10th Asian Regional Conference of IAEG (2015)

The deformation and failure mechanism of the toppling slope of the dam reservoir
area near Xinlong hydropower station
WU Jian-li(1) ，HU Xie-wen(1,2) ，LANG Jing-xuan(1) ，and LV Jun-lei (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
E-mail:45117304@qq.com(2) Key Laboratory of Ministry of education of traffic tunnel , Southwest Jiaotong university, Sichuan Chengdu
610031,China
Abstract

Through field investigation a toppling slope of the dam reservoir area near Xinlong
hydropower station, this paper proposed a reasonable planar division and section zonation of
the toppling rock mass according to its deformation degree, analyzed the deformation
feature and evolutional mechanism of the toppling deformation in the rock near the dam
reservoir, and revealed the evolution process of the toppling failure under the action ofreservoir water. The results show that the toppling rock mass can be divided into strong or
weak group in accordance with the deformation degree of toppling-bending andbending-pulling. After the deformation, the strata inclination of strong toppling slope is
10°-30°, while the weak one is 40°-70°.Under the circumstance of rainstorm or water
storage, the strong toppling rock mass will probably creep along the boundary of strong toweak, with a possibility of whole sliding.
Key words: toppling failure; deformation mechanism; planar division; reservoir area

locates about 6km upstream of Xinlong County,
has a storage capacity under normal water level
about 0.083 billion m3 and installed capacity of
240MW, with a dam crest on elevation of
3112.00m, crest length of 250m, maximum damheight of 67m, early prepared normal water levelof 3107m, and reservoir backwater length of 26km,
There is toppling slopes in reservoir area near thedam on the right bank, the length of which along
the river is about 1.4km, and the width across theriver is 170m.
From 2011 to 2012, the original selections of
dam site, the type of dam, the layout of its hydroproject and others were respectively carried out
on-site comprehensive survey. The overallplanning was based on power plant project area,
reservoir area and others with their topographic
and geologic conditions, external transport andother conditions, and then in 2013, the relevant
geological exploration test work on site ofpre-feasibility study stage was carried out, thecontents of which including the exploration and

1 Introduction
Toppling failure as a typical failure mode incountertendency layered slopes, was firstly
analyzed in the 1970s using the limit equilibrium
method proposed by Goodman and Bray
（1976,1981）which caused an extensive attention
on toppling deformation in geotechnicalengineeringandengineering
geologicalcommunities. In the recent 40 years, manygeological experts and scholars made a lot of deep
researches and analyses on this theory, and thegraphical method and numerical computation had
made widely development and application in thestability calculation of toppling mass(Ishida T and
Chigira M 1987, Hoek E and Bray 1981, Zanbak
C 1983, Wyllie DC 1980, GAO Lian-tong, YAN
E-chuan 2015).
The Xinlong hydropower station which1

10th Asian Regional Conference of IAEG (2015)

investigation of river bed and overburden
thickness on both sides, hierarchical structure andspatial distribution feature. Exploration andinvestigation was made out for river bed and thedepth and level of weathering unloading tocross-strait rock, the rock integrity, structuralplane development degree and its syntagmatic
relation, physical mechanical properties andpermeability of rocks, analyzing those impacts tothe dam foundation as well as the stability andleakage of rock-mass slope.
Engineering geological survey shows obvious
signs of deformation in the slope, and the overall
stability of the slope could be endangered if thedeformation developed wantonly, thus evaluating
the stability of the toppling slope is necessary.
Through studying the mechanism of deformationand the stability situation of toppling slope undervarious conditions, three geophysical profiles andfive adits are successively set up in the topplingmass. This paper here studies the deformation
mechanism and potential failure mode of topplingslope, and finally makes evaluation on the stabilityof toppling mass.

2 Geological conditions in the study area
The slope in dam site area locates about 1km
upstream of Xinlong County, Dilong Village of themain stream channel segment of Yalong River.
The length of the channel segment in dam site area
is about 0.5km, and Xinlong gutter imports fromthe upstream left bank in dam site area. Yalong
River flows into the dam site at an azimuth angleof S32°W, turns into S55°W near the dammonolith, and finally flows out the dam site in the
direction near "W". Valley in dam site area is theoblique~transverse valley, which is in asymmetric"U" shape, and the valley is deeply cut. The valley
is steep on the left bank, with a natural slope angleof 50°~60°, and the height of slope on the verge ofrivers 300m~350m; the valley is also steep on theright bank under 3130m, with a natural slope
angle of 40°~45°, and the valley above 3130m is
gradual, with a natural slope angle of 15°~20°, andthe height of slope on the verge of river is
130m~280m (see Fig. 1). By early crustal uplift
and unloading of rock, some deformablelandslides are embedded near the right bank of thedam.

Fig.1 Full view of the slope on the right bankIn order to identify the plane distribution
range and geology of deformable landslides,
the work to building three geophysicalprospecting sections, five adits and multiple
drilling is successively set out. The quaternary
overburden layer of toppling slope part mainlycomposes of modern alluvial deposits and rockfall deposits locates on the toe of the valley slope.
Bedrock made of slate sandstone of Lianghekou
Formationlower

section(T3ln1)

hasslightlychanged with an overall stable trend, due to thestrata steep dip angle, the tendency has slightchange (see Fig. 2).

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10th Asian Regional Conference of IAEG (2015)

Fig. 2 Geological profile of the dam axis
According to seismic inversion data, stratum
is divided into three layers: the first layer isstrongly weathered and unloading rock mass, witha covering layer of gravelly soil and residual slope
wash mainly, whose wave velocity is between
500m/s~1500m/s; the second layer is the weakrock mass on the top, whose wave velocity is
between 1500m/s~2200m/s; the third layer isbottom weak and slightly new rock mass, whose
wave velocity is greater than 2200m/s(see Fig. 3).
Take profile Z2 in Fig. 3 for example, the entiretyof first and second layer shows a fan-shaped
distribution from the bed side to the mountain top
side. The first layer burial depth is 10~55m, with aminimum burial depth at stake number Z2-65, andthe deepest at stake numberZ2-420.The burial
depth of second layer of weak unloading rockmass is 20~60m, with a minimum burial depth atstake number Z2-65, and the deepest at stake
numberZ2-440.

The slopeis located in the west limb of themultiple anticline of Yalong River, and it is amonoclinal stratum, with a stratum overalloccurrence of N10°W~N20°E/NE (or NW)
70~90°, while the regional fold deformation is
weak. No major fold development is seen in theslope, andthere are only individual small
foldssporadically exposed.No mass fault is seen in
the slope.
Rock mass unloading is relatively strong inthe slope, and fractures in all directions are
generally open in strong unloading band, withrock debris and secondary mud filled, mostfractures are 2~10cm wide, and top-collapsed
phenomenon exists in partial adit.Fractures areless open in weak unloading band, with rockdebris filled mainly, and secondary mud filled
partially, but the width is generally less than 3cm,
and secondary mud has the feature of interval
filled, that is the rock masses with more secondary
mud and less secondary mud are often distributed
alternately . In the strong unloading rock mass,rock debris or secondary mud filled arewidespread in the fractures along the slope, andalternately filled exists in weak unloading rockmass.
The groundwater of the toppling slope infront of the dam could be divided into loosely
packed layer of pore water and fissure water inbedrock.The loosely packed layer of pore water isdistributed mainly in slope wash etc., and thewater abundance is often greatly affected byseasonal changes. While thefissure water inbedrock is mainly supplied by atmosphericprecipitation, and flows into the Yalong River.

Fig. 3 The geophysical prospecting result map ofcross slope Z2 in toppling slope
Combined with adit exploration (Table 1), thefirst layer above (dark blue) actually represents
strong unloading, strong toppling deformable rockmass, and the second layer (light blue) actuallycorresponds to the weak unloading, part weak
toppling deformable rock mass.

3 Deformation features of toppling mass
3.1 Partition of topplingslope3

10th Asian Regional Conference of IAEG (2015)

Table 1The strong and weak deformable achievements of adit exploration reveal for the toppling slope in
Xinlong hydropower station

Adit
opening
elevation（m）

Covering
layer
thickness（m）

Parti
-tion

Adit
number

Adit
depth
（m）

A1

PD08

99.3

3081

A2

PD16

118

3137

Deformation
degreeUnloading

Weathering

Weak
(m)

Strongweathering (m)

Top
weak
(m)

Botto
m
weak
(m)

StrongWeak

Strong
(m)

13

30

38.5

38.5

55

38.5

55

90

1

30

72.5

72.5

85

72.5

85

102

of toppling slope front of dam are ascertained.
According to the development characteristics of
toppling slope and current slope stability, threeareas of A1, A2 and B can be divided (see Fig. 4～
5).

According to surface survey and exploration,
the division criteria between strong and weak
deformation upstream (Table 2) directed at
Xinlong hydropower station dam area are revealed,
and the planar distribution range and spatial
distribution for the strong and weak deformation

Table 2Simple table about criteria on strong and weak toppling deformation division for the slope near
dam at the right bank of Xinlong hydropower Station
Essential feature

Strong toppling deformation

Weak toppling deformation

Rock mass dip

Dip angle 10~40°

Dip angle 40~70°

Tension fissure developing

Wedge-shaped tension fissures

Wedge-shaped tension fissuresdensity between layers

generally develop,wellsporadically develop,poorconnectivity

connectivity

Rock masses generally loose,poor

Rock masses generally loose,wellself-stabilization

self-stabilization

Cataclastic structure,partially

Mainly is mosaic

loose structure

structure,partially loose structure

angle(reflecting toppling
degree)Rock mass loose degree
Rock mass structure4

10th Asian Regional Conference of IAEG (2015)

Fig. 4 The engineering geological zoning map for slopeof Xinlong near dam right bank areaa)

A1 zoneb) A2 zone

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10th Asian Regional Conference of IAEG (2015)c) B zone
Fig. 5 Cross-sectional view of the toppling slope . a) A1 zone, b) A2 zone, c) B zone
less than 40°after the stratum topples, with partial
granular media form and poor self-stabilization; 2)
thickness of weak deformation is maldistributed; 3)
topography and geomorphologyis relatively
gradual(with a slope angle of 35°~40°), andresidual colluvium deposits distribute in the footof the slope (see Fig. 6).

3.2 Essential featuresof zone A1
Zone A1 locates at the downstream segmentof the slope, and the length along the river is about
332m.Its main features are as follows: 1) strong
deformation occurred on entire slope surface, withthe thickness of 18-30m, rock mass crushed
severely, and it is nearly horizontal or the angle

Fig.6 Downstream side zone A1 of slope
Analysis shows that, this area is actually aresidual body of the early strongly toppling rockmass after surface peeling.Strong deformation has
spread throughout the slope and residual
colluvium deposits distribute in the foot of theslope.The overall stability of slope in zone A1 isgenerally poor, with poor partial stability, which isunfavorable for the slope engineering.

thickness of 33-46m. After toppling, the strata arenearly horizontal or in a dip of less than 40°, andthe rock is broken in cataclastic structure withgeneral self-stability. 2) Weak deformations are in
general distribution with a height of 39-81m. After
toppling, the strata are in a dip between 40°-70°,
and the rock is in mosaic structure. Part of thestrata are in relaxed state due to the impact of

3.3 Essential features of A2 zone

weak unloading. 3) Geomorphology of this zone is

A2 zone locates in the middle part of therelatively steep (slope greater than 45°). The

slope, which is 469m in length along the river. Its

bedrocks are uncovered except for those covered

main characteristics are: 1) Strong deformations

by residual colluvium deposits in some certain

distribute on a certain height above the highway

part of the slope (see Fig. 7).

instead of the whole surface of the slope, with a6

10th Asian Regional Conference of IAEG (2015)

Fig.7 A2 zone in the middle part of the slope
Analysis shows that, this zone is also theare: 1) Strong deformations distribute on almost

remained mass which was peeled off fromthe whole surface of the slope, but thin in

surface-layer in strong toppling rock mass in the

thickness, which is about 10-20m. After toppling,

early period in wider range and larger scale. As athe strata are nearly horizontal or in a dip of lesswhole, strong toppling rock mass in zone A2 is inthan 40°, and the rock is broken in cataclastic

a general level of overall stability and low level ofstructure with general self-stability. 2) Weak

local stability, which is a bit better when compared

deformations are in general distribution, theto zone A1. Due to that zone A2 is near the

thickness of which is close to that of A2 zone,

reservoir and dam section, the safe operation ofwhich is in the range of 35-80m. After toppling,

the dams would be threatened by bank caving after

the strata are in a dip between 40-70°, and the rockthe impoundment of reservoirs.

is in mosaic structure. Part of the strata are in
relaxed state due to the impact of weak unloading.
3) In addition to the terrace accumulation in the

3.4 Essential features of B zone
B zone locates in the upper reaches of the

middle part of the slope, there are residualslope, which is 616m in length along the river. A

colluvium deposits at the foot of the slope. The

notable distinction to A1 zone and A2 zone is that,

rest parts are also relatively steep (slope angle

terrace accumulation is reserved in the middle part

greater than 40°) (see in Fig.8).

of the slope in this zone. The main characteristics

Fig.8 B zone in the upper reaches of the slope
Distribution of earlier terrace accumulation inConsequently the overall stability of the slope

the middle part of the slope shows that this zone is

becomes better due to pressing foot or pressing

similar to A2 zone. But after a wide range ofslope. Overall stability of the slope is good. As aslump in early times, alluvial accumulations (IVwhole, overall stability of the strong toppling rocksteps of terrace) is able to accumulate due to themass on the surface of B zone is good, except thatconvexthe local stability of the slope foot is in a generalbankimpactof

YalongRiver.

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10th Asian Regional Conference of IAEG (2015)

There still are possibilities of surface slip
failure of residual slope in these three zones,
particularly in A1 zone and A2 zone. The failuremode would be pull-type, in another way, the slipof damaged front locked patches will result in theoverall slip of post median part along the fracturesurface of strong or weak toppling rock mass. As aconsequence, in the stability calculation of
toppling slope below, analysis will be conductedaccording to the slope type -- strong or weak
toppling rock mass.

level, in another way, the overall stability of B
zone is better than A2 zone. After impoundment of
the reservoir, there could be local bank caving at
the foot of the slope, but in limited scale, so thesafe operation of the dam would not be threatened.
3.5 Possible instability mode of slope
Slope is mainly located on slates, of which
the foliation is extremely developed, the lithology
is relatively weak, the spillway rock mass is steep,and the valley trend mainly intersects with stratum

4 Stability analysis of toppling mass

strike at small angles. As the Yalong River
undercut and earth crust upraises correspondingly,

4.1 Parameter selection

spillway rock mass enters the progress of
dumping~bending,bending~cracking

In

order

to

obtainthephysicalandandmechanical parameters of ground in field, we

potential slipping surface at bottom formed bycarried out 26 group indoor rock physical andbending fracture surface towards free face, andmechanical tests and 32 sets of indoor soil

current toppling slope is created (Zhang Zhuoyuan,

physical and mechanical properties tests, testing

Wang Shi tian 1994). Simultaneously, the slope

the rock density, cohesion force, shear strength,

could be classified into strong toppling and weakcompressive strength, modulus of elasticity,

toppling rock mass according to the deformation

softening coefficient and so on.

level of dumping~bending and bending~cracking.
The

dip

angle

of

strataformedbyParameters for deformable slipping surface

theare shown in table 3, which are determined by the

dumping~bending procedure of strong formablefield and laboratory test results, survey data andare about 10°-30° while that of strata formed by

the experience analogy.

the dumping~bending procedure of weak formable
are about 40°-70°.

Soilparameter
Types ofrock-soil
Gravelly Soil (col+dlQ4)
Gravel (egg stone) soil
(alQ2)
A1
Strong topplingzone
rock mass
Weak topplingrock mass
A2
Strong topplingzone
rock mass
Weak topplingrock mass
B zone Strong topplingrock mass
Weak topplingrock mass

Table 3 Parameter values of the slope
Water immersion state

Water-saturated state

C(kPa)

Φ(°)γ(kN/m3)

C(kPa)

Φ(°)γ(kN/m3)

0.0

21.0

20.0

20.0

17.0

21.0

60.0

41.0

21.8

48.0

32.9

22.8

150.0

24.2

26.8

135.0

21.8

27.3

200.0

26.6

26.8

180.0

23.9

27.3

200.0

26.6

26.8

180.0

23.9

27.3

300.0

28.8

26.8

270.0

25.9

27.3

150.0

26.6

26.8

135.0

23.9

27.3

300.0

28.8

26.8

270.0

25.9

27.3

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10th Asian Regional Conference of IAEG (2015)

4.2 Calculation model and working condition

rainfall(saturation).

The

value

of

seismic

After impoundment of the Xinlong reservoir,

influence coefficient is 0.5. X-profile, 5-profile

the water’splummet of 5m once is taken as theand 70profile are selected as typical profiles ofextreme case, that is to say, from the normal

stability calculation in A1, A2 and B zones.

storage level of 3107 meters to 3102 meters.
4.3 Result and analysis of stability calculation

Therefore three kinds of water storage working
conditions which are used in stability calculation

Potential sliding surface appeared to be

are considered: 1) before impoundment; 2) normal

broken line~arc shape, which is determined bywater level of 3107 meters; 3) plummeting 5strong and weak bottom boundary of the toppling

meters from normal water level of 3107 meters.

slope. Typical profiles from above three zones are

And superimposition conditions as follows should

calculated

be considered in the above conditions: 1) thetransmit-coefficientnatural state; 2) the earthquake; 3) continuous

results are shown in table 4 to table 6.

bybrokenline-sliding

method.

Thesurfacecalculation

Table 4 Calculation results on stability coefficient before the impoundment
ProfilesPotentialsliding surface

Water immersion

A1 zoneWorking conditions
Water immersion+earthquake

Persistent rain

Strong toppling rock
1.164
mass
Weak toppling rock
1.349
mass
A2 zoneStrong toppling rock
1.136
mass
Weak toppling rock
1.205
mass
B zoneStrong toppling rock
1.321
mass
Weak toppling rock
1.385
mass
Table 4 shows that strong and weak toppling

persistent rainfall in A2 zone; the correspondingrock mass are both stable under water immersion

weak toppling rock mass is stable all the time,and strong toppling rock mass is understable underwhich indicates that the stability of slope in A2

earthquake

beforezone is fair. All the slope in B zone is stable underimpoundment in A1 zone, which indicates that thevarious conditions, which matches the fieldstability of slope in A1 zone is poor. Strong

qualitative analysis results and indicates that thetoppling rock mass under water immersion isback pressure of terrace in the middle of the slope

stable, and is understable under earthquake andmass is of significant influence on stability.

andpersistent

rainfall

1.045

1.040

1.196

1.197

1.011

1.009

1.054

1.061

1.162

1.173

1.213

1.229

Table 5 Calculation results on stability coefficient under normal water level of 3107 meters
ProfilesPotentialsliding surface

A1 zoneStrong toppling rock mass
Weak toppling rock mass
Strong toppling rock mass
Weak toppling rock mass
Strong toppling rock mass
Weak toppling rock mass

A2 zoneB zoneWater immersion
1.132
1.279
1.124
1.128
1.312
1.345

9

Working conditions
Water immersion +earthquake
1.019
1.142
1.000
0.990
1.154
1.179

Persistent rain
1.050
1.229
1.009
1.043
1.173
1.222

10th Asian Regional Conference of IAEG (2015)

Table 6 Calculation results on stability coefficientfrom 3107m plummeting to 3102m
ProfilesPotentialsliding surface

A1 zoneStrong toppling rock mass
Weak toppling rock mass
Strong toppling rock mass
Weak toppling rock mass
Strong toppling rock mass
Weak toppling rock mass

A2 zoneB zoneWater immersion
1.117
1.251
1.121
1.121
1.308
1.340

Working conditions
Water immersion +earthquake
1.007
1.119
0.998
0.985
1.151
1.175

Persistent rain
1.032
1.202
1.006
1.037
1.169
1.218

Table 5 shows that strong toppling rock masspossibility of an overall slide. So attention should

is understable under earthquake and persistentbe paid to this and reinforcement treatment would

rainfall and weak toppling rock mass is always

be necessary.

stable under normal water level of 3107 meters,

The results according to Table 6:After a sharp

which indicates that the reservoir has somedecrease of 5m of the water level, A1 zone would

influence on the stability of slope.

be influenced to a certain extent. A2 zone would

Table 6 shows that strong toppling rock mass

have the worst stability and there would be highis understable and weak toppling rock mass ispossibility of an overall slide, reinforcement

stableunder earthquake and persistent rainfall intreatment should be done in this zone. Strong andA1 zone. It also indicates that water plummet has

weak toppling rock mass in B zone would

some influence to A1 zone, especially upstream

encounter certain decrease in their stability

side in A2 zone, the stability becomes worse.

coefficient, not in minor scale, so the whole zone

Strong toppling rock mass in A2 zone is in theis in stable state.

state of limit equilibrium under earthquake andReferences

superimposition of persistent rainfall~ unstablestate, and it is probable to have an overall sliding;weak

deformation

mass

is

stableunder

superimposition of persistent rain, but it is in thestate of limit equilibrium under earthquake
superimposition condition ~ unstable state.
5 Conclusion
Toppling slope are classified into A1 zone,
A2 zone and B zone according to the development
characteristics of the slope and the stability ofcurrent condition of the bank slope.
Before impoundment, the slope are always instable state under all kinds of working conditions.
The slope is overall stable without regard to theimpoundment condition.
The results according to Table 5:All zones inthe slope are in stable state when the water level
reached 3017m. Strong toppling rock mass in A2zone is mostly influenced. In the condition of
normal impoundment combined with rainstorm, its
stability would be weaken and there would be high

BRAY J W, GOODMAN R E.The theory of base
Friction Models.International Journal of rockmechanics
andminingsciencesandgeo-mechanics abstract,1981,18( 6) : 453-468．
GAO Lian-tong, YAN E-chuan, XIE Liang-fu.
Improved
Goodman-Bray
MethodinConsideration ofGroundwaterEffect.Journal of
Yangtzeriver
Scientificresearch
Institute.2015;32:78-83.
Goodman RE, Bray JW. Toppling of rock slopes.
In: Boulder CO, Proceedings ofthe specialty
conference on rock engineering for foundations
and slopes.New York: American Society of Civil
Engineers; 15–18 August 1976, p. 201–23.
Hoek E, Bray JW. Rock slope engineering.
London: Institute of Mining and Metallurgy;
1981.
Ishida T, Chigira M, Hibino S. Application of
the distinct element method foranalysis of
toppling observed on a fissured slope. RockMech Rock Eng1987;20:277–83.
Wyllie DC. Toppling rock slope failures examples
of analysis and stabilization.Rock Mech RockEng 1980;13:89–98.

1010th Asian Regional Conference of IAEG (2015)

Zanbak C. Design charts for rock slopes
susceptible to toppling. J GeotechEngASCE
1983;109:1039–62.
Zhang Zhuoyuan, Wang Shi tian, Wang Lan sheng.
Engineering geology analysis principles [M].
Beijing. Geology press, 1994.

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