In-Situ Direct Shear Test Research of Rock and Soil of Typical Bank Slope in Three Gorges Reservoir Area

Li Xin-zhe College of Civil Engineering, Chongqing University, Chongqing 400030, China; College of Hydraulic & Environmental Engineering, China Three Gorges University, Yichang, Hubei 443002, China; Key Laboratory of Geological Hazards in Three Gorges Reservoir Area of Ministry of Education, China Three Gorges University, Yichang, Hubei 443002, China e-mail: [email protected]

Li Jian-lin Key Laboratory of Geological Hazards in Three Gorges Reservoir Area of Ministry of Education, China Three Gorges University, Yichang, Hubei 443002,China e-mail: [email protected]

Deng Hua-feng Key Laboratory of Geological Hazards in Three Gorges Reservoir Area of Ministry of Education, China Three Gorges University, Yichang, Hubei 443002,China e-mail: [email protected]

ABSTRACT

The impoundment of the Three Gorges Dam have a certain impact on the stability of the bank slope in the reservoir area, therefore, the study on the bank slope stability has become an important topic. A basic condition for these studies is to determine the mechanical parameters of the rock and soil of the bank slope. In this paper, taking a typical soil-rock mixture bank slope as the research object, using self-developed in- situ direct shear test device， large situ direct shear tests of soil-rock mixture of slide mass and slide zone were carried out respectively. The paper intends to obtain more realistic results of rock and soil shear strength parameters of the slope in order to provide an important reference for the bank slope governance and stability evaluation. The research results show that: (1) The cohesive force of the soil-rock mixture of the slide mass is low, which is 9.7 kPa~16.7 kPa, and the internal friction angle is large, which is 25.6° ~34.0°. The shear strength of the soil-rock mixture of the slide mass decreases with elevation decreasing. （2）The cohesive force of the soil-rock mixture of the slide zone is low, which is 10.5 kPa~12.25 kPa, and the internal friction angle is small, which is 14.8°~15.8°.（3）The cohesive force difference between the slide mass and slide zone is not big, while the internal friction angle difference is big, the internal friction angle of the slide mass is much bigger than that of the slide zone.

KEYWORDS: Three Gorges Reservoir Area, bank slope, soil-rock mixture, situ direct shear test, shear strength

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Vol. 19 [2014], Bund. K 2524

INTRODUCTION

In order to solve major economic development problems such as flood protection, power generation, irrigation and water supply, a large number of various types of large-scale hydropower projects have been built in China. In addition, there are a large number of large- scale hydropower projects which are under construction or to be constructed, these projects will undoubtedly ease China's power shortage, and promote economic and social development, but the emergence of these large reservoirs might break the balance of biology, environment, geology, mechanics which formed naturally over years and thus lead to series of new geological disasters, the bank slope stability is one of outstanding problems to be solved in recent years [1].

Wu Qiong and Luo Hongming discussed the bank slope stability on theoretical analysis [2- 3]. Based on the cyclical fluctuations of the water level in the Three Gorges Reservoir, LiuXinxi and Liao Hongjian studied the adverse effects on slope stability arose by the water level fluctuation with the numerical simulation method, obtained the seepage field in the bank slope, And evaluated the slope stability[4-5]. Luo Xianqi established a landslide model test control systems which is composed of water level control system, multiple physical testing system, non-contact displacement measuring system to study deformation laws of reservoir landslide [6]. A basic condition for these studies is to determine the mechanical parameters of the rock and soil, many scholars studied the mechanical parameters of rock and soil of the slope through experiments, However, most experiments are indoor direct shear tests to field samples or remodeling sample [7-9], situ direct shear tests on mechanics parameters of rock and soil are not many [10-11]. In this paper, taking a typical soil-rock mixture bank slope as the research object, using self-developed in-situ direct shear test device large situ direct shear tests of soil-rock mixture of slide mass and slide zone were carried out respectively, more realistic results of rock and soil shear strength parameters were obtained and provide a important reference for the bank slope governance and stability evaluation.

PROJECT BACKGROUND AND TEST DESIGN

Project Background

Baishuihe landslide locates in Zigui County in the Three Gorges Reservoir Area of the Yangtze River wide valley, and extends to the Yangtze River in ladder form. The landslide mass is a loose accumulation of a large old landslide, which developed in the west wing of Zigui syncline. The outcropping strata of the landslide are thick-bedded sandstone with thin- bedded mudstones of Xiangxi group under the EC of Jurassic. it is a monoclinic forward sloping stratum, the occurrence of the stratum is 15°, and the stratum dip is 36°. The trailing edge elevation of the landslide is 410m, the leading edge elevation is 70m, and the leading edge has been submerged in water level. The length from north to south of the landslide is Vol. 19 [2014], Bund. K 2525 about 600m, the width from east to west is about 700m, the average thickness of the slide mass is about 30m, and the volume of the slide mass is about 12.6 million m3. The panorama schematic of Bai Shuihe landslide is shown as Fig.1.

Yangtze River

Bai Shuihe

Figure 1: Panorama schematic of Bai Shuihe landslide

Test design Test method The level pushing method is used in the tests, that is the acting line of the horizontal force passes through the sliding surface, the reactive force is provided by reactive device, and the shear direction is in line with the sliding direction.

Test equipment（1）Jacks with pressure gauges：Jacks are used to apply vertical pressure and horizontal force，and gauges show the size of the applied force.（2）Dial indicator：they are used to measure the horizontal displacement and the vertical displacement under the vertical pressure of samples，each sample arranged four measuring points, two of them to measure the horizontal displacement and the other two to measure the vertical displacement.（3）Magnetic bearing：they are used to erect dial indicator.（4）Shear box： it is used to make samples and composed of upper box and lower box, it is a square box with a side length of 0.5m and the its height is 0.3m(the height of upper box and lower box is 0.15m respectively)（5）Vertical pressure loading steel frame：According to site conditions，the ground anchor is used to provide the reactive force of the vertical pressure for the tests of slide mass, and the roof of the adit is used to provide the reactive force of the vertical pressure for the tests of slide zone.

Equipment installation Step 1: Installing shear box; Step 2: Installing reactive force device; Step 3: Installing vertical loading system; Step 4: Installing horizontal thrust system; Step 5: Installing measuring system. The installation image of measurement and reactive Vol. 19 [2014], Bund. K 2526 system of slide mass test is shown as Fig.2, and the panoramic image of slide mass test system is shown as Fig.3.

Figure 2: Installation image of Figure 3: Panoramic image of soil-rock measurement and reactive system of soil- mixture of slide mass test system rock mixture of slide mass test

Sample Preparation When the samples of the slide mass are prepared, firstly remove the cover layer, excavate trench around and set aside sample body in the process of excavation, then process into a sample with the size of0.5m×0.5m×0.4m , three samples need to be processed at each test point. The situ shear tests of slide zone were carried out in adits，each group test also needs three samples, and the sample size is also 0.5m×0.5m×0.4m.

Loading Control Step 1--Applying vertical pressure: Different vertical pressures were applied to the three samples of the same group with 4-5 grade. When a predetermined pressure was applied and the vertical deformation was stable, the horizontal force might be applied.

Step 2--Applying horizontal force: At the beginning the horizontal force was applied as 10% of the maximum shear load, then progressively increase the load to the maximum shear load by several grades. The force is applied once every minute and the deformation was measured before and after the load applied. When shear failure was approaching, closely and pressure changes and corresponding horizontal deformations were monitored and read closely. Throughout the shearing process, the vertical pressure should remain constant. Samples after shear failure are shown as Figure 4. According to the above steps direct shear tests of the other two samples were made under different vertical pressures. After the completion of each sample test, some soil samples were taken back to the laboratory to determine density and natural moisture content. Vol. 19 [2014], Bund. K 2527

（a）Soil - rock mixture of slide mass （b）Soil - rock mixture of slide zone

Figure 4: Samples after shear failures

RESULTS AND ANALYSIS

Test results and analysis of Soil-rock mixture of slide mass In accordance with the elevation three test points (ZJ1, ZJ2, ZJ3) were chosen from low to high, correspondingly three groups of situ direct shear tests were carried out to the soil-rock mixture of slide mass and there are three samples in each group of test. Curves between shear stress and shear displacement under different vertical pressures are shown as Fig.5. It is shown that three groups of samples have the same plastic deformation characteristics, that is to say that the stress increased rapidly in early stage, while increased slowly in late stage, curve slopes (shear stiffness) were from large to small and the stress change is a continuous process, there are peak intensities which increased with the increasing of vertical stresses. Shear strength values (i.e. peak intensity of each curve) under different vertical stress of three groups of test are summarized as in Table 2.

14 12 10 8 16Kpa 24Kpa 6 32Kpa 4

Shear stress(KPa) 2 0 -20 0 20 40 60 80 100 Shear displacement(mm)

(a) Group 1 ( ZJ1 ) Vol. 19 [2014], Bund. K 2528

10 pa) k 8 12.8Kpa 6 19.2Kpa 32.0Kpa 4

Shear stress( Shear 2 0 -20 0 20 40 60 80 100 120 Shear displacement(mm)

(b) Group 2 ( ZJ2 )

14 12 10 8 16Kpa 24Kpa 6 32Kpa 4

Shear stress(KPa) 2 0 -20 0 20 40 60 80 100 Shear displacement(mm)

Figure 5: Curves between shear stress and shear displacement under different vertical pressures of soil -rock mixture of slide mass

Table 1: Statistics of shear strength of soil -rock mixture of slide mass

Group number 1（ZJ1） 2（ZJ2） 3（ZJ3）

Test position Lower part of landslide Middle part of landslide Upper part of landslide

Sample ZJ1.1 ZJ1.2 ZJ1.3 ZJ2.1 ZJ2.2 ZJ2.3 ZJ3.1 ZJ3.2 ZJ3.3

Vertical pressure 16 24 32 12.8 19.2 32 16 24 32 （kPa）

Peak value of shear 17.6 20.8 25.28 22.8 25.5 33.6 8.2 31.5 39 stress（kPa）

Curves between shear strength and vertical pressure are shown as Fig.6. The inclination angle of the line is the internal friction (Φ) and the intercept on the ordinate is the cohesive force (C), they are summarized in Table 2. The results show that the cohesive force of the soil- Vol. 19 [2014], Bund. K 2529 rock mixture of the slide mass is low, which is 9.7 kPa~16.7 kPa, and the internal friction angle is large, which is 25.6°~34.0°. The shear strength of the soil-rock mixture of the slide mass decreases with elevation decreasing, this is due to that the mixture at lower elevation is influenced much by rainfall infiltration which lead to a deterioration of the mechanical properties of rock and soil mixture.

30 y = 0.48x + 9.7067 25 R 2= 0.9908

20 k Pa) 15 10 5 Shear stress ( 0 0 5 10 15 20 25 30 35 Vertical pressure (kPa) （a）Group 1 ( ZJ1 )

40 y = 0.5725x + 15.086 35 R 2 = 0.9918 30 25 k Pa) 20 15 10 5 Shear stress ( 0 0 10 20 30 40 Vertical pressure (kPa) (b) Group 2 ( ZJ2 )

45 40 y = 0.675x + 16.7 R2 = 0.952 35 30

k Pa) 25 20 15 10 5 Shear stress ( 0 0 5 10 15 20 25 30 35 Vertical pressure (kPa)

Figure 6: Curves between shear strength and vertical pressure of soil -rock mixture of slide mass

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Table 2: Shear strength test results of soil-rock mixture of slide mass

Group number 1（ZJ1） 2（ZJ2） 3（ZJ3）

Test position Lower part of landslide Middle part of landslide Upper part of landslide

Shear strength C(kpa) Φ(°) C(kpa) Φ(°) C(kpa) Φ(°) parameters

Test value 9.7 25.6 15.1 29.8 16.7 34.0

Test results and analysis of Soil-rock mixture of slide zone Two groups of situ direct shear tests were carried out to the rock and soil of slide zone and there are three samples in each group of test. Curves between shear stress and shear displacement under different vertical pressures are shown as Fig.7. It can be seen from the figure that two groups of samples have the same plastic deformation characteristics, the stress increases rapidly in early stage, while increases slowly in late stage, curve slopes (shear stiffness) are from large to small and the stress change is a continuous process, there are peak intensities but not very obvious and the peak intensities increase with the increasing of vertical stresses. Shear strength values (i.e. peak intensity of each curve) under different vertical stress of three groups of test are summarized as in Table 3. The shear stress come to the plastic yielding stage after reached the maximum, and in this stage no significant stress reduction occurs, this may be due to that the high-containing rock leads to the strong occlusion effect of soil-rock particles of the shear plane and long process of structural adjustment.

90.0 80.0 250kpa 70.0

_ 60.0 50.0

k Pa) _ 150kpa _ 40.0 _ _ 30.0 50kpa _ 20.0 10.0

Shear stress ( 0.0 0 5 10 15 20 25 30 Shear displacement (mm)

（a）Group 1

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90.0 80.0 250kpa 70.0

_ 60.0 _ 50.0 150kpa k Pa) _ 40.0 _ _ 30.0 50kpa _ 20.0 10.0 Shear stress ( 0.0 0 5 10 15 20 25 30 Shear displacement (mm)

（b）Group 2 Figure 7: Curves between shear stress and shear displacement under different vertical pressures of soil -rock mixture of landslide zone

Table 3: Statistics of shear strength of soil-rock mixture of slide zone

Group number 1 2

Sample JQ1 JQ2 JQ3 JQ4 JQ5 JQ6

Vertical pressure（kPa） 50 150 250 50 150 250

Peak value of shear stress 29 45 82 27.5 47 84.0 （kPa）

Curves between shear strength and vertical pressure are shown as Fig.6. Shear strength parameters are summarized in Table 4. The test results show that the cohesive force of the soil- rock mixture of the slide zone is low, which is 10.5 kPa~12.25 kPa, and the internal friction angle is small, which is 14.8°~15.8°. The comparison of results of the slide mass and slide zone shows that the cohesive force difference between the slide mass and slide zone is not big, while the internal friction angle difference between them is big, and the internal friction angle of the slide mass is much bigger than that of the slide zone.

90 80 2 70 y = 0.265x + 12.25 R = 0.950 60

k Pa) 50 40 30 20 10 Shear stress ( 0 0 50 100 150 200 250 Vertical pressure (kPa) （a）Group 1 Vol. 19 [2014], Bund. K 2532

90 80 2 70 y = 0.282x + 10.45 R = 0.969

60 k Pa) 50 40 30 20 Shear stress ( 10 0 0 50 100 150 200 250 Vertical pressure (kPa) （a）Group 2

Figure 8: Curves between shear strength and vertical pressure of soil-rock mixture of slide zone

Table 4: Shear strength test results of soil-rock mixture of slide zone Group number 1 2 Shear strength C(kPa) Φ(°) C(kPa) Φ（°） parameters Test value 12.25 14.8 10.5 15.8

Due to shallow slide of the landslide is mainly influenced by mechanical parameters of soil-rock mixture of the slide mass, the stability calculation should use the results in Table 2, the overall slide of the landslide is mainly influenced by mechanical parameters of soil-rock mixture of the slide zone, the stability of the overall slide calculation should use the results in Table 4.

CONCLUSION

According to in situ direct shear tests of soil-rock mixture of slide mass and slide zone respectively, the characteristics of rock and soil shear strength parameters of typical bank slope in Three Gorges Reservoir area were obtained as below:

(1) The cohesive force of the soil-rock mixture of the slide mass is low, which is 9.7 kPa~16.7 kPa, and the internal friction angle is large, which is 25.6°~34.0°. The shear strengths of the soil-rock mixture of the slide mass decrease with decreasing elevation.

（2）The cohesive force of the soil-rock mixture of the slide zone is low, which is 10.5 kPa~12.25 kPa, and the internal friction angle is small, which is 14.8°~15.8°. Vol. 19 [2014], Bund. K 2533

（3）The cohesive force difference between the slide mass and slide zone is not big, while the internal friction angle difference is big, the internal friction angle of the slide mass is much bigger than that of the slide zone.

（4）Due to shallow slide of the landslide is mainly influenced by mechanical parameters of soil- rock mixture of the slide mass, the stability calculation should use the results in Table 2, the overall slide of the landslide is mainly influenced by mechanical parameters of soil-rock mixture of the slide zone, the stability of the overall slide calculation should use the results in Table 4.

ACKNOWLEDGEMENTS

The authors acknowledge the financial support by the Key basic research and development program of China（973 pre planned special）, Project No. 2012CB426502.

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