Stability Analysis and Optimal Selection of Treatment Schemes of a Complex Soil High Slope

Li Dongbo Lecturer, School of Science, Xi’an University of Architecture and Technology, Xi’an, China e-mail: [email protected]

Zhao Dong Professor, School of Science, Xi’an University of Architecture and Technology, Xi’an, China e-mail: [email protected]

ABSTRACT Stability analysis and optimal selection of treatment schemes of high slope, is one of key measures to ensure the safety of high slope itself and surrounding buildings. Taking a complex soil high slope named Yinxiang river high slope for an example, finite element method (FEM) was adopted to investigate its stability under the natural condition. By investigating the stress state, stability safety factor and shear strain increment of the slope before and after treatment, three treatment schemes, which is the slope cutting , lattice anchor rod and anti-slide pile are compared. The results indicate that anti-slide pile scheme can meet the design requirement and is more reasonable and economical. The results obtained in the present paper can provide basis for further control of the Yinxiang river high slope and reference for stability analysis and treatment schemes of the similar slopes.

KEYWORDS: high slope, stability, treatment schemes, optimal selection

INTRODUCTION

Because of the large-scale infrastructure construction, the number of high slope under complicated geological conditions has increased dramatically, which leads to more and more instability disasters, such as landslide, collapse and so on. In addition, a large number of human engineering activities seriously threatened the stability of high slope. Instability of high slope can destroy the nearby buildings, block traffic and cause casualties and huge economic losses, so it has become a global natural geological disasters. Therefore, the

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Vol. 20 [2015], Bund. 2 646 stability evaluation and treatment scheme optimization have become more and more important. In terms of the theoretical study, according to the basic ideas of Sarma method, Qiang- yong Zhang, et al. [1] established the stability analysis model of rock slope by considering the earthquake action, the effect of underground fissure water and rock bolt supporting effect of rock. Zhong Wei et al. [2] evaluated the stability of complex rock high slope by using the method of engineering geology, kinematics analysis method and the rigid body limit equilibrium method. Bo Zhao et al. [3] established the safety evaluation model of various indicators and levels of Jinping first-class hydropower station left bank slope, based on the improved matter-element extension theory and the long-term monitoring data. Considering the mesoscopic structure and the contact characteristic of soil-rock-mixture interface, literature [4] analyzed the slope stability on the basis of the nonlinear elastic-ideal plasticity constitutive model. To study the long-term stability of the high rock slope, Rubin Wang et al. [5] analyzed the long-term mechanical behavior of rock slope at the normal storage level by comprehensively using Nishihara model and finite difference numerical method. Ning Li and Qihu Qian [6] proposed four criteria of rock high slope stability analysis and evaluation, namely the potential sliding surface of slope mechanical parameters of minimum value norms, the rule of maximum active slope reinforcement force as well as the numerical method on the safety factor of stability analysis and evaluation, and the lower limit criteria. Jianyong Gao et al. [7] proposed the stability prediction model of loess high slope by comprehensively considering various factors, such as the moisture content. In terms of experimental study, taking the loess high slope of Guanyin hall tunnel as an example, Yufeng Liu et al. [8] established the centrifugal test model of loess high slope, and investigated excavation retaining structure deformation characteristics and traits in different support mode. To evaluate the safety of slope, Weiyuan Zhou, et al. [9] proposed a overall rotation slope model to simulate the 3d slope cracking failure. In terms of numerical analysis, Bintang Sun [10] investigated high slope stability and the optimum scheme of slope reinforcement by using finite element method. Changlu Chen [11] analyzed the 3d dynamic stability of loess high slope by combining the method of strength parameters reduction with the finite difference method. In the present paper, taking the Yinxiang river high slope for an example, the FEM analysis model has been established to analyze its stability under the natural condition by using ABAQUS software. On the basis, by investigating the stress state, stability safety factor and potential sliding surface, three treatment schemes are compared. Finally, comprehensively considering economy and safety, a reasonable and economical treatment scheme is determined.

PROJECT SUMMARY

The high slope project is in the east coast of Yinxiang river, located in weibin district of Baoji, shaanxi province, and about 260m long and 100m wide, 45~89 m high. The landscape belongs to typical loess tableland and river terraces. By investigating the positions of boulders Vol. 20 [2015], Bund. 2 647 layers around 1km range, it is found that the elevation is roughly in the same horizontal plane, which indicates that the high slope has no obvious dislocation. Fig.1 is the close-up view of the high slope. The field survey and investigation indicate that the river erosion is the main reason for the high slope formation. Since the quaternary period, the earth's crust has risen gradually and the Yinxiang river incised, which generated the new free face and the anti-slide force decreased. In addition, with the continuous accumulation of eolian loess, the landslide probability of the slope becomes larger and larger because of the action of gravity. So the slope stability analysis in area 1 and the corresponding slope treatment scheme optimization are very important. According to the survey results, the geometry size of main slide slope section is shown in Fig. 2.

Altitude(m) Assumed height system

Q3 loess paleosol paleosol Q2 loess

Q2 loess paleosol Boulder

Sandy gravel

Sandy clay rock

Figure 1: The close-up view of the high slope Figure 2: Geological profile of main sliding section

TREATMENT SCHEMES

According to the site survey and investigation, the initial design put forward the following three alternatives.

(a) Slope cutting treatment

Slope cutting can reduce the load of upper slope, slow down falling gradient and reduce the volume of landslide, which can reduce the sliding force and improve slope stability. It is mainly used to prevent the small and medium-sized earthiness landslides and rock slope failure, and the object of the slope cutting is sliding parts. For slope shape of slope cutting in this project, ladder platform is adopted and the slope is divided into 9 levels. The slope heights of 1th~8th level are all 9m, while the 9th level is 12.78m. The slope ratios of the 1th~8th level are 1:0.26, while the 3th~8th level are 1:0.53 and the 9th level is 1:1. The slope widths of the 2th and 5th levels are 5m, while the other levels are 3m. Schematic diagram of slope cutting treatment is as shown in Fig. 3 (b). Vol. 20 [2015], Bund. 2 648

(b) Lattice anchor rod treatment

Lattice strengthening technology is to use masonry, cast-in-place reinforced concrete or precast prestressed concrete to form lattice, and the lattice is then fixed by applying of anchor rod or cable. The technology is suitable for the slope which is steep, geotechnical uniform and hard. Slope ratio and width of platform in the treatment scheme is the same as that of the slope cutting treatment. On this basis, the lattice beam and anchor rod are used to strengthen the slope, namely the lattice anchor rod treatment. The size of reinforced concrete lattice beam is 300×300 mm, and the strength grade of concrete is C25. The lattice size is 3.0×3.0m, and non prestressed full grouted bolts are set at nodes of lattice. The length of the bolts is 15m, and aperture is 130mm.The main steel is grade Ⅱ rebar whose diameter is 25mm. Schematic diagram of lattice anchor rod treatment scheme is as shown in Fig. 3 (c).

Anti-slide pile deeps into slide bed to retain landslide and reinforce slope stability. It is suitable for shallow and medium thick landslide. But for the currently active landslide, it should be more careful when piling, because the vibration of excavation may cause landslide. The scheme is based on slope cutting treatment. Following the principle of "waist", anti- slide piles with a section of 2m×2.4m are arranged in the 5th slope level, and pile space is 7m, while the length of piles is 35m. To prevent shallow slope failure, the length of anchor bolt is 10m, while the length of anchorage section is 5m. Schematic diagram of anti-slide pile treatment scheme is as shown in Fig. 3 (c).

(a) The natural slope (b) Slope cutting

Figure 3: Treatment schemes

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METHODOLOGY

a) Numerical model of the natural slope

The main sliding section of the natural slope is as shown in Fig.3 (a). In the model, the ideal elastoplastic constitutive model of non associated flow rule is adopted, and Mohr- Coulomb yield criterion is selected. Horizontal displacements of the left and right boundary are constrained, while horizontal and vertical displacements of the bottom boundary are all constrained. Rock and soil physical and mechanical parameters of each layer are as shown in Table 1. Table 1: Rock and soil physical and mechanical parameters Geotechnical Elastic Modulus Poisson ratio Cohesion Internal friction angle Weight name E/MPa υ c/kPa φ/º γ/kN/m3

Q3 loess 7.7 0.30 65 27 17.2

Q2 loess 7.9 0.31 78 27 17.3 boulder 670 0.30 20 27 21 Sandy gravel 700 0.30 65 43 20 Sandy clay rock 700 0.30 65 43 20

(b) Numerical model of lattice anchor rod treatment

In the model, the lattice beam and anchor bolt are treated as linear elastic materials. The beam element (Beam32) is adopted to simulate lattice beam, while the truss element (T3D2) is adopted to simulate anchor bolt. Binding constraint (Tie) is set between lattice beam and slope, while anchorage section and free section are set as embedding constraint (embed). Parameters of rock and soil are the same as that of the natural slope, and parameters of the lattice beam and anchor bolt are as shown in Tab.2.

Table 2: Lattice beam and anchor bolt physical and mechanical parameters Elastic Modulus Unit Weight Poisson ratio Name E/MPa γ/kN/m3 υ Free section 195000 78.5 0.3 Anchor bolt anchorage section 29000 25 0.2 Lattice beam/Anti-slide 29000 25 0.2

In the anti-slide pile treatment scheme, anti-slide piles and anchor bolts are all treated as linear elastic materials. The eight node solid element (C3D8) is adopted to simulate anti-slide pile, and hard contact is set between anti-slide pile and soil. Parameters of rock and soil and boundary conditions are the same as the former. Vol. 20 [2015], Bund. 2 650

RESULTS AND DISCUSSIONS

Based on the simulated results, stability and failure mode of the natural slope and different treatment schemes are compared, which can be used as the accordance to select the treatment scheme. To this end, stress state, safety factor and potential sliding surface are incestigated.

(a) Mises stress analysis

Fig.4 is the Mises stress contour of the natural slope and slope-cutting scheme. According to the Fig.4(a), stress concentration is obvious at the foot of the natural slope, which indicates that shear failure may occur firstly in this place. In other word, when strength of the slope rock and soil is reduced because of irregular human engineering activities or rainfall, tractive landslide may occur easily. According to the Fig.4(b), after the slope cutting treatment, stress at the foot of the slope is obviously reduced, and the color of the stress contour uniformly changed from shallow to deep, which indicates that the overall stability of the slope is improved obviously.

Stress concentration

(a)Natural slope (b)After slope-cutting treatment Figure 4: Mises stress contour

(b) Stability safety factor

Fig.5 show the relationship curves between the safety factor and horizontal displacement, namely the Fs-U1 relationship curve. According to Fig.5(a), the stability safety factor of the natural slope is 0.967, which is similar to 0.95 obtained by the limit equilibrium method. In addition, because the stability safety factor is less than 1, which indicates that the natural slope is unstable and should be treated. The stability safety factors of slope-cutting treatment, lattice anchor rod treatment and anti-slide pile treatment are 1.227, 1.752 and 1.619 respectively. According to the technical code [14], recommended stability safety factor of 2nd level slope, such as the slope in the present paper, should not be less than 1.3 in the general conditions. So, only from this aspect, slope cutting treatment can not meet the requirement, and anti-slide piles treatment is the best, then the second is the lattice anchor rod treatment.

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(a) Natural slope

(b) Slope-cutting treatment

Figure 5: Fs-U1 relationship curves

Fig.6 are the shear strain increment nephograms under limit state of different treatments. According to Fig.6(a), under limit state, shear strain increment region appears obviously in the natural slope, which indicates that the potential sliding surface will be the first to form in the region. The maximum value of shear strain increment appeared at the lower part of the Vol. 20 [2015], Bund. 2 652 potential sliding surface, which indicates that failure may occur firstly at this point, and then tractive landslide is formed.

According to Fig.6(b) ~(d), if the slope was treated by slope cutting, then the landslide may occur at the top of the 2th level. For treatment schemes of lattice anchor rod and anti-slide piles, potential sliding surface develops more deeply into the slope. So anti-slide force increases and the failure mode changes to global instability. In that case, shear failure is most likely to occur at the foot of the slope. In addition, shear strain increment of soil at the back of anti-slide pile is larger, while that is smaller in front of anti-slide piles, which indicates that anti-slide pile effectively prevent the deformation of soil at the back of anti-slide pile. For the lattice anchor rod treatment scheme, slope deformation is given priority to the overall deformation, which is more conducive to the stability of the slope.

(a) Natural slope (b) Slope-cutting treatment

(c) Lattice anchor rod treatment (d) Anti-slide piles treatment Figure 6: Shear strain increment nephograms under limit state of different treatments

(d) Comprehensive comparison and analysis

According to literature [15], when the anti-slide pile length is greater than 35m, anti-slide treatment should be comprehensively compared with other schemes, thus the reasonable slope treatment scheme are established. Comprehensively considering economy and safety, for the anti-slide pile treatment, the pile excavation is bound to affect the slope stability and may cause geological disasters, such as slope landslide. Compared with the anti-slide pile treatment, disturbance of lattice anchor rod treatment is smaller, and the excavation work and dosage of concrete is less. Vol. 20 [2015], Bund. 2 653

Therefore, according to analysis results of stress state, stability safety factor and potential sliding surface, as well as comprehensive consideration of economy and safety, the most reasonable treatment should be the lattice anchor rod scheme.

CONCLUSIONS

Based on FEM simulation results and discussions above, several conclusions can be obtained, which are listed as follows. a) Stress concentration is obvious at the foot of the natural slope, so tractive landslide may occur easily. After slope cutting treatment, stress at the foot of the slope is obviously reduced, which indicates that the overall stability of the slope is improved obviously. b) Stability safety factor 0.967 of the natural slope is less than 1, which indicates that slope is unstable. The stability safety factor of slope cutting treatment can not meet requirement of technical code, while the factors of lattice anchor rod and anti-slide pile treatment can meet the requirement. The maximum factor is that of the lattice anchor rod scheme, so it is the most secure method. c) For the slope cutting treatment, the landslide may occur at the top of the 2th level. For treatment schemes of lattice anchor rod and anti-slide piles, potential sliding surface develops more deeply into the slope. Anti-slide force increases and the failure mode changes to global instability. So they are better than cutting slope treatment. d) In a word, comprehensively considering economy and safety, the most reasonable treatment should be the lattice anchor rod scheme. The results in the present paper can provide the basis for further treatment of the Yinxiang river high slope, and can provide reference for the similar slope stability evaluation and treatment.

ACKNOWLEGEMENT The authors acknowledge the support by the open foundation of State Key Laboratory of Western Architecture and Technology under Grant No. K013319. REFERENCES

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