10th Asian Regional Conference of IAEG (2015)

Structure and Stability of Zhangmu Deposit in Tibet
HU Ruilin, ZHANG Xiaoyan, GAO Wei, MA Fengshan, ZHANG Luqing, ZHOU Jian
Key Laboratory of Shale Gas and Geoengineering, Institute of Geology and Geophysics,
Chinese Academy of Sciences, Beijing, People's Republic of China
E-mail: hurl@mail.iggcas.ac.cn
Abstract
Zhangmu Port is situated at southwest of Tibet, and is the only trading port between
China and Nepal. However, this important area has been faced the threat of geologicaldisasters like landslides occurred in Zhangmu deposit over a long time. Evaluation of thestructure and stability of the Zhangmu Deposit was undertaken to elucidate the causes of thedeposit and its engineering geological characteristics based on field survey, and the stabilityof the deposit is analyzed mainly based terrain, the surface deformation features andmaterial components of the deposit. Results show that, structure of Zhangmu deposit is nota simple single soil structure, but includes colluvial, residual and glaciofluvial deposits
amongst others, and the slope is less likely to slide as a whole, but will mainly slip partiallycontrolled by local hazard factors such as slope gradient, lithology, human engineering
activities, and concave lateral erosion action.
Key words: Zhangmu Port, Zhangmu Deposit, landslide, structure, stability.
1.

Introduction

The importance of Zhangmu Deposit cannot be
over emphasized as the Zhangmu Port which is the
only overland trading port between China and Nepal
and also the only international trade port withparticular scale in Tibet is located within its vicinity.
Zhangmu Port is situated at the southwest of Tibet
where is the end of National Highway No.318, themost southern Tibetan Plateau, left bank of BoquRiver in the southern slope on the middle of theHimalayas (Fig.1). It has a total area of about 70 km2,
80km away from Kathmandu, capital of Kingdom of
Nepal, to Shigatse 450km, and to Lhasa 780km.
Zhangmu town is not only an important border trade
port but also a significant cross-border tourism city
connecting China and Nepal and this means a lot toeconomy development of Tibet (Tubden K, Liu YJ,
2011). Because of the topographical constraints andthe needs of foreign trade, the main administrative
institutions including government offices andresidential apartments are all built on the gentle
terrain of the Zhangmu Deposit (Fig.2). In recent
years however, local deformation and landslides(Fig.3) have happened frequently on the Zhangmu
Deposit with the rapid development of the urban
construction, which severely threatens lives,properties and the economic development inZhangmu County (Ma DT, Tu JJ, Cui P, et al., 2004).

The instability of the slopes and occurrence oflandslides has become a major problem which
impacts the survival and development of the port.
Since 1993, a lot of investigation and prevention
work has been done for Zhangmu Deposit, and somuch fundamental data and research results havebeen accumulated (Xie LP, Li DH, Qiao G, et al, 2003;
Jia ZY, Liu YP, Lei Y, et al, 2006; Shang WT, Hou T,
Feng X, et al., 2011; Wang D, Feng X, Yang FX, etal., 2011; Zhu J, Cai QE, Jiang HB, 2010; Mao CW,
2008; Yi SM, Tang HM, 1996). However, previous
studies did not find out the material composition andthe structure of slope, and there is dispute on thedetermination of landslides. The disaster prevention
work stays at a “take stop-gap measures” state, whichblocks the radical cure of slope deformation andfailure. Therefore further research work to elucidate
the causes of Zhangmu Deposit and its engineering
geological characteristics thereby speeding up thecomprehensive disaster prevention has become thekey to achieve significant development for Zhangmu
County and the whole Tibet area at large. Thus, inthis work, exploration depth was increased andmaterial components and its effect to slope stability
particularly discrimination of potentially unstableslopes were focused on to provide scientific support
for overall utilization of Zhangmu Deposit.

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10th Asian Regional Conference of IAEG (2015)

amphibolite gneiss (Chen NS, Hu GS, Deng W, et al.,2013). There are two main ravines distributed at thenorth and south of the deposits respectively named
Zhangmu Gully and Dianchang Gully, and the latter
one is also the main tributary of Boqu River in thisarea (Fig.4).

2.

Geology Condition and Previous work
The study area is belonging to middle-highmountain topography type, and has a large latitudedifference from 1700m～3646m, with a cutting depth
up to 2000m. It is ravinated strongly, and the
bedrocks are mainly granite gneiss and granite

Fig.1 Traffic location map of Zhangmu Port

Fig.2 Full view of Zhangmu County

Fig.3 Talus slide in Zhangmu

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10th Asian Regional Conference of IAEG (2015)

Fig.4 Engineering geological plane of Zhangmu
Zhangmu County is on a relatively gentle slope
zone composed by quaternary loose deposits withgood vegetation cover, in which all existing surface
deformation and landslides are developed. It is very
important to identify the structure of the accumulated
layers for determining the stability of slopes.
Previous investigation works (Xie LP, Li DH, Qiao G,
et al, 2003; Jia ZY, Liu YP, Lei Y, et al, 2006) haveshown that the deposit is primarily composed ofeluvial slope materials, mainly distributed in theslopes outside of Zhangmu ancient slide area. The
lithology is gravelly soil with silty clay and blockstones with mixed sizes (the largest diameter of 5 m)which are mainly composed of mica and quartz
schists(Fig.5). The deposit is loose on the surface
layer and medium to slightly dense in the lower part.
The silty clay is in a plastic-hard plastic state andsome local landslide masses exists in the lower partof the slope and its material resources are mostly
eluvial slope materials from the upper part. The
deposit is thicker in the upper slope and thinner in thelower slope, with an average value of about 20-30 m
up to a maximum of 60 m or more. However, thegeological structure of the deposit is quite complex,
the depth of investigation from previous work is
inadequate for the precise study on the material
composition and cause types of the deposit.
3.

was carried out on the basis of preliminary
investigation. It includes 31 new boreholes (5 aredeep drilling boreholes), 14 trial trenches, and 20 test
points of in-site push shear tests, permeability tests,
sieve tests and gravity tests distributed in different
deformation areas (Fig.4). Through the field
investigation, it was observed that the constituents ofthe deposit are not so simple. The typical section
shows that the deposit is not entirely eluvial slope
materials (Fig.5). Generally, the upper part of thedeposit is mainly gravel soil, mostly tawny, withlarge content of block stones of different diametersizes from 5 cm to 5 m. The rocks are angular inmorphology and mainly composed of mica schist andquartz schist, and the fine part is mainly silty clay.
Beneath this layer of soil, a layer of grey silty clay
with a certain amount of gravel is generally
developed. This layer has a transitional relation withthe underlying bedrock, and the interface between
them is blurred (Fig.6). It became obvious that
Zhangmu Deposit is not just a single eluvial slope
material, but composed of the upper grey-tawny
colluvial deposit and lower grey weathered residual
deposit. On the east side of Line Bianfangyilian to theold fire brigade, the overall thickness of the deposit ismuch bigger. The upper colluvial deposit has amaximum thickness of 42-55 m and the particle size
ranged from 45 to 100 cm, and the block stone
content is between 60 and 80%. Large stone content
is reduced in the lower weathered residual deposit
which has a maximum thickness of 28-34m and ismainly composed of gravels of 2-10 cm sizes. On the

Discussion of material components andstructure
For further research, supplementary field work3

10th Asian Regional Conference of IAEG (2015)

west side, the overall thickness of the deposit isthinner. The upper colluvial deposit has a maximum
thickness of 20-29 m and the particle size ranged
from 20 to 40 cm, with block stone content of 20 to

25 %. Large stone is rarely seen in the lower
weathered residual deposit which has a maximum
thickness of 15-20 m, gravel content of about 60 %and average particle size of 2-6 cm.

Fig.5 A typical engineering geological section (Line II-II’)upper one, and the formation processes are shown in
Fig.9.

Fig.6 Weathered profile opposite Hongqiao Hotel
Fig.7 Two terraces on Zhangmu Deposit

The research shows that, two obvious terraced
terrains exist on the Zhangmu Deposit (Fig.7), andthe boundary is located approximately along the lineof Bianfangyilian-Bangcun neighborhood committee
building. Although the ground surfaces up and down
the scarp are all composed of colluvial deposit, there
are still some differences in material compositions.
Deposits at upper part of scarp generally has coarse
particles, big content of angular stones, small content
of fine-grained materials and dense trees on it; but atthe lower part, the slope deposit weathered severely
and has relatively small particles, less block stone,
more fine-grained materials, fewer trees and more
grass on it (Fig.8). It suggests that the colluvial
deposits up and down the scarp are formed indifferent collapsed periods. Historically there are twomain concentrative collapsing periods, and the lower
colluvial deposit is obviously formed earlier than theFig.8 Colluvium features up and down the scarp

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10th Asian Regional Conference of IAEG (2015)

It’s worthy to note that, a mélange of deposits offluvial facies and outwash facies referred to as
glaciofluvial deposit, because the two kinds ofdeposits are indistinguishable, was found on theplatform along Bianfangyilian in the lower part of theslope (Fig.10). The deposit is mainly distributed on
the terrain platform which is in the east of Hejia
Hotel north gully, west of Dianchang Gully and southof Bianfangyilian (Fig.11), with an area of about 0.14
km2, a volume of 18,400 m3 and consists of gravelly
soil, silty clay with gravel stone and silty clay incomposition, and is well cemented. Most of thegravel is rounded, and the average roundness is
sub-angular and some up to sub-rounded, with asignificant action of water flow which has obvious
differences in the appearance with other gravel incolluvium. The glaciofluvial deposit has a wide rangeof grain size, and is mainly in 2-60 cm (about
40%-50%), very large particles above 60 cm about
15%, and fine particles less than 2 mm about 35%.

Previous surveys indicate that, two big ancientlandslides respectively named as Fuliyuan Landslideand Bangcundong Landslide which also developed
some secondary landslides, and a big deformationbody named Linguanzhan Landslide exist on the
Zhangmu Deposit (Fig.4).

Fig.11 Distribution diagram of glaciofluvial deposits

Fig.9 Formation process diagramof Zhangmu colluvium

Fig.12 Causation types distributionof Zhangmu Deposit
However, this survey found that the strata inwhich these landslides developed is the same withupper colluvial deposit, thus the landslides can besaid to have formed latter than the upper colluvial
deposit and are therefore not ancient landslides. In
addition, the Bangcundong Landslide just has a lessobvious chair-like terrain in the landscape, and there
is no significant deformation on the slope except thetwo secondary landslides. The evidence of integral
slip is not enough, so Bangcundong Landslide
actually doesn’t exist, there are just two small
shallow landslides named Bangcundong NO.1 andFig.10 Glaciofluvial deposits in the north gullyof Hejia HotelIn summary, Zhangmu Deposit is composed ofcolluvial deposit, residual deposit, glaciofluvial
deposit and landslide deposit (Fig.12). However,since residual deposit is mainly buried under the
ground, it’s not marked in the figure.
4.

Slope deformation features of Zhangmu
Deposit5

10th Asian Regional Conference of IAEG (2015)

NO.2.
Based on preliminary ground surface deformation
monitoring data, deformation law of the deposit
initially studied shows that, surface deformation of
Zhangmu Deposit is mainly concentrated on the
middle and lower part of the slope along FuliyuanLinguanzhan and the east slope of Bangcundong(shown as the grey-dark part in Fig.13), and thedeformation is small on the upper and lower slope.
Reasons adduced for this situation include that: thisarea is a transition terrain with big slope gradient, andthe slope stability is poor; houses concentrate on thisarea and human engineering activities is intense; andconcave lateral erosion action of Dianchang gully is
the main reason of Bangcundong slope’s instability.
The deformation of the area around Fuliyuan

Landslide is almost as big as that of landslide area.
So, from the deformation angle, the range of theFuliyuan Landslide can be enlarged to the black line
as shown in Fig.13. On the area of so-called
Bangcundong ancient landslide, there is no uniform
deformation and the deformation amount is small,therefore, the justification for its existence isinsufficient. From the displacement vector diagram(Fig.14) we can also get the same conclusions whichshows that the displacement direction is almost thesame along Fuliyuan area and shows relatively intact
integrity and obvious landslide characteristic, towards
Boqu River (on the left side of the figure); while on
Bangcundong landslide area, the displacement vector
direction is in chaos and poor in consistency.

Fig.13 Surface displacement grading figure on Zhangmu Deposit
5.

Preliminary Analysis of Slope Stability

With regard to the stability of “Zhangmu
Landslide”, controversy of overall or local slip
always exists. The fundamental basis of the former isthe existence of a relatively protruding topography on
the landscape. From the present survey, it is believedthat Zhangmu Deposit is unlikely to have overall slip
mainly based on the following:
1) As earlier mentioned, the main materials of
Zhangmu Deposit are from collapsed accumulation of
the upper mountain. There are two concentrative
collapsing stages, and they formed the slope with two
micro-terraces. So, the protruding topography is notformed by the landslide.
2) There was no obvious sign of sliding foundaround the landslide boundary.
3) Slope surface displacement shows no integrity
but partial slip characteristic (Fig.14).
4) Most importantly, glaciofluvial deposit isFig.14 Displacement vector diagram6

10th Asian Regional Conference of IAEG (2015)engineering activities, and slides frequently.
Therefore, the slopes have poor stability, and need
necessary reinforcement. Areas outside these twoareas mentioned above have moderate stability.

found at the lower part of the slope, cemented and isformed clearly earlier than the upper colluvium, andits presence indicates that the slope has never been
overall slipped in the history; otherwise theglaciofluvial deposit would have not been preserved.
On the other hand, the glaciofluvial deposit cemented
well and formed a relatively stable gravel terrace. It
not only changed the slip shape of the deposit, butalso played a role in sliding resistance to some extent(Fig.15). The presence of gravel terrace also
separated the deposit laterally, and blocked its overallslipping (Fig.16).

Fig.17 Slope stability zoning map
of Zhangmu Deposit

Fig.15 Longitudinal section schematic viewof Zhangmu Deposit

6.

Conclusions

Evaluation of the structure and stability of
Zhangmu Deposit with the aid of field survey withdeeper borings to elucidate the causes of Zhangmu
Deposit and its engineering geological characteristics
has led to the following conclusions.
1) Zhangmu Deposit is mainly composed ofcolluvial deposit, weathered residual material andoutwash sediment presented as layer distribution insection and three concentrated parts in the plane. The
colluvial deposit is the main part of the deposit, and it
was formed in two focus collapse events while thelower part is formed earlier than the upper part andthey are different in material composition. The
weathered residual material is not exposed on thesurface, and is mostly located beneath the colluvial
deposit with the outwash sediment supposedly
formed earlier in time and showing good cementationand stability, mainly distributed on the tableland near
Bianfangyilian.
2) The surface deformation of the deposit ismainly concentrated on the middle-lower part of theslope, like Fuliyuan-Linguanzhan area and east slopeof Bangcun, while deformation in other places issmall. The direction of displacement along Fuliyuan
area is relatively consistent, showing good integrity,
while the displacement vector in Bangcundonglandslide is in chaos and poor in consistence.
However the monitoring time employed confers some
limitations on these views.
3) Subject to various factors, Zhangmu Deposit isunlikely to slip as a whole, but dominated by local
slip deformation.

Fig.16 Cross section schematic viewof Zhangmu Deposit
Therefore, overall landslide is hard to develop on
the Zhangmu Deposit, and its stability is mainly
controlled by the local hazard factors. According toanalysis, the main factors of restricting slope local
slip include slope gradient, lithology, groundwater
activity, human engineering activities, and concave
lateral erosion action. Based on these, a preliminary
evaluation of slope stability of Zhangmu Deposit wasstudied and from the result (Fig.17), it can be seen
that, the stable slope area is mainly distributed on the
top of the slope and Bianfangyilian terrace connectedwith it where has flat terrain and less disturbanceslope. The top of the slope is mainly composed of
newly collapsed deposit and Bianfangyilian terrace
mainly composed of well cemented glaciofluvial
deposit. They are both in a good topographic andgeologic condition and both have good slope stability,
suitable for urban land development and utilization.
The unstable slope area is mainly distributed on
Fuliyuan landslide and Bangcundong slope. It has
severe surface deformation and intense human7

10th Asian Regional Conference of IAEG (2015)

4) The slope stability of Zhangmu Deposit ismainly controlled by slope gradient, humanengineering activities, lithology, groundwater activity,and concave lateral erosion action. Areas with goodstability are mainly distributed on the top of the slope
and Bianfagnyilian terrace connected with it, whileareas on Fuliyuan landslide and Bangcundong havepoor slope stability.
It has to be pointed that, conclusions above aremainly some preliminary understandings obtained
from analysis of geological survey, and need to betested by further engineering geomechanics analysis.

190-202.
Wang D, Feng X, Yang FX, et al. (2011): A stability
evaluation based on monitoring and digital
analyzing for Zhangmu ancient landslide of
China-Nepal Highway. In: Yan X P, Yi P, Wu C Z, etal, eds. First International Conference on
Transportation Information and Safety (ICTIS),
Wuhan, China, pp. 1477-1491.
Xie LP, Li DH, Qiao G, et al. (2003): The added
engineering geological investigation report on
Zhangmu landslide prevention project Phase II,
Tibet,
Sichuan:Sichuan
Huadi
Building
Engineering CO., LTD. (Technical Report, in
Chinese).
Yi SM, Tang HM. (1996): The fractal feature of
Zhangmu landslides group in Tibet and its
significance, Journal of Changchun University ofEarth Science, Vol. 26, No. 4, pp. 392-397 (inChinese).
Zhu J, Cai QE, Jiang HB. (2010): Deoformation
monitoring and analysis of ancient landslide at
Zhangmu Port in Tibet. Journal of Engineering
Geology, Vol. 18, No. 1, pp. 66-70 (in Chinese).

Acknowledgements
The author appreciates the supports from Prof.
Wei Fangqiang, Prof. Chen Ningsheng, Prof. Wang
Quancai, Prof. He Siming, Doctor Li Zhanlu andDirector Dong of Land and Resources Bureau in
Nyalam County. The research reported in this paper
was supported by National Natural Science
Foundation of China (No.41330643).

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