross-sectional dimensions were determined in accordance with domestic highway tunnel design codes, and numerical simulations were conducted for tunnel spacings ranging from 10 m to 25 m. The simulation focused exclusively on stress distribution characteristics between the two tunnels passing through the fault fracture zone. This is because the strength of intact rock strata is significantly higher than that of fault zone rocks, ensuring the stability of intact rock sections even when the analysis scope is limited to the fault fracture zone. The corrected physical and mechanical properties of the rock mass (Table 4 ) were adopted for the numerical analysis. The concrete support was assigned a Poisson’s ratio of 0.17 and a density of 2500 kg/m³. The Mohr-Coulomb constitutive model was used to describe the material behavior. The load condition was defined as the self-weight of the rock mass, with boundary conditions set as full restraint at the model bottom and normal restraint on both side surfaces. The numerical model is illustrated in Fig. 4 . The Z-direction and X-direction displacement results are presented in Fig. 5 and Fig. 6 . The analysis results of Fig. 5 and Fig. 6 are summarized in Fig. 7 . As shown in Fig. 7 , the X-direction and Z-direction displacements increase sharply when the tunnel spacing is 10 m. As the spacing increases to 18 m or more, the displacements decrease and tend to stabilize. The tunnel stability criteria proposed by Gu et al. (2023) were adopted to evaluate the safety of adjacent tunnels (Fig. 8 , Table 5 ). Table 5 Block instability criteria (Gu et al. 2023) Type of block instability Displacement Completely unstable block B1 Delayed unstable block B2 Potentially unstable block B3 >45 mm 25 mm–45 mm 0 mm–25 mm Based on Fig. 7 and Table 5 , the minimum safety distance between adjacent tunnels in the fault fracture zone was determined to be 18 m. 3.2. Optimization of Excavation Method Based on FLAC3D numerical simulation results, for weak rock masses such as fault fracture zones, the recommended tunnel spacing is no less than 18 m. The optimal excavation method combines top-heading and benching (with the top heading advanced) and the Arch Support Method (ASM), following the completion of the pilot tunnel. The ASM involves first excavating pilot tunnels on both sides of the arch, followed by the remaining arch section to form the top heading. The arch section is then reinforced with integral reinforced concrete before excavating the benching. Zigzag columns are installed on the sidewalls, and integral reinforced concrete is poured between the columns to form a stable support system (Han et al. 2021 ). Figure 9 shows the vertical displacement of the surrounding rock mass after excavating the upper pilot tunnels on both sides of the arch with cycle lengths of 3 m, 6 m, 9 m, and 12 m. FLAC3D simulation results predict maximum vertical displacements of 22 mm, 27 mm, 33 mm, and 36 mm for excavation cycle lengths of 3 m, 6 m, 9 m, and 12 m, respectively. In the 8–20 m fault zone, blasting vibration-induced fracture and sliding of discontinuity surfaces necessitate immediate integral reinforced concrete reinforcement after each excavation cycle to prevent crown collapse. When the arch section is at risk of instability, temporary timber supports should be installed in a circular pattern before arch lining construction. Figure 10 shows the vertical displacement of the tunnel lining after reinforcing the arch section with 30 cm thick integral reinforced concrete. The maximum displacement is less than 3 mm, ensuring tunnel safety. This approach was adopted for the construction of the target tunnel section. 3.3. Design and Application of Support System for Tunnel Stability To enhance tunnel stability, a systematic support system was designed and implemented. Holes with a diameter of 38–42 mm were drilled on the tunnel wall at a grid spacing of 1.5×1.5 m. Two 20 mm diameter steel bars were bundled and inserted into each hole, followed by grouting with cement mixture (water-cement ratio: 0.4–0.5). The hole openings were sealed with loess wedges to prevent grout leakage. Considering the deterioration depth of the tunnel sidewalls, the anchor bolt length was set to 3.5 m. For tunnel sections with a wall height of 3–4 m, 38–42 mm diameter anchor bolts were installed at 2 m intervals. The anchor bolt length was designed to penetrate the tunnel lining and extend at least 50 cm into the underlying rock mass. The anchor bolts were fixed via 30 cm (length) × 1 m (width) supporting beams, arranged in a zigzag pattern at 3 m intervals. In hazardous sections, 60 anchor bolts were installed to reinforce the surrounding rock mass of adjacent tunnels. This support system, combined with the optimized excavation method, enabled the safe construction of adjacent tunnels in the weak rock mass of the fault fracture zone. 4. Discussion The application of the optimized adjacent tunnel excavation method and arch support system in the fault fracture zone resulted in stable post-construction displacements in the weak rock section. This confirms the effectiveness of the arch support system and staged excavation in controlling rock mass displacement. Field-measured displacements in the collapse zone were consistent with numerical simulation predictions, verifying the suitability of the FLAC3D model and displacement prediction method for on-site conditions. Increased displacements of the tunnel sidewalls and crown were observed during pilot tunnel and top heading excavation, attributed to the mechanical response of the rock mass to excavation-induced stress redistribution. Specifically, pilot tunnel excavation alters the initial stress field, leading to relatively large displacements during subsequent tunnel construction. Abnormal displacement behavior was identified as a result of conical collapse on the right side of the monitoring point. This indicates that discontinuity surfaces in the fault fracture zone dominate collapse behavior, emphasizing the importance of appropriate support systems and excavation sequences in controlling unstable rock mass behavior. 5. Conclusions This study proposed an adjacent tunnel excavation method for weak rock masses using the Arch Support Method (ASM). Field tests (PLT and ZBL-U5200 elastic wave exploration) were conducted to accurately characterize the mechanical properties of the rock mass, and FLAC3D numerical simulation was used to optimize the cross-sectional dimensions and support system of adjacent tunnels. The key findings and engineering benefits are as follows: First, the tunnel excavation period in weak rock masses was halved through the application of staged excavation, pilot tunnel construction, and standardized support system installation. This shortened construction period reduces labor costs, equipment rental fees, and other associated expenses, yielding significant economic benefits. Second, tunnel safety was improved by 2.5 times. Post-construction field monitoring showed that the maximum displacement of the surrounding rock mass was maintained below 25 mm, which falls within the stable range (Table 5 ) according to the instability criteria. This drastically reduces the risk of collapse, sliding, and other safety accidents. The proposed method is suitable for underground structure construction in weak rock masses, including fault fracture zones. Future research should focus on expanding the applicability of the method to various weak rock types and tunnel scales, and further optimizing the excavation and support parameters for broader engineering generalization. Declarations Acknowledgements The authors are grateful to acknowledge for many helps and advice of all persons for this research project and also thank for the trouble editors and reviewers have taken. Author contributions C.H. Pak Conceived and designed the analysis, Performed the analysis, Wrote and revise the paper. J.S. Son and P.M. Kim Conceived and designed the analysis, Collected the data, Wrote and revise the paper. Y.M. Ji and M.H. Ri Collected the data, Contributed data or analysis tools. C.H. Kang and R.S. Pak Conceived and designed the analysis, Collected the data, Wrote the paper. Funding This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Data availability The data used to support the findings of this study are available from the corresponding author upon request. Conflict of interest The authors declare no conflicts of interest. No significant financial support was received that could have influenced the outcome of this work. References Chen LL, Wang YQ, Wang ZF, Fan FF, Liu Y (2023) Characteristics and treatment measures of tunnel collapse in fault fracture zone during rainfall: A case study. 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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-8706282","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":580908889,"identity":"64608a8a-40da-48f1-9fab-28cae180ead1","order_by":0,"name":"Pak 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apparatus\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-8706282/v1/a7d921376f8917fe0c064110.png"},{"id":101289916,"identity":"a445744e-36f2-4347-be24-79c1b6de8a7f","added_by":"auto","created_at":"2026-01-28 07:53:13","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":367253,"visible":true,"origin":"","legend":"\u003cp\u003eZBL-U5200 ultrasonic detector\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-8706282/v1/3213e4c89f78b72fd0ce6497.png"},{"id":101289915,"identity":"56bfb086-391b-47f5-ae9f-9133df48c682","added_by":"auto","created_at":"2026-01-28 07:53:13","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":152843,"visible":true,"origin":"","legend":"\u003cp\u003eElastic wave propagation velocity in rock samples and in-situ rock masses\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-8706282/v1/dab3a0c86ab3ed210a797cc2.png"},{"id":101297482,"identity":"319d4f30-755b-46e1-b34e-21b8a0a57845","added_by":"auto","created_at":"2026-01-28 09:27:19","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":373528,"visible":true,"origin":"","legend":"\u003cp\u003eNumerical model of adjacent tunnels in the fault fracture zone\u003c/p\u003e\n\u003cp\u003e(L = distance between adjacent tunnels)\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-8706282/v1/85445759e5b3a1863345fda4.png"},{"id":101289911,"identity":"c03a0c0a-333d-4f16-926c-14c85ae920a3","added_by":"auto","created_at":"2026-01-28 07:53:13","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":709054,"visible":true,"origin":"","legend":"\u003cp\u003eZ-direction displacement of adjacent tunnels at different spacings\u003c/p\u003e\n\u003cp\u003eL=10m, 12m, 14m, 16m b): L=18m, 20m, 22m,24m\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-8706282/v1/af81377734d3dfa95bbf7340.png"},{"id":101289910,"identity":"62076b66-9d61-4e06-b196-014a1d40e5f3","added_by":"auto","created_at":"2026-01-28 07:53:12","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":492263,"visible":true,"origin":"","legend":"\u003cp\u003eX-direction displacement of adjacent tunnels at different spacings\u003c/p\u003e\n\u003cp\u003ea): L=10m, 12m, 14m, 16m b): L=18m, 20m, 22m,24m\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-8706282/v1/a1fbcf2a870743db186837dc.png"},{"id":101289917,"identity":"93fcd331-9267-4277-a048-9e8ffae8e704","added_by":"auto","created_at":"2026-01-28 07:53:14","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":103158,"visible":true,"origin":"","legend":"\u003cp\u003eSummary of X-direction and Z-direction displacements at different tunnel spacings\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-8706282/v1/7d1772fc21d87882d41f3fd5.png"},{"id":101289912,"identity":"98979a59-c8fc-450c-adff-5704deddaabd","added_by":"auto","created_at":"2026-01-28 07:53:13","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":81703,"visible":true,"origin":"","legend":"\u003cp\u003eInstability classification of roadway surrounding rock blocks\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-8706282/v1/eb6adeb73e7e7451ae1e31fe.png"},{"id":101289913,"identity":"905f877f-e58c-470e-ac29-5e8d35e79317","added_by":"auto","created_at":"2026-01-28 07:53:13","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":423197,"visible":true,"origin":"","legend":"\u003cp\u003eVertical displacement of surrounding rock mass after upper pilot tunnel excavation;\u003c/p\u003e\n\u003cp\u003ea) 3.0 m cycle length, b) 6.0 m cycle length, c) 9.0m cycle length, d) 12.0m cycle length\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-8706282/v1/444aa6b50cb811404a409de8.png"},{"id":101289914,"identity":"2fa06111-4809-4bf9-a088-e6fa320cca67","added_by":"auto","created_at":"2026-01-28 07:53:13","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":177228,"visible":true,"origin":"","legend":"\u003cp\u003eVertical displacement of tunnel lining after reinforced concrete reinforcement\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-8706282/v1/c4c3fe2a948f8371ca9629f6.png"},{"id":101397885,"identity":"c424a80a-723b-4940-9d4c-2cd2b7d5a51b","added_by":"auto","created_at":"2026-01-29 09:37:55","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4288414,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8706282/v1/5659173e-cf8c-477e-82db-4d2b1994a0c8.pdf"}],"financialInterests":"The authors declare no competing interests.","formattedTitle":"\u003cp\u003eStudy on Determining Safe Distance and Optimization of Supporting Methods for Adjacent Tunnels in Fault Fracture Zone\u003c/p\u003e","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eFaults are critical geological discontinuities that dominate the stability of underground structures during design and construction (Wei et al. \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). Structural planes, including faults, joints, and bedding planes, are the primary factors controlling the stability of deep underground structures. Under high-stress conditions, these planes can trigger rock mass instability, fracturing, and large-scale failure, leading to severe safety hazards and engineering accidents (Xu et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Yin et al. \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Huang et al. \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). Consequently, major fault zones are generally avoided in underground construction to mitigate the risks of structural collapse and rock mass failure (Xu et al. \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2018\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eHowever, complete avoidance of fault zones is often impractical due to spatial layout constraints, economic considerations, geological uncertainties (e.g., unforeseen fault zones), and complex interactions between faults and associated joints (Xu et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Chen et al. \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). Thus, the scientific analysis of fault characteristics and their impacts on geotechnical stability is a pivotal research topic in geological and geotechnical engineering (Fasching and Vanek \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). Brittle fault zones typically exhibit strong anisotropy, resulting in significant differences in stress distribution and displacement between surrounding rocks and underground structures. Such geological conditions frequently induce critical issues, including rock mass instability, premature collapse, and structural failure during tunneling (Entfellner and Moritz \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2025\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eNumerous studies have focused on the safety and stability of tunnels in fault zones. Li et al. (\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2022\u003c/span\u003e) used COMSOL Multiphysics to analyze the safety distance between adjacent tunnels under different excavation cross-sections and water pressure conditions. Their findings revealed that increased pressure in the fault fracture zone and reduced distance between the working face and the fault zone enhance groundwater connectivity with rock fractures, significantly compromising excavation face stability. Groundwater infiltration into fractured rock layers within fault zones is the primary driver of progressive rock stability deterioration post-excavation. Water erosion expands existing fractures, increases rock moisture content, and drastically weakens rock mass strength (Chen et al. \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Yao et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). Additionally, groundwater flow can trap large rock particles in fractures, reducing the bearing capacity of surrounding rocks and triggering collapse (Chen et al. \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Therefore, comprehensive geological investigations of fault zones and accurate characterization of tunnel fracture and deformation characteristics are indispensable for safe construction in such areas (Yao et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2025\u003c/span\u003e; Xie et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2025\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eStability issues and blast stress wave effects between adjacent tunnels in fault fracture zones can be evaluated using computer simulations, field experiments, and theoretical calculations. To minimize errors, these evaluations should integrate numerical predictions with field-measured data (Feng et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Xia et al. (\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2026\u003c/span\u003e) investigated the dynamic response of twin tunnels to blasting and collapse impact loads through a case study of a 24-story building demolition. Other studies have assessed the safe distance and dynamic adaptation state of adjacent tunnels considering geological structures and explosive stress wave effects (Zhang et al. \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2025\u003c/span\u003e; Wang et al. \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2025\u003c/span\u003e; Li et al. \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). Many scholars have also proposed tunnel stability assessment methods and optimized blasting designs for fault zones, which have been successfully applied in engineering practice. However, tunnel collapse induced by faults may still require permanent repairs or bypass tunnel excavation, and research on systematic excavation methods and safety distance determination for adjacent tunnels in weak fault zones remains limited.\u003c/p\u003e \u003cp\u003eIn the study area, a 2500-meter-long concrete tunnel (6 m wide, 2.3 m high) constructed in the 1980s was severely damaged by fault activity. A 100-meter section from the collapse zone to the tunnel entrance direction was completely filled with collapsed rock. Therefore, in order to carry out tunnel excavation in such fault zones, it is necessary to clarify measure the physical and mechanical properties of rocks, while carefully considering the characteristics of the fault zones.\u003c/p\u003e \u003cp\u003eTo overcome this difficulty, can select two method. First, it is to repair the collapsed tunnel and second, it is to excavate a bypass tunnel through the fault fracture zone.\u003c/p\u003e \u003cp\u003eIn this paper, we have confirmed the physical and mechanical properties of the fault zone rock and its surrounding rocks by using PLT and ZBL-U5200, based on this, were derived the safe distance for the bypass tunnel accordingly, and then, were established the supporting method of tunnel.\u003c/p\u003e"},{"header":"2. Geotechnical investigation","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1. Geological condition of study area\u003c/h2\u003e \u003cp\u003eThe study area features complex terrain with significant undulations, well-developed fault lines, and a high average altitude of 1400\u0026ndash;1800 m. Fault lines are randomly oriented relative to the rock mass axis and slope direction, with some areas experiencing intersecting faults. The dominant rock type in the study area is dolomite, classified into white dolomite and gray dolomite based on color. The fault zone is primarily composed of fractured rock masses, with clayey granular rocks dominating the fracture zone interior.\u003c/p\u003e \u003cp\u003eMost faults in the study area extend to the surface and taper with increasing depth, with a typical thickness of 8\u0026ndash;20 m. Blasting vibration and surface water infiltration triggered tunnel surface collapse, which progressed to large-scale collapse and surface subsidence. Fault slip caused severe tunnel damage, completely destroying facilities and structures within a 100-meter range. In this area, fragmented rocks (e.g., mud and soil) exhibit flow characteristics, and collapsed rock masses behave similarly to concrete mixtures. Therefore, accurate characterization of rock physical and mechanical properties and thorough consideration of fault zone characteristics are essential for safe tunnel excavation in this region.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2. Physical and mechanical properties of rocks\u003c/h2\u003e \u003cdiv id=\"Sec5\" class=\"Section3\"\u003e \u003ch2\u003e2.2.1. Determination of axial strength index by PLT\u003c/h2\u003e \u003cp\u003eThe Point Load Test (PLT) was used to determine the physical and mechanical properties of the rock mass. PLT is widely applied for measuring rock compressive and tensile strength due to its simple experimental procedure, easy sample preparation, and suitability for field applications (Han et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Paraskevopoulou and Diederichs \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; ISRM 1985). Given the brittle nature of the rocks and limited on-site instrumentation, a point load tester was employed to measure uniaxial compressive strength. The testing equipment included a 5-ton hydraulic press, an SDB-410C tensometer, a deformation sensor, and a sliding caliper (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003ePLT results are shown in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\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\u003ePhysical and mechanical properties of rock samples (PLT results)\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"9\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" 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=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eRock type\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eUniaxial\u003c/p\u003e \u003cp\u003ecompressive\u003c/p\u003e \u003cp\u003estrength,\u003c/p\u003e \u003cp\u003e(σₙₘ), MPa\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eTensile\u003c/p\u003e \u003cp\u003estrength\u003c/p\u003e \u003cp\u003e(σₜ), MPa\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eInternal\u003c/p\u003e \u003cp\u003efriction angle\u003c/p\u003e \u003cp\u003e(φ), \u0026deg;\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eCohesion\u003c/p\u003e \u003cp\u003e(c), MPa\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eDensity\u003c/p\u003e \u003cp\u003e(ρ), kg/m\u0026sup3;\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eYoung\u0026rsquo;s\u003c/p\u003e \u003cp\u003emodulus\u003c/p\u003e \u003cp\u003e(E\u003csub\u003er\u003c/sub\u003eₘ), GPa\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003eAcoustic\u003c/p\u003e \u003cp\u003evelocity\u003c/p\u003e \u003cp\u003e(cₚ), m/s\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c9\"\u003e \u003cp\u003ePoisson\u0026rsquo;s ratio\u003c/p\u003e \u003cp\u003e(\u0026micro;)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eWhite\u003c/p\u003e \u003cp\u003edolomite\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e84.02\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e8.58\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e65.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e23.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e2750\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e16.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e4329\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e \u003cp\u003e0.22\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGray\u003c/p\u003e \u003cp\u003edolomite\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e85.12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e8.89\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e66.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e25.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e2820\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e15.86\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e4520\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e \u003cp\u003e0.21\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFractured zone\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e58.31\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e5.79\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e37.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e4.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e2450\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e15.12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e2543\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e \u003cp\u003e0.32\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\u003eTable\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows no significant difference in compressive and tensile strength between dolomite and fault fracture zone rocks, indicating that the fracture zone rock is relatively hard in its intact state. However, field construction observations revealed that the fault fracture zone rock mass exhibited flow-like collapse similar to concrete mixtures, leading to tunnel collapse. This discrepancy indicates that although the fracture zone rock itself has high inherent strength, its mechanical properties are significantly degraded by the combined effects of fracturing, moisture infiltration, and blasting vibration.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section3\"\u003e \u003ch2\u003e2.2.2. Determination of rock mass strength weakening coefficient\u003c/h2\u003e \u003cp\u003eThe ZBL-U5200 non-metallic ultrasonic detector (China) was used to measure elastic wave propagation velocity. This device transmits and receives ultrasonic waves through a pair of planar transducers, records the travel time through the rock mass, and calculates the propagation velocity by dividing the wave path length by the travel time (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Elastic wave propagation velocity is a well-recognized key parameter for quantifying rock mass fracturing degree (Wang et al. \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2024\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eMeasurements were conducted by mounting sensors on the rock mass surface and applying vibration. The elastic wave propagation velocity results are presented in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eElastic wave propagation velocity in rock masses\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"3\"\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 \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eRock type\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003edensity\u003c/p\u003e \u003cp\u003eρ, ㎏/㎥\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eacoustic\u003c/p\u003e \u003cp\u003evelocity\u003c/p\u003e \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{c}_{p},\\:\\)\u003c/span\u003e\u003c/span\u003e㎧\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eWhite dolomite\u003c/p\u003e \u003cp\u003eGray dolomite\u003c/p\u003e \u003cp\u003eFractured zone\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e2750\u003c/p\u003e \u003cp\u003e2820\u003c/p\u003e \u003cp\u003e2450\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e3772\u003c/p\u003e \u003cp\u003e3706\u003c/p\u003e \u003cp\u003e1068\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\u003eComparing Tables\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, significant differences are observed between the elastic wave velocities of rock samples and in-situ rock masses. Based on these results, the rock mass strength weakening coefficient (kₛ) was calculated (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), and the relationship between elastic wave velocities of rock samples and in-situ rock masses is illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eRock mass strength weakening coefficients\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\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=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eRock type\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eRock mass, ㎧\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eRock sample,\u003c/p\u003e \u003cp\u003e㎧\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eStrength weakening\u003c/p\u003e \u003cp\u003eCoefficient,\u003c/p\u003e \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{k}_{s}\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eWhite dolomite\u003c/p\u003e \u003cp\u003eGray dolomite\u003c/p\u003e \u003cp\u003eFractured zone\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e3772\u003c/p\u003e \u003cp\u003e3706\u003c/p\u003e \u003cp\u003e1068\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e4329\u003c/p\u003e \u003cp\u003e4520\u003c/p\u003e \u003cp\u003e2543\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.89\u003c/p\u003e \u003cp\u003e0.82\u003c/p\u003e \u003cp\u003e0.42\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\u003e \u003c/p\u003e \u003cp\u003eIntegrating Tables\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, the corrected physical and mechanical properties of the in-situ rock masses were derived (Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e), accounting for strength weakening effects.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eCorrected physical and mechanical properties of in-situ rock masses\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"9\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" 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 \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eRock type\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eUniaxial\u003c/p\u003e \u003cp\u003ecompressive\u003c/p\u003e \u003cp\u003estrength,\u003c/p\u003e \u003cp\u003e(σₙₘ), MPa\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eTensile\u003c/p\u003e \u003cp\u003estrength\u003c/p\u003e \u003cp\u003e(σₜ), MPa\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eInternal\u003c/p\u003e \u003cp\u003efriction angle\u003c/p\u003e \u003cp\u003e(φ), \u0026deg;\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eCohesion\u003c/p\u003e \u003cp\u003e(c), MPa\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eDensity\u003c/p\u003e \u003cp\u003e(ρ), kg/m\u0026sup3;\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eYoung\u0026rsquo;s\u003c/p\u003e \u003cp\u003emodulus\u003c/p\u003e \u003cp\u003e(E\u003csub\u003er\u003c/sub\u003eₘ), GPa\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003eAcoustic\u003c/p\u003e \u003cp\u003evelocity\u003c/p\u003e \u003cp\u003e(cₚ), m/s\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c9\"\u003e \u003cp\u003ePoisson\u0026rsquo;s ratio\u003c/p\u003e \u003cp\u003e(\u0026micro;)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eWhite dolomite\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e74.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e7.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e65.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e21\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e2750\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e14.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e3852\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e \u003cp\u003e0.22\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGray dolomite\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e69.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e7.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e66.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e2820\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e13..0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e3706\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e \u003cp\u003e0.21\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFractured zone\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e24.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e37.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e2450\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e6.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e1068\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e \u003cp\u003e0.32\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\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e and Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, the rock mass strength\u0026mdash;particularly in the fault fracture zone\u0026mdash;has been significantly weakened. This highlights the necessity of optimizing tunnel construction processes to accommodate the degraded rock mass properties.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"3. Stability Analysis of Adjacent Tunnels","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\n \u003ch2\u003e3.1. Displacement Analysis Between Adjacent Tunnels Located in Fault Fracture Zones\u003c/h2\u003e\n \u003cp\u003eFLAC3D numerical simulation software was used to evaluate the stability of adjacent tunnels with a cross-section of 6.1 m (width) \u0026times; 5.2 m (height) (Han et al. \u003cspan class=\"CitationRef\"\u003e2021\u003c/span\u003e). The tunnel cross-sectional dimensions were determined in accordance with domestic highway tunnel design codes, and numerical simulations were conducted for tunnel spacings ranging from 10 m to 25 m.\u003c/p\u003e\n \u003cp\u003eThe simulation focused exclusively on stress distribution characteristics between the two tunnels passing through the fault fracture zone. This is because the strength of intact rock strata is significantly higher than that of fault zone rocks, ensuring the stability of intact rock sections even when the analysis scope is limited to the fault fracture zone. The corrected physical and mechanical properties of the rock mass (Table \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e) were adopted for the numerical analysis. The concrete support was assigned a Poisson\u0026rsquo;s ratio of 0.17 and a density of 2500 kg/m\u0026sup3;. The Mohr-Coulomb constitutive model was used to describe the material behavior. The load condition was defined as the self-weight of the rock mass, with boundary conditions set as full restraint at the model bottom and normal restraint on both side surfaces. The numerical model is illustrated in Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e.\u003c/p\u003e\n \u003cp\u003eThe Z-direction and X-direction displacement results are presented in Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e and Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e.\u003c/p\u003e\n \u003cp\u003eThe analysis results of Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e and Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e are summarized in Fig. \u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e.\u003c/p\u003e\n \u003cp\u003eAs shown in Fig. \u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e, the X-direction and Z-direction displacements increase sharply when the tunnel spacing is 10 m. As the spacing increases to 18 m or more, the displacements decrease and tend to stabilize. The tunnel stability criteria proposed by Gu et al. (2023) were adopted to evaluate the safety of adjacent tunnels (Fig. \u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003e, Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e).\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u003ctable id=\"Tab5\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 5\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eBlock instability criteria (Gu et al. 2023)\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\u003eType of block instability\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eDisplacement\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\u003eCompletely unstable block B1\u003c/p\u003e\n \u003cp\u003eDelayed unstable block B2\u003c/p\u003e\n \u003cp\u003ePotentially unstable block B3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026gt;45 mm\u003c/p\u003e\n \u003cp\u003e25 mm\u0026ndash;45 mm\u003c/p\u003e\n \u003cp\u003e0 mm\u0026ndash;25 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 \u003cp\u003eBased on Fig. \u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e and Table \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e, the minimum safety distance between adjacent tunnels in the fault fracture zone was determined to be 18 m.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\n \u003ch2\u003e3.2. Optimization of Excavation Method\u003c/h2\u003e\n \u003cp\u003eBased on FLAC3D numerical simulation results, for weak rock masses such as fault fracture zones, the recommended tunnel spacing is no less than 18 m. The optimal excavation method combines top-heading and benching (with the top heading advanced) and the Arch Support Method (ASM), following the completion of the pilot tunnel.\u003c/p\u003e\n \u003cp\u003eThe ASM involves first excavating pilot tunnels on both sides of the arch, followed by the remaining arch section to form the top heading. The arch section is then reinforced with integral reinforced concrete before excavating the benching. Zigzag columns are installed on the sidewalls, and integral reinforced concrete is poured between the columns to form a stable support system (Han et al. \u003cspan class=\"CitationRef\"\u003e2021\u003c/span\u003e). Figure \u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003e shows the vertical displacement of the surrounding rock mass after excavating the upper pilot tunnels on both sides of the arch with cycle lengths of 3 m, 6 m, 9 m, and 12 m.\u003c/p\u003e\n \u003cp\u003eFLAC3D simulation results predict maximum vertical displacements of 22 mm, 27 mm, 33 mm, and 36 mm for excavation cycle lengths of 3 m, 6 m, 9 m, and 12 m, respectively. In the 8\u0026ndash;20 m fault zone, blasting vibration-induced fracture and sliding of discontinuity surfaces necessitate immediate integral reinforced concrete reinforcement after each excavation cycle to prevent crown collapse. When the arch section is at risk of instability, temporary timber supports should be installed in a circular pattern before arch lining construction.\u003c/p\u003e\n \u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003e shows the vertical displacement of the tunnel lining after reinforcing the arch section with 30 cm thick integral reinforced concrete. The maximum displacement is less than 3 mm, ensuring tunnel safety. This approach was adopted for the construction of the target tunnel section.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\n \u003ch2\u003e3.3. Design and Application of Support System for Tunnel Stability\u003c/h2\u003e\n \u003cp\u003eTo enhance tunnel stability, a systematic support system was designed and implemented. Holes with a diameter of 38\u0026ndash;42 mm were drilled on the tunnel wall at a grid spacing of 1.5\u0026times;1.5 m. Two 20 mm diameter steel bars were bundled and inserted into each hole, followed by grouting with cement mixture (water-cement ratio: 0.4\u0026ndash;0.5). The hole openings were sealed with loess wedges to prevent grout leakage. Considering the deterioration depth of the tunnel sidewalls, the anchor bolt length was set to 3.5 m.\u003c/p\u003e\n \u003cp\u003eFor tunnel sections with a wall height of 3\u0026ndash;4 m, 38\u0026ndash;42 mm diameter anchor bolts were installed at 2 m intervals. The anchor bolt length was designed to penetrate the tunnel lining and extend at least 50 cm into the underlying rock mass. The anchor bolts were fixed via 30 cm (length) \u0026times; 1 m (width) supporting beams, arranged in a zigzag pattern at 3 m intervals.\u003c/p\u003e\n \u003cp\u003eIn hazardous sections, 60 anchor bolts were installed to reinforce the surrounding rock mass of adjacent tunnels. This support system, combined with the optimized excavation method, enabled the safe construction of adjacent tunnels in the weak rock mass of the fault fracture zone.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"4. Discussion","content":"\u003cp\u003eThe application of the optimized adjacent tunnel excavation method and arch support system in the fault fracture zone resulted in stable post-construction displacements in the weak rock section. This confirms the effectiveness of the arch support system and staged excavation in controlling rock mass displacement.\u003c/p\u003e \u003cp\u003eField-measured displacements in the collapse zone were consistent with numerical simulation predictions, verifying the suitability of the FLAC3D model and displacement prediction method for on-site conditions. Increased displacements of the tunnel sidewalls and crown were observed during pilot tunnel and top heading excavation, attributed to the mechanical response of the rock mass to excavation-induced stress redistribution. Specifically, pilot tunnel excavation alters the initial stress field, leading to relatively large displacements during subsequent tunnel construction.\u003c/p\u003e \u003cp\u003eAbnormal displacement behavior was identified as a result of conical collapse on the right side of the monitoring point. This indicates that discontinuity surfaces in the fault fracture zone dominate collapse behavior, emphasizing the importance of appropriate support systems and excavation sequences in controlling unstable rock mass behavior.\u003c/p\u003e"},{"header":"5. Conclusions","content":"\u003cp\u003eThis study proposed an adjacent tunnel excavation method for weak rock masses using the Arch Support Method (ASM). Field tests (PLT and ZBL-U5200 elastic wave exploration) were conducted to accurately characterize the mechanical properties of the rock mass, and FLAC3D numerical simulation was used to optimize the cross-sectional dimensions and support system of adjacent tunnels. The key findings and engineering benefits are as follows:\u003c/p\u003e \u003cp\u003eFirst, the tunnel excavation period in weak rock masses was halved through the application of staged excavation, pilot tunnel construction, and standardized support system installation. This shortened construction period reduces labor costs, equipment rental fees, and other associated expenses, yielding significant economic benefits.\u003c/p\u003e \u003cp\u003eSecond, tunnel safety was improved by 2.5 times. Post-construction field monitoring showed that the maximum displacement of the surrounding rock mass was maintained below 25 mm, which falls within the stable range (Table\u0026nbsp;\u003cspan refid=\"Tab5\" class=\"InternalRef\"\u003e5\u003c/span\u003e) according to the instability criteria. This drastically reduces the risk of collapse, sliding, and other safety accidents.\u003c/p\u003e \u003cp\u003eThe proposed method is suitable for underground structure construction in weak rock masses, including fault fracture zones. Future research should focus on expanding the applicability of the method to various weak rock types and tunnel scales, and further optimizing the excavation and support parameters for broader engineering generalization.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors are grateful to acknowledge for many helps and advice of all persons for this research project and also thank for the trouble editors and reviewers have taken.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eC.H. Pak Conceived and designed the analysis, Performed the analysis, Wrote and revise the paper. J.S. Son and P.M. Kim Conceived and designed the analysis, Collected the data, Wrote and revise the paper. Y.M. Ji and M.H. Ri Collected the data, Contributed data or analysis tools. C.H. Kang and R.S. Pak Conceived and designed the analysis, Collected the data, Wrote the paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe data used to support the findings of this study are available from the corresponding author upon request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflict of interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no conflicts of interest. No significant financial support was received that could have influenced the outcome of this work.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eChen LL, Wang YQ, Wang ZF, Fan FF, Liu Y (2023) Characteristics and treatment measures of tunnel collapse in fault fracture zone during rainfall: A case study. Eng Fail Anal. 145:1-18. htps://doi.org/10.1016/j.engfailanal.2022.107002 \u003c/li\u003e\n\u003cli\u003eChen X, He C, Xu GW, Bai RM, Yuan QY, Gaoyu Ma GY (2025) Analytical Solutions for Deep Tunnels in Viscoelastic Plastic Rock Considering Rheological Damage Effects and the Lining Influence. Rock Mech Rock Eng 58:1117\u0026ndash;1145. https://doi.org/10.1007/s00603-024-04228-3\u003c/li\u003e\n\u003cli\u003eEntfellner M, Moritz B.A (2025). Prediction of stress-induced tunnel collapses based on displacement monitoring. 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Bull Eng Geo Environ. 83:1-25. https://doi.org/10.1007/s10064-024-03738-2\u003c/li\u003e\n\u003cli\u003eZhang RL, Wu W, Li QJ, Liu J, Wang AM (2025) A dynamic calculation method for safety step distance in mechanized soft rock tunnel construction using multi-source data integration. Tunnel Undergr Space Technol.165:1-13. https://doi.org/10.1016/j.tust.2025.106867 \u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"Sariwon University of Geology","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":"Fault fracture zone, Adjacent tunnels, Safety distance, FLAC3D, Excavation method","lastPublishedDoi":"10.21203/rs.3.rs-8706282/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8706282/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe design and construction of tunnels and underground structures in deformed rock masses, such as fault fracture zones, pose significant challenges in geotechnical engineering. This study proposes an optimized excavation method for adjacent tunnels in fault fracture zones, integrating in-situ testing, numerical simulation, and field verification. First, the physical and mechanical properties of the surrounding rock mass were characterized using a Point Load Tester (PLT) and an elastic wave velocity measuring instrument (ZBL-U5200), and the rock mass strength weakening law was quantified. The rock mass strength degradation coefficient was calibrated based on field monitoring data to reflect the actual geological conditions.\u003c/p\u003e \u003cp\u003eSubsequently, FLAC3D numerical simulation was employed to evaluate the stability of adjacent tunnels in the weak rock mass. A technical scheme combining sequential excavation and gradual cross-sectional enlargement was proposed for the weak rock sections, accompanied by a parallel simultaneous construction strategy. The effectiveness of the proposed method was verified through numerical simulation and field application. Advanced excavation and support technologies were successfully implemented, significantly improving the stability of adjacent tunnels.\u003c/p\u003e \u003cp\u003eThis study provides a reliable technical approach for tunnel excavation and underground structure construction in highly fractured and weak rock masses, particularly in fault fracture zones.\u003c/p\u003e","manuscriptTitle":"Study on Determining Safe Distance and Optimization of Supporting Methods for Adjacent Tunnels in Fault Fracture Zone","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-01-28 07:53:07","doi":"10.21203/rs.3.rs-8706282/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":"39f13212-d8fe-4ee5-9055-023373ab8698","owner":[],"postedDate":"January 28th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":61794968,"name":"Geology"}],"tags":[],"updatedAt":"2026-01-30T09:49:03+00:00","versionOfRecord":[],"versionCreatedAt":"2026-01-28 07:53:07","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8706282","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8706282","identity":"rs-8706282","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}