Two important features of the high slopes at Gushui Hydropower Station are layered accumulations (rock-soil aggregate) and multilevel toppling failures of plate rock masses; the Gendakan slope is selected for case study in this paper. Geological processes of the layered accumulation of rock and soil particles are carried out by the movement of water flow; the main reasons for the toppling failure of plate rock masses are the increasing weight of the upper rock-soil aggregate and mountain erosion by river water. Indoor triaxial compression test results show that, the cohesion and friction angle of the rock-soil aggregate decreased with the increasing water content; the cohesion and the friction angle for natural rock-soil aggregate are 57.7 kPa and 31.3° and 26.1 kPa and 29.1° for saturated rock-soil aggregate, respectively. The deformation and failure mechanism of the rock-soil aggregate slope is a progressive process, and local landslides will occur step by step. Three-dimensional limit equilibrium analysis results show that the minimum safety factor of Gendakan slope is 0.953 when the rock-soil aggregate is saturated, and small scale of landslide will happen at the lower slope.
Rock-soil aggregate is widely distributed in China and worldwide [
The existence of large rock-soil aggregate slopes has a tremendous impact on projects, such as the stability of the slope for a reservoir storing water. Landslides, debris flows and other disasters are often caused by rock-soil aggregate slope under rainfall and earthquake conditions [
Rock-soil aggregate is generally considered a highly complex discontinuous material [
A large volume rock-soil aggregate slope (Gendakan slope) at the Gushui Hydropower Station is selected for case study in this paper. The geological evolution process of the rock-soil aggregate slope is analysed. Combined with a field geological survey and experimental tests based on the geological process analysis of the rock-soil aggregate slope, the layered characteristics of rock-soil aggregate and the toppling failure of plate rock masses are presented. The physical characteristics and particle size distribution of the rock-soil aggregate at the Gushui Hydropower Station are analysed. Field tests of shear strength and the relationship of cohesion, friction, and water content are presented. Finally, the deformation and failure mechanism of the rock-soil aggregate slope is analysed, and a three-dimensional limit equilibrium method is adopted to compute the safety factor of slope.
The Gushui Hydropower Station is located at the upper stream of the Lancang River, Xingzheng Village, Foshan Town, Northwest of Deqin City, Yunnan Province. Figure
Site location of the Gushui Hydropower Station.
Different types of accumulations are distributed in the project site region, including alluvium accumulation, talus accumulation, collapse accumulation, and outwash accumulation. The main type is outwash accumulation because of the rock and soil particles carried by glacier melt water. The accumulation can be divided into 4 types according to the deposit process: fluvial deposits, flood deposits, collapse or residual deposits, and outwash deposits. Fluvial deposits: Figure Flood deposits: the flood deposits located at the export of a valley or gully are distributed like a fan because of the rock and soil particles carried by the flood after the late Pleistocene. The rock-soil aggregate is composed of gravel, sand, and rock blocks. The scale of flood deposits is small, and the particle sorting is general. Collapse or residual deposits: the collapse or residual deposits are distributed at the foothills of the Lancang River. The main compositions are the deposits of rockfall or the toppling failure of slopes. Outwash deposits: outwash deposits are the main type of the rock-soil aggregate slope in this dam site region. The rock and soil particles carried by the glacier melt water have left many traces that record the outwash deposit process. Below an elevation of 3,000 m, the slope is mostly covered by rock-soil aggregate.
Typical fluvial deposit terrace of the Lancang River in the Gushui Hydropower Station region.
The main rock types are sandstone, mudstone, limestone, and basalt. The rock mass is strong and hard. The typical characteristic is that the rock layers are close to vertical, the bedrock is mostly plate rock masses, and toppling failure occurs in the plate rock masses. Figure
Rock masses in the Gushui Hydropower Station region: (a) and (b) are the exposed plate sandstone.
At the left bank of the project site region, the strong weathering depth of rock masses is less than 30 m, the horizontal depth of the weak weathering is approximately 100 m–200 m, and vertical depth is also approximately 100 m–200 m. At the right bank of the dam site region, the strong weathering depth of rock masses is less than 50 m, the horizontal depth of weak weathering is approximately 80 m–280 m, and the vertical depth is approximately 70 m–250 m. The weathering degree of rock masses is asymmetric between the left and right banks. The unloading depth of the rock masses at the left bank is approximately 30 m–50 m, and the right bank is approximately 50 m–100 m.
At the Gushui Hydropower Station region, numerous rock-soil aggregate slopes are distributed at both sides of the river valley. Figure
Rock-soil aggregate slope distribution in the dam site region of the Gushui Hydropower Station.
As shown in Figure
The Gendakan slope at the Gushui Hydropower Station: (a) photograph of the Gendakan slope; (b) three-dimensional visualization.
In the Gushui Hydropower Station region, the outwash accumulation is well developed below 4,000 m, especially below the 3,000 m. The main reason for the formation of the rock-soil aggregate is the melting of the glaciers, which generates surface water. An enormous amount of rock and soil particles are carried by the outwash, and they flow downward and are deposited. The rock-soil aggregate is mostly composed of rock block, broken stone, and clay or sandy soil. Because the geological history of the rock-soil aggregate formation is long, the slope evolution can be divided into many stages, and the layered characteristic of outwash accumulation is obvious. Figure
Layered rock-soil aggregate in the PD 33: (a) and (c) are the small size particle rock-soil aggregates; (b) and (d) are the large size particle rock-soil aggregates.
As shown in Figure
Combined with a field geological survey and an experimental test of the rock-soil aggregate, other physical characteristics of the rock-soil aggregate are as follows. The rock-soil aggregate can be simplified as a two-phase structure: soft clay and hard rock block. Soft clay is the main component, and hard rock block is the filling material. The range of the particle size is large. The different particle sizes of rock block are distributed in the soft clay randomly, exhibiting inhomogeneity and randomness. In the deposit process, the particle sizes are influenced by the terrain and the carrying capacity of the water flow. The rock block content will be very large in some regions, but in other regions the rock block content will be small.
In the Gushui Hydropower Station region, the bedrock is most of the plate rock masses, and the rock layers are thick. There are mainly plate sandstone, plate limestone, and plate basalt, and the rock layers are nearly vertical. In the deep valley region, the plate rock masses are influenced by the weight of the upper rock-soil aggregate and gravity itself. The failure occurs in the rock block, and the toppling failure occurs for the plate rock masses along the slope direction. The toppling failure of plate rock masses only occurs at a certain depth, not in the deepest parts of the slope. The erosion of the river valley and the increase of the upper rock-soil aggregate weight and thickness of the plate rock layer are the main reasons for the toppling failure of plate rock masses [
Toppling failure of plate rock masses in the PD 13: (a) the bending fracture surface in the horizontal direction and (b) the bending fracture surface in the vertical direction.
Based on the field geological survey of plate rock masses, two main physical characteristics of the toppling failure of plate rock masses are as follows. Rock mass structure characteristic: a longitudinal thin-bedded structure along the slope direction and an alternate layer of soft and hard rock mass; Spatial distribution characteristic: in the vertical section, a discontinuity surface exists for rock layers, and the bending phenomenon of plate rock masses is apparent; in the horizontal section, the toppling failure of the plate rock masses is distributed at different spacing parallel to each other, and the shear dislocation phenomenon is common.
The toppling failure of plate rock masses can be divided into different stages, and the geological history is long. The toppling failure can be divided into two types: strong toppling and weak toppling. The classification of toppling failure is according to the angle of the toppling rock layer and the normal rock layer and the physical chrematistics of the failure surface. Table
Classification of the toppling failure of plate rock masses.
Type | Dip of rock layer | Geological characteristics | Position |
---|---|---|---|
Strong toppling | Angle of toppling rock layer and normal rock layer is larger than 60°. | A clear breakage phenomenon in rock, the continuity of fracture surface is good and extends in a long length, and each surface is distribution of parallel strips in different distances. The crack in fracture zone is mainly opened, no filling of rock block or debris. The phenomenon of shear dislocation is obvious, and several sets of joints are generated by the toppling effect. | (1) Upper part of slope |
Weak toppling | Angle of toppling rock layer and normal rock layer is less than 60°. | The strata dip is abnormal, but the breakage phenomenon is not obvious; the distribution of rock mass is multilayer and continuous. Most of them maintain the organization and structure of original rock mass, but the shear strength is decreased in the local region. The crack in fracture zone is partly opened, and there is a filling of calcite crystals or calcarenite. | (1) Lower part of slope |
As shown in Table
A toppling failure example in the Baqian slope illustrates the physical characteristics of strong toppling and weak topping. The dip of the Baqian slope is approximately 20°–40°. The toppling failure occurs in the plate sandstone, limestone, and mudstone. The normal rock layer is oriented in the dip direction of 325°–335° and the dip of 75°–90°. The dip of the weak topping rock layer is 40°–50°, and the strong toppling is 20°–35°. The horizontal depth of strong toppling is approximately 29.1 m–72.5 m, and weak toppling is approximately 90.7 m–111.2 m. The fracture and bending of the rock block and the dislocation of the rock layers result in the toppling of plate rock masses.
Based on the above geological analysis of rock-soil aggregate and toppling failure, the key characteristic of rock-soil aggregate is that it is layered, and for the toppling failure of plate rock masses, there are several fracture surfaces. Figure
Geological evolution process of slope in the Gushui Hydropower Station region: (a) ancient landscape and (b) current landscape.
As shown in Figure
For the plate rock masses in the long geological evolution history, first, the deformation and strength parameters of the rock masses are decreased by the weathering and unloading effects. Second, the water flow will also impact the plate rock masses. The failure of the rock masses will occur, but these effects are not the main reasons for the toppling failure of the plate rock masses. The key reasons for the toppling failure of plate rock masses are the increasing weight of the upper rock-soil aggregate and the mountain erosion by river water. Also, the stress in the shallow plate rock masses is increased. Combined with the increasing stress and decreasing rock mass mechanics parameters, toppling failure occurs. The fracture of rock block and the dislocation of rock layers are also influenced by several geological stages. Because the weight of the upper rock-soil aggregate increases, the mountain is eroded by river water, and the weathering and unloading of rock masses are gradual processes. The result is several distinct stages in the toppling failure trace.
For the slope stability problem and its impact on the Gushui Hydropower Station, the likelihood of landslide in the rock-soil aggregate is greater than in the toppling failure plate rock masses, and the shallow landslide risk of the rock-soil aggregate is greater than the deep landslide along the joint surface. Table
An example of shear strength test results for plate basalt and joint surface.
Type | No. | Physical characteristic | Peak value | Residual value | ||
---|---|---|---|---|---|---|
Frication angle (°) | Cohesion (MPa) | Frication angle (°) | Cohesion (MPa) | |||
Basalt | R-1 | Weak weathering | 51.12 | 2.20 | 43.62 | 1.61 |
R-2 | Weak weathering | 51.78 | 2.25 | 45.00 | 1.68 | |
| ||||||
Joint | J-1 | Rigid | 36.87 | 0.50 | 35.26 | 0.30 |
J-2 | Debris silted | 22.29 | 0.21 | 21.31 | 0.17 |
As shown in Table
In this section, the Gendakan slope is selected for the geotechnical characteristics analysis of rock-soil aggregate. Forty-seven test pits, 13 vertical boreholes and 7 horizontal geological tunnels were made for the geological survey work, and 50 indoor triaxial compression experiments (the test sample is a cylinder, diameter is 300 mm, and height is 600 mm) were performed to determine the mechanical characteristics of the rock-soil aggregate. First, the physical characteristics of the rock-soil aggregate are analysed based on the field survey and experimental test results; second, the particle size distribution characteristics are analysed; finally, the shear strength of the rock-soil aggregate influenced by the water content is analysed based on the experimental test results.
The rock-soil aggregate is composed of several minerals: quartz and plagioclase are the main components; dolomite, calcite, and sericite are the secondary components; and chlorite and kaolinite are the minor components. The density of the rock-soil aggregate is approximately 1.95–2.21 g/cm3. Figure
Water content of rock-soil aggregate varied with the horizontal depth.
As shown in Figure
A field particle screening test was conducted for the particle size distribution of rock-soil aggregate in the vertical depth of 5–10 m at the Gendakan slope for 10 groups. Figure
Statistical results for particle size distribution of rock-soil aggregate at the Gendakan slope: (a) field test results and (b) indoor test results.
The field test and indoor experiment results show that the rock-soil aggregate is composed of clay breccia, fine-grained soil, and rock block. The particle size distribution characteristics of the rock-soil aggregate are as follows. The rock block content of a particle diameter less than 5 mm is approximately 32.46%, and the rock block content of a particle diameter greater than 5 mm is approximately 67.54%. The rock block content of a particle diameter greater than 60 mm is approximately 7.34%. The soil content of a particle diameter less than 0.075 mm is approximately 15.29%, and the soil content of a particle diameter less than 0.005 mm is approximately 9.7%.
The mechanical characteristics of the rock-soil aggregate are sensitivity to the water content and shear strength decreasing with the increasing water content. The stability of the rock-soil aggregate slope is affected under rainfall or water conditions. For the rock-soil aggregate slope at the Gushui Hydropower Station, parts of the rock-soil aggregate slope will be under the water level when the reservoir is working, and the shear strength of the rock-soil aggregate will decrease and impact the slope stability. The rock-soil aggregate impounded for an extended period of time will cause the shear strength to decrease further. Furthermore, the shear strength of rock-soil aggregate under heavy rainfall or high water level conditions is influenced by the microstructure of the rock-soil aggregate, the content of rock block, the particle size distribution characteristics, and other factors. Therefore, the shear strength of rock-soil aggregate under water is highly complex. Table
Some cohesion and friction angle values for rock-soil aggregate in China [
Location (position, province) | Cohesion (kPa) | Friction angle (°) | Material characteristics |
---|---|---|---|
Xiaowan, Yunnan | 50.0 | 36.0 | Mixture of rock block, boulders, and gravel soil; rock content is approximately 32%; rock diameter is 30–350 mm. |
Hutiao Valley, Yunnan | 12.6 | 36.5 | Mixture of broken stone and rock block; rock content is approximately 46%; rock diameter is 0.1–1.0 m. |
Lancang River, Yunnan | 48.0 | 35.0 | Mixture of broken stone, rock block, boulders, and silt; rock content is 20%–35%; rock diameter is 0.3–5.0 m, dense structure. |
Qingshui River (no. 1), Yunnan | 35.0 | 31.0 | Clay cementation of pebble and basalt block. |
Qingshui River (no. 2), Yunnan | 65.0 | 30.0 | Calcarenite and clay filling of broken stone and sandstone block. |
Unknown slope (no. 2), Yunnan | 60.0 | 35.0 | Mixture of slate block, limestone block, and clay; rock diameter is 30–80 mm, loose structure. |
Liangjiaren, Yunnan | 40.0 | 29.0 | An ice-water deposit, mixture of broken stone, boulders, and silt; rock content is 25%–35%. |
18.0 | 19.0 | An ice-water deposit, mixture of broken stone, rock block, and silt; rock content is 25%–35%. | |
Qianjiangping slope, Hubei | 23.0 | 20.0 | Mixture of broken stone, pebble, and clay; the maximum of rock block diameter is 1.5 m; pebble diameter is 30–100 mm. |
Baiyiyan, Hubei | 47.9 | 33.8 | Calcarenite and sandy soil filling of broken stone. |
55.5 | 35.4 | Mixture of limestone block, sandstone block, and clay. | |
Huangshi, Hubei | 50.0 | 40.0 | Mixture of limestone block, sandstone block, and clay. |
30.0 | 32.8 | Mixture of breccia block, limestone block, and clay. | |
Yunyang, Chongqing | 26.3 | 13.3 | Mixture of sandstone block, broken pebble, and silty clay; rock content is approximately 20%; rock diameter is 2–20 mm. |
Fengjie, Chongqing | 42.6 | 26.4 | Mixture of broken stone and clay; rock content is approximately 15%; rock diameter is 10–20 mm. |
94.6 | 28.8 | Mixture of breccia block, broken stone, and clay; rock content is approximately 55%; rock diameter is 30–50 mm. | |
Anle, Chongqing | 55.0 | 32.0 | Mixture of broken stone, sandstone block, and sandy clay; rock diameter is 100–800 mm, talus type. |
38.0 | 25.0 | Mixture of pebble and sandy soil; rock diameter is 20–80 mm, alluvium type. | |
Three Gorges reservoir, Chongqing | 25.0 | 30.0 | Mixture of rock block and silty clay. |
Dashiban, Sichuan | 25.0 | 13.2 | Mixture of rock block and clay; rock content is 25%–35%. |
Jinsha River, Sichuan | 48.0 | 39.0 | Mixture of limestone block, pebble, and clay; the maximum rock diameter is approximately 4 m. |
Feishuiya, Sichuan | 60.0 | 38.6 | Mixture of limestone block, sandstone block, and clay. |
Xiaoliang Mountain, Sichuan | 65.0 | 31.4 | Mixture of pebble and sandy clay. |
Dahaizi, Sichuan | 36.0 | 33.0 | Mixture of broken stone and sandy soil. |
Baishuizhai, Sichuan | 37.2 | 23.1 | Mixture of limestone block, phyllite block, and clay. |
46.5 | 28.0 | Mixture of limestone block and sandy soil, dense structure. | |
Jinsha reservoir, Sichuan | 30.0 | 17.0 | Mixture of broken stone, sandstone block, and silty clay. |
Yalong River, Sichuan | 60.0 | 35.0 | Mixture of broken stone, sandstone block, and clay; rock content is approximately 30%; rock diameter is 400–1000 mm. |
Shiwan, Sichuan | 10.0 | 25.4 | Mixture of pebble, granite block, and clay; rock diameter is 40–150 mm. |
Unknown slope, Jiangxi | 10.0 | 25.0 | Mixture of rock block and clay. |
As shown in Table
Statistical results for shear strength of rock-soil aggregate in China: (a) cohesion and (b) friction angle.
As shown in Figure
Fifty triaxial compression experimental tests were performed to determine the shear strength of the rock-soil aggregate. Some errors exist for the test results, so only 28 experimental test results of the shear strength of rock-soil aggregate are analysed here. The shear strength of rock-soil aggregate is described by two mechanical parameters, cohesion and friction angle. These experimental tests are divided into two conditions, unsaturated sample and saturated sample. The water content for unsaturated samples is approximately 9.0–13.0%, and the water content for saturated samples is approximately 13.0–18.4%. The degree or saturation for unsaturated samples is approximately 70–80% and 95–98% for saturated samples. Figure
Shear strength test results of rock-soil aggregate under different water content conditions: (a) cohesion and (b) friction angle.
As shown in Figure
The relationship of cohesion, friction angle, and water content of rock-soil aggregate at the Gendakan slope can be described by a fitting equation as follows:
The parameters for the fitting equation are as follows. Cohesion in Figure Friction angle in Figure
The stability of the rock-soil aggregate slope is mainly controlled by the bottom layer of rock-soil aggregate, which has a contact surface with bedrock. The water content of the bottom layer of rock-soil aggregate is approximately 8%–10%, so a water content of 9% is selected for natural rock-soil aggregate, and 13% is selected for the saturated rock-soil aggregate under heavy rainfall conditions or behind water level. Table
Cohesion and friction angle values of rock-soil aggregate under natural and saturated conditions for the slope stability analysis.
Condition | Water content (%) | Shear strength | |
---|---|---|---|
Cohesion (kPa) | Friction angle (°) | ||
Unsaturated (natural condition) | 9 | 57.7 | 31.3 |
Saturated (heavy rainfall) | 13 | 26.1 | 29.1 |
As shown in Table
The basic geological characteristics of Gendakan slope are shown in Section
Engineering geological condition of Gendakan slope in plane.
As shown in Figure
(a) Engineering geological condition of Gendakan slope in Section
As shown in Figure
Figure
An arc-shaped landslide of rock-soil aggregate slope under rainfall condition in the Gushui Hydropower Station region.
As shown in Figure
The three types of slope stability problems for the Gendakan slope, as shown in Figure
Two conditions for rock-soil aggregate slope are considered: natural slope and heavy rainfall. Figure
Computed results for the safety factor of Gendakan slope under different conditions.
Conditions | Natural slope | Heavy rainfall |
---|---|---|
Global slope stability | 1.435 | 1.215 |
Local slope stability case 1 | 1.368 | 1.136 |
Local slope stability case 2 | 1.159 | 0.953 |
Three-dimensional limit equilibrium computer model under different conditions: (a) global slope stability; (b) local slope stability case 2; (c) local slope stability case 1, and (d) three-dimensional mesh for (c).
Sensitivity analysis results of shear strength parameters and safety factor.
As shown in Figure
The stability problem of the rock-soil aggregate slope is the key issue for the safe construction and operation of the Gushui Hydropower Station. The stability of the rock-soil aggregate slope is poor. When some rock-soil aggregate slopes are under water when the reservoir is operational, the stability of the rock-soil aggregate slope will decrease, and a landslide will occur. A very large landslide volume of rock-soil aggregate will impact the safety of the rockfill dam. From the upper stability analysis of the rock-soil aggregate slope, the analysis results show that the landslide probability of whole rock-soil aggregate is small, and the failure of the rock-soil aggregate slope is a progressive process. The local and small volume landslides of rock-soil aggregate impact on the rockfill dam are small. Excavation and support methods for rock-soil aggregate slopes should be performed to ensure the slope stability, but the investment needed for the project will be large.
In this paper, the Gendakan slope is selected as a case study example for geotechnical characteristics and stability analysis of rock-soil aggregate slope. The glaciers are melting and generating water; large amounts of rock and soil particles at the slope surface are carried by the movement of this water, and they migrated to the lower parts of the slope. The slope evolution can be divided into many stages, so the layered characteristics of the rock-soil aggregate are obvious. The key reasons for the toppling failure of the plate rock masses are the increasing weight of the upper rock-soil aggregate and the mountain erosion by river water. Also, the stress in the shallow plate rock masses is increased. Combined with the increasing stress and decreasing rock mass mechanical parameters, the toppling failure will occur.
The shear strength of the rock-soil aggregate is influenced by the water content and the particle size distribution characteristics. The statistical results for the 30 sets of shear strength of the rock-soil aggregate show that most cohesion values are in the range of 20–60 kPa, and most friction angles are in the range of 24°–36°. The experimental test results show that the cohesion and friction angle of the rock-soil aggregate are decreased by increasing water content. Based on the fitting equation of shear strength parameters, the cohesion and the friction angle for the natural rock-soil aggregate (water content is 9%) are 57.7 kPa and 31.3°, respectively, and for the saturated rock-soil aggregate under heavy rainfall conditions (water content is 13%), are 26.1 kPa and 29.1°, respectively. Combined with engineering geological survey and the mechanical characteristics of rock-soil aggregate, there may be two landslide types for the Gendakan slope: whole slope landslides along the bottom rock-soil aggregate layer and local arc-shaped landslides at the lower slope. The local landslide at the lower rock-soil aggregate slope will occur under heavy rainfall conditions.
This paper was supported by the Key Deployment Project of Chinese Academy of Sciences (no. KZZD-EW-05-01), the National Natural Science Foundation of China (no. 41102194 and no. 51209156), the Science Foundation for Excellent Youth Scholars of Sichuan University (no. 2013SCU04A07), and the Opening Fund of State Key Laboratory of Geohazard Prevention and Geoenvironment Protection (Chengdu University of Technology) (no. SKLGP2013 K015). The authors are thankful for the help of Dr. Huai-kun Sun in the field tests.