3D Numerical Simulation in Stability Analysis of a Freeway Slope

In recent years, a three-dimensional numerical simulation has been widely used in the analysis of slope stability. In this paper, a landslide is used as the engineering background, and the FLAC3D software is used to analyze the stable state of the landslide. The relationship between the displacement and the time step of the point is analyzed. The position of the sliding surface is simulated, and the stable state of the landslide is comprehensively evaluated, thereby providing a meaningful experience for other similar projects.


Introduction
With the development of the national economy, the investment in infrastructure is increasing, especially the development of the western development.The construction of railways and highways has increased year by year, and it has continued to extend to the mountainous areas.Due to the constraints of topography and landforms, a large number of slope projects have inevitably been encountered, and some areas even have high and steep slopes.Many highways and railway lines often suffer from landslides and other geological disasters, resulting in transportation interruption, a large number of casualties, and serious economic losses.Therefore, it is very important to research slope stability.
The selection of landslide control measures is generally based on qualitative analysis [1][2][3] , which comprehensively analyzes the suitability of the project, the feasibility of the technology, the investment benefit ratio, and the difficulty of construction to select the control plan.However, such analysis methods do not have a quantitative comparison, and numerical simulation analysis precisely has the function of quantitative analysis.When conducting analysis, the FLAC3D simplified model can be used to discuss.A typical section of the landslide mass is selected.A simplified three-dimensional model is established.The stability of the simplified model under natural conditions is analyzed, and its displacement cloud map is analysed [4][5][6] .Finally, the sliding surface of the landslide mass is identified.Anti-slip retaining structures on a simplified 3D model are arranged to select more reasonable governance measures.The model to study governance plans not only simplifies the workload but also eliminates the interference of terrain factors, making it easy to compare governance plans horizontally.

Topography
The sloped body is located in the southwestern mountainous area, and its landform is a middle-mountain landform in an eroded structure.The mountain body is mainly composed of slate and phyllite.The mountains in the territory overlap, the slopes are mostly above 35°, and cliffs and cliffs can be seen everywhere.The mountains are rocky.The north side is a rocky bank slope composed of quartzite and metamorphic sandstone of the Bikou Group of the Lower Paleozoic, and the upper part is covered with a thin Quaternary system.The gravel soil is accumulated on the slope, mostly rock weathering residues.

Stratigraphic lithology
The lithology of the strata in the landslide area is mainly Quaternary sloping gravel, landslide accumulation block stone, gravel and soil, and Lower Proterozoic Bikou Group.The lithology is mainly slate, sandy slate, metamorphic sandstone, and quartz schist.From new to old, the order is as follows: A Quaternary sloping gravel layer is distributed on the gentle hillside on the north side of the landslide.Gray-yellow is mainly composed of gravel and soil.The gravel is composed of metamorphic sandstone, slate, etc., with poor classification, angular shape, loose structure, and partial overhead phenomenon.
Landslide accumulation of boulders and gravel layers is distributed on the south side, mainly composed of blocks, gravel, and soil.The surface vegetation is well-developed and rich in humus.The rock mass at the leading edge is broken, and many are bent and deformed.
Lower Proterozoic Bikou Group slate: The rock has good mechanical properties, strong weathering resistance, and compact and hard rock mass.Metamorphic sandstone intercalated with quartz schist has a fresh surface that is blue-gray and slightly schistose and has variable residual fine-grained structure, thick layered structure, single layer thickness of 0.6 ~ 0.8 m, and dense and hard lithology.

Hydrological conditions
The precipitation in the area is relatively large, and the influence on the slope is mainly in two ways: ① Bedrock fissure water.Bedrock fissure water is mainly replenished by atmospheric precipitation and seeps down to the valley along beddings, joints, and fissures.② Pore diving occurs in the Quaternary loose overburden.It is recharged by atmospheric precipitation, and the discharge is mainly underground runoff and evaporation.
Groundwater quality and hydrochemical type: The surface water is sampled and analyzed for water quality.It does not contain aggressive CO 2 , and the hydrochemical type is HCO 3 -Ca-Mg, SO 4 -Ga type water.The groundwater in the landslide area is non-corrosive to concrete and non-corrosive to steel bars in reinforced concrete structures.
At present, the landslide mass is mainly affected by natural conditions, including topography, stratum lithology, hydrogeological conditions, etc., without the influence of human activities such as construction disturbance.The engineering geological section of the landslide is shown in Figure 1.

The establishment of computational models
In this paper, FLAC3D is used to calculate the stability safety factor of the landslide.Compared with the safety factor obtained by the traditional limit equilibrium method, the numerical calculation based on the strength reduction method has many advantages.It can simulate complex boundary conditions, geological structure characteristics, various effects, loads, and various mechanical properties of rock and soil.The relationship between stress and strain of rock and soil can be considered.The strength reduction method based on FLAC3D does not need to presume the slip surface and can automatically search for the position and shape of the slip surface when solving the safety factor.It can analyze the asymptotic failure process of the landslide.
The strength reduction method applied to finite difference or finite element analysis can be expressed as keeping the gravitational acceleration of the rock and soil constant.By gradually reducing the shear strength index, the values of c and φ are divided by the reduction coefficient F s at the same time.A new set of strength indexes c i and φ i are obtained, and then the finite element analysis is carried out.The calculation is repeated until the slope reaches the critical failure state.The ratio of the strength index used at this time to the original strength index of the rock and soil mass is the slope.The safety factor F s .The formula is as follows [7][8] : The 3D calculation model is established according to the geological data.The FLAC3D modeling project is huge and cumbersome.In this paper, ANSYS is used to establish the model, and then the model is imported into FLAC3D for numerical calculation.
The foot point of the front edge of the slope is the coordinate origin (0, 0, 0), the slope is 150 m long and 150 m wide, and the initial in-situ stress is the self-weight in-situ stress field.
The boundary conditions are: X, Y, and Z directions are fixed constraints, the landslide body is mainly composed of gravel soil, and the sliding bed is mainly composed of slate.The calculation parameters are selected according to the survey data and the values in Table 1, and the calculation condition is the natural state.When calculating the Shear modulus and Bulk modulus of rock and soil mass, it shall be calculated according to the following formula [9] : ) where E is the elastic modulus of rock soil mass, B is the Bulk modulus of rock soil mass, S is the Shear modulus of rock soil mass, and μ is the material Poisson's ratio.

Numerical calculation results
Using FLAC3D [10][11] software for numerical simulation, the safety factor of landslide is calculated to be 0.97, as shown in Figure 2. The relationship between displacement, stress, strain, maximum unbalanced force, and the displacement and time step of a specific recording point is analyzed.Figure 3 is a schematic diagram of ANSYS grid division, and Figure 4 is a schematic diagram of the landslide body.
Because the main component of the landslide body is gravel soil, which has low mechanical properties, and the sliding bed is slate, which is relatively firm, the internal stress field of the landslide changes due to the influence of external conditions.
Figure 5 is the cloud map of the total displacement of the landslide mass.Figure 6 is the cloud map of the cross-section of the midline of the total displacement of the landslide mass.It can be seen from the overall displacement cloud map that the maximum displacement of the landslide mass is 15 cm.The displacement of the landslide mass gradually decreases from the center of the trailing edge to the outside, and the maximum displacement occurs at the center of the trailing edge of the landslide mass.It can be seen from the profile cloud map of the midline position that the maximum displacement decreases gradually downward from the center of the trailing edge of the landslide.
Figure 7 shows the displacement cloud map of the landslide body in the X direction.Figure 8 shows the displacement cloud map of the landslide body in the Y direction.Figure 9 shows the displacement cloud map of the landslide body in the Z direction.The maximum displacement of the landslide body in the X direction is 3 cm, and the displacement in the X direction is very small because one side of the landslide body is slightly higher than the other side.The displacement of the landslide body in the Y direction is the sliding direction of the landslide, and the maximum displacement is 11 cm.The displacement in the Y direction is mainly caused by the downward sliding of the landslide.The maximum displacement of the landslide body in the Z direction is 11 cm; that is, vertical displacement occurs in the up-down direction, and the Z-direction displacement also decreases gradually from the top to the bottom from the center position of the trailing edge of the landslide body.Figure 13 shows the change history of the maximum unbalanced force.It can be seen from Figures 10-12 that there is a slight displacement in the X direction.At the end of the calculation, the recording point has a displacement of 2.2 cm.With the increase of the time step, the displacement gradually increases from 0. The recording point has a displacement of 8.1 cm in the Y direction, which points to the free face direction and is also the sliding direction of the landslide.The recording point has a displacement of 7 cm in the Z direction, that is, a displacement of 7 cm downward.This recording point is in the center of the landslide body.The selection of record points is representative.The record points should be able to reflect the stable state of the landslide mass, and the state of the landslide mass can be explained by the change of displacement.It can be seen from Figure 13 that the maximum unbalanced force gradually tends to zero, and the calculation is convergent.IOP Publishing doi:10.1088/1742-6596/2638/1/0120127 maximum value of the stress in the Y direction appears at the foot of the slope.The stress value in the Z direction is larger and increases with the increase of depth, which is in line with the occurrence law of in-situ stress.Through quantitative analysis, it can be concluded that the failure of the landslide will be that the trailing edge will crack first, and the position of the sliding surface of the landslide body can also be determined.

Figure 1 .
Figure 1.The engineering geological section of the landslide

Figure 2 .Figure 4 .Figure 5 .Figure 6 .Figure 7 .
Figure 2. Safety factor calculation result Figure 3. Schematic diagram of ANSYS meshing Figures 14 and 15 are the displacement vector diagram and the velocity vector diagram of the landslide mass, respectively.It can be seen from Figures 14 and 15 that the vector directions of the two vector cloud images are coincident, showing the movement trend of the landslide body, that is, sliding towards the main sliding direction.

Figure 10 .Figure 11 .
Figure 10.Historical map of displacement change of point (75, 95, 120) in landslide body in X direction

Table 1 .
Mechanical parameters of rock and soil mass