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3The analysis of the failure process: a development of the failure process with time; b through f are the t and n displacements of the point couples a through e, respectively (Note t and n are the tangential and normal components of the relative displacements of the point couples, respectively; Ts is the time before the final failure of the slope)
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An in-flight excavation centrifuge model test was conducted to investigate the behavior of a loess slope under different excavation slope angles. The displacements of the loess slope were measured using the particle image velocimetry system. Measurement results showed that the slope can be divided into several zones according to the features of the...
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... A similar method was also applied in a rock slope excavation model by removing a plastic plate with a fixed pulley system at 80g . In recent years, in-flight excavation at the toe of a slope using a controlling system containing a motor and cutting blade has been widely used to investigate the effects of the soil properties (Tamrakar et al., 2006), slope inclination (Li et al., 2011), and excavation direction (Fan et al., 2016;Zhang et al., 2020b). The in-flight excavation method and the g-up pre-excavation technique in the same slope model were compared . ...
Landslides are serious geohazards that occur under a variety of climatic conditions and can cause many casualties and significant economic losses. Centrifuge modelling, as a representative type of physical modelling, provides a realistic simulation of the stress level in a small-scale model and has been applied over the last 50 years to develop a better understanding of landslides. With recent developments in this technology, the application of centrifuge modelling in landslide science has significantly increased. Here, we present an overview of physical models that can capture landslide processes during centrifuge modelling. This review focuses on (i) the experimental principles and considerations, (ii) landslide models subjected to various triggering factors, including centrifugal acceleration, rainfall, earthquakes, water level changes, thawing permafrost, excavation, external loading and miscellaneous conditions, and (iii) different methods for mitigating landslides modelled in centrifuge, such as the application of nails, piles, geotextiles, vegetation, etc. The behaviors of all the centrifuge models are discussed, with emphasis on the deformation and failure mechanisms and experimental techniques. Based on this review, we provide a best-practice methodology for preparing a centrifuge landslide test and propose further efforts in terms of the seven aspects of model materials, testing design and equipment, measurement methods, scaling laws, full-scale test applications, landslide early warning, and 3D modelling to better understand the complex behaviour of landslides.
... html). Recently, loess landslide research has primarily focused on the homogeneous slope (Fan et al. 2016;Peng et al. 2019). The loess interlayer is commonly mentioned as background in geological research (Hongbing et al. 2006;Szymanski et al. 2014). ...
Abundant water and unstable loess are favorable factors for initiating landslides; landslides induced by hidden layer instability in a residential area would cause marked damage. On March 27, 2021, a landslide caused by loess interlayer instability occurred in Dege County, Sichuan Province, China. In total, 20 houses were damaged, and the lives of 1100 residents were threatened. This study explores the failure mechanism of the landslide caused by loess interlayer instability through field investigation, laboratory tests, and circular slide stability analysis. The results show that (1) Dege landslide is a small thrust-type landslide caused by the instability of loess hidden under colluvium; (2) precipitation convergence by hollow landform and domestic water leakage increased the moisture content of the loess interlayer; domestic water leakage occupied 91.7%; (3) the loess interlayer zone has high porosity, strong collapsibility, and high clay mineral content, which causes softening by water; and (4) multiple water sources reduced the anti-sliding force by approximately 3.3%, and newly building load increased the sliding force by approximately 15.9%. The combination of multiple water sources and building load led to landslide failure. This study provides new insights into the hidden loess interlayer landslide in mountainous urban areas of western China and can be a reference for landslide prevention and mitigation.
... A series of centrifuge model tests was performed to investigate the static liquefaction of the soil slope overlying the bedrock (Take et al., 2014;Take et al., 2004). Some excavation-induced slope failures were observed in the centrifuge model tests (Fan et al., 2016;Li et al., 2011;Sabermahani et al., 2018). An in-flight excavation centrifuge model test helped confirm the significant progressive failure of the loess slope with a combination of shear and tensile failures under different excavation slope angles (Fan et al., 2016). ...
... Some excavation-induced slope failures were observed in the centrifuge model tests (Fan et al., 2016;Li et al., 2011;Sabermahani et al., 2018). An in-flight excavation centrifuge model test helped confirm the significant progressive failure of the loess slope with a combination of shear and tensile failures under different excavation slope angles (Fan et al., 2016). ...
A number of landslides occur in the soil slopes overlying bedrock due to excavation. However, the excavation-induced deformation and failure behavior and the coupling influential mechanism of the bedrock and excavation have not been clearly elucidated, although they are essential for the stability analysis of slopes overlying bedrock under excavation conditions. In this paper, a series of centrifuge model tests was conducted on the slopes overlying bedrock under excavation conditions with different bedrock shapes. The slopes overlying different bedrocks experienced a significant progressive failure process under excavation conditions, and the slip surface gradually developed from the excavation area at the slope toe to the slope top. The failure mechanism of the slope that overlies the bedrock under the excavation condition can be illustrated by the significant coupling of the deformation localization and failure. There was a shear zone, where the shear strain was concentrated with a substantial increase rate. The displacement direction of the soil in the shear zone was consistent and invariable for a slope overlying the bedrock after the shear zone appeared. Excavation led to a significant influence area inside the slope overlying the bedrock, which was significantly affected by the shape of the bedrock: steeper bedrock corresponded to a wider significant influence area, and flatter bedrock corresponded to a narrower one.
... However, Li et al. (2011) conducted centrifuge tests with an excavation angle approaching 90°to propose that local failure appeared progressively from the bottom to the top of the slope. Fan et al. (2016) researched centrifuge models at excavation angles of 63°and 72°, indicating that the tensile failure also occurs first on the slope shoulder and then shear failure occurs in the toe of the slope and gradually expands toward the trailing edge. Therefore, when the excavation angle is approaching 90°, it is prone to stress concentration at the slope toe, which causes the local failure to appear progressively from the bottom to the top of the slope. ...
... The image analysis system generated a series of measurements, including the displacement contour and the displacements in the horizontal and vertical directions of the selected point on the model slope. The displacement contour was obtained from the displacement vector using the least-squares method in the software (Fan et al. 2016). The installed CCD camera was located in the center and perpendicular to the monitoring area. ...
... According to the centrifuge model, the critical confining pressure unloading ratios Δcrc at the positions of the EPS-1 and EPS-5 sensors were 0.55 and 0.56, respectively. Fan et al. (2016) researched centrifuge models for different excavation slope gradients. In their research, the earth pressure variation diagrams at different positions in the centrifuge model indicated that the slope of the centrifuge model was destroyed at the shallow surface. ...
Excavation leads to unloading rebound and stress redistribution in the soil of a slope near the excavation face. The process of reequilibrium of the stress is followed by large deformation or even failure of the slope. In this study, the excavation-induced Yangya landslide was selected as a geological prototype. From reduced triaxial compression (RTC) tests, the characteristics of deformation and the mechanical response of unloading soil were analyzed, which were related to the instability and response mechanism of a loess slope under unloading. Furthermore, a geotechnical centrifuge was used to simulate the deformation and failure process under the condition of excavation at the slope toe. The deformation characteristics before and after the slope toe excavation and the response characteristics of the earth pressure inside the slope were studied. The results show that after excavation of the slope toe, local collapse occurs near the excavation surface. The displacement, the potential slip surface, and the tensile cracks of the slope develop progressively from the front edge to the trailing edge of the slope. Moreover, the failure of a loess slope induced by excavation is dependent on the change of pressure unloading ratio; when the confining pressure unloading ratio reaches a critical level, the shear strength decreases sharply. Differences also exist in the critical unloading confining pressure ratio between shallow and deep failures in excavation-induced landslides. Therefore, the scope of the unloading effect of the excavation on the slope directly determines the scale of the landslide. After excavation, stress relaxation clearly occurs within the slope, and the unloading effect is more obvious near the excavation surface. The excavation mainly affects the front and middle parts of the slope and has little or no effect on the rear part of the slope. The deformation and failure mechanisms of the loess slope induced by excavation are characteristic of a typical progressive retreat failure mode landslide.
... Among the estimation methods for rock slope stability, monitoring methods have been used first and became popular for directly determining the stable level and the possible risk zone of an unstable rock mass in natural and artificial slopes (Xu et al. 2014; have been used to analyze the unloading deformation mechanism, support strategies and safety factors (Hou et al. 2014;Li et al. 2014;Jiang et al., 2015;Yan et al. 2019). Additionally, physical modeling methods have been accepted for studying the excavation effect, deformation process, failure mechanism and support scheme (Liu et al. 2013;Lai et al. 2015;Fan et al. 2016). These methods have made important contributions to the stability assessment and engineering design of general rock slopes. ...
Tectonic shear belts, with their low mechanical strengths, large spatial extents and obvious shearing damage, present challenges for the excavation of high slopes. In this study, we described the basic unloading performances of shear belts induced by excavation through observational data and numerical back analysis. The unloading deformation and failure processes of deep outward-dipping shear belts exposed on a hydraulic slope nearly 300 m tall were investigated first. The field observation and monitoring data indicated that the excavation-induced deformation pattern of the outward-dipping shear belt can be divided into two stages: primarily opening deformation normal to the shear belt during the first stage and primarily shear deformation along the shear belt during the second stage, manifested as opening and shear cracks along the active shear belt. Then, the numerical and analytical analysis were performed to expose the displacement evolution of the shear belt during the multistep excavation of the slope from the top down. The back analysis showed that the grading deformation mechanism of the shear belt was primarily related to the reduction of normal compressive stress on the shear belt's plane due to slope excavation. The reinforcement schemes, including a prestressed anchor cable to limit deep unloading deformation and a prestressed rock bolt to restrain the surface relaxation of the rock mass of the slope, were also discussed. In practice, actual excavation and corresponding monitoring data indicated that the unloading mechanism and supporting measures for the shear belt presented in this study were reasonable and effective. These presented experiences can also enrich the stability analysis and support design for similar high slopes.
... The camera, which had a resolution of 5 megapixels, was installed in front of the window of the model box to record video during the test. The PIV technique was introduced by White et al. (2003) from hydromechanics, and has been widely used to quantify soil displacement behavior in geotechnical engineering (Stanier and White 2013;Cao et al. 2015;Fan et al. 2016). This system consisted of a Charge Coupled Device (CCD) camera, a data collection computer, a control computer and image analysis software. ...
... It can be reasonable inferred that these landslides were mainly induced by the irrigation. Unlike rainfallinduced loess landslides, which are characterized by sudden global failures (Ling et al. 2012), or excavation-induced loess landslides, which will be stable until reaching a critical excavation angle or height at the slope toe (Fan et al. 2016). This centrifuge model test shows that local failure at the toe of slope is first triggered (I). ...
The Heifangtai platform in Northwest China is famous for irrigation-induced loess landslides. This study conducted a centrifuge model test with reference to an irrigation-induced loess landslide that occurred in Heifangtai in 2011. The loess slope model was constructed by whittling a cubic loess block obtaining from the landslide site. The irrigation water was simulated by applying continuous infiltration from back of the slope. The deformation, earth pressure, and pore pressure were investigated during test by a series of transducers. For this particular study, the results showed that the failure processes were characterized by retrogressive landslides and cracks. The time dependent reductions of cohesion and internal friction angle at basal layer with increasing pore-water pressure were responsible for these failures. The foot part of slope is very important for slope instability and hazard prevention in the study area, where concentration of earth pressure and generation of high pore-water pressures would form before failures. The measurements of earth pressure and pore-water pressure might be effective for early warning in the study area.
Centrifuge model tests and numerical simulations were performed to study the landslide evolution process and failure mechanism. A TLJ-500 geotechnical centrifuge was used for the experiments and landslide deformation and stress was monitored using high-precision differential displacement sensors and earth pressure micro-sensors. Discrete element numerical simulations were performed using PFC2D based on the experimental results. The findings show that the landslide evolution process can be divided into three stages: 1) compaction and consolidation; 2) uniform deformation; and 3) accelerated deformation and failure. The numerical simulation results verify the distinct stage characteristics of the landslide evolution process. According to the migration of microscopic soil mass particles within the landslide, stage 3) can be further divided into a deformation development stage and instability and failure stage. The simulation displacement monitoring curves and displacement map show distinct deformation characteristics and displacement indicators from stages 2) to 3) and from the deformation development stage to the instability and failure stage. The experimental and numerical results reveal the landslide failure mechanism: the upper part of the landslide thrusts and slides; the middle part squeezes; the lower part collapses; and shear plane penetration leads to landslide failure.
The sliding mechanism of loess landslides is unique and complex; a single-scale and linear mechanical model cannot describe their failure process scientifically, thus necessitating a multiscale and nonlinear research idea. In order to monitor and observe the whole failure process of loess landslide more conveniently, a physical model test (2.0 × 0.6 × 1.5 m) is designed to simulate the landslide failure process under rainfall condition. Twelve pore water pressure sensors and twelve soil pressure sensors were arranged in three loess layers, and were used to record the numerical variation characteristics of pore water pressure and soil pressure during the failure process of Landslide physical model. The results show that, the failure process of loess landslides and the data from sensors are very random, making it impossible to use a single-scale or linear mechanical model to analyze their mechanics. However, we can well describe the failure process of loess landslide through the micro, meso, and macro scales. Therefore, the mechanical problems of loess landslides should be analyzed from six aspects, including the liquid bridge, force chain, soil mechanics test, a physical model, real landslides, and tectonic stress. In the future, nonlinear and multiscale research ideas can make loess landslide studies more scientific and effective.
Slope failure due to improper excavation is one of common engineering disasters in China. To explore the failure mechanism of soil slope induced by toe excavation, especially to investigate the influence of excavation unloading path and rate on slope stability, a numerical slope model was built via particle flow code PFC2D. The development of crack and strain during excavation were obtained and used to evaluate the deformation characteristics. Furthermore, excavation types representing different unloading paths and rates were compared in terms of crack number and strain level. Results indicate that crack number and strain level induced by horizontal column excavation are much greater than those of vertical column excavation and oblique excavation. The crack number and strain level increase with excavation unloading rate. Besides, the feasibility of taking the average strain of slope surface and the average value of maximum strain along monitoring lines to represent the global deformation characteristics were discussed. This study can provide a theoretical guidance for slope monitoring and preliminary optimal selection of excavation scheme in the design and construction of slope engineering.
Loess Plateau is one of the natural test sites to carry out the research of hazards induced by human activities. The loess landslide at Liuwanjia, Yan’an City, Shaanxi Province of China was observed to occur after continuous excavation and rainfall. The Liuwanjia landslide was selected as a case study and then detailed field mapping, 3D laser scanning and laboratory tests were carried out to study the formation characteristics and mechanism of this type of landslide. Based on the analysis of field investigation, excavation was a direct causing factor and the rainwater was a significant influencing factor in the formation of the landslide. The unloading fissures caused by excavation were preferential paths for infiltration of rainwater. Soil compression in the slope induced by excavation and the infiltration of rainwater played an significant role in the formation of local failure. Finally, the formation mechanism of Liuwanjia landslide has been discussed.
