10th Asian Regional Conference of IAEG (2015)

Patterns and Mechanism of Rockslides Induced by Rainfall in China
Qiang XU(1), Hanxiang LIU(1) and Xiujun DONG(1)(1) State Key Laboratory of Geohazards Prevention and Environment Protection, Chengdu University of Technology,
Chengdu, China
E-mail: xuqiang_68@126.com
Abstract
Since 1980s, in southwestern China, especially in red-strata regions of Sichuan Basin,tens of thousands of landslides have been triggered by heavy rainfall, which include several
large-scale deep-seated rockslides. For this kind of rockslides, based on field investigation,
laboratory tests, physical model tests and numerical calculation, conclusions can be drawn in
two aspects:(1) the movement of deep-seated rockslides triggered by heavy rainfall can be grouped
into two patterns, namely the single plane slide and the wedge block slide. For the former
pattern, a landslide occurring in nearly horizontal rock strata is called translational rockslide,
while that occurring along bedding plane with dip angle larger than 10º is called bedding
rockslide. For the latter pattern, the symmetrical and asymmetrical wedge block rockslides
are proposed based on the spatial configuration of two lateral sliding surfaces.
(2) the hydrostatic pressure in the rear edge and the uplift pressure on the bottom
sliding surface both contribute to the occurrence of a rockslide. For the initiation of atranslational rockslide, a critical water head in vertical cracks in the rear edge is established.
The development of water pressure on the bottom sliding surface is time consuming, and the
distribution mode along cracks also changes with time from triangle shape to trapezoid shape.
Just because of this, the occurrence of a rockslide is always observed with lagging behind therainfall event. This phenomenon is called hysteresis (or time delay). The present study is
expected to provide basic work for the establishment of rainfall prediction model and theearly warning of the deep-seated rockslides.
Keywords: rockslide, rainfall, slide pattern, formation mechanism
began in the Poyang Lake. After June, however, therapid altering from drought into consecutive rainstorms
not only triggered flood hazards in thirteen provinces,like Jiangxi, Hunan, Guizhou, Fujian and etc., but also
triggered a number of landslides and debris flows
occurring in groups. In Sichuan basin of western China,
a special rock stratum is widely developed and is called
red stratum due to the main apparent color of rocks (e.g.
sandstone, mudstone). The rocks are extremely
sensitive to water, and always cause severe landslide
hazards occurring in groups in rainy season, including
some well-recorded landslides like the Tiantai village
landslide (Xu et al., 2006), the Kualiangzi landslide(Fan et al., 2009) and the Ermanshan landslide (Xu et al,2012).
A wide scientific literatures reveal that short andintense rainfalls preferentially trigger soil slips, debris

1. Introduction
Heavy rainfalls always cause geological disasters
such as landslides and debris flows around the world,
either in the tropic region or the arid region, and rainfall
has become the most common triggering factor oflandslides, other than earthquakes, volcanic eruptions,human activities and etc. (Keefer et al., 1987; Crosta,
1998; Wang et al., 2002). In recent years, the weather
disorder due to global climate change is bringing about
more and more extreme weather phenomena, which
accelerates the occurrence of geological disasters. In
China, one of the extreme phenomena is the rapid
altering of flood and drought. For example, in 2011,
before June, an extream drought not seen for sixty years
hit the middle and lower reaches of the Yangtze River,causing the minimum water area since satellite records

110th Asian Regional Conference of IAEG (2015)

cliff(a) the colse - view of the Xingma Middle School landslide(b) crack in the rear edge(c) concrete rupture at thetoe
Fig.1 The close-view of the Xingma Middle School landslide and the local deformation features.

flows and shallow landslides, while prolonged and lessintense rainfalls preferentially trigger deep-seatedlandslides (Zêzere et al., 1999; Guzzetti et al., 2004). It
should be noted that, compared to shallow landslides,
the deep-seated landslides may be much less in number
but responsible for the largest economic damge in a
rainfall event. In spite of its importance, researches on
deep-seated landslides caused by rainfall are still
limited (Polemio and Sdao, 1999; Chigira, 2009; Fan etal., 2009; Tsou et al., 2011), and the specific patterns
and the corresponding mechanisms of the deep-seatedlandslides are yet to be clearly understood.
Based on the long-term researches of rainfallinduced landslides in China, especially in Sichuan basin,
where the geological and geomorphological conditions
and rainfall characteristics are favorable for deep-seatedrockslides, the purpose of this present paper is to show
the typical patterns of this kind of rockslides and thegenetic mechanisms. The research work involves
multiple methods, including field investigation,
laboratory tests, physical modeling tests and numericalcalculation. As the ultimate goal, the early signs of
sliding are expected to be identified and then the early
warning can be implemented for the deep-seatedrockslides.

Other than the well-reported landslides, another two
rockslides are herein introduced as case studies for thetwo failure patterns, and then the basic features of eachfailure pattern will be summarized.

length of 140 m, which makes a volume of 4.8×104 m3.
The XMS landslide is restricted in the vicinty of acliff. The slope angle is 65º~85º. The exposed strata of
the cliff are the huge thickness of sandstones belonging
to the Jianmenguan Formation of the middle
Cretaceous System (K2j), while a layer of silty
mudstone is located in the lower part of the cliff. The
strata have a a dip angle of only about 10º. Besides, a
set of nearly vertical structural planes are developed in
the slope with their strikes parallel to the free surface ofthe cliff. They are long enough and, together with thebottom bedding planes, they cut the slope into several
thin plates.
From the point of rock mechanics, although theslope has been disintegrated, the nearly horizontal rockstrata will still keep the slope at a stable state. In fact,many signs of deformation indicate that the occurrenceof the XMS landslide is much correlated to the rainfall.
For example, Fig.1(b) shows the tension crack in therear edge of the landslide, which was formed after theslope moved forwards 2 meters during the heavy
rainfall on July 3~5 of 2007. The opening crack is 1~3
m long, 140 m wide, 17.5m deep, and becomesnarrower from west to east. As the slope moved
forwards, the pushing force was produced in the toe
area of the landslide, and caused concrete rupture on
the playground of the Xingma Middle School (Fig.1(c)).
Another response to the landslide pushing is thedisappearance of a 40-cm-wide drainage channel which
was just next to the cliff. During the landslide process,the silty mudstone at the bottom was locally shattered,
which made the sliding body sit down 0.5 m.

2.1 Xingma Middle School Landslide, Sichuan
In 2007, a landslide occurred in the hillslope behind
the Xingma Middle School, Xingma village, Nanjiang
county, Sichuan province. The Xingma Middle Schoollandslide is hereafter referred to as the XMS landslide.
As shown in Fig.1, the landslide has a rectangle-likeplan and the longitudinal length (parallel to the sliding
direction) of only 15m is far smaller than the transverse

2.2 Chengkou Landslide, Chongqing
On the early morning of July 19th, 2010, a rockslide
was caused by heavy rainfall in the Miaoba town,
Chengkou county, Chongqing city. The landslide
moved fast, blocked the road and the river at its toe,and formed a barrier lake with the storage reaching 6
million cube meters (Fig.2). As a result, the water level
before the dam rised up quickly within half an hour and2. Patterns of rainfall-induced rockslides2

10th Asian Regional Conference of IAEG (2015)

the Miaoba town in the upstream area (500 m far fromthe landslide) was submerged, causing about ten
thousand inhabitants in emergency evacuation.
According to field survey, the rock strata in thelandslide area are mainly composed of shale interbeded
by sandstone which belong to the Reping Fomation of
the middle Silurian System (S2l), and the superfacialcovering is composed of the Quaternary Holocene
residuals (Q4dl+el). Because the landslide area is justlocated within the axial part of a syncline, the dip
direction and dip angle of rock structural planes (1# and2# planes in Fig.3) in the syncline’s two limbs aredifferent. Accordingly, together with another one set ofsteep cracks in the rear edge of the landslide, the sliding
body finally presents a wedge block having twosymmetric flanks, as shown in Figs.2 and 3.
A spring spot can be found at the crown of thelandslide (Fig.3), and after the hazard took place, theflow quantity from the spot was measured to be 0.8L/s.
A small trough also exists at the back of the landslide,which provides a natural channel for surface run-off
into the slope. Beginning on July 16th of 2010, heavy
rainfall continued in Chengkou town, and the total
precipitation on July 19th when the landslide occurredreached about 200 mm. The spring water and thesurface runoff due to rainfall entered into the trough,and then into the interior slope. This process produced
water pressure around the boundaries of the sliding
body and reduced the effective stress of soils, which isbelieved as the main cause of the landslide.

1# plane:
50 º∠45 º

2# plane:
232 º∠36 º

Fig. 3 The V-shape trough and the spring water
exposed in the rear edge of the Chengkou landslide
It can be seen that, the single plane slide is classified
into two subtypes of movements. For α < 10º, mostly
ranging between 3º-5º, a landslide occurs in nearlyhorizontal rock stratum and is called translational
rockslide, while for α > 10º, a landslide occurs along
the bedding planes of rocks and is called the bedding
rockslide. In the latter subtype, if α is still smaller than
20º, the slope will keep stable under natural condition,
but fails in heavy rainfall along the bedding plane. This
type of rockslide is commonly seen in red-strata regionsof Sichuan Basin. If α exceeds 20º, the steep rock strata
is difficult to keep stable under natural condition, letalone subjected to other disturbances such likeexcavation or rainfall. Comparing the two subtypes of
movements, the translational rockslide has attracted
many attentions since long due to its unusual
occurrence conditions and genetic mechanisms (Zhang
et al., 1994). As an important part of this study, thiswill be stressed in the following section.

Luojiang River

3. Genetic Mechanism of rainfall-induced rockslides
It is widely accepted that rainfall mainly produces
two effects on the occurrence of a landslide (Chen etal.,2006; Tu et al., 2009). The first effect, namely themechanical effect, is caused by increase of waterpressure as the rainwater infiltrates into a slope. During
the same process, the second effect, namely physicaleffect, is caused by the softening of slope materials. Fora rock slope, water is limited to flow through cracks
because of the impermeability of rocks. Due to space
limit, only the mechanical effect is analyzed based onfield monitoring and laboratory tests, and the emphasis
will be put on the development of water pressure incracks.

Fig. 2 The close-view of the Chengkou landslide
2.3 Typical patterns of rainfall-induced rockslides
Based on descriptions of the above two rockslides,
the basic features of the single plane slide and thewedge block slide, two typical patterns caused byrainfall, are acquired. More detailed classification has
been made for each failure pattern according to the dip
angle (α) of rock stratum, as presented in Table 1.

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10th Asian Regional Conference of IAEG (2015)

Table 1(1) Typical patterns of deep-seated rockslides triggered by heavy rainfall(First type of failure pattern)
Subtype

DiagramMain features and typical cases

First type of failure pattern: single plane rockslide(a) Plate-shaped rockslide

Subtype Ⅰ:
translational
rockslides(α < 10º)(b) One-step rockslide(c) Multi-step rockslide

Subtype Ⅱ：
beddingrockslides(α > 10º)(a) along the gentle bedding
plane (10º <α < 20º)(b) along the steep bedding
planes ( α > 20º)

This type of rockslide has a rectangular slip plane, and thelongitudinal length in sliding direction is far smaller than thetransverse length. It usually occurs in rock strata with dip angle
lower than 10º, and a set of steep cracks develop parallel to the freesurface, which cut the slope into thin plates. During heavy rainfall,
the tension cracks are filled with water and water pressures are
rapidly increased to a critical value for sliding. After that, the waterpressures decrease and the slide stops. The rockslide often occurs in
Sichuan basin composed of red-strata. The typical cases include the2007 Xingma Middle School landslide and the Dahe Middle Schoollandslide in Nanjiang county, Sichuan province (Xu et al., 2010).
This type of rockslide usually occurs in a gentle slope with agradient lower than 15º and a dip angle of rock stratum lower than
10º. A nearly vertical crack develop in the rear edge. During heavy
rainfall, the initiation and cease conditions for the slide is the samewith type (a). After sliding, a long and large tension trough wasformed where the vertical crack develops, and spring spots or waterflows occur at the toe, but the sliding body still looks intact without
disintegration. The rockslide also often occurs in Sichuan basincomposed of red-strata. The typical cases include the Kualiangzi
landslide in Zhongjiang county, Sichuan province (Fan et al., 2009).
This type of rockslide has the same geomorphological andgeological conditions with type (b). The difference is the slope iscut by several sets of nearly vertical cracks. During heavy rainfall,
1# block is first pushed out by water pressure in crack, which makes
the blocks at its back lose support and slide out successively fromthe front to the back and from the center to the sides. After sliding,several tension troughs will occur in the slope. The typical cases
include the 2004 Tiantai village landslide in Xuanhan county,
Sichuan province (Xu et al., 2006).
A bedding rockslide occurs along the bedding planes of rocks.
When the dip angle (α) is smaller than 20º, the slope will be kept
stable under natural condition, but fails in heavy rainfall along the
bedding plane. This type of rockslide is commonly seen in redstrata regions of Sichuan Basin. The typical cases include the 2010
Niumachang landslide in Fengyi village and the 2011 Shibangou
landslide in Jiangying village, Nanjiang county, Sichuan province(Hu, 2013).
When the dip angle (α) exceeds 20º, especially if the slope contains
weak interlayers, the steep rock strata is hardly kept stable undernatural condition, let alone subjected to other disturbances such likeexcavation or rainfall.

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10th Asian Regional Conference of IAEG (2015)

Table 1(2) Typical patterns of deep-seated rockslides triggered by heavy rainfall(Second type of failure pattern)
Second type of failure pattern: wedge block rockslide
Differing from a bedding rockslide in which the sliding surfaces arecontrolled both by bedding plane and rock structure, the sliding
surfaces of this type of rockslide are two sets of rock structuralplanes which have the same dip angle but totally reverse dip
direction. Both of them intersect with the slope surface. In special
cases, like the 2010 Chengkou landslide of Chongqing, the sliding
surfaces just develop in two limbs of a syncline, and form a Vshaped spatial configuration. Another example is the 2010 Jinping
Power Station landslide in Kangding county, Sichuan province (Sunet al., 2013).

Subtype Ⅰ:
symmetric
wedge blockrockslides

Subtype Ⅱ:
asymmetricwedge blockrockslides(a) Ⅱ-1(b) Ⅱ-2

This type of rockslide has the same slope structure with thesymmetric type, but the two sliding surfaces have different dip
angles. The slide is controlled by the gentler surface, but will be
restricted by the steep surface during movement. That is to say, thesteep surface acts not only as a sliding surface, but also as a lateral
cutting surface. The typical case is the Shiaolin landslide in Taiwan(Tsou et al., 2011).
This type of rockslide often occurs in the prominent part of a valley
slope. A set of gentle structural planes (mostly bedding planes) dip
outwards, while another set of steep planes have strikes parallel tofree surface. They together cut the slope into a sliding body. During
heavy rainfall, the run-off infiltrates along the steep planes into thebottom of sliding surface, which causes soil softening and increasein water pressure. Due to the small contact area in sliding direction,
once water infiltrates, the rockslide can easily occur. The typicalcase is the 2010 Ermanshan landslide in Sichuan province (Xu etal., 2012) and the 2010 Guanling rockslide in Guizhou province(Yin et al., 2010).

As mentioned in the previous section, thetranslational landslide is controlled by one set orseveral sets of nearly vertical cracks and a nearlyhorizontal bedding plane (dip angle < 10 º ) which
afterwards forms the sliding surface. During rainfall,water infiltrates along those opening structures andgenerates hydrostatic water pressure on the verticalcracks and uplift pressure on the potential sliding
surface. Fig.4 gives the concept model of one-steptranslational landslide as well as the water pressure
distribution in cracks. Based on a limit equilibrium
method, Zhang et al. (1994) proposed a critical height(hcr) of water head in the vertical crack as the initiation
criterion for one-step translation landslide and thenbuilt the estimation equation of hcr as follows:(1)
where hcr is the critical water head value; W is theweight of a one-meter wide slide block (t/m); α is the
dip angle of the sliding surface (a downslope dip gives
a positive value, an upslope dip gives a negative value),α < 10º; L is the length of the slide block; φ is thefriction angle of the sliding surface; γw = the bulk
density of water.

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10th Asian Regional Conference of IAEG (2015)

Fig.4 The concept model of translational landslide
and the water pressure distribution

H1=136cm

According to them, when the water head in thevertical crack exceeds hcr, a translational landslide will
occur. This opinion and the equation are further
verified by Fan et al. (2009) who performed physicalmodel test and numerical calculation for a one-step anda multi-step translational landslide in Sichuan basin.
With this opinion, once the sliding body moves, thewater in cracks will rapidly dissipate. As a result, thehydrostatic pressure and uplift pressure are both
reduced, and the movement immediately stops as thewater level drops to lower than hcr. Actually, in the field,
not large displacement can be observed for atranslatonal rockslide, and even the maximum value isonly from tens of meters to several meters. The above
mechanism is simultaneously applicable to the plateshaped translational landslide. More detailed analysis
can be found in Xu et al. (2006) and Fan et al. (2009).
A serials of physical model tests have been
conducted to study the developing process of waterpressure and its correlationship with time and waterhead difference between cracks. The tests were basedon the wedge block slide. As shown in Fig.5, the test
system consists of model container, water pouring
device and pressure monitoring device. The wedge
block is designed to be symmetric and waterproof. The
water pouring device is composed of water pouringholes and water box. Three pouring holes are designed
at the back of container with gound-based height (H1 in
Fig.5) of 71cm, 101cm and 131cm.

H2=131cm

Each pouring height corrresponds to various waterlevels in the water box, which models different levelsof water supply in the rear edge of the block. Besides
those, Six piezometric tubes are installed along thesliding direction of each symmetric plane with different
ground-based height (H2 in Fig.5) of 65.2cm, 59.3cm,
53.3cm, 47.0cm, 41.4cm and 35.5cm. With the design,when the water head at the rear edge changes, the waterpressure along the two symmetric planes can beobtained.

wa

ter
tab
le

groundH2=131cm

H1=136cm(a) 1minute after

wat
er

tab
le

groundH2=131cm

H1=136cm(b) 10 minutes after

water

table

groundH2=131cm

H1=136cm(c) 20 minutes after

water

table

ground(d) 40 minutes afterFig. 6 Development of water pressure in cracks of
the wedge block slide
Herein, due to space limit, only the water pressure
development with time will be displayed when thewater pouring height is 131cm and the water level inwater box is 136cm. From Fig.6, it can be seen that, theFig.5 Physical model test of the wedge block landslide

610th Asian Regional Conference of IAEG (2015)

version of the manuscript are thanked sincerely.

water on the bottom plane is gradually flowing
forwards and the water pressure is also increasing.
Before 20 minutes has passed, the water pressure
demonstrates a triangle distribution mode, but after 20
minutes, it keeps a trapezoid mode. The process
indicates the development of water pressure on cracks
is a time consuming process. For a deep-seatedrockslide, just because it costs time to develop the upliftpressure on the bottom sliding surface, it always occurs
behind the rainfall event, and this phenomenon is called
the hysteresis effect.

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4. Conclusions
In this study, the typical patterns are firstsummarized based on tens of deep-seated rockslides
caused by rainfall in China. Then the genetic
mechanisms of those rockslides are analyzed mainly
based on field investigation, physical model tests andlaboratory tests. The results can be concluded asfollows:(1) According to the spatial configuration of sliding
surfaces, the rockslides can be classified into twopatterns of movements, the single plane slide and thewedge block slide. For each pattern, deeper
classification is made. For example, the single planeslide can be further divided into the translational
rockslide and bedding rockslides. The wedge blockslide can be further divided into the symmetric andasymmetric block rockslides. The basic features of eachpattern of movement are described including the slope
structures and rainfall conditions.
(2) for a translational landslide, it occurs once thewater head in the vertical crack in the rear edge reaches
a critical value (hcr) and stops once the water headdecreases bellow hcr. Hence, in the field, large
displacement can not be observed for a translational
rockslide.
(3) The physical model test of the wedge block slide
shows the development of water pressure around thesliding body is responsible for the occurrence of arockslide in a rainfall event. As time goes, the waterflows forwards and the uplift pressure along the bottom
sliding surface gradually increases with the distribution
mode changing from triangle to trapezoid.
Acknowledgments
This research is financially supported by the
Science and Technology Support Program of Sichuan,
China on "Formation mechanism of rainfall triggeredlandslides occurring in groups in gentle strata of theeast Sichuan & demonstration of landslides' monitoringand early warning work" (Grant Number 2012SZ0066).
All reviewers for their valuable comments on the early710th Asian Regional Conference of IAEG (2015)

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