Stability and Failure Mechanism Analyses of the Zhenggang Landslide in Southwestern China

+e Zhenggang landslide is an ancient complex landslide located at southeastern Tibetan Plateau, China. Due to intensive rainfalls in 2008 and heavy snowfalls in 2009, the Zhenggang landslide exhibited a high probability of reactivation once again. In this study, geological structure, matter features, and macrodeformations of the Zhenggang landslide (including Zone I and Zone II) were investigated for uncovering its formation mechanism and evolution tendency first, and then the stability and failure mechanism analyses of the Zhenggang landslide were conducted in detail by a combined limit equilibrium and finite element analysis method. Results of geological investigations indicate that the Zhenggang landslide has undergone sliding several times and is in a metastable state now. +e distribution of the activity of the landslide is a retrogressive landslide in Zone I but an advancing landslide in Zone II. Such conclusions are further proved by the numerical stability and failure analyses.


Introduction
Uplift of tectonically active mountain belts, such as Himalayas (also known as Tibetan Plateau in China), Southern Alps of New Zealand, and Andes, generates high topographic relief and incised river gorges, resulting in not only abundant potential hydroelectric resources but also sufficient gravitational potential for landslides to occur [1].Landslides thus have been one of the most destructive nature hazards, especially large-scale landslides in mountainous areas [2][3][4].Causes of landslides in mountainous areas indicate that the occurrence of landslides should be a result of complex hydromechanical process related to geomaterial properties, hydrogeological conditions, earthquakes, and rainfalls [5][6][7][8][9][10][11].However, due to the frequency and adequacy of rainstorms, rainfall-induced landslides are one of the major geological hazards at present [12,13], especially when landslides occur in coarse-grained soils [14][15][16].Some studies pointed out that intense short-duration rainfalls always cause surface erosions or shallow landslides in a homogeneous slope, while preexisting surface cracks or weak interlayered clays are much more easily triggering a deep-seated landslide failure during prolonged rainfalls [17].Such difference is chiefly because potential sliding surfaces related to preexisting cracks on ground surface are prone to concentrated water infiltration, leading to positive water pressure overlying the interlayered clays or bedrock interfaces during or after rainfalls [18][19][20].At the same time, the water that exists in the landslide continuously softens the effective strength of the soil, resulting in large deformation and long-distance rapid movement.
e Zhenggang landslide (E98 °55′-98 °90′, N28 °30′-28 °40′) is a huge ancient landslide located at the right bank of the downstream of the Gushui hydropower station in the southeastern Tibetan Plateau of China (Figure 1) and has a serious impact on the safety of diversion tunnels and flood discharge tunnels of the hydropower station.Unfortunately, due to the intensive rainfalls in October 2008 and the heavy snowfalls in February 2009, the Zhenggang landslide exhibited large deformations along an interlayered clay once again.e instability analysis of the landslide is thus highly urgent to dam construction.In this study, the formation mechanism and evolution tendency of the Zhenggang landslide will be analyzed rst.
en the stability and deformation features of the Zhenggang landslide will be studied by a combined limit equilibrium and nite element analysis method.Finally, a comprehensive treatment scheme will be discussed.

Regional Setting of the Zhenggang Landslide
2.1.Regional Topographic and Geomorphic Conditions.Due to the uplift of the Tibetan Plateau caused by India-Asia collision, the regional geological structures of the Zhenggang area are subjected to a strong extrusion activity.ree parallel rivers are an outstanding representative reminder of the major stages and incidents in the evolution history of this zone, as shown in Figure 1. e regional geomorphic attributes near the Zhenggang landslide are high mountains and deep valleys, with the north side higher than the south.
e main direction of mountains is NNW and nearly SN. e Lancang River ows from north to south.e shape of incised river valleys turns on a "V" (Figure 2).e natural slope of both sides of the river is 20 °-45 °.River terraces are developed.Figure 3(a) shows that the external boundary of the Zhenggang landslide shows a tongue shape on a plane, and the crown of the whole landslide takes on a shape of a round-backed armchair.
ree main gullies: Upstream gully, Zhenggang gully, and Yagong gully scatter on the ground surface.e Zhenggang gully su ered from the most intensive gully erosion, leading to the deepest terrain incision depth.So the whole landslide in geomorphologic structure can be divided into two subdomains: Zone I and Zone II.Geological survey and eld reconnaissance indicate   Advances in Civil Engineering contact relationships are given priority to tectonic faults in this area.Results of borehole show that the sliding surface of the landslide developed mainly along an interlayered clay.It is inward sunken overall but slightly doming at some places inside.e shape of the whole sliding surface approximately takes on a spoon shape, as shown as in Figure 4(b).e occurrence of the sliding surface is N30 °-50 °W, NE∠40 °-60 °at the top part, N30 °-40 °W, NE∠20 °-40 °at the middle part, and N30 °-40 °W, NE∠10 °-20 °at the toe part.e results of the borehole also indicated that the Zhenggang landslide formed relatively early and had experienced several times of large sliding in the past.erefore, the landslide shall be a multistage complex landslide.

Engineering Geology and Hydrogeological Conditions
Figure 5 indicates that there are mainly three layers of materials from top to bottom, including landslide deposit, interlayered clay (henceforth referred to as slip zone), and bedrocks.e landslide deposit mainly consists of Quaternary sediments, including diluvium layer of Quaternary System (Q dl ), glacio uvial deposit of Holocene Series (Q fgl ), and landslide deposit (Q del ).Resluts from borehole exploration (all of them exposed the layer of the landslide deposit) show that the thickness of the landslide deposit in Zone I is about

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Lantsang River  Advances in Civil Engineering 20-30 m below the elevation of 2300 m (e.g., ZK44), about 30-40 m between the elevation of 2250 to 2750 m (e.g., ZK40 and ZK42), and about 20-30 m above the elevation of 2600 m; the thickness of the landslide deposit in Zone II is about 30-50 m below the elevation of 2300 m (e.g., ZK208), about 50-100 m between the elevation of 2300 to 2500 m (e.g., ZK118 and ZK120), and about 30-40 m above the elevation of 2500 m (e.g., ZK116 and ZK205).e thickness of the landslide deposit near the Zhenggang gully and other small gullies is relatively thin.Results from exploratory adits show that the thickness of the landslide deposit in Zone I is about 20-30 m below the elevation of 2300 m, about 80-90 m between the elevation of 2300 to 2600 m (e.g., PD1704 and PD1705), and about 30-40 m above the elevation of 2600 m (e.g., PD205); the thickness of the landslide deposit in Zone II is about 40-60 m below the elevation of 2300 m (e.g., PD204), about 120-170 m between the elevation of 2300 to 2500 m (e.g., PD144, it exposed the bottom boundary of the landslide in the depth of 168.20 m), and about 30-40 m above the elevation of 2500 m (e.g., PD142, PD203, and PD207).Results from both borehole exploration and exploratory adits show that the materials of the landslide deposit are composed by fine sandy soil and lumps of weathered basalt gravels ranging in size from a few centimeters to more than 20 m. e cementation is weak.e features of rhythm structure are obvious with an occurrence of N5 °W, NE∠30 °-40 °.Due to the loose structure and high permeability of the landslide deposit, melting water and atmospheric precipitation can infiltrate into the landslide deposit rapidly.e landslide deposit bulges obviously near the toe of the sliding surface and has been subjected to disintegration many times.
e slip zone is a thin layer of fine-grained material between the bottom of the landslide deposit and the underlying bedrock (or toppling deformation rock mass), as shown in Figures 6(a)-6(c).Results from both borehole exploration and exploratory adits show that the occurrence of the slip zone is N30 °-50 °W, NE∠40 °-60 °at the top part, N30 °-40 °W, NE∠20 °-40 °at the middle part, and N30 °-40 °W, NE∠10 °-20 °at the toe part.e thickness of slip zones, exposed at exploratory adits, is approximately 20-200 cm. e materials of them as a whole are clayey and compacted wet fine soils, mixed with 10-30% small rock fragments that have well psephicity and range in sizes from 1 to 3 cm.e phenomena of shearing slip, specifically in some local areas, are visible as shown in Figure 6(d).e material composition of the slip zone is different in different parts.e material in a dry state at the top part has a high content of rock fragments, the lithology of which is slate and sandstone.e material in a plastic state at the middle part has a high content of clayey soil.e mingled rock fragments at the middle part are mainly composed of slate and sandstone, but with a small amount of limestone and mudstone.e material at the toe part has a high content of rock fragments, the lithology of which is slate, sandstone, limestone, and basalt.erefore, the slip zone has a high shear strength at the top and toe part but a relatively low shear strength in the middle part.Results disclosed at exploratory adit (PD204) indicate that the bedrock underlying the toe of the landslide deposit is the Lower Permian celadon basalts (P 1j 3 ) (Figure 6(e)) and the metamorphic sandstone (T 3hn ) (Figure 6(f)).ey are strongly unloading and rebound toppling rock masses and are very fractured and weathered.e shear and tensile fractures orienting downward slope are developed and have the characteristics of intensive and equalinterval distributions.e rock mass takes on special blocklayered structure.e average spacing of opening fracture distributions is approximately 20-40 cm.No filling material or only little amount of small rock debris are in opening fractures.e overhead phenomenon of rock mass is obvious.
e hammering sound on adit's wall is stuffy.Compared with the other results of exploratory adits basically at the same elevation, the depths of strongly unloading and rebound zones are obviously deepened, which indicate that the landslide thrust coming from the sliding body leads to the shear breaking and the structure disintegration of basalt.
e hydrogeological conditions around the Zhenggang area are simple.e groundwater is mainly fissure water flowing in rock fractures.Since the joints of the rock mass are well developed due to serious unloading, rebound, and stress relief of bedrocks, the permeability of the rock mass is relatively higher than the slip zone.However, since the groundwater table is normally lower than the slip zone, the influence of groundwater on the landslide stability is not considered in this study.

Deformation History and Evolution Tendency of the Landslide.
e geological survey indicates that the tectonic deformation system in the landslide area is controlled mainly by the dextral shear between India Plate and Asia Plate.Geostatic stress, disadvantage structure surface, and unloading deformation are the controlling factors for the landslide deformation and evolution.Glaciation, groundwater, and earthquake are inducements for the landslide instability and failure.Results of borehole exploration (e.g., ZK44 and ZK208) and exploratory adits (e.g., PD204 and PD144) indicate that the limestone belt distributed at the toe of landslide deposit is belonging to the residual deposits of P 1j 5 rock formation, which also is exposed at the flank of the Zhenggang landslide.erefore, it can be concluded that the sliding distance of the landslide sliding towards to the Lancang River has reached to 400-500 m.
e geological survey also disclosed the stratigraphic contact relationship of the Zhenggang area, as shown in Figure 7. e glacial deposit was formed later than the formation of the third terrace.e formation of the old landslide was formed later than the glacial period.So the evolution process of the Zhenggang landslide can be divided into three dynamic stages.
e first stage was the formation of the third terrace.e valley incision, unloading of jointed rock mass, tectonic movement, and climatic change intensified the rock weathering.e landslide deposit started to form.e strong intermittent crustal uplift was the main tectonic movement.e second stage was a largest-scale bedrock landslide formed by early strong bending and toppling deformation of rock masses.With the coming of glacial period, the bending and toppling rock mass near the ground surface increasingly disintegrated and became to move to the river valley.Fracture surface of bending and toppling rock mass was formed.With e former was induced by an unexpected long-term intensive rainfall (three days and three nights, the maximum rainfall amount is 78 mm per day), and the latter was triggered by a heavy disastrous snowfall.Both of these incidents caused pore water pressures overlying the sliding surface ranging from 1 m to 9 m.Both of Zone I and Zone II appeared new signs of losing stability.More new tensional cracks appeared on the upper and middle part of the ground surface.In Figures 9(a), 9(b), and 9(e), the main scarp is much more visible; in Figure 9(f), the anks adjacent to the sides of the sliding surface are more developed, and the bulging deformation accompanied by radial cracks distributed like a sector was more seriously near the toe of the sliding surface.e deep-seated movements along the preexisting sliding surface exposed at geological adits were clearly visible.
e outward-dipping deformations and breaks of wood used for adit support were more prominent, as shown in Figures 9(c), 9(d), and 9(h).e collapse of the landslide deposit at the toe part is more serious and exposes the toe of bottom-sliding surface clearly.All evidence at that time indicated that the landslide was more likely to be reactivated again.Fortunately, such pore water pressures were dissipated e ectively by some preimplemented drainage measures, for example, drainage holes.e landslide therefore got stabilized again but with a very slow creeping deformation along the slip zone.

Stability Analysis of the Zhenggang Landslide
Limit equilibrium for rigid body analysis and numerical simulation are the most popular approaches for landslide stability assessment [5,6,18].However, the limit equilibrium for rigid body analysis provides no information regarding the magnitudes of the strains within the slope, nor any indication about how the strains may vary along the sliding surface, so that the progressive failure of the slope along the full length of the sliding surface cannot be known.
e assumed inclinations of the side forces between slices result in the degree of computational accuracy not as high as the methods that satisfy all conditions of equilibrium so that the calculated safety factor of a slope may be less than 1.0, but the slope is actually stable.In contrast, although the numerical simulation can not only give the stress-strain state of the slope but also re ect the process of progressive failure of the slope, the calculation of safety factor is an inevitable weakness.erefore, how to give consideration to both two aspects shall be more convincing and more comprehensive.

Calculation Method for Safety
Factor.In this study, a method employing the same de nition of safety factor (SF shear strength of soil/shear stress required for equilibrium) but combining the limit equilibrium and nite element analysis together will be used for landslide stability analysis.
e main idea of this method in two-dimensional domain is (a) to divide a sliding surface into short line segments according to the intersection of all nite elements and the sliding surface rst; (b) to link all those line segments in turn as the calculating sliding surface; and (c) to calculate the resistance and sliding forces along the sliding surface through integrating the forces along each short line segment by the way of the length of the line segment times the stress-tensor component of each line segment's midpoint (Figure 10).e safety factor for a landslide can thus be expressed as follows:   Advances in Civil Engineering where L is the whole path of the sliding surface; τ f is the shear strength of soil mass; σ α and τ α are the normal and shear stresses on an oblique plane of one soil element, respectively; ΔL k is the length of the kth line segment; n is the total number of line segments; and m is the total number of intersections between all elements and the sliding surface.If a landslide fails towards the right (taking the counterclockwise direction to be positive), the included angle of the sliding surface and the x-axis should be always an obtuse angle α, as shown in Figure 11.Taking a coordinate rotation on the original stress tensor of the midpoint of each line segment on the sliding surface, the normal and shear stresses on one plane through the midpoint of each line segment (the stress point) and the orientation of line segment can be expressed as follows: where σ α and τ α are the normal and shear stresses on one plane through the stress point; σ x , σ y , and τ xy are the normal and shear stresses of the stress point; and α is the orientation of the plane through the stress point.
If the pore water pressure at the stress point is p, the effective shear strength, τ f , should be where c is the effective cohesion of the soil and φ is the effective friction angle.

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Hence, the safety factor for the sliding surface composed by a series of short line segments can be (5)

Calculation Model and Boundary Conditions.
Since the calculation method for safety factors in this study is based on the stress state of the slope, the nite element analysis is very necessary.e nite element meshes of two-mentioned typical pro les are shown in Figure 12, where each model includes three layers and is meshed by quadrilateral elements.Considering that the landslide deposit is a kind of coarse material, the elastic-plastic properties of the landslide deposit are described by the Mohr-Coulomb model.Roller boundary conditions are applied along the model bottom and the vertical borders.

Parameter Determination.
e reasonableness of shear strength of geomaterials always has a vital e ect on the  10 Advances in Civil Engineering results of landslide stability analysis, especially when the shear strength of the slip zone controls the slope stability seriously.In this study, the physical and mechanical parameters of the undisturbed slip zone clay were determined through laboratory tests, as shown in Figure 13.Due to the limits of the experimental condition and cost, the shear strength of the natural landslide deposit removing the basalt gravels in size larger than 2 cm was always tested as the reference value rst.en an empirical value of shear strength higher than the reference value was used for the landslide deposit.However, such shear strength could not always stand for the true strength of the landslide deposit.Back analysis is often used to ensure the proper selection of shear strength for the landslide deposit, combining with the slow creeping deformation features of the landslide (1.00 < safety factor (F s ) < 1.05, minimally stable).In this study, both of these two typical cross sections were studied by back analysis taking F s 1.05 as the level of slope stability state.Outcomes illustrated that the shear strength of the landslide deposit gained from back analysis was greater than the results of the laboratory test.So the shear strength of the landslide deposit employed the results of the back analysis.
e shear strength of the bedrock was obtained from laboratory tests.So did the bulk densities, Young's modulus, and Poisson's ratio for three layers of the Zhenggang landslide deposit, as shown in Table 1. 2 illustrates the result of the safety factors of the Zhenggang landslide in natural and rainfall conditions.A simpli cation for applying pore water pressure on the sliding surface was used here for substituting the e ect of rainfall conditions.e simpli cation is primarily attributed to the signi cant di erences between the landslide deposit and the slip zone in structure and permeability and the simplicities of calculation.e higher permeability of the landslide deposit and the compacted structure of the slip   zone always lead to a higher pore water pressure overlying the sliding surface rapidly, which actually results in gradual destabilization of the landslides.In this study, we adopt a water pressure of 3 m on the sliding surface for rainfall conditions. is value is based on the real condition of the project and the suggestion of engineers.e distributions of pore water pressures along the sliding surface are assumed to be proportional to the depth of the sliding surface.Outcomes show that both the results of the limit equilibrium for rigid body analysis and the proposed method are in good consistence with each other.

Results of Landslide Stability Analysis. Table
e safety factors of all stages of the landslide larger than 1.05 indicate that the landslide in both natural and rainfall conditions is in a basically stable state (1.05 < F s < F st , where F st is a safety factor threshold of the landslide under different conditions, decided by the specification).e safety factors of the third stage in Zone I and the first stage in Zone II having the lowest values in rainfall state demonstrate that the landslide deposit may have an evident probability of instability when there are pore water pressures existing on the sliding surfaces.erefore, it is necessary to enhance the deformation monitoring of the landslide in rainy season.
e main purpose of sensitivity analysis is to determine which controlling parameters have greater influences on landslide stability.In this study, the sliding surfaces of the third stage in Zone I and the first stage in Zone II are selected for calculating safety factors with different shear parameter combinations.Results in Table 3 and Figure 14 show that when the internal frictional angle of the slip zone increases one degree, the safety factor increases by 0.0404 for the third stage in Zone I but by 0.0321 for the first stage in Zone II.When the cohesion of the slip zone increases 1 kPa, the safety factor increases by 0.0017 for the third stage in Zone I but by 0.0007 for the first stage in Zone II.
e average sensitivity coefficient of the internal frictional angle 3.63% is 30.25 times the effect of the cohesion 0.12%.erefore, the internal frictional angle of the slip zone has a much greater influence on landslide stability.
Pore water pressure induced by rainfalls is always the main culprit of landslide failure.Melting water and atmospheric precipitation infiltrating into the landslide deposit rapidly, on the one hand, causes an obvious increase in the soil moisture content, resulting in an evident reduction of the shear strength of the soil; on the other hand, they increase soil unit weight and pore water pressure leading to an increase in the driving forces but a decrease in the resisting forces on a landslide so as to aggravate landslide deformation Figure 15 shows that the safety factors of landslides decrease quickly with the increase of the pore water pressure.When the pore water pressure reaches 5 m, the third stage in Zone I loses stability in advance.4.2.Landslide Failure Mechanism Analysis.Figure 17 shows that the plastic zones of the landslide deposit in natural state are mainly distributed along the downstream boundary of the landslide and the Zhenggang gully.e plastic zones of the slip zone are interconnected locally.So the whole landslide is in a basically stable state in the natural state.e plastic zones more obvious at the upper part of Zone II than the other parts illustrate that the large deformation occurs, which is in keeping with the actual situation of the engineering.Since the rainfall in ltration, resulting in high pore water pressure on the sliding surface, is the most signi cant external triggering factor for the failure of the Zhenggang landslide, the water pressure of 3 m on the sliding surface is implemented for substituting the e ect of rainfall conditions.e water pressures on the sliding surface are also assumed to be proportional to the depth of the sliding surface, as shown in Figure 18. Figure 19 shows that the middle-lower part of Zone I and the upper part of Zone II have the most serious plastic deformations under rainfall   19(b), it can be inferred that higher pore water pressure has a significant influence on the landslide stability.e third stage of Zone I is much more sensitive to the pore water pressure than other landslides and may lose stability earlier than other landslides.So the landslide in Zone I has a higher risk than that in Zone II during or after rainfalls.Effective drainage measures are therefore beneficial for improving the safety of the landslide deposit.Figure 20 shows that the deformations at the lower part of Zone I and the upper part of Zone II are the most remarkable, which actually proves the foregoing conclusions again.

Comprehensive Treatment Scheme
Since the Zhenggang landslide has a serious impact on the safety of the dam, three treatment schemes taking both engineering safety and cost into consideration have been proposed by designers.e first treatment scheme mainly focuses on conventional slope control measures, such as slope cutting, anchorage, and slope drainage.e second treatment scheme is to reserve exits for diversion tunnels, tailrace tunnels, and emptying tunnels at the upstream of the Zhenggang landslide first and then use three underground drainage tunnels to bring water to the downstream of the Zhenggang landslide.And the third treatment scheme is to set all exits for diversion tunnels, flood discharge tunnel, tailrace tunnels, and emptying tunnels at the downstream of the Zhenggang landslide.Because both the second and third treatment schemes have the same influence on the stability of the landslide deposit, only the first and third treatment schemes will be discussed in this study.Table 4 shows the detailed treatment designing for landslide stabilization.
Results of landslide stability analysis indicate that although the stability of all landslides in both Zone I and Zone II is improved under the first scheme, the safety factors of the landslides in Zone II do not meet the requirement of     Advances in Civil Engineering hydroelectric project.A landslide may occur during the process of slope cutting.So e ective measures for deformation monitoring must be emphasized in the rst scheme.In contrast, the stability of all landslides under the third scheme can meet the requirement of hydroelectric project after deep drainage well.Although the engineering cost of the third scheme is more than that of the rst scheme, the third scheme has the minimum potential disturbance to the landslide and can reduce the di culty of engineering construction, leading to much less risk of landslides.Reasons: even though the landslide occurs after building construction, it will not have in uence on the safety of reservoir operation.Measures: intercepting drain; deep drainage; crack closed.

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erefore, the treatment effect of the third scheme is better than that of the first scheme.

Conclusions
e Zhenggang landslide is an ancient large-scale landslide.e geological investigation on the stratigraphic contact relationship of the Zhenggang area indicated that the formation of the landslide had experienced three stages, including the formation of the third terrace, glacial period, and the late reformation of old landslide.e deformation and failure of the sliding surface at depth show that the distribution of the activity of the landslide is a retrogressive landslide in Zone I but an advancing landslide in Zone II.
e results of borehole exploration and exploratory adits demonstrated that the landslide deposit was under a metastable state and much more likely to lose stability along the weak interlayered clay during intense or prolonged rainfall infiltration.Outcomes from the numerical stability analysis and the failure mechanism of the Zhenggang landslide prove the same conclusions and indicate that the internal frictional angle of the slip zone has a much greater influence on landslide stability.Results of comparing different treatment schemes indicate that it is much better to set all exits of diversion tunnels, flood discharge tunnel, tailrace tunnels, and emptying tunnels at the downstream of the Zhenggang landslide for the dam safety, in spite of more investment.

Figure 4 (
a) shows that the landslide has an elevation of 2180-3220 m, with a maximum valley width about 1300 m. e area of the landslide is 1.7 million•m 2 .e volume of the landslide is approximately 47.2 million•m 3 (including 9.4 million•m 3 in the Zone I and 38.1 million•m 3 in the Zone II).Results of geological survey indicates that all kinds of rock outcrops, including magmatic, sedimentary, and metamorphic rocks, are visible in this area, as shown in Figure 4(c).enormal tectonic formation under the landslide deposit is a monocline structure with the occurrence of N20 °-30 °, SW∠65 °-85 °.Due to strong bending and toppling deformation, the occurrence of the topping rock masses near the landslide deposit is N30 °-35 °, SW∠15 °-30 °.Compressive structural surfaces are developed so that the stratigraphic

Figure 3 :
Figure 3: e topographic and geomorphic features of the Zhenggang landslide.

Figure 5 :
Figure 5: Typical pro les of the Zhenggang landslide: (a) A-A′ in Zone I; (b) B-B′ in Zone II.

Figure 7 :
Figure 7: e stratigraphic contact relationship of the Zhenggang area.

Figure 9 :
Figure 9: Warning signs appeared at the ground surface and exploratory adits.

Figure 10 :
Figure 10: e sliding surface composed by small line segments.

2 Figure 11 :
Figure 11: e normal and shear stresses on one plane through the midpoint of each line segment and the orientation of line segment.

Figure 12 :
Figure 12: Finite element models of two typical cross sections.

5 Figure 13 :
Figure 13: Shear strength of the slip zone clay with di erent water contents.

4. 1 .
Calculation Model and Boundary Conditions.In order to interpret the deformation mode and failure mechanism of the Zhenggang landslide, a three dimensional mesh model was employed.e model is 1900 m × 1230 m × 1855 m in dimensions and composed of 8080 triangular prism elements, as shown in Figure 16.e Mohr-Coulomb model is applied to describe the elastic-plastic properties of the materials.Roller boundary conditions are applied along the model bottom and the vertical borders.

Figure 14 :
Figure 14: Safety factors under di erent shear strength: (a) the 3rd in Zone I; (b) the 1st in Zone II.

Figure 15 :
Figure 15: Safety factors for landslides under di erent pore water pressures.

Figure 17 :
Figure 17: e plastic zones of the landslide deposit (a) and the slip zone (b) in natural condition.

Figure 18 :
Figure 18: e design sketch of pore water pressures.

Figure 19 :Figure 20 :
Figure 19: e plastic zones of the landslide deposit (a) and the slip zone (b) in rainfall condition.

Table 4 :
Treatment designing for landslide stabilization.Zone I Zone II e rst scheme Reasons: the exits of ood discharge tunnel lie below the foot of Zone I. Measures: dig out all landslide deposit above the strong weathered line; take slope rates of 1 : 1.6, 1 : 1.4, 1 : 1 and 1 : 0.8; add prestressed anchor cable; necessary slope protection and intercepting drain arrangement; deformation monitoring.Reasons: better in stability than Zone I; a large volume of the landslide; impossible to dig out all landslide deposit and add supports widely in Zone II.Measures: reducing load by slope cutting in elevations of 2800-3150 m; applying support of antislide pile; deep drainage; deformation monitoring.e third scheme

Table 1 :
Recommended shear parameters of three layers for Zhenggang landslide.

Table 2 :
Safety factors of the Zhenggang landslide deposit in natural and rainfall conditions.

Table 3 :
Factors of safety for different landslide stages in Zone I and Zone II.