NHESSDNatural Hazards and Earth System Sciences DiscussionsNHESSDNat. Hazards Earth Syst. Sci. Discuss.2195-9269Copernicus GmbHGöttingen, Germany10.5194/nhessd-3-2537-2015Movement of the Donglingxin landslide, China, induced by reservoir inundation and rainfallYuJ.WangR. B.rbwang_hhu@foxmail.comhttps://orcid.org/0000-0002-2535-0862XuW. Y.YanL.ZhangJ. C.MengQ. X.Research Institute of Geotechnical Engineering, Hohai University, Nanjing, ChinaKey Laboratory of the Ministry of Education for Geomechanics and Embankment Engineering, Hohai University, Nanjing, ChinaR. B. Wang (rbwang_hhu@foxmail.com)15April2015342537256422March201531March2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://nhess.copernicus.org/articles/.htmlThe full text article is available as a PDF file from https://nhess.copernicus.org/articles/.pdf
With numerous high mountains, deep valleys and turbulent rivers,
many hydropower plants have been constructed in the south-west
China. Reservoir bank slopes are very common in this area, these
slopes are widespread and quite often involved in deformation that
can result in serious damage and casualties. In case of the
Donglingxin landslide, for an in-depth study of processes that can
trigger these events, the deformation characteristics and the
failure mechanisms of the slope were performed on a detail scale,
based on an intensive monitoring of rainfall events, reservoir level
fluctuation and groundwater movement. The deformation of the upper
part of slope is mainly induced by rainfall events, reservoir level
fluctuation affects the deformation of the lower part of slope. The
increase of pore water pressure may result in the failure of
slope. The filed investigation suggest that the slope is
unstable. Drainages is the only stabilization measure which can be
implemented, due to very complex geological and geomorphology
condition.
Introduction
In recent years, a large number hydropower plants have been
constructed in south-west China, and many landslides are
reactivated by initial impoundment of the reservoirs, so
instabilities of reservoir bank slopes have been a challenging
issue for hydropower projects. Many failure events of reservoir
bank slope have been reported in China (Huang, 2009). In 1961,
a reservoir-induced landslide near the Zhaxi dam claimed the lives
of over 70 workers (Jin and Wang, 1988). In 2003, the Qianjiangping
landslide killed 24 people and destroyed 346 houses (Wang
et al., 2004). The landslide-related direct and indirect economic
losses in China costs more than 20 billion Yuan every year (Bai
et al., 2014).
Researchers in China studying the deformation characteristics and
triggering mechanisms of landslides (Qi, 2006) found that reservoir
level fluctuation and rainfall cause deformation leading to
landslides (Wang et al., 2008).Infiltration of rainfall accelerates
the deformation of the landslide and deteriorates its stability
(Wang et al., 2012).
Sanbanxi procject is located on the lower reach of the Qingshui
River (Fig. 1), the main stream of the Yuanjiang River. It is the
sole hydropower plants with multi-year regulating ability on the
Yuanjiang river. It has a concrete face rockfill dam with maximum
dam height of 185.5 m and a total installed capacity of
1000 MW (4×250MW). The reservoir has
a normal pool level 475 m and a total capacity of
3.75 billionm3. The total area of the reservoir is
79.56 km3, and the backwater of the reservoir is
120 km long. The Donglingxin landslide is located on the
south bank of the Qingshui River, southeast of Liuchuan, Jianhe
Country, Guizhou Province. It's is 80 km upstream of the
Sanbanxi dam (Fig. 1), on which there is a village of dozens of
inhabitants. With a volume of 20.7 millionm3, it is
the largest ancient slide in the reservoir. The slide was
reactivated by the first impoundment of the reservoir in
July 2007. Due to the narrow valley at the slide location, the
supports and protections measure is difficult to be used. The
Liuchuan town in the immediate vicinity will be destroyed, once the
slide fails. Therefore, the study of its deformation
characteristics and the failure mechanisms is significant for
assuring the safety of reservoir and dam.
In this paper, we discuss the deformation characteristics and
triggering mechanism of the Donglingxin landslide through
geological investigation and comprehensive analysis of the 2 year
observation data. The deformation tendency was also assessed.
Description of the Donglingxin landslide
The Donglingxin landslide is a reservoir bank slope, its natural
slope surface mainly tends toward N 35∘ W. There are two
gullies on the edge of landslide. Geological survey indicated that
the Donglingxin landslide is a large-scale ancient landslide,
which is an accumulation of several consequent slides along the
bank slope, several secondary slumps occurred in its front, most
of the landslide area is underlain by coarse silty clay and stone,
relaxed and broken rock mass was found only at the foot of the
excavation slope and the central ridge. The landslide can be
divided into two zones, Fig. 2 shows the topography and zoning of
the Donglingxin landslide (Google earth), (i) zone 1 extends along
the Qingshui River with approximately 280 m in height and
310 km2 in area, it is 800 m wide and lies
between the riverbed and elevation 700 m, and the landform
looks like a round-backed armchair (ii) zone 2 is located in the
upper southern region of zone 1 with 120–220 m in height
and 68 km2 in area it is 200 m wide and lies
between elevation 600 and 825 m. Borehole data indicate
that the landslide is generally between 18 and 96 m thick,
the thickest being about 140 m. The landslide volume is
about 20 millionm3. The slope of zone 1 ranges from
22∘ in the upper part to 40∘ in the lower
part. The slope of zone 2 ranges from 17 to
35∘. Houses are built at elevations of
670–702 m. Figure 3 shows the local village and rice
paddies at an elevation of 692 m.
The landslide is located in a sub-humid subtropical climate zone
with a mean annual precipitation of 1280.5 mm. Most of
precipitation occurs in the months from April to August with
a maxmum daily precipitation up to 133.3 mm. The lowest
water level of the Sanbanxi Reservoir is about 425 m,
while the highest water level of the reservoir after inundation is
472 m. Groundwater in slope is pore phreatic in
Quaternary and bedrock fissure water. The precipitation recharges
groundwater within the landslide. There are two drainage paths:
(1) through fracture network downwards into the underlying
bedrock, (2) discharge into the gully directly.
Landslide geology
The stratum outcropped in this region are Banxi group of Algonkian
system and Quaternary. The lithology at the landslide is
illustrated in three profiles presented in Fig. 5 along the
sections marked by I-I′, II-II′, and III-III′ in
Fig. 4. Stratigraphic geo-materials differ in compositions and
characteristics as described in detail below.
Member 1 of Qingshui formation of Banxi Group of Algonkian
system (Ptbq1): mainly consisting of blastobedding
tuffaceous sandstone, tuffaceous silty slate and blastobedding tuff.
Quaternary, including eluvium and diluvium
(Q4edl), accumulated layer landslide
(Q4del) and human accumulation layer (Q4s)
Eluvium and diluvium (Q4edl) is comprised of silt clay,
with mixture of broken stones, it is found in the slope mostly behind
the village and on both sides of the ridge with thickness of
1–5 m.
Accumulated layer landslide (Q4del) is composed of four
types of geological materials. It is 18–96 m thick. The
stratigraphic physical and mechanical properties are given in Table 1
(provided by HydroChina Zhongnan Engineering).
Coarse-grained silty clay with mixture of strongly
weathered broken stones are distributed on the surface of the
landslide except at the lower part of zone 1 and east of zone 2. The
content rate of rock is 20–40 %. The thickness is generally
5–12 m with some places at the upper part of the deposit over
20 m thick.
Strongly weathered broken stones and gravel with mixture
of silty clay was observed in the middle of the landslide, and covered
with layer i. The content rate of rock in this strata is high. The
thickness of this strata varies greatly, from 6.3–12 m in the
west and north to 51.2–79.1 m in the central and southern
parts of the landslide.
Cataclastic rock mass comprised of blastobedding
tuffaceous sandstone, tuffaceous silty slate and blastobedding tuff,
is distributed mainly at the lower part of the landslide. This strata
has moderate or strong weathering, with good rock mass integrity. The
thickness is over 120 m.
The sliding zone mainly consists of yellow brown and dust
color silty clay with thickness of 1.3–11.3 m, a small amount
of gravel distributed in the front.
Human accumulation layer (Q4s) is gravel, about 5 m
thick, laid during the highway excavations.
Deformation features and stabilization measuresDeformation features
The first impoundment of the Sanbanxi reservoir began in
January 2006. At the end of July 2007 the reservoir level reached
472, 3 m below the normal storage level. Field
investigations showed that cracks appeared at the concrete
foundation of house (Fig. 6) and the west boundary of zone 1 at
elevations of 665–700 m where the head of gully at the
end of July 2007. The deformation tendency of the surface of
landslide aggravated in 2008. This newly developed deformation is
described below.
In the eastern gully, where the landslide is covered by quaternary overburden, a few cracks 1–5 cm wide and about 2 m long were observed along N 10∘ W–N 20∘ E.
In the rice paddies at elevation 692 m, along N 10∘ W–N 5∘ E, a tension crack was observed; it was 15–20 cm wide, more than 2 m long, and 10–30 cm in visible depth (Fig. 6).
The landslide experienced prolonged periods of intense rainfall, muddy water was observed coming out of the toe of zone 1 (Fig. 7).
The field investigation suggested that there are two sliding surface
in the slope (Fig. 5). One is shallow located within the lower
accumulated layer landslide, which has a rotational slide mode. The
other is deep through the sliding zone. It is a translational slide,
and the slope may appear bedding landslide. The deformation
characteristics show: the landslide belongs to thrust load caused
landslide, the landform and material buildup is geological condition
of landslide, abundant rainfall and reservoir fluctuation are
primary induction factors of landslide.
Stabilization measures
Due to the narrow valley at the landslide location, the supports
and protections measure is difficult to be used. Many researches
(Fleming et al., 1989; Iverson et al., 1997) suggested that, the
increase of pore water pressure caused the decrease of shear
strength, therefore, drainage is only stabilization measure which
can be used.
Figure 8 shows a drainage tunnel at 500 m elevation,
excavated through the bottom of the landslide. To reduce the
saturation level at the sliding zone and increase landslide
stability, drainage holes were drilled in the tunnel for draining
the groundwater around the sliding zone, and the bedrock fissure
water. The drainage tunnel construction began in
March 2010. Figure 9a shows the drainage tunnel under construction,
and the water draining out of the tunnel wall can be seen in
Fig. 9b. The main tunnel was completed in May 2011.
Monitoring system
A large number of monitoring instruments were installed to enable
real-time recording of the slope deformation and analysis of the
deformation tendencies. Two monitoring methods – geodetic
detection and borehole monitoring – were used to establish the
vectors of the dome displacements and to measure the groundwater
level of the slope, respectively.
For geodetic detection, three horizontal displacement datum points
and three vertical displacement datum points made of concrete were
constructed, as well as nine observation points at the surface of
the slope (Fig. 8). In October 2009, observation data of the slope
deformation were obtained. Additionally, eight boreholes were
drilled for measuring groundwater level.
Monitoring results and analysisGroundwater
The data of groundwater level from the eight boreholes and the
reservoir, rainfall data are plotted in Fig. 10. Looking at the
groundwater level of boreholes ZK11 and ZK14 located in the lower
part of landslide, four fluctuations can be identified.
From November 2009 to February 2010 the groundwater level at ZK11 and ZK14 dropped from 452.32 and 475.83 to 428.19 and 455.70 m, respectively. In this period, rainfall was low and the reservoir drainage system had been operating for three months; hence, these decreases in groundwater level are related to reservoir drainage.
From May 2010 to July 2010, the groundwater level at ZK11 and ZK14 rose by 36.17 and 8.89 m, respectively, because of increased reservoir water level. This was a period of heavy rainfall in the region, which also affected the groundwater level of boreholes ZK11 and ZK14.
From December 2010 to March 2011, the groundwater level at ZK11 and ZK14 dropped from 466.14 and 470.37 to 434.67 and 455.23 m, respectively. The decreases in groundwater during this period were induced by reservoir drainage.
From April 2011 to July 2011, the groundwater level at ZK11 and ZK14 increased from 428.52 and 453.48 to 454.57 and 461.08 m, respectively. The influence of the heavy rainfall on the groundwater level is more noticeable, because although the water level of the reservoir decreased during this period, the groundwater level increased.
At the upper part of the landslide (borehole ZK2; Fig. 10), three
distinct phases can be identified in the groundwater level curve of
KZ2, with May 2010 and May 2011 being the demarcation points.
Before May 2010, the groundwater level at ZK2 gradually dropped
from 645.49 to 617.91 m. In this period, low rainfall levels
led to a decrease in the groundwater level.
From May 2010 to May 2011, the groundwater level at ZK2
increased from 617.91 to 647.88 m following heavy rainfall,
and then fell to 629.09 m.
Since May 2010, the groundwater level at ZK2 increased from 629.09 to 652.21 m because of heavy rainfall, and then fell to 638.40 m.
Overall, as shown in Fig. 10, changes in the reservoir levels
mainly affected the groundwater level at the lower part of the
landslide, showing s positive correlation between the two levels.
The permeability of the lower part of the landslide is good. In the
upper part of the landslide, the groundwater accumulates because of
rainfall; therefore, the permeability of the upper part is poor.
Deformation and triggering events
Figure 11 shows the monitoring results of displacement over the
2 year recording period between October 2009 and
September 2011. The rainfall near the landslide and the reservoir
water level are also shown. Taking, for example, the deformation
trend of the landside, three stages can be identified.
From October 2009 to May 2010 the reservoir water level dropped
from 466 to 426 m. The displacements at DLXG02, DLXG03,
DLXG05, and DLXG06 (Fig. 11) increased rapidly because of reservoir
drainage. The biggest deformation occurred in the lower part of the
landslide, after January 2010. There is no significant change in the
reservoir water level during this time and the displacements at
these four observational points decrease slowly.
From May 2010 to March 2011 the reservoir water level rose from
426 to 466 m at first and then dropped from 470 to
428 m. There were many fluctuations in the reservoir water
level during this period and great increases in the displacements of
DLXG02, DLXG03, DLXG05, and DLXG06 from May 2010 to July 2010. The
biggest displacement increment was about 12.08 mm. From
July 2010 to September 2010 the reservoir water level gradually
dropped by 6 m, but the deformation of these four
observational points decreased by 5.77, 4.10, 2.17, and
4.22 mm, respectively. There were two increases in the
displacements of DLXG01, DLXG08, and DLXG09 located in the upper
part of the landslide; one occurred after the rainfall events in
June 2010 and the other occurred after the rainfall events in
September 2010.
After March 2011 the displacements of DLXG02, DLXG03, DLXG05, and DLXG06, induced by fluctuations of the reservoir water, increased; the displacements of DLXG01, DLXG08, and DLXG09 increased sharply after rainfall events.
Before this period, the displacement of DLXG04 increased slowly;
after April 2011, once the main drainage tunnel was completed, the
displacement of DLXG04 decreased by 7.32 mm in May 2011 and
rapidly increased after May 2011.
Movement tendency
With geological investigation and monitored data, it is found that
movement of the Donglingxin landslide can be divided into two
phases:
Creep deformation: before April 2010, as shown in Fig. 11. The rate of this creep movement of landslide was increased slowly and gradually with a movement rate of 0.72–2.12 mmmonth-1.
Instantaneous deformation: roughly since April 2010. The execution of work and inundation were the two principal triggers which activated the landslide, furthermore, rainfall induced the shallow rotational slide.
Stability analysis
The movement of the Donglingxin landslide can be attributed to the
strength drop of the sliding zone caused by the increase of pore
water pressure, the shear strength of the sliding zone,
characterized by the cohesion c and the friction angle φ
are given in Table 1 (provided by HydroChina Zhongnan
Engineering). The shear strength decreased from 23 kPa and
29.4∘ for the natural state to 20 kPa and
28∘ for saturated state.
Three representative engineering profiles: preflies I-I′, II-II′,
and III-III′ (Fig. 5) were selected for stability analysis by
modified Janbu method under three different conditions: a natural
state, a rainfall and an inundation with rainfall. The safety
factors of two sliding surfaces were calculated, as given in
Table 2.
From Table 2, it is interesting to find that, rainfall had a great
negative influence on the stability of the shallow sliding
surface, but a minor influence on the stability of the deep
sliding surface. Inundation was the opposite, had a great
negative influence on the stability of the deep sliding surface.
Disaster prediction
The consequence of numerical simulation by YADE (Fig. 12) shows
that once the Donglingxin landslide fails, stones and soil with the
total debris volume reaching several millions and waves surging to
a height of about 34 m. the upriver water level will be
rised to 478–490 m by river blocking up in the Qingshui
river, the affected area is about 23 km
range. Consequently, the Liuchuan town which is 1.5 km away
from the landslide with 22 434 inhabitants will be destoryed by
surge. 14 km away from the landslide, a small hydropower
station with 486 m in elevation will be submerged. The
Jianhe county town with 44 057 inhabitants and 476.6 m in
elevation is 20 km away from the landslide, it will also be
submerged.
Conclusions
Based on monitoring measurements over a period of about
2 years and stability assessment we conclude the
following.
The geological investigation indicated that the Donglingxin landslide is a large-scale ancient landslide, which is an accumulation of several consequent slides along the bank slope, the slope may appear bedding landslide, and the deformation characteristics shows that the landslide belongs to thrust load caused landslide.
Because the accumulated layer landslide is rich in broken stone and gravel, it has good permeability, the rainfall water and reservoir water are prone to permeate through this stratum into the depth of the slope. Thus, they have effect on the strength reduction of the landslide.
Due to the narrow valley at the landslide location, the supports and protections measure is difficult to be used, but the movement near the drainage tunnel may be more active after a few months of drainage.
Movement of the lower part of the Donglingxin landslide corresponds to water level changes in the Sanbanxi reservoir. The changes are more evident when the reservoir water level rises. Movement of the upper part of Donglingxin landslide corresponds to rainfall events. The movement is greater during and after the wet season.
The groundwater level at the lower part of the Donglingxin landslide correlates well with the reservoir water level. The groundwater at the upper part of the Donglingxin landslide corresponds to rainfall events.
Stability assessment indicate that the rainfall had a great negative influence on the stability of the shallow sliding surface, but a minor influence on the stability of the deep sliding surface. The inundation was the opposite, had a great negative influence on the stability of the deep sliding surface.
Once the Donglingxin landslide occurrs, the affected area which will be submerged is about 23 km range.
Acknowledgements
This work is financially supported by National Key Technology
Research and Development Program (Grant No. 2013BAB06B01), and the
National Natural Science Foundation of China (Grant no. 51209075,
51479049,51409082), and the Fundamental Research Funds for the
Central Universities (Grant no. 2014B17714).
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Stratigraphic physical and mechanical
parameters.
Triaxial shear strength Strata c (kPa) φ (∘) Density (gcm-3) NaturalSaturatedNaturalSaturatedNaturalSaturatedQ4deli. Coarse-grained silty clay with mixture of broken stones201826242.02.1ii. Broken stones and gravel with mixture of silty clay201535302.152.22iv. Sliding zone232029.4282.032.08
Results of stability analysis with modified
Janbu method.
ProfileSliding surfaceNaturalrainfallInundation with rainfallI-I′shallow1.1331.0851.014deep1.1551.1051.023II-II′shallow1.1401.0770.995deep1.1701.1321.035III-III′shallow1.1721.1271.028deep1.2091.1181.045
Location map of the Sanbanxi dam and the
Donglingxin landslide, China.
The Donglingxin landslide facing the Qingshui
River (taken form Google earth).
Photo of local village and farmland.
Topography and monitoring locations of the
Donglingxin landslide.
Geological profile of the landslide deposit.
Photo of cracks in farmland at elevation 692 m.
The phenomenon of muddy water coming from the
landslide.
Location of the observation points in the
Donglingxin landslide.
Construction of drainage tunnel EL 500.
Monitoring results of groundwater levels at
boreholes ZK1–ZK5, ZK11,
ZK13, and ZK14 for the period October 2009 to September 2011 in
the Donglingxin landslide. Rainfall for this
period and water level in the Sanbanxi reservoir are also
shown.
Monitoring results of displacements for the
period from October 2009 to September 2011 in the Donglingxin
landslide. Rainfall for this period and the water level
in the Sanbanxi Reservoir are also shown.