Open Geosci. 2015; 1:637–645
Research Article
Open Access
Thanakrit Thongkhao, Sumet Phantuwongraj, Montri Choowong*, Thanop Thitimakorn, and
Punya Charusiri
Geological and engineering analysis of residual
soil for forewarning landslide from highland area
in northern Thailand
DOI 10.1515/geo-2015-0059
Received Jan 14, 2015; accepted Aug 27, 2015
1 Introduction
Abstract: One devastating landslide event in northern
Thailand occurred in 2006 at Ban Nong Pla village, Chiang
Klang highland of Nan province after, a massive amount
of residual soil moved from upstream to downstream, via
creek tributaries, into a main stream after five days of unusual heavy rainfall. In this paper, the geological and engineering properties of residual soil derived from sedimentary rocks were analyzed and integrated. Geological mapping, electrical resistivity survey and test pits were carried out along three transect lines together with systematic collection of undisturbed and disturbed residual soil
samples. As a result, the average moisture content in soil
is 24.83% with average specific gravity of 2.68, whereas the
liquid limit is 44.93%, plastic limit is 29.35% and plastic index is 15.58%. The cohesion of soil ranges between 0.096–
1.196 ksc and the angle of internal friction is between 11.51
and 35.78 degrees. This suggests that the toughness properties of soil change when moisture content increases. Results from electrical resistivity survey reveal that soil thicknesses above the bedrock along three transects range from
2 to 9 m. The soil shear strength reach the rate of high decreases in the range of 72 to 95.6% for residual soil from
shale, siltstone and sandstone, respectively. Strength of
soil decreases when the moisture content in soil increases.
Shear strength also decreases when the moisture content
changes. Therefore, the natural soil slope in the study area
will be stable when the moisture content in soil level is
equal to one, but when the moisture content between soil
particle increases, strength of soil will decrease resulting
in soil strength decreasing.
Landslide is a worldwide natural hazard, which in most
cases, occurs as a debris flow and soil creep. In a tropical climate region, landslide is commonly caused by intense and continuous heavy rainfall [1]. Therefore, a better
understanding of landslide processes requires the precise
characterisation of the triggering factors and relationship
among geological and engineering parameters. The triggering factors are often time dependent and for this reason
it is complicate to have a quantitative approach without a
permanent in-situ investigation measurement [2]. In most
of the landslide processes, loss of soil equilibrium caused
by heavy rainfall is considered as one of the most important triggering factors.
Thailand is located in a warm and tropical climate
region. The tropical monsoons and typhoons from both
the Andaman Sea and the South China Sea contribute
to the heavy rain inducing landslide to occur around the
country. Rainy season starts in June and commonly produces over discharge in the basins locating in the northern part of the country. Beside the effect of heavy rain in
the granitic terranes from the north that triggered landslides, but long and continue heavy rain in southern Thailand also causes the overbank full discharge leading to soil
creep in many places [3–5]. The average annual rainfall
ranges from 1,000 to 1,500 mm for northern, northeastern
and central parts of the country, whereas at the eastern
and southern part of the country, rainfall is between 2,000
to 3,000 mm. Rainfall inducing landslides normally occurs
in mountainous area due to single intent or long period
of rain. In case of prolonged rainfall, the flood and debris
flow commonly follow landslide and cause more damages
to villages along flow passages and on the alluvial plain.
Keywords: landslide; multi-stage direct shear test; shear
strength; Nan; northern Thailand
Thanakrit Thongkhao, Sumet Phantuwongraj, Thanop Thitimakorn, Punya Charusiri: Department of Geology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand
*Corresponding Author: Montri Choowong: Department of Geology, Faculty of Science, Chulalongkorn University, Bangkok 10330,
Thailand; Email: [email protected]; [email protected]
© 2015 Thanakrit Thongkhao et al., published by De Gruyter Open.
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License.
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failure of slope from the highland or steep terrain in northern Thailand is often reported to
occur mostly in conjunction with flash flooding from heavy rainfall and in those areas where
human activities such as exploitation of land by changing soil slope for agriculture purpose
are present. The Ban Nong Pla area is considered to be situated in one of the landslide risk
zones.
638 | Thanakrit Thongkhao et al.
Figure 3. Location of a village and present condition of highland forest and agriculture.
Figure 1. Digital Elevation Model (DEM) showing the study area (red square) covering the
Figure 1:
Digital
Elevation
Model
showing
the(left)
study
areamap
(red
northern
provinces
of Thailand
and(DEM)
Thai-Laos
PDR border
and index
of
indicating
where the study
area is located
(black square)
square)Thailand
covering
the northern
provinces
of Thailand
and(right).
Thai-Laos
PDR border (left) and index map of Thailand indicating where the
study area is located (black square) (right).
Figure2.2:
Land
useland
andcover
land
cover
fromarea,
theNan
study
area,
Nan
highFigure
Land
use and
from
the study
highland.
Ban
Nong
Pla
community
of two smallconsists
villages. One
located
on upper
part and
the other
land. Ban
Nongconsists
Pla community
ofistwo
small
villages.
One
is in the lower part of the mountain.
is located on upper part and the other is in the lower part of the
mountain.
Apart from 2006 Nan landslide, other well-known
landslide events in northern Thailand, in the past decades,
has been recorded. One of these events occurred on 4 May
Figure 3: Location of a village and present condition of highland
forest
and agriculture.
THE METHODOLOGIES
Satellite image and interpretation
2001TheatTHEOS
Wang
Chin
Phrae and
province.
ThisDevelopment
megasatellite
imagedistrict,
from Geo-Informatics
Space Technology
Agency is firstly applied in this study for interpreting and classifying patterns of landslide
landslide
wascover,
onetype
of ofthe
country’s
worst
records.
Land-of
scars, land-use/land
common
and unusual
agricultures,
and boundary
existing forest. Geographical reference data were digitized with the coordinates either in
slide
subsequently
extended
to
Phee
Pun
Num
mountain
terms of latitude and longitude or columns and rows. Attribute data associate a numerical
code to each
cell or set of
coordinates
and for each variable,
or to represent actual
values (900
range
covering
Phrae,
Lampang
and Sukhothai
provinces.
m elevation, 25 degrees slope gradient) or to connote categorical data types (land-uses,
vegetation
type,
land
cover,
rock
type)
[12].
The number of casualties has reached more than 30 peoAfter geomorphological map has been created, test pits and shallow excavation along
ples
and caused
losses
of more
milthe prospected
transects the
were economic
conducted. These
combined
methods than
provide300
an academic
mean of acquiring a very detailed record of the complex soil conditions which often exist
lion
baht
[6].
Soon
after,
at
Nam
Kor-Nam
Chun
area,
Lomnear to the ground surface. It is worth remembering, however, that test pits and other outcrops
can also be used for in situ testing and to obtain high quality samples. Disturbed and
sak
district, Phetchabun province, a landslide occurred
undisturbed soil samples from these methods were analyzed for both physical and
engineering
properties.
in August
2001, killing 150 people and damaging about
Two-dimensional
resistivity
survey of 645 million baht of eco600
houses electrical
with total
amount
nomic
loss. During
time,
the amounts
of rainfalls
Two-dimensional
electricalthat
resistivity
measurement
has been carried
out aiming to
distinguish a layer of residual soil from rock basement. Basic modes of operation include the
were
reported
up
to
150
mm
[7].
In
May,
2004
at
Mae
Raprofiling (mapping), vertical electrical sounding (VES), combined sounding and profiling
(two-dimensional
resistivity
imaging),
and
electrical
resistivity
tomography
(ERT).
The
mat district, Tak province, one mega-landslide determined
images which are obtained (apparent resistivity pseudo sections) are processed by inversion
the loss of 400 casualties, 2,500 damaged houses, and
500 million baht of economic loss. There were more than
250 mm of rainfall prior to the landslide movement [8].
After these events, interested areas were included in the
“landslide hazard map” produced by the Land Development Department (1:250,000 scale) [10] and associated disasters from nine districts of Nan province [11]. This is the
main reason why scientists seem to consider the amount
of rainfall as the main trigger mechanism for mass movement. Notably, all landslide events occur in rainy season.
2 The study area
The study area lies within a small highland village named
Ban Nong Pla that is the center of the study area and is
located in a valley with a steep slope (Figure 1). The location of the village is reported as one of high potential
landslide risk areas in Nan province [9]. The area is surrounded by mountains with elevation ranging from 700
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test (ASTM D 4221-11), Atterberg’s limits test (ASTM D 4318), standard compaction test
(ASTM D 698-78) and permeability test (ASTM D 2434-68).
Undisturbed soil samples were collected at the interface level between bed rock and
residual soil for testing the shear strength parameter, cohesion and friction angle including
consolidation drained test from total 8 pits (also see locations in Figure 4). Consolidation
Analysis
residual
undrained test was made only from BNP ST9 because only one multi-stage
direct of
shear
test is soil for forewarning landslide from highland area
needed to find shear strength parameter.
|
639
After geomorphological map has been created, test
pits and shallow excavation along the prospected transects were conducted. These combined methods provide
an academic mean of acquiring a very detailed record of
the complex soil conditions which often exist near to the
ground surface. It is worth remembering, however, that
test pits and other outcrops can also be used for in situ
testing and to obtain high quality samples. Disturbed and
undisturbed soil samples from these methods were analyzed for both physical and engineering properties.
3.2 Two-dimensional electrical resistivity
survey
Figure 4. Two-dimensional resistivity line surveys (dark lines) and locations of pits and soil
Figure
4: (red
Two-dimensional
resistivity line surveys (dark lines) and
sampling
dots)
Two-dimensional
locations of pits and soil sampling (red dots).
to 1,200 m above the mean sea level. Streams within subwatershed flow from southeastern to northwestern direction into the Huai Nam Puea main stream (Figure 2). Land
use in this area can be classified mostly as temporary agriculture such as short-life rice, lychee, and corn for animal
feeding. Some areas are still dense forest without landslide
scar (Figure 3). Considering that the failure of slope from
the highland or steep terrain in northern Thailand is often
reported to occur mostly in conjunction with flash flooding
from heavy rainfall and in those areas where human activities such as exploitation of land by changing soil slope for
agriculture purpose are present. The Ban Nong Pla area is
considered to be situated in one of the landslide risk zones.
electrical resistivity measurement has
been carried out aiming to distinguish a layer of residual
soil from rock basement. Basic modes of operation include
the profiling (mapping), vertical electrical sounding (VES),
combined sounding and profiling (two-dimensional resistivity imaging), and electrical resistivity tomography
(ERT). The images which are obtained (apparent resistivity pseudo sections) are processed by inversion software.
The multi-electrode resistivity technique consist a multicore cable with as many conductors (24, 48, 72, 96 and so
on) as electrodes plugged into the ground at a fixed spacing, every 5 m. Three resistivity lines were carried out in
this study including BNP-1, BNP-2, and BNP-3 in the upper
and lower slopes of a village (Figure 4).
3.3 Measurement of soil shear strength
3 The methodologies
3.1 Satellite image and interpretation
The THEOS satellite image from Geo-Informatics and
Space Technology Development Agency is firstly applied
in this study for interpreting and classifying patterns of
landslide scars, land-use/land cover, type of common and
unusual agricultures, and boundary of existing forest. Geographical reference data were digitized with the coordinates either in terms of latitude and longitude or columns
and rows. Attribute data associate a numerical code to
each cell or set of coordinates and for each variable, or to
represent actual values (900 m elevation, 25 degrees slope
gradient) or to connote categorical data types (land-uses,
vegetation type, land cover, rock type) [12].
The shear strength of soil mass is an internal resistance
per unit area. Soil mass can offer to resist failure and sliding along any planes inside it. One must understand the
nature of shearing resistance in order to analyze soil stability problems such as bearing capacity, slope stability,
and lateral pressure on earth-retain structure [13].
The unsaturated shear strength characteristic of residual soils as function of the degree of saturation, was studied in the laboratory using the multi-stage direct shear
test. The shear strength was completed by using multistage loading over a range of net normal stresses and shear
stress values. The physical properties of residual soil samples were analyzed in geotechnical engineering laboratory
by using the testing method of American Society for Testing and Materials (ASTM) including total unit weight, water content test (ASTM D 2216), specific gravity test (ASTM
D 854), sieve analysis test (ASTM D 422), hydrometer test
(ASTM D 4221-11), Atterberg’s limits test (ASTM D 4318),
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640 | Thanakrit Thongkhao et al.
using both consolidated and unconsolidated, undrained
triaxial tests with pore pressure measurements. At each
stage, a single soil sample was sheared until failure, followed by an increase in confining pressure leading to
shearing [14]. Multi-stage direct shear test should be limited to soil that is not sensitive to change in structure. Consolidated drained triaxial and consolidated undrained triaxial tests were performed with pore pressure measurement on both undisturbed and remolded soils [15]. This
test is not recommended for drained compression test on
sensitive soils. However, there is benefit in using multistage triaxial test on undisturbed residual soils mainly
composed of weathered rhyolite and granite, which have
great variation in their properties [16]. The results are practically indistinguishable from conventional test.
Unlike conventional shear strength test for soils that
use several soil specimens, a multi-stage test uses a single
soil specimen and shears a sample in stage with increasing confining stress. The multi-stage test is not an ASTM
standard method for obtaining total or effective stress parameters, but has been widely used in practice. The multistage test has been adapted to both triaxial and direct
shear tests, especially when there are difficulties in sampling and sample preparation. It has been applied to rocks,
undisturbed submarine soils, undisturbed silty sand and
undisturbed residual soils [17]. The benefit of multi-stage
test we expected in this study includes (1) the effects of
sample variability are eliminated, (2) the time required for
sample preparation and testing is minimized, and (3) the
overall cost for the tests is reduced [18].
Figure 5. Boundary of residual soil and rock basements (dark dash line) from resistivity line
Figure
5: Boundary
soil
and
basements
(dark2-9dash
BNP-1
(A), BNP-2of
(B)residual
and BNP-3
(C).
Soilrock
thicknesses
range from
m
line) from resistivity line BNP-1 (A), BNP-2 (B) and BNP-3 (C). Soil
thicknesses range from 2–9 m.
standard compaction test (ASTM D 698-78) and permeability test (ASTM D 2434-68).
Undisturbed soil samples were collected at the interface level between bed rock and residual soil for testing the
shear strength parameter, cohesion and friction angle including consolidation drained test from total 8 pits (also
see locations in Figure 4). Consolidation undrained test
was made only from BNP ST9 because only one multi-stage
direct shear test is needed to find shear strength parameter.
3.4 Multi-stage direct shear test
Multi-stage direct shear test is a method to measure shear
strength of undisturbed, partially saturated silty clay by
4 Results
4.1 Soil thickness from two-dimensional
electrical resistivity tomography (ERT)
Ban Nong Pla village is located in a small hill bounded by
steep slope at higher elevation from a village and is surrounded by deep valleys. Dense forest has been largely
removed for agriculture purposes. The development of
land exploitation tends to move to higher elevation of the
mountains, causing a reduction of dense forestry. In this
study, we operated two-dimensional resistivity survey to
distinguish layers of residual soil from bed rock. The evaluation of the thickness of soil overlying on rock basement
is important for calculating the possible volume of soil that
can collapse. Three ERT lines we preformed (BNP-1, BNP-2,
and BNP-3, Figure 5). BNP-1 is located from the southeastern to northwestern direction (SE-NW), BNP-2 is designed
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Analysis of residual soil for forewarning landslide from highland area
| 641
Table 1: Classification of soil types according to size and grain size distribution from 11 disturbed soil samples.
Type of soil
Gravel
Sand
Silt
> 2.0
2.0–0.075
0.075–0.002
Shale
BNP-ST1
15.34%
12.64%
41.74%
Shale
BNP-ST2
14.34%
13.62%
41.46%
Shale
BNP-ST3
27.12%
8.92%
34.95%
Shale
BNP-ST4
11.86%
9.49%
42.56%
Siltstone
BNP-ST5
6.14%
14.74%
35.63%
Siltstone
BNP-ST6
0.37%
13.49%
35.89%
Shale
BNP-ST7
19.19%
13.94%
34.06%
Siltstone
BNP-ST8
3.32%
30.55%
35.07%
Siltstone
BNP-ST9
0.12%
37.71%
29.92%
Shale
BNP-ST10
17.51%
11.48%
38.64%
Sandstone
BNP-ST11
0.37%
65.82%
25.91%
Cu is Uniformity Coeflcient, Cc is Coeflcient of gradation
Parent rock
Test Pit
Clay
< 0.002
30.28%
30.58%
29.01%
36.09%
43.49%
50.25%
32.81%
31.06%
32.25%
32.37%
7.9%
Cu
Cc
51.42
0.0083
Figure 6. Grain size distribution of residual soil from weathered parent rocks in the area
Figure 6: Grain size distribution of residual soil from weathered parent rocks in the area.
The coefficient permeability values of residual soil from sandstone, shale and siltstone
between
10-7 cm/sec.
Therefore,
wateronfrom
rain fallcharts
to soilby
surface
from the east to the are
west
(E-W)10-6
andtoBNP-3
is oriented
in whenever
sified based
plasticity
Unified Soil Classificawith higherdirection
rate than (SE-NW).
permeability of soil, water
in soil
will CL-ML
increaseand
and SM.
then They indicate that
southeastern to northwestern
tionlevel
System
aslayer
ML-CL,
causing a decrease in stability of soil slope, and subsequently increase the risk for landslide.
the toughness
of size
soil and
cangrain
change when the moisThe classification in type of soil from 11 disturbed
soil samplesproperties
according to
ture content increases [19].
size distribution is summarized in Table 1.
From Table
of gravel,
17.36% of values
sand, of residual soil
4.2 Physical laboratory
tests1, residual soil from shale consists
The15.22%
coefficient
permeability
30.98% of silt and 36.42% of clay. Residual soil from siltstone includes quantity of 7.84 %
fromofsandstone,
shale
and sandstone
siltstonehas
are between 10–6 to
gravel, 15.95% of sand, 38.65% of silt and 37.55%
clay. Residual
soil from
Residual soil from weathered
parent
rocksofsuch
shale,
cm/sec.
Therefore,
whenever
0.37% of gravel,
65.82%
sand,as7.9%
of silt 10–7
and 25.91%
of clay.
It can be
concludedwater
that from rain fall to
residual
weathered
parent rocks
such as
shale,
siltstone
and
sandstone
in
siltstone and sandstone
in soil
thefrom
study
area includes
high
soil
surface
with
higher
raterecognized
than permeability
of soil, wathis study area has high percentage of fine grained texture and can be qualified to poor
percentage of fine grained
texture
(Figure
6).
It
is
character
level
in
soil
layer
will
increase
and
then
causing
a dedrainage that can absorb high amount of water during rain. Furthermore, shear strength
terized as gap and uniform
grades.
Most
of
residual
soils
crease
in
stability
of
soil
slope,
and
subsequently
increase
decreases, increasing the possibility of the soil to slide.
shear strength
testLimit,
of soil at
content
using in type of soil from
in the study area showsResults
a low of
plasticity
(Liquid
thenatural
risk formoisture
landslide.
The condition
classification
consolidated drained test are shown in Table 2. The consolidated drained direct shear test in
LL < 47% and Plasticity
Index, PI = 15.58%). Soils are clasnatural water content condition shows a cohesion value that tends to increase when a degree
of saturation is low. The cohesion of soil is between 0.223 to 0.714 ksc. Friction angle ranges
from 11.51 to 25.30 degrees.
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ar strength of soil and volume of moisture content in soil mass
Analysis of engineering properties of residual soil from parent rocks was carried out for
uating the possibility of landslide. Shear strength of soil in natural condition decreases
642 | Thanakrit Thongkhao et al.
n moisture content increases [25]. Based on shear strength from unconsolidated drained
(from 8 test pits), the cohesion of soil ranges from 0.096 to 1.196 ksc. The angle of
2: Shear
strength
of soil at
natural moisture
content condition
usingtest
the consolidated
rnal friction is Table
between
11.51
to 35.78
degrees.
Consolidated
undrained
(from 1 testdrained test.
) ranges from 0.18 to 0.617 ksc and angle of internal friction is between 19.45 to 29.80
Test
pit
WCshear strengthDepth
Degree
of
c′
rees. Therefore, the
monitoring
of soil
moisture content
in natural
ditions is necessary for analyzing soil %
stability problems.(m)In addition, weathering
shear strength
(ksc)
meter (i.e., cohesionTPand
friction
angle)
should
be
used
to
analyze
stability
model
in
1
22.02
2.0
Grade V
0.223
er to find a plane of failure
factor of soil slope2.5
and this information
TP 2 and a safety23.75
Grademay
V be
0.315
d in combination withTP
geophysical
survey
(i.e. ERT survey). 2.5
3
22.85
Grade V
0.501
In this study, it hasTP
been
shown
how
shear
strength
of
soil
decreases
when
the
moisture
4
33.04
0.8
Grade VI
0.433
ent changes. Rate ofTPshear
72.11% to 98.78%,
to
5 strength decreases
27.57 in a range of0.6
Grade83%
V
0.455
20% and 79.6% to TP95.6%
for residual
sandstone,
6
26.87soil from shale,
1.0 siltstone and
Grade
VI
0.525
ectively. Strength ofTPsoil
decreases when
the moisture content
in soil mass
increases
7
19.98
0.8
Grade VI
0.714
inuously. Therefore, it can be concluded that a failure of natural slope in the study area
TP 8
15.81
1.2
Grade VI
0.408
occur when the moisture content between soil particles increases leading to a decrease in
ngth of soil.
′
Φ
(degree)
14.77
20.47
25.30
23.08
11.51
17.82
15.17
21.90
is low. The cohesion of soil is between 0.223 to 0.714 ksc.
Friction angle ranges from 11.51 to 25.30 degrees.
5 Discussions
5.1 Types of landslide and their controlling
factors
Criteria for classifying possible landslide and erosion of
hillside in highland area are based on several mechanisms such as types of sediments, trace and shape of landslide and estimated amount of water involved in the landFigure 7: Total cohesion and degree of saturation (TP 1, TP 4 and TP
[20]. Field mapping and landslide classification were
Figure 7. Total cohesion and degree of saturation (TP 1, TP 2 andslide
TP 3)
8).
applied in this study, and they are based on a principle
of classification of avalanche material and type of move11 disturbed soil samples according to size and grain size ment [1]. Possible types of landslide in the study area can
be classified as creep and lateral spread. The two possible
distribution is summarized in Table 1.
From Table 1, residual soil from shale consists 15.22% types reflect the diversity of factors which are responsible
of gravel, 17.36% of sand, 30.98% of silt and 36.42% of clay. for their origin. Following a detailed geological field mapResidual soil from siltstone includes quantity of 7.84% ping, possible factors producing slope movements in this
gravel, 15.95% of sand, 38.65% of silt and 37.55% of clay. area are classified as (1) change of slope gradient [22], (2)
Residual soil from sandstone has 0.37% of gravel, 65.82% rock types and degree of weathering [23] and (3) changes
of sand, 7.9% of silt and 25.91% of clay. It can be con- of water content in soil due to heavy rainfall [24].
cluded that residual soil from weathered parent rocks such
as shale, siltstone and sandstone recognized in this study
area has high percentage of fine grained texture and can be
qualified to poor drainage that can absorb high amount of
water during rain. Furthermore, shear strength decreases,
increasing the possibility of the soil to slide.
Results of shear strength test of soil at natural moisture content condition using consolidated drained test are
shown in Table 2. The consolidated drained direct shear
test in natural water content condition shows a cohesion
value that tends to increase when a degree of saturation
5.2 Relation between physical and
engineering properties of residual soil
5.2.1 Total cohesion of soil and degree of saturation
From Figure 7, it can be recognized that shear strength of
residual soil decreases when moisture content increases.
The consolidated drained test of a cohesion in soil ranges
from 0.096 to 1.196 ksc of residual soil derived from shale
and siltstone and 0.18 to 0.617 ksc from sandstone. There-
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reaches a critical thickness (equal or greater than 1 m). In this area, a degree of saturation is
between 60-70%. This range of saturation makes factor of safety greater than 1. However, if a
degree of saturation is greater than or equal 80%, factor of safety of soil slope will be less
Analysis of
than 1 and this can generate failure (Figure
8).residual soil for forewarning landslide from highland area | 643
Figure 8: Plots ofFigure
factor of safety
versus
angleof(left)
and thickness
8. Plots
ofslope
factor
safety
versus (right).
slope
angle (left) and thickness (right)
fore, a total cohesion of soil decreases when volume mois- 83% to 91.20% and 79.6% to 95.6% for residual soil from
CONCLUSION
ture content
in soil increases. Also, it is believed that mois- shale, siltstone and sandstone, respectively. Strength of
ture content is an important factor affecting shear strength soil decreases when the moisture content in soil mass inis a severe
natural
disaster
often continuously.
devastates highland
of soil. ShearLandslide
strength decreases
when
moisture
contentthatcreases
Therefore,areas
it canof
beThailand.
concluded that
Landslide
often
occurs
after
heavy
rainfall
resulting
in
a
loss
of
stability
and
equilibrium
in soil increases. An increase in volume of water in soil a failure of natural slope in the study area
can occur when
among
residual
soilmaking
and weathered
basements.
The geological
properties
of residual
soilleadwill disturb
surface
tension,
cohesion ofrock
soil to
de- the moisture
content between
soil particles
increases
collected
in
this
area
includes
specific
gravity,
grain
size
distribution,
liquid
limit,
plastic
crease and the volume of water, then, increases. A friction ing to a decrease in strength of soil.
limit,
unit weight
and
moisture content. Engineering properties includes
between
soil plastic
particlesindex,
reducestotal
to a decrease
in soil
shear
shear
and
multi-state
direct
shear
tests. Also, a geophysical survey by means of ERT
strength
thatstrength
can impact
directly
to a stability
of soil
slope.
has been carried out. Results can be summarized5.2.3
as follows.
Factor of safety, slope angle, soil thickness and
(1) Residual soil in the study area is mainly degree
derived
from weathering processes of
of saturation
clastic
sedimentary
including shale, siltstone and sandstone. Most of residual
5.2.2 dominant
Shear strength
of soil
and volumerocks
of moisture
soil
is in
characterized
by low plasticity (LL <The
47%
and PI
This
content
soil mass
behavior
for =
soil15.58%).
slope failure
in means
upstreamthat
area of
properties of soil changes when moisture content
increases.
indicates
area
may
Huai
Nam PueaThis
basin,
Ban Nongthat
Pla the
village
is discussed
Analysis
of aengineering
properties
of residual soil from in terms of factor of safety (FS) and slope angle. Accordhave
high risk of
landslide.
parent rocks
carried
outtwo-dimensional
for evaluating the possibiling to the undisturbed
soil samples
a failurethat
of slope
(2)was
Result
from
electrical resistivity
survey (BNP-1,
2 andtest,
3) shows
ity of soil
landslide.
Shear
strength
of
soil
in
natural
concan
occur
where
slope
angle
of
the
area
is
greater
than
layer overlying on rock basements has a thickness between 2 to 9 m.
dition decreases when moisture content increases [25]. or equal 25 degrees. Based on analysis in stability of soil
Based on shear strength from unconsolidated drained test slope using a method of infinite slope, values of slope an(from 8 test pits), the cohesion of soil ranges from 0.096 gle that can cause a failure of slope is between 25–45 deto 1.196 ksc. The angle of internal friction is between 11.51 grees. This range of slope angle is in agreement with literto 35.78 degrees. Consolidated undrained test (from 1 test ature values [26]. Factor of safety of cut slope is also lower
pits) ranges from 0.18 to 0.617 ksc and angle of internal fric- than a natural slope. Critical angle of slope that can trigger
tion is between 19.45 to 29.80 degrees. Therefore, the mon- landslide is 17.1 degrees, which corresponds to FS equal to
itoring of soil shear strength moisture content in natural 1.3. Therefore, any construction having slope angle greater
conditions is necessary for analyzing soil stability prob- than 17.1 degrees may be at risk of failure slope. Analysis of
lems. In addition, shear strength parameter (i.e., cohe- slope stability by method of infinite slope shows that facsion and friction angle) should be used to analyze stability tor of safety decreases when soil slope reaches a critical
model in order to find a plane of failure and a safety factor thickness (equal or greater than 1 m). In this area, a degree
of soil slope and this information may be used in combi- of saturation is between 60–70%. This range of saturation
nation with geophysical survey (i.e. ERT survey).
makes factor of safety greater than 1. However, if a degree
In this study, it has been shown how shear strength of of saturation is greater than or equal 80%, factor of safety
soil decreases when the moisture content changes. Rate of of soil slope will be less than 1 and this can generate failure
shear strength decreases in a range of 72.11% to 98.78%, (Figure 8).
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644 | Thanakrit Thongkhao et al.
6 Conclusion
Landslide is a severe natural disaster that often devastates
highland areas of Thailand. Landslide often occurs after
heavy rainfall resulting in a loss of stability and equilibrium among residual soil and weathered rock basements.
The geological properties of residual soil collected in this
area includes specific gravity, grain size distribution, liquid limit, plastic limit, plastic index, total unit weight and
moisture content. Engineering properties includes shear
strength and multi-state direct shear tests. Also, a geophysical survey by means of ERT has been carried out. Results can be summarized as follows.
(1) Residual soil in the study area is mainly derived from
weathering processes of dominant clastic sedimentary rocks including shale, siltstone and sandstone.
Most of residual soil is characterized by low plasticity (LL < 47% and PI = 15.58%). This means that
properties of soil changes when moisture content increases. This indicates that the area may have a high
risk of landslide.
(2) Result from two-dimensional electrical resistivity
survey (BNP-1, 2 and 3) shows that soil layer overlying on rock basements has a thickness between 2 to
9 m.
(3) Size of soils in this area is characterized mostly as
fine-grained, which is sensitive for sliding whenever
water is induced.
(4) Based on unconsolidated undrained test, cohesion
of soil ranges from 0.096 to 1.196 ksc and angle of internal friction is between 11.51 to 35.78 degrees. Consolidated undrained test shows that cohesion of soil
ranges from 0.18 to 0.617 ksc and angle of internal
friction is between 19.45 to 29.80 degrees.
(5) Shear strength of soil will reach high decrease rate
in a range of 72.11% to 98.78%, 83% to 91.20% and
79.6% to 95.6% for residual soil from shale, siltstone
and sandstone, respectively. It reveals a relationship between geological and engineering properties
that shear strength decreases when moisture content changes.
(6) Strength of soil decreases when moisture content in
soil increases. Therefore, a natural soil slope in the
study area can be stable when moisture content in
soil level is equal to one, but when moisture content
in soil particles increases, strength of soil can be decreased, then, resulting in soil strength decreases.
An integration of geological and engineering field
method and laboratory testing in this paper highlighted
some significant relations among their properties. This integrated analysis of both perspectives, definitely, can be
used to forewarn future landslide, not only in this area,
but can also be extended to some other landslide high risk
areas that own a similarity in terms of geological and geographical conditions.
Acknowledgement: National Research University Project,
Office of Higher Education Commission provided fund to
MC and SP (WCU-045-CC-57). Graduate School of Chulalongkorn University provided M.Sc. research fund to TT.
We thanks Professor S. Soralump for discussing soil engineering properties. Thanks are also to Editor-in-chief and
anonymous reviewers that improve this manuscript significantly.
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