Stabilization of Slope for Hill Road at Chorla Ghat
Karpe, V.M. Sarang, P.Y. Dias, N.P. G. Student P. G. Student P. G. Student
e-mail: [email protected] e-mail:[email protected] e-mail: [email protected]
ME Foundation Engineering, Goa College of Engineering, Ponda - Goa
ABSTRACT
This paper presents a case study of slope failure and the possible remedial measures to be undertaken at the
Chorla Ghat. The site is situated on the State Highway (SH31) between Goa – Karnataka passing through a hilly
terrain named “Chorla Ghat”. The highway is flanked by a steep (20 m high) slope retained by 4.3 m gravity wall
on one side and a very deep valley on the other. In the present study, the two alternatives are suggested to prevent
recurrence of slope failure during rains. These include providing a gentle slope (15°) with pitching for 3m from
top of existing retaining wall and then constructing reinforced gabion wall for remaining height. This would drain
all the infiltrated water which can be collected in drain at the base of the gabion wall. The other alternative is to
construct reinforced soil wall using Geocomposites. This can be done by providing Geogrid sandwiched between
two layers of Geotextile.
Indian Geotechnical Conference – 2010, GEOtrendz
December 16–18, 2010
IGS Mumbai Chapter & IIT Bombay
1 INTRODUCTION
The movement of mass of a soil in a downward and outward
direction of a slope is called a slide or a slope failure. The
failure of a natural slope is a common geological
phenomenon occurring whenever an imbalance takes place
between shear strength and shear stress in the ground. The
first sign of an imminent landslide is the appearance of
surface cracks in the upper part of the slope, perpendicular
to the direction of the movement. The instability is either
due to increase in seepage pressure, due to excavation of
slope toe material, due to increase of shear stress from
surface loading as a result of construction or traffic or due
to slow time dependent deterioration of material leading to
acceleration of creep rate. The slip may occur through the
fill, through the base or through foundation. The analysis
of slope stability can be done by force equilibrium or
moment equilibrium conditions.
A number of design approaches are available. However,
in this case study the most commonly used approach based
on Limit Equilibrium Method has been used.
The analysis consists of three parts,
(1) Internal Stability Analysis (Local Stability
Analysis): An assumed Rankine’s Failure surface
is used with consideration of possible failure modes
of reinforced soil mass such as pullout, connection
failure and creep. The analysis is mainly aimed at
determining tension and pullout resistance in the
reinforcement, length of reinforcement and
integrity of facing element.
(2) External Stability Analysis (Global Stability
Analysis) of reinforced soil mass is checked
including sliding, overturning, load-bearing
capacity failure and deep seated slope failure.
(3) Analysis of Facing System including its
attachment to thereinforcement.
Traditionally retaining walls of masonry, concrete or
RCC are used to hold back earth along cut or fill where
safe side slopes cannot be provided due to inadequacy of
space. However earth reinforced structures with
geosynthetics and metal reinforcements are finding
increasing use in modern construction due to their flexibility
in design, flexibility of construction over poor subsoil
condition and ability to withstand differential settlement.
Geosynthetics are primarily manufactured from polymers
such as Polyethylene, Polypropylene, Polyester, Polyamide,
Polyvinyl Chloride (PVC), etc. The most common types
are - Geotextiles, Geogrids, Geonets, Geomembranes, and
GeoComposites which are used in contact with soil, rock
and/or any other Civil Engineering related material, as an
integral part of manmade structures. They are utilised in a
range of applications in many areas of Civil Engineering
like geotechnical, transportation, water resources,
environmental, coastal and erosion control engineering for
achieving technical or economic benefits. The various
708 V.M. Karpe, P.Y. Sarang and N. Dias
functions performed are separation, reinforcement,
filtration, drainage, and moisture barrier. Various methods
used to stabilise slopes using Geosynthetics include
Wraparound type of wall, Gabion wall, Concrete panel
(facia blocks) wall, etc.
2. SLOPE FAILURE OF HILL ROAD AT CHORLA
The site is situated on State Highway (SH 31) between Goa
- Karnataka passing through hilly terrain named Chorla
Ghat. The site is located 27 km from Sanquelim Village in
Bicholim Taluka of State of Goa and 70 km from Khanapur
in Belgaum District in Karnataka. The highway is flanked
by a steep (20 m high) slope retained by 4.3 m gravity wall
on one side and a very deep valley on the other as shown in
Figure 1. The topographical feature of the site is such that,
the large amount of water from the catchment area on the
upstream side is discharged through this part of the terrain,
where landslides occur very frequently.The soil is non
homogenous consisting of boulders, gravels and silty soil
which allows immediate percolation of surface water during
the rainy season. Inadequate drainage and soil
characteristics cause saturation of ground soil, which leads
to the development of pore pressure and surface cracks.
Also water percolates through the cracks thereby
accelerating the failure.
Two springs were initially flowing down on either side
of the slope. One of the springs changed its course after a
series of landslides. It now flows below the road level i.e in
the deep valley section indicating that there has been a
change in the water course.
Fig. 1: Profile of the Site at Chorla Ghat
The first failure of the slope occurred in the year 2007,
after the curved road which followed the contour of the
valley was flattened. This failure was progressive with slow
movement of soil, which was accelerated during the rainy
season. The most significant indication of this failure was
the formation of number of tension cracks on the upper
part of the slope, resulting in a number of successive wedges
as shown in Figure 2. Other indications like trees leaning
outward over the slope and the change in the water course
of an existing spring was also observed. Long term creep
was also observed.
Fig. 2: Failure in the Form of Successive Wedges
(Source: PWD, Goa)
As a protective measure, the concerned authority
constructed a gravity retaining wall of varying height (to
suite the topography of the site) of 2.4m to 3.7m along the
road. But this measure shifted the slip surface from the
base to the top of the retaining wall. Subsequently, in the
year 2008, the height of the retaining wall was further
increased to a total of 4.3m. Also, a trench was dug on the
upstream side to arrest and divert the water as shown in
the Figure 1. In spite of this protective measure, there was
another failure.
As a remedy to this, in the year 2010, a part of the
slope was flattened and grids with granite pitching were
constructed as shown in Figure 3(a). Each panel was of
3.5m x 3.5m with concrete beams of 70cm x 22cm cross
section. These panels were constructed along the slope for
a length of 22.5m and inclined width of 17.5mts. However,
the slope failed again after a spell of incessant rains in July
2010 as shown in Figure 3(b).
(a) (b) Fig. 3 (a) Slope protection with grids and Fig. 3: (a) Slope Protection with Grids and Granite Pitching
(Source: PWD, Goa), (b) Failure of the Slope Protected with
Grids and Granite Pitching
3. ALTERNATIVE METHODS TO PREVENT
SLOPE FAILURE
As shown in Figure 4, it is proposed to provide a gentle
slope of 15o with pitching for about 12m from the top of
4.3m retaining wall. Pitching includes placing of rubble or
stone blocks with cement mortar to prevent water from
infiltrating through the surface. Longitudinal and traverse
drains may be provided to trap and drain the surface water.
At the top of the pitched portion i.e. at Location 1, a
retaining structure of height 6.0 m is proposed to be
constructed followed by a gentle slope of 15o for a stretch
of 15 m. Another retaining structure of 4.0 m height is
proposed to be constructed at Location 2. As effective
drainage is essential for achieving and maintaining soil
stability, the backfill soil is to be replaced by cohesion less
soil.
Stabilization of Slope for Hill Road at Chorla Ghat 709
Alternative I: Mechanically Stabilised Earth (MSE)
Gabion Wall
It is proposed to provide a MSE Gabion wall at Location 1
& Location 2 as shown in Figure 4.
Fig. 4: Alternative Methods in the Form of Retaining
Structures Proposed at Location 1 & Location 2
Gabions are modules or cages formed of wire mesh,
jointed to form square, rectangular or trapezoidal shaped
units. Each module has to be connected with lacing wire,
helicals and/or rings to adjacent modules, to form
monolithic structure. The modules may be divided into cells
by means of diaphragms positioned at 1m centres. These
modules are filled with durable rock fragments or river
cobbles, size exceeding the mesh size but not larger than
half the depth of the individual basket so as to produce a
neat front of the structure. The meshes are made of 2 -
5mm diameter wire, with openings from 60mm to 100 mm.
Welded mesh gabions have square mesh where the
longitudinal wires are welded to the cross wires at their
intersection points. Woven mesh gabions has hexagonal
openings which is formed by twisting pairs of wires together
with a double or triple twist. They are galvanised or coated
with Polyvinyl chloride (PVC) for protection against
corrosion. A filter layer in the form of non woven Geotextile
may be placed between the Gabion and the backfill as there
is a danger of soil particles being washed out through the
rock fill by seepage.
MSE Gabion walls consists horizontal layers of welded
wire mesh used as tie backs for soil reinforcement, attached
to the back face of the Gabion and embedded in the backfill.
These layers extend beyond a rupture plane by an effective
length (le) as shown in the Figure 5 (a) Rankine’s method
is used for the design of this wire mesh tiebacks. These
layers will resist the active soil force, by a combination of
friction on the wire mesh surface and mechanical interlock
with the soil. Reinforcing mesh may fail by pullout, if
frictional resistance developed along the surface is less than
force to which this reinforcement is subjected.
Alternative II: Geocomposite Reinforced Earth Wall
(Wrap Around Type)
Alternative II is to provide a Reinforced earth retaining
wall with Geocomposites (Wrap Around Type) in Location
1 and 2 as shown in Figure 4.
The Geocomposite suggested is Geogrid sandwiched
between two layers of Geotextile. Geogrid will satisfy the
reinforcement function whereas Geotextile will drain the
unfiltered water. Depending on requirement of strength
biaxial or monoaxial Geogrids can be used. Usually biaxial
Geogrids are placed at the base. At a calculated height
monoaxial Geogrids are placed. These layers extend beyond
a rupture plane by an effective length (le) as shown in the
Figure 5 (b).Rankine’s Method is used for the design.
Geocomposites provides additional shear strength to the
soil. This is by virtue of friction mobilised at the interface
of Geocomposite and soil. Geocomposites has to be placed
continuous throughout the length of the retaining structure.
Geocomposite may fail by pullout if frictional resistance
developed along the surface is less than force to which this
reinforcement is subjected.
Fig. 5(a): Details of MSE Gabion Wall
4. ANALYSIS AND DESIGN
Design Assumptions
(common for Alternative I and II)
Seismic loads have not been considered in the design.
Soil Parameters
The existing soil on site has the following properties:
Cohesion (C)=5KN/m2, Angle of internal friction(ø)= 17o,
Unit weight (γ) = 16KN/m3.
The replaced backfill is assumed to have the following
properties: Cohesion (C) = 0, Angle of internal friction
(øb) = 30o, Unit weight (γ
b) = 18KN/m3.
(a) MSE Gabion Wall
Gabion walls are analysed as gravity retaining walls, that
is, walls which use their own weight to resist the lateral
earth pressure. The lateral earth pressure is calculated by
Coulombs Theory. The wall is checked for Stability against
overturning, sliding and bearing capacity failure.
The design values taken are as follows:
Unit weight of rock fill (γg) = 15.5KN/m3, Porosity of
Gabions = 40%, Back face slope angle to the vertical (batter)
(β) = –6o
Tensile strength of the mesh divided by factor of safety
of 1.85 = (σg) = 45/1.85 = 24KN/m
710 V.M. Karpe, P.Y. Sarang and N. Dias
At Location 1:
Gabion wall thickness (T) = 1.0m
Total wall height (H) = 7.0mts (6.0m +1.0m embedment)
Total length of wire mesh attached to the back face of
the gabion and embedded in the backfill (B) = 5.0m.
Vertical spacing of the layers of wire mesh attached at
the back face of the gabion (Sv) = 1.0m for z =0 to z =4.0m
and (Sv) = 0.6m for z =4.0m to z =7.0m
At Location 2:
Gabion wall thickness (T) = 0.6m
Total wall height (H) = 5.0mts (4.0m +1.0m embedment)
Total length of wire mesh attached to the back face of
the gabion and embedded in the backfill (B) = 3.5m
Vertical spacing of the layers of wire mesh attached at
the back face of the gabion (Sv) = 1.0m for the full height of
the wall.
A granular subbase layer of 300mm thick may also be
provided at the base of the gabion walls.
(b) Geocomposite Reinforced Earth Wall (Wrap Around
Type)
The method suggested by Koerner is adopted in the design.
Wide width strength of Geocomposites is taken as 20KN/m.
At Location 1:
Total wall height (H) = 6.0mts, Total length of
Geocomposite varies from 6.0m to 4.0m. However for ease
of construction total length may be kept equal to 6.0m. The
vertical spacing of the layers of Geocomposite (lift height)
Sv =1.0m
Lap length= 1.0m
At Location 2:
Total wall height (H) = 4.0mts, Total length of
Geocomposite varies from 5.0m to 3.5m. However for ease
of construction total length may be kept equal to 5.0m.
Vertical spacing of the layers of Geocomposite (lift height)
=1.0m, Lap length= 1.0m.
The wall is checked for Stability against overturning,
sliding and bearing capacity failure. It is also proposed to
protect the Geocomposite from Ultraviolet rays of the sun by
growing grass such as khus and vetiver can also be grown on
the wall. This will not only add to the aesthetics but also help
in blending with the natural landscape.
5. CONCLUSIONS
It may be concluded that the slope failure at Chorla Ghat
occurred due to a number of causes. The primary causes being
infiltration of rain water due to non- homogenous and
widening of the road. Improper drainage of water and steep
slope compounded the failure. The remedial measures
adopted consisting of concrete grid beams with granite
pitching of the panels have not been successful.
Two solutions have been proposed in this case study to
prevent any further slope failure. Alternative I is a MSE
Gabion Wall and Alternative II is a Geocomposite Reinforced
Earth Wall (Wrap Around Type). However it is recommend
that a MSE Gabion wall may be constructed taking into
consideration local site conditions, and topography. Gabions
are highly permeable and prevent the build up of water
pressure. They are flexible and conform to difficult site
geometry and can adjust to differential settlement and lateral
movement. They are also cost effective as the rock fill
available at the site may be used and can be constructed in
small time fame. It is also an eco friendly structure and
permits the growth of vegetation.
Fig. 5 (b): Details of Reinforced Earth Retaining Wall with
Geocomposites (Wrap Around Type)
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