Slope stability analysis in the Three Gorges Reservoir Area considering effect ofantecedent rainfallDong Tang, Dian-Qing Li and Zi-Jun Cao

State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University, Wuhan, People’s Republic of China

ABSTRACTEffects of different initial conditions of pore water pressure distribution on slope stability areinvestigated based on rainfall data in the Three Gorges Reservoir Area. A method to incorporatethe initial condition of pore water pressure distribution into the slope stability analysis issuggested. Then, sandy and clayey slopes are taken as examples to investigate the effect ofantecedent rainfall on slope stability. Results indicate that the influence of antecedent rainfall onthe slope stability increases as the saturated permeability coefficient of the soil decreases.

ARTICLE HISTORYReceived 15 March 2016Accepted 19 May 2016

KEYWORDSAntecedent rainfall; slopestability; safety factor;unsaturated seepage analysis

1. Introduction

Landslides are one of the most common natural hazards(Ho and Ko 2009). Statistical data show that most of thelandslides occurred during or after the rainfall. Numer-ous studies have been conducted on rainfall-inducedlandslides, but few of them focus on antecedent rainfall.Effects of the antecedent rainfall remain unclear to thegeotechnical community. Research on landslides inHong Kong (Brand 1984, 1992), Italy (Aleotti 2004), Sin-gapore (Pitts 1985), and Spain (Corominas and Moya1999) showed that there is no direct relationship betweenantecedent rainfall and slope failure. However, Tan et al.(1987) concluded that the antecedent rainfall is one ofthe key triggering factors of landslides. The BukitBatok landslide in Singapore (Wei et al. 1991), whichremained stable during the rainfall but failed after therainfall, provides a persuasive evidence for the key roleof antecedent rainfall. Rahardjo, Leong, and Rezaur(2008) concluded that the antecedent rainfall with a dur-ation longer than five days affects the slope stabilitybased on the field observation of pore water pressure dis-tribution in four slopes in Singapore. These inconsistentconclusions from the literature may be due to differentpermeability of soils since the permeability plays animportant role in the seepage analysis.

As the antecedent rainfall has potential influence onslope stability, it is necessary to consider a sufficientlong period of antecedent rainfall in the slope stabilityanalysis, and thus, tominimise the errors in the numericalanalysis. However, the duration of antecedent rainfall

considered varies in previous studies. Lumb (1975) useda 15-day antecedent rainfall for the slope stability analysisin Hong Kong. Rahardjo, Leong, and Rezaur (2008),Rahardjo et al. (2001) and Rahimi, Rahardjo, and Leong(2011) used a five-day antecedent rainfall for the slopestability analysis in Singapore. Guzzetti et al. (2007) sum-marised the threshold of rainfall triggering landslides andfound that the duration of antecedent rainfall used byresearchers varies from 1 to 120 days. Therefore, howmany days of antecedent rainfall should be consideredfor the slope stability analysis is still an open question.Moreover, the antecedent rainfall used in most of pre-vious studies is hypothetical (Ng, Wang, and Tung2001). It is worth investigating the effect of antecedentrainfall on slope stability using real precipitation data.

Based on the daily rainfall data in the Three GorgesReservoir Area, this paper investigates effects of differentinitial conditions of pore water pressure distribution onslope stability. The method to obtain the initial con-dition, which reflects different responses of initial porewater pressure distribution of different soils to the cli-mate environment, is suggested. Then, sandy and clayeyslopes are used as examples to investigate effects of ante-cedent rainfall on the slope stability.

2. Methodology

2.1. Model and boundary conditions

Since this paper focuses on the antecedent rainfall, ahomogenous slope model with simple geometry from

© 2016 Informa UK Limited, trading as Taylor & Francis Group

CONTACT Dian-Qing Li dianqing@whu.edu.cn State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University, 8Donghu South Road, Wuhan 430072, People’s Republic of China

GEORISK, 2017VOL. 11, NO. 2, 161–172http://dx.doi.org/10.1080/17499518.2016.1193205

Rahardjo et al. (2007) is used in the study, as shown inFigure 1. Rahardjo et al. (2007) studied effects of soilproperties, rainfall intensity, ground water table andshape of the slope on slope stability. It was found thatsoil properties and rainfall intensity are the most influen-tial factors for slope stability. Ground water table andslope shape, however, have less significant effects onthe slope stability. As shown in Figure 1, AB, BC, andCD are flux boundaries. The unit flux is set to be equalto the rainfall intensity if the rainfall intensity is lessthan or equal to the permeability coefficient of the soil.Otherwise, it is taken as equal to the permeability coeffi-cient of the soil when the rainfall intensity is greater thanthe permeability coefficient of the soil. Water pond is notallowed in the analysis. AH, DE, and FG are imperme-able boundaries. EF and GH are water head boundaries,and the total heads are equal to heights of point H and E,respectively.

Figure 2 shows the finite element mesh of the modelused for the seepage analysis. There are a total of 7133nodes and 7143 elements. To guarantee the accuracyand reduce the computational efforts in the meantime,the soil mass close to the slope is refined using 0.2 m-

element, while the part far away from the slope are dis-cretised into 0.4 m-element. The bottom part of themodel is meshed using even larger elements, as shownin Figure 2 since most of the bottom part is saturatedin the analysis. Using the homogenous slope modelalso saves computational efforts compared with using areal slope. Nevertheless, it is worthwhile to furtherexplore the effect of antecedent rainfall on real slopesin the future study.

The real daily rainfall of the Three Gorges ReservoirArea shown in Figure 3 (China Meteorological Adminis-tration 2012), is used in this study. The Three GorgesReservoir is located at the subtropical monsoon climatezone, where 70% of the annual rainfall concentrates inthe period from May to September. The daily rainfalldata of 153 days from May to September (see dashedbox shown in Figure 3) in 1998 are used in this study.The rainfall of each day is averaged over 24 hours.

2.2. Soil properties

The SEEP/W (GEO-SLOPE International Ltd 2010a)module of GeoStudio is used to perform the unsaturated

Figure 1. Geometry of the slope.

Figure 2. Finite element mesh of the slope model.

162 D. TANG ET AL.

seepage analysis. The governing equation for the two-dimensional seepage analysis is given by:

∂

∂xkx

∂H∂x

( )+ ∂

∂yky∂H∂y

( )+ Q = mw

2 gw∂H∂t

, (1)

where H is the total head; x and y are the horizontal andvertical directions of the model, respectively; kx and kyare permeability coefficients in the x- and y-directions,respectively; Q is the boundary flux; gw is the unit weightof water; t is the time; and mw

2 is the slope of the Soil–Water Characteristic Curve (SWCC). The SWCCmodel proposed by Fredlund and Xing (1994) is adoptedin this study:

u = usC(w)1

[ln (e+ (w/a)n)]m

{ }(2)

with the correction factor C(w) defined as

C(w) = 1− ln 1+ w

wr

[ ]/ln 1+ 106

wr

[ ], (3)

where u is the volumetric water content; us is the satu-rated volumetric water content; e is the natural number;a, n, andm are fitting parameters which are related to theair-entry value, the maximum slope of SWCC, and theresidual water content, respectively; w is the matricsuction; and wr is the matric suction at residual state.In this study, C(w) is taken as equal to 1, as suggestedby Leong and Rahardjo (1997).

The permeability function is estimated using themethod proposed by Fredlund, Xing, and Huang(1994), by which means, the permeability function isobtained by integrating along the SWCC:

kw(w) = ks

�bln (w)

u(ey)− u(w)ey

u′(ey)dy

�bln (waev)

u(ey)− u(w)ey

u′(ey)dy

, (4)

where b is equal to ln(106) kPa; y is a dummy variable ofintegration representing ln (w); waev is the air-entryvalue; u′ is the derivative of Equation (2) with respectto w; and ks is saturated permeability coefficient.

Two soils with relatively high and low saturated per-meability coefficients, namely sandy soil and clayeysoil, are chosen from three types of soils in Rahardjoet al. (2007). Table 1 summarises the hydraulic proper-ties of the two soils. The sandy soil has a saturated per-meability coefficient ks of 8.64 m/d and SWCC fittingparameter a of 10 kPa. The clayey soil has a saturatedpermeability ks of 0.0864 m/d and SWCC fitting par-ameter a of 100 kPa. The other parameters, that is, m,n, and us, are taken as the same values for both soilsbecause they are relatively less influential. The respectiveSWCCs and permeability coefficient curves for the twosoils are plotted in Figures 4 and 5. According to thesoil statistics in the Three Gorges Reservoir Area (Liuet al. 2009; Tang et al. 2002), the saturated permeabilitycoefficient ranges from 0.028 to 10 m/d. It is shown thatthe saturated permeability coefficients of the two soils inthis study fall within this range.

The shear strength equation adopted in the SLOPE/W(GEO-SLOPE International Ltd 2010b) module of GeoS-tudio for the slope stability analysis is (Fredlund,

Figure 3. Daily rainfall of year 1998 in the Three Gorges ReservoirArea and the data (dashed box) adopted for this study.

Figure 4. SWCCs of the two soils.

Table 1. Properties of unsaturated soils used in this study.

Soil type

SWCC fitting parameters

Saturatedpermeabilitycoefficient

a (kPa) m n θs ks (m/s) ks (m/d)

Sandy soil 10 1 1 0.45 10−4 8.64Clayey soil 100 1 1 0.45 10−6 0.0864

GEORISK 163

Morgenstern, and Widger 1978):

t = c′ + (sn − ua) tan w′ + (ua − uw) tan wb, (5)

where t is the shear strength of unsaturated soils; c′ is theeffective cohesion; (sn − ua) is the net normal stress;(ua − uw) is the matric suction; w′ is the effective internalfriction angle; and wb is the friction angle correspondingto the part of shear strength due to matric suction. In thispaper, the shear strength parameters c′, w′, and wb are10 kPa, 26°, and 26°, respectively. In addition, the unitweight of the soil g is taken as 20 kN/m3.

2.3. Initial conditions

The initial condition of pore water pressure distributionis the key input information for transient seepage analy-sis. Luo (2008) investigated the effect of different initialpore water pressure distributions on slope stabilitywith different permeability coefficients. It was assumed

that the maximum negative pore water pressure at thesurface of the slope is −75, −50, and −25 kPa, respect-ively, and the pore water pressure distribution followshydrostatic pressure at around the water table. However,pore water pressure distributions for soils with differentpermeability coefficients may be different under thesame climate environment. The actual humidity situ-ation of different soils cannot be reflected by simplyassuming the same initial pore water pressure distri-bution. To make the initial pore water pressure distri-bution consistent with the real situation in the soilprofile, Rahardjo et al. (2007) obtained the initial con-dition from field observation. Rahimi, Rahardjo, andLeong (2011) applied a small rainfall on the slope inthe steady seepage analysis, and used the result of thesteady seepage analysis as the initial condition for thetransient seepage analysis. Zhan et al. (2012) usedthe average annual rainfall as the boundary conditionin the steady seepage analysis to obtain the initial con-dition of the transient seepage analysis. The GeoStudiomanual (GEO-SLOPE International Ltd 2010a) alsosuggests that the initial condition obtained from thesteady seepage analysis with a small, but nonzero, rainfallas the boundary condition is more reasonable comparedwith the initial condition by applying a zero rainfall asthe boundary condition.

To explore the influence of different methods fordetermining the initial condition of pore water pressuredistribution on slope stability, two methods, namely set-ting the maximum negative pore water pressure directlyand using the result of the steady seepage analysis withthe average annual rainfall as the boundary condition,are compared in this paper.

2.4. Procedure

The SEEP/W and SLOPE/W modules of GeoStudio areused to perform seepage analysis and slope stabilityanalysis, respectively. The main procedures are summar-ised as below.

Firstly, the initial condition for the transient seepageanalysis is determined. By applying the average annualrainfall as the boundary condition on the slope, thepore water pressure distribution is obtained from thesteady seepage analysis and is then used as the initialcondition for the transient seepage analysis. In contrast,for the case that a maximum negative pore waterpressure is assumed as the initial condition, the initialcondition is set directly.

Secondly, the transient seepage analysis is performedusing SEEP/W module after applying different durationsof antecedent rainfall on the slope as boundary con-ditions. The pore water pressure distribution at the end

Figure 5. Permeability coefficient curves of the two soils.

Figure 6. Annual rainfall in the Three Gorges Reservoir Areaduring 1995–2000.

164 D. TANG ET AL.

of the analysis is used as input in the slope stability. Forexample, to obtain the pore water pressure distributionof the slope at the end of the 20th day considering 10-day antecedent rainfall, the rainfall between 11th and20th days is applied as boundary condition for the tran-sient seepage analysis.

Thirdly, the SLOPE/W module is used to performslope stability analysis. The result of pore water pressuredistribution from the transient seepage analysis is used asinput in slope stability analysis. Then, Bishop’s simplifiedmethod is used to calculate the safety factor of slopestability.

This study makes use of deterministic methods toexplore effects of antecedent rainfall on safety marginof slope stability, in which various uncertainties (e.g.uncertainties in soil properties) in rainfall-induced land-slides are not taken into account. Nevertheless, it isworthwhile to point out that these uncertainties can beincorporated into rainfall-induced landslide analysis byprobabilistic methods (e.g. Ali et al. 2014; Huang et al.2015). Effects of antecedent rainfall on reliability ofslope stability shall be systematically explored in futurestudy under a probabilistic framework.

3. Results and discussion

3.1. Comparison of initial conditions

A sensitivity study is performed to explore the feasibilityof four different initial conditions in reflecting the cli-mate environment. The four initial conditions includethose with the maximum negative pore water pressureof −25, −45, and −75 kPa, and the pore water pressuredistribution obtained from steady seepage analysis byapplying the average annual rainfall as the boundary

condition. Figure 6 shows the annual rainfall in theThree Gorges Reservoir Area from 1995 to 2000(China Meteorological Administration 2012). The aver-age daily rainfall is calculated as ratio of the averageannual rainfall over 365 days, and it is 2.6 mm in thisarea.

Figure 7 presents the pore water pressure distributionfor the two soils under the four different initial con-ditions. For sandy soil, the pore water pressure distri-bution for the case with average annual rainfall asboundary condition is close to that of directly settingmaximum negative pore water pressure to be −45 kPa.For clayey soil, however, the pore water pressure distri-bution for the case with average annual rainfall asboundary condition is, on average, close to that ofdirectly setting maximum negative pore water pressureas −25 kPa.

As a conclusion, when conducting transient seepageanalysis, setting the maximum negative pore waterpressure is the most direct and efficient method pro-vided that the field observation data are available. How-ever, if there are no field observation data, assuming thesame maximum negative pore water pressure for differ-ent soils cannot reflect the real response of soils to theclimate environment. It is suggested applying the aver-age annual rainfall on the slope for steady seepageanalysis to obtain the initial pore water pressuredistribution.

3.2. Effect of the initial condition on slope stability

The procedure described in Section 2.4 is adopted in theanalysis for sandy and clayey slopes, respectively, todemonstrate effects of the initial condition on slopestability.

Figure 7. Pore water pressure distributions under different initial conditions in: (a) sandy soil; (b) clayey soil.

GEORISK 165

3.2.1. Sandy slopeFigure 8(a)–(d) show the variation of safety factor ofsandy slope as a function of time for 20-day, 30-day,40-day, and 50-day antecedent rainfall, respectively. Ineach subfigure, four curves plot safety factors underdifferent initial conditions, including those with themaximum negative pore water pressure of −25, −45,and −75 kPa, and that with the pore water pressure dis-tribution obtained from steady seepage analysis. Forexample, point A in Figure 8(a) represents the safetyfactor evaluated at the end of the 40th day, duringwhich 20-day antecedent rainfall is considered and themaximum negative pore water pressure is taken as−75 kPa. The curves of safety factor in different subfi-gures start at different time because different durationsof antecedent rainfall are considered. For example, thecurves of safety factor start at the 20th day when consid-ering 20-day antecedent rainfall (see Figure 8(a)), while

curves of safety factor start at the 30th day when consid-ering 30-day antecedent rainfall (see Figure 8(b)).

Some conclusions can be drawn from the resultsshown in Figure 8, including (1) as the duration of ante-cedent rainfall considered increases, the safety factorscorresponding to different initial conditions become clo-ser. This indicates that the influence of initial conditionon the slope stability decreases as the duration of antece-dent rainfall increases; (2) when the duration of antece-dent rainfall is longer than 50 days, the safety factorscalculated from different initial conditions tend to con-verge. This means that the initial condition has minimalinfluence on the slope stability as the duration of antece-dent rainfall is longer than 50 days; (3) the safety factorsfor the initial condition with the maximum negative porewater pressure of −45 kPa plot close to those obtained byapplying the average annual rainfall as boundary con-dition. It is not surprising to see this since the pore

Figure 8. Influence of initial conditions on safety factor of sandy slope with time considering different antecedent rainfalls: (a) 20 days;(b) 30 days; (c) 40 days; (d) 50 days.

166 D. TANG ET AL.

water pressure distributions are close to each other in thetwo cases, as shown in Figure 7(a).

3.2.2. Clayey slopeSimilarly, the safety factors for clayey slope are calcu-lated. As shown in Figure 9, as the duration of antecedentrainfall considered increases, the safety factors for differ-ent initial conditions tend to converge. The influence ofinitial condition on the stability of clayey slope is moresignificant than that for sandy slope. For example,when the 50-day antecedent rainfall is considered forsandy slope, there is obvious difference among safety fac-tors corresponding to different initial conditions, asshown in Figure 9(d). This is different from the obser-vation in Figure 8(d) for sandy slope. The initial con-ditions still have considerable effects on the slopestability for clayey slope even if the duration of antece-dent rainfall is relatively long. In addition, curves ofsafety factor for the case with initial condition obtainedby applying the average annual rainfall as boundary con-dition on the slope is almost identical with those

corresponding to the case that the initial conditionobtained by setting the maximum negative pore waterpressure as −25 kPa. This might be attributed to similaraverage pore water pressure in the two cases, as shown inFigure 7(b).

Comparing safety factors for sandy slope shown inFigure 8 with those of clayey slope shown in Figure 9,it is found that effects of initial conditions on slope stab-ility depend on the soil type. Appropriate initial con-ditions should be chosen to obtain reasonable results ofslope stability analysis. Under the same climate con-dition, assuming the same maximum negative porewater pressure as the initial condition for different slopesmight be inappropriate. The initial condition, which hasthe pore water pressure distribution obtained from thesteady seepage analysis by applying average annual rain-fall as boundary condition, is similar to the cases with themaximum negative pore water pressure of −45 and−25 kPa for sandy slope and clayey slope in this study,respectively. This is attributed to the fact that the waterstorage capability of clayey slope is greater than that of

Figure 9. Influence of initial conditions on safety factor of clayey slope with time considering different antecedent rainfalls: (a) 20 days;(b) 30 days; (c) 40 days; (d) 50 days.

GEORISK 167

sandy slope. Thus, the clayey slope is more humid thansandy slope. It is recommended to apply the averageannual rainfall as boundary condition in the steady see-page analysis and then use the result of pore waterpressure distribution as the initial condition for transientseepage analysis if there are no field observation data.

3.3. Effect of the antecedent rainfall on slopestability

3.3.1. Sandy slopeBased on the above analysis, the average annual rainfall isapplied on the slope in the steady seepage analysis. Then,the pore water pressure distribution obtained from thesteady seepage analysis is used as the initial conditionin the transient seepage analysis. The safety factor isfinally calculated using the result of transient seepageanalysis. Figure 10 shows the safety factor of the sandyslope versus time with the consideration of different dur-ations of antecedent rainfall. For comparison, the histo-gram of the daily rainfall is also included in the figure.Some conclusions can be drawn as follows.

(1) When the duration of antecedent rainfall exceeds 30days, the curve of safety factor versus time tends tobe stable. This means that 30-day antecedent rainfallshould be considered in the analysis to generatesafety factor with reasonable accuracy.

(2) The safety factors at the end of the 63rd day and108th day are the minimum values during the rain-fall process at around 61st day and 101st day,respectively. When the duration of antecedent rain-fall considered exceeds 15 days, the minimum safetyfactors for different periods of antecedent rainfall

almost overlap. This indicates that the minimumsafety factors after heavy rainfalls are stable if 15-day antecedent rainfall are considered. The safetyfactors calculated with consideration of antecedentrainfall less than 15 days cannot reflect the actualsafety margin of slope stability. Thus, it is suggestedusing the duration of antecedent rainfall longer than15 days in the engineering design.

(3) The influence of antecedent rainfall on sandy slopeis minimal. Safety factors considering different dur-ations of antecedent rainfall are not significantlydifferent. The influence is particularly small betweenthe 1st and 60th days when the rainfall intensity isrelatively small.

(4) Different types of antecedent rainfall have differentinfluence on slope stability. Although the total rain-fall for the two series of rainfall at around the 61stday and the 101st day are similar (say 300 and390 mm, respectively), they have different charac-teristics. The rainfall at around the 61st day has ahigh intensity but a short duration, while the rain-fall at around the 101st day has a low intensity buta long duration. Consider, for example, the safetyfactor at the 68th day. If 5-day antecedent rainfallis considered, the antecedent rainfall at around61st day is not included, but it is included for thecase with 10-day antecedent rainfall. Effects of theantecedent rainfall at around 61st day can beexplored by comparing the safety factors at the68th day considering different durations of theantecedent rainfall. The maximum differencebetween the safety factor obtained by consideringthe antecedent rainfall at around 61st day andthat obtained without considering it is about 0.25,

Figure 10. Safety factor of the sandy slope versus time with the consideration of different durations of antecedent rainfall.

168 D. TANG ET AL.

but the difference for the antecedent rainfall ataround the 101st day is only 0.22. To conclude,under the similar total amount of antecedent rain-fall, the antecedent rainfall which has high intensitybut short duration has more significant impacts onsandy slope.

As shown in Figure 10, the minimum safety factordoes not occur at the time with the highest rainfall inten-sity, but at the end of the rainfall process. Hence, theminimum safety factor has no direct relationship withthe maximum daily rainfall. Figure 11 shows therelationship between the safety factor of sandy slopeand the accumulating rainfall with different durations.The safety factor generally decreases as accumulatingantecedent rainfall increases. Hence, the scale value ofthe right vertical axis is intentionally reversed so thatthe trend of accumulating antecedent rainfall and thesafety factor can be compared conveniently. It is shownthat the variation of safety factor is generally consistentwith that of 10-day accumulating rainfall. According tothe curve of 10-day accumulating rainfall, the accumulat-ing antecedent rainfall at around 63rd day is greater thanthat at around the 108th day. This is consistent with theobservation that the minimum safety factor at aroundthe 63rd day is smaller than that at around the 108thday. Thus, for sandy slope, the 10-day accumulatingantecedent rainfall is closely related to the minimumsafety factor in this study.

3.3.2. Clayey slopeSimilar to the sandy slope, the safety factors of clayeyslope considering different durations of antecedent rain-fall are calculated and are plotted versus time in

Figure 12. The histogram of daily rainfall intensity isalso included for comparison. Some conclusions can bedrawn from Figure 12.

(1) When the duration of antecedent rainfall exceeds 40days, curves of safety factor tend to be stable, indi-cating that more than 40-day antecedent rainfallshall be considered to obtain reasonable estimatesof safety factor for clayey slope.

(2) The safety factors at the end of the 63rd day and the108th day are the minimum values during the rain-fall process at around 61st day and 101st day, whichis the same as the observation obtained from thesandy slope. When the duration of antecedent rain-fall exceeds 20 days, the minimum safety factors arealmost the same. In other words, the minimumsafety factor obtained by considering 20-day antece-dent rainfall is stable, while the safety factor consid-ering less than 20-day antecedent rainfall cannotreflect the real safety margin of slope stability. It issuggested using 20-day antecedent rainfall in theengineering application.

(3) Comparing the overall trend of the safety factorvarying with time, the difference in safety factorsof clayey slope considering different durations ofantecedent rainfall is greater than that of sandyslope. In other words, the influence of antecedentrainfall on clayey slope is more significant thanthat on sandy slope.

(4) Comparing the difference between the safety factorsobtained by considering the antecedent rainfall ataround the 61st day and the 101st day, respectively,and that obtained without considering them, it isfound that the influence of antecedent rainfall at

Figure 11. Relationship between the safety factor of sandy slope and the accumulating rainfall with different durations.

GEORISK 169

around the 101st day on the slope stability is greaterthan that at around the 61st day. Hence, the antece-dent rainfall in a long term and with a low intensityhasmore significant influence on stability of the clayeyslope. This is different from the case of the sandy slope.

Figure 13 shows the relationship between the safetyfactor and the accumulating antecedent rainfall of differ-ent durations. It is found that the variation of safety fac-tor is generally consistent with the variation of 15-dayaccumulating antecedent rainfall. When the 15-dayaccumulating antecedent rainfall reaches the peak, theminimum safety factor is obtained. When the 15-dayaccumulating antecedent rainfall decreases, the safetyfactor of the clayey slope increases. As shown inFigure 13, the accumulating antecedent rainfall at

around the 108th day is greater than that at around the63rd day, and the safety factor at the 108th day is smallerthan that at the 63rd day. Thus, for clayey slope, the 15-day accumulating antecedent rainfall is closely related tothe minimum safety factor.

4. Conclusions

This paper investigated effects of initial condition andantecedent rainfall on slope stability based on the dailyrainfall data in the Three Gorges Reservoir Area. Severalconclusions are drawn from this study:

(1) The initial condition of the transient seepage analy-sis is important for the slope stability analysis. It issuggested using the pore water pressure distribution

Figure 12. Safety factor of the clayey slope versus time with the consideration of different durations of antecedent rainfall.

Figure 13. Relationship between the safety factor of clayey slope and the accumulating rainfall with different durations.

170 D. TANG ET AL.

obtained from the steady seepage analysis under theaverage annual rainfall as the initial condition forthe transient seepage analysis if there are no obser-vation data.

(2) The influence of antecedent rainfall on the slopestability becomes more and more significant as thepermeability coefficient of the soil decreases. Forsandy and clayey slopes, the influence of antecedentrainfall on slope stability becomes minimal as the30-day and 40-day antecedent rainfalls are con-sidered, respectively. To obtain reasonable safetyfactor of sandy and clayey slopes, 30-day and 40-day antecedent rainfall shall be considered in theanalysis.

(3) As for the minimum safety factor after a heavy rain-fall, the safety factor calculated by considering ante-cedent rainfall less than 15 days for sandy slope and20 days for clayey slope cannot reflect the real safetymargin of slope stability.

(4) Under the similar amount of total antecedent rain-fall, antecedent rainfall in a short duration andwith a high intensity has more significant impactson the sandy slope, while antecedent rainfall in along duration and with a low intensity affects theclayey slope more significantly.

(5) The accumulating antecedent rainfall can be relatedto the minimum safety factor of slope stability. Thetime when the minimum safety factor occurs is closeto those corresponding to the maximum 10-day and15-day accumulating antecedent rainfall for sandyand clayey slopes, respectively.

Disclosure statement

No potential conflict of interest was reported by the authors.

Funding information

This work was supported by the National Science Fund forDistinguished Young Scholars (Project No. 51225903), theNational Natural Science Foundation of China (Project Nos.51329901, 51579190) and the Natural Science Foundation ofHubei Province of China (Project No. 2014CFA001).

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