Strain-softening Characteristics Research Articles

Investigation of strain-softening behaviours and development of constitutive model of red clays in Guiyang, China

Red clay is widely used in engineering projects; it is characterized by high moisture content, high plasticity, large porosity, high strength and low compressibility. The engineering properties and deformation modelling of red clay in Guiyang differ significantly from those of ordinary soils. Consequently, it is essential to conduct a detailed study of the stress–strain relationship of red clay in order to obtain accurate engineering parameters. Hence, a series of unconsolidated undrained triaxial compression tests (UU) was conducted on red clay in Guiyang under different water contents and confining pressures. The test results show that the stress–strain curve of the clay undergoes three stages: elastic deformation, plastic yielding and stress reduction after failure. After reaching peak deviatoric stress, the deviatoric stress decreases gradually and stabilizes, showing noticeable strain-softening characteristics. The non-linear model from the Nanjing Hydraulic Research Institute (NHRI) was introduced for fitting and obtaining relevant parameters. Then, based on the FLAC 3D platform, numerical simulation was carried out on a standard soil sample. The research results show that the calculated curve from the non-linear model fits the experimental curve quite well, and the numerical test curve also agrees with it. This indicates that the non-linear model from NHRI can effectively reflect the strain-softening behaviour of clay and is highly applicable to Guiyang red clay.

Prediction of retrogressive landslide in sensitive clays by incorporating a novel strain softening law into the Material Point Method

Sensitive clays, when subjected to large strains, exhibit a unique strain-softening behavior, transforming into a remolded liquid with remarkably low shear strength. When a slope fails, this behavior leads to the remolded clay moving away from its original position, triggering subsequent failures and catastrophic outcomes. To accurately predict such scenarios, it is crucial to incorporate realistic strain-softening characteristics into the constitutive soil model. This paper presents a novel yet practical strain-softening law developed by the authors that effectively captures the post-peak behavior of sensitive clays down to their remolded strength. The softening law is implemented in an elastoplastic Mohr-Coulomb model and incorporated into Anura3D, an open-source software that uses the Material Point Method to simulate large deformations. The constitutive model is calibrated by simulating the stress-strain behavior through direct shear tests conducted at three sensitive clay landslide locations. The accuracy of the overall numerical framework is assessed by predicting the post-failure movements of these landslides. Notably, two of the landslides, Sainte-Monique (1994) and Saint-Jude (2010), have previously been analyzed using other numerical tools, allowing for a comparative analysis with the method presented here. The third landslide, the Saint-Luc-de-Vincennes landslide, representing a composite flow slide and spread, has been numerically simulated for the first time. The post-failure behavior observed in the landslide events is compared with field observations and other numerical analyses. The results show that the MPM models with the proposed strain-softening law can reasonably predict post-failure retrogression and runout distance, which are crucial parameters for determining the risk of landslides in sensitive clays.

A nonlinear dynamic constitutive model of cohesive soil-captured cyclic strain softening characteristics

The dynamic constitutive relationship of soil is essential for seismic dynamic calculations in geotechnical seismic engineering. Numerous experiments have demonstrated that cohesive soil exhibits cyclic strain softening characteristics under dynamic loading (equal-amplitude loading and unequal-amplitude loading) conditions. However, the Masing criteria (Masing's 2-fold method) cannot describe the cyclic strain softening behaviour of soil for equal-amplitude loading and often fail to estimate the damping ratio accurately (underestimation of the damping ratio at low strain; overestimation of the damping ratio at high strain). Furthermore, the Masing criteria are not applicable for nonequal-amplitude loading. To solve these problems, the research in this paper is based on the Davidenkov skeleton curve, and Masing's 2-fold method is utilized to establish a hysteretic curve. The cyclic strain softening coefficient and damping ratio adjusting coefficient were proposed for equal-amplitude loading to accurately capture the cyclic strain softening properties of cohesive soil. For nonequal-amplitude loading, Masing's n-fold method is employed to establish stress‒strain hysteresis curves, and using the customized constitutive interface reserved by FLAC3D, the development of this constitutive model is achieved on the Visual Studio (VS)2015 platform. The calculation accuracy of the model is verified by comparison with the theoretical values and experimental data, and the rationality of the constitutive model is verified by cyclic direct simple shear tests and cyclic torsional shear tests of soil subjected to equal-amplitude loading and centrifuge model tests of ground bearing unequal-amplitude loading, which provides effective help for the study of earthquake disasters in cohesive soil sites.

Numerical analysis of downward progressive landslides in long natural slopes with sensitive clay

Landslides occurring in sensitive clay often result in widespread destruction, posing a significant risk to human lives and property due to the substantial decrease in undrained shear strength during deformation. Assessing the consequences of these landslides is challenging and necessitates robust numerical methods to comprehensively investigate their failure mechanisms. While studies have extensively explored upward progressive landslides in sensitive clays, understanding downward progressive cases remains limited. In this study, we utilised the nodal integration-based particle finite element method (N-PFEM) with a nonlinear strain-softening model to analyse downward progressive landslides in sensitive clay on elongated slopes, induced by surcharge loads near the crest. We focused on elucidating the underlying failure mechanisms and evaluating the effects of different soil parameters and strain-softening characteristics. The simulation results revealed the typical pattern for downward landslides, which typically start with a localised failure in proximity to the surcharge loads, followed by a combination of different types of failure mechanisms, including single flow slides, translational progressive landslides, progressive flow slides, and spread failures. Additionally, inclined shear bands occur within spread failures, often adopting distinctive ploughing patterns characterised by triangular shapes. The sensitive clay thickness at the base, the clay strength gradient, the sensitivity, and the softening rate significantly influence the failure mechanisms and the extent of diffused displacement. Remarkably, some of these effects mirror those observed in upward progressive landslides, underscoring the interconnectedness of these phenomena. This study contributes valuable insights into the complex dynamics of sensitive clay landslides, shedding light on the intricate interplay of factors governing their behaviour and progression.

Experimental study on tensile and fracture properties of continuous directional steel fibre reactive powder concrete

In this paper, an orientation method with pre-tensioning at both ends is proposed to successfully prepare continuously directional steel fibre reactive powder concrete (DSFRPC). The tensile and fracture properties are determined by splitting tensile, axial tensile and three-point notch fracture tests. The paper sets three different groups named C0, Group S and Group D, which presents non-fibre, randomly distributed fibre and directional fibre, respectively. The test results showed that: the splitting strength of Group D specimens increased quite significantly with the increase of fibre dosage. Compared to the Group S, the splitting strength of Group D was improved by 46.5%, 69.8%, and 66.1%, respectively. The specimens of C0 and Group S in the axial tensile test suffered typical single-seam brittle failure, which showed obvious strain softening characteristics. With the increase of fibre dosage, the damage mode of specimens in Group D changed from single-seam brittle damage to multi-seam ductile damage, showing better energy dissipation capacity. Under the same converted fibre dosage, the fracture energy enhancement ratios of Group D specimens were 14.7, 18.9 and 19.55, respectively. At a lower water-cement ratio, the toughening effect of continuous directional steel fibre specimens is more obvious. The toughening effect of the reactive powder concrete (RPC) of with continuous directional steel fibre is mainly reflected in the directional distribution of steel fibre produced by the interfacial adhesion and friction caused by the increase in energy dissipation, as well as the maintenance of a stronger grip between the “dispersed rebar” and the matrix. Based on current study, the future research should be focused on a wider range of materials, structural design, and environmental conditions to verify the effectiveness and reliability of DSFRPC in various applications.

Study on progressive failure mode of surrounding rock of shallow buried bias tunnel considering strain-softening characteristics

Mountain tunnels portal often have to pass through slope terrain unavoidably, thus forming a shallow buried bias tunnel. During the construction of shallow buried bias tunnel, disasters such as slope sliding and tunnel collapse frequently occur. The failure mode of surrounding rock obtained by current research is based on the limit equilibrium theory, which cannot reflect the progressive failure characteristics of the surrounding rock of shallow buried bias tunnel. In order to reveal the failure mechanism of the gradual instability of surrounding rock of shallow buried bias tunnel, the problem of gradual failure of the surrounding rock is reduced to an elastic–plastic analysis problem for surrounding rock considering the strain-softening characteristics. Based on the elastic–plastic analysis of the failure process of shallow buried bias tunnel, MATLAB was used to compile a program to read the finite-difference calculation result file, extract the effective information such as shear strain and tensile strain at the center point of each unit, and establish the analysis method of the progressive failure mode of shallow buried bias tunnel. The reliability of the method proposed was verified by comparing the failure process of the model test with the development process of shear strain increment. Under the condition of no support, the formation mechanism of failure plane of surrounding rock on both sides of shallow buried bias tunnel is different. The shallow buried side is the shear failure plane formed by the collapse of surrounding rock, while the deep buried side of the tunnel is the shear failure plane formed by the collapse of surrounding rock and slope sliding. Under the conditions of excavation and support, the failure plane of the shallow buried bias tunnel can be divided into three parts according to the formation sequence and reasons. The part I is the failure plane, which is formed by active shear under the influence of tunnel excavation. The part II is the failure plane formed by tensile crack of slope top. The part III is the failure plane formed by passive shear under the push of the soil in the upper part of the slope.

Effect of thermal cycling on the mechanics and microstructure of ultra-high performance concrete

This study investigates the mechanical and microstructural responses of Ultra-High Performance Concrete (UHPC) subjected to a substantial number of thermal cycles upto 300 times from ambient temperature to 60°C. The experimental findings reveal a distinctive pattern of behavior in the static compressive strength, characterized by an initial increase, followed by a subsequent decline, and a subsequent modest resurgence as the thermal cycling progressed. Notably, the peak compressive and flexural strengths were attained after 120 thermal cycles, whereas the maximum dynamic compressive strength was observed after 60 thermal cycles. It is worth highlighting that the dynamic increase factor (DIF) exhibited an inverse trajectory in comparison to the static compressive strength. After 120 thermal cycles, the stress-strain curves under impact loads exhibited pronounced strain softening characteristics, with an oscillation state duration of 250μs, which surpassed that of the ascending and descending phases. The observed variances in macroscopic mechanical properties can be attributed to several pivotal factors, including the quantity of hydration product, the mean chain length (MCL) of the calcium-silicate-hydrate (C-S-H) gel, the distribution of pore volume, and the presence of micro-cracks. This comprehensive examination serves as a valuable theoretical and empirical foundation for advancing the utilization of UHPC in applications subjected to prolonged and intricate thermal conditions.

Strain softening characteristics and stress–strain relationship of Guiyang carbonate laterite

Strain softening characteristics and stress–strain relationship of Guiyang carbonate laterite

Macroscopic and microscopic analysis of the effects of moisture content and dry density on the strength of loess.

To clarify the impact of moisture content and dry density on the strength of loess, the remolded loess samples with different moisture content and dry density were prepared, and the influence of moisture content and dry density on loess strength was explored from the macro level by direct shear test without suction control. On this basis, the mechanism of the influence of moisture content and dry density on loess strength was explored from the micro level by nuclear magnetic resonance method. The research results indicate that: In the case of low water content, there are peak points in the stress-strain curve of remolded loess, exhibiting strain softening characteristics. In the case of high water content, there is no obvious peak in the stress-strain curve, exhibiting strain hardening characteristics. Moisture has a significant impact on the shear strength of remolded loess. As the moisture content of the soil sample increases, the cohesion decreases significantly, and the change in internal friction angle is not obvious. As the moisture content continues to increase, the free water content continues to increase. Free water will continuously soften the soil particle structure, reduce the bonding force between soil particles, and cause the cohesion to decrease with the increase of moisture content. The change in dry density also has a significant impact on the shear strength parameters of remolded loess. As the dry density of the soil sample increases, the cohesion increases. The smaller the dry density, the larger the pore ratio, and the looser the contact between soil particles, weakening the bonding effect. The larger the pore ratio, the more bound water is converted to free water, and the strong bonding force between the water film and soil particles disappears. Both of these microscopic factors can lead to a decrease in cohesion with a decrease in dry density.

Laun's rule for predicting the first normal stress coefficient in complex fluids: A comprehensive investigation using fractional calculus

Laun's rule [H. M. Laun, “Prediction of elastic strains of polymer melts in shear and elongation,” J. Rheol. 30, 459–501 (1986).] is commonly used for evaluating the rate-dependent first normal stress coefficient from the frequency dependence of the complex modulus. We investigate the mathematical conditions underlying the validity of Laun's relationship by employing the time-strain–separable Wagner constitutive formulation to develop an integral expression for the first normal stress coefficient of a complex fluid in steady shear flow. We utilize the fractional Maxwell liquid model to describe the linear relaxation dynamics compactly and accurately and incorporate material nonlinearities using a generalized damping function of Soskey–Winter form. We evaluate this integral representation of the first normal stress coefficient numerically and compare the predictions with Laun's empirical expression. For materials with a broad relaxation spectrum and sufficiently strong strain softening, Laun's relationship enables measurements of linear viscoelastic data to predict the general functional form of the first normal stress coefficient but often with a noticeable quantitative offset. Its predictive power can be enhanced by augmenting the original expression with an adjustable power-law index that is based on the linear viscoelastic characteristics of the specific material being considered. We develop an analytical expression enabling the calculation of the optimal power-law index from the frequency dependence of the viscoelastic spectrum and the strain-softening characteristics of the material. To illustrate this new framework, we analyze published data for an entangled polymer melt and for a semiflexible polymer solution; in both cases our new approach shows significantly improved prediction of the experimentally measured first normal stress coefficient.

Characterization of Freeze-Thaw Cycle Damage to Mudstone in Open Pit in Cold Regions—Based on Nuclear Magnetic Resonance Method

Damage deterioration of rocks in cold regions under seasonal changes and daily cycles of freezing and thawing generate a series of engineering geological problems. These problems will seriously affect the safe and efficient production of open-pit mines. In this paper, a freeze–thaw cycle test and uniaxial compression test considering the natural conditions of the slope were carried out. Mechanical properties and damage mechanisms of open-pit mine mudstone under freeze–thaw conditions were investigated based on nuclear magnetic resonance (NMR) technology. The test results show that the microscopic internal pore structure of mudstone was changed under the superimposed effect of freeze–thaw damage and hydration damage. The internal pore size of mudstone increased with the number of freeze–thaw cycles, while the average pore size of the natural mudstone test increased more. Macroscopically, the compressive strength and modulus of elasticity of mudstone varied linearly with the number of freezing cycles, and the compressive strength and modulus of elasticity showed a decreasing trend. The strain-softening characteristics of mudstone samples were significant for more freeze–thaw cycles. The study explains the microscopic causes of mudstone deterioration in open-pit mines in cold regions and offers guidance for solving engineering disasters caused by mudstone deterioration.

Influence of sand content on mechanical properties of sand-silt mixtures from check dam deposits in the Loess Hilly of Ningxia, China.

An accurate description of the stress-strain relationships of sand-fine mixtures is very important to analyze the soil's mechanical properties. Hence, a series of consolidated drained (CD) triaxial tests were performed on reconstructed sand-silt mixtures with sand contents of 0%, 16.67%, 28.57%, 50%, and 60% in the paper to examine the effect of the sand content on the stress-strain curves of the soil. Results show that for sand-fine mixtures with different sand contents, the stress-strain curves are also mainly strain softening though there exist different degrees of softening. In order to quantitatively describe the strain-softening characteristics of sand-fine mixtures, a modified Duncan-Chang model was developed. To verify the applicability of the modified mode, examples such as coral clay and undisturbed loess are described and predicted. There is a high consistency between theoretical and experimental values. Finally, a sand-content-dependent constitutive model that considered the effects of sand content and confining pressure was proposed based on the modified Duncan-Chang model by constructing the relationship between model parameters and confining pressure and sand content. The constitutive model was implemented in ABAQUS software and verified by comparing the calculated results with the triaxial test data of sand-fine mixtures under the confining pressure of 500 kPa. The comparison results indicate that the constitutive model can reflect the real characteristics of sand-fine mixtures.

Elastic perfect plastic rock mass analysis and equivalent GSI determination considering strain-softening behavior and three-dimensional strength

The elastic perfect plastic model (EPP) fails to accurately capture the strain-softening mechanical behaviors of rock masses, while the post-peak constitutive model of elastic strain softening (ESS) has great application limitations in practical engineering for complex numerical algorithms. In this paper, an EPP constitutive approximation of rock mass considering three-dimensional strength and strain-softening characteristics is proposed. The forward analysis method of the equivalent Geological Strength Index (GSIeq) is given considering the impact of in-situ stress, rock mass properties and engineering excavation. Extensive case studies demonstrate that the radial stress and displacement obtained by the proposed method agree well with those of the ESS model, with only small errors in the Ground Response Curve. The rock mass elastic modulus E and the initial stress level p0 significantly affect GSIeq, while the support pressure ps contributes little to that. As Rp,EPP/R0 > 3.0, usually GSI < 25, the residual strength parameter GSIr can be directly used as the calculation parameter of the EPP model, while for a highly qualified rock mass, such as GSI > 75, the peak strength parameter GSIp can be directly used as an approximate value of GSIeq. Furthermore, with the increase of the in-situ stress level, GSIeq gradually decreases to GSIr, and the difference of plastic stress distribution law based on ESS and EPP models gradually diminishes. Moreover, the proportion of plastic deformation to total deformation increases nonlinearly with buried depth. The proposed method offers a new calculation idea for the simplified analysis of strain-softening rock masses.

Effect of Temperature on Interface Characteristics between Compacted Recycled Asphalt Pavement and Geogrid

This study presents an experimental investigation of the interface behaviors of recycled asphalt pavement (RAP) reinforced with biaxial geogrid. A series of direct shear tests were conducted on compacted RAP specimens with or without geogrid under different normal stresses (50, 75, and 100 kPa) and test temperatures (0, 20, and 35°C) using an improved temperature-controlled direct shear apparatus. The effect of test temperature on interface shear strength and strength parameters was systematically examined. Test results showed that shear stress versus shear displacement curves of RAP specimens with or without geogrid show strain-softening characteristics. Under the same normal stress, the curve of the reinforced RAP sample with a high temperature is always below that with a low temperature, which indicates the test temperature has an adverse effect on the strength of the reinforced RAP sample. The shear strength consistently increases linearly with the increase of applied normal stress for all RAP samples. The shear strength with geogrid is greater than pure RAP under a given test temperature and normal stress. The shear strength of RAP samples at lower test temperatures is higher than that at a higher temperature. Both apparent adhesion intercept of RAP-geogrid and cohesion of RAP show a decreasing trend with the increase of the test temperature and tend to be stable with increasing temperature. Both the interface friction angle of RAP-geogrid and the internal friction angle of RAP slightly decrease with the increase of test temperature. For RAP-geogrid, the value of the apparent adhesion intercept is higher than that for sand and gravel with geosynthetics. The interface friction angle is close to that of sand and gravel with geosynthetics. Therefore, RAP material can be well used as alternative backfill materials in lieu of natural aggregates such as gravel and sand in geotechnical applications.

A rate-dependent constitutive model for saturated frozen soil considering local breakage mechanism

A rate-dependent constitutive model for saturated frozen soil is vital in frozen soil mechanics, especially when simultaneously describing the nonlinearity, dilatancy and strain-softening characteristics. The distribution of the non-uniform strain rate of saturated frozen soil at the meso-scale due to the local ice-cementation breakage is described by a newly binary-medium-based homogenization equation. Based on the field-equation-based approach of the meso-mechanics theory, the interaction expression of the strain rate at macro- and meso-scale is derived, which can give the strain rate concentration tensor at different crushed degrees. With the thermodynamics and empirical assumption, a breakage ratio in the rate-dependent form is determined. This overcomes the limitations of the existing binary-medium-based models that are difficult to simulate rate-dependent mechanical response. Based on these assumptions, a newly binary-medium-based rate-dependent model is proposed considering both the ice bond breakage and material composition characteristics of saturated frozen soil. The proposed constitutive model has been validated by the test results on frozen soils with different temperatures and strain rates.

Research on the effects of heating and cooling processes on the mechanical properties of yellow rust granite

Geological hazards caused by high-temperature rocks cooling down after encountering water are closely related to underground mining and tunneling projects. To fully understand the impact of temperature changes on the mechanical properties of rocks, yellow rust granite samples were subjected to heating-natural cooling and heating-water cooling cycles to experimentally study the effects of these processes on the mechanical properties of the samples. The mechanism of the heating-cooling process on the macromechanical properties of the rock was discussed. Based on the Drucker-Prager criterion and Weibull distribution function, a damage variable correction factor was introduced to reflect the post-peak strain softening characteristics, and a thermo-mechanical coupled damage constitutive model of the granite was established. The results showed that in the natural cooling mode, the mechanical properties deteriorate significantly when the temperature exceeded 600 ​°C, and the failure mode changed from brittle failure to ductile failure. In the water cooling mode, the peak strength and deformation modulus increased at temperatures below 400 ​°C with an increase in the cycle number, while at 600 ​°C, the peak strength and elastic modulus notably decreased. The peak strain increased with the increase of the cycle number and temperature at all temperatures, and the failure mode of the granite tended to change from tensile failure mode to shear failure mode. The experimental results were used to validate the damage constitutive model. The shape parameter r and scale parameter S in the Weibull distribution function of the model were used as indicators to reflect the brittleness degree and peak strength. This study helps to understand the behavior of rocks in high-temperature environments, in order to prevent and mitigate potential geological hazards.

Study on the settlement law of tunnel in diatomite stratum based on Strain Softening model

Nowadays, there is no precedent for building a high-speed railway in diatomite area. Due to the complex structure and poor mechanical properties of diatomite as well as the lack of relevant engineering experience, more attention has been paid to the proper constitutive model of the tunnel in diatomite layer using the numerical calculation method, while the traditional Elastoplastic calculation model is the most used yet. Therefore, relying on the Feifengshan tunnel, through FLAC3D software as well as the on-site monitoring, the analysis of the settlement law about tunnelling in diatomite stratum is carried out based on different constitutive models. The research results show that diatomite has obvious strain-softening characteristics. The calculated surface settlement and vault settlement based on the Strain Softening model was greater than that based on the Mohr Coulomb model. When compared with the on-site monitoring data, it was found that the Strain Softening model would more accurately show the settlement law of the tunnel in diatomite and has better applicability in the diatomite area. The above-mentioned research results may provide some references for the construction and design of tunnels in similar strata in the future.

Failure investigation and treatments of tunnel entrance collapse in weak diatomaceous soil induced by heavy rainfall through coupling surface and groundwater flows

Rainfall is a great threat to the stabilities of the slope at tunnel entrances located in mountainous areas because it can reduce the shear strength of soil and, as a result, the stabilities of tunnel entrances. Although conventional rainfall-induced slope damage analyses commonly overlook the impact of runoff, it is crucial to consider it to assess slope instability accurately. To assess the factor of safety distribution about the slope of the Feifengshan Tunnel in Zhejiang Province, China, and analyze the impact of runoff on slope instabilities, this paper uses a coupling model of surface runoff, groundwater flow, as well as soil mechanics, through the shallow water equation, Richards equation, and the local safety factor (LFS) method. Furthermore, given the strain-softening characteristics of diatomaceous soil, designing and analyzing them according to peak strength may result in potential hazards. Thus, this study analyzes the distribution of the slope safety factor under different constitutive models and recommends suitable slope reinforcement measures. The study also investigates the slope reinforcement measures required to avoid the secondary collapse of tunnel entrance during rainfall, verifying numerical simulation results regarding the section’s volumetric water content and surface settlement values. The findings of this study offer a dependable approach for evaluating the stability of tunnel entrance slopes and are significant for tunnel site selection and reinforcement effect assessment. The numerical results emphasize the effects of surface runoff from rainfall water distribution. Furthermore, the strain softening model has significant advantages in calculating surface settlement and tunnel slope failure.

Experimental Study on Engineering Properties of Cemented Soil with High Water Content

A series of unconsolidated-undrained triaxial tests and unconfined compressive strength (UCS) tests for cemented soils with different curing times were carried out in this research. In total, three cemented soil mixtures with different cement contents were adopted in the tests, and the confining pressure was controlled in the range of 100–1600 kPa. The influence of curing time, cemented soil mixture ratio and confining pressure on the compressive and shear capacity of cemented soil was analyzed based on the test results. The test results indicate that the cement content and curing time both had a great influence on the strength of cemented soil, the UCS of the cemented soil increased linearly with the curing time under the semi-logarithmic coordinate, the cemented soil exhibited strain softening characteristics in the axial shear tests, and the maximum deviatoric stress of the cemented soil increased with the increase in confining pressure. A linear correlation was found between the cohesion and the UCS of cemented soil, and the cohesion was about 0.40 times the compressive strength.

Shear behavior of bedding fault material on the basal layer of DGB landslide

Daguangbao (DGB) Landslide (12 × 108 m3) is the largest landslide triggered by the 2008 Ms8.0 Wenchuan Earthquake, in which basal shear failure develops on an interlayer fault at 400 m deep under the ground. After the landslide, a 1.8 km long (in the sliding direction) oblique shear face is exposed. Different kinds of materials in the interlayer fault of DGB landslide are taken for direct shear test, medium scale shear test, in-situ shear test and ring shear test. The test results show that fault material cohesion ranges from 20 to 320 kPa and internal friction angle from 15° to 41°. Shearing strength of interlayer fault materials is related to fragmentation degree of structure. The lower fragmentation degree the more obvious strain softening characteristics of materials, the higher fragmentation degree the poorer shearing resistance of materials. Compared with argillaceous materials in the same fault, the mylonitic materials are of higher shear strength and internal friction angle. Both mylonitic materials and breccia materials are strong in liquefying. In saturated undrained cases, shear strength of fault materials could drop to 9.7°, with S3 down to 0. In saturated undrained dynamic shear conditions, fault internal friction angle could be reduced to 23.1° and 4.2°. It is concluded that low friction feature of fault materials caused by the influence of groundwater is the main reason for destabilization of DGB landslide.