10th Asian Regional Conference of IAEG (2015)

Analysis of the post-earthquake stability and research on deformation and failure mechanismof the talus slope located in front of the dam of Zipingpu hydraulic project
LANG Jing-xuan(1), HU Xie-wen(1,2)，WU Jian-li(1) and DIAO Ren-hui(1)(1) Engineering Laboratory combined with national and local of spatial information technology of high speed railway
operation safety, Southwest Jiaotong university, Sichuan Chengdu 610031, China(2) Key Laboratory of Ministry of education of traffic tunnel, Southwest Jiaotong university, Sichuan Chengdu
610031, China
Abstract
In order to figure out the influence on the talus slope located at left riverbank in front of the dam of
Zipingpu hydraulic under the “5.12”Wenchun earthquake, the stability of the talus slope under different
conditions is calculated based on field investigation. The results show that the talus slope is generally
stable under various conditions, while only the front part of Dengzhan plateau collapsed under theearthquake. The research of its deformation characteristics show that, the deformation and failure modeof the talus slope is in the charge of the micro-landform, and earthquake-induced slope failures usuallytake place on where the slope is deep and where the deep part joins the gentle part.
Key words: talus slope, stability analysis, deformation and failure mechanism, Wenchuan earthquakeof the dam on the left bank locates a large older

1. Introduction
A large amount of avalanches and landslides was
triggered by the earthquake occurred in 2008 in
Wenchuan with a Ms=8.0(Huang Runqiu,2009). The
Zipingpu hydraulic project which located about 17km
away from the earthquake epicenter suffered a intensityof Ⅸ~Ⅹ(Zheng Sheng’an et al, 2008), a large oldlandslide on its left bank maintained stable while only
where on the slope the steep part joints the gentle’s
slumped. So it is of great importance to find out thelandslide, it has a total capacity of 35 million m3 ~ 45
million m3, whose stability after the earthquake directly
influences the normal operation of the dam project.
Therefore, find out the deformation and failure
performance and use the Limit equilibrium theory toanalyze the deformation characteristics and stability ofthe older landslide is of universal significance on
investigation of older landslide, engineering control andother aspects of theory and practice.

deformation and failure performance of the older

2. The basic characteristics of the older landslide

landslide and analyze its stability and mechanism under

2.1 Terrain features and engineering partition
The older landslide locates within the dam of Shakinsevere earthquakes.
The Zipingpu hydraulic project locates on theto the Maxi of the reservoir(that is at the left bank in

upstream of Minjiang River, 67km northwest offront of the dam) , its upstream boundary reaches to theChengdu, Sichuan. It mainly provides us with

Tang Jialin gully and Peaches plateau, while theagricultural irrigation and water supply, it also has thedownstream boundary reaches to Jiajia gully, the scarp

functioncontrol,

is near the watershed with a elevation up to 1300m, its

environmental protection and tourism, what’s more, it is

toe reaches to the Minjiang River. The older landslide

the water source of the city of Dujiangyan andhas a longitudinal length of 1600m, a width of 300 ~Chengdu(Zheng Shengan et al, 2008). 618m upstream

870m,

of

power

generation,

flood

1

and

a

horizontal

distribution

area

of

10th Asian Regional Conference of IAEG (2015)

approximately 1.0km2 in the main study area.

Fig.1 Location of the study area(from Google Earth)
The frontier of the talus slope is relatively narrow,
while widened as go into the back of the slope, it is as awhole like a round chair. Several plateaus on different
elevations and slopes that connect them constitute theolder landslide. The plateaus mainly consist the
Guanyinplateau(elevation

of

1110

~

1150m),

Baimiaozi(elevation of 1000 ~ 1040m), Dengzhan
plateau(elevation of 960m) and Hudou plateau(elevationof 800 ~ 820m), the terrain of the talus slope isrelatively moderate, usually 20 ~ 30 °, while theFig.2 Geological zonation of talus slope

peripheral bedrock up to 40 °.
The talus slope according to its origin, composition(Yang Jian et al,2006)

characteristics and engineering significance could be

Outcrops and pit holes by exploration revealed

divided into two parts: zoneⅠis constructed by several

that the talus slope is mainly piled up by gravel andbedrock falling bodies, it contains the Mid-Devonian

clay. Block gravel mainly composed of limestone and

(D2)

which

dolomitic limestone, with a small amount of fineconstitutes disconnected buttes, and zone Ⅰ is not

sandstone and diabase can be divided into 7 layers

directly related to the project. ZoneⅡis bounded toaccording to its obvious vertical zonation, from the

Tangjialin gully and can be divided to zoneⅡ-1 andnew to the old respectively: Q ⑦ (gravel containing

zoneⅡ-2.

clay), Q ⑥ (clay containing gravel), Q ⑤ (gravel

dolomiteanddolomitic

limestone,

ZoneⅡ–1: locates in the vicinity of the Baimiaozi

containing clay), Q ④ (gravel containing clay), Q ③

and Peaches plateau, the upstream of the Tangjialin(gravel), Q ② (gravel containing clay), Q ① (claygully. Almost there is no change to the boundary

containing gravel). The bedrock below has a

conditions after water impoundment. Tangjialin gully ismorphology of steep-to-slow shape on downslope

deeply cut, the rockmass along the Minjiang River(ie,direction (similar to the surface shape of the slope)the Dengzhan Plateau direction) in the area has no freewith a tendency out of the slope, and in thesuface, and would not probably collapse.

longitudinal view the interface between rock bed and

ZoneⅡ–2: locates in the vicinity of the Dengzhan

overlying talus at both ends is steep while the middle

and Hudou plateau, the downstream of the Tangjialin

is relief, inclination angle could up to 13.5° ~ 16°,

gully. The maximum thickness of the talus slope here iswith a minimum of 5.5°~ 6°, the surface of rupture on

7

3

about 128.0m, the total volume is (2.5 ~ 3.0) × 10 m ,and the front edge of Dengzhan plateau has crept
several times, so it is more likely to deform after theearthquake.

2

the back edge of the Dengzhan plateau is 42 °.

10th Asian Regional Conference of IAEG (2015)

platforms with a gradient of 1:3.0 between them wasset(Wang Jialin et al, 2009).
Though suffered from the “5.12” Wenchuan
earthquake in 2008, the whole talus slope did not slide,yet only four small-scale collapses and two surface
GuanyinPlateaudeformations appeared under the road on elevation ofDengzhanPlateau890m on the front edge of the Dengzhan Plateau. The
small-scale collapses are less than 30m high; while thesurface deformations located downstream outside theBaimiaoziroad, the visible length of the fracture is about 30m,
the maximum width is about 50cm with a dislocation
about 20cm and a depth of about 1m.

Fig.3 Landform characteristics of theⅡzone of the talus
slope
The layer Q①, mainly contains fine particles mixed
with the coarse particles started from interface
between rock bed and overlying talus under the front
part of Dengzhan plateau, extended to the front edgeof Hudou plateau. The layer which is weak, of low
shear strength, coupled with the extremely weakpermeability, is the key layer affecting the stability ofthe talus slope(Xu Xiangtao et al, 2008).
2.2 Deformation and failure of the older landslide
after the earthquake

Fig.4 Fissure on the surface in the vicinity of 890m
elevation

The analysis and calculation of the stability of the

Besides the pavement was damaged by there threetalus slope on the left bank of Dengzhan Plateau after

shear failure on the road on elevation of 950m andimpoundment shows that the stability of the whole

980m. There was no deformation phenomenon on

slope is stable, yet only the front shallow part of theother parts, the overall talus slope is stable.

slope is of poor stability, so before impoundment thefront edge of the slope was heaped. The ballast bodyhas a length of about 470m and width of 330m at thebottom and 60m at the top respectively, what’s more,on the elevation of 845m, 815m and 790m three310th Asian Regional Conference of IAEG (2015)

Fig.5 Longitudinal cross-section 1-1 of the talus slope
Xiewen et al,2009; Yan Zhixin et al, 2010), based on
3. Analysis of the stability
static equilibrium theory of force acted on slide body or
3.1 Selection of calculation condition and parameters
block, is to evaluate the stability of the slope throughof rockmass
analyzing stress conditions under various failure modes
When "5.12" earthquake occurred, the water level inand relationship between stabilizing force and sliding
Zipingpu reservoir is approximately 835m, after theforce(Kang Yaming et al, 2006).
earthquake the main water kept a normal level of 877m
According to the deformation and failure modes ofand probably drop from 877m to 872m. Calculation
talus slope, potential mainscarp and toe of surface ofusing Limit equilibrium includes three kinds offracture generally appear in where the steep side joinsconditions:1) water level of 835m when earthquake
the gentle, therefore a total of 18 potential surfaces ofoccurred; 2) the normal water level of 877m; 3)therupture was found in calculation of stability using
normal water level of 877m plunge 5m. Each condition
search method of arcuate surface of rupture(Fig.4).
includes three states: 1) natural state; 2)earthquake; 3)
Table2 Stability calculation results of the talus slope
continuous rainfall (full of water).
under the 835m water lever with its front loaded when the
The field actually suffered from a basic intensity ofearthquake occurred.
Stability factor
Ⅸ, equivalent to a level acceleration of 392gal when
Section Potential
Normalslide
Normal"5.12" earthquake occurred; while after the earthquake
number surface Normal+earthquake(Ⅸ)+earthquake(Ⅷ)
the intensity dropped to Ⅷ, and acceleration to 185gal.
CI
1.537
1.084
1.286
According to relevant regulations, The composite effect
BI
1.442
1.031
1.216
AI
1.429
1.015
1.201
factor of the earthquake is 0.25. Learn from experiment
CH
2.181
1.373
1.710
results, physical and mechanical parameters of the soil
BH
1.976
1.282
1.577
was obtained through sample testing(Table 1).
AH
1.909
1.240
1.524
CG
2.199
1.457
1.776
Table1 Physical-mechanical parameters of the talus slope
BG
1.958
1.328
1.603
Strength
Physical Properties
AG
1.878
1.268
1.533
Soil type in
1-1'
NaturalSaturation
CF
1.648
1.227
1.421
density γ
density γ
slide zone
C（kpa） ()
BF
1.544
1.149
1.331
（kN/m3） （kN/m3）
AF
1.516
1.106
1.292
Soil
24
24.3
20
31
CE
1.458
1.150
1.297
Soft slide
22
22.4
10
16
zone
BE
1.609
1.209
1.395
(Q①)
AE
1.561
1.132
1.327
Ballast
20
50
27
CD
1.214
0.975
1.091
3.2 Analysis of limit equilibrium
BD
2.421
1.741
2.047
AD
1.839
1.289
1.533
Limit equilibrium(Zheng Yingren et al, 2009; Hu
Because the groundwater in slope body is rich and410th Asian Regional Conference of IAEG (2015)

shallow and the stability coefficient of each potential

in the slope is poor, its stability coefficients in different

surface of rupture is essentially the same in rainstorm andwater levels were 0.975, 0.974 and 0.913, the stability

natural state, so the rainstorm state was excluded from thecoefficients of the whole slope (along the interface

analysis. The calculation results are shown in Table2~3.

between the rock bed and overlying soil AI) were 1.015,

In the typical1-1 'profile, because of the ballast on the1.058 and 1.045, also in less stable to basic stable state.

front edge, all potential surfaces of rupture are stable in

While the rest of the potential surfaces of rupture cutting

natural state under different water levels(835m,877m andthrough the overlying soil are in stable state. The

plunge 5m) .

calculation results are consistent with the actual situationof deformation and failure after the earthquake.

Under the Intensity= Ⅸ in “5.12” earthquake, in
addition to the stability of potential surface of rupture CD

Fig.6 Schematic diagram of calculation cross-section 1-1 of the talus slope under the 835m water level when theearthquake occurred.
Table3 Stability calculation results of the talus slope under the 877m water lever and a rapid drawdown of 5m with its
front loaded.
Stability factor of the water level of 877m
plunged 5m
Potential Stability factor of the water level of 877m
Section slide
number surface
NormalNormalNormalNormal+earthquake+earthquake
NormalNormal+earthquake(Ⅸ)+earthquake(Ⅸ)
(Ⅷ)
(Ⅷ)
CI
1.540
1.083
1.286
1.531
1.079
1.281
BI
1.520
1.072
1.272
1.492
1.057
1.252
AI
1.513
1.058
1.260
1.488
1.045
1.242
CH
2.047
1.314
1.623
2.021
1.303
1.607
BH
1.945
1.269
1.557
1.939
1.266
1.553
AH
1.902
1.238
1.520
1.897
1.236
1.517
CG
2.037
1.379
1.665
2.006
1.365
1.645
BG
1.921
1.312
1.579
1.914
1.309
1.574
AG
1.873
1.268
1.531
1.867
1.266
1.528
1-1'
CF
1.623
1.209
1.400
1.592
1.192
1.377
BF
1.614
1.190
1.385
1.607
1.186
1.380
AF
1.601
1.155
1.357
1.596
1.152
1.353
CE
1.432
1.134
1.277
1.336
1.069
1.198
BE
1.839
1.344
1.570
1.797
1.320
1.539
AE
1.776
1.249
1.484
1.746
1.233
1.462
CD
1.197
0.974
1.083
1.113
0.913
1.011
BD
3.181
2.128
2.583
3.036
2.057
2.483
AD
2.315
1.520
1.859
2.248
1.487
1.814
Table 3 shows that, before the earthquake, eachpotential surfaces of rapture in the talus slope kept stable5

10th Asian Regional Conference of IAEG (2015)

under different water levels, which illustrates that

long-term adjustment and releasing, stress redistribution

underground water plays a less important role in various

in the talus slope has been completed at this time, thefactors that influence the stability of the slope. Strong

stress tends to form the original stress field in vertical andearthquake affects the front edge of the Dengzhan

horizontal direction. Rock and soil are compacted andPlateau severely, results in collapse and deformation byterrain becomes flat(Zhang Zhuoyuan et al, 1994), thereducing its stability; but the whole terrain and theslope is in a very stable state, External factors (rainfall,interface between rock bed and overlying soil are

earthquakes, etc.) has little effect on its stability(Qingenerally gentle, though dislocation might appear along

Siqing et al, 2005).

the interface between the rock bed and overlying soil,

The front edge of Dengzhan Plateau is relatively steep,

the short duration of vibration and the resistance fromwhich results in the concentration of stress in the vicinity

the gentle interface made the whole slope stable. Theof the free surface, so a tension area was formed in the

results from the analysis point out that the whole talus

shoulder of the slope, where deformation and failure areslope kept stable though suffered from the “5.12”

easily to generate because of the maximum stressearthquake, while only the front edge where the steep

difference or the maximum shear stress here. In addition,

part joints the gentle’s(part of CD) of Dengzhan Plateau

seismic wave will produce instant tension stress(Zhang

collapsed. This phenomenon has a close relation to theZhuoyuan et al, 1994), so the shoulder of the slope will

micro-terrain, especially the gradient of the slope. The

probably generate relative large displacement or collapse,

angle of the talus slope and interface between rock bed

while those gentle slope will keep stable or only generate

and overlying soil are generally about 20°and 13°

small displacement.

respectively, while the deformation part is 30～40°. This

Tens of thousands of landslides and avalanches, etc.

is because the protrusion and gradient transition part ofwere induced by “5.12” Wenchuan earthquake in 2008,

the slope has an amplification of seismic waves on

Huang Runqiu analyzed the regulation of the distribution

vertical and free face(Xu Guangxing et al, 2008), whichof the hazards and pointed out that most of the hazards

made the slope suffered more intense vibration damage.

were originated on the slope angle of 20～50°, and the
maximum density of the hazard appeared on slopes
steeper than 40°(Huang Runqiu et al, 2008). Several
typical large landslides were initiated by this earthquake,for example, the original slope inclination of Daguangbao
landslide is 55～60°(Huang Runqiu et al, 2008), slope
inclination of Donghekou landslide is 70～80°, and thatof Tangjiashan landslide is 40°,etc.
In the contrast, all of the older landslides in theFig.7 Collapse deformation on the front of the Dengzhan
plateau after the earthquake

seismic region have not been reactivated, e.g. Malingyan

4. Reactivation mechanism under the earthquake

landslide in Wenchuan County and the Zipingpu talus

landslide in Tangjiashan dammed lake, Qingtushan
slope discussed in this paper.

The evolution of the talus slope is a long termThis phenomenon indicates that the materials made up

progressive failure process, usually it can be divided into

the older landslides have already been failed, the energy

four stages(Xu Xiangtao et al, 2008): creep stage,

has been released, the slope has become gentle. The

pressing stage, sliding stage and stable stage. After theZipingpu talus slope is not steeper than 20°, while the"5.12" Wenchuan earthquake, there is no significant

interface between the rock bed and overlying soil is notdeformation on the entire talus slope, the calculation ofsteeper than 13°, which made the reactivation impossible

stability also shows that, the whole slope kept stable, yet

and only collapsed on where the steep part changes into

only the front edge of Dengzhan Plateau is unstable,

the gentle’s.

which resulted in some small scale collapse. Through its

6

10th Asian Regional Conference of IAEG (2015)

dam of Zipingpu hydraulic project [J]. Chinese Journal

5. Conclusion
Study of the stability of the Zipingpu talus slopeof Rock Mechanics and Engineering，2009，28(6)：affected by earthquake under various conditions shows

1279-1287.

that:

Zheng Yingren，Ye Hailin，Huang Runqiu. Analysis and(1)In natural and rainfall states, each potential sliding

discussion of failure mechanism and fracture surface of

surface kept stable under different water levels before theslope under earthquake[J]. Chinese Journal of Rockearthquake,andMechanics and Engineering，2009，28(8)：1714-1723.

groundwater are less important to the stability of the talus

Hu Xiewen ， Huang Runqiu ， Zhu Haiyong ， et al.

whichillustrates

that

rainfallslope.

Earthquake reactivation effects and stability study of(2)The whole talus slope also kept stable though

Malingyan landslide in Tangjiashan dammed lake[J].

suffered from the earthquake with Intensity=Ⅸ, yet only

Chinese Journal of Rock Mechanics and Engineering，

four small-scale collapses and surface fractures occurred

2009，28(6)：1270-1278.

on the front edge of Dengzhan Plateau.

Yan Zhixin，Zhang Sen，Zhang Xuedong，et al. Failure(3)The deformation and failure mode of the slope ismechanism and stability analysis of slope undercontrolled by micro terrain(Gan Jianjun, 2014). The

earthquake[J]. Journal of Engineering Geology，2010，average inclination of the Zipingpu talus slope and the18(6)：844-850.

interface between bedrock and overlying soil is gentle,

Kang Yaming，Yang Mingcheng，Hu Yanxiang. Mixed

which is not susceptible to failure caused by earthquake;analysis on slope stability by limit equilibrium method

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2006,15(3)：74-77.

of the concentration of stress or the area of tension stress

Xu Guangxing，Yao Lingkan，Lizhaohong，et al. Dynamic

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