Landslides (2016) 13:1139–1149DOI 10.1007/s10346-015-0644-8Received: 13 November 2014Accepted: 7 October 2015Published online: 15 October 2015© Springer-Verlag Berlin Heidelberg 2015

Q.X. Huang I J.L. Wang I X. Xue

Interpreting the influence of rainfall and reservoirinfilling on a landslide

Abstract This paper focuses on the potential influence of rainfallparameters and reservoir levels on displacement of a reservoirlandslide. Displacements were monitored by a borehole inclinom-eter and four surface survey points. Monitoring data were obtain-ed from 1985 to 2010, covering the periods before, during, and afterimpoundment of the reservoir. A simple stability analysis has beenperformed to understand the real influence of reservoir levelchanges on slope stability and to interpret the movement patternduring the impoundment period. Analyses of available data indi-cate that the overall trend of the displacement could be character-ized by three distinct periods, corresponding to a slow creep phasebefore infilling, a sharp increase in velocity during the infilling,and another slow creep phase after infilling. Displacement ratesshow slight fluctuation in both Bslow creep^ episodes. Beforeimpoundment, the displacement pattern was clearly controlledby rainfall, in which the displacement rate during the rainy periodwas twice that in the dry period. Displacement response to rainfallhad a time lag of about 1–2 months. During the first 75 days ofimpoundment, displacement increased quickly with a linear trend,following the rising of the reservoir level with a rate of 2 m/day.Then, the displacement rate gradually decreased after the infillingof the reservoir until velocities slightly higher than that weremeasured before the infill. The result of the stability analysiscoincides with this trend, indicating the complex effect of theimpoundment: First, the decrease of stability of the landslideprevails over the increase in lateral support of the reservoir lake,while at a critical level, the increase in lateral support prevails overthe decrease in stability. During the period of normal operations,an inverse relation between water level fluctuations and velocityfluctuations was observed, and rainfall had no significant effect onthe sliding motion, with the exception that surface displacementsshowed a slight increase.

Keywords Reservoir landslide . Water impoundment . Reservoirlevel fluctuation . Monitoring . Displacement behavior

IntroductionLandslides can be triggered by a variety of external factors. Sea-sonal and torrential rainfall is the most important meteorologicalfactor that can activate or accelerate movement of the slidingmass, as has been reported in numerous studies (e.g., Vita et al.1998; Hong et al. 2005; Tsai 2008). Other factors than precipitationcan influence the deformation of slopes, which are located adja-cent to reservoirs or lakes. In different parts of the world, increas-ing attention has been focused on the stability of reservoir slopes,which may generate hazards to reservoirs and dams. In the early1960s, Jones et al. (1961) studied about 500 landslides that wererelated to the construction of the Grand Coulee Dam in theWashington State (USA). Results of these studies show that 49 %of the landslides occurred during the reservoir-filling period and30 % during periods of drawdown. Nakamura (1990) calculated fora Japanese case history that about 60 % of reservoir landslides

occurred during a sudden drawdown of the water level, andanother 40 % were triggered during a rising period of the water.Schuster (1979) noted that landslides induced by water level fluc-tuations in reservoirs involve various types of movement andgeologic materials. All these studies strongly demonstrate theimpact of reservoirs on the stability of bordering slopes.

The Vajont landslide of 9 October 1963 represents an outstand-ing and valuable case history for improving our knowledge of theinfluence of reservoirs on the stability of adjacent slopes (Kilburnand Petley 2003). To examine the influence of reservoir operations(filling and drawdown) on the stability of the slope of Monte Toc,the Vajont landslide has been back-analyzed in detail by Paronuzziet al. (2013). The authors found that the main destabilizing con-tributor to the landslide was the rate of filling and drawdown,which had a strong impact on the slope stability. More studiesevaluated the results of a sudden drawdown of the water level.Lane and Griffiths (2000) used the finite element method toproduce operating charts applicable to real structures subjectedto drawdown operations. Liao et al. (2005) obtained a relationshipbetween slope stability and the rate of water level drawdown.Other studies have been done to understand the importance ofchanges in reservoir levels for slope stability. Zhan et al. (2006)confirmed the importance of reservoir level changes on slopestability. Zangerl et al. (2010) reported the influence of fluctuatingreservoir levels on a deep-seated rockslide above the GepatschDam reservoir in the Austrian Alps. Research also showed thatsome existing old landslides in China were reactivated and beganto deform more noticeably during the fill of a reservoir (Wanget al. 2008; Li et al. 2010). Singh et al. (2012) investigated thetriggering of a huge landslide after a 20-m increase of the reservoirlevel of the Baglihar Dam. Pinyol et al. (2012) reported that a largelandslide was reactivated on the left bank of the Canelles Reser-voir, triggered by a drawdown of the water level, after several yearswith high water levels, with rates reaching values between 0.5 and1.2 m/day.

All these studies revealed some correlations of slope displace-ment or stability with reservoir level change. However, there havebeen few studies with simultaneous monitoring of surface andinternal displacements of landslides, in combination with rainfalland reservoir level changes. Furthermore, in most of these studies,the investigated relationship between reservoir level change andlandslide displacements covers only short time periods, and theyare mainly related to surface displacements. Despite the qualityand quantity of data available, some issues have not beencompletely analyzed and require further investigation. These is-sues include the differences in slope response to rainfall beforeand after impoundment, and displacements as a consequence ofreservoir level variations during various periods of initial im-poundment and normal operations.

In this paper, we will investigate the influence of rainfall pa-rameters and reservoir level changes using displacements of aborehole inclinometer and precise measurements at the surface

Landslides 13 & (2016) 1139

Original Paper

of four reference points. Data are from 1985 to 2010, coveringperiods before, during, and after reservoir impoundment. Thelandslide response to rainfall or reservoir level changes duringvarious periods is discussed in detail. In addition, we will discusssome of our unexpected results in relation to outcomes obtainedfrom previous research.

Location and geological setting of the study areaThe Jinlongshan slide is situated on the left bank of the YalongRiver, about 1.1-km upstream of the dam of the Ertan HydropowerStation in Southwest China. The slide is confined by two valleys onboth east and west sides, while on the south is Yalong River(Fig. 1a). Given the position of the slide, its potential for slidingis of great concern for reservoir users and state authorities. For abetter understanding of its probable evolution and the relationshipbetween reservoir operation and landslide motion and implica-tions for future reservoir management, a series of reconnaissanceefforts were made and a monitoring system was installed longbefore the impoundment of the reservoir. Based on the conclusionof geological and geotechnical investigations and according togeological conditions, developmental stages, and differentialmovements, the slide has three distinct segments, as shown inthe Fig. 1a. In this paper, we selected only zone I as the study area.

The landslide (hereinafter referred to as zone I) ranges inelevation from 1030 to 1395 m a.s.l. and is 200 m wide in an east-west direction parallel to the Yalong River and has a maximumlongitudinal length of about 900 m in a south-north direction.The failure surface was found at varying depths between 25and 30 m. The estimated volume is around 3.5×106 m3.

Almost half the landslide was submerged after filling of thereservoir.

Mean annual temperature varies from 11.50 to 19.70 °C in thestudy area, so nearly all precipitation fall as rain. The rainy periodis from June to October and 85–95 % of annual rainfalls from Julyto September.

Geological investigations included an emplacement of eightboreholes, one of which was equipped with inclinometer casings.Investigations before the dam construction identified the landslideas an ancient slide, with the slip surface developed roughly parallelto the topographic surface.

Bedrock outcrops in the area consist of a sequence of sedimen-tary rock units that correspond to the transition from the LateSinian to the Early Permian, overlapped by basalt formations ofthe Middle Permian. The stratigraphic column from youngest tooldest units is shown in Fig. 2 and covers the area between theAbulangdang and Jinlong valleys (see Fig. 1a):

1. Landslide deposits of quaternary material, Q4del

2. Basalt, P2β, Middle Permian3. Limestone, Yangxin group (P1y), Early Permian4. Claystone, Liangshan group (P1l), Early Permian5. Predominantly limestone with few claystones, Cretaceous6. Dolostone, Dengying group (Zbd), Sinian

Geomorphological observations and analysis of geotechnicaldrillings indicate that the slope cover is mainly composed oflandslide deposits which predominantly consist of stones and finer

Fig. 1 a Geo-morphological environment and location of the Jinlongshan slide with sliding zone I under study indicated on the orthophotograph. b Detailed geologicmap of the study area (zone I). Also shown are locations of boreholes, inclinometer, and survey points

Original Paper

Landslides 13 & (2016)1140

soil material with relatively high permeability. The deposits show aclearly layered structure, the upper part contains limestone blocksof the Yangxin group (P1y), and the lower part includes gravelclaystone of the Liangshan group (P1l).

Results of borehole interpretation and surface investigationsuggest that the slide zone is mainly composed of partly weath-ered claystone fragments, compressed tightly with a thicknessbetween 0.1 and 0.3 m. The slip surfaces developed along theclaystone layer of the Early Permian Liangshan group, first atthe clay/bedrock interface where the bedrock is close to thesurface and then within the clay layer, as confirmed by incli-nometer monitoring. Generally, the slide moves in a south-westdirection around 25°.

In order to evaluate the characteristics of the slip zone, the siteinvestigation was supplemented by a detailed laboratory testingprogram. Both undisturbed and disturbed samples were collectedfrom boreholes or outcrops and cut faces at different position. Toobtain the shear strength, three kinds shear tests were carried outwith direct shear apparatus: (1) consolidated quick test, (2) con-solidated slow shear test, and (3) reversal direct shear test, used tomeasure the drained residual strength. The specimen is shearedforward and then backward until a minimum shear resistance ismeasured.

The saturated condition was chosen for strength measure-ment, and the specimens were carefully saturated with vacu-um before the tests. Every test is repeated four or moreidentical specimens under different normal loads: 0.1, 0.2,0.3, and 0.4 MPa, respectively. From the results, the shearstrength parameters can be determined. Shear envelopes un-der different condition are shown in Fig. 3.

Based on the lab results, the critical geotechnical parame-ters were initially evaluated, and the range of shear strengthparameters values is given in Table 1. For slope stabilityanalysis, considering the influence of infill, shear strengthparameters were suggested choose among consolidated-quicktest shear strength and reversal direct shear test shearstrength.

Installation of instrumentation to monitor displacementLandslide investigations before construction of the dam identifiedthis mass movement as an ancient slow-creeping slide. Given itslocation and activity, the landslide posed a serious threat andrequired a careful study of its stability conditions.

In the mid-1980s and early 1990s, surface and subsurfacedeformations were continuously measured. Most monitoringrecords extend over 15 years, with some time series as long as20 years. Figure 2 gives an overview of the measurementsetup.

On April 23, 1985, an inclinometer system (IN-1) wasinstalled, with the main purpose of determining the depth ofthe sliding surface and to monitor subsurface movement ratesof the landslide. A guide casing for an inclinometer wasinstalled in a borehole that was drilled in the lower part, atelevation 1179 m.

From April 1992 to November 1997, four survey points (prefixedby SP-; data sampled by total station) were installed at elevations1161, 1231, 1327, and 1372 m, respectively (Fig. 2).

The local hydrological station situated near the reservoir pro-vided the rainfall time series. In this way, landslide displacements,reservoir levels, and rainfall were measured simultaneously.

Fig. 2 Geological cross sections A–B (see Fig. 1b), with the slide surface and monitoring system. IN-1 is the inclinometer used to monitor internal slope displacements; SP-1 to SP-4 are survey markers used to monitor surface displacement

Landslides 13 & (2016) 1141

Monitoring was done with variable frequencies, depending onmeasurement type and conditions in different periods. During theinitial filling period, inclinometer data were read every 2 days,starting in 1998. Reading frequency changed to once a day, until 10August when the interval changed again to 2 days until midNovember 1998. Survey points were observed four to six times,from 1 May to 22 July 1998.

Analysis of the internal displacements is limited to a perioduntil 2007, because of destruction of the inclinometer in Decemberof that year. All displacements mentioned in this case study referto the horizontal component.

Inclinometer IN-1 detected the depth of the rupture sur-face between 29 and 31 m (around elevation 1150 m, Fig. 4).The displacement profiles over 10 years show a thin defor-mation zone with a large rigid plug on top. The maximumdeformation was found at a depth of around 29 m. Since thesliding section moved as a rigid block, displacements at thesurface can be considered consistent with the internaldisplacements.

Overview of monitoring resultsA summary of monitoring results over 25 years (1985–2010) isshown in Fig. 5. Creep rates measured by the inclinometerbefore impoundment of the reservoir were relatively constant,with an average velocity over 1985–1998 of 0.72 mm/month.From early May to late October 1999 during the filling period,the rise of the reservoir level coincided with a sharp rise of thedisplacement measured by both the inclinometer and surveypoints SP-1–SP-4. From December 1999, displacements gradual-ly returned to a slow creeping phase, when the reservoir reachedthe normal operation phase, and the annual reservoir levelvariation was between 1155 and 1200 m (Fig. 5).

Both surface displacement and subsurface displacement ratesshowed strong correlations with the reservoir level variations.Figure 5 shows a clear response of the displacement to thereservoir level rise, during the initial infill. With a rise of thereservoir level from 1030 to 1175 m, the graphs show a continuousincrease in displacement during that period, varying from 20 mmto over 60 mm at the surface and 44 mm in the subsurface.

Fig. 3 Shear envelopes for direct shear tests. a, b Weathered claystone fragments, sampled from crushed zone; c, d claystone, sampled from boring core. Consolidated-quick test (1), consolidated-slow shear test (2), and reversal direct shear test (3)

Table 1 Summary of the shear test results

Type of direct shear test Range of shear strengthc (kPa) f (tgφ)

Consolidated-quick test 19–36 0.32–0.51

Consolidated-slow shear test 11–24 0.27–0.44

Reversal direct shear test 3–12 0.24–0.36

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Landslides 13 & (2016)1142

Figure 6 also clearly shows that overall velocities during theinfill period (May 1998–October 1999) were significantly largerthan in the foregoing period (November 1985–April 1998). Theaverage velocity measured with the inclinometer was 7.5 mm/month, more than ten times greater than in the period prior toimpoundment. Surface displacement rates increased to between 3.1

and 11.3 mm/month, representing an 8 to 19.5 times increasedvelocity.

In summary, the displacement pattern is characterized by threedistinct periods, corresponding to a slow creep phase before res-ervoir filling, a sharp increase in speed during infilling, and an-other slow creep phase after filling. During the impoundmentphase, the displacement rates were fully controlled by the risingwater level. The question remains what factors influenced theslight fluctuations during both creep periods.

Temporal variations and behavior during different periods

Displacement characteristics before impoundmentBefore the impoundment of the reservoir, the displacement pat-tern was clearly controlled by rainfall. The displacement rateduring the rainy season (average 0.97 mm/month) was two timesgreater than in the dry season (average 0.48 mm/month). Figure 7shows a higher resolution of the displacement data for the period1985–1997. It shows a close correlation of displacement rate fluc-tuations with seasonal fluctuation patterns of the rain. The periodof heavy rainfall is common from May to October, whereas themaximum displacements are from September to October. Thus,there is a delay in response of maximummonthly displacements tothe maximum monthly rainfall of about 1–2 months.

The response time to rainfall depends on several factors. Themoisture content and the difference in permeability of the differ-ent layers can explain the time lag. In this study, as describedearlier, the rainy period is from June to October. Much of theprecipitation received in June and July is absorbed by the dryunsaturated zone. During this period, the water flux toward thesaturated groundwater zone is negligible. By September, the mois-ture content in the unsaturated zone is sufficient to induce adownward flux to the saturated zone resulting in a rise of the

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Landslides 13 & (2016) 1143

groundwater level (Bogaard and Van Asch 2002) and a delayedincrease in the displacement rate halfway the rainy season.

Furthermore, the low permeability of the clay layer in which theslip surface has developed may have contributed also to the de-layed reaction of the landslide to rainfall, reflecting that it takestime to adequately saturate the clay and to facilitate a flux to thegroundwater and reduce the effective strength of the slip surface.

Displacement variations during initial impoundmentThe initial impoundment here refers to the period from the start ofthe fill of the reservoir (May 1998, level at 1031 m) until the time thereservoir has reached the normal operating level (1200 m) on 20October 1999. The initial impoundment procedure can be dividedinto three phases (Fig. 5), and the development of displacementduring this period was analyzed based on data recorded from theinclinometer.

Phase 1 (first of May until about mid July1998) can be describedwith a linear increase of the displacement both with time (Fig. 8a)and the rise of the reservoir level (Fig. 9). The level increased about85 m in the first 2 weeks (up to EL.1115 m) and then with a more orless constant velocity of 2 m/day until EL.1175 m. As a conse-quence, there is about 4.9 mm displacement occurred in theformer part (can be seen in Fig. 9), and then the displacement rateshows an average value of about 10 mm/month in the followingpart, which means that there is a delay of 2 weeks in response towater level increase. The generated displacement rate is

significantly higher than in the period before the filling of thereservoir (Fig. 5). During this period, about 25-mm displacementoccurred.

The subsequent displacements during phase 2 (between midJuly 1998 and mid October1999) reached approximately 20 mm,while the reservoir level fluctuated between 1155 and 1180 m. Alinear trend line gives a good description of the displacementresponse to time in this period, as shown in Fig. 8b, with anaverage displacement rate of 6 mm/month. It is interesting to seethat after a rapid rise of the water level, the subsequent more orless constant high water level in the reservoir has a damping effecton the displacement velocity of the landslide.

Phase 3 (about October 1998 to October 1999) is characterizedby also a more or less constant trend (Fig. 8c) in the displacementvelocity, but the velocity is significantly lower than in phase 2. It iscomparable with the creep velocities measured before the infill ofthe reservoir (see Fig. 5). This is more or less in accordance withthe theory of slope equilibrium where a slope inundated (partly)by the static water of a lake has the same stability as anonsubmerged slope.

During the period of filling, the increase of the reservoir levelhas an effect on the mechanical conditions of the reservoir slopes,such as a decrease of the resistance parameters, and changes inpore pressure. The displacement behavior simply demonstratesthat the filling can decrease the stability, corresponding with anincrease in displacement rate, particularly in the phase1 period.

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Fig. 7 Pre-impoundment relationship between monthly displacement and monthly rainfall (from November 1985 to December 1997), data from inclinometer

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Landslides 13 & (2016)1144

Furthermore, the difference between the three phases also indi-cates that the influences of impoundment varied under differentcondition. We will try to give below, by means of a stabilityanalysis, an explanation of these differences.

Displacement trend during post-impoundmentMan-made impoundments are characterized by filling-drawdownoperations that determine cyclic pore water pressure variations

influencing the stability conditions of the reservoir banks overtime resulting in different displacement patterns. Both surfaceand subsurface data collected during the period November 1999to June 2012 were used to analyze the displacement characteristicduring the normal operation period.

Results show that the surface displacements reached 151, 177,169, and 143 mm (corresponding to SP-1, SP-2, SP-3, and SP-4,respectively) until early June 2010, while the subsurface displace-ment (IN-1) amounts to 92 mm until mid June 2007 (see Fig. 5).The data in Fig. 5 also indicates that the slide showed a long-termslightly decreasing trend rate in the order of 10–20 mm/year.

The pattern of the subsurface displacement is clearly shown inFig. 10. A nonlinear trend line provided the best fit to the data witha high coefficient of determination, also indicating a slight de-creasing trend in the velocity (Fig. 10a). The overall trend shows acreeping movement, which is similar to the displacements at thesurface. However, the detailed data clearly shows the influence ofreservoir level fluctuation on displacement during 1999–2007.Typical examples are given in Fig. 10b, c, where the subsurfacedisplacement rate increased to a maximum when reservoir leveldescended to a minimum and vice versa.

Figure 11 shows in combination with the yearly precipitation thedisplacements of both surface and subsurface devices also a slight-ly declining trend in the period after reservoir impoundment (1999

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Landslides 13 & (2016) 1145

to 2009). In general, we see over the whole period 1999–2009 anexponentially declining trend in the displacement rate which is inaccordance with an exponentially declining trend of the yearlyprecipitation. In detail, the year 2005 differed from the generaltrend, with an increase of annual displacement caused by anextraordinary amount of rainfall that year. Cumulative annualrainfall was nearly constant during 2003–2008 (about 870 mm/year), except in 2005 with an increase of 150 mm. In that year, anincreased displacement was recorded in the upper part by SP-2and SP-3. However, this was not recorded by the inclinometerlocated in the inundated section. Figure 11 shows that from theyear 2003 movement rates in all zones has slowed down to a levelof 0.3 mm to 1.0 mm per month (III-2), while the rate duringperiod III-1 was between 1.2 and 1.9 mm/month (also see in Fig. 6).

The slight but not uniform reaction of the velocity in years withrelative higher precipitation and the differences in velocities in theperiod III-1 and III-2 needs a further explanation.

A case can be made that both displacement questions are acomplex response involving different responses to rainfall andreservoir fluctuation by segments of the landslides. During thenormal operation period, the reservoir level fluctuated between1150 and 1200 m, and the total slide can be considered to have threedifferent segments: an upper segment which is never inundated(elevation range from 1200 to 1400 m), a periodically inundated

middle segment (elevation range from 1150 to 1200 m), and apermanently inundated lower segment (elevation range from 980to 1130 m).

The difference between displacements by the three segments isinterpreted to be responses to rainfall and reservoir fluctuation inthe following manner. For the upper segment, rainfall was thepredominate factor. When there is an increase in rainfall, conse-quently, a higher displacement occurred, as indicated by SP-2 andSP-3; especially in 2005, there is a clear response to a higherprecipitation. For the middle segment, rainfall may be the second-ary factor compared to the effect of reservoir level fluctuations.The IN-1 data located in the middle segment (Fig. 2) clearlydemonstrates the influence of the fluctuation of the water level(Fig. 10). The lower segment was completely submerged since thefilling during a long time and the softening effect on the slidingsurface make displacement less sensitive to rainfall and the fluc-tuating reservoir level. This explanation also helps in interpretingthe decrease in annual displacement, if we surmise that the time toreach the maximum extent of the softening effect is 4 years.

Simplified stability analysesAs a consequence of reservoir level and hence pore water pressurevariations, the stability of the reservoir banks changes over time.In order to understand the real influence of reservoir level changes

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Fig. 10 Comparison of reservoir elevation and displacement during period of normal operation (a), with two typical periodic cycles shown in detail (b and c). Data arefrom inclinometer IN-1 at 29-m depth (Fig. 4)

Original Paper

Landslides 13 & (2016)1146

on slope stability, simplified stability analyses have been carriedout, combined with seepage analyses. The main objective of thiscombined seepage-slope stability approach is to evaluate the factorof safety (FOS) variations during the initial impoundment period.

The evolution of the reservoir-induced groundwater level with-in the slope and the related change in pore water pressure issimulated using the program SEEP/W (Geo-Slope InternationalLtd., 2007), whereas limited equilibrium slope stability analyseshave been carried out employing the program SLOPE/W (Geo-Slope International Ltd., 2007), adopting Morgenstern and Price’s(1965) method with a half-sine inter-slice function. Following thisapproach, the results of the two-dimensional transient seepagesimulation have subsequently been used for the slope stabilityanalysis employing the limit equilibrium method (LEM) in orderto evaluate the variations in the FOS caused by reservoir filling anddrawdown. The geometry and location of the slip surface of thesimplified model adopted for the LEM were chosen according tothe inclinometer information and the morphological and strati-graphic structure along sections A–B in Fig. 1b.

In the slope stability analyses, only the influence of the waterlevel of the reservoir has been considered, and rainfall has beenneglected. Three cases are simulated: (1) There is no effect on theshear strength of the shear zone, while the effect of the lateralsupport of the reservoir was simulated with buoyancy by changingmaterial unit weight of the submerged segment, (2) a softeningeffect was taken into account, by decreasing the cohesion in thesubmerged section, and (3) the softening effect was taken intoaccount, by a decrease of both the cohesion and the coefficientof internal friction.

For the analysis, the variation of the shear strength parametersof the landslide material were determined for different shearstrength pairs. After co-evaluation of laboratory tests for mechan-ical properties, the characteristic range was estimated, and prop-erties have been assigned to the slide surface as shown in Fig. 12.

The simulation begins with the original landslide before theimpoundment, and the FOS was calculated for a stepwise increase

of the water level with 20 m. The variation of the FOS in relation tothe rise of the reservoir level for the three cases is shown in Fig. 12.

During impoundment, the slope stability initially decreases, butat a certain reservoir level, the FOS starts to increase (Fig. 12). Thewater level belonging to the minimum FOS value (circled points inFig. 12) is called the critical level hereafter. For case I, the finalvalue of the FOS is higher than the initial value before the im-poundment (Fig. 12), which is an indication of the buttressingeffect of the reservoir lake, also shown in case 2 and case 3.

Figure 12 shows a significant variation in the critical level of80 m (EL. 1110 m), 100 m (EL. 1130 m), and 140 m (EL. 1170 m)respectively for the three scenarios. This suggests that, owing tothe influence of the softening effect, the time required to reach thecritical level is variable.

These calculated results help to explain the differences in dis-placement rates between the three phases (Fig. 8) during the entirefilling period. The examined simulation cases do not correspondto reality completely. However, the slope behavior in FOS agreeswith the general trend observed from the monitoring.

During the impoundment, highly permeable materials facilitatethe formation of reservoir-induced groundwater storage, which

0

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case 3 c=10 kPa, f=0.3

Fig. 12 Variation of the factor of safety during filling. Different values of the shearzone parameters are considered

Landslides 13 & (2016) 1147

deeply penetrates inside the slope, resulting in a decrease of thestability of the landslide mass. At the start of the filling process, thedecrease in resistance as a consequence of an increase in porepressure prevails strongly over the stabilization by the buttressingeffect of the reservoir lake. As a consequence, the decrease of theFOS induced an increase in displacement rate, during the first75 days (phase 1). When the reservoir reached the critical level,for example 1175 m, the FOS starts slightly to increase, whichmeans an increasing effect of the lateral force exerted by the lakeon the stability of the landslide (compare phase 1 with phase 2).The buttressing effect reached its maximum at 1200 m, when themiddle segment and the lower segment were completely sub-merged (Fig. 2). The final velocities are higher after the fill thanbefore the fill.

Discussion and conclusionIn this paper, the behavior of the landslide at the Ertan DamReservoir and its main influencing factors have been investigatedand analyzed based on monitoring data over 25 years. It covers theperiod before, during, and after impoundment of the reservoir.The overall displacement trend was characterized by a creep peri-od before filling, a sharp increase in velocity during filling, and anattenuation of the velocity followed by another creep phase afterfilling of the reservoir.

Given the observed displacement rates, before impoundment(7–15 mm/year, Figs. 5 and 6) and during the normal operationperiod (10–20 mm/year), the landslide complex can be classified asBvery slow^ (IUGS 1995; Dikau et al., 1996), i.e., with a typicalvelocity of 16 mm/year (Hungr et al. 2014). The pattern of dis-placement was dominantly controlled by rainfall, while the reser-voir level was the main impact factor since the filling of thereservoir.

Rainfall can have a significant effect on slide displacement ratesthrough loading of the slope and reduction of effective shearstrength of the failure surfaces (Alonso et al. 2003; Hong et al.2005). Rough data of the water level observed in the inclinometerbore hole indicates that the groundwater level was 1–2 m above theslide surface during the rainy period and under the slide surfaceduring the dry period before impoundment. The displacement ratein the rainy period was twice that in the dry period, clearlyrevealing the influence of rainfall on displacement.

The influence of rainfall on the movement often shows a timelag. The response time to rainfall depends on several factors(Corominas and Moya, 1999; Coe et al., 2003; Ayalew, 1999; Sorbinoand Nicotera 2013; Wilkinson et al., 2002; Dixon and Brook, 2007;Bernardie et al., 2014). In this study, the monitoring result of thiscase study shows a time lag of 1–2 months between maximummonthly precipitation and maximum monthly displacement(Fig. 7), resulting from a buffering effect of the dry unsaturatedzone and the lower permeability of the clay layers below the higherpermeable landslide deposits and the reducing effective shearstrength during continuous displacement.

The displacement shows a sudden increase in displacementduring the first 75 days (phase 1) of the fill of the reservoir. Itpoints to a decrease of the stability of the landslide body, due to anincrease in pore pressure by the infiltrating reservoir water, whichat first could not be counterbalanced by the increase in lateralsupport of the lake. In phase 2 and especially in phase 3, when thereservoir has been filled up to nearly the normal operating level,

an attenuation of the velocity was observed probably because thelateral support of the lake became more effective (phase 3). Thedelayed effect of the water level of the lake on the stability of theland slide was also evident from the results of the stability analysisas shown in Fig. 12.

The displacement reduces again to a creep velocity slightlyhigher than the velocity before the impoundment of the reservoir,which was ascribed to a reduction of the intrinsic shear strengthdue to a permanent submergence of a part of the slip surface.

The strong decrease in displacement rate after the reservoirwater reached that the 1170 level appeared to be a strong prooffor a Btheoretical critical level,^ also demonstrated in our stabilityanalyses, above which further impoundment leads to an increasein slope stability, first defined by Kenney (1992) and later byBromhead et al. (1999), Lane and Griffiths (2000), andMichalowski (2009). In our case, 1170 m may be the critical level,when the landslide is inundated over about one third of its totallength (see Fig. 2).

Internal monitoring data shows that the slide has a thindeformation zone with a large rigid plug on top with nointernal shear deformation. A comparison of the inclinometerresults before and after the filling shows an increase in thethickness of displacement zone, as shown in Fig. 4. This mayhave also contributed to the increase in velocity during thefill of the reservoir (Van Asch et al. 2009) and may alsoexplain the difference in final (creep) velocity before and afterimpoundment of the reservoir.

On a more detailed scale, the temporal displacement character-istics of an active sliding body are clearly controlled by seasonalfluctuations of the reservoir level, with peaks in the velocity whenthe water levels are at their lowest (and vice versa) as is demon-strated in our case by Fig. 10b, c. This was also reported in otherstudies (e.g., Kümpel et al. 2001; Zangerl et al. 2010). It means thatat a fall to a minimum reservoir level and thus an immediatedecrease in lateral support of the water, pore pressures in thelandslide are still high which generate an increase in velocity. Ata rise to a maximum level, the lateral support has an immediatepositive effect on the stability of the landslide inducing a decreasein the velocity.

AcknowledgmentsThis research was financed by the Research Foundation of SKLGP(No. SKLGP2011Z011) and the National Foundation for NaturalScience of China (No. 41402262 and No. 41302245). Investigationswere implemented and coordinated by Chengdu Engineering Cor-poration Limited. We express our gratitude to Dr. Th. W. J. vanAsch (Utrecht University, The Netherlands) for helping improvingthe structure and the language of the paper and advices about theinterpretation of the data. The authors thank the anonymousreviewers for their helpful suggestions for improving the paper.

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Q. Huang : J. Wang ())State Key Lab of Geohazard Prevention and Geoenvironment Protection,Chengdu, 610059, Chinae-mail: geo_cdut@163.com

Q. Huang : J. Wang : X. XueChengdu University of Technology,Chengdu, 610059, China

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