10th Asian Regional Conference of IAEG (2015)

Analyzing Failure Characteristics and Potential of Landslides in Hai Van Mountain,
VietnamTien PHAM(1), Kyoji SASSA(2), Kaoru TAKARA(1), Khang DANG(1) and Loi DOAN(3)(1) Disaster Prevention Research Institute, Kyoto University, Japan
E-mail:phamtiengtvt@gmail.com(2) International Consortium on Landslides, (3) Institute of Transport Science and Technology, Vietnam
Abstract
Many slope failures and reactive landslides took place in Hai Van mountain where islocated in weathered granite rocks and monsoon tropical region of Vietnam. Those
landslide disasters not only destroyed the infrastructure of the national railway but also it
posed a serious threat to safe railway operations in many times. The objectives of thisresearch topic are to study failure characteristics and to assess the potential of landslide
occurrences on slopes induced by factors of rainfalls and earthquakes. Some causative
factors of landslide events such as geology and topography were explored during a site
survey in 2014. In this trip, two of landslide prone soil samples, namely less weathered
granite materials and strongly weathered granite materials, were taken at failed slopes tofurther investigate their shear behaviors by performing laboratory ring shear tests. The test
results revealed that the mobility behavior of two landslide samples in the same conditions
are quite different and dependent on the liquefaction at sliding surface under un-drained
conditions (Sassa, 2000). In this crucial regard, only landslide samples of the slightly
weathered granitic sands were highly susceptible to the sliding surface liquefaction andrapid motion while landslides of the strongly weathered granitic sands were not apt to moveat the high speed. In addition, an assessment of the potential of landslide occurrences
triggered by only rainfalls or earthquakes with ring shear tests showed that failures are more
likely to occur in less weathered granite material areas than the occurrence on slopes ofheavily weathered granite materials due to the smaller value of their shear strengthparameters under un-drained testing conditions.
Keywords: landslides, characteristics, granite materials, ring shear apparatus, Hai Van,
Vietnam1.

Introduction2006), Hiroshima in 1999 (Chigira, 2001), Shikoku
Island in 2004 (Dahal, 2008) in Japan. Similar
phenomena have occurred in southern Italy(Calcaterra et al., 1996), northeastern Spain (Palacios
et al., 2003) and Central Korea (Lee et al., 2002).
Among researches, some authors tried to correlated
the relationships between the distribution and densityof landslides, and precipitation or to analyzed
causative factors and failure characteristics in term of
petrologic texture and the weathering types of granite
rocks. While others attempted to employ methods ofslope
stability
analysisbyobtaining
physical-mechanical soil properties from laboratory
and filed tests. In addition, with an empiricalapproach, landslide-triggering thresholds dependingon rainfall intensities and duration were also
investigated so as interpret and predict its occurrencesin granite areas. Specifically, some results from

Since granite rocks are very susceptible to theprocesses of weathering and decomposition, a great
number of landslides have commonly occurred in
mountainous terrains of weathered granite rocks
worldwide. For example, numerous rainfall-induced
shallow landslides occurred or reoccurred in granitic
areas in Japan, which resulted in more than 1500
casualties over the last 70 years (Chigira, 2011). As
the most recent natural disaster, the Hiroshimalandslide triggered by the historically largest rainfall
intensity of 217 mm per 3 hours that caused 50
fatalities and 38 missing on 20 August, 2014 (Sassa etal., 2014). Due to great losses of people and property
from disasters, such kind of landslides have been
taking a great interest of many researchers, such as
Shimane in 1964 (Oyagi, 1968), Aichi in 1972 (Tobe,1

10th Asian Regional Conference of IAEG (2015)previous researches revealed that the landslide
occurrences are greatly related to the weatheringmanner and grade of granitic rock materials and thedisasters mostly occurred in slightly and/or
moderately weathered granitic rock regions. It meansthat landslides took place frequently in areas of
weakly weathered granite than in areas of strongly
weathered granite. Such characteristics andmechanism of landslides were reported by Durgin(1977), Oyagi, (1968) Yairi, (1973) and Suzuki et al.
(2002).
Although landslide occurrence are commonly
known in many granitic terrain places, its mechanismof failure has not been studied deeply because of thecomplexity of weathering materials and triggeringfactors like rainfalls. Therefore, this study firstly
presents some influencing factors on landslide
phenomena and then try to analyze the potential of its
occurrence in the slope behind the Hai Van station in
granitic rock area in Hai Van Mountain, Vietnam
through ring shear tests in order to interpret initiationand motion mechanism of landslides due to triggers
of rainfall or earthquake. The simulation of naturallandslides so far has been the first time to be applied

to a study case in Vietnam with ring shear apparatus
under un-drained conditions.
2.

The study area and a case study
The study area is located on Hai Van Mountain(Fig. 1), which is one part of the Annamite Range
dominated by a complex geological structure ofweathering granitic rocks and very high precipitationin monsoon tropical climate region. The annual
average rainfall mostly ranges from 2000 mm to 3600
mm and about 63 % to 85% of this amount falls
during the wet season between September andDecember. According to rainfall data collected at a
Da Nang rain gauge station being far away 17 km
southeast of Hai Van Mountain in the 38-year period
from 1975 to 2013, the annually 3-day maximum
accumulated precipitation varies between 200 mm
and 748.4 mm except for 133.8 mm in the year 2004.
The topography runs across dangerous cleavage
terrain due to tectogenesis with many fault systemfracture zones. There are a lot of slopes that arehighly susceptible to landslides around Hai Van area
in which an active slope failure on the slope behind
Hai Van station of the national railway at Km 766+

Fig. 1 Location of study area and the landslide in Hai Van Mountain
770 (hereinafter called as Hai Van station landslide)damages.
is selected as a case study for a detail investigation of
The main trigger of heavy and prolonged rainfalls
the mechanism. Currently, a set of equipments foris directly responsible to all previous events in 1999,
landslide movement monitoring and data collecting
2004 and 2005 (according to statistical data ofhas been setting up on the back slope of Hai Van
Management Unit for roads and Tam et al., 2008).
station, such as extensometers, alarming and earning
Besides, there is still a medium potential of landslide
warning extensometers, GPS stations, and rain
occurrence
triggeringby seismicity
duringgauges. A clear evidence of the movement of Hai
earthquakes because the study area is very near a
Van station landslide which is approximately 19 mm
large Da Nang-Khe Sanh fault zone with a maximum
was recorded due to a precipitation accumulation
magnitude of 5.5 degree in the Richter scale.
during the rainy season in 2013 (Asano, 2014). The
Therefore, both of rainfalls and earthquakes arelocation of Hai Van station has a great significance tocommonly known as one of the potential trigger that
increasing traffic flow efficiency along the national
can result in landslides in the Hai Van area.
railway. However, traffic safety and infrastructures ofthe station are always threatened by landslide hazard
3. Material and Methods
impacts, namely traffic congestions and structural2

10th Asian Regional Conference of IAEG (2015)

3.1 Site investigation and soil sampling
To take an overview of the latest landslide status
and its causative factors, an site survey was
conducted in May, 2014. During this trip, twolandslide prone soil samples were taken from thesurface of failed slopes near the Hai Van station ofwhich materials are assumed to be the same at thepotential sliding surface of the station landslides,
namely strongly weathered granite materials (HV1

sample) and slightly/moderately weathered granite
materials (HV2 sample). HV2 sample was taken at
the middle position of the mountain while HV1sample was from the mountain ridge (Fig. 2). HV1
sample is slightly silty sands and fine to course
grained in red brown color whereas HV2 is coarse
grained sands with whitish grey spotted black andsilk white. Grain-size distribution of samples ispresented in the Fig. 3.

Fig. 2 Location of soil samplingstate of their specimens with pre-decided normal andshear stress, depending on the specimen depth, slope
angle and unit weight of the soil. The depth ofpotential sliding surfaces of the Hai Van station
landslide was estimated in site investigation from 15
m to 20 m. Thus, the parameters of 230 kPa fornormal stress and 120 kPa for shear stress were usedin the calculation of the initial stress corresponding to
15 m of a depth and a slope angle of 26 degree.

Fig. 3 Grain-size distribution of the samples
3.2 Portable ring shear apparatus ICL-1
The un-drained portable ring shear apparatus
ICL-1 developed by Prof. Sassa and his colleagues inInternational Consortium on Landslides (ICL) in2010 for SAPTREPS projects in Croatia wasemployed in this study (Fig. 4). The capacity of
ICL-1 to keep un-drained condition is up to 1 MPa of
normal stress and pore-water pressure, that is almost
double than in previous versions (up to 500 kPa in
DPRI 5, 6 and 7) (Ostric, 2012). The maximum shear
velocity is only 5.3 cm/sec which is smaller than thatin previous versions due to a reduction in dimensions.
In order to investigate and compare failure
characteristics as well as the mechanism of two
different landslide prone samples, these samples werenormally consolidated under the same initial stressFig. 4 Portable ring shear apparatus ICL-1 with its
instrument box, monitoring box and control box
4.

Results

4.1 Site investigation
Field investigations indicated an active deep
landslide with main scarp (a black color line) andbody parts (a red color dotted line) on the slope3

10th Asian Regional Conference of IAEG (2015)

behind the Hai Van station (Fig. 5). It is very clear tosee that the landslide body have a tongue-shape and aconvex surface. Estimated sizeand extent oflandslide are about 150 m and 250 m of width andlength, respectively. Landslide types are translationalor rotational modes like a tongue-like shape landslide,which has a potential sliding surface existing in thelayer of weathered granitic rocks with a depth morethan 15 m.

materials like sands, rainwater is easily to infiltrate
into sub-surface resulting in increasing of pore-waterpressure of which poses the slope to failures.
(2) The complexity of morphological settings
The terrain in Hai Van Mountain is very complex
due to its topography running across dangerous
cleavage terrain. Although the angle of slopes iscommonly between 20 degree and 35 degree, entire
area is cleaved into a lot of valleys, gullies anderosion slots. Those factors pose the study areas tooccurrence of slope failures with diverse
mechanisms.
(3) Monsoon tropical climate
The climate firstly increases the weathering of
granite resulting in decreasing of shear resistance ofslope materials. Heavy and prolonged rainfalls
brought from monsoon climate generate pore waterpressure in soil layer on the slope resulting in theslides. Rainwater not only makes the weathering
more quickly by infiltration into rock cracks but also
it can erode loose materials on slopes and trigger
debris flow, earth flow or mud flow.
(4) The anthropic factor
Former construction and widening of the Hai Van
station on the national railway resulted in losing theequilibrium state of slopes through cutting down
foots of slopes or grading side slopes. According to areport on causes of the failure (Tam, 2005), after
undercutting the foot of the slope up to 50,000 cubic
meters for constructing Hai Van station, manytransversal long cracks of 20 centimeters occurred
sparsely in a large slope. Then the slope and its minor
blocks started to move down slowly until temporarily
constructing countermeasures were implemented
urgently in the year 2005.

Fig. 5 Landslide description in May, 2014 and its pastcondition (a photo taken by Institute of Transport
Science and Technology in Vietnam)
4.2Causative factors of landslides(1) Internal changes of slope material properties
Various materials of granite rocks with different
weathering grades were found around the landslidearea in the site survey such as a thin layer ofcolluviums, loose soil-like materials, slightly soft andfine grained materials, clayed or silty soil-likematerial and sand-like materials. The detailed
description of various weathered granite materials ispresented in the Fig. 6. Among them, granites are inboth fresh or decomposed types completely withstrong or less weathering degree in different layers
and directions. The weathering of granitic rocks not
only results in decreasing of material strength on theslopes but also the materials are very sensitive to

4.3 Results of ring shear test and its interpretation(1) Un-drained shear stress control tests
The shear stress control tests are not only tomeasure physical mechanical soil properties but thesimulation is also very useful to investigate failure
behavior of landslide during motion and post-failure
period (Sassa, 2004). The test results of two samples
are presented in the Fig. 7 that shows the effective
stress path (a red color line) and the total stress path(a black color line) as well as physical parameters
monitored including shear stress at peak τp and atsteady state τss, friction angle during motion φm, peakfriction angle φP, apparent friction angle φa(ss). For
HV2 sample, stress path that reached the failure line
(red color line) and then moved down rapidly along
the failure line to the steady state stress point at avery low value of residual strength (24.1 kPa). In
contrast, after reaching the failure line, shear
resistance of HV1 sample gradually decreased to a
constant value of a higher residual strength (93.2
kPa) before the stress path had a tendency to go
inversely along with a negative in pore-water

Fig. 6 Materials of weathered granite rocks
slide. In addition, in the layer of high permeable410th Asian Regional Conference of IAEG (2015)

pressure value. The reason is because there wasexcess pore-water pressure generation during shear
displacement after the failure of HV2 sample whereas
there was not much increasing of pore pressure for
HV1 sample. The peak friction angle (φP) of HV1
sample and HV2 sample are around 41.0° and 36.0°

with 143.5 kPa and 133.1 kPa of maximum shear
resistance, respectively. The friction angle duringmotion (φm) of two landslide samples at large
displacement are about 38.0° for HV1 soil sample
and 33.5° for HV2 sand sample.
(2) Rainfall-induced landslide simulation with pore-Fig. 7 Effective stress path of un-drained shear stress control test for HV1 sample (left) and HV2 sample (right)

Fig. 8 Effective stress path of pore-water pressure control tests on HV1 sample (left) and HV2 sample (right)
water pressure control tests
Rainfall-induced landslides were produced byincreasing gradually pore-water pressure simulating
the rise of ground-water level during rainfalls. In
these tests, both samples were consolidated to 230kPa
in normal stress and 120 kPa in shear stress. Thenpore-water pressure was increased up to 200 kPa at a
rate of 1.5 kPa/sec for HV2 sample and at a lower
rate of 0.2 kPa/sec for HV1 sample due to its low
permeability. The test results of two landslide
samples are shown in the Fig. 8, with red color line
showing effective stress path and black color line
showing total stress path.
The results show that HV1 sample failed around
95 kPa of pore-water pressure increment while failure
occurrence of HV2 sample was earlier to occur with
about 80 kPa of additional pore-water pressure value.
The critical pore pressure ratios of HV1 sample andHV2 sample (ru1 and ru2) are corresponding to 0.41
and 0.34. For the HV1 sample, the friction angle atpeak (φP) stayed at about 41.3° and for the HV2sample, this angle value was only 36.4°.
(3) Earthquake-induced landslide simulation with
un-drained cyclic loading tests
Initially the initial stress condition on sliding

surface was created at 230 kPa of normal stress and at
120 kPa of shear stress in the drained condition. Next,
the initial cycle of shear stress increment ±10 kPa insine curve and the second cycle of shear stressincrement is also ±10 kPa until 7th cycle and 5th cycleof shear stress increment for HV1 sample and HV2sample respectively under un-drained condition. The
results of cyclic tests of HV1 sample and HV2sample are presented in the Fig. 9&10, respectively.
Figures 9-(a) and 10-(a) present time series data ofcyclic tests, where the black line represents normal
stress, the red line shows the shear resistancemobilized on the sliding surface and the green line
shows the control signal for shear stress which wasgiven by the servo-stress control motor. During tests,
the pore-water pressure (a blue color line) wasmonitored along with the shear displacement (apurple color line). Figures 9-(b) and 10-(b) show thestress path, with a red color line showing effective
stress path and a blue color line showing total stresspath. Similarly to the results of un-drained shear
stress control tests, friction angles at peak oflandslides are approximately 41.1° for HV1 sample
and 36.5° for the HV2 sample.
Although shear stress of two samples reached the5

10th Asian Regional Conference of IAEG (2015)

failure line at the last cycles of cyclic loading, shear
behaviors of two samples are very different inpost-failure stage. Shear displacement ceased at large
displacement of 10 m for HV2 sample landslides
while movement of HV1 sample landslides stopped
after cyclic loading and start to move very slowly at
the next cycles. The rapidly accelerated motion was
produced after failure of HV2 sample. By contrast,

shear displacement and deformation of HV1 sample
were accumulated only during cyclic loading.
Consequently, at the last cycle of shear stress loading,there was no more shear displacement and pore-waterpressure generation. Thus, landslide movement
stopped at around 8.4 m of shear displacement before
the termination of simulation tests.

Fig. 9 Effective stress path (left) and time series of cyclic loading test for HV1 sample

Fig. 10 Effective stress path (left) and time series of cyclic loading test for HV2 sample
5.

Discussions

Those values show that HV1 sample is stronger than
HV2 sample in strength during shearing.
Potential of landslides occurrences can berevealed from its failure mechanism that is mainly
controlled by the nature of the weathered material
and mass structure of granite rocks. Based on the
results from the simulation of rainfall/earthquake
induced landslides, it showed that failures are more
likely to occur on slopes of less weathering granitic
rocks than that in slopes of strong weathering granitic
rocks due to less shear strength parameters of thesample in the less weathering granitic rocks region.
Basically, the motion of the landslides on theslope behind Hai Van station can be dominatedsignificantly by shear behaviors of both HV1 sample
soil layer and the HV2 sample soil layer. Amongthem, HV2 sample is highly susceptible to rapid
motion while HV1 sample shows a low mobility
which can trigger a slow motion of landslides only.
Since a very slow movement of the Hai Van station
landslide monitored in the 2013 rainy season, a highpossibility of potential sliding surface of landslides
exists in the layer of HV1 sample. From the point of

Test results show that HV1 sample have notgenerated much amount of pore-water pressureduring shearing while HV2 sample produced excess
pore-water pressure. Similarly, there was a much
reduction in shear strength of HV2 sample whereas
HV1 samples showed a small decreasing in shear
strength. It means that only HV2 sample experiencedliquefaction at sliding surface (Sassa, 1996 and 2000)
while the phenomena did not occur in HV1 sample.
Basically, this different characteristic mainly resultsfrom particular shear behaviors of two landslide
prone samples of weathered granite rocks.
As seen in the results, the effective friction angle
and peak angle of HV1 sample (41.0° and 36.0°,
respectively) is higher than that in HV2 sample(38.0° in effective friction angle and 33.5° in peakangle). Similarly, the apparent friction angle 22.1° for
HV1 sample is also larger than 5.9° of apparent
friction angle of HV2 sample. Moreover, the peak
shear strength of HV1 sample is about 143.5 kPa
while that value is only 133.1 kPa for HV2 sample.

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10th Asian Regional Conference of IAEG (2015)

view, the landslide will continue moving at very slow
speed like a creep landslide under impacts of rainfalls
along a sliding surface existing in the layer ofweathered granite rocks.
6.

Chigira, M. (2001): Micro-sheeting of granite and its
relationship with landslide specifically after theheavy rainstorm in June 1999, Hiroshima Prefecture,
Japan. Engineering Geology 59, pp. 219-231.
Chigira, M., Mohamad, Z., et al. (2011): Landslides
in weathered granitic rocks in Japan and Malaysia,
Bulletin of the Geological Society of Malaysia 57(2011) 1-6.
Dahal, R.K., Hasegawa, S., et al. (2008): Failure
characteristics
ofrainfall-induced
shallowlandslides in granitic terrains of Shikoku Island ofJapan, Environmental Geology 56 (7): 1295-1310.
Durgin, P.B. (1977): Landslides and the weatheringof granitic rocks. Geological Society of America
Reviews in Engineering Geology, 3, pp. 127-131.
Ikeda, H. (1975): Geomorphology and weathering
condition of granite in the upper Didogawa
Mountains, in the area southern Shigaraki and Tarao,
the report of Ministry of construction, pp.1-39.
Ostric, M., Liutic, K. (2012): Portable Ring Shear
Apparatus and its application on Croatia Landslides,
Annuals of Disaster Prevention Research Institute,
Kyoto University, No. 55B, pp. 57-65.
Oyagi, N. (1968): Weathering-zone structure andlandslides of the area of granitic rocks in
Kamo-Daito, Shimane Prefecture. Reports of
Cooperative Research for Disaster Prevention,
National Research Center for Disaster Prevention,
Vol.14, pp. 113-127.
Ochiai, H. (2014): Development of landslide
monitoring and early warning system at Hai Van
station. Progress report of Working Group 4.
Proceedings of SATREPS Workshop on Landslides,
pp. 26-31.
Palacios, D., Garcia, R., Rubio, V. and Vigil, R.
(2003) Debris flows in a weathered granitic massif:
Sierra degredos, Spain. Catena, 51, 115–14
Lee, S., Chwae, U. and Min, K. (2002) Landslide
susceptibility mapping by correlation between
topography and geological structure the Janghungarea, Korea. Geomorphology, 1153.
Sassa, K. (1996): Prediction of earthquake induced
landslides. Proceedings of 7th International
Symposium on Landslides. A.A. Balkema.
Trondheim, 17-21 June, Vol 1, pp. 115-132.
Sassa, K. (2000): Mechanism of flows in granularsoils. Proceedings of GeoEng2000, Melbourne, Vol
1, pp. 1671-1702.
Sassa, K., Fukuoka, H., Wang, G., Ishikawa, N.
(2004): Un-drained dynamic loading ring shear
apparatus and its application to landslide dynamics.
Landslides, 1(1), pp. 7-19.
Sassa, K., Fukuoka, H., et al. (2014): Initiation
Mechanism of Rapid and Long Run out Landslideand Simulation of Hiroshima Landslide Disasters
using the Integrated Simulation Model (LS-RAPID)
Proceeding of International Forum“Urbanization

Conclusions

The understanding of failure mechanisms andassessment of landslide potential can be examinedreliably by using ring shear apparatus ICL-1 in thisresearch, including shear stress control tests,
pore-water pressure control test and cyclic loading
tests. As presented in the results and discussion partsof this paper, slope failures are more susceptible tooccur in the layer of HV2 sample than theoccurrences in HV1 sample because the clayed-likesoil sample is stronger than the sand sample in shear
resistance as well as HV1 sample is more resilient tosliding surface liquefaction than the same behavior of
HV2 sample. Consequently, the landslide movement
is very rapid for landslides of HV2 samples but isvery slow for landslides of HV1 samples. The failure
characteristics of different landslide soil samples
show that landslides are more likely to occur on
slopes of less weathering granitic rocks than theiroccurrences in slopes of strong weathering granitic
rocks under loading of rainfalls and earthquakes due
to lower shear strength parameters of the sample in
the less weathering granitic rocks region. This finding
is similar like findings from previous researches onlandslides in weathered granitic rocks regions, which
was found by other researches before. In short,landslides are highly susceptible to occur in bothslopes of Hai Van station and others in Hai Van
Mountain in the future. Therefore, it is very necessary
to take countermeasures to control the movement of
the slope as using structural measures as well as
monitoring and early warning system.
In addition, this research presented and analyzed
some causative factors of landslides in Hai Van
Mountain on which Hai Van landslides were inducedby combination of contributing factors including
climatic, morphological and geological conditions ofweathered granitic area, human activities as well as
rainfalls.
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10th Asian Regional Conference of IAEG (2015)

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