complex rock slope deformation at laxiwa hydropower...

ORIGINAL PAPER

Complex rock slope deformation at Laxiwa Hydropower Station, China:background, characterization, and mechanism

Min Xia1,2 & Guang Ming Ren1& Tian Bin Li1 & Ming Cai2,3 & Tian Jun Yang4

& Zong Li Wan4

Received: 11 August 2017 /Accepted: 10 August 2018 /Published online: 1 September 2018# Springer-Verlag GmbH Germany, part of Springer Nature 2018

AbstractAn unstable rock slope called the Guobu landslide, observed in granite with a maximum cumulative displacement of 45 m,located on the upper stream of the dam of the Laxiwa Hydropower Station, China, and directly threatening the safety of the damand the people living downstream, was investigated. Detailed field surveys, geological structure investigations, remote sensingimage analysis, and GPS displacement monitoringwere carried out to investigate the deformation characteristics of the slope afterreservoir impoundment, historical deformation, ancient landslide reactivation, the influence factors of the large deformation ofthe slope, and the mechanism. Based on the results of the above analyses, it was seen that there is a clear geological structuraldependence of the failure mechanisms in the study area, and that the complex failure mechanisms include toppling, subsidingwedge failure, rockfalls, and tension cracking in deep-seated rock masses. Moreover, analysis of the remote sensing imagesindicated that the ancient landslide had been reactivated between 2005 and 2008. Detailed field displacement monitoring datashowed that the whole slope was deforming significantly and continuously with the increase of the water level in the reservoir.The displacement rate had a positive correlation with the variation of the reservoir water level, increasing and decreasing as thethe latter rose and fell, respectively. The vulnerable geological conditions, water infiltration, and the reservoir impoundment werethe main factors causing the large deformation of the Guobu landslide. Water infiltration was the driving force resulting in thereactivation of the ancient landslide, and the reservoir impoundment accelerated the slope deformation. The mechanism of thelarge deformation of the rock slope was a combination of the upper pushing deformation induced by wedging and toppling, dueto the ancient landslide reactivation, and the lower traction deformation due to the reservoir impoundment.

Keywords Landslide . Bank slope .Mechanism . Reservoir impoundment

Introduction

In a mountain environment, construction of a dam creates areservoir that can affect the stability of adjacent valley slopes.

Many landslides have been triggered by reservoir fillingand drawdown operations (Schuster and Wieczorek 2002;Zhu et al. 2011; Paronuzzi et al. 2013; Gu et al. 2017). Theimportance of changes in reservoir water level for bankstability was emphasized by Jones et al. (1961), who ana-lyzed the frequency of landslides related to the GrandCoulee dam from 1941 to 1953, and found that about 50and 30% of landslides occurred during water filling anddrawdown operations, respectively (1961). Nakamura(1990) noted that about 40% of reservoir landslides inJapan were triggered in the periods of rising water levelsin the reservoirs. The Vajont landslide represents an impor-tant and valuable case history for improving knowledge onlarge-scale rock slope failures and understanding of theinfluence of reservoirs on the stability of adjacent slopes(Nonveiller 1967; Müller 1968; Tika and Hutchinson1999; Petley and Petley 2006; Alonso and Pinyol 2010).

* Guang Ming [email protected]

1 State Key Laboratory of Geohazard Prevention and GeoenvironmentProtection, Chengdu University of Technology, Chengdu 610059,China

2 MIRARCO, Laurentian University, Sudbury, ON P3E 2C6, Canada3 Bharti School of Engineering, Laurentian University,

Sudbury, ON P3E 2C6, Canada4 China Hydropower Consulting Group, Northwest Institute of Survey

and Design, Xian 710065, China

Bulletin of Engineering Geology and the Environment (2019) 78:3323–3336https://doi.org/10.1007/s10064-018-1371-x

http://crossmark.crossref.org/dialog/?doi=10.1007/s10064-018-1371-x&domain=pdf

mailto:[email protected]

A reservoir-induced landslide near the Zhaxi dam inChina in 1961 resulted in more than 70 casualties (Jinand Wang 1988). In recent years, the Qianjiangping land-slide occurred in the Three Gorges areas in 2003 andclaimed 14 lives with 10 people missing, and resulted insevere damage to the local environment and economy

(Wang et al. 2004; Yin 2007). At other project sites, someexisting old landslides have been reactivated and begun todeform noticeably, including the Shiliushubao, Shuping,and Maoping landslides (Wang et al. 2005; Qi et al.2006; Xia et al. 2013; Xia and Ren 2015).

The reactivation of a large rock slope (the Guobu slope),located 500 m upstream of the Laxiwa hydropower dam, isintroduced in this paper (Fig. 1). The reservoir impound-ment date was March 1, 2009. In late May 2009, the Guobuslope showed significant deformation, with new cracksappearing on the crest of the slope and frequentlyoccurring rockfalls. This is a representative case studyrelated to rock slope deformation associated withreservoir impoundment, and a few studies have beenperformed on this unstable slope. For example, Zhang etal. (2013) and Shi et al. (2017) used satellite radar remotesensing to investigate the displacement history of theGuobu lands l ide and the re l a t ionsh ip be tweendeformation and impoundment. Lin and Liu (2016)adopted a fluid–solid coupling model to investigate thelarge deformation mechanism of the Guobu landslide.

Compared with other case studies in the literature, theGuobu slope has an extremely large displacement, and itsfailure mechanism is quite complex. Firstly, up to July2014, the maximum cumulative displacement had reached45 m, and further development of the deformation of theslope was a great concern for the safety of the dam and ofthe people living downstream. Secondly, there is a cleargeological structural dependence of the failure mecha-nism, which include toppling, subsiding wedge failure,rockfalls, and tension cracking in deep-seated rockmasses. Geological structures such as joints, shears, andfaults influence slope deformation (Xia 2015). This com-plex failure mechanism is similar to case studies reportedby some researchers (e.g., Norrish and Wyllie 1996;

Fig. 1 a Google Earth image of the research area, b view of the unstableslope

Fig. 2 Geology map of Laxiwadam site area

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Braathen et al. 2004). They noted that the deformationboundary was constituted of large-scale cracks or faults,which was the precondition for such a large slope defor-mation to occur. Large rock slopes associated with reser-voir impoundment with such a large displacement occur-ring at the Guobu slope have not been reported to abroader international community.

To improve our understanding of complex rock slope fail-ure mechanisms, historical deformation and reactivation ofancient landslides, and the influence factors for large defor-mation and related mechanisms, we have carried out detailedfield surveys, geological structure field investigations, remotesensing using satellite imaging technology, and displace-ment monitoring since 2009 at the Guobu slope site.Detailed monitoring data and remote sensing imageswere used to analyze the reactivated time of the ancientlandslide and the triggering mechanism, understand therelationship between the reservoir water level and thelarge deformation of the slope, and to investigate theinfluencing factors and the mechanism.

Geology and setting

The unstable rock slope is located 500 m upstream of theLaxiwa hydropower dam, along the east bank of the YellowRiver (Fig. 1a). The elevations of the deformed slope rangefrom 2370 to 2950 m a.s.l. (above sea level). The average

Fig. 3 Geological map of theunstable slope: 1 Mesozoicgranite, 2 boundary of unstableslope, 3 cracks observed on theground surface of the slope, 4crack (LF1), which defines theupper boundary of the unstableslope, 5 large-scale grabenstructures located at the crest ofthe slope, 6 profile A–A’, 7 areaunder the water level of thereservoir, 8 ancient landslide, 9the location of the photo in Fig. 4b

Fig. 4 Back scarp: a the back scarp, traced along crack LF1, defines theupper boundary of the unstable slope; b movement plunges observed atthe back scarp (orientation is expressed as dip direction and dip in degree)

Complex rock slope deformation at Laxiwa Hydropower Station, China: background, characterization, and... 3325

slope gradient is 42°. The slope has a maximum longitudinallength of 1050 m along the river flow direction and a width of350–470 m, covering an area of 11.5 × 104 m2. As can be seenfrom Fig. 1b, the crest of slope is triangular in plain view, arelatively flat platform which is 750 m long and 50–290 m

wide. With a maximum horizontal thickness of over 250 m,the slope has a volume of 3 × 107 m3.

The bedrock in the study area is Mesozoic granite (Fig. 2).Figure 3 presents the geological map of the unstable slope. It isbounded by a distinct scarp, LF1, at the rear with an ele-vation difference of 30 m, which defines the upper bound-ary of the deformed block (Fig. 4a). The back cliff tracesalong the pre-existing crack, LF1, which is approximately700 m long. The attitude of movement plunge of the crackLF1 is 300/70 ± 10° (Fig. 4b) (orientations of discontinu-ities are expressed as dip direction and dip in degree). Thestructure of the slope is similar to cases reported by Norrishand Wyllie (1996) and Braathen et al. (2004). We agreewith others that a deformation boundary constituted oflarge-scale cracks or faults is a precondition for large slopedeformations to occur.

Three major (J1, J2, J3) and one minor (J4) discontinu-ity sets were identified in the strata based on the fieldmeasurements (Figs. 5, 6), including two sets of steeplydipping joints and two sets of slightly to moderately dip-ping joints. The mean orientations of each discontinuityset are presented in Table 1. Joint set J1 dips steeply in thesame direction as the slope and joint set J2 dips steeply

Fig. 5 Stereographic projection of the four discontinuity sets

Fig. 6 The four discontinuity setsobserved in the slope (seeorientations in Table 1)

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into the slope. Both have a very high persistence and cutthe blocky strata into a platy block structure.

Moreover, four grabens (LF54, LF55, LF53, andLF56), traced along the strike of the slope, 400–450 mlong and 20–50 m wide, were identified at the crest ofthe slope (Figs. 3 and 7; Table 2). These graben struc-tures were formed from subsiding wedge failures trig-gered by a combined effect of toppling and slidingalong joint sets J2 and J1.

Three types of rock mass structures, i.e., loose,cataclastic, and blocky structures, were identified withinthe unstable slope based on the data revealed by explo-ration adits, boreholes, and geophysical explorations(Fig. 8). Meanwhile, the deformed boundary betweenthe unstable block and undisturbed rock masses was alsoidentified.

Historical deformation and reactivationof the ancient landslide

Historical deformation

In 1989, an ancient landslide at the crest of Guobu slopewas identified during the geological survey conducted be-fore the construction of the dam started. The ancient land-slide ranges from 2750 to 2950 m a.s.l., with a back scarp

that has an elevation difference of 20 m (Fig. 9a). The backscarp (LF1) and the low-angle fault (HF104) define theupper and lower boundaries of the ancient landslide, re-spectively (Fig. 9a, b). Under the back scarp, there was aflat platform where four large-scale grabens were identi-fied. These grabens, with the maximal length up to 500 m,traced along the strike of the slope. Figure 10 shows pho-tographs of the crest taken in 1989 and 2003. As can beseen from Fig. 10, no visible cracks or ground deformationcould be identified on the crest between 1989 and 2003.Moreover, simple observation piles had been set from 1991to 1997 to monitor the slope displacement. The displace-ment observation piles were installed on both sides of thegraben structure (Fig. 11). The monitoring results indicatethat the displacement increase was not significant from1991 to 1997 (Fig. 12).

Reactivation of ancient landslide

The reservoir impoundment date was March 1, 2009.After the impoundment, some tension cracks were foundon the crest of the Guobu slope in late May 2009 alongwith the occurrence of some small-scale rockfalls. To in-vestigate the deformation feature at different times and thecause of the ancient landslide reactivation, remote sensingimages were used to analyze the ground surface deforma-tion before and after the reservoir impoundment from2004 to 2010. The high-resolution satellite images(Fig. 13a–c) were from the America QuickBird May2004, WordView-1 May 2008, and the WordView-2June 2010, respectively.

The cracks on the ground and the back scarpinterpreted by the remote sensing images are shown inFig. 13. As can be seen from Figs. 13a–c, there was nosignificant change of the number of cracks on the groundsurface from 2004 to 2008; however, there was a largeincrease in 2010 after the reservoir impoundment. The

Table 1 Summary of discontinuity sets identified in the study area

Dip (°) Dip direction (°) Strike (°)

J1 60–80 NW NE30

J2 60–70 NE–E NNW–SW

J3 15–30 NW NE30–60

J4 20–35 SE–SW NW340–NE30

a LF56 b LF53

Fig. 7 The crest of the unstableslope


deformation feature of the back scarp from 2004 to 2010is shown in Figs. 13d–g. The back scarp was clearly seenin 2004 (Fig. 13d), but no visible tension deformationcould be identified. Figure 13e shows that new discon-tinuous tension cracks appeared at the back scarp in2005, and extended their lengths in 2008 (Fig. 13f). Acontinuous crack was exposed at the back scarp in 2010(Fig. 13g). Based on the above analysis, it was concludedthat the ancient landslide had been reactivated between2005 and 2008 before the reservoir impoundment.However, the numbers of cracks on the ground surfaceincreased significantly after the reservoir impoundment,and the new tension cracks at the rear extended in theupstream and downstream directions. It is evident that thereservoir impoundment accelerated the slope deformation.

Deformation characteristics after reservoirimpoundment

1) Toppling

Toppling along joint set J2 occurred commonly over thewhole unstable area, especially in the convex ridge topog-raphy (Fig. 14a), resulting in a series of tension cracksparallel to the strike of the slope (Fig. 14b). Some of thesecracks extended tens to hundreds of meters, and appearedas a grooved landform with the outside scarp higher thanthe inside one (Fig. 14b).

2) Subsiding wedge failure

Four deep grabens (LF54, LF55, LF53, and LF56),traced along the strike of the slope, 400–450 m longand 20–50 m wide, were identified at the crest of theslope (see Figs. 3, 7; Table 2). As mentioned above,two main joint sets (J1 and J2), which are perpendicularto each other, dip steeply with a very high persistenceacross the whole unstable area, creating wedge structuresthat are favorable to toppling and sliding failures.Therefore, these graben structures were formed fromthe subsiding wedge failure triggered by the combinedeffect of toppling and sliding along joint sets J2 and J1(Fig. 14c, d).

Table 2 Characteristics of the cracks or graben structures observed atthe crest of the slope

Length (m) Width (m) Strike (°)

LF1 700 – NE8–NE15

LF56 400–450 20–50 NE2–NE10

LF53 400 25–50 NE17–NE26

LF54 280–350 15–40 NE16–NE32

LF55 150 30–35 NE15–NE21

Fig. 8 Geological profile A–A’(profile location is shown in Fig. 3)

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3) Rockfalls

In late May 2009, the Guobu slope showed a largedeformation, with new cracks appearing on the crest ofthe slope and the occurrence of frequent rockfalls. Therockfalls occurred at different parts of the slope from the

toe to the crest, distributed mainly in shallow loose rockmasses and convex ridges (Fig. 15). These rockfallswere the results of the completely disintegrated rockmass structures induced by toppling and sliding alongjoint sets J1 and J3 that dip in the same direction ofthe slope.

Fig. 9 a Sketch of the ancientlandslide, b lower boundary of theancient landslide

Fig. 10 The crest of –Guobuslope: a, b the back scarp, c thegraben structure

Fig. 11 Monitoring pointsinstalled at the crest of the slope(profile B–B’ is a part of profileA–A’, located at the crest of theslope, as shown in Fig. 3)


4) Tension cracking in deep-seated rock mass

In deep-seated rock masses, tension cracks developedalong the steeply dipping joints were revealed by the ex-ploration adits. These tension cracks were generally openwith apertures from a few millimeters to several centime-ters (Fig. 16). The presence of these deep-seated crackswas likely caused by the lateral unloading of the slopeduring the incision of the Yellow River, toppling failure,and tensile deformation of the rock mass.

In conclusion, all analysis results including previouswork by Xia (2015) indicate that there is a clear structuraldependence of the failure mechanisms in the study area.The complex failure mechanism was a combination of

toppling, subsiding wedge failure, rockfalls, and tensioncracking in deep-seated rock masses. The geological struc-tures, mainly pre-existing joint sets, faults, and large-scalediscontinuities, play an important role in influencing slopedeformation (Xia 2015). Moreover, this complex failuremechanism is similar to case studies reported elsewhere(e.g., Norrish and Wyllie 1996; Braathen et al. 2004),showing that the deformation boundary constituted oflarge-scale cracks or faults was a precondition for such alarge deformation of the slope to occur.

Influencing factors of large deformationof the rock slope

Vulnerable geological conditions

Due to the vulnerable geological conditions in situ, thedeformation of the rock mass was triggered by someunfavorable factors. Four vulnerable geological condi-tions are discussed in the following.

1) High and steep slope

A high and steep slope is one of the key controlling factorsof rock slope instability (Pritchard and Savigny 1990, 1991).The unstable slope is 700 m high from the toe to the crest (Fig.8), and the V-shaped valley is narrow with slope angles vary-ing from 38° to 47°. In fact, partial slope surfaces are nearly

Fig. 12 Variation of the cumulative displacements from the displacementobservation piles on the platform of the ancient landslide between 1991and 1997 (the monitoring locations are marked in Fig. 11)

Fig. 13 Remote sensing imagesfrom 2004–2010: a–c cracks onthe ground, d–g deformationfeatures of the back scarp

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vertical and the average slope gradient is greater than 42°,which can cause high slope instability problems.

2) Rapid river dissection at the foot of the slope

Table 3 shows the characteristics of the river terrace and thecrustal uplift rate in the study area. It ican bes seen that the areais an uplifted zone with a rate of approximately 5.16 mm/yearsince the late Pleistocene. Rock mass relaxation and tension-ing along the steeply dipping joints were caused by the releaseof lateral support of the slope in the process of river incision.Therefore, many tensile cracks parallel to the strike of theslope were generated as the result of unloading, creating afavorable condition for the large deformation of the slope.

3) Unfavorable combination of two joint sets

The two steeply dip joint sets (J1 and J2) are well devel-oped in the strata. Joint set J1 trends NWand dips in the samedirection as the slope, and this facilitates sliding and formationof the upper boundary of the unstable slope. Joint set J2 trendsNE and dips into the slope, which facilitates toppling. The twojoint sets are nearly perpendicular to each other and cut thestrata into discrete blocks, which made the rock mass in thestudy area very vulnerable and fragile.

4) Presence of ancient landslide

The ancient landslide, located at the upper part of Guobuslope, was a potential threat to the whole slope’s stability.Firstly, the rock mass quality had been gradually weakenedduring the process of the formation of the ancient landslide.The rock mass had suffered a long-term strength degradationsince the formation of the ancient landslide due to the effect ofvarious external factors such as rainfall, weathering, andunloading. Therefore, the degradation of the rock massstrength directly decreases the stability of the ancientlandslide.

Secondly, the upper and lower boundaries were definedby the back crack, LF1, and the low-angle fault, HF104,respectively. The reactivation of the ancient landslide is avery low likelihood under the effect of gravitation alonebecause fault HF104 dips into the slope with a low dipranging from 20° to 25°. However, the back crack LF1,with widths varying from a few centimeters to tens ofcentimeters, is an infiltration path of rainfall. Therefore,

Fig. 14 Toppling and subsidingfailures: a toppling along joint setJ2, b groove landform due totoppling, c subsiding wedgefailure, d a series of grabens at thecrest of the slope

Fig. 15 Rockfalls induced by toppling and slide along joint sets J1 and J3


the ancient landslide quite possibly was reactivated underthe effect of hydraulic pressure or any other external forcesuch as horizontal seismic force. Because there was noseismicity in recent years before the reactivation of theancient landslide, it can be seen that water infiltration isthe main factor causing the reactivation of the ancientlandslide. The influence of water infiltration on the reac-tivation of the ancient landslide will be elaborated in thenext section.

Water infiltration

To prevent water from affecting the dam construction in thearea, a small dam was constructed near the back crack LF1on the crest of the slope during the construction period (Fig.11). Because, since the formation of the ancient landslide, theGuobu platform had become the lowest place compared withthe rear planation surface in the region (see Fig. 1), all the waterfrom the rear region from heavy rainfall and snow melt couldeasily flow and infiltrate into the back crack LF1 or otheropened tension cracks (see Fig. 17). Based on the meteorolog-ical observation data, the annual rainfall on the crest of theunstable slope is approximately 300 mm (Fig. 18). In addition,as can be seen from Fig. 17, the drainage channel connectingthe small dam was damaged due to the opened tension cracks.The large amount of water stored in this small artificial

reservoir infiltrating into the open cracks is a threat to the sta-bility of the ancient landslide. On the other hand, due to theheavily broken rock mass, the water infiltrating into the ancientlandslide had not only degraded the strength of the rock massbut also caused seepage forces and increased pore water pres-sures. Meanwhile, the hydrostatic pressure within these crackswould increase with the increase of the filling water level. Theancient landslide would reactivate when the water head withincracks was higher than that of the critical water head.

Reservoir impoundment

Detailed displacement monitoring of the unstable slope beganin August 2009 and the data showed that the whole slope wasdeforming significantly and continuously with the increase ofthe reservoir water level. The displacement monitoring resultsof the GPS points at the crest of the slope (as shown in Fig. 11)are presented in Fig. 19. Figure 19a shows the variations of thecumulative displacements of the GPS points in the profile B–B’ on the crest of the unstable slope from August 2009 to July2014. According to the surface displacement monitoring data,the maximum cumulative displacement was more than40,000 mm in the period. Based on the analysis, we can con-clude that there is a decrease trend of displacement from theslope shoulder to the back scarp LF1. The GPS points outsidethe unstable slope, for example, QC4, QC3 and IP12, show

Fig. 16 Tension cracking in deep-seated rock mass developed alongthe steeply dipping joints: a PD8at the depth of 94 m, b PD10–4 atthe depth of 23 m

Table 3 Development of the riverterrace and crustal uplift rate inthe study area (Pan and Su 2009)

River terrace Elevation(m) a.s.l.

Formingtimes (year)

Crustal upliftrate (mm/yr)

Average crustaluplift rate (mm/yr)

I 2235–2240 5.16 2.72II 2270 Q4:10,650 ± 140

III 2315 5.16 approximatelyIV 2365 Late Q3:28,000

V 2420–2430 Middle Q3:93,000 1.01

VI 2480 2.90Planation surface 3000 Late Q2:250,000

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much smaller cumulative displacements and some have nodisplacement. The monitoring results demonstrate the defor-mation boundary of the unstable slope, which is in goodagreement with results exposed by the exploration adits andboreholes.

Figure 19b presents the relationship between the daily dis-placement velocity and the reservoir water level from August2009 to July 2014. The water level fluctuated from 2380 to2420 m from August 2009 to March 2011, and this resulted in

a marked increase of displacement velocity, with the maxi-mum displacement velocity of 80–90 mm/day occurring inDecember 2009. In general, the displacement velocity in-creased as the reservoir water level rose, whereas it decreasedas the water level fell. After March 2011, the displacementvelocity gradually decreased and finally became steadily witha displacement velocity less than 5 mm/day when the reser-voir water level remained approximately constant at 2448 m.

The results from the remote sensing images demonstratedthat the ancient landslide reactivation occurred between 2005and 2008 before the reservoir impoundment (Fig. 13). Theyalso indicated that there was a large increase of the number ofcracks on the ground surface of the slope after the reservoirimpoundment. Meanwhile, detailed displacement monitoringdata showed that the whole slope was deforming significantlyand continuously with the increase of the reservoir waterlevel (Fig. 19a, b). In general, the displacement rate has apositive correlation with the variation of the reservoir waterlevel, increasing and decreasing as the reservoir water levelrose and fell, respectively (Fig. 19b). Consequently, we con-cluded that the water infiltration was the driving force for thereactivation of the ancient landslide, and that the increase ofthe reservoir water level accelerated the slope deformation.

Mechanism of large deformation of rock slope

The mechanism for the large deformation of the Guobu slope isanalyzed in the following. Asmentioned above, water infiltrationis the dominant factor for the reactivation of the ancient landslide.When the water head within cracks was higher than the criticalwater head, the ancient landslide begun to reactivate due to acombined deformation of wedging and toppling failure resultingfrom the unfavorable combination of joint sets J1 and J2.Meanwhile, the reactivated ancient landslide gradually pushedthe relaxed or loose rock mass under fault HF104. Under theeffect of pushing from the ancient landslide, the lower superficialloose rock mass begun to deform, with displacement decreasingfrom top to bottom. This reveals that the ancient landslide reac-tivation accelerated the deformation of the loose rockmass underfault HF104, and that this deformation extended from the top tothe bottom of the slope (Fig. 20a).

After the reservoir impoundment, the superficial relax-ation zone or loose rock mass, which had earlier experi-enced long-term deformation, begun to deform significant-ly with the increase of the reservoir water level. The dis-placement, which was attributed to traction deformationtriggered by reservoir filling, decreased from the toe tothe top of the slope (Fig. 20b). Once these two types ofdeformations (pushing and traction deformations) had been

Fig. 17 Opened tension crack at the crest of the slope (taken on May 12,2007)

Fig. 18 Annual precipitation on the crest of the unstable slope between2005 and 2016


linked together within the slope, the displacement rate ofthe whole slope would have a positive correlation with thevariation of reservoir water level (Fig. 20c).

Consequently, we conclude that the mechanism of the largedeformation of the unstable slope is a combination of the upperpushing deformation induced by wedging and toppling failureassociated with the ancient landslide reactivation and the lowertraction deformation due to the reservoir impoundment. In otherwords, the large deformation is the result of a combination of theupper pushing deformation and the lower traction deformation.As of now, the potential sliding surface is not identified withinthe slope according to the observation results revealed by theexploration adits and boreholes in situ.

Conclusion

In this research, a complex unstable rock slope in a reservoirbank slope on the upper stream of the dam of the Laxiwa

Hydropower Station in China was investigated and analyzedto understand the deformation characteristics, influencing fac-tors and the mechanism of the large deformation. The resultsfrom the integrated field survey and structural investigationsindicate that the complex mechanism was a combination oftoppling, subsiding wedge failure, rockfalls, and tensioncracking in deep-seated rock masses.

Vulnerable geological conditions, water infiltration, and thereservoir impoundment were the main factors causing the largedeformation of the Guobu landslide.Water infiltration is the driv-ing force resulting in the reactivation of the ancient landslide, andthe reservoir impoundment accelerated the slope deformation. Ingeneral, the displacement rate has a positive correlation with thevariation of the reservoir water level, increasing and decreasingas the reservoir water level rises and falls, respectively.

The mechanism of the large deformation of the unstableslope is a combination of the upper pushing deformation in-duced by wedging and toppling failures due to the ancientlandslide reactivation and the lower traction deformation due

a) Cumulative displacement

b) Relation between reservoir water level and daily displacement velocity

Fig. 19 Cumulativedisplacements and dailydisplacement velocities of GPSpoints in profile B–B’ on the crestof the unstable slope from August2009 to July 2014

3334 M. Xia et al.

Fig. 20 Schematics showing thedevelopment of the largedeformation of the Guobulandslide: a the ancient landslidewas reactivated due to waterinfiltration, pushing the lowerloose rock mass; b the reservoirimpoundment caused the tractiondeformation of the superficialloose rock mass; c these two typesof deformations have been linkedwithin the slope


to the reservoir impoundment. As of now, the potential slidingsurface has not been identified within the slope according tothe observation results revealed by the exploration adits andboreholes in situ.

Acknowledgments This study was funded by the State Key Laboratoryof Geohazard Prevention and Geoenvironment Protection (Grant No.SKLGP2015Z021). The financial supports are gratefully acknowledged.

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