LandslidesDOI 10.1007/s10346-014-0500-2Received: 12 January 2014Accepted: 30 May 2014© Springer-Verlag Berlin Heidelberg 2014

Huiming Tang I Changdong Li I Xinli Hu I Aijun Su I Liangqing Wang I Yiping Wu I RobertCriss I Chengren Xiong I Yunan Li

Evolution characteristics of the Huangtupo landslidebased on in situ tunneling and monitoring

Abstract Huangtupo landslide, volumetrically the largest, mostcomplex landslide in the Three Gorges Reservoir region ofChina, is a dangerous mass on which the district of Badong hasbeen inadvertently situated. Risk remediation efforts include theconstruction of a large observational tunnel and monitoring sys-tem that are unique in the world. This tunnel and its side branchespermit detailed mapping of its 3D structure while providing sam-ples for laboratory analysis. The new investigations validate thatthe Huangtupo landslide is a composite of several independentlandslides and that movement occurs along the major rupturezones as well as on interlayer sliding zones in the underlyingBadong Formation. Uranium–thorium disequilibrium dating es-tablishes that the northern part of the landslide, called theRiverside Slump, underwent at least two periods of movement atabout 100 and 40ka (ka stands for a thousand years). These eventswere induced by the steep slope created by the downcutting of theYangzte River. The results from in situ displacement monitoringover a 7-year period confirm that the central part of the landslideis creeping at a slow, relatively stable rate of about 15mm/yearrather than being in a stage of acceleration under the protection ofanchored concrete beams and other defense structures at its toe.Available data suggest that engineering measures can control theindependent landslides that together constitute the hugeHuangtupo mass, which will avoid the need for costly relocationof thousands of people.

Keywords Huangtupo landslide . Three GorgesReservoir . Landslide evolution . Landslide stability .

In situ displacement monitoring

IntroductionBadong District, in the Hubei Province of China is famous for itsunique geological conditions and large-scale landslides that haverequired relocation of thousands of people. Old Xinling Town,the original seat of Badong County, was relocated to theHuangtupo area because of rising water levels following theimpoundment of the Three Gorges in the 1980s. Subsequently,when the Huangtupo area was found to be situated on anextremely large landslide, a second costly relocation of nearly16,000 people to the Shennongxi area of Guandukou Town wasplanned. In addition, special structures for protection, landslidestabilization, and research were constructed at Huangtupo, in-cluding the tunnel under investigation and monitoring systemdiscussed in this report.

Several previous researchers have used the conventional rigidequilibrium limit method and numerical modeling to study thestability of reservoir landslides (Muller 1964; Breth 1967; Liu et al.2005). Dramis and Sorriso-Valvo (1994) discussed the effect ofdeep-seated gravitational slope changes on landslides. Chen and

Lee (2003) presented a dynamic model for rainfall-induced land-slides. A thermo-poro-elastic approach was used by Goren andAharonov (2009) to study the stability of landslides. Jian et al.(2009) pointed out that the main factors contributing to the Anlesilandslide are recent tectonic activity, incompetent beds, and in-tensive rainfall. Li et al. (2012, 2014) conducted the evaluation andcontrol strategy on reservoir bank landslide in the Three GorgesReservoir region. Cruden and Varnes (1996) and Hungr et al.(2013) describe landslide features and presented a useful classifi-cation of landslide types.

Many have included the effects of water level fluctuation onlandslide stability (Van Asch et al. 1996; Hewitt 1998; Zhu et al.2002; Franco and Claudio 2003; Ding et al. 2004; Hu 2005; Tangand Zhang 2005; Liao et al. 2005; Chai and Li 2004). Wu et al.(2001) focused on the mechanism and stability of the landslides inthe area of the Three Gorges Project, Yangzte River. Deng et al.(2000) and Ni et al. (2013) discussed the mass rock creep anddeformation and stability of the Huangtupo landslide.Mosselman et al. (2000) studied the effects of bank stabilizationon bend scour in anabranches of braided rivers. Panizzo et al.(2005) made a detailed analysis of the great landslide events inItalian artificial reservoirs. Jiang et al. (2012) performed the water-rock (soil) interaction test and mechanism analysis of Huangtuporiverside landslide in Three Gorges Reservoir.

A limitation of previous studies is that a few boreholes ortrenches provide insufficient access to accurately represent thegeological structure and evolution of large, complex reservoirlandslides. In order to address this issue, a large tunnel wasconstructed and an extensive monitoring system was installed todetermine the characteristics, stability, and evolution of theHuangtupo landslide. In particular, a 908-m-long, in situ investi-gation tunnel with multiple side branches was constructed, and theworld’s first 3D, multi-field landslide monitoring system was im-plemented in the Huangtupo landslide, all accomplished via the“Three Gorges Reservoir Geo-hazards Research StatePreponderant Subjects Innovation Platform.” This paper uses re-sults from this unique facility to better understand the complexdynamics, characteristics, and evolution of the Huangtupolandslide.

MethodsNumerous techniques were used to study the Huangtupolandslide. First, standard field techniques, facilitated by sub-surface access provided by the investigation tunnel system,were used to define the extent of various slumps; the attitudeof bedding; the location, attitude, and character of rupturesurfaces, etc.

Second, multiple sites were selected for boreholes and moni-toring stations. Repeated downhole surveys in the HZ6 borehole

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were made, and multiple stations were surveyed every month toprovide details about landslide creep rates. The instrument preci-sion rate is about ±0.02 mm per 500 mm.

Third, several samples were collected for mineralogicalanalysis by standard X-ray diffraction techniques. In addition,three calcite samples were selected for uranium–thorium (U-Th) disequilibrium dating utilizing a MAT-262 instrument atthe State Key Laboratory of Geological Process and MineralResource; normal precision is better than 5 ka (ka stands fora thousand years).

Geological backgroundHuangtupo landslide is developed in the strata of the MiddleTriassic Badong Formation (T2b

2 and T2b3), mainly composed of

mudstone, pelitic siltstone, and argillaceous limestone. The crownelevation of the landslide is about 600 masl, while its toe variesfrom 50 to 90 m but is submerged by the Yangzte River, whosestage is regulated by the Three Gorges Dam and now varies from145 to 175 m (Fig. 1). This composite landslide covers an area of1.35 km2 and its volume of nearly 70 million m3 makes it the largestreservoir landslide in China.

The Huangtupo landslide can be divided into four parts, herecalled Riverside Slump I#, Riverside Slump II#, SubstationLandslide, and Garden Spot Landslide (Figs. 1 and 2; Hu et al.2012a, b). Garden Spot Landslide covers an area of 32.6×104 m2

and has a volume of nearly 14 million m3. Substation Landslidecovers an area of 38.1×104 m2, has a volume of 13 million m3, and is“boot-shaped” in plan.

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Fig. 1 Engineering geological map of the Huangtupo landslide. Monitoring sites (triangles) and sample locations discussed in this paper are shown. Inset map showsthe general location in China

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The western boundary of Riverside Slump I# extends fromBadong New Dock to Shennongxi High School and CountyHospital, and its eastern boundary is adjacent to Slump II# (seeFig. 1). The crown of Slump I# is covered by Garden SpotLandslide. The elevation of the toe of Slump I# varies from 90to 70 m and is now submerged, and it covers an area of 32.50×104 m2, being 770 m long and 450 to 500 m wide. Given itsaverage thickness of 69.4 m, the volume of Riverside Slump I#is nearly 23 million m3. Both Slump I# and Slump II# are

composed of rock block, rock fragments, and cataclastic rockwith clay (Hu et al. 2012a, b).

Among of all the above four landslides, Slump I# and Slump II#are most affected by the fluctuation of Three Gorges Reservoir andhuman activities; therefore, their deformation rates are greaterthan those of Substation Landslide and Garden Spot Landslide.Furthermore, according to the site monitoring results, the defor-mation of Slump I# is more obvious than the others. Therefore,Slump I# is a typical accumulative landslide and was chosen for

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the great in situ tunnel project and monitoring system. Thissystem has provided valuable new findings about landslidecharacteristics.

Landslide characteristics exposed by in situ investigation

Layout of in situ tunneling and monitoringThe detailed characteristics of the Huangtupo landslide must bedocumented before its behavior and evolution can be understood.In situ tunneling provides many advantages over conventionaldrilling as it is a more effective and accurate way of fine explora-tion. The in situ tunnel system in Huangtupo landslide is com-posed of a main investigation tunnel with a total length of 908 m(Fig. 2). Positions along the main tunnel are referenced as footagesrelative to a benchmark near its western portal, defined as positionK0+000 m. Five crosscuts, called branch tunnels BR-1 to BR-5,intersect the main tunnel at positions K0+320, K0+420, K0+460,K0+520, and K0+570. Footages along these branches are de-scribed in a different system intrinsic to each, for example, K3+45 m would be 45 m from the intersection of BR-3 with the maintunnel.

In addition, a series of devices were fixed on the surface or intothe sliding mass of the Huangtupo landslide to monitor the defor-mation both on the slide surface and within its interior, includinga number of GPS surface monitoring sites, namely G1, G2, G7, G9,G11, G18, G20, and G22, and some deep displacement monitoringsites, namely HZK5, HZ6, HZK7, and HZK30 (see Fig. 1). A 3D laserscanner system and measuring robot were used to collect themulti-field information (Fig. 3).

Exposed weak rock layers and interlayer sliding zonesDuring the recent excavation of investigation tunnels, both weakrock layers, termed interlayer sliding zones, and major rupturezones located at or above the bedrock surface, discussed in thefollowing section, were easily identified. The soft interlayer slidingzones contrast with the generally hard lithology of the Badong

Formation (see Fig. 4). Twelve interlayer slide zones were recog-nized in the investigation tunnels, with five intersected by themain tunnel, one by BR-2, and six by BR-3.

Along the main investigation tunnel, the interlayer slidingzones were intersected at positions K0+117 to K0+131 m, K0+175to K0+259 m, K0+438.5 to K0+484.5 m, K0+512.4 to K0+578.4 m,and K0+601.1 to K0+620.4 m. Figure 5 shows the rock at theheading of K0+556.8 m as seen during the tunnel excavation,where the dip is 18° in the dip direction of NE20°. The weak rocklayer is a 50-cm-thick, grayish-yellow, angular to sub-angularbreccia. Another clear sliding trace is seen at the interface betweenthe weak rock layer and the overlying limestone near the end ofBR-3, just below the main rupture zones (Fig. 5).

Figure 5 illustrates that there are five weak layers and one majorrupture zone in BR-3. The five weak layers all occur within thebedrock and have almost the same attitude as the major rupturezone. The joint spacing of the bedrock proximal to the mainrupture zone is much smaller than in the rock at distance. Thus,the weakest zone is the major rupture zone at the end of the BR-3.

Major rupture zonesMajor rupture zones generally occur immediately above the bed-rock surface, but sometimes cut the chaotic material of earlierslide masses. Excavations show that the rupture zones are consti-tuted of three lithologies, namely brownish-yellow, gravelly soil inthe slide mass; yellow and light gray clay with gravel along thesliding zone; and the cyan-brown and brownish-yellow argilla-ceous limestone that forms the subjacent stable bedrock(Fig. 6a). Gravel clasts ranging from 0.5 to 3.0 cm constitute about20 % of the sliding zone, with relatively good psephicity.Slickensides are common in the matrix material that hosts theclasts (Fig. 6b, c).

In particular, along the main tunnel, a major rupture zonewas intersected between K0+654.1 and K0+723.2 m, where ithad a dip of 34° and dip direction of 354° (see Fig. 7). Onlyabout 100 m away, in BR-5, the same rupture zone was found

Yan

gtze

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Fig. 3 3D laser scanning graph of the NW entrance of the main tunnel, here situated in the Badong Formation. The white triangle is the site benchmark K0+000 whichlies 4 m from the tunnel entrance. Yangtze River lies to the left

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between K5+14.5 and K5+21.7 m, where the dip was 30° towardNE28°. The above evidence indicates that the eastern section ofRiverside Slump I# is an independent landslide with a curvedrupture surface.

In the No. 3 branch tunnel, a separate, structurally highersliding zone was found between K3+135.9 and K3+139.6 m, which

had a dip of 47° and dip direction of 355°. If this planar surface isprojected southward to the position of the main tunnel, it wouldlie more than 110 m above the tunnel at that point. Note that thedip of this rupture surface is considerably larger than thatintersected in the main tunnel and in BR-5. In addition, thissurface has a younger apparent age than the latter (see below).We conclude that the rupture zone exposed in BR-3 is a structur-ally higher, independent sliding zone.

Mineralogy of the sliding zoneThe two major rupture zones were sampled in BR-3 at K3+135.9 to K3+139.6 m and in BR-5 between K5+14.5 and K5+21.7 m. X-ray diffraction analysis results show that the min-eralogy in both slide zones is dominated by quartz andcalcite, with subordinate amounts of chlorite, illite, and mont-morillonite (Fig. 8).

Dating of landslideU-Th dating is a radiometric technique commonly used to deter-mine the age of calcium carbonate materials. U-Th disequilibriumdating of secondary calcite can provide information on the timingof the Huangtupo landslides.

Three calcite samples were collected from the Huangtupo tun-nel during excavation, yielding a U-Th apparent age of 100 ka forthe Riverside Slump I# and II#, 40 ka for calcite in BR-3, and 50 kafor the structural fracture zone at the crown (Fig. 9).

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Fig. 5 Sketch of the right side of the no. 3 branch tunnel. Six interlayer sliding zones were encountered, and an important rupture surface was encountered at K3+138

Limestone JointWeak rock

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Fig. 4 Sketch of the tunnel face at K0+556.8 m looking east made duringexcavation, showing a weak interlayer sliding zone with a dip of 18° to NE20

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Evolution process analysis

Evolution characteristic of spatial morphologyIn situ monitoring indicates that Riverside Slump I# is steadi-ly creeping. The maximum deformation direction is NE20° infour of five deep displacement monitoring sites in RiversideSlump I#. This is the general bedrock dip direction and canbe considered as the main sliding direction, so longitudinalprofiles (a–a′) were constructed in this direction (Fig. 10).

However, the maximum deformation direction in the HZ6borehole is NE45°.

In the following, profile a–a′ is selected as typical. This profile isclose to the sliding zone exposed in BR-3, intercepts the maintunnel at K0+478.96 m, and crosses boreholes HZK7, HZK5, andHZK30 (see Fig. 11). The collar of borehole HZK7 is 195.7 m, andthe bedrock surface was intersected at an elevation of 119.1 m; thedeep-seated displacement monitoring result shows that the max-imum displacement occurs at elevation 127.2 m, which can be usedto determine the possible sliding zone position. The elevation ofthe lower shear outlet near Yangzte River is about 83 m. In order tounderstand the spatial morphology analysis of Riverside Slump I#,an elevation contour graph of bedrock was constructed using datafrom the investigation tunnels (see Fig. 12).

The lithology of bedrock below the Riverside Slump I# ismainly marlite, argillaceous limestone, dolomitic limestone, lime-stone, and dolomite with an age of T2b

3–1. The dip of the rocks isgenerally to the NE, although some small folds are present.

On the basis of the above analysis, the rupture zone exposed inthe main tunnel and in BR-5 should represent the same surface,but the rupture zone in BR-3 is a different, structurally highersurface. Furthermore, the elevation of the shear outlet in thewestern side of New Dock in Badong County ranges from 83 to90 m, while the elevation of the shear outlet at the eastern side ofNew Dock is only about 60 m, so there must be elevation mutablesites near the New Dock. From the conventional point of view,once the slide event of Riverside Slump I# occurs, the rupture zoneshould be continuous and shear out at a stable elevation.Therefore, the difference of 23 to 30 m in elevation of the shearoutlet shows that the Riverside Slump I# is composed of twoindependent sub-landslides, named Riverside Slump I#-1 andRiverside Slump I#-2 (Fig. 12).

The western boundary of Riverside Slump I#-1 coincides withthat of the original Riverside Slump I#, and its crown extends fromthe old government building to Xinhua book store in JintangRoad; its eastern boundary reaches Shengzi storehouse, and theshear outlet elevation is about 80 m near the New Dock. Thesliding zone exposed in BR-3 is located near the crown ofRiverside Slump I#-1. Regarding Riverside Slump I#-2, its easternboundary is the same as that of the original Riverside Slump I#, thewestern boundary is near the old port office of Badong County,and its crown lies near the Third Primary School and JinlingMiddle School. The above investigations validate that theHuangtupo landslide is a composite structure that includes severalindependent landslides.

Evolution characteristic of sliding mass deformationThe field and tunnel observations of the Huangtupo landslide wereaugmented by data from several monitoring instruments fixed onits surface that include surface and deep displacement monitoringat several sites along the central part of Riverside Slump I#(Fig. 13).

Figure 13 illustrates a clear, slowly increasing trend of cumula-tive displacement at monitoring points G18 and HZ6 located nearthe center of Slump I#. The creep rate measured over a 7-yearperiod is about 15 mm/year. Overall, the deformation of theHuangtupo slide mass has been quite steady since 2007, so the

Sliding mass:Brownish yellow gravelly soil

Major rupture zone:Yellow and light gray clay with gravel

Bedrock: Cyan brown argillaceous limestone

Bedrock: Brownish yellow argillaceous limestone a

b

c

Fig. 6 Sliding zone and related features exposed during tunnel excavation at K0+690. a Tunnel face. b Detail of the rupture zone. c Slickensides in the rupture zone

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mass can be considered to be in a stage of uniform motion.However, the rate of movement has jumped during every rainyseason since 2007, suggesting that a combination of intense rain-fall and reservoir operations have affected the movement of thelandslide mass (Tang et al. 2014, in revision).

Badong County is located in the west of Hubei Province andhas a subtropical climate, with heavy rain in the rainy seasonfrom June to August. During this time, the water level in thereservoir is sharply lowered for flood control, effected by in-creased discharge from the giant Three Gorges Dam. Figure 13illustrates that landslide creep is accelerated during this seasonalrainy period.

Evolution mechanism of Huangtupo landslideThe above evidence provides many new insights on the evolutionand behavior of the Huangtupo landslide. First, the apparent ages

of secondary calcite suggest that Riverside Slump I# and II# areolder than the sliding zone encountered in BR-3. Moreover, the ageof fault structural fracture zone at the crown is probably differentthan these, all proving the existence of multiple evolutionarystages and events in the Huangtupo landslide.

Second, the lithologic character and orientation of thesliding zones provide more evidence for a complex history.Argillized, breccia argillization and breccia zones are all com-mon in the interlayer sliding zones. For instance, in the K3+127.2 to K3+132.0 m section of BR-3, the dip of the interlayersliding belt is 25° in direction 9°. Sliding zone material isreddish brown silty clay in a zone that varies from 20 to40 cm thick that includes small round gravel clasts andslickensides. Interactions with ground water and other effectshave reduced the strength of such soft layers, providing shearsurfaces for the landslide.

Fig. 8 X-ray diffractogram of sample from the major rupture zone at position K3+138. The mineralogy is dominated by quartz and calcite, with minor clays as identified,Q (quartz), cc (calcite), M (montmorillonite), I (illite), K (kaolinite), F (feldspar)

a b

Fig. 7 Sliding zones intersected by main tunnel. a Tunnel face at K0+683.4 m, looking NE, showing a major rupture zone; b tunnel face at K0+708.6, looking NE,showing material of the overlying slide mass

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The existence of soft rock layers in the Badong Formation thatdip toward the river provides the necessary geologic condition forthe occurrence of this landslide. Downcutting of the Yangzte Riverwill continue to steepen the front slope of Riverside Slump I#-1,inducing slide reactivation. Additional downcutting action couldreactivate Riverside Slump I#-2 (see Fig. 14). Landslide evolutionalong rivers is a complex response to many governing processesand involves multiple active sliding events.

DiscussionThe large, complex Huangtupo landslide has great social andeconomic importance, and accordingly has attracted much at-tention from government officials, scientists, and engineers. Thelandslide was originally considered to be a single huge masswith a deep, intact sliding zone that would render it verydifficult to control. This viewpoint was adopted by decision-makers, who necessarily supported the costly relocation of thou-sands of people from the Huangtupo area to the Shennongxiarea in Guandukou Town.

However, the new findings presented in this paper indicate thatthe Huangtupo landslide is composed of several independentFig. 9 U-Th apparent ages on calcite from two major rupture surfaces and an

interlayer sliding zone

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Fig. 10 Attitude of sliding zones encountered in the tunnel system. Garden Spot Landslide is on top of the Riverside Slump, see Fig. 11 for section a–a′

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Fig. 12 Map of the two sub-landslides in Riverside Slump I#, showing topographic contours. Sub-landslide 1#-2 on the west is above sub-landslide 1#1

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landslides. On the whole, the landslide mass is creeping slowly andrelatively steady and can be considered to be in a stage of uniformmotion rather than in a stage of acceleration under the protectionof the defense structures (anchored concrete beams and otherdefense structures) along its toe.

In short, new data suggest that the smaller independent land-slides can be controlled by proper engineering measures. Futuremonitoring and construction of mitigation works undertaken bythe planning departments should be prioritized on the indepen-dent landslides that exhibit the fastest movement.

ConclusionsHuangtupo landslide is volumetrically the largest, most complex,and most economically significant landslide in the Three GorgesReservoir region. Consequently, the “Three Gorges Reservoir Geo-hazards Research” State Preponderant Subjects InnovationPlatform constructed a large tunnel system and implemented the

first 3D, multi-fields landslide monitoring system in the world. Themonitoring system gathers data on ground water levels, displace-ment, and strain and stress. Furthermore, a 3D laser scannersystem, GPS monitoring system, and deep displacement monitor-ing equipment collect information that reveals the detailed behav-ior and character of the sliding body, sliding zone, and bedrock.

New data clarify the behavior and evolution of the Huangtupolandslide. The U-Th apparent ages of secondary calcite show thatRiverside Slump I# and II# are older than the sliding zone encoun-tered in branch tunnel BR-3. Moreover, the apparent age of thestructural fracture zone at the crown is different than these, pro-viding additional evidence for the complex evolution of theHuangtupo landslide. These data support field observations thatmultiple rupture surfaces and several independent slide massesare present.

Argillized, breccia argillization, and breccia zones are commonin the interlayer sliding zones. Interactions with groundwater

Fig. 13 Displacement curve of two proximal monitoring sites in the central part of Riverside Slump I# (see Fig. 1). HZ6 represents inclinometer data from the boreholecollar; G18 is a surface site

Fig. 14 Evolution of Riverside Slump I#, showing the two constituent sub-landslides. Topographic contours are in meters above sea level

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weaken these soft layers, which are important shear surfaces in thelandslide. The existence of soft rock layers and interlayer slidingzones in the Badong Formation that dip toward the Yangzte Riveris the essential condition that generated the Huangtupo landslide.Continued downcutting of the Yangzte River will steepen theyoung, already steep front, reactivating Riverside Slump I#-1.Further downcutting will reactivate Riverside Slump I#-2.

Furthermore, the latest tunneling investigations indicate thatthe Huangtupo landslide is composed of several independentlandslides instead of a single huge one. Displacement monitoringover a 7-year period reveals that the center of Riverside Slump 1# isslowly creeping at a rate of about 15 mm/year. The current defor-mation is very steady and the landslide, whose toe is defended byengineering structures, exhibits uniform annual creep rates.Available data suggest that the smaller independent landslides thattogether constitute the Huangtupo landslide can be controlled byproper engineering measures, which can circumvent the necessityto relocate an entire city.

AcknowledgmentsThe work was funded by the National Basic Research Program ofChina (973 Program) (No. 2011CB710600) and the Key NationalNatural Science Foundation of China (Nos. 41230637 and41202198). The authors appreciate the help provided by friendsduring the study.

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H. Tang ()) : C. Li : X. Hu : L. Wang : Y. Wu : Y. LiFaculty of Engineering,China University of Geosciences,430074, Wuhan, Chinae-mail: tanghm@cug.edu.cn

A. Su : C. XiongThree Gorges Research Center for Geo-hazard, Ministry of Education,430074, Wuhan, China

R. CrissDepartment of Earth and Planetary Sciences,Washington University,One Brookings Drive, Saint Louis, USA

Landslides