Analysis of landslide stability under the impact of rainfall and reservoir water: a landslide inSouthwestern China as an example
Bowen ZHENG(1)(2), Kai YU(3)(4), Yongshuang ZHANG(3), Xin YAO(3), Shishu ZHANG(5), and Shengwen QI(1)(1) Institute of Geolgoy and Geophysics, Chinese Academy of Sciences, China
E-mail: zhengbowen@mail.iggcas.ac.cn(2) University of Chinese Academy of Sciences, China(3) Institute of Geological Mechanics, Chinese Academy of Geological Sciences, China(4) CCCC Second Highway Consultant Corporation Limited, China(5) PowerChina Chengdu Engineering Corporation Limited, China
Abstract
Reservoir landslides seriously affect the people's life and property safety. There is an
important reality significance to carry out the work of reservoir landslide monitoring andearly warning and stability analysis. In order to study the impact of rainfall and variation ofreservoir water level on the landslide stability and provide the theoretical foundations ofprevention and reduction of reservoir landslide, it takes a landslide of Southwestern China
as an example. Based on the monitoring data of rainfall, reservoir water level, ground waterlevel and surface displacement, the SEEP module and SLOPE analysis module of
GeoStudio software are used to calculate the safety coefficient of landslide. The results
show that the safety coefficient increases to the peak value gradually and then decreases toa certain value when the reservoir water level rises. The rise of reservoir water level has thelarger rate and the peak value of safety coefficient is bigger, and vice versa. The safety
coefficient decreases to the minimum and then increases gradually when the reservoir waterlevel declines. The decline of reservoir water level has the larger rate and the minimum ofsafety coefficient is smaller, and vice versa. Fractures development of landslide surface
providing channels for the rainfall infiltration is the main reason of landslide instability. The
combination of heavy rainfall and water level declining is the most disadvantageouscondition for landslide stability. Storage and drainage of reservoir should be arranged
reasonably and the drainage of landslide surface should be strengthened at the same time.

Keywords: reservoir landslides, rainfall, reservoir water level fluctuation, safety coefficient1.

Introduction

Reservoir landslides happen occasionally with theconstructions of large and medium sized reservoirs
located in alpine and gorge regions of Southwestern
China. Reservoir landslides have a serious impact on
the people's life and property safety, such as
Tangyanguang landslide in the Zhexi Reservoir of
Hunan province in March 1961 and Qianjiangping
landslide in the Three Gorges Reservoir in July 2003.
(Jin and Wang, 1986; Yin and Peng, 2007) Hence,
there is an important theoretical and practicalsignificance to carry out the work of reservoir
landslide monitoring and early warning and stability
analysis. Based on the monitoring contents, the workof reservoir landslide monitoring concludes
monitoring of absolute displacement and relative
displacement, monitoring of deep displacement,
monitoring of physical field, monitoring of groundwater, monitoring of external trigger factors, and soon. (Zhou, 2004; Peng et al., 2012) Some newtechnologies such as three-dimensional laser scanner,
GPS-RTK, remote sensing InSAR and BOTDR are
applied into the landslide monitoring fields, which
enrich the monitoring method and improve themonitoring accuracy. Chen et al. (2005) studied therelationship of landslides of Three Gorges Reservoir
and the condition of rainfall, which shows that thelandslides of Three Gorges Reservoir mainly
occurred in the rainy season from April to October,especially in heavy rainy season from June to August.
Wang et al. (2010) summarized the characteristics ofthe landslides of Three Gorges Reservoir after
impounding, which shows that rainfall is the common
factor to cause the reservoir landslides to happen.
Forty-nine percents of landslides are induced byrainfall and thirty-nine percents of landslides arecaused by storage and drainage of reservoir, whiletwelve percents of landslides induced by engineering
activities. Liao et al. (2012) studied the application ofdata of SAR in the monitoring of landslides of Three
Gorges Reservoir. The results show that there is not
too large deformation of landslide whenever the
reservoir has high water level or low water level.
However, the deformation of landslide changes
intensely with the reservoir water level rising or
declining quickly.
In order to study the impact of rainfall andvariation of reservoir water level on the landslide
stability and provide the theoretical foundations ofprevention and reduction of reservoir landslide, ittakes a landslide of Southwestern China as an
example. Based on the monitoring data of rainfall,reservoir water level, ground water level and surface
displacement, the SEEP module and SLOPE analysis
module of GeoStudio software are used to calculate
the safety coefficient of landslide.
2.

Engineeringcharacteristics

Fig. 1 Profile of engineering geology of reservoir
landslide (study area)

geology

The reservoir landslide of Southwestern China
belongs to the landslide of bedding multi-level creep
type, the length and width of which are both about
eight hundred meters and the mean slope of which isfifteen degrees. In this paper, the study like a long
tongue area is the east part of the landslide, which has
the most intense activity. The length of study area isabout four hundred and fifty meters. The width isabout one hundred and thirty meters. The mean
thickness of study area is about twenty five meters
and the total volume is about one point five million
cubic meters.
Based on the investigation of adjacent stratum
lithology and the analysis of drilling data, thelandslide dipping South East is monoclinic, including
loose weak sandstone, siltstone, shale, mudstone andcoal seam of Triassic System and overlying residual
deposits and slope deposits material of about two to
ten meters. There are slope deposits of gravel soil in
the front of landslide, the gravel compositions are
made up of rock debris of Xujiahe Formation. The
sliding bed is composed of feldspar, lithic sandstoneand siltstone mingled with shale and coal seam. There
are two-stage sliding faces. (Fig. 1)
Due to the long-term creep of landslide, the main
generated engineering geology problems include
surface tension fracture, surface collapse, threshingground sedimentation, house cracking and so on, as
are seen in Fig. 2.
3. Hydrogeololgy characteristics
The study area of landslide is located in thetransitional zones of basin marginal climate zone andWestern Sichuan province plateau and mountainclimate zone. The general meteorological(a) Surface tension fracture(b) Surface collapse(c) Threshing groundsedimentation(d) House cracking

Fig. 2 Engineering geology problems
characteristics show that the rainfall is abundant. The
annual precipitation is ranged from 1001.9 millimeterto 1264.7 millimeter. The rainfall of June to August
accounts for sixty-six percent to seventy-six percent
of annual precipitation. The annual evaporation isranged from 733.1 millimeter to 920.8 millimeter.
The climate of study area is warm and wet. The
annual average temperature is ranged from 15.2 ℃ to15.9 ℃. The annual average relative humidity isranged from eighty-one percent to eighty-four percent.
The annual average sunshine rate is ranged fromtwenty-four percent to thirty percent (Wang, 2001).
The abundant surface runoff provides beneficial
conditions for the formation of ground water. The
exposure of landslide stratum is relatively good. The
geological structure, weathering and corrosion haveeffects on the fissures development of landslide. The
loose deposits of landslide surface transformed into
terraced fields are widely distributed, which provides
beneficial conditions for the rainfall collection andseepage. The ground water of landslide is divided
into three categories, which are fracture pore water ofclastic rocks, karstic water of carbonatite and porewater of loose accumulation. The supplies of groundwater include lateral supply and vertical supply. The
lateral supplies of ground water are mainly fromkarstic water and fissure water of overlying klippe.
The vertical supply is mainly from rainfall, which is
the important supply source. The discharge way ofground water is relatively complicated, which
includes evaporation, subsurface runoff and so on.
The supply and discharge of ground water oflandslide is affected by the seasons.
4.

Richards Equation is nonlinear partial differential
equation. To solve this equation, it’s necessary toconfirm the initial condition and boundary condition.
The initial condition is initial ground water level,
which is determined by measured data.
The boundary condition of water head is
H | г1 = f1 (x, y, t)(2)

Monitoring results analysis

The monitoring results of rainfall, reservoir waterlevel, ground water level and surface displacementshow that the variations of ground water level datafrom three water holes were similar from July 2012 toMarch 2014. The impact of rainfall on the variationof ground water level was obvious. The ground waterlevel fluctuated in the rainy seasons and declined in
the dry seasons. Short-term heavy rainfall resulted in
the rapidly rise of ground water level, which declined
rapidly after rainfall. Due to the rainstorm on July 10,2013, the landslide formed large deformations, which
was related to the influences of the erosion, full waterload and water pressure from heavy rainfall on thelandslide. As a result, rainfall is one of major factorsof deformation of landslide. There were twice
storages and drainages from July 2012 to March 2014,
the highest water level and lowest water level were
841 meters and 785 meters respectively. When thereservoir water level declined rapidly from middle ofJuly to middle of August 2013, the deformation oflandslide increased. The reason is that there was awater head difference between ground water andreservoir water due to the hysteresis effect of declineof ground water when the reservoir water level
declined rapidly, which generated the seepage force
pointing to the outside of landslide. There were few
deformations when the reservoir water level remained
unchanged. As is mentioned above, the variation ofreservoir water level especially the rapid decline ofreservoir water level is another major factor oflandslide deformation.
5.

is water unit weight, t is time.

Seepage and stability analysis
method

5.1 Unsaturated - saturated seepage theory
The Richards Equation (Richards, 1931) is used
to solve the seepage problem in the unsaturated soil,
as can be seen in formula (1).
∂[kx·(∂H / ∂x)] / ∂x + ∂[ky·(∂H / ∂y)] / ∂y + Q= mw·γm·(∂H / ∂t)(1)
Where H is the absolute height of water head, kx is thepermeability coefficient in the direction of x, ky is thepermeability coefficient in the direction of y, Q is theitem of boundary source, mw is density than water, γm

The boundary condition of flow is∂H / ∂n | г1 = - f2 (x, y, t) / k(3)

5.2 Unsaturated soil strength theory
Vanapalli et al. (1996) put forward unsaturated
soil strength criterion formula:τ = c’ + (σ - ua) tanΦ’ + (ua - uw)·[(θw - θr) / (θs θr)·tanΦ’](4)
Where c’ is effective cohesion, σ is total stress, ua isbarometric pressure on the sliding surface, Φ’ iseffective internal friction angle, uw is hydrostaticpressure on the sliding surface, θw is volumetric watercontent, θr is residual volumetric water content.
5.3 Stability analysis theory
The angle variation of trailing edge of landslide
sliding surface trailing edge is more than 10 degrees.
In addition, the landslide sliding surface is mansard
sliding surface. So authors adopt Morgenstern – Pricemethod to solve the security coefficient.
The normal force En and moment Mn between soil
stripes integrated by equilibrium differential
equations need satisfy the condition:
En (Fs, λ) = 0(5)
Mn = ∫ (X - E·dy / dx) dx = 0(6)Where y=Ax+B, λ、A and B are constants, X istangential force between soil stripes, E is normal
force between soil stripes.
Suppose a λ and Fs, the En and Mn are integrated
by stripes. If the values of En and Mn are not zero, λand Fs are corrected by iteration. When the iterativevalues of En and Mn reach zero simultaneously, thesafety coefficient can be obtained.
6.

Numerical simulation of stability
under rainfall and reservoir waterlevel variation conditions

6.1 Reservoir landslide simplified model
Based on the engineering geological andhydrogeological data and monitoring data, thelandslide simplified model containing maingeological and hydrological characteristics isestablished. To satisfy the requirements of numericalsimulation, boundary conditions are set reasonably, as
can be seen in Fig. 3 and Tab. 1.

initial ground water level was analyzed by annual
average rainfall infiltration capacity. Considering theplant intercept and infiltration rate, the value of daily
average rainfall infiltration is 8.0×10-4 m/day.
The soil-water characteristic curve andpermeability function curve of landslide can be seenas Fig. 4 and Fig. 5.

Fig. 3 Reservoir landslide simplified model
The rainfall of a hydrologic year from January 1,
2013 to December 31, 2013 was set as the boundary
condition. The calculations show that the influence
depth of rainfall infiltration was limited, the influence
depth of which was ranged from 2 to 4 meters. The
influence depth of rainfall infiltration is different with
the increase of rainfall time and difference of rainfall
distribution. When the rainfall time was longer, theinfluence depth was deeper. The long-term rainfall
could form continuous ground water in a certain
depth. As a result, the steady-state of initial groundwater level was analyzed by the annual average
rainfall infiltration.
6.2 Numerical calculation parameters
There are not obvious sliding zones, the physicaland mechanical properties of which are similar to
those of sliding mass. Therefore the test parametersof weak layers of sliding mass are regarded as theparameters of sliding zones. The sliding mass
parameters of physical and mechanical parameters are
obtained from large-scale direct shear test. The
calculation parameters of trailing edge residual
deposits and leading edge slope deposits are acquiredby engineering analogy method, as can be seen in Tab.
2. The impact of initial position of ground water levelon the seepage computation is maximal. Suppose
reservoir water level accepts the infiltration of rainfall
after it reaches a lowest point. When the ground waterlevel reaches a steady state, it will be set as the initial
ground water level of landslide. The steady state ofFig. 4 Soil-water characteristic curve

Fig. 5 Permeability function curve
6.3 Stability calculation under the condition ofwater level rising
To study the effect of different rising rate ofreservoir water level on the landslide stability, thecalculation situations can be seen in Tab. 3.
There are two sliding surface in the landslide (Fig.
3). The Fig. 6 shows that the safety coefficients ofsliding surface NO.1 and NO.2 are 1.262 and 1.160
respectively at the beginning of reservoir water level
rising, which become 1.264 and 1.182 when thereservoir water level reach the maximum water level
and remain stable. The variation trends with time ofsafety coefficients of sliding surface NO.1 and NO.2
are similar. The safety coefficient increases to thepeak value gradually and then decreases to a certain

Tab. 1 Simplified model boundary condition
Boundary
AB
BC

Condition
Rainfall flow boundary
Water head boundary of reservoir water levelvariation
Overflow boundary

CD、EA

Flow boundary

DE

Flow boundary

Remark—Reservoir water level rises
Water level declines
Constrain in the horizontal direction
free in the vertical direction
Constrain in the vertical direction
free in the horizontal direction

Tab. 2 Calculation parameters
Density (g/cm3)
Natural Saturate

Title
Trailing edge residual
deposits
Leading edge slope deposits
Sliding mass
Sliding zones
Sliding bed

Total stress parametersc (kPa)Φ (°)

Effective stress parametersc’ (kPa)Φ’ (°)

2.20

2.24

20.0

31.0

—

—

2.15
2.21
2.19
2.30

2.22
2.23
2.23
2.32

30.0
35.0
25.0
45.5

31.0
33.0
22.7
35.5

—
20
9.7
—

—
35.2
20.1
—

value when the reservoir water level rises. The timeto peak of safety coefficient is corresponds to that ofreservoir water level rising to zenith. The rising slopeof curve of safety coefficient varying with time is lessthan the declining slope, the tendency of which
weakens gradually with the increase of rising rate ofreservoir water level. The rise of reservoir water levelhas the larger rate and the peak value of safety
coefficient is bigger, and vice versa. If the rising rateis larger, the curve of safety coefficient varying withtime is steeper, and vice versa, which indicates thatthe rise of ground water level lags the rise of reservoir
water level. The safety coefficient of reservoir waterlevel rising to zenith is larger than the initial safety
coefficient, which shows that the rise of reservoir
water level is beneficial to the landslide stability.
6.4 Stability calculation under the condition ofwater level declining
The calculation situations can be seen in Tab. 4.
The Fig. 7 shows that the variation trends withtime of safety coefficients of sliding surface NO.1
and NO.2 are similar. The safety coefficientdecreases to the minimum and then increases
gradually when the reservoir water level declines. If
the declining rate is larger, the curve of safety
coefficient varying with time is steeper and theoccurrence time of minimum of safety coefficient issooner, and vice versa. The decline of reservoir waterlevel has the larger rate and the minimum of safety
coefficient is smaller, and vice versa. The safety
coefficient of reservoir water level declining to nadir
is smaller than the initial safety coefficient, whichshows that the decline of reservoir water level isadverse to the landslide stability. The conclusion isconsistent with previous researches (Liu et al., 2005;
Luo et al., 2008; Lu et al., 2014; Xiang et al., 2014).

(a) Safety coefficient of sliding surface NO.1 varying
under different rising rate of reservoir water level(b) Safety coefficient of sliding surface NO.2 varying
under different rising rate of reservoir water level
Fig. 6 Safety coefficient varying under different
rising rate of reservoir water level
The Tab. 5 shows that if the declining rate is larger,
the maximum seepage rate of ground water and theseepage force are larger. Comparing sliding surface
NO.1 with NO.2, the safety coefficient of sliding
surface NO.2 is smaller than that of sliding surface
NO.1, which indicates that the impact of sliding
surface NO.2 on the landslide stability is greater thanthat of sliding surface NO.1.

Tab. 3 Stability calculation situations of reservoir water level rising
Reservoir water level (m)Initial water
Final water
level
level
785

841

Calculation
situations

Water rising rate(m/d)

Completing days(d)Operating days
(d)

Ⅰ
Ⅱ
Ⅲ
Ⅳ

1
2
3
4

56
28
18.7
14

94
122
131.3
136

Tab. 4 Stability calculation situations of reservoir water level declining
Reservoir water level (m)Initial water
Final water
level
level
841

785

Calculation
situations

Water declining rate(m/d)

Completing days(d)Operating days
(d)

Ⅰ
Ⅱ
Ⅲ
Ⅳ

1
2
3
4

56
28
18.7
14

94
122
131.3
136

(a) Safety coefficient of sliding surface NO.1 varying
under different declining rate of reservoir water level

Fig. 8 Local collapse of landslide leading edge
March and the end of August, landslide is in the limit
equilibrium state. The variation trend of safety
coefficient is consistent with reservoir water levelvariation. The influence of rainfall on the safety isless than that of reservoir water level variation. Dueto the climatic variation and tectonic movement, thejoints and fissures develop, which destroy the global
stability of landslide and provide the advantages tothe rainfall infiltration.

(b) Safety coefficient of sliding surface NO.2 varying
under different declining rate of reservoir water level
Fig. 7 Safety coefficient varying under different
declining rate of reservoir water level
The ground water level of landslide leading edgechanges with reservoir water level varying. Under theaction of ground water and alternation of wetting anddrying, the physical and mechanical properties oflandslide lower. When the reservoir water level
declines, the direction of seepage rate of landslide
leading edge is pointing to the outside of landslide,which is unfavourable to the stability of landslide
leading edge, as is can be seen in Fig. 8.
6.5 Stability calculation based on the monitoringdata of rainfall and reservoir water level
The calculations (Fig. 9) show that the safety
coefficient of sliding surface NO.2 changes with therainfall and reservoir water level varying, the value of
which is ranged from 1.038 to 1.25. Because ofreservoir water level declining in the beginning ofFig. 9 Landslide safety coefficient variation under the
combined effect of rainfall and reservoir water in ahydrologic year (sliding surface NO.2)
7.

Conclusion

In order to study the impact law of rainfall andvariation of reservoir water level on the landslide
stability, it takes a landslide of Southwestern China as
an example. Based on the monitoring data of rainfall,reservoir water level, ground water level and surface
displacement, the SEEP module and SLOPE analysis
module of GeoStudio software are used to calculate
the safety coefficient of landslide.

Tab. 5 Safety coefficient and seepage rate under different declining rate of reservoir water level
Declining rate
(m/d)
1
2
3
4

Occurrence timeof minimum(d)Surface
Surface
NO.1
NO.2
53
50
27
27
18
18
16
16

Minimum ofsafety coefficient
Surface
Surface
NO.1
NO.2
1.159
1.037
1.139
0.999
1.130
0.983
1.126
0.977

The safety coefficient increases to the peak value
gradually and then decreases to a certain value when
the reservoir water level rises. The time to peak ofsafety coefficient is corresponds to that of reservoir
water level rising to zenith. The rising slope of curveof safety coefficient varying with time is less than the
declining slope, the tendency of which weakens
gradually with the increase of rising rate of reservoir
water level. The rise of reservoir water level has thelarger rate and the peak value of safety coefficient isbigger, and vice versa. If the rising rate is larger, thecurve of safety coefficient varying with time issteeper, and vice versa, which indicates that the riseof ground water level lags the rise of reservoir waterlevel. The safety coefficient of reservoir water level
rising to zenith is larger than the initial safety
coefficient, which shows that the rise of reservoir
water level is beneficial to the landslide stability.
The safety coefficient decreases to the minimum
and then increases gradually when the reservoir waterlevel declines. If the declining rate is larger, the curveof safety coefficient varying with time is steeper andthe occurrence time of minimum of safety coefficient
is sooner, and vice versa. The decline of reservoir
water level has the larger rate and the minimum ofsafety coefficient is smaller, and vice versa. The
safety coefficient of reservoir water level declining tonadir is smaller than the initial safety coefficient,which shows that the decline of reservoir water levelis adverse to the landslide stability.
The developed joints and fissures provide theadvantages to the rainfall infiltration, which has anadverse influence on the landslide stability. The
combination of heavy rainfall and decline of reservoir
water level is the most unfavourable condition for the
instability of landslide.
Storage and drainage of reservoir should bearranged reasonably and too fast drainage should be
avoided (especially in the rainy season). The drainageof landslide surface should be strengthened at thesame time. To avoid the surface ponding infiltratinglandslide from the fracture channel, escape canals aresupposed to build. Surface fissures of landslide
leading edge should be filled and tamped. Pay
attention to landslide closely in the period of rapid
variation of reservoir water level and rainy season.

Maximum seepage rate
(m/d)
1.171
1.229
1.236
1.245
Acknowledgements

This research is supported by the Project of the
12th Five-year National Sci-Tech Support Plan ofChina (2011BAK12B09), China Geological Survey(1212010914025).
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