Infiltration in two types of embankments and the effects of rainfall...

Infiltration in two types of embankments and the effects of rainfalltime on the stability of slopes

Guoxiang Tu* & Runqiu HuangState Key Laboratory of Geohazard Prevention and Geoenvironment Protection, Chengdu University of Technology, Chengdu610059, China*Correspondence: [email protected]

Abstract: We have studied discrepancies between rainfall infiltration and the effects of rainfall time on slope stability usingtwo expressway embankments consisting of soil with different permeability as examples to monitor variations in water contentand porewater pressure of the embankments during two typical rainfall events. The results were as follows. (1) Rainfallinfiltration in the sandy clay embankment occurred in three phases: complete infiltration, rate-reducing infiltration and stableinfiltration. Rainfall infiltration in the gravel embankment could be divided into two phases: complete infiltration and watertable rise. The wetting front in the sandy clay embankment moved down slowly with time, and the volumetric moisture contentbetween the infiltration surface and wetting front ranged from 20 to 42% and decreased with depth. In the gravel embankment,the wetting front dropped rapidly, and the soil volumetric moisture content between the infiltration surface and wetting frontremained within a low range of 2.8 – 3.5% during the rainfall event. (2) During the rainfall event, the variation of porewaterpressure in the sandy clay embankment lagged behind rainfall and was affected mostly by the wetting front moving down. Incontrast, in the gravel embankment porewater pressure variation did not lag far behind rainfall. (3) The effect of infiltration onslope stability was greater for the gravel embankment than for the sandy clay embankment. The factor of safety for the formerdecreased rapidly during the rainfall events and recovered with almost no delay after the rainfall event ended, whereas the factorof safety for the latter decreased slowly and continued to decrease for 2 – 9 days after the rainfall event ended. (4) If the totalrainfall was constant, for the sandy clay embankment the effect of rain intensity on the factor of safety of the slope was smallerthan that for the gravel embankment, and the factor of safety for the sandy clay embankment was controlled by the rainfallintensity and duration.

Received 2 March 2015; revised 12 September 2016; accepted 21 September 2016

Rainfall is one of the most important factors inducing slope failure.More than 90 slope failure cases were listed by Sun (1988), and ofthese more than 95% were caused by or closely related to rainfall.According to Zhong (1999), 15 geohazards in 27 studied cases wereinduced by rainstorms. Recently, more attention has been paid toissues such as rain-induced slope failure and its mechanism (Au1998; Ng & Shi 1998a; Cho & Lee 2001; Ying et al. 2002; Sunet al. 2008; Lin et al. 2009; Hossain 2010; Schnellmann 2010),methods of evaluating slope stability (Ping et al. 2004; Wei et al.2006; Zhu et al. 2006; Liu et al. 2007; Lou 2007; Wu et al. 2008;Zhou et al. 2008) and forecasting slope failure owing to rainfall (Ng& Shi 1998b; Zhong 1998; Gao & Yin 2007; Miller et al. 2009).These studies have indicated that rainfall can cause a change in theslope seepage field, decrease the matric suction of soil, and increasethe porewater pressure and soil unit weight. Also, the increase inwater content softens soil and induces an increase in shear stress and adecrease in shear strength of the slope sliding surface, which causesslope failure. Therefore, slope stability is affected by variousparameters including total rainfall, rain intensity, duration of rainfall,patterns of rainfall, soil properties and the topography of the slope.

Many researchers have found that slope failure often lags behindrainfall. Wu et al. (2008) showed that slope failure caused by rainfalltypically occurred 0.3 – 0.8 days after the rainfall event ended.Zhong (1998) found that landslides caused by rainfall usuallyoccurred after long periods of rainfall or after a rainstorm ended, andnoted that the delay was generally less than 10 days, based oninvestigations of slope failure owing to rainfall in the mountain areaof western Hubei, China. Gao & Yin (2007) found that landslidesusually occurred in rainstorms or shortly afterward, and the delaywas no more than 4 days, based on a study of landslides caused by

rainfall in Shenzhen. Zhan & Liu (2011) concluded that the factor ofsafety of a slope approached its minimum 8–12 h after rainfall,based on a study of slopes in Jiangxi, China.

When the total rainfall is large enough, or the duration of rain islong enough and rain intensity is great enough (Zhang et al. 2003;Godt et al. 2006), it can change slope stability. Therefore, rainintensity and duration are the two most important factors forevaluating slope stability (Guzzetti et al. 2008; Tsai 2008; Milleret al. 2009). Many studies have shown that the lower bound of totalrainfall to cause slope failure is 100 mm, and the critical rainfallintensity is 50 mm d−1. In most cases, the infiltration capacity of theslope is less than the critical rainfall intensity, so it takes time forrainwater to seep into deep soil (more than 5 m below the surface).The physical and mechanical properties of deep soil including watercontent, porewater pressure and strength vary with time. The factorof safety of a slope is a function of time, and from this perspective,slope stability should change with rainfall duration. Therefore,investigating rainfall infiltration and the effects of rainfall time onthe stability of embankment slopes is important for evaluating slopestability and forecasting slope failure.

The deformation of the slope and the variation of slope stabilityduring a rainfall event are closely related to the interaction betweengas, liquid and solid phases in soil (Hu et al. 2011). Manyresearchers have studied the coupling interaction of the three phasesand their effects on slope stability using experimental tests (Mein &Larson 1973; Ochiai et al. 2004; Qian et al. 2011) and numericalsimulation (Feng et al. 1998; Qi et al. 2003; Cai & Ugai 2004;Collins & Znidarcic 2004; Huang et al. 2010). Some have employeda gas–water two-phase model to investigate the effects of rainfallinfiltration on slope stability and the effects of groundwater seepage

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Thematic set:Landslide Research in China Quarterly Journal of Engineering Geology and Hydrogeology

Published online November 17, 2016 https://doi.org/10.1144/qjegh2015-027 | Vol. 49 | 2016 | pp. 286–297

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on soil properties, including physical, chemical and mechanicalproperties (Tsaparas et al. 2002; Rouainia et al. 2009; Sun et al.2009; Zhang et al. 2009; Davies et al. 2014).

In this paper, two expressway embankments consisting of soilwith different permeabilities were used as examples to monitor thevariations in soil water content and porewater pressure during tworainfall events. Using the monitoring data and finite-elementanalysis, we investigate the process of rainfall infiltration into theembankments, and the variation in water content and porewaterpressure in the embankments from a dry condition to a moistcondition during the rainfall event. We then discuss, on the basis ofthe seepage and strength theory of unsaturated soil, the discrepan-cies between rainfall infiltration in the two embankments, the effectsof rainfall intensity and duration on the slope stability, and therelation of embankment stability to rainfall time when the totalprecipitation is constant.

Materials and methods

The two test embankments were built during the construction of anexpressway. One, with a length of 50 m, was formed of gravel,whereas the other, with a length of 45 m, consisted of sandy clay.Figure 1 shows the grain-size distributions of the soils. Theembankment cross-sections were identical, as shown in Figure 2.Both were covered by a road tarmac surface at the embankmentcrest; this surface was considered as an impermeable layer in themodelling described in this paper. A monitoring cross-section wasselected for each embankment, and nine porewater pressure gaugesor water content monitors were installed to monitor the variations ofporewater pressure in each cross-section. During a rainfall event, the

porewater suction would drop below −100 kPa, which was thelower limit of the tensiometers (the range of measurement of thetensiometers is from −100 to 0 kPa), and the elevated water tablewithin the embankment would then induce positive porewaterpressure. Therefore, to measure these positive porewater pressures, aGI-PW vibrating wire osmometer from Earth Products China Ltd(with a range of measurement from 0 to 1000 kPa) was used, and a503DR moisture analyzer from InstroTek/CPN was employed tomeasure volumetric water content. The matric suctions of soilscould then be read according to the soil-water characteristic curves.It was presumed that the matric suction was equal to the negativeporewater pressure. Thus, we could obtain the variations ofporewater pressure (including both negative and positive porewaterpressure) during the process of rainfall.

In three years (from 2011 to 2013), the variations in water contentand porewater pressure in the embankments induced by multiplerainfall events were monitored. This paper focuses on studying howrainfall infiltrated into the soil in the two embankments and thevariations of water content and porewater pressure during a period ofwetting, and comparing the influence of rainfall on the stability ofthe two embankments. Considering these objectives, the rainfallevents selected in this study should meet the following require-ments. First, rainfall intensities should be sufficient to allowinfiltration deep into the embankments (at least 5 m below thesurface). Second, prior to the selected rainfall event, there should bea long period of low or zero precipitation (suggested to be at least20 days), and the rainfall events were chosen to have matchingantecedent conditions so that we could concentrate on the relativeeffects of these events on embankment pore pressure responsewithout having to account for differences in antecedent conditions.

Fig. 1. Grain-size distribution of soil of embankments: (a) gravel soil; (b) sandy clay soil.

Fig. 2. Location of the monitoring site inthe embankments.

287Rainfall effects on slope stability


Third, the total magnitudes of precipitation of the rainfall eventsshould be similar, so that the effects of rain intensity and duration onthe stability of the embankments could be compared.

On the basis of these requirements, two rainfall events wereselected for study from the monitoring data. One was the rainfallevent from 16 to 19 July 2011, which had an average daily rainfall of145.3 mm and a total rainfall of 435.9 mm (Fig. 3a). The other wasthe rainfall event from 7 to 16 April 2013, which had an averagedaily rainfall of 50.2 mm and a total rainfall of 451.8 mm (Fig. 3b).Before these rainfalls, no rain occurred for 53 days before the formerand 68 days before the latter, so these rainfall events wereconsidered independent from others in terms of effects on porewaterpressure and water content. Additionally, the total precipitations ofthese two rainfall events were similar, and both of the rainfallintensities were more than 40.2 mm d−1.

The monitoring data did not clearly reflect how the rainwaterinfiltrated the embankments and the infiltration differences atdifferent locations; it was also difficult to show the generaldistribution characteristics of porewater pressure and water contentin the embankments. Therefore, a model was built using the finite-element method (FEM) to simulate how rainwater infiltrates intosoils and the resultant seepage within the embankments. The modelwas validated and updated using the measured data. The governingequation (Richards equation) of water seepage in unsaturated orsaturated soil is as follows (Mao et al. 1999):

@

@xi

1

gwkijkr(p)

@p

@xjþ ki3kr(p)

� �¼ [C(p)þ aSs] @p

@t(1)

where xi and xj are the coordinate axes, kij is the tensor ofpermeability coefficients of saturated soil, kr( p) is the relativecoefficient of permeability, p is the porewater pressure, γw is the unitweight of water, Ss is the specific storativity, C( p) is the specificwater capacity of the soil and α is a parameter that expresses thesaturated or unsaturated condition of the soil (α = 0 means that thesoil is unsaturated and α = 1 means that the soil is saturated). Thedefinite condition of equation (1) is listed below.

(1) Initial condition:

p(x, y)t¼0 ¼ p0(x, y): (2)(2) Boundary condition:

p(x, y, t) G1�� ¼ p0(x, y, t)

� kr(p)kij @p@xj

þ ki3kr(p)� �

ni G2�� ¼ qn

� kr(p)kij @p@xj

þ ki3kr(p)� �

ni G3�� ¼ 0, p G3�� ¼ 0

� kr(p)kij @p@xj

þ ki3kr(p)� �

ni G4�� , 0, p G4�� , 0

8>>>>>>>>>><>>>>>>>>>>:

(3)

where ni (i = 1, 2, 3) is the direction cosine, Γ1 is the pressurecondition, Γ2 is the flux boundary condition, Γ3 is the saturatedescape boundary condition, Γ4 is the unsaturated escape boundarycondition and qn is the normal flux of the boundary, where thedirection to the outside is positive.

During a rainfall event, the maximum infiltration capacity of thesoil was calculated using the following equation:

R(t) ¼ kr(p) kij @p@xj

þ ki3� �

ni: (4)

If rain intensity q(t) was less than R(t), it indicated that rainfallcompletely infiltrated into the soil, then setting R(t) = q(t). If rainintensity q(t) was larger than R(t), it indicated that rainwateraccumulated on the embankment surface. It was difficult forponding of rainwater to occur on the surfaces of the twoembankment slopes, and ponding water on the crests could notinfiltrate into the soils as the crests were covered by a road tarmacsurface. Therefore ponding water on the crests and side slopes of theembankments was not considered in our model. This paper focusedon how rainwater infiltrated into the soil and affected the stability ofthe slope, and therefore the effect of runoff was not consideredeither. R(t) was calculated using equation (4) and setting p = 0 as theinfiltration boundary condition (Wei et al. 2006) because theembankment crest was covered by a road tarmac surface and wasassumed to be impermeable (p = 0).

From the FEM calculations, the results including porewaterpressure and volumetric water content were input into another FEMmodel built in ANSYS for the analysis of slope stability of theembankments so that the variation in slope stability with time couldbe obtained using the strength reduction method. Therefore, thevariation in slope stability could be obtained for different rainintensities and durations. In the FEMmodel, the soil was assumed toyield to the unsaturated soil Mohr–Coulomb criterion (Fredlund &Rahardjo 1993)

t ¼ (s� ua) tanf0 þ (ua � uw) tanfb þ c0 (5)where τ is the shear strength of soil, c′ is the effective cohesion, w′ isthe effective angle of internal friction, σ is the total stress, wb is thefriction angle related to matric suction, and ua and uw are the pore airpressure and porewater pressure, respectively. If ua < uw, then ua = 0and we set wb = w′.

The physical and mechanical parameters of the soils in theembankments are listed in Table 1. All these parameters exceptpermeability coefficients of saturated soils were tested in thelaboratory, and the saturated permeability coefficients wereobtained by in situ testing.

The shear strength parameters of the soils were tested by triaxialtest in the laboratory. The confining pressures were set as 80, 100,150, 200, 300, 400 and 500 kPa for the sandy clay soil, and 100,200, 250, 300, 400 and 500 kPa for the gravel soil. The test resultsare shown in Figure 4. The parameter wb was set according toFredlund & Rahardjo (1993).

Matric suction data for the two soils with different water contentswere measured in the laboratory (Fig. 5) by a suction double-cellextractor (made by Earth Products China Ltd) to determine thestress-dependent soil-water characteristic curve. The two curveswere derived using the model of Van Genuchten (1980) to fit themeasured data. The derived parameters in equation (6) are listed inTable 2, and the soil-water characteristic curves are shown in

Fig. 3. Daily rainfall for the two rainfallevents: (a) first rainfall; (b) secondrainfall.

288 G. Tu & R. Huang


Figure 5.

u ¼ ur þ (us � ur)[1þ (apc)n]m (6)

where θ is the volume water content of soil, θr is the residual watercontent of soil, θs is the saturated water content of soil, pc is thematric suction, and α, n and m are curve fitting parameters (α hasunits of 1/pressure, and m = 1− (1/n)).

The permeability coefficient of unsaturated soil shown inFigure 6 was calculated from Figure 5 and Table 1 using thefollowing equation of Fredlund & Rahardjo (1993):

kw(u)i ¼kskse

AdXmj¼i

(2j þ 1� 2i)(ua � uw)�2jn o

ð7Þ

where kw(θ)i is the predicted water coefficient of permeability for avolumetric water content, θi, corresponding to the ith interval (m s

−1)(i is the interval number, which increases as the volumetric watercontent decreases; for example, i = 1 identifies the first interval, whichis close to the saturated volumetricwater content, θs, and i =m identifiesthe last interval, corresponding to the lowest volumetric water contenton the experimental soil-water characteristic curve, θr); j is a counterfrom i tom; ks is the measured saturated coefficient of permeability (ms−1); kse is the calculated saturated coefficient of permeability (m s

−1);mis the total number of intervals between the saturated volumetric watercontent, θs, and the lowest volumetric water content, θr, on the

experimental soil-water characteristic curve (i.e. m = 20); (ua − uw)j isthe matric suction corresponding to the jth interval (kPa); Ad is anadjusting constant, assumed to be equal to unity in this paper.

Results

The variations in porewater pressure and volumetric watercontent of the embankments

As mentioned above, the first of the studied rainfall events lasted3 days with an average precipitation of 145.3 mm d−1, and thesecond continued for 9 days with an average precipitation of50.2 mm d−1. The variation of water content at each monitoringposition in the embankment is shown in Figure 7. During the firstrainfall event, for the gravel embankment the water content of thesoil at 2 m depth increased from 1.3 to 3.3% after 1 day, and thewater content of soil from 4 to 6 m depth increased by less than 1%.However, the soil at 8 m depth underwent a large increase inmoisture content and approached full saturation. After 3 days fromthe rainfall start, soil at 6 m depth underwent a large increase inmoisture content and approached full saturation, but the watercontent of soil at 4 m depth was still relatively low, generally lessthan 3%. After the rainfall event ended, the water content decreasedrapidly to about 2%, close to its original value, in 2 days.

For the sandy clay embankment, the water content of the soil at2 m depth increased from 10 to 26.7% (over 15%), but that of soil at

Table 1. The physical and mechanical parameters of soil in the embankment

Name of soilUnit weight of saturated soil,

γ (kN m−3)Effective cohesion,

c′ (kPa)Effective angle of internal

friction, w′ (°) wbPermeability coefficient of saturated

soil, kas (m s−1)

Porosity ofsoil

Gravel soil 21.4 0 35.2 12.5 1 × 10−3 0.3Sandy clay soil 17.6 17.2 18.3 10 1 × 10−5 0.45

Fig. 4. Triaxial test results: (a) sandy claysoil; (b) gravel soil.

Fig. 5. Soil-water characteristic curves.VG model is model of Van Genuchten(1980).

Table 2. The derived parameters of the Van Genuchten model

Name of soil Saturated volume water content, θs Residual volume water content, θr α (kPa−1) n m

Gravel soil 0.3 0.013 0.82 3.71 0.73Sandy clay soil 0.45 0.041 0.06 1.89 0.47



4 m depth increased by

decreased with depth, whereas after 2 days, it was higher in the soilat 4 m depth than in the soil above 2 m depth, and it decreased by c.1% in the soil above 2 m depth from the previous value. After 7 daysfrom the start of the rainfall event, the water content in soil at 8 mdepth increased to 8.3%, and 2 days later, it approached saturation.After the rainfall event ended, the water content of all soil above 8 mdepth decreased to c. 2% in 2 days.

For the sandy clay embankment, the water content of soil at 2 mdepth changed little in the first day. Three days later, it showed alarge increase in soil above 4 m depth, especially for soil above 2 mdepth, where it increased about 25% from the previous value. After5 days, the water content of soil above 6 m depth increased by morethan 3% from the previous value, and it increased over 17% in soilabove 4 m depth, whereas it changed little in soil above 2 m depth.Seven days later, the water content in soil above 8 m depth began toincrease, and it increased over 14% from the previous value in soilabove 6 m depth, whereas it changed little (

embankments, and consequently variation of soil density owing tothe change of water content occurred, and of strength with time. As aresult of these variations, the stability of the slope varied with timeduring the rainfall event, and this response differed for the twoembankment materials.

As shown in Figures 8 and 9, the porewater pressure obtainedfrom the FEM simulation correlated well with the monitored resultsfor the embankment.

The seepage field of the embankments under various rainconditions was obtained and the results were input into anotherstress–strain FEM model. Finally, the variation of the factor ofsafety of the slope with time was calculated based on the strength-reduction method in FEM, which was originally used by Matsui &San (1992) and Griffiths & Lane (1999).

The embankment considered in this paper is located in Ya’an,Sichuan Province. According to network data (www.baidu.com),the typical annual precipitation is c. 2000 mm in the study area, andthe largest daily precipitation on record was 430 mm on 14 July1974. The total precipitations of the two monitored events described

in this paper were 435.9 and 451.8 mm, and were of 3 and 9 daysduration respectively. For comparison of the calculated results, inthis study we assumed that the total precipitation was 450 mm andthat there were four types of rainfall: (1) rain intensity 50 mm d−1

and duration 9 days; (2) rain intensity 90 mm d−1 and duration5 days; (3) rain intensity 150 mm d−1 and duration 3 days; (4) rainintensity 450 mm d−1 and duration 1 day. Figure 10 shows thefactor of safety variation with time during the rainfall event for thetwo embankments.

For the gravel embankment, the factor of safety decreased mostrapidly in the first 1–2 days, from 1.83 to 1.21, 1.16, 1.09 and 1.07for the four rainfall types respectively, then decreased gradually to aminimum value. The minimum factor of safety was 1.16, 1.13, 1.09and 1.06 for the four rainfall types respectively. The greater therainfall intensity, the larger the decrease in factor of safety.Following the cessation of rainfall, the factor of safety increasedrapidly from theminimum value over a period of 1 day to 1.35, 1.37,1.37 and 1.34 for the four types of rainfall respectively. Therefollowed a lower rate of increase in the factor of safety over time.

Fig. 8. Variation of porewater pressure of soil in the embankments during the rainfall from 16 to 19 July 2011.



http://www.baidu.com

For the sandy clay embankment, the factor of safety of the slopedecreased differently for the four types of rainfall. At the time ofrainfall cessation, it decreased by 0.02 for the first type, by 0.01 forthe second type, by 0.01 for the third type and by 0.02 for the fourthtype. After rainfall ended, the factor of safety continued to decreasefor some time. The minimum factor of safety was 1.27, 1.26, 1.27and 1.33 respectively during the four types of rainfall.

Discussion

The infiltration of rainwater in the slope was divided into threestages (Feng et al. 1998).

(1) The first stage was the stage of wetting or completeinfiltration. At the beginning of rainfall, the soil was relatively dryand had lower permeability, lower water content, high matricpotential and high capacity to retain water. Therefore, rainwatercompletely infiltrated the soil; this was the wetting stage for thesurface layer.

(2) The second stage was the transition stage. In the surfacelayer, water content increased with continuous rainwater infiltration,and the capacity to retain additional rainwater decreased. Therefore,the rate of rainwater infiltration decreased, and the rainwater thatcould not infiltrate the soil started to accumulate on the surface ofthe slope.

(3) The third stage was the stage of stable infiltration. At thispoint, the surface soil layer tended to be saturated, and the rate ofinfiltration was reduced to its minimum value. The rate ofinfiltration was closely related to the saturated soil infiltrationcharacteristic.

However, under the influence of the characteristics of rainfall andthe infiltration of soil, not all three stages of infiltration were presentin every slope during the rainfall events. For example, if the rainintensity was small and the permeability of soil was high, only thefirst stage was observed during the rainfall event.

From Figure 7, it can be noted that the water infiltrated rapidlyinto the gravel embankment, and the water content in deep soilincreased rapidly and approached full saturation during the rainfallevent, whereas the water content in shallow soil remained at a lowvalue, only slightly larger than the initial condition. This indicatesthat the wetting front was able to move rapidly down through the fillin the gravel embankment, caused the deep soil to be saturated andmade the groundwater table rise from the embankment base,whereas the water content in soil above the groundwater table couldnot maintain an elevated water content, owing to the low waterretention capacity of the gravel fill. After the rainfall event ended,water was able to rapidly drain from the fill, and the soil watercontent at depth rapidly returned to values approximating the initialconditions (Fig. 11). Presumably, this was also a function of thehigher permeability of the gravel.

For the sandy clay embankment, the wetting front moved downslowly during the rainfall event. After rainfall ended, it continued tomove down to deep soil for a few days, whereas the shallow soilbegan to lose water over time. This may be explained by the lowpermeability and large capacity to retain water of sandy clay soil.

Figure 8 shows that the increase in porewater pressure within thegravel embankment varied consistently over time during the firstrainfall event. The middle of the embankment from depths below6 m to the base showed a positive porewater pressure that increasedwith depth. This indicates that the groundwater table had risen abovea depth of 6 m at the middle step of the embankment. After therainfall event ended, the elevated pore pressures began to dissipate,rapidly for the first 1–2 days and then at a much slower rate (or not atall) for the rest of the monitoring period. Furthermore, withapproach to the toe of the slope, the faster was the elevated porepressure dissipated and the greater its decrease. This may be because

the water in deep soil drained from the toe of embankment slope(Fig. 11), as noted above.

For the sandy clay embankment, the variation of porewaterpressure lagged behind the rainfall start, and the delay increased asthe depth increased. During the first rainfall event, only theporewater pressure in the surface layer (i.e. 8 m increased to a value over 0 kPa for the later part of the rainfallevent. In the toe of the embankment at depths >4 m, porewaterpressure increased to relatively high positive values immediatelyafter the start of rainfall. After the rainfall ended, porewater pressuredissipated over time, and with approach to the toe of the slope, thefaster it dissipated. This indicates that water could infiltrate easilyinto deep soil in the gravel embankment and cause the groundwatertable to rise because of the high permeability of the soil. Owing to itslow capacity to retain water, the gravel soil lost water rapidly, andgroundwater seeped away from the toe of the slope.

For the sandy clay embankment, the variation of porewaterpressure of the soil at depths >2 m lagged behind the rainfall start,and the delay increased with depth. However, it responded muchmore rapidly than in any of the other sections, and lagged behind therainfall start by only c. 0.5 – 1 days. After the rainfall event ended,the dissipation of porewater pressure in soil depths over 2 m clearlylagged behind the rainfall cessation, and porewater pressurecontinued to increase for a few days during the second rainfall.This could be because of the low permeability and good capacity toretain water of the sandy clay soil. Then the water infiltrated into thesoil slowly and the wetting front moved down slowly during therainfall. The porewater pressure responded similarly to the responseof the gravel embankment from the surface to a depth of 2 m; thereason could be that the soil was relatively dry, and that at a lowerwater content, high matric potential and high capacity to retainwater, rainwater was absorbed rapidly by the soil at shallow depths.After the rainfall event ended, the sandy clay soil could retain water,then cause the porewater to seep down to deep soil slowly and after alag in time.

Comparing Figure 8 with Figure 9, under constant total rainfall,the stronger the rain intensity, the greater the porewater pressure wasin the gravel embankment. Especially at the middle step and toe ofthe embankment, as the rain intensity increased, there was a positiveporewater pressure decrease immediately under the 6 m depth belowthe surface; this could be explained by the groundwater table risingafter a greater rainfall intensity.

The maximum value of porewater pressure did not vary with thechange in rain intensity in the sandy clay embankment. However,after longer duration and lower rain intensity, elevated porewaterpressures were maintained for a longer time. During the two rainfallevents, the porewater pressure decreased as the depth increased in



Fig. 9. Variation of porewater pressure of soil in the embankments during the rainfall from 7 to 16 April 2013.

Fig. 10. The relation between the factor of safety for the two embankment slopes and time.



the sandy clay embankment. This implies that the wetting front inthe sandy clay soil moved down slowly during the rainfall eventowing to the soil’s low permeability and good capacity to retainwater, and longer duration of rainfall was favourable for theinfiltration of rainwater into the soil.

From the above discussion, the infiltration of rainwater in thesandy clay soil can be divided into three phases: completeinfiltration, rate-reducing infiltration and stable infiltration. Thewater content usually first reached or approached the maximumvalue (i.e. saturated soil) in the surface layer, and then the wettingfront gradually moved down and the water content of deep soilsstarted to gradually increase over time. Hence, the porewaterpressure decreased with depth during the rainfall event. During therainfall event and after rainfall ended, the variation of water contentand the porewater pressure in the sandy clay embankment alwayslagged behind the rainfall.

In contrast, for the gravel embankment, owing to the soil’s highpermeability and small capacity to retain water, the infiltration and themoving down of the wetting front were faster than in the sandy claysoil. Unlike the sandy clay embankment, the water content of soilbetween the infiltration surface and the water table always stayed at alow value, which was much less than the water content of saturatedsoil. If the initial water table was shallow in the embankment, it could

readily rise. The porewater pressure usually increased with depth atthe middle step and toe of embankment slope, and this was clearlygoverned by the water table rise. Hence, unlike the sandy clay soil orother fine soil, rate-reducing infiltration and stable infiltration werenot observed during the rainfall event, and the infiltration can bedivided into only two stages in the gravel embankment: the completeinfiltration stage and the rising groundwater stage.

The issue of slope stability related to rainfall has been extensivelystudied for many years by many researchers and from differentperspectives. These studies have mainly focused on the effects ofthe precipitation, rain intensity and duration of rainfall (Ng & Shi1998a, b; Zhong 1998; Gao & Yin 2007; Miller et al. 2009).However, little attention has been paid to rainwater infiltration indifferent types of soils and the effects of time on slope stabilityrelated to rainfall.

According to the FEM results (Fig. 10), the effects of infiltrationon slope stability were different for the two embankments. Withrainwater infiltration the factor of safety of the gravel embankmentslope decreased by c. 0.67 – 0.77, whereas it decreased by only c.0.01 – 0.08, and not over 0.1, for the sandy clay embankment. Thisis because the rainfall could cause the groundwater table to riserapidly to a high elevation in the gravel embankment, whereas thiscould not happen in the sandy clay embankment because of its low

Fig. 11. The relation between cumulative infiltration flux and time in the surface layer of the embankments.



permeability. It should be noted that the runoff of rainwater on thesurface of the embankments was not considered during thecalculation process in this paper; therefore its effect on the factorof safety could not be investigated.

The variations of stability over time for the two embankmentswere also different. The factor of safety for the gravel embankmentslope decreased rapidly during a rainfall event and recovered withno delay almost immediately after the rainfall event ended, whereasthe factor of safety for the sandy clay embankment slope decreasedslowly during the rainfall event and continued to decrease forseveral days after the rainfall event ended. A possible reason is thatthe rainwater could infiltrate into the gravel embankment during therainfall event and seep out after the event ended because of thegravel’s high permeability and low capacity to retain water, whereasthe wetting front in the sandy clay embankment moved down slowlyduring the rainfall event and the groundwater needed a longer timeto seep out after the rainfall event ended because of the soil’s lowpermeability and good capacity to retain water.

From Figure 12, the minimum factor of safety was reduced byc. 0.1 when the rain intensity increased from 50 to 450 mm d−1. Inparticular, it decreased by c. 0.07 when the rain intensity increasedfrom 50 to 150 mm d−1, because the greater rain intensity causedthe groundwater to move to a higher elevation in a short time. Forthe sandy clay embankment, the effect of rain intensity on the factorof safety of the slope was smaller than that for the gravelembankment; when the rain intensity varied from 50 to150 mm d−1, the difference between the minimum and maximumfactors of safety was c. 0.06. This indicates that rainfall exceedingthe infiltration capacity of the sandy clay embankment should runaway along the surface of embankment owing to the soil’s lowpermeability. The factor of safety for the sandy clay embankmentwas controlled by two factors: rain intensity and duration. In thisstudy, the minimum factor of safety occurred during the rainfallevent with 90 mm d−1 and duration of 5 days, because this rainfallintensity and duration allowed enough rainfall to infiltrate into theembankment and over a long enough time to allow the wetting frontto reach a great enough depth to affect the factor of safety of the slope.

Conclusions

The infiltration of rainwater and its effects on slope stability arecomplicated issues with many factors interacting and influencing eachother. The following conclusions may be drawn from our results.

(1) The characteristics of infiltration in the two embankmentswere different. The infiltration process in the sandy clay

embankment could be divided into three stages: the wetting orcomplete infiltration stage, the transition stage and the steadyinfiltration stage. In contrast, for the gravel embankment, the rate-reducing infiltration and stable infiltration were not observed duringthe rainfall event, and the infiltration could be divided into only twostages: the complete infiltration stage and the rising groundwaterstage. With continuing rain, the wetting front in the sandy clayembankment moved down slowly and gradually, and the watercontent of soil decreased with depth. Thewetting front moved downvery rapidly in the gravel embankment, and the water content of thesoil between the infiltration surface and thewetting front maintaineda similar value and was far lower than the saturated water contentduring the rainfall events.

(2) The variation of porewater pressure in the sandy clayembankment lagged behind the start of the rainfall event and wasaffected mostly by the wetting front moving down. However, it didnot lag far behind rainfall in the gravel embankment, and thegroundwater table moved up rapidly owing to the infiltration ofrainwater.

(3) Owing to the differences in permeability and the capacity toretain water, the effect of infiltration on slope stability was greaterfor the gravel embankment than for the sandy clay embankment. Forthe infiltration of rainwater, the factor of safety of the gravelembankment slope decreased by c. 0.67 – 0.77, whereas itdecreased by only c. 0.01 – 0.08, and not over 0.1, for the sandyclay embankment. The variations of stability over time for the twoembankments were also different. The factor of safety for the gravelembankment slope decreased rapidly during a rainfall event andrecovered with almost no delay after rainfall ended, whereas thefactor of safety for the sandy clay embankment slope decreasedslowly during the rainfall event and continued to decrease for 2 –9 days after rainfall ended.

(4) The effect of rain intensity on the factor of safety of the slopewas smaller for the sandy clay embankment than for the gravelembankment, and the factor of safety for the sandy clayembankment was controlled by two factors: rain intensity andduration.

The infiltration of rain and its effect on the slope stability wereclosely related to the type of rainfall. The deductions made in thisinvestigation were based on the monitoring data and calculationresults for porewater pressure and water content in the soil of twoembankments during two rainfall events. Although both events hadthe same characteristics, such as intensity during thewhole period ofrain and a long duration, the results obtained may be not suitable forinvestigating the infiltration properties of rain and its effects onslope stability during complicated rainfall conditions, and furtherwork should be considered.

Acknowledgements and FundingWe are very grateful to the academic and technical staff of the State KeyLaboratory of Geo-hazard Prevention and Geo-environment Protection (SKLGP)of the Chengdu University of Technology, Sichuan Province, China. Thisresearch was financially supported by the Major State Basic ResearchDevelopment Program of China (973 Program: 2013CB733200), the NationalNatural Science Foundation of China (41202212, 41472274), and the YoungPeople’s Foundation of SKLGP (SKLGP2012Z008).

Scientific editing by Tom Dijkstra; Mike Winter

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