10th Asian Regional Conference of IAEG (2015)

Stability analysis of Shijiaping landslide and its effect on the safety of a 750KV
substation in Tianshui, China
Lihui LI, Xiaolong DENG, Yufang TAN, Ruilin HU
Key laboratory of Shale gas and Geoengineering, Institute of Geology and Geophysics, CAS, China
E-mail:lhli2942@mail.igcas.ac.cn
Abstract
As a part of Lanzhou East-Tianshui-Qian county electric power transmission project,
one 750KV substation engineering was planned to build in Gangu County, Tianshui, China.
However, field investigation revealed that an ancient Shijiaping landslide is situated on theleft bank of the substation site. In order to assess the suitability of the substation site andensure its safe operation, the evaluation of landslide stability and its effects on the site wasperformed. By using limit equilibrium method and numerical calculation, the results weredrawn, which indicated that in natural state, the Shijiaping landslide is stable at present, yetcreeping may occur in rainstorm season due to the softening of underlying mudstone. In themost adverse condition as the combination of rainstorm and earthquake, the landslide would
be unstable. However, the landslide does not pose threat to the safety of the transformer
substation site, for there is on chance for the occurrence of long run-out landslide. The
results in the paper played an important role in guiding the siting of the substation and thetransmission line.
Keywords: transformer substation, Shijiaping landslide, stability
1.

Introduction
The planned substation was to be built at 300
meters away from north of Shijiaping, Xinxing
town, Gangu county. It is 11.5km away fromGangu county (Fig.1). The traffic is convenient,
the Gangu-Tongwei highway passes through on
the 30 meters to the west of substation.
The landform in the studied region is mainly
loess mountain ridge and fluvial terrace. Ancientlandslides and debris flows are relative highly
developed,
which
has
reformed
thegeomorphology of this area. The substation is
located on the loess deposit, near the substation
there is the Sandu River with an incision depthof 30m. According to geological survey, thisregion belongs to geohazard incidental zone,
Shijiaping large-scaled landslide is on the westof the substation. Meanwhile, this area situatedin Ⅷ degree seismic intensity area. Hereby, thestability of the landslide will have a great
influence on the safety operation of thesubstation.
The purpose of the paper is to answer
whether the landslide would affect the safety ofthe substation. Site investigation, drilling, test
pitting and trenching were carried out. Basedon the above investigation, the engineering
geological conditions of the site was evaluated.
Limit equilibrium method and numericalstimulation were used to calculate the stability of
Shijiaping landslide.

As a part of Lanzhou East-Tianshui-Qian
county electric power transmission project, one
750KV substation engineering was planned tobuild in Gangu County, Tianshui, China. With
the development of modern large-scale
high-voltage transmission line laying projects,
calls for strictly controlled foundation
displacement of superstructures is becoming
more and more important. Yet there exist many
ancient landslides in northwest China, whose
stability exerts great influence on theconstruction of the transmission lines. Related
projects had been studied(Cui et. al., 2007; Li etal., 2007; Ren, 2012; Li, 2014) and yieldedsome profound results.

Fig.1 Location of Shijiaping Landslide110th Asian Regional Conference of IAEG (2015)

2.

Geological settings

River, larger branches include Zaojiaoshu gully,
Bailin gully, Ma’er’yu gully and Gongjiachuan
gully. The Sandu River is rich in salt, its water
quality is poor and inappropriate for irrigation.
Lithology of phreatic aquifer is featured byglutenite with a buried depth over 20 meters, andthe aquifer depth is about 2-4m. Groundwaterflows from north to south, which is parallel tothe strike of river valleys, both on the east of thesite and right side of the Sandu River there isnonexistence of groundwater on the surface. TheSandu River is charged by rainfalls, and therainfall intensity directly influence river level.
The salinity of groundwater is relatively higher,
and the TDS reaches 3.89g/L, which belongs tosaline water. As to its pH value, it belongs toneutral water with pH value of 7.48. The water
hardness is computed as 8.419meq/L, which
belongs to hard water.

2.1 Climate
The study area belongs to warm temperatezone, with a subhumid climate. The annual
average temperature is 10.2°C, extreme
maximum temperature is 36.3°C while extreme
minimum temperature is -19.2°C. The rainfall,
about 480.8mm per year, presents an uneven
distribution in a year. Annual average
evapotranspiration is recorded as 1481.5mm.
Geohazards such as landslides, rockfalls, debris
flows are easily triggered by rainfalls with shortduration and heavy intensity. As a typicalseasonally frozen region, the maximum depth ofpermafrost is 51cm.
2.2 Tectonics
The convergence part of Longxi vortexstructure incorporates with Qilv arcuate fold belt
at this region, and structural features are relativecomplex (GDMG, 1971). Major structural traces
are structural belt with NW direction and arcuate
compressive belt with NNW direction. The
dominated regional trend from northwesterly,
which is parallel to Weihe River. Folds andfaults are fairly developed, however, moststructural traces are covered by thickwide-spread loess. Neotectonic movement in thisregion has resulted in strong uplift of crust anddownward erosion of rivers.

2.5 Earthquake
Five earthquakes have been recordedhistorically due to their significant regionalinfluence. The basic earthquake intensity of thisarea is Ⅷ
degree, which belongs tomeizoseismal area. The PGA of this area is
0.20g, and characteristic period of the seismic
response spectrum is 0.45s.
3.

Characteristics of Shijiaping Landslide

The Shijiaping landslide lies at the left bankof Sandu River, east of the site. The slope body
is about 1500m in length, and the maximum
width is about 1500m, its level difference is180m approximately. The front part of slope
body is gently with a gradient of 10°, while its
back part is steep, with a gradient of 17°. At
the backscarp, the slope angle is generally 65°~70 ° . Which presents an obvious armchair
shaped structure, as Figure 2 shows.

2.3 Strata
All the systems in Cenozoic, ranging fromthe Paleogene to Quaternary, are represented inthis region. The Paleogene rocks are essentiallyamaranthine shaly sandstone with pelitic texture
and calcite cementation, it is sporadicallyexposed on both sides of Sandu riverbed and thegullies. The Neogene rocks are red clastic rocks
with loess overlies them, which form theskeleton of loess mountain ridge. Outcrops areoccasionally observed on gullies, heavy eroded
mountains and Sandu River banks. TheQuaternary rocks themselves are loose
accumulations, they include black clay(Q2), theMalan loess(Q3pr), first terrace and floodplan(Q4al-pl), redeposited loess(Q41al), gravityaccumulation(Q42del) and alluvial-pluvial sandand pebble(Q43al ＋ pl).The gravity accumulation
spreads over the gullies and alluvial-pluvial fans,the materials mainly consist of deposits fromlandslides, rockfalls and debris flows, which ischaracterized by a mixed lithology with messy
structure and different thickness(GPIHEG,2007).

Fig.2 Overall sketch of Shijiaping landslide
Field investigations revealed that sinkholes
and gullies are highly developed in landslide
body. There are sinkholes parallel to the trailingedge and sidewall of the landslide, as Figure 3
illustrates. The sinkholes almost string-shaped
distributed along the gullies, trailing edge andsidewalls of the landslide. Moreover, several
rockfalls have been observed on the frontal parts

2.4 Hydrogeology
The main stream in the study area is Sandu

210th Asian Regional Conference of IAEG (2015)

near southern sidewall of the landslide.
The strata of landslide body contain Neogene
rocks and Quaternary accumulations. The
outcropped Neogene strata are mainly in gulliesof the landslide body and partial areas alongbanks of Sandu River sporadically. The rocks
are mainly mudstone with thickness more than
50cm. In the upper mudstone layer, verticaljoints are highly developed. The rock would
soften when it absorbed water, and cracked afterlosing water, i.e., it has the feature ofwater-expanding and dry dehydration. TheQuaternary strata rocks are mainly the Malanloess, redeposited loess, silty clay, gravel soil,
calcareous cemented conglomerate and flood
plain deposits. Additionally, black clay withhumus outcrops could be observed at local area.
The Malan loess is thicker with a maximum
thickness of 40m, vertical joints and sinkholes
are highly developed in this layer. Mudstoneinterbedded with silty clay is also found inlandslide accumulation. Six interlayers can befound from the borehole and trial pit exploration.
Striation and mirror surface, as Figure 4 shows,
could be easily observed, which implies that thelandslide has suffered several slides.

comprises of mudstone, due to the existence ofperched groundwater, partial soil becomessaturated and its strength is relatively low.
Consequently, hole collapse and necking haveoccurred during drilling.

Fig.4 Mirror surface and striation in drill cores
4.

LEM stability analysis of Shijiaping
landslide

Field geological survey and exploration
reveal that the landslide terrain has been steadydue to alterations of human activities, and nosliding events have been recorded since 100years before. Besides, obvious deformation
traces are nonexistent in trailing edge of thelandslide. Therefore, it is believed that thelandslide stays in stable state at present.
However, with the increase of sinkholes, anddownward erosion of gullies, accompanied bycollapses of Sandu River banks, the landslide
body would be severely fragmented and then its
mechanical properties would be degenerated aswell. Therefore, unexpected complex geologicalconditions would occur and thus exert anadverse effect on the stability of the wholeslopes. To evaluate the safety of this substation
site, limit equilibrium method (LEM) and finite
difference method (FDM) (Chen et. al., 1995,Chen, 2003) were utilized to analyze the stabilityof the landslide.
Three kinds of state were considered in thecalculation, natural state, rainstorm state,earthquake state and rainstorm simultaneouswith earthquake state. Calculation methods ofMorgenstern-Price, Janbu and Bishop areadopted respectively. I-I ′ and II-II ′ profiles

Fig.3 Sinkholes in the leading edge andbackscarp of Shijiaping Landslide
In the landslide accumulation body,groundwater is mainly perched groundwater incolluvial deposits and pore-phreatic groundwaterin sand gravel layer. It is directly supplied byrainfalls.
Relativewater-resistinglayer310th Asian Regional Conference of IAEG (2015)

were chosen to conduct the calculation (Figure 5and Figure 6). With regard to I-I′ profile,
two-stage sliding surfaces, the H1-1 and H1-2
slices are both calculated. There is only theperched water in landslide body without anunified underground water level, therefore, theimpact of groundwater on slope stability can beignored during calculations.
In natural state, the intensity of rainfall isweak, with a small permeability coefficient of
the soil, the impact of rainfall is ignored when
performing the calculation. In rainstorm state,
the most unfavorable factors are taken into
account, and the landslide accumulation isconsidered as saturated soil.
Due to theexistence of perched water and nonexistence of
unified groundwater table, in addition, seepagesof groundwater in toe of the slope are not seen.
Therefore, seepage influence is excluded when
performing stability calculation under the fourstate. When considering earthquake, a Ⅷ

degree of the local basic seismic intensity is setfor calculation. The factor of safety (FOS) under
different conditions is listed in Table 1.

Fig.5 Topography of Shijiaping landslide

unit:mI

NE 85°

I'

1565
1535
1505
1475
1445
1415
1385

Sandu River

1355

1307

1325
1295

1390m

(unit:m)1654

II1624

NE 89°

II '

1594
1564
1534
1504
1474
1444

Q4

1414

del1384

Sandu River

1354

1309

1324
1294

1730m

LEGEND
Q4
delLandslide accumulation
Gravel soil

Loess

Sliding surface

Silt

Silty clay

Mudstone

Shaly sandstone

Glutenite

Speculated form line

Fig.6 Engineering geological profiles of Shijiaping landslide
5.

(1100-1690m). Fig.8 illustrates 3-dimensionalmesh plot of the model including 166320
elements and 93500 nodes.
(2) Boundary and initial conditions
A simple boundary condition wasconfigured for calculation, which implies that aroller boundaries are placed on the four lateralsides of the model, while all the base nodes on
the bottom are fixed in both vertical andhorizontal directions, and the surface nodes areset free.

Numerical stimulation stability analysis

5.1 Modelling and boundary conditions(1) Studied regions
Figure 7 showed the 3D terrain of therectangular region to calculate, which includes
the Shijiaping landslide and substation site. The
east-west length is 2720m, and its south-north
width is 2330m, the vertical scope range fromsurface to a certain distance of bed rocks

4

10th Asian Regional Conference of IAEG (2015)(3) Parameters and model configuration
The type of Shijiaping belongs toloess-mudstone landslide, its slide mass could beregarded as elasto-plastic materials. Therefore,

the Mohr-Coulomb constitutive model isassigned to perform the calculation. The
parameters used in the calculation were listed inTable 2.
corresponding to cases in LEM stability
calculation. According to the influences ofdifferent triggered factors in the evolution oflandslide, deformation characters of the slope arestimulated under the condition of natural state,
rainstorm state, earthquake state and rainstorm
accompanied with earthquake state.
5.3 Results(1) Stress analysis
Figures 9~12 illustrate the vector plots ofprinciple stress in I-I’, II-II’ profiles under thecondition of natural state and earthquake state.
As the figures demonstrate, the slope shapeexerts directly influence on directions ofprinciple stress in the slope, the direction
deflects significantly near the slope surface, theminimum principle stress approaches tohorizontal direction, while the direction ofmaximum principle stress is parallel to slope
surface. Under the condition of natural state andrainstorm state, the minimum principle stresspresents as compressive state. While in thesituation of earthquake, earthquake & rainstorm,
the minimum principle stress turns to tensile.
Moreover, influenced by horizontal seismicforce, the direction of principle stresses haschanged significantly.

Fig.7 3D topographic map of Shijiaping
landslide(unit:m)Fig.8 Mesh plot of Shijiaping landslide model

5.2 Design of numerical stimulation
The numerical stimulation is designed
Table 1 Calculated factor of safety (FOS) of Shijiaping landslide under different conditions
Profile No.

H1-1

H1-2

H2

1.73

Rainstorm
state1.266

Earthquakestate
1.292

Rainstorm &earthquake state
0.945

bishop

1.747

1.278

1.307

0.956

Janbu

1.71

1.251

1.276

0.934

Morgenstern-Price

1.900

1.394

1.389

1.019

bishop

1.887

1.384

1.384

1.015

Janbu

1.851

1.357

1.354

0.993

Morgenstern-Price

2.006

1.442

1.486

1.068

bishop

2.023

1.454

1.498

1.077

Janbu

1.996

1.435

1.479

1.063

Calculation method

Natural state

Morgenstern-Price5

10th Asian Regional Conference of IAEG (2015)

Table 2 Parameters used in the stimulation
Lithology

Elasticmodulus(GPa)Poisson’sratioνDensity(KN/m3)

C(kPa)

Φ(°)

TensileStrength(MPa)accumulations

0.03

0.37

17.64

28.2

12.3

-slip soil

0.08

0.35

1838

15-mudstone

130.22

20

38.68

16.46

0.086

covering layer

0.02

0.38

1620

25.7

-

Section I-I’

Section II-II’
Fig.9 Principle stress plots in natural state

Section I-I’
Section II-II’
Fig.10 Principle stress plots in rainstorm state

Section I-I’
Section II-II’
Fig.11 Principle stress plots in earthquake state

610th Asian Regional Conference of IAEG (2015)

Section I-I’
Section II-II’
Fig.12 Principle stress plots in earthquake & rainstorm state(2) Deformation analysis
Figures 13~16 demonstrate displacement

vector plot of I-I’ profile and II-II’ profile under
different conditions.

Section I-I’
Section II-II’
Fig.13 Displacement vector plots in natural state

Section I-I’

Section II-II’

Fig.14 Displacement vector plots in rainstorm state

Section I-I’

Section II-II’

Fig.15 Displacement vector plots in earthquake state7

10th Asian Regional Conference of IAEG (2015)

Section I-I’

Section II-II’

Fig.16 Displacement vector plots in rainstorm & earthquake state
As the figures illustrate, in natural state, due
to the impact of slope shape, maximum
displacement occurs at trailing edges. In I-I’profile, maximum displacement trends towardoutside of the slope, which exerts no adverse
effect on the slope stability. However, maximum
displacement in II-II’ profile trend toward insideof the slope, which puts an adverse influence on
slope stability. In the situation of earthquake, thetrend of displacement in the slope alters, andboth the maximum displacements in I-I’ profileand II-II’ profile are parallel to their respective
slop surfaces, and trend toward the inside ofslope, which is unfavorable for the stability ofthe trailing edges.
6.

References
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Conclusions

Both from the perspective of geologicalmicroscopic assessment and quantitative
calculation, it is concluded that the Shijiaping
landslide can maintain its stability at present innatural state, which implies that the landslide
could not fail thoroughly. However, as thestrength of underlying mudstone decreases whensoften in water, creeping would occur in theslope. Besides, under the condition of rainstorm
and earthquake, the landslide would be unstable.
Due to a gentle surface of the slope, there arelittle chance for Shijiaping landslide evolved
into a remote landslide if it revives or partially
fails. In summary, the landslide puts no direct
threat to the site of substation.
Acknowledgements
This work has been supported by the key
research project supported by National Natural
Science Foundation of China ( No. 41330643)
and the National Natural Science Foundation ofChina (No. 41172269 and No. 41372323) andThe authors are also thankful to the reviewers
for their kind suggestions, and all supports aregratefully acknowledged.

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