Experimental Study on Failure Mechanisms of Steep Loess Fill Slopes with Large Gradient

Experimental Study on Failure Mechanisms of Steep Loess Fill Slopes with Large Gradient | 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 Article Experimental Study on Failure Mechanisms of Steep Loess Fill Slopes with Large Gradient Guangyuan Dou, Shuoyu Zhu, Hao Zhang, Linwan Chen, Xing Chen This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7704393/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 To explore the influence of steep slopes on the deformation and failure mechanisms as well as instability modes of loess-filled slopes under rainfall conditions, this study conducted indoor simulated rainfall experiments, combined with matrix suction sensors, moisture content sensors and 3D laser scanning technology for research. The results showed that under rainfall, the water movement rate of steep fill slopes follows the pattern of shoulder > toe > middle of the slope, and the moisture content exhibits a "stable-rapid increase-stable" trend. The slope destabilization induced by rainfall exhibits hysteretic nature. Under the same rainfall intensity, the steeper the slope, the shallower the slide surface, the smaller the critical rainfall amount for landslide initiation, the quicker the landslide onset, the smaller the landslide volume, and the higher the landslide probability. The failure mode of the 45° slope is "toe collapse-deep-seated creeping cracking failure", while that of the 60° slope is "slope shoulder collapse-multi-stage retrusive failure". This study provides theoretical references for the engineering construction and landslide disaster prevention of steep loess-filled slopes. Physical sciences/Engineering Earth and environmental sciences/Environmental sciences Earth and environmental sciences/Natural hazards Loess filled slope rainfall infiltration slope gradient failure mode physical model test Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Introduction Loess is a porous and weakly cemented sediment from the fourth century, characterized by grayish-yellow, brownish-yellow, and even reddish-brown colors. Its particle composition is mainly composed of fine particles, with a uniform texture and no stratification [ 1 – 3 ] . Loess is widely distributed in China, accounting for approximately 6.6% of China's total land area.. It is mostly found in regions such as Shaanxi, Shanxi, and Gansu [ 4 – 5 ] . With the development of society and the advancement of urbanization in various regions, the original construction land in urban areas can no longer meet the demands of development. Therefore, large-scale major engineering projects such as land reclamation and mountain flattening have been conducted in the loess plateau area surrounding Yan'an. These projects not only brought development opportunities for local people but also posed significant risks of major disasters. Due to the highly water-sensitive and collapsible nature of loess soil, the loess fill slope formed by extensive construction projects are prone to geological hazards such as landslides under rainy conditions [ 6 – 9 ] . A study conducted by scholars investigated and analyzed the occurrences of loess landslides in the northern region of Shaanxi Province from 1990 to 2010. They found that the frequency of landslides was highest during the months of July to October. Additionally, it was observed that the rainy season in the Loess Plateau also concentrated most heavily from July to September [ 10 ] . A large and concentrated amount of rainfall can lead to an increase in the water content of loess, the formation of cracks at the rear edge of slopes, and other conditions. In addition, the loess fill slope, which alters the hydrogeological conditions of the original terrain, hinders the drainage of groundwater. Under the infiltration of rainwater, the groundwater level can easily rise, causing soil softening and eventually triggering landslides [ 11 – 16 ] . The stability of loess slopes has been a focal point of research within the field of engineering geology, with numerous scholars conducting in-depth studies from various perspectives. Extreme heavy rainfall is a significant inducement for instability in loess slopes [ 17 – 18 ] . During the process of rainwater infiltration, the soil water content continuously increases, gradually transforming the soil from an unsaturated to a saturated state, which decreases effective stress in the soil and consequently reduces the shear strength of the slope, eventually leading to slope instability and destruction [ 19 ] . Meanwhile, the process and rate of water infiltration have substantial impacts on slope stability. The wetting front is a critical indicator for assessing the rate of soil infiltration; its movement speed is influenced by the initial physical state of the soil mass, soil characteristics, and rainfall conditions [ 20 ] . A rapidly moving wetting front can generate significant hydraulic gradients locally within the slope body in a short period, destabilizing the soil structure; whereas a slowly moving wetting front allows time for stress adjustment within the soil but prolonged water accumulation can continually degrade soil strength, ultimately affecting slope stability [ 21 ] . The slope structure and types of rainfall also significantly influence the mechanisms of shallow loess destruction. Different slope structures result in varied internal stress distributions within the slope, while different types of rainfall manifest differently in terms of infiltration rates, methods, and soil wetting processes [ 22 – 23 ] . Their interaction leads to diverse modes of destruction, such as concentrated rainfall causing rapid saturation of superficial soils in specific structured slopes leading to sudden failure, or continuous rainfall potentially softening deeper soils in other structured slopes, forming potential sliding surfaces that threaten overall stability [ 24 ] . With the intensification of human activities, an increasing number of artificial filled slopes have entered the living range of people, making the safety and stability of these filled slopes a subject of considerable research significance [ 23 , 25 ] . Chen studied linear-type loess filled slopes and derived insights into the failure patterns and the impact of fine particle movement on filled slope failure [ 26 ] . Additionally, based on Zhang's laboratory model tests, it was concluded that under rainfall conditions, deformation of the loess filled slope exhibits complex spatiotemporal distribution characteristics. Rainfall infiltration causes uneven settlement, triggering crack generation and propagation, accelerating deep-soil softening, and reducing strength, which may ultimately lead to overall instability. By monitoring key parameters such as pore water pressure and displacement rates in real-time, an innovative early warning threshold model and framework were proposed, offering new ideas for early warning and risk prevention of slope disasters [ 27 ] . The construction slope of loess fill slopes varies according to project requirements. In areas with limited construction space, numerous steep fill slopes have emerged. Based on Chinese scholars' statistical analysis of loess geological hazards in Yan'an, Shaanxi Province, unstable slopes exceeding 50° account for 90.6% of cases. Among slopes less than 60°, the primary failure mechanism is landslides, exhibiting a significant nonlinear relationship between slope instability risk and gradient: The probability of landslide occurrence increases exponentially with slope angle. At 40°, the probability is only 0.01, but it surges to 0.16 at 60° [ 28 – 29 ] . This data reveals that steep slope angles may be one of the core controlling factors inducing instability in loess fill slopes. Moreover, the causes and failure patterns of landslides differ across varying slope angles. Therefore, investigating the impact of steeper slopes on the instability mechanisms of loess fill slopes under rainfall infiltration conditions is crucial for reducing casualties and economic losses caused by landslides. To this end, this study investigates loess fill slopes as the research subject. Utilizing indoor rainfall simulation experiments combined with 3D laser scanning and sensor monitoring, it examines the deformation and failure processes, instability characteristics, and failure patterns of loess fill slopes under varying steep slopes. The study also analyzes the influence of slope gradient on the instability mechanism of loess fill slopes under rainfall conditions. The findings aim to provide theoretical guidance for the prevention and control engineering of loess fill slopes. Materials and Methods Experimental Instruments The location of this model experiment was Sichuan Province State Key Laboratory of Geohazard Prevention and Geoenvironment Protection (Chengdu University of Technology). The experimental setup was a self-developed rainfall simulation test system. The experimental instruments mainly include a model box, a Rainfall simulation system, and a measurement system(Fig. 1 ). The specific details are as follows: Model Box: The dimensions of the model are 1.2 meters in length, 0.5 meters in width, and 0.7 meters in height. It consisted of three-sided transparent organic glass and a water-impermeable high-strength bottom board. The glass was connected to the bottom board with angle steel, and the connection is sealed with glass glue. At the left side of the model box, a 10cm square grid is drawn on the surface of the organic glass for coordinate reference and benchmark point plotting. To reduce the boundary effects of the slope in the model box, Vaseline is applied to the inner surface of the organic glass before the experiment. Rainfall Simulation System: The rainfall simulation system is self-designed and mainly consists of a water tank, low-pressure ultrafine mist rainfall nozzles, water meter valves, water supply pipes, and pressure gauges. The low-pressure ultrafine mist rainfall nozzles are arranged in four rows, with four nozzles per row, totaling 16 nozzles. The distance between each nozzle is 30cm. Measurement system: The measurement system consists of five components: volumetric water content sensor, matric suction sensor, high-definition digital camera, data acquisition device, and 3D laser scanner. The volumetric water content sensor and matric suction sensor used in the experiment are EC volumetric water content sensor and MPS matric suction sensor developed and produced by Decagon, a company based in the United States. The data acquisition system is EM50 acquisition system, and the 3D laser scanner is riegl 3D laser scanner developed by RIEGL, a company based in Austria. The sensor parameters are shown in Table 1 , and the experimental instruments are shown in Fig. 1 . Table 1 Sensor parameters. Type Quantities Precision Range MPS 6 ±(10% +2 kPa) -9kPa~-100,000 kPa EC 6 ± 2% 0-100% Experimental content Basic properties of the experimental soil sample The study area is located in Anse County, Yan'an City, Shaanxi Province, China, with a total area of 2,950 km 2 , between longitude 108°5′44″-109°26′18″ E and latitude 36°30′45″-37°19′3″ N. The area is intersected by gullies and rounded with beams and mounts, with an average elevation of 1,371.9 m above sea level, and it is typical of the Liangxuan-like yellow earth mound landform. The materials used in this model experiment were taken from the fresh profile of Malan loess at Nangou, Ansai County, Yan'an City(Fig. 2 ). The basic physical parameters measured are shown in Table 2 . To ensure that the test results are not affected by the size effect of the model box, all selected materials were passed through a 2mm standard sieve. Table 2 Basic physical parameters of the test soil. Type specific gravity w l w p I p Porosity K/ (m·s − 1 ) the loess of Yan'an 2.70 30.6 21.4 9.2 0.85 1.26×10 − 6 Experimental procedure The experiment aimed to investigate the infiltration process of water in loess filled slopes and the impact of gradient on the failure modes of these slopes under natural rainfall conditions. The specific experimental design is as follows: (1)Variable Control and Parameter Setting Maintaining constant rainfall intensity and original slope gradient, this study used the fill slope gradient as the sole variable. Based on field investigations of the study area, the gradient of loess filled slopes predominantly ranges between 30° and 60°, characterized by a "three surfaces, two bodies, one water" structure. Accordingly, the initial slope gradient was designed to be 65°, with two additional fill gradients set at 45° and 60° for experimentation. Rainfall intensity was determined by considering regional extreme rainfall intensity and uniformity of experimental rainfall. According to statistical data from the Yan'an Meteorological Bureau over the past decade, the maximum rainfall intensity in the region is 60 mm/h. After testing for rainfall uniformity, it was found that when the rainfall intensity is 30.1 mm/h, the uniformity reaches 81%, meeting the experimental requirements. Therefore, the rainfall intensity was set at 30.1 mm/h, using continuous uniform rainfall. (2)Model Construction and Preparation The filled slope model was constructed using a layered filling method, with each layer having a thickness of 10 cm, totaling six layers. During construction, scarifying treatment was applied between layers to ensure adequate contact between soil layers while achieving a compaction degree of 90% of the maximum density for each filled soil layer, with moisture content controlled to a predetermined standard value. After completing the layered filling, the model was statically cured for more than 24 hours until sensor readings stabilized, allowing for subsequent experimental operations. (3)Sensor Deployment and Data Acquisition A total of six sets of sensors were deployed for the experiment, each set comprising a matric suction sensor and a volumetric water content sensor spaced horizontally 20 cm apart. Sensors numbered 1, 3, 5, and 6 were buried at a depth of 10 cm, while sensors 2 and 4 were placed at a depth of 20 cm. Taking the 45° slope as an example, the sensor layout is shown in Fig. 1 (the 60° slope experiment has only a different gradient compared to the 45° slope, with identical principles for sensor deployment). The frequency of data collection was set at once per minute. When significant deformation occurred in the slope body, timely three-dimensional laser scanning of the slope was conducted to obtain detailed information about slope deformation. This experimental setup provides a structured approach to studying the effects of gradient on the stability of loess filled slopes under rainfall conditions, contributing valuable insights into slope management and protection strategies. Results Analysis of the failure process of 60° fill slopes Slope failure process The deformation and failure process of the 60-degree fill slope, as well as the deformation response cloud map, are shown in the Fig. 3 . During the initial rainfall period, the moisture content of the slope soil was low, and the rainwater infiltrated rapidly. With rainfall intensity lower than the slope infiltration rate, there was no significant deformation in the slope. After a certain duration of continuous rainfall, the surface moisture content of the slope gradually increases, while the infiltration rate decreases. When rainfall intensity exceeds the slope infiltration rate, a small amount of rainwater accumulates at the slope top, and fine flows appear on the slope surface. The flowing rainwater erodes the slope surface, causing soil particles to gather towards the toe of the slope. As the rainfall amount gradually increases, multiple micro-cracks develop at the slope top. Subsequently, the slope top collapses and numerous tension cracks form, with a width of approximately 1.5 cm, crossing the entire slope top. These cracks provide advantageous channels for rainwater infiltration, accelerating the slope deformation. As the cracks continue to expand, two levels of settlement deformation occur at the crack locations on the slope top. The first level of settlement deformation reaches as much as 3.5 cm. Multiple oblique cracks appear on the slope surface, accompanied by minor gullies. When the cracks reach a certain extent, multiple shear discontinuities occur at the top of the slope, leading to localized collapse. The development and expansion of oblique cracks at the rear edge of the slope result in a arc-shaped slump. Rainwater enters the slope through the cracks formed by the slump, increasing the moisture content of the internal soil mass and accelerating its softening. When the slip surface inside the soil mass becomes fully connected, the slope shoulder collapses, and a large amount of soil, along with rainwater erosion, slides toward the toe of the slope. The overall instability and destruction of the slope occur, accompanied by a continuous increase in rainfall, eventually transforming into a mudflow. Characterisation of the hydraulic response of slopes under rainfall infiltration The Fig. 4 shows the variation in readings of matric suction sensors and moisture content sensors on a 60-degree fill slope. During the initial rainfall period, the moisture content remained unchanged. This was because the wetting front had not reached the location of the sensors. In comparison to horizontal infiltration, the movement of the wetting front on the slope is parallel to the slope surface. Hence, the rate of wetting front movement on the slope can be calculated using the following equation [ 26 ] : where v s represents the rate of wetting front movement on the slope, θ is the slope angle, and t is the time taken for the wetting front to move. The overall moisture content variation curve can be divided into three stages. During the initial rainfall period, the wetting front has not reached the location of the sensors, and thus the moisture content remains unchanged, indicating a stable stage. Once the wetting front reaches the sensors, the soil moisture content rapidly increases, resulting in a sharp upward trend in the curve. Due to differences in depth and location, the response time also varies. The order of increasing response time for the moisture content is as follows: EC-6 = EC-5 < EC-1 < EC-3 < EC-4 < EC-2, corresponding to times of 14 minutes, 14 minutes, 16 minutes, 21 minutes, 24 minutes, and 42 minutes. The average movement rates of the wetting front are 7.14 mm/min, 3.57 mm/min, 3.13 mm/min, 2.38 mm/min, 4.16 mm/min, and 2.38 mm/min, respectively. From these findings, it can be concluded that under rainfall conditions, the water infiltration rate on the 60-degree fill slope is higher at the slope shoulder, followed by the slope toe and then the slope middle. When the moisture content increases to a certain extent, its growth rate gradually slows down until the readings stabilize. At this point, the sensors in various locations of the slope body have reached a saturated state, with the moisture content ranked from high to low as follows: EC-6 > EC-2 > EC-4 > EC-1 > EC-3 > EC-5. During the stable stage of moisture content, there are two noticeable drops: one is when cracks at the top of the slope connect, leading to redistribution of moisture within the slope; the other is when rainfall stops, causing a rapid decrease in moisture content due to continuous downward infiltration without replenishment from the upper part. After the upper part collapses, Sensors EC-1, EC-3, EC-4, and EC-6 all remain stable in terms of moisture content, while EC-2 continues to increase due to accelerated water penetration caused by circular cracks. EC-5 experiences a sudden drop at 112 minutes and returns to a stable state after 231 minutes. This is because the collapse of the upper part exposes EC-5 to the air, followed by the overall instability of the slope and subsequent reburial into the soil. The trend of matric suction change is similar to that of moisture content, but the difference is that it transitions from a rapid increase to a rapid decrease in the second stage. MPS-6 is exposed to the air due to landslide collapse, resulting in no data, while the order of matric suction change for the remaining sensors corresponds to the order of moisture content change. The response time sequence is as follows: MPS-5 < MPS-1 < MPS-3 < MPS-4 < MPS-2, with times of 14.5 minutes, 16 minutes, 21 minutes, 24 minutes, and 42 minutes, respectively. Comparing the moisture content and matric suction, we find that the matric suction still changes during the crack propagation period, remaining stable before the collapse of the upper part of the landslide. In the stable state, the order of matric suction magnitude is as follows: MPS-3 > MPS-1 > MPS-4 > MPS-2 > MPS-5. Except for the fluctuation in moisture content data caused by the collapse of Sensor MPS-5, the other sensors all exhibit the phenomenon that as moisture content increases, matric suction decreases, consistent with the findings of many researchers [ 30 – 34 ] . Analysis of the failure process of 45° fill slopes Slope failure process The process of deformation and failure of the 45° slope, along with the deformation response cloud map, is shown in the Fig. 5 . In the early stage of rainfall, the deformation process of the slope is similar to that of a 60° slope, which will not be described in detail here. As the rainfall duration increased, the super-saturated runoff generated from the top and upper part of the slope converged at the toe of the slope, causing infiltration at the toe of the slope. This led to a rapid increase in moisture content and a rapid decrease in effective stress at the toe of the slope. At 113 minutes, a arc-shaped collapse occurs, and the collapsed area rapidly expands on both sides due to continuous erosion by rainwater, gradually leaving the toe of the slope suspended. When the rainfall continues until 146 minutes, two tension cracks develop at the top of the slope, gradually expanding from micro-cracks. The cracks are parallel to the slope surface and numerous micro-cracks develop around them. These cracks provide a pathway for infiltrating rainwater, and they continue to develop with increasing rainfall, rapidly increasing the moisture content inside the soil, then reducing the internal shear strength of the soil mass. As a result, the slope undergoes creep, tensile cracking, and failure. At this point, the maximum width of the top tension crack can reach 1.5cm. When the volume of the softened material inside the slope reaches a certain extent, a sliding surface forms, causing a large-scale depression at the top of the slope, overall instability of the slope, and a large amount of soil sliding downward with rainwater, ultimately resulting in a flow-slide failure. Characterisation of the hydraulic response of slopes under rainfall infiltration The Fig. 6 shows the variations in readings of matric suction sensors and moisture content sensors on a 45° fill slope, as indicated. The overall moisture content curve of the 45° slope is similar to that of the 60° slope, exhibiting a stable-rapid rise-stable trend. The response time of moisture content from smallest to largest is EC-6 = EC-5 < EC-1 < EC-3 < EC-4 < EC-2, corresponding to 17 minutes, 17 minutes, 22 minutes, 38 minutes, 59 minutes, and 71 minutes. The average movement rates of the wetting front are as follows: 5.88mm/min, 4.16mm/min, 3.21mm/min, 1.86mm/min, 1.19mm/min, and 0.99mm/min for sensors EC-6, EC-5, EC-3, EC-1, EC-4, and EC-2 respectively. From these observations, it can be concluded that under rainfall conditions, the infiltration rate of water in the 45° fill slope is highest at the shoulder, followed by the toe, and then the middle of the slope. The stable moisture content decreases in the following order: EC-6 > EC-5 > EC-3 > EC-1 > EC-4 > EC-2. The lower moisture content at the toe compared to the middle of the slope is caused by localized failure and partial suspension at the toe, resulting in loss of moisture. Sensor 1 experiences a steep drop after the rainfall stops, because the rainwater that gathers at the toe due to super-saturated runoff disappears, causing a sudden decrease in moisture content. The variation curve of matric suction in the 45° fill slope is similar to that of the 60° slope, and the sequence of sensor variations is consistent with the sequence of moisture content variations on the 45° slope. Further elaboration is unnecessary. Slope Failure Patterns at Different Slope Gradients 60°. Fill Slope Failure Patterns The failure mode of a 60° loess fill slope is characterized by slope shoulder collapse—multi-stage retrusive failure. The deformation process exhibits characteristics such as settlement deformation, obvious tensile cracks, deep sliding surfaces, multi-stage sliding, and rapid failure time. It can be divided into the following processes: (1)Formation and propagation of tension cracks at the slope crest: During the initial rainfall stage, the moisture content of the slope is low. There is a large difference in moisture content between the two sides of the wetting peak, and the potential energy difference drives the water to penetrate rapidly, and the soil infiltration rate is greater than the rainfall rate. At this time, the slope is in the state of non-pressure infiltration, and there is no runoff on the slope. As water continues to infiltrate, the moisture content at the slope crest increases, and the soil matric suction decreases. The slope crest, as the tensile stress concentration zone, develops tension cracks when the effective shear strength of the soil is smaller than the tensile stress. At the same time, the formation of cracks provides favorable pathways for rainwater infiltration, accelerating erosion towards the interior, and enlarging the plastic zone of the slope crest, gradually expanding the cracks. (2)Settlement deformation of the slope crest: As the rainfall increases, water accumulates at the slope crest, creating a temporarily saturated zone around the tension cracks. Water columns can be observed within the cracks, indicating the existence of excess pore water pressure around the cracks. Loess soil exhibits strong sensitivity to water, causing softening of the surrounding soil near the cracks under the combined effect of self-weight and excess pore water pressure, resulting in internal sliding of slope and settlement deformation of the slope crest. (3)Sliding failure of the slope shoulder: The inclined cracks formed at the slope shoulder gradually expand into a circular or horseshoe shape during the rainfall process. Rainwater accelerates the softening of the soil around the circular or horseshoe-shaped cracks. Under the influence of self-weight and seepage forces, the soil tends to slide towards the open face of the slope. As the internal moisture content gradually increases, the effective stress of the soil decreases, and the plastic zone expands until the sliding surface is fully connected. The development of inclined cracks at the slope shoulder forms a circular or horseshoe-shaped Backwall of landslide. Eventually, the slope shoulder undergoes sliding failure. (4)Collapse of the slope shoulder and overall slope instability failure: The sliding surface generated by the failure of the slope shoulder forms a new open face, leading to stress redistribution at the slope crest. The supporting force of the soil at the slope crest decreases. Meanwhile, rainwater further softens the internal soil as it accelerates along the cracks, causing the cracks to gradually extend to wider and deeper areas. When the cracks extend to a certain extent, the combined action of self-weight stress, dynamic water pressure, and static water pressure exceeds the supporting force of the soil. This results in the collapse of the slope shoulder towards the open face, triggering overall instability and failure of the slope. 45°. Fill Slope Failure Patterns The failure mode of a 45° loess fill slope is characterized by toe collapse—deep-seated creeping cracking failure. The deformation and failure process of the slope exhibit distinct features such as toe collapse, obvious tension cracks, deep sliding surfaces, and prolonged duration of failure. The deformation and failure process of the slope has the following characteristics: (1)Slope toe infiltration to collapse: In the initial stage of rainfall, the slope has a low moisture content and the soil infiltration rate equals the rainfall rate, resulting in no surface runoff. As rainwater continues to infiltrate, the moisture content of the slope increases, and the soil infiltration rate gradually becomes smaller than the rainfall rate, leading to surface runoff. The runoff from the slope crest and upper of slope accumulates at the toe, increasing the moisture content of the soil at the toe. This reduces the matric suction and shear strength of the soil, causing local soil sliding force greater than anti-sliding force. As a result, the toe experiences a a circular or horseshoe-shaped collapse, and with the continuous accumulation of runoff, the volume of the collapse extends gradually to both sides parallel to the slope surface. (2)Formation and expansion of tension cracks at the slope crest: Similar to the first stage observed in the 60°slope, I will not elaborate on it again here. (3)Slope creeping cracking failure: The tension cracks formed at the top of the slope provide advantageous channels for rainwater infiltration, allowing rainwater to quickly enter the interior of the slope, reducing the matric suction of the internal soil matrix. Meanwhile, the open face formed at the toe of the slope causes the slope soil to have a tendency to slide downward under the combined action of self-weight stress and the dynamic and static water pressures generated by rainfall. As the shear strength of the soil continues to decrease, the slope experiences creep, characterized by increased tension cracks at the top of the slope and shear failure of the soil at the toe of the slope. (4)Overall slope instability failure: With the continuous expansion of the cracks, rainwater accelerates into the interior of the slope along the cracks, preventing timely drainage of internal water content. The soil gradually becomes saturated and generates excess pore water pressure internally. According to the principle of effective stress in soil mechanics, the effective stress of the soil gradually decreases as the excess pore water pressure increases [ 35 ] . As a result, the shear strength of the soil decreases, and the internal plastic zone expands. When the shear strength becomes smaller than the soil's resistance to sliding, the sliding surface becomes fully developed, leading to the overall instability failure of the slope. Discussion Comparing the time of tension crack formation at the crest, the onset of creep deformation, and the cessation of rainfall for overall flow slide between the 60° and 45° slopes, it was found that the 60° slope required less time in all aspects. This indicates that under rainfall conditions, a steeper gradient leads to landslides with less accumulated rainfall and increases the probability of landslide occurrence. Observations on the variation of wetting fronts at different positions on both slopes revealed that the rate of change is consistently faster at the slope shoulder than at the toe and mid-slope, indicating that the progression of the wetting front is fastest at the shoulder and slowest at the mid-slope under rainfall conditions. Before the formation of tension cracks at the crest, sensors 2 or 3 at the mid-slope were always the last to show an increase, suggesting that changes in moisture content at the mid-slope influence the generation of cracks and the development of slip surfaces. Comparing the distance between the rear edge of the landslide and the original shoulder, as well as the depth of the slip surface, it was found that for the 45° slope, the rear edge is further from the original shoulder and the slip surface is deeper compared to the 60° slope, indicating that gentler gradients lead to deeper slip surfaces. Prior to the onset of creep deformation within the slope body, all sensors showed an increase in moisture content and a decrease in matric suction, which stabilized for some time, demonstrating that increased moisture content is a primary factor in inducing landslides, with a certain lag effect—a finding consistent with Zhang et al.'s research [ 25 , 27 , 30 ] . Differences in slope stress distribution form the core mechanical basis for the distinct failure patterns exhibited by slopes of varying inclines [ 36 ] . For the 60° steep slope, the shoulder region becomes a critical stress concentration zone due to the significantly increased tangential component of gravitational force along the slope surface. Here, vertical compressive stress from the overlying soil exists, while the steep inclination causes rapid accumulation of tangential shear stress. Without sufficient lateral support from surrounding soil, this forms a “tension-shear composite stress zone.” In the experiments, oblique tensile cracks first appeared at the shoulder of the 60° slope (with a sudden change in moisture content detected by sensors at 14 minutes). This directly reflects how tensile stress first exceeded the soil's tensile strength after effective stress rapidly decreased due to rainwater infiltration. As cracks propagated, the shear resistance of the shoulder soil continued to decline. When shear stress surpassed residual shear strength, the shoulder collapsed first. The newly exposed surface after collapse further intensified stress redistribution in the upper soil mass, triggering a chain reaction of “collapse-stress release-new collapse,” ultimately resulting in shoulder collapse followed by multi-stage retreating failure. In contrast, the 45° moderate slope exhibits smaller gravitational tangential components, resulting in a stress state more dominated by vertical compressive forces. Consequently, the toe becomes the stress-concentrated weak link. As slope runoff converges at the toe, the toe soil remains saturated over time. A sudden drop in matrix suction causes a significant reduction in shear strength. Simultaneously, as the primary zone bearing vertical stresses, the toe experiences continuously accumulating shear stress from both self-weight and upper soil loads. When shear stress exceeds the soil's shear strength, localized failure and collapse occur first, forming an exposed face. The exposed face disrupts the slope's original stress equilibrium, inducing an upward creep toward the face. This triggers tensile cracking at the crest, ultimately leading to a failure sequence: toe collapse → deep-seated creep → tensile failure. Additionally, 45° slopes are more prone to developing deep slip surfaces. This is fundamentally due to the lower tangential component allowing rainwater sufficient time to infiltrate deep into the slope, leading to a reduction in the strength of the deep soil layers. In contrast, 60° slopes experience a larger tangential component and faster onset of instability, resulting in slip surfaces that are mostly confined to the shallow layers. Slope gradient directly influences the spatiotemporal distribution of slope moisture content by altering the residence time and infiltration pathways of rainfall on the slope surface, thereby indirectly regulating failure patterns [ 37 ] . Experimental data from slopes with two different gradients both show that rainfall transport rates follow the pattern “slope shoulder > toe > mid-slope.” However, the differing seepage effects caused by slope gradient variations ultimately lead to differences in the initiation location and evolution process of slope failure. For the 60° steep slope, the steep gradient significantly increases surface runoff velocity (experiments observed stream formation 8–10 minutes earlier than on the 45° slope). Rainwater dwells briefly in the mid-slope zone with limited infiltration, resulting in slow moisture content increase (Sensor 3 response time: 21 minutes, 7 minutes later than the slope shoulder). In contrast, the slope shoulder, situated at the terminal convergence point of runoff and characterized by steep inclination, facilitates concentrated infiltration along shoulder fissures (sensor No. 6 exhibited a sudden moisture content change at 14 minutes in the experiment), rapidly forming transient saturated zones. The formation of saturated zones drastically reduces effective stress in shoulder soils, accelerating tensile crack propagation and shoulder collapse. New fissures generated during multi-stage collapses further serve as preferential pathways for rainfall infiltration, driving rapid failure propagation into the slope interior. On the gentle slope of the 45° slope, surface runoff flows more slowly, prolonging rainfall retention time on the slope surface. This not only allows sufficient infiltration in the shoulder area (as indicated by Sensor 6's response at 17 minutes) but also creates a long-term waterlogged environment at the toe due to its low-lying topography, where runoff accumulates (experimental measurements showed Sensor 5 at the toe reached a stable moisture content of 32%–35%, approaching saturation). Sustained rainfall infiltration gradually softens the soil at the toe, causing continuous shear strength degradation and ultimately triggering the first failure. Simultaneously, the slower runoff velocity allows ample time for rainwater to infiltrate deep into the slope body. The wetting front advances slowly downward, progressively reducing the strength of deep-seated soils and creating conditions for subsequent deep-seated creep and fracturing. Furthermore, the mid-slope zone of the 45° slope, where rainfall retention time was moderate, exhibited a gradual increase in moisture content. This zone functioned as a “transition zone” connecting stress transfer between the slope crest and toe. Its strength degradation process synchronously regulated the coordinated evolution of crack propagation at the crest and collapse at the toe, ultimately forming a failure path characterized by “toe collapse first, crest cracking later, and deep-seated breakthrough.” The failure mode of the 60° loess filled slope is characterized by shoulder collapse followed by multi-level retreat-type destruction, featuring settlement deformation, prominent tensile cracking, deep sliding surfaces, multiple levels of sliding, and short duration of destruction. In contrast, the 45° loess filled slope exhibits a failure mode of toe collapse and deep-seated creep-induced tensile fracture, marked by toe collapse, pronounced tensile cracking, deep sliding surfaces, and prolonged destruction duration. For the 60° slope, reinforcement should focus on the top and shoulder areas, such as installing anti-slide piles and prestressed anchor cables to strengthen the shoulder and prevent overall instability due to shoulder collapse; effective drainage systems should be implemented to reduce water accumulation at the top. For the 45° slope, emphasis should be placed on toe protection through measures like retaining walls and slope protection works to enhance stability and prevent toe collapse; improving drainage at the top and on the slope face can mitigate rainwater infiltration and runoff collection. Moreover, vegetation restoration can be carried out on both types of slopes to improve soil resistance to erosion, reduce the rate of rainfall infiltration, and enhance overall slope stability. Conclusions This study investigated the instability and failure of loess-filled slopes with different inclinations under rainfall conditions through indoor simulated experiments. The preliminary conclusions are as follows: (1)Under rainfall conditions, the rate of water movement in the loess-filled slope shows a pattern of shoulder > toe > middle of the slope for slopes with different inclinations. At the same time, the trend of volumetric water content exhibits a stable-rapid increase-stable pattern, while the matric suction shows a stable-rapid decrease-stable pattern. After a stable period of time for both factors, slope failure occurs, indicating a lag in the destabilization of loess-filled slopes induced by rainfall. (2)Under the same rainfall intensity, the steeper the slope, the greater the probability of landslide occurrence, the quicker the onset of landslide, and the smaller the critical rainfall amount required for landslide initiation. Moreover, for lower inclinations, the depth of surface of rupture is deeper and overall volume of landslides are larger. (3)The failure mode of the loess-filled slope with a 60° inclination is characterized by slope shoulder collapse—multi-stage retrusive failure. The failure mode of the loess-filled slope with a 45° inclination involves toe collapse—deep-seated creeping cracking failure.. Smaller inclinations make the toe more susceptible to infiltration-induced collapse, while larger inclinations are more prone to settlement deformation of the slope crest under the influence of self-weight stress. Declarations Data availability statement Readers with reasonable demands can obtain data from the corresponding author. Research funding No funding Acknowledgements We would like to express our gratitude to the State Key Laboratory of Geohazard Prevention and Geoenvironment Protection for providing the instrument and venue support. At the same time, we also thank the proofreaders and editors for their assistance. Author contributions Guangyuan Dou: writing, editing and methodology. 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1","display":"","copyAsset":false,"role":"figure","size":310468,"visible":true,"origin":"","legend":"\u003cp\u003eTest Models and Instruments. (a) Rainfall nozzle layout, (b) Model box, (c) 3D laser scanner and data acquisition, (d) Sensor distribution, (e) Sensors\u003c/p\u003e","description":"","filename":"floatimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7704393/v1/d32116f91991a028c800f484.jpeg"},{"id":94026970,"identity":"3fc5a76e-cfb6-44c8-bf8e-0a27308920ae","added_by":"auto","created_at":"2025-10-21 13:40:40","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":893185,"visible":true,"origin":"","legend":"\u003cp\u003eLocation map of sampling points. (a) Sampling site, Shanxi Province, (b) loess fill in Yanan, (c) soil samples for test\u003c/p\u003e","description":"","filename":"floatimage2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7704393/v1/4c8cc21103c1ea8af875cd31.jpeg"},{"id":94026975,"identity":"5328759e-c05d-41cd-89e1-9340608e47b8","added_by":"auto","created_at":"2025-10-21 13:40:40","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":209746,"visible":true,"origin":"","legend":"\u003cp\u003eThe failure process of 60°loess fill slopes.(a) Initial state, (b) Settlement deformation at the slope crest, (c) Collapse occurring at the slope shoulder, (d) Cracks at the top of the slope, (e) Landslide occurring at the slope shoulder，Instability failure of the slope, (f) mudflow\u003c/p\u003e","description":"","filename":"floatimage3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7704393/v1/9f658a2465aeecc761fd2a5c.jpeg"},{"id":94027451,"identity":"70e48dc3-3ef3-494f-9e71-209d40ad855d","added_by":"auto","created_at":"2025-10-21 13:48:40","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":108839,"visible":true,"origin":"","legend":"\u003cp\u003eSensor variation curve of a 60° loess fill slope. (a) Variation curve of moisture content, (b) Variation curve of matric suction.\u003c/p\u003e","description":"","filename":"floatimage4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7704393/v1/e675fb63ed68c77f9a39dd27.jpeg"},{"id":94026977,"identity":"8d8a7efd-af19-4075-b5b4-2be5344fa556","added_by":"auto","created_at":"2025-10-21 13:40:40","extension":"jpeg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":149255,"visible":true,"origin":"","legend":"\u003cp\u003eThe failure process of 45°loess fill slopes. (a) Initial state, (b) Collapse of the toe of slope, (c) Cracks at the top of the slope, (d) creeping cracking failure of slope, (e) Instability failure of the slope, (f) mudflow\u003c/p\u003e","description":"","filename":"floatimage5.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7704393/v1/c0f6dc5ec85230bb5e53eed6.jpeg"},{"id":94028075,"identity":"d7f0c328-e78b-4cff-9d20-2da969b8054c","added_by":"auto","created_at":"2025-10-21 13:56:40","extension":"jpeg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":94814,"visible":true,"origin":"","legend":"\u003cp\u003eSensor variation curve of a 45° loess fill slope. (a) Variation curve of moisture content, (b) Variation curve of matric suction\u003c/p\u003e","description":"","filename":"floatimage6.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7704393/v1/5d6682b29f73d89117aba292.jpeg"},{"id":94026972,"identity":"84bef386-28b0-4f3c-b7e5-f7e8056500c3","added_by":"auto","created_at":"2025-10-21 13:40:40","extension":"jpeg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":80000,"visible":true,"origin":"","legend":"\u003cp\u003eThe failure Patterns of 60° loess fill slopes. (a) Formation and propagation of tension cracks at the slope crest, (b) Settlement deformation of the slope crest, (c) Sliding failure of the slope shoulder, (d) Collapse of the slope shoulder and overall slope instability failure\u003c/p\u003e","description":"","filename":"floatimage7.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7704393/v1/1c2c751923a505fc323b3230.jpeg"},{"id":94028077,"identity":"b2802a39-f9af-4328-8f72-38c9028b2bd3","added_by":"auto","created_at":"2025-10-21 13:56:40","extension":"jpeg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":76592,"visible":true,"origin":"","legend":"\u003cp\u003eThe failure Patterns of 45° loess fill slopes. (a) Slope toe infiltration to collapse, (b) Formation and expansion of tension cracks at the slope crest, (c) Slope creeping cracking failure, (d) Overall slope instability failure\u003c/p\u003e","description":"","filename":"floatimage8.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7704393/v1/6e6cb561dfd7a7ebb3fba05e.jpeg"},{"id":97664830,"identity":"cde64b4d-a82e-428f-b320-b3bf0a7dc5c0","added_by":"auto","created_at":"2025-12-08 09:14:52","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2685506,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7704393/v1/62780b03-c470-4f57-b291-9ab8634c1ba3.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Experimental Study on Failure Mechanisms of Steep Loess Fill Slopes with Large Gradient","fulltext":[{"header":"Introduction","content":"\u003cp\u003eLoess is a porous and weakly cemented sediment from the fourth century, characterized by grayish-yellow, brownish-yellow, and even reddish-brown colors. Its particle composition is mainly composed of fine particles, with a uniform texture and no stratification\u003csup\u003e[\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]\u003c/sup\u003e. Loess is widely distributed in China, accounting for approximately 6.6% of China's total land area.. It is mostly found in regions such as Shaanxi, Shanxi, and Gansu\u003csup\u003e[\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]\u003c/sup\u003e. With the development of society and the advancement of urbanization in various regions, the original construction land in urban areas can no longer meet the demands of development. Therefore, large-scale major engineering projects such as land reclamation and mountain flattening have been conducted in the loess plateau area surrounding Yan'an. These projects not only brought development opportunities for local people but also posed significant risks of major disasters. Due to the highly water-sensitive and collapsible nature of loess soil, the loess fill slope formed by extensive construction projects are prone to geological hazards such as landslides under rainy conditions\u003csup\u003e[\u003cspan additionalcitationids=\"CR7 CR8\" citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]\u003c/sup\u003e. A study conducted by scholars investigated and analyzed the occurrences of loess landslides in the northern region of Shaanxi Province from 1990 to 2010. They found that the frequency of landslides was highest during the months of July to October. Additionally, it was observed that the rainy season in the Loess Plateau also concentrated most heavily from July to September\u003csup\u003e[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]\u003c/sup\u003e. A large and concentrated amount of rainfall can lead to an increase in the water content of loess, the formation of cracks at the rear edge of slopes, and other conditions. In addition, the loess fill slope, which alters the hydrogeological conditions of the original terrain, hinders the drainage of groundwater. Under the infiltration of rainwater, the groundwater level can easily rise, causing soil softening and eventually triggering landslides\u003csup\u003e[\u003cspan additionalcitationids=\"CR12 CR13 CR14 CR15\" citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eThe stability of loess slopes has been a focal point of research within the field of engineering geology, with numerous scholars conducting in-depth studies from various perspectives. Extreme heavy rainfall is a significant inducement for instability in loess slopes\u003csup\u003e[\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e. During the process of rainwater infiltration, the soil water content continuously increases, gradually transforming the soil from an unsaturated to a saturated state, which decreases effective stress in the soil and consequently reduces the shear strength of the slope, eventually leading to slope instability and destruction\u003csup\u003e[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]\u003c/sup\u003e. Meanwhile, the process and rate of water infiltration have substantial impacts on slope stability. The wetting front is a critical indicator for assessing the rate of soil infiltration; its movement speed is influenced by the initial physical state of the soil mass, soil characteristics, and rainfall conditions\u003csup\u003e[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/sup\u003e. A rapidly moving wetting front can generate significant hydraulic gradients locally within the slope body in a short period, destabilizing the soil structure; whereas a slowly moving wetting front allows time for stress adjustment within the soil but prolonged water accumulation can continually degrade soil strength, ultimately affecting slope stability\u003csup\u003e[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]\u003c/sup\u003e. The slope structure and types of rainfall also significantly influence the mechanisms of shallow loess destruction. Different slope structures result in varied internal stress distributions within the slope, while different types of rainfall manifest differently in terms of infiltration rates, methods, and soil wetting processes\u003csup\u003e[\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]\u003c/sup\u003e. Their interaction leads to diverse modes of destruction, such as concentrated rainfall causing rapid saturation of superficial soils in specific structured slopes leading to sudden failure, or continuous rainfall potentially softening deeper soils in other structured slopes, forming potential sliding surfaces that threaten overall stability \u003csup\u003e[\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eWith the intensification of human activities, an increasing number of artificial filled slopes have entered the living range of people, making the safety and stability of these filled slopes a subject of considerable research significance\u003csup\u003e[\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]\u003c/sup\u003e. Chen studied linear-type loess filled slopes and derived insights into the failure patterns and the impact of fine particle movement on filled slope failure\u003csup\u003e[\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]\u003c/sup\u003e. Additionally, based on Zhang's laboratory model tests, it was concluded that under rainfall conditions, deformation of the loess filled slope exhibits complex spatiotemporal distribution characteristics. Rainfall infiltration causes uneven settlement, triggering crack generation and propagation, accelerating deep-soil softening, and reducing strength, which may ultimately lead to overall instability. By monitoring key parameters such as pore water pressure and displacement rates in real-time, an innovative early warning threshold model and framework were proposed, offering new ideas for early warning and risk prevention of slope disasters\u003csup\u003e[\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eThe construction slope of loess fill slopes varies according to project requirements. In areas with limited construction space, numerous steep fill slopes have emerged. Based on Chinese scholars' statistical analysis of loess geological hazards in Yan'an, Shaanxi Province, unstable slopes exceeding 50\u0026deg; account for 90.6% of cases. Among slopes less than 60\u0026deg;, the primary failure mechanism is landslides, exhibiting a significant nonlinear relationship between slope instability risk and gradient: The probability of landslide occurrence increases exponentially with slope angle. At 40\u0026deg;, the probability is only 0.01, but it surges to 0.16 at 60\u0026deg;\u003csup\u003e[\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]\u003c/sup\u003e. This data reveals that steep slope angles may be one of the core controlling factors inducing instability in loess fill slopes. Moreover, the causes and failure patterns of landslides differ across varying slope angles. Therefore, investigating the impact of steeper slopes on the instability mechanisms of loess fill slopes under rainfall infiltration conditions is crucial for reducing casualties and economic losses caused by landslides. To this end, this study investigates loess fill slopes as the research subject. Utilizing indoor rainfall simulation experiments combined with 3D laser scanning and sensor monitoring, it examines the deformation and failure processes, instability characteristics, and failure patterns of loess fill slopes under varying steep slopes. The study also analyzes the influence of slope gradient on the instability mechanism of loess fill slopes under rainfall conditions. The findings aim to provide theoretical guidance for the prevention and control engineering of loess fill slopes.\u003c/p\u003e"},{"header":"Materials and Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003eExperimental Instruments\u003c/h2\u003e\u003cp\u003eThe location of this model experiment was Sichuan Province State Key Laboratory of Geohazard Prevention and Geoenvironment Protection (Chengdu University of Technology). The experimental setup was a self-developed rainfall simulation test system. The experimental instruments mainly include a model box, a Rainfall simulation system, and a measurement system(Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The specific details are as follows:\u003c/p\u003e\u003cp\u003eModel Box: The dimensions of the model are 1.2 meters in length, 0.5 meters in width, and 0.7 meters in height. It consisted of three-sided transparent organic glass and a water-impermeable high-strength bottom board. The glass was connected to the bottom board with angle steel, and the connection is sealed with glass glue. At the left side of the model box, a 10cm square grid is drawn on the surface of the organic glass for coordinate reference and benchmark point plotting. To reduce the boundary effects of the slope in the model box, Vaseline is applied to the inner surface of the organic glass before the experiment.\u003c/p\u003e\u003cp\u003eRainfall Simulation System: The rainfall simulation system is self-designed and mainly consists of a water tank, low-pressure ultrafine mist rainfall nozzles, water meter valves, water supply pipes, and pressure gauges. The low-pressure ultrafine mist rainfall nozzles are arranged in four rows, with four nozzles per row, totaling 16 nozzles. The distance between each nozzle is 30cm.\u003c/p\u003e\u003cp\u003eMeasurement system: The measurement system consists of five components: volumetric water content sensor, matric suction sensor, high-definition digital camera, data acquisition device, and 3D laser scanner. The volumetric water content sensor and matric suction sensor used in the experiment are EC volumetric water content sensor and MPS matric suction sensor developed and produced by Decagon, a company based in the United States. The data acquisition system is EM50 acquisition system, and the 3D laser scanner is riegl 3D laser scanner developed by RIEGL, a company based in Austria. The sensor parameters are shown in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, and the experimental instruments are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e\u003cp\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\u003eSensor parameters.\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=\"char\" char=\".\" 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\u003eType\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eQuantities\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003ePrecision\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eRange\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eMPS\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e\u0026plusmn;(10% +2 kPa)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e-9kPa~-100,000 kPa\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eEC\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;2%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0-100%\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003eExperimental content\u003c/h3\u003e\n\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003eBasic properties of the experimental soil sample\u003c/h2\u003e\u003cp\u003eThe study area is located in Anse County, Yan'an City, Shaanxi Province, China, with a total area of 2,950 km\u003csup\u003e2\u003c/sup\u003e, between longitude 108\u0026deg;5\u0026prime;44\u0026Prime;-109\u0026deg;26\u0026prime;18\u0026Prime; E and latitude 36\u0026deg;30\u0026prime;45\u0026Prime;-37\u0026deg;19\u0026prime;3\u0026Prime; N. The area is intersected by gullies and rounded with beams and mounts, with an average elevation of 1,371.9 m above sea level, and it is typical of the Liangxuan-like yellow earth mound landform. The materials used in this model experiment were taken from the fresh profile of Malan loess at Nangou, Ansai County, Yan'an City(Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). The basic physical parameters measured are shown in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. To ensure that the test results are not affected by the size effect of the model box, all selected materials were passed through a 2mm standard sieve.\u003c/p\u003e\u003cp\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\u003eBasic physical parameters of the test soil.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"7\"\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\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eType\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003especific gravity\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003e\u003cem\u003ew\u003c/em\u003e\u003csub\u003e\u003cem\u003el\u003c/em\u003e\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003e\u003cem\u003ew\u003c/em\u003e\u003csub\u003e\u003cem\u003ep\u003c/em\u003e\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003e\u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003ep\u003c/em\u003e\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003ePorosity\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c7\"\u003e\u003cp\u003e\u003cem\u003eK/\u003c/em\u003e(m\u0026middot;s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ethe loess of Yan'an\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e2.70\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e30.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e21.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e9.2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.85\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e1.26\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003eExperimental procedure\u003c/h3\u003e\n\u003cp\u003eThe experiment aimed to investigate the infiltration process of water in loess filled slopes and the impact of gradient on the failure modes of these slopes under natural rainfall conditions. The specific experimental design is as follows:\u003c/p\u003e\u003cp\u003e(1)Variable Control and Parameter Setting\u003c/p\u003e\u003cp\u003eMaintaining constant rainfall intensity and original slope gradient, this study used the fill slope gradient as the sole variable. Based on field investigations of the study area, the gradient of loess filled slopes predominantly ranges between 30\u0026deg; and 60\u0026deg;, characterized by a \"three surfaces, two bodies, one water\" structure. Accordingly, the initial slope gradient was designed to be 65\u0026deg;, with two additional fill gradients set at 45\u0026deg; and 60\u0026deg; for experimentation. Rainfall intensity was determined by considering regional extreme rainfall intensity and uniformity of experimental rainfall. According to statistical data from the Yan'an Meteorological Bureau over the past decade, the maximum rainfall intensity in the region is 60 mm/h. After testing for rainfall uniformity, it was found that when the rainfall intensity is 30.1 mm/h, the uniformity reaches 81%, meeting the experimental requirements. Therefore, the rainfall intensity was set at 30.1 mm/h, using continuous uniform rainfall.\u003c/p\u003e\u003cp\u003e(2)Model Construction and Preparation\u003c/p\u003e\u003cp\u003eThe filled slope model was constructed using a layered filling method, with each layer having a thickness of 10 cm, totaling six layers. During construction, scarifying treatment was applied between layers to ensure adequate contact between soil layers while achieving a compaction degree of 90% of the maximum density for each filled soil layer, with moisture content controlled to a predetermined standard value. After completing the layered filling, the model was statically cured for more than 24 hours until sensor readings stabilized, allowing for subsequent experimental operations.\u003c/p\u003e\u003cp\u003e(3)Sensor Deployment and Data Acquisition\u003c/p\u003e\u003cp\u003eA total of six sets of sensors were deployed for the experiment, each set comprising a matric suction sensor and a volumetric water content sensor spaced horizontally 20 cm apart. Sensors numbered 1, 3, 5, and 6 were buried at a depth of 10 cm, while sensors 2 and 4 were placed at a depth of 20 cm. Taking the 45\u0026deg; slope as an example, the sensor layout is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e (the 60\u0026deg; slope experiment has only a different gradient compared to the 45\u0026deg; slope, with identical principles for sensor deployment). The frequency of data collection was set at once per minute. When significant deformation occurred in the slope body, timely three-dimensional laser scanning of the slope was conducted to obtain detailed information about slope deformation.\u003c/p\u003e\u003cp\u003eThis experimental setup provides a structured approach to studying the effects of gradient on the stability of loess filled slopes under rainfall conditions, contributing valuable insights into slope management and protection strategies.\u003c/p\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003eAnalysis of the failure process of 60\u0026deg; fill slopes\u003c/h2\u003e\u003cdiv id=\"Sec9\" class=\"Section3\"\u003e\u003ch2\u003eSlope failure process\u003c/h2\u003e\u003cp\u003eThe deformation and failure process of the 60-degree fill slope, as well as the deformation response cloud map, are shown in the Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. During the initial rainfall period, the moisture content of the slope soil was low, and the rainwater infiltrated rapidly. With rainfall intensity lower than the slope infiltration rate, there was no significant deformation in the slope. After a certain duration of continuous rainfall, the surface moisture content of the slope gradually increases, while the infiltration rate decreases. When rainfall intensity exceeds the slope infiltration rate, a small amount of rainwater accumulates at the slope top, and fine flows appear on the slope surface. The flowing rainwater erodes the slope surface, causing soil particles to gather towards the toe of the slope. As the rainfall amount gradually increases, multiple micro-cracks develop at the slope top. Subsequently, the slope top collapses and numerous tension cracks form, with a width of approximately 1.5 cm, crossing the entire slope top. These cracks provide advantageous channels for rainwater infiltration, accelerating the slope deformation. As the cracks continue to expand, two levels of settlement deformation occur at the crack locations on the slope top. The first level of settlement deformation reaches as much as 3.5 cm. Multiple oblique cracks appear on the slope surface, accompanied by minor gullies. When the cracks reach a certain extent, multiple shear discontinuities occur at the top of the slope, leading to localized collapse. The development and expansion of oblique cracks at the rear edge of the slope result in a arc-shaped slump. Rainwater enters the slope through the cracks formed by the slump, increasing the moisture content of the internal soil mass and accelerating its softening. When the slip surface inside the soil mass becomes fully connected, the slope shoulder collapses, and a large amount of soil, along with rainwater erosion, slides toward the toe of the slope. The overall instability and destruction of the slope occur, accompanied by a continuous increase in rainfall, eventually transforming into a mudflow.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\n\u003ch3\u003eCharacterisation of the hydraulic response of slopes under rainfall infiltration\u003c/h3\u003e\n\u003cp\u003eThe Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e shows the variation in readings of matric suction sensors and moisture content sensors on a 60-degree fill slope. During the initial rainfall period, the moisture content remained unchanged. This was because the wetting front had not reached the location of the sensors. In comparison to horizontal infiltration, the movement of the wetting front on the slope is parallel to the slope surface. Hence, the rate of wetting front movement on the slope can be calculated using the following equation\u003csup\u003e[\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]\u003c/sup\u003e:\u003c/p\u003e\u003cp\u003e\u003cimg src=\"data:image/png;base64,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\" width=\"177\" height=\"85\"\u003e\u003c/p\u003e\u003cp\u003ewhere \u003cem\u003ev\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e\u003c/sub\u003e represents the rate of wetting front movement on the slope, \u003cem\u003eθ\u003c/em\u003e is the slope angle, and \u003cem\u003et\u003c/em\u003e is the time taken for the wetting front to move.\u003c/p\u003e\u003cp\u003eThe overall moisture content variation curve can be divided into three stages. During the initial rainfall period, the wetting front has not reached the location of the sensors, and thus the moisture content remains unchanged, indicating a stable stage. Once the wetting front reaches the sensors, the soil moisture content rapidly increases, resulting in a sharp upward trend in the curve. Due to differences in depth and location, the response time also varies. The order of increasing response time for the moisture content is as follows: EC-6\u0026thinsp;=\u0026thinsp;EC-5\u0026thinsp;\u0026lt;\u0026thinsp;EC-1\u0026thinsp;\u0026lt;\u0026thinsp;EC-3\u0026thinsp;\u0026lt;\u0026thinsp;EC-4\u0026thinsp;\u0026lt;\u0026thinsp;EC-2, corresponding to times of 14 minutes, 14 minutes, 16 minutes, 21 minutes, 24 minutes, and 42 minutes. The average movement rates of the wetting front are 7.14 mm/min, 3.57 mm/min, 3.13 mm/min, 2.38 mm/min, 4.16 mm/min, and 2.38 mm/min, respectively. From these findings, it can be concluded that under rainfall conditions, the water infiltration rate on the 60-degree fill slope is higher at the slope shoulder, followed by the slope toe and then the slope middle. When the moisture content increases to a certain extent, its growth rate gradually slows down until the readings stabilize. At this point, the sensors in various locations of the slope body have reached a saturated state, with the moisture content ranked from high to low as follows: EC-6\u0026thinsp;\u0026gt;\u0026thinsp;EC-2\u0026thinsp;\u0026gt;\u0026thinsp;EC-4\u0026thinsp;\u0026gt;\u0026thinsp;EC-1\u0026thinsp;\u0026gt;\u0026thinsp;EC-3\u0026thinsp;\u0026gt;\u0026thinsp;EC-5. During the stable stage of moisture content, there are two noticeable drops: one is when cracks at the top of the slope connect, leading to redistribution of moisture within the slope; the other is when rainfall stops, causing a rapid decrease in moisture content due to continuous downward infiltration without replenishment from the upper part. After the upper part collapses, Sensors EC-1, EC-3, EC-4, and EC-6 all remain stable in terms of moisture content, while EC-2 continues to increase due to accelerated water penetration caused by circular cracks. EC-5 experiences a sudden drop at 112 minutes and returns to a stable state after 231 minutes. This is because the collapse of the upper part exposes EC-5 to the air, followed by the overall instability of the slope and subsequent reburial into the soil. The trend of matric suction change is similar to that of moisture content, but the difference is that it transitions from a rapid increase to a rapid decrease in the second stage. MPS-6 is exposed to the air due to landslide collapse, resulting in no data, while the order of matric suction change for the remaining sensors corresponds to the order of moisture content change. The response time sequence is as follows: MPS-5\u0026thinsp;\u0026lt;\u0026thinsp;MPS-1\u0026thinsp;\u0026lt;\u0026thinsp;MPS-3\u0026thinsp;\u0026lt;\u0026thinsp;MPS-4\u0026thinsp;\u0026lt;\u0026thinsp;MPS-2, with times of 14.5 minutes, 16 minutes, 21 minutes, 24 minutes, and 42 minutes, respectively. Comparing the moisture content and matric suction, we find that the matric suction still changes during the crack propagation period, remaining stable before the collapse of the upper part of the landslide. In the stable state, the order of matric suction magnitude is as follows: MPS-3\u0026thinsp;\u0026gt;\u0026thinsp;MPS-1\u0026thinsp;\u0026gt;\u0026thinsp;MPS-4\u0026thinsp;\u0026gt;\u0026thinsp;MPS-2\u0026thinsp;\u0026gt;\u0026thinsp;MPS-5. Except for the fluctuation in moisture content data caused by the collapse of Sensor MPS-5, the other sensors all exhibit the phenomenon that as moisture content increases, matric suction decreases, consistent with the findings of many researchers\u003csup\u003e[\u003cspan additionalcitationids=\"CR31 CR32 CR33\" citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\u003ch2\u003eAnalysis of the failure process of 45\u0026deg; fill slopes\u003c/h2\u003e\u003cdiv id=\"Sec12\" class=\"Section3\"\u003e\u003ch2\u003eSlope failure process\u003c/h2\u003e\u003cp\u003eThe process of deformation and failure of the 45\u0026deg; slope, along with the deformation response cloud map, is shown in the Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. In the early stage of rainfall, the deformation process of the slope is similar to that of a 60\u0026deg; slope, which will not be described in detail here. As the rainfall duration increased, the super-saturated runoff generated from the top and upper part of the slope converged at the toe of the slope, causing infiltration at the toe of the slope. This led to a rapid increase in moisture content and a rapid decrease in effective stress at the toe of the slope. At 113 minutes, a arc-shaped collapse occurs, and the collapsed area rapidly expands on both sides due to continuous erosion by rainwater, gradually leaving the toe of the slope suspended. When the rainfall continues until 146 minutes, two tension cracks develop at the top of the slope, gradually expanding from micro-cracks. The cracks are parallel to the slope surface and numerous micro-cracks develop around them. These cracks provide a pathway for infiltrating rainwater, and they continue to develop with increasing rainfall, rapidly increasing the moisture content inside the soil, then reducing the internal shear strength of the soil mass. As a result, the slope undergoes creep, tensile cracking, and failure. At this point, the maximum width of the top tension crack can reach 1.5cm. When the volume of the softened material inside the slope reaches a certain extent, a sliding surface forms, causing a large-scale depression at the top of the slope, overall instability of the slope, and a large amount of soil sliding downward with rainwater, ultimately resulting in a flow-slide failure.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\u003ch2\u003eCharacterisation of the hydraulic response of slopes under rainfall infiltration\u003c/h2\u003e\u003cp\u003eThe Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e shows the variations in readings of matric suction sensors and moisture content sensors on a 45\u0026deg; fill slope, as indicated. The overall moisture content curve of the 45\u0026deg; slope is similar to that of the 60\u0026deg; slope, exhibiting a stable-rapid rise-stable trend. The response time of moisture content from smallest to largest is EC-6\u0026thinsp;=\u0026thinsp;EC-5\u0026thinsp;\u0026lt;\u0026thinsp;EC-1\u0026thinsp;\u0026lt;\u0026thinsp;EC-3\u0026thinsp;\u0026lt;\u0026thinsp;EC-4\u0026thinsp;\u0026lt;\u0026thinsp;EC-2, corresponding to 17 minutes, 17 minutes, 22 minutes, 38 minutes, 59 minutes, and 71 minutes. The average movement rates of the wetting front are as follows: 5.88mm/min, 4.16mm/min, 3.21mm/min, 1.86mm/min, 1.19mm/min, and 0.99mm/min for sensors EC-6, EC-5, EC-3, EC-1, EC-4, and EC-2 respectively. From these observations, it can be concluded that under rainfall conditions, the infiltration rate of water in the 45\u0026deg; fill slope is highest at the shoulder, followed by the toe, and then the middle of the slope. The stable moisture content decreases in the following order: EC-6\u0026thinsp;\u0026gt;\u0026thinsp;EC-5\u0026thinsp;\u0026gt;\u0026thinsp;EC-3\u0026thinsp;\u0026gt;\u0026thinsp;EC-1\u0026thinsp;\u0026gt;\u0026thinsp;EC-4\u0026thinsp;\u0026gt;\u0026thinsp;EC-2. The lower moisture content at the toe compared to the middle of the slope is caused by localized failure and partial suspension at the toe, resulting in loss of moisture. Sensor 1 experiences a steep drop after the rainfall stops, because the rainwater that gathers at the toe due to super-saturated runoff disappears, causing a sudden decrease in moisture content. The variation curve of matric suction in the 45\u0026deg; fill slope is similar to that of the 60\u0026deg; slope, and the sequence of sensor variations is consistent with the sequence of moisture content variations on the 45\u0026deg; slope. Further elaboration is unnecessary.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec14\" class=\"Section2\"\u003e\u003ch2\u003eSlope Failure Patterns at Different Slope Gradients\u003c/h2\u003e\u003cp\u003e\u003cb\u003e60\u0026deg;. Fill Slope Failure Patterns\u003c/b\u003e\u003c/p\u003e\u003cp\u003eThe failure mode of a 60\u0026deg; loess fill slope is characterized by slope shoulder collapse\u0026mdash;multi-stage retrusive failure. The deformation process exhibits characteristics such as settlement deformation, obvious tensile cracks, deep sliding surfaces, multi-stage sliding, and rapid failure time. It can be divided into the following processes:\u003c/p\u003e\u003cp\u003e(1)Formation and propagation of tension cracks at the slope crest: During the initial rainfall stage, the moisture content of the slope is low. There is a large difference in moisture content between the two sides of the wetting peak, and the potential energy difference drives the water to penetrate rapidly, and the soil infiltration rate is greater than the rainfall rate. At this time, the slope is in the state of non-pressure infiltration, and there is no runoff on the slope. As water continues to infiltrate, the moisture content at the slope crest increases, and the soil matric suction decreases. The slope crest, as the tensile stress concentration zone, develops tension cracks when the effective shear strength of the soil is smaller than the tensile stress. At the same time, the formation of cracks provides favorable pathways for rainwater infiltration, accelerating erosion towards the interior, and enlarging the plastic zone of the slope crest, gradually expanding the cracks.\u003c/p\u003e\u003cp\u003e(2)Settlement deformation of the slope crest: As the rainfall increases, water accumulates at the slope crest, creating a temporarily saturated zone around the tension cracks. Water columns can be observed within the cracks, indicating the existence of excess pore water pressure around the cracks. Loess soil exhibits strong sensitivity to water, causing softening of the surrounding soil near the cracks under the combined effect of self-weight and excess pore water pressure, resulting in internal sliding of slope and settlement deformation of the slope crest.\u003c/p\u003e\u003cp\u003e(3)Sliding failure of the slope shoulder: The inclined cracks formed at the slope shoulder gradually expand into a circular or horseshoe shape during the rainfall process. Rainwater accelerates the softening of the soil around the circular or horseshoe-shaped cracks. Under the influence of self-weight and seepage forces, the soil tends to slide towards the open face of the slope. As the internal moisture content gradually increases, the effective stress of the soil decreases, and the plastic zone expands until the sliding surface is fully connected. The development of inclined cracks at the slope shoulder forms a circular or horseshoe-shaped Backwall of landslide. Eventually, the slope shoulder undergoes sliding failure.\u003c/p\u003e\u003cp\u003e(4)Collapse of the slope shoulder and overall slope instability failure: The sliding surface generated by the failure of the slope shoulder forms a new open face, leading to stress redistribution at the slope crest. The supporting force of the soil at the slope crest decreases. Meanwhile, rainwater further softens the internal soil as it accelerates along the cracks, causing the cracks to gradually extend to wider and deeper areas. When the cracks extend to a certain extent, the combined action of self-weight stress, dynamic water pressure, and static water pressure exceeds the supporting force of the soil. This results in the collapse of the slope shoulder towards the open face, triggering overall instability and failure of the slope.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cb\u003e45\u0026deg;. Fill Slope Failure Patterns\u003c/b\u003e\u003c/p\u003e\u003cp\u003eThe failure mode of a 45\u0026deg; loess fill slope is characterized by toe collapse\u0026mdash;deep-seated creeping cracking failure. The deformation and failure process of the slope exhibit distinct features such as toe collapse, obvious tension cracks, deep sliding surfaces, and prolonged duration of failure. The deformation and failure process of the slope has the following characteristics:\u003c/p\u003e\u003cp\u003e(1)Slope toe infiltration to collapse: In the initial stage of rainfall, the slope has a low moisture content and the soil infiltration rate equals the rainfall rate, resulting in no surface runoff. As rainwater continues to infiltrate, the moisture content of the slope increases, and the soil infiltration rate gradually becomes smaller than the rainfall rate, leading to surface runoff. The runoff from the slope crest and upper of slope accumulates at the toe, increasing the moisture content of the soil at the toe. This reduces the matric suction and shear strength of the soil, causing local soil sliding force greater than anti-sliding force. As a result, the toe experiences a a circular or horseshoe-shaped collapse, and with the continuous accumulation of runoff, the volume of the collapse extends gradually to both sides parallel to the slope surface.\u003c/p\u003e\u003cp\u003e(2)Formation and expansion of tension cracks at the slope crest: Similar to the first stage observed in the 60\u0026deg;slope, I will not elaborate on it again here.\u003c/p\u003e\u003cp\u003e(3)Slope creeping cracking failure: The tension cracks formed at the top of the slope provide advantageous channels for rainwater infiltration, allowing rainwater to quickly enter the interior of the slope, reducing the matric suction of the internal soil matrix. Meanwhile, the open face formed at the toe of the slope causes the slope soil to have a tendency to slide downward under the combined action of self-weight stress and the dynamic and static water pressures generated by rainfall. As the shear strength of the soil continues to decrease, the slope experiences creep, characterized by increased tension cracks at the top of the slope and shear failure of the soil at the toe of the slope.\u003c/p\u003e\u003cp\u003e(4)Overall slope instability failure: With the continuous expansion of the cracks, rainwater accelerates into the interior of the slope along the cracks, preventing timely drainage of internal water content. The soil gradually becomes saturated and generates excess pore water pressure internally. According to the principle of effective stress in soil mechanics, the effective stress of the soil gradually decreases as the excess pore water pressure increases\u003csup\u003e[\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]\u003c/sup\u003e. As a result, the shear strength of the soil decreases, and the internal plastic zone expands. When the shear strength becomes smaller than the soil's resistance to sliding, the sliding surface becomes fully developed, leading to the overall instability failure of the slope.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"Discussion","content":"\u003cp\u003eComparing the time of tension crack formation at the crest, the onset of creep deformation, and the cessation of rainfall for overall flow slide between the 60\u0026deg; and 45\u0026deg; slopes, it was found that the 60\u0026deg; slope required less time in all aspects. This indicates that under rainfall conditions, a steeper gradient leads to landslides with less accumulated rainfall and increases the probability of landslide occurrence. Observations on the variation of wetting fronts at different positions on both slopes revealed that the rate of change is consistently faster at the slope shoulder than at the toe and mid-slope, indicating that the progression of the wetting front is fastest at the shoulder and slowest at the mid-slope under rainfall conditions. Before the formation of tension cracks at the crest, sensors 2 or 3 at the mid-slope were always the last to show an increase, suggesting that changes in moisture content at the mid-slope influence the generation of cracks and the development of slip surfaces. Comparing the distance between the rear edge of the landslide and the original shoulder, as well as the depth of the slip surface, it was found that for the 45\u0026deg; slope, the rear edge is further from the original shoulder and the slip surface is deeper compared to the 60\u0026deg; slope, indicating that gentler gradients lead to deeper slip surfaces. Prior to the onset of creep deformation within the slope body, all sensors showed an increase in moisture content and a decrease in matric suction, which stabilized for some time, demonstrating that increased moisture content is a primary factor in inducing landslides, with a certain lag effect\u0026mdash;a finding consistent with Zhang et al.'s research\u003csup\u003e[\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eDifferences in slope stress distribution form the core mechanical basis for the distinct failure patterns exhibited by slopes of varying inclines\u003csup\u003e[\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]\u003c/sup\u003e. For the 60\u0026deg; steep slope, the shoulder region becomes a critical stress concentration zone due to the significantly increased tangential component of gravitational force along the slope surface. Here, vertical compressive stress from the overlying soil exists, while the steep inclination causes rapid accumulation of tangential shear stress. Without sufficient lateral support from surrounding soil, this forms a \u0026ldquo;tension-shear composite stress zone.\u0026rdquo; In the experiments, oblique tensile cracks first appeared at the shoulder of the 60\u0026deg; slope (with a sudden change in moisture content detected by sensors at 14 minutes). This directly reflects how tensile stress first exceeded the soil's tensile strength after effective stress rapidly decreased due to rainwater infiltration. As cracks propagated, the shear resistance of the shoulder soil continued to decline. When shear stress surpassed residual shear strength, the shoulder collapsed first. The newly exposed surface after collapse further intensified stress redistribution in the upper soil mass, triggering a chain reaction of \u0026ldquo;collapse-stress release-new collapse,\u0026rdquo; ultimately resulting in shoulder collapse followed by multi-stage retreating failure. In contrast, the 45\u0026deg; moderate slope exhibits smaller gravitational tangential components, resulting in a stress state more dominated by vertical compressive forces. Consequently, the toe becomes the stress-concentrated weak link. As slope runoff converges at the toe, the toe soil remains saturated over time. A sudden drop in matrix suction causes a significant reduction in shear strength. Simultaneously, as the primary zone bearing vertical stresses, the toe experiences continuously accumulating shear stress from both self-weight and upper soil loads. When shear stress exceeds the soil's shear strength, localized failure and collapse occur first, forming an exposed face. The exposed face disrupts the slope's original stress equilibrium, inducing an upward creep toward the face. This triggers tensile cracking at the crest, ultimately leading to a failure sequence: toe collapse \u0026rarr; deep-seated creep \u0026rarr; tensile failure. Additionally, 45\u0026deg; slopes are more prone to developing deep slip surfaces. This is fundamentally due to the lower tangential component allowing rainwater sufficient time to infiltrate deep into the slope, leading to a reduction in the strength of the deep soil layers. In contrast, 60\u0026deg; slopes experience a larger tangential component and faster onset of instability, resulting in slip surfaces that are mostly confined to the shallow layers.\u003c/p\u003e\u003cp\u003eSlope gradient directly influences the spatiotemporal distribution of slope moisture content by altering the residence time and infiltration pathways of rainfall on the slope surface, thereby indirectly regulating failure patterns\u003csup\u003e[\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]\u003c/sup\u003e. Experimental data from slopes with two different gradients both show that rainfall transport rates follow the pattern \u0026ldquo;slope shoulder\u0026thinsp;\u0026gt;\u0026thinsp;toe\u0026thinsp;\u0026gt;\u0026thinsp;mid-slope.\u0026rdquo; However, the differing seepage effects caused by slope gradient variations ultimately lead to differences in the initiation location and evolution process of slope failure. For the 60\u0026deg; steep slope, the steep gradient significantly increases surface runoff velocity (experiments observed stream formation 8\u0026ndash;10 minutes earlier than on the 45\u0026deg; slope). Rainwater dwells briefly in the mid-slope zone with limited infiltration, resulting in slow moisture content increase (Sensor 3 response time: 21 minutes, 7 minutes later than the slope shoulder). In contrast, the slope shoulder, situated at the terminal convergence point of runoff and characterized by steep inclination, facilitates concentrated infiltration along shoulder fissures (sensor No. 6 exhibited a sudden moisture content change at 14 minutes in the experiment), rapidly forming transient saturated zones. The formation of saturated zones drastically reduces effective stress in shoulder soils, accelerating tensile crack propagation and shoulder collapse. New fissures generated during multi-stage collapses further serve as preferential pathways for rainfall infiltration, driving rapid failure propagation into the slope interior. On the gentle slope of the 45\u0026deg; slope, surface runoff flows more slowly, prolonging rainfall retention time on the slope surface. This not only allows sufficient infiltration in the shoulder area (as indicated by Sensor 6's response at 17 minutes) but also creates a long-term waterlogged environment at the toe due to its low-lying topography, where runoff accumulates (experimental measurements showed Sensor 5 at the toe reached a stable moisture content of 32%\u0026ndash;35%, approaching saturation). Sustained rainfall infiltration gradually softens the soil at the toe, causing continuous shear strength degradation and ultimately triggering the first failure. Simultaneously, the slower runoff velocity allows ample time for rainwater to infiltrate deep into the slope body. The wetting front advances slowly downward, progressively reducing the strength of deep-seated soils and creating conditions for subsequent deep-seated creep and fracturing. Furthermore, the mid-slope zone of the 45\u0026deg; slope, where rainfall retention time was moderate, exhibited a gradual increase in moisture content. This zone functioned as a \u0026ldquo;transition zone\u0026rdquo; connecting stress transfer between the slope crest and toe. Its strength degradation process synchronously regulated the coordinated evolution of crack propagation at the crest and collapse at the toe, ultimately forming a failure path characterized by \u0026ldquo;toe collapse first, crest cracking later, and deep-seated breakthrough.\u0026rdquo;\u003c/p\u003e\u003cp\u003eThe failure mode of the 60\u0026deg; loess filled slope is characterized by shoulder collapse followed by multi-level retreat-type destruction, featuring settlement deformation, prominent tensile cracking, deep sliding surfaces, multiple levels of sliding, and short duration of destruction. In contrast, the 45\u0026deg; loess filled slope exhibits a failure mode of toe collapse and deep-seated creep-induced tensile fracture, marked by toe collapse, pronounced tensile cracking, deep sliding surfaces, and prolonged destruction duration. For the 60\u0026deg; slope, reinforcement should focus on the top and shoulder areas, such as installing anti-slide piles and prestressed anchor cables to strengthen the shoulder and prevent overall instability due to shoulder collapse; effective drainage systems should be implemented to reduce water accumulation at the top. For the 45\u0026deg; slope, emphasis should be placed on toe protection through measures like retaining walls and slope protection works to enhance stability and prevent toe collapse; improving drainage at the top and on the slope face can mitigate rainwater infiltration and runoff collection. Moreover, vegetation restoration can be carried out on both types of slopes to improve soil resistance to erosion, reduce the rate of rainfall infiltration, and enhance overall slope stability.\u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eThis study investigated the instability and failure of loess-filled slopes with different inclinations under rainfall conditions through indoor simulated experiments. The preliminary conclusions are as follows:\u003c/p\u003e\u003cp\u003e(1)Under rainfall conditions, the rate of water movement in the loess-filled slope shows a pattern of shoulder\u0026thinsp;\u0026gt;\u0026thinsp;toe\u0026thinsp;\u0026gt;\u0026thinsp;middle of the slope for slopes with different inclinations. At the same time, the trend of volumetric water content exhibits a stable-rapid increase-stable pattern, while the matric suction shows a stable-rapid decrease-stable pattern. After a stable period of time for both factors, slope failure occurs, indicating a lag in the destabilization of loess-filled slopes induced by rainfall.\u003c/p\u003e\u003cp\u003e(2)Under the same rainfall intensity, the steeper the slope, the greater the probability of landslide occurrence, the quicker the onset of landslide, and the smaller the critical rainfall amount required for landslide initiation. Moreover, for lower inclinations, the depth of surface of rupture is deeper and overall volume of landslides are larger.\u003c/p\u003e\u003cp\u003e(3)The failure mode of the loess-filled slope with a 60\u0026deg; inclination is characterized by slope shoulder collapse\u0026mdash;multi-stage retrusive failure. The failure mode of the loess-filled slope with a 45\u0026deg; inclination involves toe collapse\u0026mdash;deep-seated creeping cracking failure.. Smaller inclinations make the toe more susceptible to infiltration-induced collapse, while larger inclinations are more prone to settlement deformation of the slope crest under the influence of self-weight stress.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003eData availability statement\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;Readers with reasonable demands can obtain data from the corresponding author.\u003c/p\u003e\n\u003cp\u003eResearch funding\u003c/p\u003e\n\u003cp\u003eNo funding\u003c/p\u003e\n\u003cp\u003eAcknowledgements\u003c/p\u003e\n\u003cp\u003eWe would like to express our gratitude to the State Key Laboratory of Geohazard Prevention and Geoenvironment Protection for providing the instrument and venue support. At the same time, we also thank the proofreaders and editors for their assistance.\u003c/p\u003e\n\u003cp\u003eAuthor contributions\u003c/p\u003e\n\u003cp\u003eGuangyuan Dou: writing, editing and methodology.\u0026nbsp;Shuoyu Zhu: supervision, experiments, calculations, review, equipment provision, checking, methodology, and funding. Hao Zhang: experiments, calculations. Linwan Chen: Calculations.\u0026nbsp;Xing Chen: data management, Experiments and data management.\u003c/p\u003e\n\u003cp\u003eCompeting interests\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eWang, R., Li, Y., Guan, F., Fan, W. \u0026amp; Ma, T. Mode I fracturing behavior of Malan loess and implications for the toppling failure of loess slopes. \u003cem\u003eEng. 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Laboratory Model Tests on the Deformation and Failure of Terraced Loess Slopes Induced by Extreme Rainfall. \u003cem\u003eLand\u003c/em\u003e \u003cb\u003e13\u003c/b\u003e, 1631. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3390/land13101631\u003c/span\u003e\u003cspan address=\"10.3390/land13101631\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2024).\u003c/span\u003e\u003c/li\u003e\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":"Loess filled slope, rainfall infiltration, slope gradient, failure mode, physical model test","lastPublishedDoi":"10.21203/rs.3.rs-7704393/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7704393/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eTo explore the influence of steep slopes on the deformation and failure mechanisms as well as instability modes of loess-filled slopes under rainfall conditions, this study conducted indoor simulated rainfall experiments, combined with matrix suction sensors, moisture content sensors and 3D laser scanning technology for research. The results showed that under rainfall, the water movement rate of steep fill slopes follows the pattern of shoulder\u0026thinsp;\u0026gt;\u0026thinsp;toe\u0026thinsp;\u0026gt;\u0026thinsp;middle of the slope, and the moisture content exhibits a \"stable-rapid increase-stable\" trend. The slope destabilization induced by rainfall exhibits hysteretic nature. Under the same rainfall intensity, the steeper the slope, the shallower the slide surface, the smaller the critical rainfall amount for landslide initiation, the quicker the landslide onset, the smaller the landslide volume, and the higher the landslide probability. The failure mode of the 45\u0026deg; slope is \"toe collapse-deep-seated creeping cracking failure\", while that of the 60\u0026deg; slope is \"slope shoulder collapse-multi-stage retrusive failure\". This study provides theoretical references for the engineering construction and landslide disaster prevention of steep loess-filled slopes.\u003c/p\u003e","manuscriptTitle":"Experimental Study on Failure Mechanisms of Steep Loess Fill Slopes with Large Gradient","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-10-21 13:40:36","doi":"10.21203/rs.3.rs-7704393/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":"40eee7fb-6797-459e-90fe-0d88db4381c4","owner":[],"postedDate":"October 21st, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":56523579,"name":"Physical sciences/Engineering"},{"id":56523580,"name":"Earth and environmental sciences/Environmental sciences"},{"id":56523581,"name":"Earth and environmental sciences/Natural hazards"}],"tags":[],"updatedAt":"2025-12-03T17:23:54+00:00","versionOfRecord":[],"versionCreatedAt":"2025-10-21 13:40:36","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7704393","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7704393","identity":"rs-7704393","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}