credit seminar iii

8/6/2019 Credit Seminar III

1/28

Page 1

1 Introduction

1.1 Principle Theory of Slope Failure

Every year there are approximately a thousand slope failure cases around the globe.

On average, death tolls of many thousands of people, as well as economic losses related tolandslide events are common. Therefore, it is evident that there is a clear need to investigate

the cause of devastating slope failures.

Slope failure is related to various causes, these include: the rise of ground water table,

soil properties and geological characteristics of slopes. These causes of slope failures are

often interrelated and can influence each other, collectively deteriorating the stability of the

slope. The combination of these failure modes forms the principle elements related to slope

failure.

Slope failure is driven by slope slip surface which is caused by gravitational andseepage forces that push the slip surface and causes slope instability (Ortigao, 2004)

According to Abramson (2002), there are various types of slope failure which are driven by

slip surfaces, namely: circular/rotational slip, non-circular slip, translational slip and

compound slip.

(a)Circular slip surface

(b) Non Circular slip surface


2/28

Studies on Application of Stone Column for Slope Stability

Page 2

(c) Translation slip surface

(d) Rotational slip surface

Figure 1: Types of slip surfaces (Das, 2006)

Figure 2: Rain water flow and seepage on slope (Murthy, 2000)

The most common type of slope failure mode is circular/rotational slip. This is

described as a circular shaped slip surface which is mobilised across a homogenous &

isotropic soil condition, whereas a non-circular slip surface is mobilized in a non-

Rain

Surface runoff

Seepage


3/28


Page 3

homogenous condition (Ortigao, 2004). On the other hand, according to Ortigao, (2004)

described that slope failure driven by translational and compound slip surface is developed

due to the presence of a rigid layer (for example a bedrock layer), or the presence of

discontinuities such as fissures and pre-existing slips.

1.2 Factors Affecting the Slope Stability

There are many factors which affects the slope stability. According to Ortigao,

(2004) described that one of the main factors is the geometrical changes. This is described

as a change in the gravitational force. The main force responsible for movement is gravity.

Gravity is the internal force that acts on body, pulling mass object in a direction toward the

center of the earth. If the object is on a flat surface then the gravitational force will act

downward. In another words, if the objects is located on the flat surface it will not move

under the gravity force.

However, in the case of a sloping ground, according to Ortigao (2004) described

that the force of gravity can be divided into two vector components, one component isacting normal to the slope and the other component is acting tangent to the slope. The slope

gains its stability from the strength properties of the soil. These include the shear strength,

frictional resistance and cohesion among the soil particles that make up the soil mass

(Ortigao, 2004). As the applied shear stress which occurs under gravitational force

becomes greater than the combination of forces holding the soil mass on the slope, the

object will move down the slope. In geotechnical engineering, this movement is called

slope failure or landslide.

Thus, this slope movement is favoured by steeper slope angles which increase the

shear stresses on the soil. The slope stability is threatened by anything that reduces the

shear strength, such as lowering the cohesion among the particles or lowering the frictional

resistance. The tenancy of slope failure is expressed in terms of the ratio of shear strength

to shear stress, which is known as Safety Factor (Cornforth, 2005).

Safety Factor = Shear Strength/Shear stress.

If the safety factor becomes less than 1.0, slope failure is expected.

The other factor that causes slope failure is an increase in water pressure. This is

caused by the increase in groundwater level. Consequently, an increase of water pressure

adds an increased internal water force inside the slope. Although water is not alwaysdirectly involved as the transporting medium in mass-wasting processes (Ortigao, 2004), it

does play an important role. For exemplary reasons, a sand castle on the beach may be

used. If the sand is dry, it is impossible to build a steep face like a castle wall. If the sand is

wet, vertical wall can be build. If the sand is too wet, then it flows like a fluid and cannot

stay as a wall.

For the case of dry sand, the sand can form a slope with a slope angle relative to the

flat ground that is equal to its Friction angle. The friction angle is the steepest angle at

which the sand slope can remain stable (Liu, 2008). In this case, the stability of the sand

slope is purely dictated by the frictional contact between the soil grains. In general, the


4/28


Page 4

friction angle increases with increasing grain size. However, different soil types contain

different soil friction angles. This mechanical soil parameter can be usually obtained from

experiments, for example, Triaxial test and direct shear test.

In the partially saturated soil, water particle and the sand particle are interlocked by

an internal suction force between them. This suction force assists in building up apparent

cohesion in cohesionless material. It should be noted that, excessive water will break the

suction force between the soil particles.

The other factor that affects the slope stability is the additional loads (surcharge)

applied on the top of the slope. This external loading can increase the disturbing force and

cause slope instability.

Another reason that affecting slope stability is water pressure. Water pressure is

common on a general slope where a water table might usually exist. When water pressure

increases, the effective stresses, shear strength decrease and can lead to slope failure. An

increase in the water pressure may be due to many uncertain reasons. Usually, the most

common reasons that cause slope failure relate to water pressure increases due to elevated

rainfall intensity and increases in the water content in slope, such as water pipe leakage.

These are the main factors that can affect the slope stability. These are also the

main items which one has to focus on when dealing with reducing the presence of slope

instability.

There is another factor that can induce instability to a slope, which is an

earthquake. However this factor is relatively uncommon when compared to the otherfactors mentioned above. Slope instability caused by an earthquake only happens during

earthquakes in active earthquake zones, such as in China and Japan. This factor causes

slope displacement and changes the gravity condition of slope material. During the

displacement and change of gravity of slope, the body of slope mass no longer is in a

balance condition, and slope will no longer be in a stable condition.

In many seismic regions of the world, slope displacements caused by earthquakes

have led to disaster situations. Examples of magnitude 7.8 earthquake-induced landslides

are the landslide events in the area of Sichuan in China, which were caused by a major

earth movement event near the belt of Sichuan region in May 2008.

According to CEDD (2008) & Ortigao, (2004), the causes of slope instability can

be summarised as follows:

External force that causes slope instability:

Geometrical changes (Undercutting, erosion, changes in slope height, length andsteepness)

Surcharge (Addition of material, Increase in slope height and increasedevelopment at slope crest)


5/28


Page 5

Shocks and vibrations (earth quake) Drawdown (lowering of water in lake or reservoir) Change in water regime ( rainfall , increase in weight , pore pressure )

Internal forces that causes slope instability:

Progressive failure (following lateral expansion of fissuring and erosion) Weathering (reduction of cohesion, desiccation) Seepage erosion (solution , piping)

Moreover, there are some other non-natural factor cause slope instability:

Removal of vegetation; Interference with, or changes to, natural drainage; Modification of slopes by construction of roads, railways, buildings, etc; Overloading slopes; Mining and quarrying activities; Vibrations from heavy traffic, blasting, etc; and Excavation or displacement of rocks.

1.3 Slope Failure Hazard

The term "landslide" describes a wide variety of processes that result in the

downward and outward movement of slope-forming materials including rock, soil,

artificial fill, or a combination of these. The materials may move by falling, toppling,

sliding, spreading, or flowing.

The various types of landslides (Highland & Bobrowaky, 2008) can be

differentiated by the kinds of material involved and the mode of movement. A

classification system based on these parameters is shown in (Figure 3). Other classification

systems incorporate additional variables, such as the rate of movement and the water, air,

or ice content of the landslide material.

Although landslides are primarily associated with mountainous regions, they can

also occur in areas of generally low relief. In low-relief areas, landslides occur as cut-and-fill failures (roadway and building excavations), river bluff failures, lateral spreading

landslides, collapse of mine-waste piles (especially coal), and a wide variety of slope

failures associated with quarries and open-pit mines. The most common types of landslides

are described as follows and are illustrated in (Figure 3).


6/28


Page 6

Table 1: Type of landslides (Highland & Bobrowaky, 2008)

Types of movement Bedrock Engineering soil

Falls Rock fall Debris fall Earth fall

Topples Rock Topple Debris Topple Earth Topple

slides RotationalRock Slide Debris Slide Earth Slide

Translational

Lateral spreads Rock Spread Debris Spread Earth Spread

Flows Rock Flow

(Deep Creep)

Debris Flow Earth Flow

(Soil Creep)

Complex Combination of two or more types of movement

(a)Falls (b)Topples (c)Debris flow

(d)Debris avalanche (e) Earth flow (f) CreepFigure 3: Types of landslides (Highland & Bobrowaky, 2008)

FALLS

Falls are abrupt movements of masses of geologic materials, such as rocks and boulders,

that become detached from steep slopes or cliffs (Figure 3a). Separation occurs along

discontinuities such as fractures, joints, and bedding planes and movement occurs by free-

fall, bouncing, and rolling. Falls are strongly influenced by gravity, mechanical

weathering, and the presence of interstitial water.

Tilted pole

Curved tree trunk

Fence out of

alignment

Soil ripples


7/28


Page 7

TOPPLES

Toppling failures are distinguished by the forward rotation of a unit or units about some

pivotal point, below or low in the unit, under the actions of gravity and forces exerted by

adjacent units or by fluids in cracks (Figure 3b).

FLOWS

There are five basic categories of flows that differ from one another in fundamental ways.

(a) Debris flow

A debris flow is a form of rapid mass movement in which a combination of loose soil,

rock, organic matter, air, and water mobilize as slurry that flows down slope (Figure 3c).

Debris flows include


8/28


Page 8

where movement is within the depth of soil affected by seasonal changes in soil moisture

and soil temperature; (2) continuous, where shear stress continuously exceeds the strength

of the material; and (3) progressive, where slopes are reaching the point of failure as other

types of mass movements. Creep is indicated by curved tree trunks, bent fences or

retaining walls, tilted poles or fences, and small soil ripples or ridges (Figure 3f).

1.4 Landslide hazard identification

According to Highland & Bobrowaky (2008), the identification of landslide hazard

involved following procedure:

Desk study- An aerial photograph is an important aspect of landslide hazardidentification. The study of aerial photographs assists in cataloguing of historical

landslides, describing and evaluating the geomorphology and determining the site

history particularly with respect to human activities on natural slopes.

Engineering geological reconnaissance Mapping- The mapping providedadditional landslide information data which was not visible on the aerial photos and

enables ground truthing of some of the geomorphologic interpretations made from

aerial photographs.

Ground Investigation - In order to understand the ground model better, groundinvestigation was carried out to explore the soil properties and the condition of the

groundwater regime.

Site investigation - site visits and field measurements were taken of the slopegeometry (eg. Slope height, angle, seepage). Therefore, the collected data can be used

to provide the most precise information and representative the real slope geometry

for further design.

Engineering Geological synthesis - An engineering geological synthesis of thefinding from the desk study, engineering geological mapping, ground investigation

fieldwork, site investigation fieldwork and laboratory tests was conducted to

produce a geological model and representative geological sections.


9/28


Page 9

1.5 India Landslide Risk Zone Category

Figure 4: Landslide risk zone category of India (Sarkar, 2005)

1.6 Methods of Slope Failure Prevention

A number of factors and parameters such as soil properties, pore water

pressure resume, slope geometry, earthquake, and vibration can influence the slope

stability. Engineering slope stabilization is generally referred to stop or decrease the

possible of instability process of slopes. Preventing the movement of a slope or

increasing the safety factor (SF) is possible by using structural or geotechnical

methods. Among techniques which increase resisting forces and basically act

externally on the soils or rocks sliding are geometrical methods (USDA, 1994),

hydraulic improvement, surface and subsurface drainage (USDA, 1994), structural


10/28


Page 10

barriers such as rigid walls and piles (Hassiotis et al., 1997; Ausilio et al., 2001),

physical and mechanical improvement (Komak Panah, 1994), chemical improvement

(Ghazav, 2008), reinforcement with geosynthetics (Jorge and Zornberg, 2002; Kousik

Deb et al., 2007), soil nailing (Turner and Jensen, 2005; Sugawara, 2006), etc.

Before 1990, chuman surface and non-reinforcing shotcrete surfaces were a

common use of material for slope stability improvement. For some steep slopes, a

stone pitching surface was most widely used, or masonry facing for rigid surface

cover. Some of them were installed weep holes to reduce the pore water pressure

inside the slope. However, the main purpose of this was to achieve an impervious

interface for prevention of the surface erosion and the rainfall entry into the slope in

order to reduce the pore water pressure inside the slope. This method is easy in terms

of construction and maintenance and was also cost efficient.

However, if the slope had inherent instability due to internal soil, shear failure

and sliding would still occur. This method would not provide an enough structural

external force against the movement of the slope failure wedge. On the other hand,

this method usually uses a concrete or stone base construction material, which is

usually grey or white in colour. This triggers an environmental problem, as the finish

is very inconsistent with the surrounding natural landscape.

1.6.1 Shotcrete surface method

Shotcrete is a process where concrete is sprayed onto slope surface using ashotcrete feeder gun to form rigid surface. Usually, shotcrete surface slopes have

approximate 50-150 mm thick and provide wire mash reinforcement to prevent surface

crack and shrinkage.

(a) Laying of wire mesh (b)Spreading of concreteFigure 5: Shotcrete surface (Jade, 1993)


11/28


Page 11

1.6.2 Masnory surface method

Use stone pitching as a rigid surface cover for prevent erosion and surface runoff. This

method is easy for maintenance and construction.

(a) (b)

Figure 6: Masonry surface (Cheng, 2008)

1.6.3 Chuman surface metod

Use of cement sand mix material for surface protection. No reinforcement and wire

mash required. Poor crack and shrinkage resistance.

Figure 7: Chuman surface (Cheng, 2008)

1.6.4 Soil Nailing

It is a new technique in which soil slopes, excavations or retaining walls are

reinforced by the insertion steel reinforcing bars. According to Ortigao (2004) noted that

the first use of the soil nailing application was in 1972 and now this method is a well-

established technique around the world. Sometimes, soil nailing can combine different type

of retaining methods such as soil nailing on retaining walls and with greening surfaces.

Soil nailing can provide a cost efficient, quick and standard technique for slope

improvement solution.


12/28


Page 12

Figure 8: Typical Soil nailing method (Tommy, 1988)

1.6.5 Bio-Engineering

Itis one of the most innovative technologies for slope improvements in the world.

According to Ducan et al. (1987) described that Bio-Engineering includes the use of tree

roots or plant roots to retain shallow slope failure. This method has an advantage as it is

natural and environmental friendly (Ducan et al., 1987). However, many factors can

influence the effectiveness of Bio-engineering for slope stabilisation. This method is in an

early stage of development, and needs a period of time for technology proving and

development.

Figure 9: Vetiver Grass System (Greenwood, 2008)

1.6.6 Soil Re-Compaction and No-fine Replacement

For some loose material slopes such as fill slope, soil nailing is not a suitable

stabilisation method. Some technologies such as soil re-compaction and soil re-placement

are more suitable and are usually applied. Soil re-compaction involves the excavation of

the loose soil, backfilling and re-compacting to improve the friction angle. However, the

soil re-compaction method has some restrictions such as every backfill and re-compaction

has to be carried out in a 300mm thick layer (NAS, 2004), layer by layer, and every single

layer needs an individual soil test for compaction ratio checking. Moreover, this method is

highly influenced by weather conditions. The soil has to be placed in thinner lifts and

requires moisture control for compaction. As a result, this method will increase the

construction cost and time period.

The other method is the soil replacement method. This design approach includes

using other materials such as no-fine concrete or gravel to replace the loose soil. Removalof the original loose soil on the slope is carried out, then forming a slope with a design


13/28


Page 13

slope angle by backfilling with no-fine concrete or gravel. After that, a thin layer of soil

with hydroseeding is applied to the surface as a cover and for landscaping. This method

can reduce the construction period, hence alleviating labour costs and operation costs

which then compare with the soil re-compaction method.

However, these replacement and re-compaction methods are constrained in that the

construction sequence has to be scheduled for the dry season when the groundwater levels

are lower than they were at the time of active landsliding. Alternatively, temporary

groundwater lowering through the use of a raking drain may be needed prior to, and during

construction work.

(a)No-Fine concrete replacement (b) Completed no-fine replacement slopeFigure 10: Soil Re-Compaction and No-fine Replacement (NAS, 2004)

1.6.7 Subsurface Drainage

Of all stabilisation methods considered for the prevention of landslides, a reduction

of pore water pressure behind the slope is the most important. According to

Cornforth(2005) described that the subsurface drainage method can reduce the

destabilising hydrostatic and seepage water pressures on the slope as well as the risk of

sliding or flow.

Figure 11: Subsurface drainage (Cornforth, 2005)

For large, unstable slopes, a drainage tunnel can be applied to draw down the water table

and minimise the risk of slope failure. In Hong Kong, the Lung Fu Shan drainage tunnel

and vertical drainage system is under construction. This drainage tunnel can prevent the

failure of a 200m high natural slope which could be triggered by water pressure. Other


14/28


Page 14

subsurface drainage methods include: Drain blanket, Trenches, Cut-off drains, Horizontal

Drains, Relief Drains and Raking Drains.

1.6.8 Shear Piles

According to Cornforth, (2005) described that shear piles are reinforced concretecylindrical piles that pass through the slide plant and anchored at lower end stable soils or

bedrock. This shear pile anchorage can provide lateral bearing resistance near the base of

ground movement (Cornforth, 2005).

Figure 12: Shear pile for slope stability (Saroglou, 2008)

This method is effective for a large instability zone and can provide the flexibility of

selecting an installation location. However, this method has limitations such as being

costly and cannot be installed in moving landslide.

1.6.9 Stone Columns

Based on Cornforth, (2005) described that this ground improvement method can

increase the average shear resistance of soil along a potential slip surface by replacing or

displacing the in situ soil with a series of closely spaced and large diameter columns of

compacted stone. However, this method requires the use of a boring machine and material

delivery, which would result in an access problem if the slope is inaccessible.

Stone columns (Barksdale & Bachus, 1983; Cheung, 1998; Kousik Deb et al., 2007;Ambily, 2007) are another method for slope stabilization. Such columns have been used

since 1950 normally for cohesive soil improvement. It is a hole with circular section which

is filled by gravel, rubble and etc and is an effective method to increase the shear strength

on the slip surface of clayey slopes. The most important cases for utilizing stone columns

(Barksdale & Bachus, 1983) are:

1) Improving slopes stability of both embankment and natural slopes2) Increasing the bearing capacity of shallow foundations constructed on soft soils3) Reducing total and differential settlements4) Decreasing the liquefaction potential of sandy soils


15/28


Page 15

Figure 13: Slope prevention by stone column (Saroglou, 2008)

The performance of stone columns for reinforced and improved soil is easier and cheaper

than other methods such as geotextile, grouting, and compaction (Barksdale & Bachus,m

1983). The diameter of stone column usually varies between 0.3 to 1.2 m and their intervals

between 1.5 to 3 m. Stone columns are normally constructed in multiple rows, depending on

the soil condition.

2 Design of stone column for slope stability prevention

2.1 Methods for slope stability analysis with stone column

Generally, for the analysis of slope treated with the stone column, two methods are applied:

The profile method and the average shear strength method.

(1) In the profile method (or discrete soil-stone column element method), each row of the

stone columns (Christoulas et al., 1997) is converted into an equivalent continuous strip.

Each strip of granular and cohesive soils is then analyzed using its actual geometry and

material properties. The stress concentration in the stone columns results in an increase in

resisting shear force.

Figure 14: Plan of grouped stone columns (Barksdale & Bachus, 1983)

Row 1 2 3 4

S

R2

S

S

Equivalent stone column strip

Stone column


16/28


Page 16

(2) In the average shear strength method, the weighted average material properties are

calculated for the material within the unit cell. The soil having the fictitious weighted

material properties is then used in a stability analysis. It is important to remember that

stone columns must actually be located over the entire zone of material having weighted

shear properties through which the circular arc passes.

To evaluate the factors of safety of the treated soil (after the installation of the

columns) composite values of unit weight, , and strength parameters c, and were used,

replacing the real composite material (soft soilstone columns) with a homogeneous

material of equivalent strength behaviour. The values of , c, were determined by the

formulae proposed by Dimaggio (1978):

csss aa 1

csss cacac 1

csss aa

tantan1tan

A

Aa

ss

Where;

sa = replacement ratio

= unit weight of composite material

s = unit weight of untreated soilc = unit weight of stone column material

c = cohesion of composite material

cs = cohesion of untreated soil

cc = cohesion of stone column material

= angle of internal friction of composite material

s = angle of internal friction of untreated soil

c = angle of internal friction of stone column material


17/28


Page 17

2.2 Closed form solution for slope stability analysis

(1)Factor of safety equation for un-reinforced slope (F.S. no-col):

Figure 15: Static slope stability analysis of untreated soil (Das, 2006)

Figure 16: Force exerted on the strip of soil (Das, 2006)

A

B

C

D

Ws

Ts

NS

USs

hs

l

b

O

A

B

C

D


18/28


Page 18

Ws = Weight of soil slice

NS = Total normal force on the failure surface of soil slice

Ts = Total tangential force on the failure surface of soil slice

US = Pore water pressure on the base of soil slice = usl

s = Inclination of failure surface of soil slice to the horizontal

hs = Height of soil slice between surface of slope and slip surface

Ws = sat,s hs b

Where; sat,s = saturated unit weight of soil slice

NS = Wscoss

= sat,s hsb coss

TS = Wssins

= sat,s hsb sins

s = Normal stress on the base of soil slice

s = Shear stress on the base of soil slice

1

lNSs =

lbh ssssat cos,

=l

bh ssssat cos,

= sssat b 2, cos

1

l

NSs =

l

bh ssssat sin, =

l

bh ssssat sin, = sssat b 2, sin

The equation of shear strength of soil slice, Ss is;

'''tan sssCss

'sC

= effective cohesion of soil

's = effective normal stress on soil

's = effective angle of internal friction of soil

''tan)( ssss uCss

Resisting force on the base of the soil slice is ( ss l = FR,s)

FR,s = '

11

'

tan)(ssss ll uC


19/28


Page 19

s

bl

cos

FR,s ='

sec'

tan)coscos

coscos

(22

, ssssb ss

ws

s

ssats

bh

bhC

Driving force on the base of the soil slice is FD,S

FD,S = Wssins= sat,s hsb sins

F.S. no-col =F

,sF

D,S

R

=ss

sssss

bh

hbC

sat,s

sb

sin

tan)cos(''

sec'

(2) Factor of safety equation for reinforced slope (F.S. soil-col):

Figure 17: Static slope stability analysis of soil reinforced with a row of stone column

Wc = sat,c hc b

Nc = Wccosc = sat,c hcS

R2cosc

Tc = Wcsinc = sat,c hcS

R2

sinc

GH =S

R2

ccos

1

O

E

F

G

H

Stone column strip


20/28


Page 20

Figure 18: Force exerted on the strip of soil-column system

1

cos

12

c

c

c

S

R

N

=

1cos

1

cos

2

2

,

c

ccsat

S

R

hS

Rc

= cccsat h 2, cos

1cos

1

T2

c

c

c

S

R

=

1cos

1

sin

2

2

,

c

ccsat

S

R

hS

Rc

= ccccsat h sincos,

Resisting force offered by soil-column system is

FR = FR,s + FR,c

FR,c = Resisting force offered by stone column

FR,c ='

tantan)( cGHusc

'tan

cos

1)coscos(

222

, c

c

scwsccsat

S

Rhh

= '' tan)cos(2

cccc

S

Rh

FR = FR,s + FR,c

S

R2

Wc

Tc

Nc

Ucc

hc

E

F

G

H


21/28


22/28


Page 22

3 Parametric studies

Parametric study is carried out to find the factor of safety for untreated as well as

for treated slope using the software Slide of Rocscience Inc. Factor of safety of treated

slope is found out by discrete soil element method considering the effect of single as well

as multiple row of stone column. Factor of safety of untreated soil comes out equal to0.926 by Fellenius method. (Table 2 & 3) shows the factor of safety with respect to

distance from the crest of the slope for single and multiple row of stone column.

Figure 19: Location of single row of stone column

Table 2: Effective location of stone column for single row

Distance: x (m) F.S. soil-col

30.24 0.928

0 0.956

22.68 0.990

7.56 1.012

15.12 1.024

Table 3: Effective location of stone column for multiple rows

Distance: x (m) F.S. soil-col

0, 30.24 0.944

15.12, 30.24 1.015

0, 15.12 1.047

15.12, 22.68 1.089

7.56, 15.12 1.097

0, 7.56, 15.12 1.12

7.56, 15.12, 22.68 1.165

15.12, 18.9, 22.68 1.176

7.56, 11.34, 15.12 1.181

L

x

Stone column


23/28


Page 23

Figure 20: Factor of safety v/s distance for single row

Parametric study is being carried out to locate the position of stone column for single row

of stone column. From the (Figure 20), it is shown that the effective location of stone

column is about x=0.5L, if single row of stone column is being used.

Figure 21: Factor of safety of soil column system v/s distance for multiple rows

Factor of safety is also being found out considering the multiple rows (Figure 21) of stone

columns. According to Cornforth (2005), permissible limit for the factor of safety for

natural slope range between 1.1 to 1.2 and that for engineered slope in the range of 1.5 to2.


24/28


Page 24

Figure 22: Spacing of stone column v/s Factor of safety of soil column system

The effect of spacing on the factor of safety is depicted in the (Figure 22), considering the

different value of cohesion of the untreated soil with multiple rows of stone columns at

distances 0.25L, 0.37L & 0.50L from the crest of the slope. Factor of safety increases

about 80% if the cohesion of untreated soil increases 50%.

Figure 23: Factor of safety of soil column system v/s Spacing of stone column

Effect of angle of internal friction on factor of safety is as shown in the (Figure 23).

Considerable increase in safety factor is observed for higher angle of internal friction of

stone column material.


25/28


Page 25

Figure 24: Angle of slope v/s Factor of safety of soil column system

A considerable increase in safety factor (Figure 24) is observed for gentle slope. For higher

value of slope, large decrease in safety factor occurs. Around 32% increase in factor of

safety occurs if slope angle decrease from 40 to 10.

4 Concluding remark

Through the parametric study, it is concluded that the effective location of a single

row of stone column is at a distance of 0. 50L from the crest and that for three rows of

stone column is 0.25L, 0.37L, 0.50L having factor of safety within permissible limit. Both

shear parameters of untreated soil and stone column having a significant effect on

stabilization of slope. For a gentle slope higher factor of safety is achieved than of steeper

slope.


26/28


Page 26

Notation

Ws

= Weight of soil slicesat,s

= Saturated unit weight of soil slice

NS = Total normal force on the failure surface of soil sliceTs

= Total tangential force on the failure surface of soil slice

US

= Pore water pressure on the base of soil slice

s

= Inclination of failure surface of soil slice to the horizontalhs

= Height of soil slice between surface of slope and slip surface

FR,c = Resisting force offered by stone column

FR,s = Resisting force offered by stone column

'sC

= Effective cohesion of soil

's

= Effective normal stress on soil

's

= Effective angle of internal friction of soil

sa

= Replacement ratio = Unit weight of composite material

c = Submerge unit weight of stone column material

s = Submerge unit weight of untreated soil

s

= Unit weight of untreated soilc

= Unit weight of stone column material

c

= Cohesion of composite materialcs

= Cohesion of untreated soilcc = Cohesion of stone column material

= Angle of internal friction of composite material

s = Angle of internal friction of untreated soil

c = Angle of internal friction of stone column material

F.S. soil-col = Factor of safety of soil column system

F.S. no-col = Factor of safety of untreated soil

S = Spacing of stone column

x = Distance of stone column row from the crest of slope

= Angle of slopeb = Width of soil slice

l = Width of soil slice along the slope angle

L = Horizontal length of slope


27/28


Page 27

References

Abramson, L., Thomas S. L, Sunil G. M. (2002). Slope Stability and Stabilisationmethods. John Villey & Sons, Inc., New York.

Ambily A.P., Gandhi S.R. (2007). Behaviour of stone columns based on experimentaland FEM analyses. Journal of Geotechnical and Geoenvironmental Engineering, (4),

405-415.

Ausilio E., Conte E., Dente G. (2001). Stability analysis of slope reinforced with piles,Computers and Geotechnics, (28), 591-611.

Barksdale R.D., Bachus R.C. (1983). Design and construction of stone columns. FederalHighway Administration Office of Engineering and Highway Operations, Volume and

, Washington, DC.

Cheng, Y. M., Lau, C. K. (2008). Slope Stability Analysis and Stabilization: Newmethods and insight. Routledge, New York.

Christoulas, ST, Giannaros CH, Tsiambaos G. (1997). Stabilization of embankmentfoundations by using stone columns. Geotechnical and Geological Engineering, 15, 247-258.

Civil Engineering and Development Department (CEDD), Geotechnical EngineeringOffice (2008). Geoguide 7 Guide to soil nail design and Construction, Hong Kong.

Civil Engineering and Development Department (CEDD), Geotechnical EngineeringOffice. (1995). Construction Specification 2. Hong Kong.

Cornforth, D. (2005). Landslides in Practice: Investigation, Analysis, Remedial andPreventive Options in Soils. John Villey & Sons, Inc., New York.

Das, B. M. (2006). Principles of Geotechnical Engineering. Thomson, Canada.Dimaggio, J.A. (1978). Stone Columns: A Foundation Treatment. Demonstration

project No 46, FHWA, Washington DC.

Ducan, J. M., Buchignani, A. L., Marius De Wet (1987). An Engineering Manual forslope Stability Studies. Report of a study performed by the Virginia Tech Center for

Geotechnical Practice and Research.

Emergency Management Australia Database, Australia Landslide Historic Events.Viewed 1

stSeptember 2008, ttp://www.ema.gov.au/ema/emadisasters.nsf/ Web Events by

Category?

Ghazavi M., Shahmandi, A. (2008). Analytical Static Stability Analysis of SlopesReinforced by Stone Columns. International Association for Computer Methods andAdvances in Geomechanics, 3530-3537.

Greenwood, J. R. (2008). SLIP4EX A Program for Routine Slope Stability Analysis toInclude the Effects of Vegetation, Reinforcement and Hydrological Changes.

Geotechnical and Geological Engineering, 24, 449465.

Hassiotis S., Chameau J.L., Gunarante M. (1997). Design method for stabilization ofSlopes with Piles, Journal of Geotechnical and Geoenvironmental Engineering, 123(4),

314-323.

Highland, L. M., Bobrowsky. P. (2008). The Landslide Handbook - A Guide toUnderstanding Landslides. U.S. Geological Survey, Reston, Virginia.


28/28

credit seminar iii

Documents