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: vanda_karpe@rediffmail.com e-mail:poonam46sarang@yahoo.co.in e-mail: siqueira.nancy@gmail.com

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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