10th Asian Regional Conference of IAEG (2015)

Study on the deformation characteristics and dynamic mechanism of seismic fractures in
Wenchuan earthquake shattered slopes

Jin Ke YUAN, Run Qiu HUANG and Xiang jun PEIState Key Laboratory of Geohazard Prevention and Geoenvironment Protection,
Chengdu University of Technology, Chengdu 610059, Sichuan
E-mail:yuanjingke@163.com
Abstract
Numerous shattered slopes triggered by wenchuan earthquake, the shattering area
were large and the seismic fractures were wide distribution, mostly threatened thepost-hazard reconstruction. According to the characteristics of seismic fractures in shatteredslopes, the research results show: (1)The seismic fractures was divided into six types:
surface type, transfixing type, strike type, tendency type, closure type, extension type fromthrough survey the morphological characteristics and deformation indications; (2)On thewhole the Seismic fractures was N30º～60ºE trending, which accords with the whole strikeof active fault in wenchuan earthquake; (3)mainly distributed in seismogenic faults area orridge、and steep with slow border area; (4)well extension, high open degree, upper wide andlower narrow of profile morphology, fillings less or no, mainly mixed stone fragments, withaerial phenomena. Based on the theory of fracture mechanics, the mechanism of seismic
fractures was belong to tensile crack, the deformation and failure process of structuralplanes under dynamic action was divided into rupture, expansion and arrestability threestages.

Keywords: wenchuan earthquake, shattered rockmass, seismic fracture, geohazards

1.

taken place frequently. Study these seismic fractures
has important significance to recognize the deformation and failure, formation mechanisms of shattered
rockmass correctly.

Introduction

Tens of thousands of collapse, landslides andsecondary geological disasters triggered by
Wenchuan Earthquake. Earthquake induced a large
number of slope geological disasters, also caused apotential disaster—Slope shattered deformable bodywhich namely shattered rockmass. The shattered
rockmass as a geological phenomenon, it was mainly
affected by the earthquake force under the certain
geological conditions (Liang et al. 2003; Feng et al.
2009; Liang et al. 2009). The dynamic effect ofseismic wave in rockmass destroyed the rockmass
structure, its whole structure varied (Shao 2005; Zuo
et al. 2008). The most typical feature of slope
shatter-destroyed is that the seismic fracturesdeveloped
The deformation, instability of shattered
rockmass has controlled by seismic fractures, it is apotential disaster. The shattered rockmass has no
macro destruction features such as large deformation,
large displacement as other destruction forms, it was
an inherent rockmass damage, also it was the maincause of secondary disasters after the earthquake

2.

Seismic fracture classifications
and failure characteristics

The rockmass structure were cracked open byearthquake force, so the seismic fracture formed. The
classification of seismic fracture should corresponds
to its formation mechanisms, now the classification
research to fractures mainly based on the visual,
intuitive shape of survey(Xie 1988). The
classification analysis to fractures mainly based on its
distribution, development characteristics, deformation features, and make it possible to predict its
deformation and destruction. Based on thisconsideration, the seismic fractures was divided into
six types: surface type, transfixing type, strike type,
tendency type, closure typ, extension type from themorphological characteristics and deformation
features.

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10th Asian Regional Conference of IAEG (2015)
groups of seismic fractures whose extension direction
were different, cross each other, the distribution
shape on the plane was "X" or "Y"shaped(Fig 3).

2.1 Surface type seismic fracture features andfailure characteristics
Surface type seismic fracture was refer to the
rockmass surface shattered longer length from its
characteristics, but its width and visible depth isgenerally small. Small scale of such fractures weredeveloped in all kinds of slope structure, and there isno obvious directional distribution, it distributed in
the rockmass surface, and had poor deep
interconnectivity (Fig 1).

(Ⅰ) “X” shaped seismic fracture (Ⅱ) “Y” shaped seismic fracture

Fig. 3 Crossover type seismic fracture(Ⅰ)

Surface seismic fracture were exist in the
rockmass surface, they can't control the deformation、
instability of rockmass. However, this type ofseismic fractures distributed generally, higher density
developed in local position, thereby it destroyed theoverall structure of rock mass. And it easy to becomea good way to rainwater infiltration when experiencerainfall conditions.

(Ⅱ)

Fig. 1 Surface seismic fracture

In accordance with the extension characteristics
of surface type seismic fracture can be divided into
broken line type, Linear type. From its combination
features can be divided into en-echelon type,crossover type.
[1] broken line type: The spreading direction in planeof seismic fractures was turning、bent point of thepolyline or arc-shaped, and then continue toextend(Fig. 1Ⅰ).
[2] Linear type: The overall spreading direction inplane of seismic fractures was extended straight, littlechange in direction, no significant turning or bending
in the process of extending(Fig. 1Ⅱ).
[3] En-echelon type: it formed by a series of seismic
fractures whose direction substantially the same, enechelon distribution presented on the plane, seismic
fractures were broken line or linear type(Fig 2).

2.2 Transfixing type seismic fracture features andfailure characteristics
Comparatively speaking to seismic fracturesurface, the transfixing seismic fracture not only longextending length, but also large opening degree,
visible depth was up to 10m. The fracture had goodinterconnectivity, linear extension in plane, joint
interface jagged, throughout inside the rockmass by
the form of pinch-out. For survey, the majority ofseismic fractures belong to this type(Fig. 4).

Mainseismic
fracture
主干震裂缝Secondaryseismic
fracture次级震裂缝Secondaryseismic fracture次级震裂缝Fig. 4 Seismic fracture throughout inside rockmass
Fig. 2 Distribution schematic diagram of en-echelon

The development scale of ransfixing seismic
fractures were large and had good interconnectivity,
it caused significant damage to rockmass structure
and affect the deformation and instability of
rockmass. Through the investigation of fractures ontype seismic fracture[4] crossover type: it formed by the two or three210th Asian Regional Conference of IAEG (2015)
slope, such fractures developed in better position as
slope top, steep slow border, free face to formed thetrailing edge or side edge boundary of cutting rockmass, it become the main controlling factors ofshattered rockmass stability. Seismic fractures
cutting rockmass to formed the dangerous rock
body on slope(Fig. 5).

Fig. 6 Seismic fracture developed near the fault

Strike type seismic fractures exposed on slope bylesser extent of subjected to the influence of terrain
and surface features, their extension length along the
strike direction was large. Seismic fractures weremainly distributed in the vicinity of fault. It was dueto the strong earthquake force existed at the fault,
higher degree of rockmass shattered, fractures could
easily developed along the strike direction of fault.
And seismic fractures which existed at otherlocations, owning to their extend process was limitedby the influence of terrain and surface features, so thecharacteristics along the strike extension was not
obvious. The strike of this type seismic fracture was
N30º～60ºE by field investigation, seismic fractures
which developed along their strike direction werelocated in the vicinity of fault.
The length along strike of strike type seismic
fracture was longer, it could constituted the instability
range boundary of shattered rockmass and controlled
a wide range of rockmass unstable. The rockmass
structure around this type of seismic fractures waspoor, higher degree of shatter-damaged. Once theshattered rockmass produced deformation andinstability, major destructive formed.

Fig. 5 Dangerous rock formed by seismic fracture
cutting

Seismic fractures was filling less or no. They
could further extended when encountered the rainfall
or vibrate interference. When the fractures continued
to run through the locked segment in shattered
rockmass, the locked segment could suddenly
snipped once the tensile stress of rockmass was
greater than the tensile strengt under the action ofgravity, earthquake force even wind. Then dangerous
rock separated from the maternal rock to drop andformed rolling rock. Through the collapses
investigation, the perforate seismic fractures became
the main controlling factor of collapse disasters after
earthquake. Such seismic fractures were developed ina large scal 、 strong interconnectivity, they oftencaused great collapse hazard.
2.3 Strike type seismic fracture features andfailure characteristics

2.4 Tendency type seismic fracture features andfailure characteristics

According to the extension direction of seismic
fractures, the strike type seismic fracture was refer to
its length along the strike direction was larger thanalong the dip direction, fracture developed mainly
along its strike. The extension direction of strike typeseismic fracture was identical or similar to thedirection of tectonic force (such as the strike of fault).
These seismic fractures extended along its strike, they
had strong directional (Fig 6). Since this type ofseismic fractures developed was mainly due to theeffect of tectonic force, so their developmental scale
were generally larger.

The tendency type seismic fracture was refer toits extension direction mainly along its dip direction.
For the developmental morphology, the length along
dip direction was greater than along its strikedirection, its development was extended into the deepof rockmass. This type of seismic fractures developedin various locations on slope, especially developed
at ridge、steep with slow border area.
Most seismic fractures belonged to this tendency
type through investigated on slope. Although its
length exposed on ground was short, owning to it was
extended into the deep along its dip direction, its
visible depth was up to 10m in the deep of rockmass.
There are certain distribution characteristics in such3

10th Asian Regional Conference of IAEG (2015)
seismic fractures, the Ladder type、“Y”-shape type
and Trace-back type had presented in the morphology
of profile combination (Fig. 7). Ladder type seismic
fractures was characterized by a series of fractures
which inclined 、 extended to the same direction,parallel or nearly parallel to each other. Due to thevisible depth limited in such seismic fractures, so thistype fractures may also intersected in the deeper
position of rockmass. The characteristics of“Y”-shape type seismic fractures was that a series ofdifferent tendencies seismic fractures crossed in theextend process to the deep, “Y”-shape presented in
the profile. Trace-back type seismic fracture wasrefer to the fractures occurred one or more branches,
the secondary seismic fractures which continuous orintermittent appeared in its extend process into thedeep.

Fig .8

Closure type seismic fracture

Tendency type seismic fractures cutting rockmass,constituted the instability boundaries of shattered
rockmass, formed dangerous rock hanged on theslope. Owning to the length of seismic fracture
extending to deep, good continuity with aerial
phenomenon, it often become a good channel for
rainwater 、 groundwater infiltration, could easily
cause the instability of shattered rock mass tocollapse. This type of seismic fractures was wide
distribution on slope, so the formation of damage isrelatively large.

Closure type seismic fractures were mainly
distributed in the better parts of rockmass structure.
The better the overall structure of rockmass, the lessdamage extent by seismic forces to the rockmass
structure. Seismic fractures occurred by rockmasstension crack in the extending process was restricted,
it discontinuous distributed by the form ofbeaded-shape in slope surface. This fracture wassmall degree opening, the joint interface on bothsides in rockmass gradually filling at later under theaction of gravity、fracture deposits.
Since this type of seismic fractures intermittent
distributed, poor continuity, small degree opening,
the joint interface gradually healing later, devastating
limited, but it discontinuous distributed in the slope
surface, with a certain degree of ductility, it destroied
the overall structure of slope. Seismic fracture fillingless or no, overhead rockmass, it could constitute a
good channel for rainwater、groundwater infiltration.
Once it connect with other fractures which was large
opening、good continuity、strong interconnectivity,
these fractures could cause cracking scaling,
accelerating the rockmass deformation instability, so
destructive.

2.5 Closure type seismic fracture features andfailure characteristics

2.6 Extended type seismic fracture features andfailure characteristics

For the extending trendence in the distribution
plane of seismic fractures, the joint interface on thistype of fractures uneven in extension process, at bothends of the fracture closed or nearly closed, but theintermediate portion is open, small opening width(Fig. 8).

Extended type seismic fracture refers to its
extension in the section, one end was gradually open,
and extended to the slope surface with opening
gradually large, one end gradually pinch out into thedeep, and had the wedge-shaped gap of wide
upper-narrow lower. This kinds of fractures was large
opening degree, filling less or no, the joint interface
was rough and jagged(Fig.9).

Fig. 7 Combination schematic diagram of tendency
type seismic fractures4

10th Asian Regional Conference of IAEG (2015)
triggering seismic faults in Wenchuan earthquake, theextension direction of fractures has obviously
effected by the triggering seismic fault. Some larger
degree of shattered opening fractures spread by linear
shaped, good extension, generally not subjected tothe influence of the terrain and surface features in its
extending direction. Some seismic fractures werecorresponding with the fault which influenced theirdevelopment. American scholars (Leonard 1929;
Holzer et al. 1987)put forwarded the view of tectonic
origin in seismic fractures by analysis the earthquake
force, they thinked the tectonic is the dominant factor
in formation and development of seismic fractures.
Fig. 9 Extended type seismic fracture

Through slope seismic fractures investigation, thefracture which was wide upper-narrow lower shaped
in section, large degree opening, overhead rockmass.
It mostly belong to the extension seismic fracture.
This seismic fracture formated by the high degree ofshatter damaged caused by seismic forces to the
rockmass structure, severely damaged, thedevelopment of fractures extended along thedirection of seismic force. Using the theory of
fracture mechanics, fractures begin to expand when
its tip stress intensity factor is greater than rockmassintensity factor by seismic force, this can be roughly
divided into three stages: initial shattered stage,
fracture formed, development stage, fracture
propagation、extending to surface stage(Lin et al.
1986).
Extended type seismic fractures were widely
developed on the slopes according to the survey,
especially developed at crest、ridge or steep withslow border area, a certain size distributed. This
seismic fractures not only destroied the rockmass
structure, but also large degree opening, overhead
rockmass. After the earthquake, frequent aftershocks、
rainfall and wind forces were constantly changed thestate of shattered rockmass balance, these made theseismic fractures continues to expand、extend. These
fractures lead the rockmass instability and collapse.
3.

Seismic fracture
characteristics

Fig. 10 Statistical graph of typical seismic fractures

The distribution of seismic fractures were mainly
due to the effect of triggering seismic faults, they
were mainly distributed in the vicinity of causative
fault or ridges position and steep slow border in slope,these positions were also the site of strong seismic
force.
3.2 Extended length characteristics
The extension length of seismic fracture refers to
its continuous length exposed on the slope surface.
The fractures extended length generally longer, morethan 6m. Some shattered serious fractures was up to
10m in length, portion extended to the top of slope.
Collapse pit、 tensional slot were exposed in theprocess of extending(Fig.11). Large number of
secondary seismic fractures were distributed aroundthe main seismic fractures. The secondary seismic
fractures discontinuous distributed and poor
continuity.

development

Investigated the basic characteristics of seismic
fractures, their spread、development characteristics、
deformation signs had some corresponding
characteristics.
3.1 Spatial distribution characteristics
Seismic fractures was N30 º ～ 60 º E
trending(Fig.10), it is basically the same with theFig. 11 Tensile groove developed

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10th Asian Regional Conference of IAEG (2015)
3.3 Opening degree characteristics
The opening degree of seismic fracture mainlyrefers to the width of crack in rock and soil mass byshocked tension in the earthquake force. Through thesurvey, the opening degree of seismic fracture was
the form of wide upper-narrow lower shaped, pinch
out into the deep(Fig.12). Cai and Zhang (2001)
indicated that the seen seismic fracture was wedge
shaped fractures on the wide upper-narrow lower
form. For some better overall structure of rockmass,
the opening degree was small, generally only a few
centimeters to a dozen centimeters; Some rockmasswith poor structure and structural plane significantly
developed, their fractures opened a greater degree.
And the seismic fractures located around thetriggering seismic faults had the wedge-shaped gap ofwide upper-narrow lower, the opening degree in top
was closed to 1m, it is illustrated the rockmass
around the faults was serious shattered.

Fig. 13 Collapse induced by the slope rockmass
were shattered
4.

Seismic
fracturemechanism analysis

mechanics

Seismic fracture was mainly caused by theearthquake force. Refraction and reflection of seismic
waves occurred in the structural plane when theseismic waves affected it (Fig.14), so the rockmass in
structural plane shattered crack due to the tension
crack which caused by the accumulated stress(Zhang
et al. 1993). The slope damage caused by seismic
force was closely related to the effect of seismic
waves, seismic wave is divided into P-wave(longitudinal wave) and S-wave (shear wave), bothjoint action at the structural plane of rockmass.
P-wave and S-wave with frequency up-down andhorizontal vibration, this generated n-pull、anti-pull
repeated stress to rockmass in the structural plane,
and rockmass in the structural plane cracked open
along the structure plane under the dynamic action of
powerful earthquake force(Zhang et al. 2005; Feng etal. 2009; Newmark 1965). The shear strength ofstructure plane quickly lower owning to it sufferedrepeated shear stress, rockmass was cutted along thestructure plane by the action of strong shear stress,
rockmass opened and deformed. Myer (1990) studied
the propagation characteristics of seismic waves
through the rockmass joint surface and the force
between the rockmass interface controlled by the
Coulomb criterion. Liang (2003) holded that thestructure plane formed shattered face by theearthquake force, the rockmass above the surface
would out of the parent rock and continue tomovement, and the rockmass below the surface
would generate a new free surface due to the
influence of upper shattered face.

Fig. 12 Schematic section of seismic fracture

The shattered width of Surveied fractures were
20cm or more, inclination angle greater than 60 º,
extended to deep by jagged-shaped, but it extending
to the deep, the shattered width became smaller
gradually.
3.4 Filling characteristics
These seismic fractures were large opening
degree generally, filling less or no, overhead
rockmass. The joint interface was relatively new andbedrock exposed, straight or jagged, extending longer.
Wide upper-narrow lower shaped in the form of
fractures, shattered stone fragments were filled in it.
Both sides of rock and soil collapsed was caused bysome seismic fractures which had large degree
opening(Fig.13).

Fig. 14 Schematic diagram of stress wave
propagation effected by the structural plane6

10th Asian Regional Conference of IAEG (2015)
Seismic fracture formed by the process of rupture、
perforation、extend in rockmass, internal fracture wasfurther developed into large fracture in the slope atlast. The ruptured of rockmass was mainly due toseismic waves, Fig 15 shows a simplified seismic
waves schedule figure, Fig 16 is a figure of simplified
equivalent stress intensity factor curves. Use thefracture mechanics theory to analysis the process of
rupture、propagation and crack arrest in rockmass (Li2010).

4.3 Crack arrest evidence
As the seismic intensity decreases, the energy
reduce, the fracture stop rupture when the stressintensity factor is less than the fracture toughness.
For the structural plane effected by strong earthquake
force, it may have higher stress intensity factor after
the earthquake stop. The fracture in the static stressfield may continue to extend、until the crack arrest
conditions is met.

5.

Fig. 15

Curve diagram ofof seismic wave course

Conclusions

In summary, through detailed investigation andanalysis to the deformation characteristics、
deformation and failure mechanisms、deformation
kinematic characteristics of slope seismic fracture,
the results show that:
Based on the morphological characteristics anddeformation deformation features of slope seismic
fracture, the seismic fractures was divided into six
types: surface type、transfixing type、strike type、
tendency type、closure type、extension type. Surface
seismic fracture in accordance with its extension
characteristics could be divided into broken line type、Linear type. From its combination features can bedivided into en-echelon type、crossover type.
Slope seismic fractures was N30º～60ºE trending,they were mainly distributed in the vicinity of
causative fault or ridges position and steep slow
border in slope. Seismic fracture was well extension、
high open degree、obvious connectivity and large
scale. Upper wide-lower narrow of profile
morphology, pinch out into the deep, fillings less orno.
Based on the rock fracture mechanics mechanism,combined with the stress intensity factor to analysis
the shatter damage mechanism of the structure plane.
The mechanics type of slope fractures mainly belong
to the tension crack, rock structure plane would
rupture to generate crack under the dynamic action,
crack gradually extend to become the penetrating
tensional seismic fracture.

Fig. 16 Curve diagram
stress intensity factor

4.1 Fracture criterion
Combined with fracture mechanics, the rupture
criterion of rockmass is the maximum stress intensity
factor criterion, law of K, the rockmass will produce
tensile failure if the stress intensity is greater than thefracture toughness. At time t=0, seismic waves is not
yet in effect, the structure plane only in the static
stress field, KI＜KIc1 (KIc1fracture toughness), it is in
the closed state. With the seismic waves apply to thestructure plane, the earthquake intensity gradually
apply to the structure surface. Increase over time, theintensity gradually increase, the equivalent stressintensity factor will also increase accordingly, thestress intensity factor reaches KIc1 with the increaseof earthquake intensity until t=t1 the rockmass will
rupture along the structural plane if it met the fracture
conditions.
4.2 Extension process
The fracture generated in the internal of rockmass
after it rupture. With the continuing effect of seismic
waves (t1<t<t2), the fracture continue to expand, thisis driven by the equivalent stress intensity factor KI,
the maximum rate of expansion obtained by thefracture when the seismic waves reach the peakintensity at t=t2, KI reach maximum KIc2. The stressintensity factor KI is gradually reduce with theweakening of seismic intensity after t>t2, the effectcourse of seismic waves will end until t=t3. The effectof seismic waves will stop after t>t3, extended
fracture is in the static stress field. If the stressintensity factor is still greater than the fracture
toughness of rock mass, the fracture will continue toextend in the static stress field, it stop the extension
until t=t4.

Acknowledgements
Financial support from key project of the Natural
Science Foundation of China (NSFC, NO. 40972195)
and the Independent research of the State key
Laboratory of Geological Hazard Prevention &Geological Environment Protection (HGY-2012-08)
is gratefully acknowledged.

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10th Asian Regional Conference of IAEG (2015)
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