The slip zone soil sample was taken from a high-speed subgrade landslide of about 511∼661 m on the right side near the K51 + 450∼K51 + 680 section of a proposed highway route in Hunan, and the block soil sample was taken by the borehole sampling method, and the sampling borehole number was HPZK2-3, and the sampling depth was about 22.5∼23.0 m. The test soil samples are yellowish, grayish-brown, plastic, and have high water content, and are identified as typical silty clay in the field, as shown in Figure 1c. The basic physical properties of silty clay measured by the laboratory basic physical properties test are shown in Table 1, indicating that the silty clay has high saturation and plasticity.

Figure 2a shows the instrument for this ring shear test, which provides a maximum axial pressure of 4 kN, a maximum shear stress of 1,000 kPa, and a maximum shear rate of 32 mm/min. The cutting box is a ring box with an inner diameter of 70 mm, an outer diameter of 100 mm and a height of 20 mm. The specimen specification required for this ring shear test is annular, as shown in Figure 2b. Both the shear stress and the normal stress can be measured by a sensor installed in the pressurization system, and the data obtained from the test can be transmitted to the computer through a fully automatic acquisition device.

Firstly, the slip zone soil sample was dried at 105°C for more than 8 h, and then passed through a 2 mm sieve after crushing, and the part less than 2 mm was taken from it, and the corresponding moisture content was calculated according to the dry density and the volume of the shear box to obtain the required target moisture content state, and then sealed with a polyethylene plastic bag for more than 24 h. Considering the actual situation of the project, the dry density of the soil sample is consistent with the undisturbed soil with the moisture content, that is, the remolded silty clay sample with a dry density of 1.82 g/cm³ and a natural moisture content of 18% is prepared for ring shear test, and the ring shear test scheme is shown in Table 2.

The shear test of 45 mm displacement was carried out on each slip zone soil sample, and after shearing to about 20 mm, the shear stress began to remain stable and unchanged, and it can be considered that the shear of slip zone soil has basically entered the residual strength state, and the specific results are shown in Figure 3. At the beginning of the test, slip zone soil enters the elastic deformation stage under the small shear displacement, and immediately reaches the peak strength state, which is due to the weak ability to resist the elastoplastic deformation with slip zone soil, and the yield deformation occurs under the action of small shear. Then, slip zone soil enters the post-peak strain softening stage, and then enters the residual stage, and the shear to the residual state of slip zone soil is to obtain the residual strength of slip zone soil, and the test results also show that the residual strength is almost constant. With the increase of normal stress, the shear strength of slip zone soil also increases, which indicates that the normal stress has an inhibition effect on soil shear.

Based on the four sets of shear strength-shear displacement curves and combined with the mol-Coulomb strength equation, the evolution curves of friction angle and cohesion with shear displacement in slip zone soil can be arbitrarily obtained. The shear displacement at any place corresponds to the four shear strengths under the condition of four normal stresses, and the linear fitting of the shear strength and normal stress can be realized by the Coulomb formula, as shown in Figure 4.

Figure 5 shows the internal friction angle as a function of shear displacement. With the increase of shear displacement, the internal friction angle increases first and reaches the peak, indicating that the friction between soil particles in the slip zone resists shear deformation in the soil, and the soil exhibits the characteristics of plastic hardening. After reaching the peak, the internal friction angle gradually decays until it stabilizes in the residual state, and with the continuous advancement of the shear deformation process, the soil structure is completely destroyed, and finally the shear strength is maintained only by the friction between the particles, and the soil shows the characteristics of strain softening. Here, the peak internal friction angle is 18.85°; The residual internal friction angle is 13.47°. In order to quantitatively evaluate the variation trend of the internal friction angle with shear displacement in the sliding zone soil, the Logistic function model was used for fitting, and the results of Figure 5 showed that the fitting degree was as high as 0.988, and the fitting effect was good, indicating that the attenuation law of the internal friction angle was in line with the Logistic function change relationship.

Figure 6 shows the internal friction angle as a function of shear displacement. With the increase of shear displacement, the cohesion of the soil in the sliding zone is increasing rapidly, which indicates that the cohesion of soil particles in the sliding zone is resisting the external shear of the soil. After reaching the peak, the cohesion of the sliding zone soil decreases, indicating that the original stress structure in the sliding zone soil is continuously destroyed during the shearing process until the structural property is completely lost. The peak cohesion of the slip zone soil was 40.475 kPa, and the residual cohesion was constantly decreasing. In order to quantitatively evaluate the attenuation law of cohesion in the slip zone, the power function model was used for nonlinear fitting, and it can be seen from the results of Figure 6 that the fitting degree is 0.94, and the fitting effect is good, indicating that the attenuation law of cohesion obeys the power function change relationship.

In general, the results of the change of internal friction angle and cohesion are first increased and then decayed. Considering that the internal friction angle and cohesion essentially represent the strength of the soil and have nothing to do with the shear evolution process of the soil, the plastic hardening process before the peak internal friction angle and the peak cohesion is appropriately corrected, and the correction results are shown in Figures 5, 6. Correspondingly, according to the evolution fitting curve of friction angle and cohesion in the slip zone soil, combined with the Moore-Coulomb formula, the shear strength curve of the slip zone soil under the action of arbitrary normal stress can be obtained. Firstly, according to the evolution model of friction angle and cohesion in the soil of the sliding zone, as shown in Equation 1 and Equation 2: tan φ = 0.342 , u ≤ 5 m m ; 0.237 + 0.116 / 1 + u / 9.75 2.83 , u ≥ 5 m m ; (1) c = 40.475 , u ≤ 2 m m ; 44.36 * u − 0.189 , u ≥ 2 m m ; (2)

Secondly, by substituting Equation 1 and Equation 2 into Equation 3, the dynamic variation model of shear strength of slip zone soil can be obtained: τ = c + σ n ⁡ tan φ (3)

According to the above steps, the shear strength of the sliding zone soil under normal stress of 100 kPa, 200 kPa, 300 kPa, 400 kPa, 500 kPa and 600 kPa was plotted with shear displacement, as shown in Figure 7. With the increase of normal stress, the shear strength increases, the attenuation amplitude of shear strength from peak strength to residual strength increases, the longer the strain softening state lasts, the greater the deformation needs to be constant, and the peak strength decays to the residual strength needs to experience greater displacement. The above process can obtain the shear strength value of slip zone soil through shear displacement under arbitrary normal stress, which lays a foundation for the study of landslide dynamic stability from the perspective of strength.

Based on the dynamic variation curve of shear strength of the sliding zone obtained by the Moore-Coulomb theory, combined with the typical residual thrust method (Song and Xu, 2012), the improved mechanical model of landslide stability is established. In addition, the finite element numerical calculation model of the landslide was established by using the Geo-studio finite element software, and the dynamic stability evaluation of the landslide was compared and studied. The specific solution is divided into two aspects and the calculation process of landslide stability is shown in Figure 8.

The improved mechanical model of landslide stability: 1) The basic geometric parameters of the strip are obtained by the strip of the sliding body, including the width of the strip l _i, the volume of the strip V _i, the inclination angle α _i at the bottom of the strip, and the effective average gravity γ and gravity W _i of the strip. 2) under the static equilibrium condition of any bar i along the direction of the vertical sliding surface, the normal stress σ _ni of the vertical sliding surface of the bar and the partial gliding force T _i along the sliding surface are obtained; Based on the shear strength curve of the sliding belt based on the mol-Coulomb strength criterion under different normal stress conditions, the dynamic value of the shear strength of the shear belt at the bottom of the i block with shear displacement is obtained, and the dynamic anti-slip force value R _i of the i block along the sliding surface is obtained. 3) the difference between the sliding force T _i of any block i and the anti-sliding force R _i after the strength reduction is used to obtain the recursive equation of the residual thrust of the i block of the landslide, see Equations 4, 5; 4) Set the reduction factor F _r, and then solve the residual thrust of each block in turn, so that the residual thrust of the last block is equal to or close to 0, that is, the landslide stability calculation converges.

Numerical calculation model of finite element: 1) Establish a two-dimensional finite element numerical model of landslide, assign material parameters of sliding body and bedrock, set boundary conditions, and mesh the model; 2) The friction angle and cohesion weakening value in the soil in the slip zone were selected as the corresponding representative landslide deformation conditions to assign the strength parameters of the slip zone, and the numerical simulation analysis of landslide deformation and failure was carried out. 3) The contour of the landslide deformation evolution result and the dynamic stability coefficient of the landslide are stopped until the stability coefficient of the landslide remains stable, indicating that the calculation of the stability of the landslide is convergent.

Based on the dynamic evolution model of mol-Coulomb strength and the typical residual thrust method, a computational mechanical model of landslide stability with dynamic changes of internal friction angle and cohesion force is established. With the change of shear displacement, the friction angle and cohesion in the slip zone also change, and the shear strength will also weaken, so the landslide thrust and stability calculated by the model will have a dynamic evolution. The 2–2 section of the landslide was selected as the typical section for strip calculation. Taking any bar i as the research object for specific analysis, as shown in Figure 9, according to the overall force balance condition along the sliding surface of any bar i, the residual thrust Pi of any bar i is: P i = P i − 1 ⁡ cos α i − 1 − α i + W i ⁡ sin ⁡ α i − τ s i l i / F r (4) where P _i-1 is the residual thrust of bar i-1; α _i-1 and α _i represent the horizontal dip angles of the bottom sliding surface at bar i-1 and i, respectively, and the inclination angle of the sliding surface of the reverse warping section is negative. W _i is the weight of strip i, and severe γ _i is taken as natural severity. F _r is the strength reduction factor, also known as the safety factor for landslide stability calculation, it is the critical value for a landslide to maintain the ultimate equilibrium state; τ _si is the shear strength of the sliding zone at the bottom of the bar, which can be obtained from the dynamic evolution model of mol-coulomb strength. l _i is the length of the bottom of bar i; h _i is the average height of bar i.

From the static equilibrium condition perpendicular to the bottom of bar i, it can be seen that the normal stress at the bottom of bar i becomes: σ n i = P i − 1 ⁡ sin α i − 1 − α i + W i ⁡ cos ⁡ α i l i (5)

In the landslide stability analysis, it is generally considered that the landslide speeds do not detach and overlap with each other, that is, the horizontal displacement of each block is equal by default (Zhang and Wang, 2010), that is, the displacement between adjacent blocks satisfies as shown in Equation 6. u i − 1 ⁡ cos ⁡ α i − 1 = u i ⁡ cos ⁡ α i (6) where u _i and u _i-1 are the displacements of bar and i-1, respectively, and α _i and α _i-1 are the inclination angles of bar i and bar i-1, respectively.

The 2–2 section of the landslide was selected as the typical geological section, and the two-dimensional numerical calculation model of the landslide was established by the Geo-Studio finite element software, as shown in Figure 10. According to the spatial structure of the landslide, the landslide is divided into three materials, the ideal elastoplastic model is used for the sliding body and the sliding belt material, and the ideal elastic model is used for the bedrock sliding bed. In order to accurately reveal the deformation and failure of the landslide, the mesh near the slip zone material was further subdivided, and a total of 6482 grid elements and 6541 grid nodes were divided. According to the results of indoor physical and mechanical tests and geotechnical engineering investigations, the physical and mechanical calculation parameters of each layer of the numerical calculation model are comprehensively determined, as shown in Table 3.

In order to accurately and comprehensively reveal the results of landslide deformation evolution, the internal friction angle and cohesion weakening values of six groups of slip zone soils were selected as the corresponding representative landslide deformation conditions for numerical simulation analysis, and the specific scheme is shown in Table 4.

With the evolution of landslide, the shear displacement of slip zone also shows a trend of weakening in space. In order to quantitatively reveal the weakening degree of the strength of slip zone, the relationship between the strength of the bottom of slide mass and the shear displacement was calculated and analyzed. Figure 11 shows the spatial distribution of slip zone strength at each position at the bottom of the landslide and the variation with shear displacement.

Spatially, from the trailing edge to the front edge of the landslide, the strength distribution of slip zone generally shows a pattern of first increasing and then decreasing, and the strength of slip zone is the highest at 139.6 m. In the middle part of the 139.6 m sliding body, the thickness of slide mass is the deepest, reaching a maximum depth of 10.9 m, extending to the front edge and the trailing edge, the thickness of slide mass is about thin, and the buried depth of slip zone is shallow. The deeper the slip zone is embedded, the greater the normal stress, and the greater the shear strength of slip zone, and vice versa. It is worth noting that the strength of the leading edge of slip zone increases abruptly, which is due to the reverse warping of the dip angle of slip zone, which increases the normal stress of slip zone, resulting in an increase in strength.

In terms of time, the strength of slip zone showed a gradual trend. With the increase of landslide displacement, the peak strength of slip zone gradually decays, and the strain softening begins to occur in slip zone, which gradually tends to be stable after about 25 mm, and remains in their respective residual strength states.

With the continuous increase of landslide displacement, the shear strength at each slip zone is strained and softened and attenuated to different degrees, and the deeper the buried depth of the slip zone, the greater the normal stress of the upper sliding body, and the faster the shear strength attenuation of the slip zone.

By means of numerical simulation, the different attenuation values of friction angle and cohesion in slip zone are taken as the deformation conditions, and the deformation displacement of the landslide and the maximum shear strain increment of slip zone are taken as the evaluation indexes, and the deformation evolution mechanism of the landslide is analyzed through dynamic calculation. The trailing edge of the landslide.

Figure 12 shows the evolution contour of landslide deformation under different shear displacement conditions. With the attenuation of the strength index of slip zone, the deformation evolution of the landslide gradually intensifies. When the shear displacement of slip zone is 1 mm, the deformation of the landslide is mainly manifested in the leading edge and the trailing edge, and the middle part is almost not deformed. With the increasing shear displacement, the friction angle and cohesion in slip zone are continuously attenuated, and the deformation of the landslide is intensified, and the deformation of the leading and trailing edges of the landslide extends to the middle part. When the shear displacement reaches 30 mm, the friction angle and cohesion in slip zone are stabilized in their respective residual states, and the landslide produces an overall sliding. The simulation results show the deformation of the front and trailing edges, and then extend the deformation to the middle sliding body, which is consistent with the deformation and failure signs of the collapse of the trailing edge due to the steep slope and the slip of the leading edge due to river erosion and cutting. The simulation results can accurately reveal the deformation evolution process of the landslide.

Figure 13 shows the incremental evolution of shear strain in slip zone under different deformation states of the landslide. As shown in Figure 13, with the attenuation of friction angle and cohesion in slip zone, the shear failure of slip zone is gradually penetrated. The shear strain increment of slip zone has a good correspondence with the deformation and displacement evolution state of the landslide. When the shear displacement is 1 mm, the shear strain of slip zone is sporadically distributed at the leading and trailing edges of the landslide, and the shear strain in the middle of the slip zone is not penetrated, so the leading edge of the landslide shows traction slip, and the trailing edge of the landslide shows stacked deformation. With the increasing shear displacement, the shear strain of the slip zone expands from both sides to the middle, and the landslide gradually shows a trend of gradual failure. When the shear displacement reaches 30 mm, the shear strain is integrated through, and the landslide manifests as overall deformation and failure.

The improved stability computational mechanical model and finite element numerical calculation model were used to obtain the dynamic stability of the landslide, respectively. Figure 14 shows the evolution of landslide stability as a function of displacement. As shown in Figure 14, the stability calculated by the two models has the same evolutionary law and the difference is small, and both gradually decrease from peak stability to residual stability. In the initial sliding state of the landslide, the peak stability of the landslide is 1.25, with the increase of the landslide displacement, the strain of slip zone begins to soften, the strength is gradually weakened, the anti-sliding force of the landslide gradually decreases, and the stability of the landslide gradually decreases from the peak; when the strain softening phenomenon of slip zone is over, the shear strength of slip zone is stable at the respective residual strength, and the landslide is gradually stabilized in the residual state, and the residual stability is between 1.07 and 1.08.

The residual stability of the landslide is between 1.07 and 1.08, indicating that the landslide is currently in a basically stable state. The stability of the landslide changes from time to time, and it is possible to slide again in the later stage of the landslide under the induction of a combination of rainfall infiltration, river erosion at the front edge and steep slope at the back edge. Therefore, the trailing edge of the landslide should be appropriately cut and the leading edge of the landslide should be diverted. In addition, necessary measures such as monitoring and early warning should be carried out irregularly in the later stage, especially in the high incidence of the local rainy season, so as to prevent large-scale landslide sliding under heavy rainfall infiltration conditions and ensure the normal operation of highway traffic.

In this study, the samples in slip zone soil were screened with coarse particles, and the fine particles were retained. In its actual state, the slip zone soil is accompanied by a large number of coarse grains. Therefore, the shear strength of slip zone soil tested by the ring shear test is obviously weaker than that of the actual slip zone soil, and the calculation results of landslide stability are also low. Therefore, the ring shear test in situ can be carried out in order to obtain a more accurate shear strength of slip zone soil and accurately analyze the stability evaluation of the landslide.