View
0
Download
0
Category
Preview:
Citation preview
50
th
IG
C
50th
INDIAN GEOTECHNICAL CONFERENCE
17th
– 19th
DECEMBER 2015, Pune, Maharashtra, India
Venue: College of Engineering (Estd. 1854), Pune, India
LANDSLIDE EARLY WARNING BASED ON GEOTECHNICAL SLOPE STABILITY
MODEL FOR THE GUWAHATI REGION
Chiranjib Prasad Sarma1, Arindam Dey
2, A. Murali Krishna
3
ABSTRACT
Landslides constitute a significant geohazard within the region of Guwahati city, rainfall being the
triggering factor for such occurrences. Comparison of historical records of rainfall and landslides in the
hill slopes of Guwahati highlights and establishes this correlation. The impact of rainwater infiltration in
causing landslides is widely recognized. Rainfall infiltration results in increase in the water content and
decrease in the matric suction thereby raising the unit weight and reducing the shear strength of soil in the
colluvium of the landslide. Hill slopes within the city of Guwahati consist of residual soils, often in
unsaturated condition, and therefore the conventional saturated soil mechanics approach to assessing the
stability of these slopes is inadequate. To assess the potential susceptibility to rainfall-induced landslide,
an effective modeling of changes in water content and matric suction in response to rainfall infiltration is
essential.
Speedy and unplanned urbanization within the city of Guwahati due to the phenomenal influx of
population over the past four decades have resulted in continuous conversion of land from non-urban use
to urban use, leading to rampant encroachment and earth-cutting on the hillslopes. Shrinkage of forest
cover has bought about a change in the surface and sub-surface hydrology. Economical incapacitated
societies are increasingly reluctant to invest money in structural measures that can reduce this natural risk.
Hence, the new issue is to implement a reliable decision support based on early warning systems aimed at
minimizing the loss of lives and property without investing in long-term, costly projects of ground
stabilization. The precursor to development of such a system is to quantify the correlation of landslide
occurrences to the destabilizing factors.
Geotechnical slope stability models are capable of providing detailed description of potential instability
under changing environmental and climatic conditions and are able to establish threshold values of the
triggering phenomenon; however, they are only rarely incorporated into landslide early warning systems.
1Chiranjib Prasad Sarma, Research Scholar, Indian Institute of Technology Guwahati, India, s.chiranjib@iitg.ernet.in
2Arindam Dey, Assistant Professor, Indian Institute of Technology Guwahati, India, arindam.dey@iitg.ernet.in
3A. Murali Krishna, Associate Professor, Indian Institute of Technology Guwahati, India, amurali@iitg.ernet.in
Chiranjib Prasad Sarma, Arindam Dey & A. Murali Krishna
In the context of constructing early warning systems in regions where extensive data on landslide
occurrences and associated rainfall are inexistent, such tools offer the possibility to establish thresholds
for measurable geotechnical parameters.
This study constitutes an investigation into the infiltration processes, and the mechanism leading to
reduction in hill slope stability. Seepage analysis is performed using SEEP/W for transient/steady state
conditions considering saturated / unsaturated material model and the computed pore-water pressure are
then used in SLOPE/W to evaluate the changes in stability with time applying limit equilibrium methods.
Two homogenous and isotropic slopes of height 30 m with an inclination of 45⁰ (1H:1V) and assigned
properties of residual soil typical to the hillslopes of Guwahati region was assumed for the analysis.
Rainfall infiltration consistent to storm events prevalent in this region has been considered and the factor
of safety of the slopes are evaluated and plotted against time. Figure 1 (a) gives the rainfall data of the
storm event of 4th
– 7th
October, 2004, while Figure 1 (b) gives the degradation of the stability condition
in the form of reducing factor of safety.
Fig. 1(a) Rainfall storm event history (afternoon of
4th October to the midnight of 7th October, 2004)
Fig. 1(b) Corresponding factor of safety vs. time
for slope composed of reddish silty clay (RS) and
pale yellow silty sand (PYS)
The study highlights the importance of considering the behavior of unsaturated soil in analysis and
predicting the stability of unsaturated residual soil slopes. Numerical transient seepage and slope stability
analysis was able to quantify the effect of infiltration on the stability of such natural slopes with time and
thus has the potential to be used in an early warning system against landslide hazard. The variation of
safety factor of the slope with time can be provided as a basis to develop method for the real-time
prediction of the rain-induced instability of slopes. However the analysis results depends greatly on the
input parameters and thus extensive field investigations, laboratory soil testing, and rainfall data needs to
be collected.
Keywords: Landslide early warning, Slope Stability, Rainfall Infiltration, Seepage, Permeability, Matric
Suction, SEEP/W, SLOPE/W
50
th
IG
C
50th
INDIAN GEOTECHNICAL CONFERENCE
17th
– 19th
DECEMBER 2015, Pune, Maharashtra, India
Venue: College of Engineering (Estd. 1854), Pune, India
LANDSLIDE EARLY WARNING BASED ON GEOTECHNICAL SLOPE
STABILITY MODEL FOR THE GUWAHATI REGION
Chiranjib Prasad Sarma, Research Scholar, Indian Institute of Technology Guwahati, s.chiranjib@iitg.ernet.in
Arindam Dey, Assistant Professor, Indian Institute of Technology Guwahati, arindamdeyiitk@gmail.com
A. Murali Krishna, Associate Professor, Indian Institute of Technology Guwahati, amurali@iitg.ernet.in
ABSTRACT: Rainfall-triggered landslides are a major geohazard in Guwahati region which cannot be
convincingly addressed by conventional slope-stability approaches. This study attempts to address this issue with
the help of advanced analysis considering the concepts of unsaturated soil mechanics involving transient/steady
state phenomenon aided by the variation of pore-water pressures for providing a predictive model for the potential
instability under rainfall infiltration process. It is observed that the matric suction and its effect on the variation of
permeability and degree of saturation along with the intensity and duration of rainfall event are critical factors in
assessing the state of instability.
INTRODUCTION
Historical records of landslide events suggest that
the phenomena in the Guwahati region are rain–
triggered. All landslides were reported to occur
when monsoon is at its peak or nearing completion,
i.e., by the month of July-August to the end of
September to mid-October [1,2]. A total
cumulative rainfall of 750 mm over three days,
between 5th
and 8th
October 2004, triggered many
landslides around Guwahati hillslope areas,
causing death of 17 persons and destruction to
property worth millions of rupees. The same storm
event triggered as many as 100 landslides of
varying dimensions along NH 44, Guwahati-
Shillong road within a reach of 80 km [3]. In the
month of June, 2012, several landslides occurred,
triggered by intense rainfall events throughout the
month.
Speedy and unplanned urbanization within the city
of Guwahati due to the phenomenal influx of
population over the past four decades have resulted
in continuous conversion of land from non-urban
use to urban use, leading to rampant encroachment
and earth-cutting on the hillslopes. Shrinkage of
forest cover has bought about a change in the
surface and sub-surface hydrology. Economical
incapacitated societies are increasingly reluctant to
invest in structural measures that can reduce this
natural risk. Hence, the new issue is to implement a
reliable decision support based on early warning
systems aimed at minimizing the loss of lives and
property without investing in long-term, costly
projects of ground stabilization. The precursor to
development of such a system is to quantify the
correlation of landslide occurrences to the
destabilizing factors.
The impact of rainwater infiltration in causing
landslides is widely recognized [4,5,6,7]. Rainfall
infiltration results in increase of the water content
and decrease in the matric suction thereby raising
the unit weight and reducing the shear strength of
soil within the colluvium of the landslide. Hill
slopes within the city of Guwahati consist of
residual soils in an unsaturated state [8,9],
therefore the conventional soil mechanics approach
in assessing the stability of these slopes proves to
be inadequate. To assess the potential susceptibility
to rainfall-induced landslide, an effective
modelling of changes in water content and matric
suction in response to rainfall infiltration is
essential.
Geotechnical slope stability models are capable of
providing detailed description of potential
instability under changing environmental and
Chiranjib Prasad Sarma, Arindam Dey & A. Murali Krishna
climatic conditions and are able to establish
threshold values of the triggering phenomenon;
however, they are only rarely incorporated into
landslide early warning systems. Several studies
concerning rainfall, infiltration and landslide
mechanism can be found in literature. Numerical
models were developed to study the variation of
infiltration in a slope with respect to rainfall
intensity and its effect on the slope stability
[10,11,12,13]. Moreover, in the context of
constructing early warning systems in regions
where extensive data on landslide occurrences and
associated rainfall are inexistent, such tools offer
the possibility to establish thresholds for the
measurable geotechnical parameters [14].
Before being able to conduct such an analysis
proper characterization of the behaviour of the
unsaturated residual soils is essential. However,
severe limitation is observed as far as literature
pertaining to characterizing the unsaturated soil
behaviour of this region is concerned. The study
attempts to apply unsaturated soil mechanics
approach for explaining the rainfall induced
landslides in this region with whatever limited
literature is available.
STUDY AREA
Location and Topography
Guwahati falls approximately within latitude
(91⁰33' – 91⁰52'6'') E and longitude (26⁰4'45'' –
26⁰14') N, with an approximate area of 328 sq. km
spread across both banks of the river Brahmaputra
[15].
Hills composed of residual soils, dotting around the
alluvial plains and the marshy wetlands, mainly
comprises of three prominent geomorphological
features that can be easily identified. The hills,
ranging in altitude of 100–300 meter are
interspersed among elongated low–lying alluvial
plain with varying altitudes of 49–56 meter above
mean sea level (MSL). The hillslope angles varies
from a gentle slope of 10⁰ to as steep as 70⁰ [16].
Geology
Gneiss and granite bodies affected by joints,
intruded by quartz–feldsphetic veins, aplite dykes
and pegmatite constitute the hills in this region.
Thin bands or lenses of quartzite, amphibolites and
biotite schists, are found parallel to the foliation
[17,18,19,20]. Physical and chemical weathering
process on the parent rock vis., granite, gneiss and
porphyritic granite produce the residual soils of
this region. Concluding from field investigation,
two types of commonly found overburden residual
soils constituting the hill slopes around Guwahati
are reported in literature [8,9]. A top laterite
formation of reddish residual silty clay is observed,
varying in thickness from few centimetres to few
meters, underlain by a saprolite formation of pale
yellowish residual soil, which has been classified
by the researchers as a poorly graded silty sand.
Photograph 1 Soil profile of a cut slope at the IIT
Guwahati Campus
Saprolite formation is the layer of residual soil
derived from isovolumetric weathering of the
bedrock and retains much of the parent rock
structure and fabric but with a much lower density.
Thus, undisturbed saprolite formation looks very
compact but actually is very porous and friable and
can be easily crumbled with minimum of effort.
Saprolite formations when exposed to profuse
rainfall infiltration can lose the clayey fraction
through the seepage of water leading to loss in the
cohesive component of shear strength [4,21,22].
Residual soil layers, up to depth of 30 m from bed
rock [8,9,16] are commonly formed in the zones of
well drained regions, while in the zones of
50
th
IG
C
50th
INDIAN GEOTECHNICAL CONFERENCE
17th
– 19th
DECEMBER 2015, Pune, Maharashtra, India
Venue: College of Engineering (Estd. 1854), Pune, India
moderate, imperfectly and poorly drained regions
erosion and mantle stripping occurs, leading to
exposed rock layers and formation of etchforms
and inselbergs. A typical formation of the soil type
is shown in Photograph 1 as obtained from a hill
slope excavation at IIT Guwahati.
The North Eastern Region of India experiences
heavy rainfall during the monsoon season spread
across the month of April to September. The
wetting cycle starts from the month of April to mid
of October and drying cycle starts from of
November up to March, March being the hottest
and driest month of the year. High evaporation rate
during the drying cycle leads to the development of
high matric suction in the hilly areas covered by
residual soils around Guwahati. This phenomenon
enables the soil slopes to remain stable at much
steeper angles. However, during monsoon season,
infiltration of rain water into the soil slopes brings
about a reduction in the matric suction and
ultimately may lead to slope failures [8,9].
There is a lack of attention to this issue from an
analytical standpoint. The major difficulty is the
assessment of the soil behaviour in the unsaturated
state. Understanding of unsaturated soil behaviour
and more importantly assessing the unsaturated soil
parameters is imperative in formulating a correct
analytical solution to the problem. In order to
determine the decrease of the suction pressures, the
relationship between the negative pore–water
pressure of the soil and the water content, referred
to as soil–water characteristic curve (SWCC) is
required and can be obtained through laboratory
test. However most of these methods are costly and
time intensive and would require skilled personnel
for its measurement. More difficult than that is to
experimentally obtain the unsaturated hydraulic
conductivity curve (UHCC), which gives the
relationship of soil permeability with matric
suction. The shear strength parameters of
unsaturated soils which require suction controlled
direct shear or triaxial test to be conducted, which
is another impasse towards the solution of this
problem. Exhaustive treatises on unsaturated soil
mechanics are found in literature but only a
handful of them are concerned to the soils found in
this region.
Soil Parameters Adopted for this Study
Literature [8,9] presents the properties of the
unsaturated soil properties of typical residual soils
(RS - Red Silty Clay, PYS - Pale Yellow Silty
Sand) commonly found in the hill slopes of
Guwahati and the same is been adopted in this
study. Table 1 gives the index properties of the
soils, while the relevant geotechnical parameters
adopted for this study are presented in Table 2.
Table 1 Index properties of the soils used in this study [8,9]
Property (RS) (PYS)
Colour Reddish Light
Yellowish
Specific Gravity 2.44 2.64
Field Bulk Density (g/cc) 1.65 1.79
Field Dry Density (g/cc) 1.49 1.63
Plastic Limit 27% Non-plastic
Porosity 0.34 0.38
Classification Silty
Clay
Poorly
Graded
Silty Sand Table 2 Geotechnical parameters of the soils adopted for this study [8,9]
Property (RS) (PYS)
Effective friction angle, φ' 31⁰ 38.5⁰
Effective cohesion, c' 10 kPa 0 kPa
Friction angle related to
matric suction, φb
16.7⁰ 7.5⁰
Permeability, ks (m/s) 1.86×10-6
1.21×10-5
The reported soil water characteristic curves
(SWCC) for both types of soils are presented in
Fig. 1. The unsaturated hydraulic conductivity
curve (UHCC) is derived empirically by
integrating along the entire curve of the volumetric
water content function (SWCC) [23] and
programmed within the GeoStudio Software Suite
Chiranjib Prasad Sarma, Arindam Dey & A. Murali Krishna
[24]. Figure 2 presents the unsaturated hydraulic
conductivity curve derived for both type of soils.
Fig. 1 Soil water characteristic curves for the two
types of soil [8,9]
Fig. 2 Unsaturated hydraulic conductivity curves
for the two types of soil
PARAMETRIC STUDY
Methodology
The parametric study involves two parts. The first
part is transient seepage analysis to investigate the
infiltration mechanism under different rate of
infiltration and the simultaneous reduction in the
matric suction. The second part involves a slope
stability analysis applying limit equilibrium
method to determine the degradation of the
stability condition of the slope with time.
The infiltration is modeled using the seepage
analysis module SEEP/W of the commercially
available software suite GeoStudio [24]. The
coefficient of permeability is a non–linear function
of the matric suction (Fig. 3) within the soil under
unsaturated condition. SEEP/W applies finite
element method to solve the governing differential
equation describing the flow through soil, which in
this case is a modified form of the Richard’s
equation and is of the following form.
2x y w w
h h hk k q m
x x y y t
(1)
where mw2 is the coefficient of volumetric water
change with respect to a change in matric suction
and is equal to the slope of the SWCC at that
particular suction value, γw is the unit weight of
water and h is the hydraulic head. For efficient
solution of this problem, the SWCC, UHCC, the
unit boundary flux (q) on the surface of slope, and
the initial hydraulic head at all nodes of the mesh
needs to be defined before solving the model.
Two slopes of height 30 m with an inclination of
45⁰ (1H:1V) (Fig. 3) composed of homogenous and
isotropic residual soil was assumed. One slope was
assigned the red silty clay (RS) properties and the
other was assigned the pale yellow silty sand
(PYS) properties.
Fig. 3 Slope Geometry used in this study
The idealized slope isolates the influence of
complex hydro-geological conditions on the
seepage analysis and enables to understand the
individual response of each type of soil to a
particular applied infiltration. The slope was
discretized with a finite element mesh of combined
4-noded quadrilateral elements and 3-noded
triangular elements with a fineness of 0.5 m.
Surface elements of 0.1 m thickness were applied
to define the ground surface of the slope. A time–
50
th
IG
C
50th
INDIAN GEOTECHNICAL CONFERENCE
17th
– 19th
DECEMBER 2015, Pune, Maharashtra, India
Venue: College of Engineering (Estd. 1854), Pune, India
step of 1 hour was found to be sufficient for
efficient solution of the problem.
Initial groundwater level and the initial pore–water
pressure distribution were defined by applying a
phreatic line and restricting the maximum suction
to a limit of 80 kPa (Fig. 4). The maximum suction
value is chosen to reflect the in-situ moisture
content typical to the above mentioned residual
soils within the study area.
Fig. 4 Initial pore pressure distribution
The infiltration rate is modeled as a unit flux
boundary (q) along the nodes at the ground surface.
SEEP/W calculates the hydraulic head at each node
based on the nodal boundary flux converted from
the applied unit boundary flux. In the case of
ponding, i.e., development of positive pore-
pressure occurring at a ground surface node, the
hydraulic head at those nodes are reset to the
elevation of that node and the flux is determined.
As such the excess flux is lost as runoff.
For the slope stability analyses, Morgenstern-Price
method of slices with a half–sine inter slice force
function was applied. Morgenstern-Price method
satisfies both moment as well as force equilibrium.
The pore–water pressures determined in the
seepage analysis by SEEP/W are used as input data
in the slope stability analysis. The SLOPE/W
program considers unsaturated shear strength
conditions by implementing the modification of the
Mohr–Coulomb material model [25,26,27] and is
expressed in the following form.
ba a wf f ff
c' u tan ' u u tan (2)
where c' is the effective cohesion of the soil, σf is
the normal stress on the failure plain, ua is pore-air
pressure, uw is pore-water pressure, φ' is the
effective friction angle of the soil and φb is the
angle defining the increase in strength with
increase in matric suction. The parameter φb varies
with the degree of saturation. φb is equal to the
effective friction angle within the capillary zone
where the soil is saturated, but the pore-water
pressure is still negative; φb decreases as the soil
becomes unsaturated. Similar behavior is reported
for soils of the Guwahati hillslope regions [8,9].
However, as SLOPE/W is programmed to input Φb
as a constant value, the parameter is treated as a
constant and the adopted value is given in Table 2.
Five different rates of infiltration consistent to
storm events prevalent in this region vis. 50
mm/day, 100 mm/day, 150 mm/day, 200 mm/day
and 250 mm/day have been considered and applied
for duration of 5 days. The factor of safety of the
slopes are evaluated at an interval of 6 hours and
plotted against time.
Fig. 5(a) Variation of factor of safety with time for
the slope composed of red silty clay (RS)
Results
Figure 5 (a) & (b) gives the degradation of the
factor of safety of the slope composed of the silty
Chiranjib Prasad Sarma, Arindam Dey & A. Murali Krishna
clay (RS) and the silty sand (PYS) respectively.
The most prominent observation that can be made
from figures is that slope stability analysis
applying conventional soil mechanics deliberately
ignoring soil suction gives an unrealistic estimate
of the stability condition and thus fails completely
to give an analytical description of the rainfall
induced slope failure mechanism. Comparing both
the figures, it can be understood that matric suction
and the cohesion component plays a significant
role in providing a much greater stability to the
slope composed of the silty clay (RS).
Fig. 5(b) Variation of factor of safety with time for
the slope composed of pale yellow silty sand (PYS)
Even after intense infiltration of 250 mm/day
applied for duration of 5 days, the slope composed
of silty clay (RS) is still stable, though very
marginally. On the other hand, such infiltration
caused the slope composed of the silty sand (PYS)
to undergo failure within duration of 1 day and 6
hours. Moreover, it is to be noted that the silty clay
(RS) has a much higher suction value at similar
volumetric water content range. This renders the
slope composed of the silty clay (RS) a much
greater stability. Figure 6 (a) and (b) gives a
graphical representation of the pore pressure
scenario across the entire slope. A gradual
development of pore pressure can be observed
within the slope composed of silty clay (RS). For
the silty sand (PYS) a distinct zone of wetting can
be observed.
Fig. 6(a) Pore Pressures (kPa) developed within
the slope composed of Red Silty Clay (RS) due to
an infiltration of 200 mm/day for a duration of 3
days
Fig. 6(b) Pore Pressures (kPa) developed within
the slope composed of Pale Yellow Silty Sand
(PYS) due to an infiltration of 200 mm/day for a
duration of 3 days
Figure 7 (a) and (b) gives the Factor of Safety for
the corresponding slopes at that particular time
step. Effect of the cohesive component of the silty
clay (RS) on the shape of the critical slip surface
can be easily identified (Fig. 7a). A circular and
moderately deep critical surface is observed,
however the factor of safety is still sufficient
enough to prevent failure of the slope. For the
slope composed of silty sand (PYS), the factor of
safety dips just below 1 indicating slippage. The
shape and depth of the critical slip surface indicates
a shallow translational slide which can be expected
considering the shear strength properties (Table 2)
of the particular type of soil. A close observation of
Figure 7(b) along with Figure 6(b) gives a clear
50
th
IG
C
50th
INDIAN GEOTECHNICAL CONFERENCE
17th
– 19th
DECEMBER 2015, Pune, Maharashtra, India
Venue: College of Engineering (Estd. 1854), Pune, India
picture of the failure occurring within the wetting
zone.
Fig. 7(a) Factor of Safety of the slope composed of
Red Silty Clay (RS) after an infiltration of 200
mm/day for duration of 3 days
Fig. 7(b) Factor of Safety of the slope composed of
Pale Yellow Silty Sand (PYS) after an infiltration
of 200 mm/day for duration of 3 days
Following the fact that the slope composed of the
silty sand (PYS) had undergone failure, the pore
pressures developed at the moment of the failure
are plotted in Fig. 8. The total applied cumulative
infiltration required to initiate failure was
calculated and plotted against rate of applied
infiltration in Fig. 9.
A linear relation was observed between rate of
applied infiltration and total cumulative rainfall
required to initiate failure (Fig. 10). This follows
from the fact that with decreasing rate of applied
infiltration, more amount rain water can infiltrate
into the slope minimizing the runoff.
Fig. 8 Pore Pressure within the slope composed of
Pale Yellow Silty Sand (PYS) at failure
Fig. 9 Total cumulative Rainfall required for
initiating slipping within the slope composed of
Pale Yellow Silty Sand (PYS) against the rate of
applied infiltration
Actual rainfall data was then input into the
simulation to assess the response of the model to
rainfall infiltration. The rainfall data used as input
is obtained from the Tropical Rainfall Measuring
Mission (TRMM) 3-hourly rainfall estimate (3B42
V7) data set for the Guwahati city location. The
time series rainfall data for duration of 84 hours (3
Chiranjib Prasad Sarma, Arindam Dey & A. Murali Krishna
days and 12 hours) from afternoon of 4th
October
to the midnight of 7th
October, 2004, which has
been used in the analysis is obtained from Goddard
Earth Sciences Data and Information Services
Center (GES DISC) maintained web portal
Giovanni (http://disc.sci.gsfc.nasa.gov/giovanni).
Figure 10 gives the rainfall intensity versus time in
hours, used in the simulation.
Fig. 10 Rainfall storm event history (afternoon of
4th October to the midnight of 7th October, 2004)
Fig. 11 Factor of Safety vs. Time for slope
composed of Red Silty Clay (RS) and Pale Yellow
Silty Sand (PYS) for actual rainfall infiltration
The factor of safety of the slopes are then
calculated at intervals of 2 hours and plotted
against time. It can observe in the Fig. 11 that the
slope composed of the silty sand (PYS) is in near
failure condition at around 76 hours and undergoes
failure by 80 hours. The analysis shows that the
stability condition of the slopes can be well
predicted for actual rainfall condition with the
application of such simulation techniques. Such an
analysis can serve as basis for prediction of rain-
triggered landslides and develop early warning
where data on based on actual observation of
landslide occurrences and coupled rainfall storm
histories are not available.
CONCLUSIONS The most important and obvious conclusion that
can be drawn from the above analysis is that
suction within a soil layer plays a very significant
role in stabilizing a slope. Deliberately ignoring
suction in the stability analysis can not only give
an unrealistic result in terms of Factor of Safety but
also fail to explain how such steep natural slopes
can remain stable during the dry season.
Application of unsaturated soil mechanics helps in
understanding the mechanism that leads to the
failure of unsaturated soil slopes, providing a
quantified relationship between infiltration and the
destabilization of such soil slopes.
The other important conclusion that can be drawn
from this analysis is the fact that, saturation of the
soil layers need not be attained for failure to occur.
In the case of the slope composed of the silty sand
it can be observed that the pore water pressure was
still negative when failure occurred (Fig. 7). The
reddish silty clay (RS), due to its cohesion
component of shear strength and higher suction
values at comparable volumetric water content,
showed greater resilience than the slope composed
of pale yellow silty sand (PYS).
Coupling transient seepage analysis using the finite
element method in SEEP/W and then using the
output for slope stability analysis using the limit
equilibrium method in SLOPE/W, a predictive
model for rain-induced slope instability of natural
residual soil slopes can be developed to estimate
the degradation of stability condition ultimately
leading to failure. However the analysis results
depends greatly on the input parameters and thus
extensive field investigations, laboratory soil
testing, and rainfall data needs to be collected.
50
th
IG
C
50th
INDIAN GEOTECHNICAL CONFERENCE
17th
– 19th
DECEMBER 2015, Pune, Maharashtra, India
Venue: College of Engineering (Estd. 1854), Pune, India
The comparison shows that the variations in pore
water pressures distributed within the soil are
highly dependent on the soil water characteristics
and hydraulic conductivity function of the type of
soil. The results show that such a simulation of
saturated/unsaturated flow coupled with a limit
equilibrium slope stability model can provide
useful insight into development of pore pressure
scenario and rainfall induced slope failure.
The study suggests that slopes within the city of
Guwahati thus have the potential of undergoing
rainfall induced landsliding, highlighting the
observations made by previous researchers.
However, the potential will vary depending on the
type of soil and its response to infiltration.
This study cannot be regarded as a conclusive
effort to analyzing and understanding the
occurrence of rainfall induced landslide within the
Guwahati region. On the contrary, this study can be
considered as an effort to present the problem in
hand from an analytical point of view and laying
the emphasis on the physics of the problem.
REFERENCES
1. Kalita, U. C. (2001), A study of landslide
hazards in North Eastern India, Proceedings of
the Fifteenth International Conference on Soil
Mechanics and Geotechnical Engineering,
Istanbul, Turkey, 1-3, 1167-1170.
2. Sarma, A.K., and Bora, P.K. (1994), Influence
of Rainfall on Landslide, International
Conference on Landslides, Slope Stability and
the Safety of Infra-Structures, Malaysia.
3. GSI (2013), Geological Survey of India,
http://www.portal.gsi.gov.in, Landslide Home,
Post-disaster studies, accessed on 31 October
2013.
4. Selby, M. J. (1993), Hillslope Materials and
Processes, Oxford University Press Inc., New
York.
5. Glade, T. and Crozier, M. J. (2005), The nature
of landslide hazard impact, Landslide hazard
and risk, T. Glade, M. Anderson, M.J. Crozier,
(eds.), John Wiley and Sons, 43-74.
6. Glade, T. and Crozier, M. J. (2005), A review
of Scale Dependency in Landslide Hazard and
Risk Analysis, Landslide hazard and risk, T.
Glade, M. Anderson, M. J. Crozier, (eds.),.
John Wiley and Sons, 75-138.
7. Wesley, L. D. (2010), Geotechnical
Engineering in Residual Soils, John Wiley &
Sons Inc.
8. Das, U.K. and Saikia, B.D. (2010), Shear
Strength of Unsaturated Residual Soils of the
Hills in Guwahati, Proceedings of Indian
Geotechnical Conference, GEOtrendz, 679-682
9. Das, U.K. and Saikia, B.D. (2011), Evaluation
of a Prediction Model for Shear Strength of
Unsaturated Soils, Proceedings of Indian
Geotechnical Conference, Kochi, 643-646
10. Gasmo, J.M., Rahardjo, H. and Leong, E.C.
(2000), Infiltration effects on stability of a
residual soil slope, Computers and
Geotechnics, 26, 145-165
11. Tsaparas, I., Rahardjo, H., Toll, D.G. and
Leong, E.C. (2002), Controlling parameters for
rainfall-induced landslides, Computers and
Geotechnics, 29, 1–27
12. Huat, B. B. K., Ali, F. Hj. and Rajoo, R. S. K.
(2006), Stability Analysis and Stability Chart
for Unsaturated Residual Soil Slope, American
Journal of Environmental Sciences, 2(4), 154-
160.
13. Rahardjo, H., Ong, T.H., Rezaur, R.B., Leong,
E.C. (2007), Factors Controlling Instability of
Homogeneous Soil Slopes under Rainfall,
Journal of Geotechnical and Geoenvironmental
Engineering, 133(12), 1532-1543
14. Thiebes, B., Bell, R., Glade, T., Jäger, S.,
Anderson, M. and Holcombe, L. (2013), A
WebGIS decision-support system for slope
stability based on limit-equilibrium modelling,
Engineering Geology, 158, 109–118
Chiranjib Prasad Sarma, Arindam Dey & A. Murali Krishna
15. Guwahati Metropolitan Development Authority
(2009), Master Plan for Guwahati
Metropolitan Area - 2025 (Part - 1), Guwahati.
16. Saikia, B.D. (2002), Geotechnical Investigation
of Probable Landslide Spots within Guwahati
City Area.
17. Maswood, Md. and Goswami, D. N. D. (1974),
Basic rocks from the Precambrian Terrain
around Guwahati, Assam, The Indian
mineralogist: journal of the Mineralogical
Society of India, 15, 55–62
18. Maswood, Md. (1981), Granite Gneisses
around Guwahati, Assam, Journal of
Geological & Mineralogical Society of India,
53(3, 4), 115–124.
19. Maswood, Md. (1982), Structural history of the
Precambrian rocks around Guwahati, Assam,
Quarterly Journal of Geological &
Mineralogical Society of India, 54(1, 2), 33–38.
20. Shukla, R.C. (1989), Study of Granite Rocks
around Kamakhya hill and adjoining area,
Geological Survey of India, Assam, Record–
122, IV, 72-73.
21. Armirza, S. (2004), Problems Of Sampling And
Essential Test In Tropical Residual Soils, e-
USU Repository Universitas Sumatera Utara.
22. Durgin, P.B. (1977), Landslides and the
weathering of granitic rocks, Reviews in
Engineering Geology, 3, 127-131
23. Fredlund, D. G., Xing, A. Fredlund M. D. and
Barbour, S. L. (1996), The Relationship of the
Unsaturated Soil Shear Strength to the Soil-
water Characteristic Curve, Canadian
Geotechnical Journal, 33, 440-448
24. GeoSlope (2007), Manuals of Geostudio 2007
software suite, GEO-SLOPE International Ltd
25. Krahn, J., and Fredlund, D. G. (1972), On total
and osmotic suction, Journal of Soil Science,
114(5), 339-348
26. Fredlund, D. G., Morgenstern, N. R. and
Widger, R. A. (1978), Shear strength of
unsaturated soils, Canadian Geotechnical
Journal, 15, 313-321
27. Fredlund, D. G. and Rahardjo, H. (1993), Soil
Mechanics for Unsaturated Soils, John Wiley
and Sons Inc.
Recommended