Landslides and Engineered Slopes – Chen et al. (eds)
© 2008 Taylor & Francis Group, London, ISBN 978-0-415-41196-7
Importance of study of creep sliding mechanism to prevention
and treatment of reservoir landslide
Junguang Bai, Shengdi Lu & Jianshe Han
Northwest Hydro Consulting Engineers, CHECC, Xi’an, China
ABSTRACT: Creep sliding mechanism of the reservoir landslide and creepage property of the sliding-zone
soil are theoretically studied from the viewpoint of dynamics. The results show that the increase and decrease of
the shearing stress ratio for the sliding plane of a reservoir landslide and the viscosity coefficient of the soil in
the sliding zone are the essential conditions which can define whether the creep sliding of the landslide will turn
to a violent sliding. It has proved that the slide acceleration test by simulating the operational conditions of the
reservoir with a representative sliding block under appropriate environmental conditions is a reliable approach
to verify the creep sliding mechanism of reservoir landslide and an effective method to treat the landslide and
eliminate in advance the danger of landslide surge.
1
GENERAL
Most of the large-sized hydropower and water conservancy projects are constructed in mountain valleys.
Deformed or destabilized rock mass or landslide
widely distributed at natural bank slopes in the valley
area, which is caused by the combined actions of various geological factors, is defined as one of the most
severe engineering geological problems encountered
during the planning, construction and operation of
hydroelectric projects. As the most common geologic
phenomenon, reservoir landslide may lead to severe
damage, for instance, the Vajont reservoir landslide in
Italy occurred on October 9, 1963 killed 1925 people
and completely destroyed the reservoir. Therefore, the
stability prediction and study of reservoir landslide as
well as the prevention and treatment of the destructive effect shall be paid with special attention all the
time. In the case of the Lijiaxia hydropower station,
creep sliding mechanism of the reservoir landslide and
creepage property of the soil in the sliding zone are
theoretically studied from the viewpoint of dynamics
and on the basis of systematic analyses of the monitored landslide deformation data.. The results show
that the increase and decrease of the shearing stress
ratio for the sliding plane of a reservoir landslide and
the viscosity coefficient of the soil in the sliding zone
are the essential conditions which can define whether
the creep sliding of the landslide will turn to a violent
sliding. On this basis, a representative sliding block
is selected to execute the slide acceleration test by
simulating the operational conditions of the reservoir
and the creep sliding mechanism is proved again. The
test results successfully predict the deformation trends
of the landslide during the later impoundment, at the
same time it proves that the slide acceleration is an
effective method to treat the landslide and eliminate in
advance the danger of landslide surge.
2
2.1
EXPERIMENTAL RESEARCH
ON LANDSLIDE CREEP SLIDING
CHARACTERISTICS
Experimental research on creep
characteristics of the sliding-zone soil
From the viewpoint of dynamics, the magnitude of
the sliding speed of a sliding mass, with the exception of a small part of the slope bodies which behave
by themselves, is mainly governed by the physical mechanics characteristics, especially the creep
characteristics of the sliding-zone soil. Therefore,
experimental researches were carried out on the creep
characteristics of the sliding-zone soil of the reservoir
landslide before and after initial impounding.
(1) Direct shear test on sliding-zone soil
Represented by the bedding strata sliding-zone soil
of the No.2 landslide, the relation between shearing
force (τ) and shear displacement (ε) falls into plastic
curve type, and the content of fine grain and clay grain
of less than 2 mm is higher (accounting for over 90%).
The result of direct shear test is as follow:
Yield strength/peak strength (τy/τp) = 0.76∼0.85
Residual strength/peak strength (τr/τp) ≈ 1
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It shows that the shearing strength of the slidingzone soil and the physical parameters (water content,
dry density) remain unchanged, during sliding of
the slide mass, the shear strength parameters of the
sliding-zone soil have become stable basically, and
reached or closed up to the residual strength in most
cases; the change in micro-structure have little effect
on the strength of the sliding-zone soil.(2)
(2) Servo shear test on sliding-zone soil
Servo shear test indicates that: under natural
environmental conditions, the water content of the
sliding-zone soil is basically in saturation state
(ω = 15∼20%), and density varies in the range of
2.0∼2.25 g/cm3 in most cases; shearing speed has a
very small impact on the strength of the sliding-zone
soil when it is larger than 16 mm/h, and both values
of f and c tend to be stable; when shearing speed is
0.6∼16 mm/h, friction coefficient K increases at first
and then decreases, however c value decreases at first
and then increases, with less change of comprehensive
shear strength.
The test result is basically consistent with that of
direct shear test, namely when the water content and
dry density of the sliding-zone soil remain unchanged,
shear strength parameters of the sliding-zone soil will
not significantly decrease during the sliding of the
slide mass, and no governing factors of intensive sliding would be formed.
(3) Shear rheology test on sliding-zone soil
In testing, a water content of 15.21% is simulated by use of ‘‘Chen’s loading method’’(3) ; a density of 2.23 g/cm3 , a normal stress of Grade 4
(0.1∼0.4 MPa), and an increment of shear load of
1/8∼1/11 P (the peak strength in the case of different
grades of normal stress) are adopted; loads with various grades last 5 days. The result is listed in Table 1.
Relation expressions of shear stress and normal
stress are obtained from analysis of the rheology
test result of the sliding-zone soil of No.1 and No.2
landslides:
Table 1.
Critical strength τ0 = 0.325σ + 0.890
Peak strength τp = 0.320σ + 0.767
Long-term strength τ∞ = 0.162σ + 0.0175
When down sliding shear stress (τ) lies between
long-term strength (f ∞) and critical strength (f0) of
sliding-zone soil, landslide would not develop to the
accelerating creepage stage; meanwhile, coefficient
of viscosity (η = τ/ε, of which ε = strain rate) will
decrease with the increase of shear stress (τ); viscosity will increase with the increase of time; when
shear stress remains unchanged (or decreases slightly),
regeneration phenomenon may happen to coefficient
of viscosity.
Compared with the change amplitude of the landslide coefficient of viscosity (η) of Gepatsch reservoir
in Austria (102 ∼106 MPa·S) (4) , the change amplitude
of the landslide coefficient of viscosity (η) of Lijiaxia
(105 ∼106 MPa · S) is smaller to some extent. The reason is that the normal stress in the Lijiaxia rheology
test (0.1–0.4 MPa) is on the low side, and the shearing
time is shorter.
During impounding, the slide mass has a loose
structure, and coefficient of viscosity η decreases.
After stabilization of water level, τ can be considered
unchanged on the whole; as time lasts, η will increase,
and the rate will decrease; due to limited increasing
of η, creep sliding would not stop, maintaining at a
low rate stable creep status.
2.2 Landslide creep sliding mechanism analysis
The tests indicate that when the shear stress acting
on the sliding-zone soil lies between the long-term
strength and critical strength, shear creepage of the
sliding-zone soil is actually deceleration creepage. In
case this deceleration creepage has a long duration and
the displacement quantity of sliding mass is less than
the accuracy of observation, it is deemed to turn into
Peak strength and long-term strength for a sliding-zone soil test of Lijiaxia.
Sliding-zone
soil
Mud
sandwiched
with debris
Debris
sandwiched
with mud
Peak
strength
fp
Long-term
strength
f∞
Critical
strength
f0
Cp
(MPa)
C∞
(MPa)
C0
(MPa)
0.320
0.162
0.325
0.023
0.020
0.025
0.415
0.325
0.418
0.040
0.036
0.042
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steady creep. Because the actions of external factors
such as reservoir level change, rainfall as well as earthquake, etc., will finally be reflected by the action of
shear stress, each change of shear stress may result in a
new adjustment of creepage state of sliding-zone soil,
namely a new deceleration creep sliding would occur.
This phenomenon has been verified by many times
of water level rising and sudden lowering. When this
deceleration creep sliding movement is simulated by
use of logarithmic curve, the monitored displacement
procedure can be perfectly fitted; when its physical
characteristics is illustrated by coefficient of viscosity, the conclusion is that it has a similar ‘‘viscosity
coefficient regeneration’’ as that of the sliding-zone
soil test result for the Gepatsch reservoir, i.e. ‘‘coefficient of viscosity decreases with the increase of shear
stress; viscosity increases with the increase of time’’.
A lot of case histories indicate that the reason
of state shifting of landslide from creep sliding to
intensive creep sliding is because the shear stress on
slide planes suddenly exceeds the critical strength, the
positional adjustment caused by landslide displacement failed to duly fit the situation of shear load
increase(5) . This may contribute to the sudden decreasing of strength of slide planes and sudden increase
of main sliding load. Moreover, it is also related to
geometrical factors such as the shape of slide plane.
The reservoir landslide in front of Lijiaxia Dam is an
ancient landside always in movement, its slide plane is
steep in the upper portion and flat in the lower portion,
and the sliding-zone soil is of residual strength. Sliding
mass has a loose structure and strong water permeability, the change of reservoir level can be responded
by slide mass movement at any time, which makes it
impossible all the time that the strength on slide plane
exceeds the critical strength. Based on this analysis,
it is impossible that the reservoir landslide in front of
Lijiaxia Dam will come into the stage of acceleration
creep, i.e. intensive creep sliding is not likely to occur
to result in surging hazard.
3
3.1
PRODUCTIVE LANDSLIDE
ACCELERATION TEST TO SIMULATE
RESERVOIR OPERATION ENVIRONMENT(1)
landslide can be triggered by increasing the static
hydraulic pressure in the landslide body, it will be
the most cost-effective method.
2. Landslide body under the most unfavorable environment: During initial reservoir impounding,
reservoir level rose about 90 m, and apart from the
front superficial zone of the landslide that had large
displacement (generally, no more than 10 m), the
middle and deep zones of the slide mass commonly
got dislocation of several tens of centimeters along
the rear edge cracks and cracks of the sub-slidingbody or creep sliding rate increased, however,
it returned to decelerated creepage or constant
creepage when the reservoir level remained at EL.
2145 m, which indicates that large displacement
for the integral landslide will not necessarily occur
during landslide acceleration test. The most unfavorable environment for the slide mass is such that
the reservoir level abruptly drops to EL. 2,145 m
from the normal water level, and the corresponding
case of the landslide acceleration test is such that
the underground water in the sliding mass reaches
to the normal pool level of EL. 2,180 m or even
higher.
3. Verification of down-sliding mechanism of landslide: The stability of landslide of the reservoir
in front of the dam has gone through several
abrupt environmental changes, e.g., the extreme
high flood (rainfall) on the Yellow River in 1981,
the Yellow River break off during impounding of
the Longyangxia reservoir in 1986, the river closure by the Lijiaxia cofferdam in 1991 (water level
increased by 8∼10 m, and the reservoir impounding in 1997 (reservoir level increased by 90 m).
The general rule is that the stability of the slide
mass is closely associated with water action; every
environmental change triggers increase of creep
sliding rate of the slide mass and gradual extension
of surface tensile crack, but gradually returned to
constant creep sliding with small magnitude after
accelerated creep sliding of short duration.(6) If the
landslide acceleration test had the slide mass undergone these changes again, it will be of realistic
engineering significance to the stability prediction
and engineering remedial work during subsequent
reservoir level rising.
Test Assumption
1. Stability improvement alternative by excavation:
Static stability analysis shows that the most unstable water level on the calculated profile of landslide during reservoir impounding is at elevations
between 2,130∼2,145 m, and the stability of landslide will increase when water level reaches EL.
2,145 m, however, the required design safety
allowance still cannot be met, and necessary excavation work shall be done. If the sliding of a
3.2
In-situ landslide acceleration test
The test zone is located on the No. 5 longitudinal profile of the downstream No.2 landslide, the landslide
acceleration plane is 200 m in length and 200∼250 m
in width, the sliding plane is the secondary sliding
plane on No.2 landslide corresponding to No.3 crack
on the surface (refer to Figure 1), with a total volume
of 1.53 × 106 m3 , of which, the part above water level
2,145 m is about 780 × 103 m3 .
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Figure 2.
Ground water Duration Curve in Test Zone.
Figure 1. Schematic profile of productive environment
simulation test zone.
Test pits, test boreholes and ground water sprinkling irrigation are jointly used for the test. A total of
three rows of water pits (32 pits) spacing at 5 m and
a total of 91 boreholes are provided. Bottom elevations of the boreholes from the first row to the third
row are EL. 2,145 m and 2,170 m respectively; 10
underground water observation holes and 21 surface
displacement monuments are provided in the test zone;
1 monitoring adit (PD No.70) consisting of 3 subadits is provided parallel to the sliding plane of the
test zone at the downstream side EL. 2,163 m, all 3
sub-adits run through the sliding plane and the secondary sliding plane in the test zone. The test and
monitoring system provides complete monitoring with
respect to hydraulic pressure, discharge, underground
water, creep behavior, etc., and some samples are taken
from the sub-adits to check the physical and mechanical parameters of the soil in the sliding zone for further
study and verification of the creep sliding property of
the slide mass and the variation behavior of the strength
of the soil in the sliding zone. Landslide acceleration
test was carried out in afternoon on 9th June, 1998
and ended on 30th September, 1998, water was supplied during consecutive 114 days. Average daily water
injected from 9th June to 16th September is about
5.5 × 103 m3 /d (200 m3 /h), and the water supplied
from 17th September to 30th September increased to
20 × 103 m3 /d (about 800 m3 /h).
3.3
Analysis of landslide acceleration test
(1) Groundwater behavior
The groundwater in the test zone before water was
injected is almost as same as reservoir water level,
and it began to rise 3 days after water was injected
(duration curves of typical groundwater observation
holes during water filling refers to Figure 2). The
discharge of the water injected at early stage of the
test is about 200 m3 /h, and that during later stage is
Figure 3. Isoline of underground water level in test zone
(1998.09.30).
higher than 800 m3 /h, and the isoline of groundwater
in the test zone is shown in Figure 3. The behavior of
groundwater dynamic is as follows:
① The groundwater in the test zone has an obvious
hump curve at the third row of the boreholes, forming groundwater dividing line, and the water level is
largely above El. 2,180 m and the overall groundwater level in the test zone is not less than 2,175 m. It
can be seen from Figure 6 that the groundwater seeps
toward the outer side of the reservoir bank and groundwater level gradually drops to about EL. 2,150 m,
and the average hydraulic gradient is 40∼50%; for
groundwater seeps toward No. 3 crack at the inner
side of the reservoir bank, the average hydraulic gradient is 60∼80%; and the groundwater, controlled by
non-linear property of the sliding mass, distributes
irregularly.
② Figures 2 and 3 show that although the underground water level in the test zone didn’t fully reach
the expected EL. 2,180 m, the effect of saturation of
slide mass and underground water seepage behavior in
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the test zone on the stability of reservoir banks at least
equals to the case that the reservoir level lows to EL.
2145 m from EL. 2,175 m at a very short duration,
almost the most unfavorable operation environment
for the reservoir in the near future.
③ Since water is injected through pits and boreholes
and sprinkling irrigation, for the superficial zone of the
slide mass, the intensity of water injection is far higher
than the infiltration intensity of future heavy rainfall.
Calculated based on the injected water and the applied
area, the water injection intensity at early stage equals
to the rainfall (11.6 mm/d) within 100 consecutive
days; and the intensity at later stage equals to rainfall (42.8 mm/d) within 14 consecutive days. Hence it
is deemed that saturation and instant pore water pressure of the slide mass is far higher than the capacity of
long term rainfall.
(2) Displacement behavior
Figure 4 shows that the displacement rate measured by instruments during landslide acceleration test
apparently increased. The displacement behavior is as
follows:
① When small quantity of water injected at early
stage of the test, the creep sliding rate is only 2∼3
times the initial value; when water quantity was
increased, creep sliding rate may increase by 10 times
approximately, and the value is about 1/2∼1/3 of
the maximum rate measured during initial reservoir
impounding.
② It is found during the test that some localized disintegrated loose slide mass on the surface of the bank
slope failed, and the displacement measured by the
instruments provided at the platform outside of No.
3 crack in the test zone increased to 220∼270 mm,
and that at the inside of No. 3 crack is about 150 mm.
Some fresh radial tension cracks occurred along No. 3
crack and the outside platform, further reflecting the
characteristics of the behavior of the slide mass during
landslide, such as pervasive crack, loose disintegration
and displacement and concurrently reflecting the creep
sliding rate of the slide mass undergone acceleration
and deceleration at a second time when underground
water level rises with increase of quantity of the
injected water, which coincides with the displacement
during initial impounding.
(3) Stability calculation check
The result of the sample test carried out in the adit
(PD70) for landslide acceleration test shows that the
strength parameter of the soil in the sliding zone was
reduced at a certain extent during the test, basically
reflecting the real strength of the soil in the sliding
zone during subsequent reservoir impounding. Since
the strength value of the soil in the sliding zone is
almost the same as the value verified before impounding, the verified parameters used in the test is adopted
in the stability calculation check for the slide mass for
easy comparison. The result of calculation check is
shown as follows:
The landslide acceleration test environment has
lowered the safety factor of the testing slide mass by
3.6∼11%; the safety factor of the secondary sliding
mass with No. 2 crack as its bottom plane reduced
by 3.5∼4.86%, its maximum sliding rate was approximately equal to 1/2∼1/3 of the value during initial
impounding and didn’t result in abrupt landslide. Calculation result shows that the stability factor of the
slide mass during further increase of reservoir water
level above EL. 2,145 m has a tendency of increase
at a certain extent, which is in compliance with the
calculation results obtained before reservoir impounding. Therefore, analyzed from perspective of stability,
surge wave will not occur in the future at other
locations having similar stability conditions as the
reservoir landslide acceleration test zone.
4
Figure 4. Duration Curve of Displacement Measured at
Typical Monitoring Points in Test Zone.
CONCLUSIONS
The landslide dynamic analysis should incorporate
mechanics theory of creep sliding mass. Study and
monitoring data of the Lijiaxia reservoir give evidence that relaxation and displacement of landslide
adjust the stability state of sliding mass. Landslide
in this area experiencing creep sliding acceleration
and deceleration before and after reservoir impounding proves that the viscosity of the sliding zone soil
increases during creep, which restrains the shear stress
acting on the sliding plane within critical state so
that accelerating creep and abrupt sliding will not
occur and accordingly, surge wave hazard will not be
incurred.
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At an appropriate environment, a typical sliding
mass can be selected for landslide acceleration test
under simulated reservoir operation conditions, which
is a reliable solution for verification of landslide creep
sliding mechanism and also one of the effective ways
to improve the stability of landslide and eliminate
landslide surge wave risk.
REFERENCES
Northwest Hydro Consulting Engineers, CHECC, Stability
Forecast and Study Report for No.1 and No.2 Landslides of Lijiaxia Hydropower Station after Preliminary
Impoundment, December, 1999.
Xiao Shufang & K. Akilov, 1991. Structural Fabric and
Strength Creep Characteristics of Mudded Intercalation,
Jilin Science & Technology Press.
Liu Xiong, 1994. An Introduction to Rock Rheology, Geological Publishing House.
Henderlun, A.J. & Patton, Jr.F. 1988. Geotechnical Analysis of New Geological Observation Data on the Failure
Plane of Vajont Landslide (Final Report, June, 1985),
reprinted in the ‘‘Translation Collection of Reservoir
Landslide Articles’’ (for internal exchange and reference
only), Northwest Hydropower Investigation, Design and
Research Institute.
Wang Lansheng, et al., 1988. Preliminary Study on Development Characteristics and Mechanism of Initiation, Sliding
and Braking of Xintan Landslide, Typical Landslides in
China, Science Press.
Jin Delian & Wang Gengfu, 1988. Tangyanguang Landslide
of Zhexi Reservoir, Typical Landslides in China, Science
Press.
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