Keywords

1 Introduction

The Western Sichuan Plateau Gradient Zone is located on the eastern edge of the Tibetan Plateau and is an area with extremely complex topographic and geological conditions in China. The region has steep topography, high seismic intensity, fragile geological environment and frequent geological disasters. Influenced by the complex topography and geological structure of the Western Sichuan Plateau Gradient Zone, landslides, avalanches and mudslide disasters have developed along the highways in the region, which have serious impacts on their normal operation and traffic safety. The Lexi Expressway is located at the edge of the Western Sichuan Plateau Gradient Zone. During the construction process, due to the influence of excavation disturbance, landslide disasters occur frequently along the route, and slope management has become an urgent problem to be solved in the construction of the expressway. Therefore, it is very important to study the whole process of deformation-sliding evolution of slopes under engineering disturbances so as to take timely and effective management measures for slopes.

The deformation and failure mechanism and stability analysis of slopes is a classical research direction in engineering geology [1,2,3]. Domestic and foreign scholars have done more research on this issue and have achieved rich research results. On the basis of analyzing the geological environmental conditions, structural characteristics, and deformation and fracture characteristics of slopes, Li et al. analyzed the instability mechanism and stability of soil-like slopes [4]. Li W G proposed a mechanical model for the instability and failure of soil slopes based on the theory of elasticity, and obtained the ultimate length of soil slopes in the ultimate equilibrium state [5]. Li A H classified bedding slope according to slope lithology, rock combination characteristics, rock dip angle, and rock thickness, and summarized 8 deformation and failure modes of bedding slope [6]. Dong used similarity experiments to study the deformation, development, and failure process of soft and hard interlayer anti tilting slopes, and analyzed the influence of different soft and hard lithology on the tilting deformation process and failure law through 3DEC [7]. Zhang K Y used the finite element strength reduction method to calculate soil slopes, analyzed the variation law of unit stress states at typical positions on the sliding surface, established the relationship between unit stress states and the overall stability of the slope, proposed unit instability criteria, and applied the proposed criteria and program to conduct finite element numerical simulation of the progressive failure process of the slope [8]. Wang G S studied the impact of the progressive failure process of slopes on stability, proposed a new contact element model to simulate the contact friction state on the sliding surface, and conducted numerical simulation and stability analysis of the progressive failure process of slope [9]. Wu and Hsieh simulated the debris movement and deposition process of slope in the Taiwan Chi-Chi earthquake, and better realized the damage pattern of the actual slope after the earthquake [10]. Zhao B Q Used the SLOPE/W module in Geostudio software to analyze the evolution law of bedding rock slopes controlled by weak interlayers from local failure to overall failure sliding [11]. Zeng Y W combined the finite element method with the limit equilibrium method to study the stability analysis of slope and analyzed the relationship between stability and deformation of slope [12]. Liu X R combined shaking table test and numerical simulation method to analyze the deformation destabilization mechanism and stability of down-gradient slope in the Three Gorges reservoir area [13]. Zhang J X used FLAC3D to simulate the deformation and damage process of rocky slope with weak interlayers under different working conditions, analyzed the deformation and failure mechanism of the slope, and put forward the corresponding management countermeasures accordingly [14].

Existing research has focused more on the deformation characteristics and instability mechanisms of slopes, and there is less research on the deformation, failure, and development evolution of slopes disturbed by engineering along highways. This article selects the high slope of soil accumulation along the ZK125 + 654–ZK125 + 775 section of the Lexi Expressway. Based on on-site geological investigation and relevant rock and soil experimental data collection, a calculation model for excavation stability of soil accumulation slope is established. The three-dimensional numerical simulation analysis method is used to simulate the evolution characteristics and stability of slope deformation and failure under various working conditions such as unsupported excavation and supported excavation. This can provide a theoretical basis for the layout of subsequent monitoring points and the evaluation of support schemes for the slope.

2 Lexi Expressway ZK125 + 654–ZK125 + 775 Section Slope Introduction

2.1 Slope Engineering Geological Conditions

The proposed Layimu Interchange of Mabian to Zhaojue section of Leshan to Xichang Expressway is located in Layimu Village, Layimu Township, Zhaojue County, Liangshan Prefecture, Sichuan Province. The slope is located in the site of Layimu Interchange, near the exit end of Layimu Tunnel, on the left side of the section with pile number ZK125 + 654–ZK125 + 775, as shown in Fig. 1.

Fig. 1.
figure 1

Accumulation Slope of ZK125 + 654–ZK125 + 775 Section of Lexi Expressway

Full size image

The slope is located in southwest Sichuan Hengduan mountain system northeast edge of Daliangshan high mountain, Sichuan basin to the southwest mountain transition zone. The mountains in the area are mostly oriented north-south, between the ridge and valley, the site is a middle mountain area in the erosion tectonics, the valley is “V” type, the highest peak near the Longtoushan, the elevation of up to 3500 m.

The site is located in the middle and lower part of the left slope of Xigou (a tributary of Zhuhe River), with a gentle transverse slope and a slope angle of 10–25°, with local areas reaching 40–60°. The slope surface is mainly composed of colluvial gravel and silty clay containing gravel, with locally exposed bedrock and underdeveloped vegetation. There are a large number of residential buildings distributed at the lower part of the slope.

The stratigraphy of the site mainly consists of Cenozoic Quaternary new avalanche slope accumulation (Q4c+dl)gravelly soil, pulverized clay and Mesozoic Triassic Lower Feixianguan Formation (T1f) siltstone, the section is shown in Fig. 2.

Fig. 2.
figure 2

(Modified by Sichuan Provincial Highway Planning, Survey, Design and Research Institute)

Profile of Accumulation Slope

Full size image

2.2 Slope Disposal Measures

This slope is a fifth grade slope, with a slope ratio of 1:1.25 for the second to fifth grade slopes. The width of the first grade slope platform is 5.5 m, the width of the second grade slope platform is 10 m, and the width of the third and fourth slope platforms is 5 m. According to the design documents, a comprehensive treatment method of setting anchor cable piles on the outer side of the cutting ditch platform, setting pressure grouting steel anchor pipe frame beams after the slope grading slows down, and setting steel pipe piles on the slope platform for support and protection is adopted.

  1. (1)

    Steel anchor pipe frame beam

The slope is protected by 4 × 3 m pressure grouting steel anchor pipe frame, and the frame girder is cast-in-place C30 concrete, with a beam width of 40 cm and a thickness of 50 cm, and the beam is set up with an expansion and contraction joint every 15–20 m, with a joint width of 2 cm, and filled with asphalt sisal wadding. The steel anchor pipe is made of φ70 × 5 mm steel pipe with a length of 15 m.

  1. (2)

    Steel Pipe Piles

Three rows of steel pipe piles are set up in the first and third level slope platforms respectively, with an outer diameter of 140 mm and a length of 23.9 m, which are laid out in plum blossom type. The steel pipe is made of No.3 steel with yield strength not less than 240 MPa and tensile strength 380–470 MPa.

  1. (3)

    Pile slab wall

Anti-slip piles are laid at 15.25 m from the left side of the highway center line, the designed pile diameter is: pile width × pile height = 2.5 m × 3.5 m, pile length is 32–34 m, anchoring section is 21–23 m, pile top elevation is 1660.79–1661.79 m, pile spacing is 5.5 m, there are 16 piles in total, which are poured with C30 concrete, and the retaining plate is hung outside between the piles. The prestressing anchor cable in the anti-slip pile adopts 6 bundles of φ15.2 mm low relaxation strand (1860 MPa).

3 Model Establishment and Parameter Selection

3.1 Establishment of Calculation Model

By using Rhino6.0 software and Griddle built-in plugins, a three-dimensional mesh model of the slope was established using three cross-sectional views, one plan view, and one elevation view of the left accumulation slope of ZK125 + 654–ZK125 + 775 as references. The model was imported into FLAC 3D 6.0 software for calculation and analysis. The model has a length of 302 m, a width of 291 m and a height of 116 m, with a total of 946,921 meshes. There are four types of rock and soil mass in total: crushed stone soil, clay, strongly weathered silty mudstone, and moderately weathered silty mudstone, calculation using the Mohr-Coulomb elastoplasticity criterion. The left and right sides of the X-axis and the front and back sides of the Y-axis of the model are set as normal displacement constraints, the upper surface is set as a free interface, and the ground surface is set as a fixed constraint. The specific model diagrams are shown in Figs. 3 and 4.

Fig. 3.
figure 3

Slope model in initial state

Full size image
Fig. 4.
figure 4

Slope model after excavation and support

Full size image

3.2 Calculation Parameter Selection

Based on the preliminary investigation data of the slope, the geotechnical test data of on-site drilling and sampling, and then through the experience of analogy and reference to the neighboring areas in the same region and other comprehensive considerations [15, 16], the basic physical and mechanical parameters of the geotechnical body involved in the calculation model are shown in Tables 1 and 2.

Table 1. Physical and mechanical parameters of the foundation of the geotechnical body
Full size table
Table 2. Physical and mechanical parameters of geotechnical bodies
Full size table

The relevant parameters of the support structure are mainly obtained through the design documents, as shown in Tables 3, 4 and 5. The rainstorm condition adopts the calculation method of parameter reduction, and the parameter values are selected based on the investigation report, the data of neighboring areas in the same region [16], and “the Technical Specification for Building Slope Engineering” GB50330-2013 combined with the site conditions.

Table 3. Mechanical parameters of steel anchor pipe and anchor cable
Full size table
Table 4. Mechanical parameters of steel pipe pile
Full size table
Table 5. Mechanical parameters of frame beams and pile-slab walls
Full size table

4 Analysis of Deformation and Failure Process of Unsupported Excavation State of Slope

4.1 Natural Slope Characterization

The pre-excavation morphology of the slope was selected as the first stage of deformation characterization, and the slope model was subjected to initial ground stress equilibrium and its displacement and shear strain increments were obtained as shown in Fig. 5.

Fig. 5.
figure 5

Cloud maps of slope simulation in initial state

Full size image

It can be seen that the overall stability of the accumulation slope in the initial state is very high, and the safety reserve is good, Fs = 1.61. There is no obvious shear strain increment zone or plastic zone within the slope. If only subjected to gravity, the slope will not experience large-scale overall instability and failure in the future for a long period of time.

4.2 Characteristics of Deformation and Failure of Unsupported Excavation State of Slope

In order to investigate the potential deformation damage mechanism and mode of this slope, an unsupported excavation model was set up. The simulation cloud maps under each step of excavation are shown in Fig. 6.

Fig. 6.
figure 6figure 6

Cloud maps of slope displacement after unsupported excavation in the first–fifth steps and under rainstorm condition (unit: m)

Full size image

After the first step, second step, third step and fourth step excavation of the slope, the displacement of the rock and soil body of the slope under the action of gravity mainly occurs around the excavation surface, and the maximum displacement values are 1.82 cm, 3.64 cm, 5.10 cm, 5.00 cm, respectively. Comparing with the cloud diagram of the slope displacement under the initial state, the maximum displacement of the slope body of the slope after excavation is shifted from the top of the slope to the excavation platform, and with the excavation proceeding, its distribution pattern is almost the same, and the range has increased. After the fifth step of excavation of the slope, the increase of its maximum displacement value becomes larger than before, increasing nearly three times, with the maximum value of 14.56 cm, and the distribution range of the slope displacement cloud map increases slightly further. The distribution range of the maximum value of displacement is significantly increased to include the entire shallow surface layer of the excavation profile. The stability coefficients Fs of the five excavations are 1.53, 1.47, 1.32, 1.11, and 1.03, respectively, indicating that the slope is continuing to develop in a direction that is not conducive to stability.

After the slope excavation under the rainstorm condition, the value of slope deformation changed dramatically compared with the previous excavation condition, and the maximum displacement value of the rock and soil body changed from 14.56 cm to 10 m, an increase of nearly 100 times. The stability coefficient Fs = 0.97 and the slope is unstable. From the contour map, the displacement distribution range is more concentrated, in the three-dimensional map is concentrated in the whole excavation surface, in the topography of the circle closed, the contour in the profile map is concentrated in the back of the excavation surface is distributed in the form of a circular arc, presenting the characteristics of the landslide, from the map there are two slip surfaces formed, one of which will be included in the whole excavation surface, and the other is distributed in the back of the third, fourth, and fifth level slopes, and the amount of the sliding body displacement is greater on this slip surface.

4.3 Analysis of Slope Deformation and Failure Mechanism

Through on-site investigation, data collection and analysis of the slope, and comprehensive numerical simulation calculation results, a deep understanding of the left accumulation slope of ZK125 + 654–ZK125 + 775 has been obtained, and its potential instability failure mode and deformation mechanism have been determined:

  1. (1)

    Natural slope period

The slope is mainly composed of three types of lithologies: gravelly soil, silty clay, and silty mudstone. The silty mudstone is divided into two types: strongly weathered and moderately weathered. Gravel soil and silty clay are interbedded, with a thickness of about 60 m. The underlying silty mudstone is a typical soil accumulation slope. According to the excavation exposure, the accumulation on the slope is relatively loose, and rainfall and engineering disturbance are unfavorable factors for the stability of the slope.

  1. (2)

    Excavation period from first step to fourth step

In actual construction excavation, support and excavation are carried out simultaneously, and even support is carried out before excavation. In order to explore the potential deformation and failure mechanism of the slope and identify the locations with high potential risks, an unsupported excavation mode is set up. After the first to fourth steps of excavation, due to the unloading effect, small-scale tensile cracks and plastic zones are formed locally behind the excavation surface. As the slope toe is excavated, the plastic zone and cracks expand, but no large through cracks or plastic zones are formed. The slope is still in a creep period, and the slope soil continues to shear and creep towards the excavation direction under gravity. At this time, there is no controlled sliding surface in the slope body, and the slope is still stable.

  1. (3)

    Fifth step excavation period

After the fifth step of unsupported excavation, the displacement and shear strain increment of the slope have significantly increased, and the plastic zone has also expanded significantly. The plastic zone of the first level slope has developed to a certain depth, and at the same time, the plastic zones of all levels of slopes are also tending to connect. Under the action of excavation unloading and gravity, the creep of the surface layer of the slope tends to accelerate, and the surface of the slope sinks. At the same time, the tensile cracks at the rear edge also accelerate to develop deeper and have a trend of continuity.

Based on the above analysis, the potential deformation and failure mechanism and mode of the left side accumulation slope of ZK125 + 654–ZK125 + 775 are defined as a creep tension type soil landslide where the geotechnical body on the surface of the slope are sheared off at the potential shear sliding surface under the multiple effects of human engineering disturbance, rainfall, and gravity. The slope is pulled by the front edge unloading and pushed by the rear edge, sliding along the circular arc shear sliding surface in the accumulation, The deformation and failure process is shown in Fig. 7.

Fig. 7.
figure 7

Schematic diagram of slope deformation and failure mechanism

Full size image

5 Analysis of Deformation and Failure Process of Slope Support Excavation State

5.1 Characteristics of Deformation and Failure of Supported Excavation State of Slope

According to the design documents, the fifth step of excavation requires excavation and pouring of anti-skid piles. After the anti-skid piles are formed, the soil is excavated and retaining plates are applied step by step during the excavation project. This process is complex and difficult to replicate using simulation software. Therefore, this process is directly set as the fifth step of excavation and support for calculation and analysis. The simulated cloud maps is shown in Fig. 8.

Fig. 8.
figure 8figure 8

Cloud maps of slope displacement after supported excavation in the first–fifth steps and under rainstorm condition (unit: m)

Full size image

The maximum displacement after the first step of support is 0.92 cm, which is distributed below the fourth level slope platform and in the middle and lower part of the fifth level slope. The maximum values after the second, third, fourth, and fifth steps of excavation support are 3.23 cm, 4.17 cm, 5.19 cm, and 5.44 cm, respectively. Compared with the pre support cloud maps, after excavation and support, the displacement distribution range and maximum value distribution range in the 3D cloud map under each excavation step are significantly reduced, and they are no longer connected like when there is no support; This feature is also present in the profile, where the displacement contour line is divided from a continuous patch distribution into shallow surfaces concentrated on all levels of slopes and slope platforms. The stability coefficients Fs of the five excavations are 1.59, 1.57, 1.56, and 1.54, respectively, indicating that the slope has high stability and will not experience large-scale instability failure.

Under the rainstorm condition, the displacement distribution range in the three-dimensional map after the fifth step of excavation is similar to that without support, but the value is less than 1% of that without support. The distribution pattern of displacement contour lines in the profile map is also significantly different from that without support, still maintaining the distribution characteristics of natural working conditions, and there is no obvious convergence of displacement contour lines into strips. This indicates that the surface displacement and deep displacement of the slope have significantly decreased compared to before support. The stability coefficient Fs = 1.42, and the slope stability is good, no different from that under the natural condition.

5.2 Analysis of Deformation Characteristics of Support Structures

The displacement cloud maps of the support structure under the multi-step excavation steps of the slope is shown in Fig. 9.

Fig. 9.
figure 9

Cloud maps of supporting structure displacement after supported excavation in the first–fifth steps and under rainstorm condition (unit: m)

Full size image

During the excavation and support process, the maximum displacement of steel anchor pipes and prestressed anchor cables is about 3.6 cm, with a very small distribution range; The maximum displacement of the frame beam is about 5 cm, and the distribution range of the maximum value is small, and the overall displacement distribution is relatively uniform; The maximum displacement of the steel pipe pile is 2 cm, which is mainly distributed in the shallow surface area of the fourth level slope platform and the middle of the second level slope platform, and gradually decreases around them, with the overall distribution still relatively uniform; The overall maximum displacement of the pile sheet wall is 5.4 cm, with the maximum value mainly distributed in the upper part of the retaining plate between piles A7 # and A11 #. The overall displacement distribution is centered around this and gradually decreases towards the surrounding areas. The deformation of the entire support structure is relatively small and evenly distributed, resulting in good support effect.

6 Conclusions

The deformation and failure mechanism and support stability of the left accumulation slope of ZK125 + 654–ZK125 + 775 were analyzed through numerical simulation methods, and the following conclusions were obtained:

  1. (1)

    Under natural conditions, unsupported excavation of slopes will not cause large-scale instability and failure, mainly due to creep deformation on the surface of the slope. The overall impact of excavation from the first to fourth steps on the slope is relatively small. After excavation in the fifth step, the creep range of the slope increases, the creep speed accelerates, and the slope is in an unstable state.

  2. (2)

    The potential deformation and failure mechanism and mode of the slope are that under the multiple influences of human engineering disturbance, rainfall, and gravity, the geotechnical body on the surface of the slope are sheared off at the potential shear sliding surface inside the slope. The slope is pulled by the front edge unloading and pushed by the rear edge, resulting in a creep pull fracture type soil landslide sliding along the arc shaped shear sliding surface in the accumulation body.

  3. (3)

    During the excavation and support process, the slope will not experience overall or local instability and failure; Under the rainstorm condition after the support, the slope still has a high safety reserve and high stability. This support structure has a good support effect on the slope of the accumulation body.