Indian Geotechnical Conference (IGC-2010)igs/ldh/files/igc 2015 pune/THEME 3...5 0 th 50th INDIAN GEOTECHNICAL CONFERENCE 17 th–19 DECEMBER 2015, Pune, Maharashtra, India Venue:

50

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INDIAN GEOTECHNICAL CONFERENCE

17th

– 19th

DECEMBER 2015, Pune, Maharashtra, India

Venue: College of Engineering (Estd. 1854), Pune, India

[IMPROVEMENT OF BEARING CAPACITY OF MODEL FOOTING WITH

STRUCTURAL SKIRTS ON SOFT GROUND]

Author1:[Dr.(Mrs) Nayanmoni Chetia][Asstt Prof, Civil Engineering Department, Jorhat Engineering College. Assam, India][

[email protected]]

Author2:[Prof. Dr. Bibha Das Saikia][Prof and HOD, Civil Engineering Department. Royal School of Engineering and

Technology, Guwahati, Assam, India (IGS LM 0569)][[email protected]]

ABSTRACT

Improvement of bearing capacity and reduction in settlement are two important aspects while the

performance of a shallow foundation is taken into consideration. Both can be achieved by providing a

structural skirt to a conventional shallow foundation. A structural skirt essentially consists of a shell

beneath the footing and may have any shape. The footing slab may be circular, square or rectangular in

shape and accordingly the skirt wall will have the same shape in section as that of the footing slab. The

improvement in bearing capacity depends on the geometrical and structural properties of the skirt and

footing such as shape and depth of the footing, skirt geometry etc; soil characteristics namely index

properties of soil and the interface conditions of the soil-skirt-foundation systems such as roughness and

material of the skirt etc. An attempt was made to determine the bearing capacity of shallow foundations

and skirted foundations using both analytical and experimental methods on both cohesive ground, loose

sand and medium sand . At first experiments were conducted on model footings (MF) in a test tank

without skirt at different depths on cohesive soil, loose sand and medium sand. The experimental results

of ultimate bearing capacity for model footing were compared to analytically found results of Terzaghi,

Meyehof, Hansen and Vesic equations from literature. The clay was collected from Titabor region of

Assam and belongs to CL type. Attempt was made to maintain near field condition throughout all the load

tests. For that purpose precalibrated uniform consolidating energy was supplied in layers in the test tank.

For the preparation of the sand bed, sand raining technique was used. The height of free fall was

calibrated from a number of trials. The proximity of the experimental and analytical results confirm that

the test set up provides reproducible data within the expected experimental error range. The pressure

settlement characteristic was then studied for Model Skirted Foundations (MSF) of different shape ratio

(L/B), depth (Df/B) ratio and skirt ratios (Ds/B). Improvement in the performance in terms of increase in

the Bearing Capacity corresponding to each case was calculated from the graph. Ultimate bearing

capacity was increased by an improvement factor of maximum 2.0 in case of soft clay, 3.49 in loose sand

of relative density 35% and 2.28 in medium sand of relative density 65%. From the available literature,

the value of improvement factor in cohesionless soil of different relative densities varies from 1.5 to 3.9.

For cohesive ground, no previous data was found on the performance of skirted foundation on soft clay.

From the experimental results skirt adhesion factors corresponding to various variable parameters (L/B,

Dr.(Mrs)Nayanmoni Chetia, Dr.(Mrs)Bibha Das Saikia

Df/B and Ds/B) were calculated out for soft clay. Reduction in the settlement for providing skirt was also

noted. Settlement reduction factors were 16%-68% in soft clay, 44%-98% in loose sand and 37%-93% in

medium sand. Settlement reduction factors are observed to be mostly dependent upon the skirt factor

(Ds/B).

Keywords: skirted foundation, shape factor, depth factor, skirt factor, improvement factor.

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IMPROVEMENT OF BEARING CAPACITY OF MODEL FOOTING WITH

STRUCTURAL SKIRT ON SOFT GROUND

Dr.(Mrs) Nayanmoni Chetia, Asstt Prof. Jorhat Engineering College.Assam. [email protected]

Dr.(Mrs) Bibha Das Saikia. Prof and H.O.D, Civil Engg Deptt. RSET. Assam [email protected]

ABSTRACT: Improvement of bearing capacity and reduction in settlement are two important aspects while the

performance of a shallow foundation is considered and both can be achieved by providing a structural skirt to it.

The improvement in bearing capacity depends on the geometrical and structural properties of the skirt and footing

such as shape and depth of the footing, skirt geometry etc; soil characteristics namely index properties of soil and

the interface conditions of the soil-skirt-foundation systems such as roughness and material of the skirt etc. An

attempt was made to determine the bearing capacity of model footing (MF) and model skirted foundations (MSF)

using analytical and experimental methods on cohesive ground, loose sand and medium sand. The proximity of the

experimental and analytical results for MF confirms that the test set-up provides reproducible data within the

expected experimental error range. The pressure settlement characteristic of MF and MSF were studied for

different shape ratio (L/B), depth ratio (Df/B) and skirt ratios (Ds/B). Ultimate bearing capacity was increased by an

improvement factor of maximum 2.0 in case of soft clay, 3.49 in case of loose sand and 2.28 in case of medium

sand .

INTRODUCTION

A structural skirt essentially consists of a slab and

a shell and may have any shape. The skirt shape

may be circular, square or rectangular depending

upon the shape of the footing slab and the wall

angle with the vertical may vary from zero to a

value less than 90 degree. It is recommended by

researchers that the proposed foundation can be

used as foundation for small buildings, industrial

floors, storage tank, grain silos, industrial

chimneys etc. Moreover single skirt can be

provided under footing resting on soil bed where

the soil underneath is having variation in its

properties in horizontal direction. Sometimes the

architectural requirements of a structure need some

portion of the structure to be on weak or filled up

soil. In such cases if a skirt is provided on one side

of the footing, the soil particles near the skirt will

be prevented from lateral movement. Bearing

capacity of the foundation is increased and the tilt

of the footing can be reduced by providing skirted

foundation partially under the structure when needed.

In 1993, Rao and Jain[1] studied performance of

various ground treatment approaches through full

scale installation and insitu testing in deep deposits

by using various materials like RCC skirt using

mild steel bars, ferrocement shells, GI sheet

reinforcement, prefabricated brick panel skirting,

interlocking pipe unit skirting, steel or hume pipes

etc. The field study has indicated the best

assurance of safety, speed and economy. In 2004,

Aghbari and Mohamedzein[2]performed laboratory

model test on skirted strip foundation on dense

sand and found that the use of structural skirts can

improve the bearing capacity by a factor of 1.5 to

3.9 depending on the geometrical and structural

properties of the skirts and foundations, soil

characteristics and interface condition of soil skirt

foundation system. In 2006, Aghbari and

Mohamedzein[3]carried out tests on skirted

circular footing models resting on sand. The results

showed that the use of structural skirts improved

the bearing capacity by a factor up to 3. In 2007,

Singh, Prasad and Agarwal[4] studied the effect the

soil confinement on ultimate bearing capacity on

square footing under eccentric inclined load. The ultimate bearing capacity is found to increase by a

factor of 6.75 as compared to the unconfined case.

The bearing capacity ratio was highly dependent

mailto:[email protected]


on the geometrical ratio of Bs/B (skirt width /

footing width). The optimum ratio was 1.5 beyond

which the improvement decreases as the ratio

increases. In 2010, Wakil[5] studied on the

horizontal capacity of skirted circular shallow

footings on sand. The effects of skirt length and the

relative density of sand on the performance of the

footing were investigated through laboratory

testing program. A comparative experimental study

between ultimate horizontal loads attained by

skirted and unskirted footings with the same

properties was conducted and it was found that the

skirts changed the failure mode of circular shallow

footings from sliding mechanism into rotational

mechanism and appreciably increased the ultimate

horizontal capacity of shallow footings.

RESEARCH METHODOLOGY

Test set-up

The methodology consists of performing load tests

for Model Footing (MF) and Model Skirted

Foundations (MSF) in the test tank of size 1.0m

x1.0m x1.0 m for different shape and depth of the

footing and depth of skirt. The test set up

comprises of the loading frame, inverted hydraulic

jack, pumping unit and the test tank essentially

with the various footing assembly. The loading

frame comprises of the reaction frame properly

loaded with cement concrete cubes. A

mechanically operated hydraulic jack of 100kN

capacity is clamped to it. The load from hydraulic

jack is transferred to the subgrade soil through

footings of different shapes and sizes. Pre

calibrated pressure gauge is used to measure the

magnitude of the applied load. The deflection dial

gauges of 0.01mm least count were placed on the

plates with the help of horizontal datum bars to

measure settlements of plates due to loading. This

datum bar arrangement is free from any connection

to the loading arrangement so that any disturbance

in the loading arrangement does not affect the

deflection dial gauge system. The soil used in the

test tank are soft clay for which attempt has been

made to get near field condition, fine sand with low

and medium relative density. For preparation of

cohesive ground the test tank is filled in five layers

with pre-calibrated moisture content and

compacting energy. After five days maturity period

the load tests were performed. The plate represents

Model Footing (MF) and the plate with skirt

represents Model Skirted Foundation (MSF).

Fig 1: Schematic diagram of loading frame

assembly and test tank with MSF at surface.

Photo 1:Loading frame assembly

There are basically two series of load tests. In first

series load tests on MF and in the second series on

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MSF were conducted. The model foundations were

made up of steel plate of thickness 6mm. The

surfaces of plate as well as skirt were made rough

before placing in the test tank. Three different sizes

of steel plates 150mmx150mm, 150mmx300mm

and 150mmx900mm were used as model footings

to represent various shapes of square, rectangular

and strip footing with L/B=1, 2 and 6. Truncated

wooden pyramid was used to ensure uniform

distribution of the load on the model footing from

the spindle of the inverted jack. The variable

geometrical parameters used in the experimental

programme are as follows L/B, Df/B and Ds/B

where

Df = Depth of plate from the surface

Ds=Depth of the skirt

L=Length of the plate

B=Breath of the plate

Photo.2 Different skirts used under plates with

L/B=1, L/B=2, L/B=6.

Material

A locally available natural clayey soil was used to

prepare the clay subgrade. The clay was collected

from Titabor in Jorhat District in disturbed

condition and some samples in undisturbed

conditions. The disturbed samples were used to

find field density, moisture content, grain size

distribution etc for the purpose of preparing the test

bed at near field conditions. The samples used for

Triaxial test was collected in pipes of 0.20m

diameter and 0.60 meter length. To avoid loss of

moisture from the collected soil, both ends of the

pipes were sealed with molten wax and then

covered with high density polythene (Photo. 3).

Photo. 3 Collection of clay samples for finding

out infield soil properties.

The collected clay sample was tested and results

of various tests to find out the index properties are

listed in Table1. From sieve analysis 62.4% of

sample is found finner than 75 micron. The amount

of water to be added and the amount of static load

to be applied was pre calculated through a set of

trials. To serve the purpose, a set of boxes with

dimensions 0.3mX0.3mX0.3m were constructed.

A range of static load were applied to achieve the

required field bulk density at the field moisture

content. With the different amount of

predetermined water mixed with the dried ground

soil mass, the boxes were kept airtight to achieve

moisture equilibrium. The samples were prepared

with varying percentage of moisture content and


various static loads were applied in multiples of

50N.

Table 1 properties of clay

Property

values Relevant IS

Code

Field moisture

content

38.60% 2720 Pt II

Field bulk

density

18.7kN/m3

2720 Pt XIV

Specific Gravity

2.63 2720 Pt III

% fines

62.40 2720 Pt IV

Liquid limit

39.80(%)

9259-1979

Plastic limit

21.00(%)

2720 Pt V

Plasticity Index

19.80(%)

2720 Pt V

Cohesion

5.1kN/m2

2720 Pt XI

Angle of

internal friction

0 degree 2720 Pt XI

Photo. 4 Prepared clay bed on the test tank

The process of filling the test tank starts with air

drying the soil, pulverised and then ground finely

with wooden hammer. The test bed in tank has

been prepared with moisture content 40% and

corresponding compacting pressure 0.0030 N/mm2.

The next step is to mix the soil with predetermined

amount of water and to keep the moist soil in

airtight condition to achieve moisture equilibrium

condition. To prepare the test bed wet soil was

placed in the test box in 0.20m thick layers. Each

layer is put under the predetermined pressure as

found out from previous trials, ie 0.0030N/mm2.

This was done with the help of four iron plates,

with the lowermost plate having the same size as

that of the tank. The properties of the prepared test

bed were found out and are listed in table 2.

Table. 2 Properties of prepared clay bed in tank

Property Range Average

value

Moisture content

(%)

38.0-39.8 38.4

Bulk density

(kN/m3)

17.88-18.82 18.70

Vane shear

strength(kPa)

4.9-5.4 5.1

For preparation of the sand bed the sand used for

this purpose is locally available fine sand in Jorhat.

The various properties of sand determined in the

laboratory are listed in table 3.2. For the

preparation of the sand bed, sand raining technique

was used. The height of free fall was calibrated

from a number of trials. Consistency in the

placement density was checked with small

aluminium cans of known volumes at different

locations and at different heights. It has a hopper

connected to 690mm long pipe with an inverted cone at the bottom. The sand passes through the 31

mm internal diameter pipe and disperses at bottom

by a 60˚ inverted cone. The placement density of

the sand can be varied by changing the height of

free fall. To achieve lower relative density, sand is

poured through a wire mesh fixed at a certain

height. Two test series with relative densities 35%

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and 65% was done. The corresponding densities

are 17.2 KN/m3 and 18.2KN/m3.

Table 3 properties of sand used in the study

Property Test

values

Relevant IS

Code

Specific Gravity 2.68 2720 Pt 3

D10 (mm) 0.19 460-1962

D30 (mm) 0.31 2720 Pt 4

D50 (mm) 0.44 2720 Pt 4

D60 (mm) 0.48 2720 Pt 4

Co-efficient of uniformity,

Cu

2.52 2720 Pt 4

Co-efficient of curvature,

Cc

1.05 2720 Pt 4

Max dry density,γdmax

19.41 2720 Pt 14

Min dry density,γdmin 15.98 2720 Pt 4

Friction angle from direct

shear test, ϕ (ID= 35%)

32o 2720 Pt 13

Friction angle from direct

shear test, ϕ (ID=65%)

35o

2720 Pt 13

Plan of the test series

The load settlement characteristics are dependent

upon many factors such as, sub grade material,

footing shape and size, depth of footing, presence

or absence of skirt, roughness and stiffness of the

footing as well as that of skirt etc. Since it is not

possible to study the effect of all the factors, load-

settlement characteristics were studied by varying

ratios of L/B, Df/B and Ds/B. Twenty seven

number of load tests were done on clay medium as

detailed in table 4.The properties of prepared clay

bed is listed in Table 4.

Assumptions

1. The foundation is rough and rigid.

2. No slippage occurs between footing and skirt.

3. The friction is considered only on the outer side

of the skirt wall.

4. No relative movement of the soil inside the skirt

takes place with respect to the skirt wall.

5. The load is transferred from the column to the

footing and to the skirt.

Table 4 Test Details for clay.

Series γ in

kN/cum

L/B Df/B Ds/B

C1 18.70 1 0.0 0.0

C2 18.68 1 0.0 0.5

C3 18.70 1 0.0 1.0

C4 18.70 1 0.5 0.0

C5 18.70 1 0.5 0.5

C6 18.66 1 0.5 1.0

C7 18.70 1 1.0 0.0

C8 18.70 1 1.0 0.5

C9 18.70 1 1.0 1.0

C10 18.66 2 0.0 0.0

C11 18.70 2 0.0 0.5

C12 18.70 2 0.0 1.0

C13 18.70 2 0.5 0.0

C14 18.70 2 0.5 0.5

C15 18.68 2 0.5 1.0

C16 18.70 2 1.0 0.0

C17 18.70 2 1.0 0.5

C18 18.70 2 1.0 1.0

C19 18.70 6 0.0 0.0

C20 18.68 6 0.0 0.5

C21 18.66 6 0.0 1.0

C22 18.70 6 0.5 0.0

C23 18.70 6 0.5 0.5

C24 18.70 6 0.5 1.0

C25 18.70 6 1.0 0.0

C26 18.70 6 1.0 0.5

C27 18.70 6 1.0 1.0

Similarly 27 no of tests were conducted on fine

sand in loose condition and the respective values

of C, ϕ, and γave are 0, 32°, 17.22 kN/cum . For fine

sand with medium relative density, another set of

tests (27 nos) were conducted with medium

relative density and the parametric variation was as

in table 4. The respective values of of C, ϕ, and γave

are 0, 36°, 18.2kN/cum respectively.

EXPERIMENTAL RESULTS

The results of 27 nos of model load test on soft

clay sub grade are presented in Fig 3,4 and 5

which shows the pressure-settlement


characteristics of square (L/B=1), rectangular

(L/B=2) and strip (L/B=6). For each shape ratio of

the plate (L/B), 9 nos of tests were performed for

different conditions of depth ratio and skirt ratios.

Fig.3 Comparison of performance of MSF with

L/B=1 on clay

Pressure settlement characteristics have been

studied for three different footing depths with and

without providing the skirt. Attempts were made to

prepare test bed for each load test at same density

and moisture content so that undrained shear

strength of soil bed could be maintained

approximately at same value (5.1kPa). The

ultimate bearing capacity of MF are calculated

using Terzaghi, Meyerhof, Hansen and Vesic

Methods. The different geometrical parameters

required for finding out the 𝑞𝑢𝑙𝑡 in the study for different shapes and depths of MF have been

calculated by using Terzaghi (T), Meyerhof (M),

Hansen (H) and Vesic (V) equations. The solutions

for tilted base and footing on slope are available in

the literature but they are not included here. In the

laboratory, plate load tests were performed only

when the loads were vertical, concentric and MF

was perfectly horizontal. The analytical and

experimental values for MF are listed in table 5.

Fig. 4 Comparison of performance of MSF with

L/B=2 on clay

Fig. 5 Comparison of performance of MSF with

L/B=6 on clay

The ultimate bearing capacity of MF are

calculated using Terzaghi, Meyerhof, Hansen and

Vesic Methods. The different geometrical

-2

0

2

4

6

8

10

12

14

16

18

0 0.05 0.1

Sett

lem

en

t,s/

B in

%

Bearing pressure in N/sq.mm

Df/B= 0, Ds/B= 0

Df/B= 0, Ds/B=0.5

Df/B= 0, Ds/B=1

Df/B=0.5, Ds/B= 0

Df/B= 0.5, Ds/B= 0.5

Df/B=0.5, Ds/B= 1

Df/ B=1, Ds/B= 0

Df/B=1, Ds/B=0.5

Df/B=1, Ds/B=1

0

5

10

15

20

25

0 0.02 0.04

Set

tlem

ent,

s/B

in

%

Bearing pressure in N/sqmm

Df/B=0,Ds/B=0

Df/B=0,Ds/B=0.5

Df/B=0.Ds/B=1

Df/B=0.5, Ds/B=0

Df/B=0.5, Ds/B=0.5

Df/B=0.5, Ds/B=1

Df/B=1, Ds/B=0

Df/B=1, Ds/B=0.5

Df/B=1,Ds/B=1

0

5

10

15

20

25

30

35

0 0.02 0.04

Sett

lem

en

t,s/

B in

%

Bearing pressure in N/sqmm

Df/B=0, Ds/B=0

Df/B=0, Ds/B=0.5

Df/B=0, Ds/B=1

Df/B=0.5, Ds/B=0

Df/B=0.5, Ds/B=0.5Df/B=0.5, Ds/B=1

Df/B=1, Ds/B=0

Df/B=1,Ds/B=0.5

Df/B=1, Ds/B=1

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parameters required for finding out the 𝑞𝑢𝑙𝑡 in the study for different shapes and depths of MF have

been calculated by using Terzaghi (T), Meyerhof

(M), Hansen (H) and Vesic (V) equations. The

solutions for tilted base and footing on slope are

available in the literature but they are not included

here. In the laboratory, plate load tests were

performed only when the loads were vertical,

concentric and MF was perfectly horizontal. The

analytical and experimental values for MF are

listed in table 5.

Table 5 Comparison of experimental and

theoretical values of ultimate bearing capacities

(qult) in kN/m2 for MF

C=5.1kN/m2, φ= 0°, γ= 18.7 kN/m

3

L/

B

Df/

B

qult

(EX)

qult(T) qult(M) qult(H) qult(V)

1 0.0

22.22 25.20 31.45 31.19 31.19

1 0.5

26.66 26.60 36.00 38.83 38.83

1 1.0

31.11 28.00 40.50 46.48 46.48

2 0.0

20.00 22.29 28.84 28.75 28.75

2 0.5

20.00 23.69 33.11 35.90 35.90

2 1.0

22.22 25.09 37.40 41.65 41.65

6 0.0

17.78 19.38 27.00 27.05 27.05

6 0.5

17.78 20.78 31.10 33.86 33.86

6 1.0

20.00 22.20 35.20 40.67 40.67

It is observed that experimental data shows more

proximity to the results obtained by Terzaghi’s

analytical equations. The proximity of the

experimental results ensures the reliability of the

test set-up. Likewise tests were performed on fine

sand in loose condition as stated before and the

results are presented in Fig 6, 7 and 8


L/B=1 on loose sand


L/B=2 on loose sand

0.001

0.01

0.1

1

0.01 0.1 1 10B

eari

ng

Pre

ssu

re in

N/s

q.m

mSettlement in mm

Df/B=0, Ds/B=0

Df/B=0, Ds/B=0.5

Df/B=0, Ds/B=1

Df/B=0.5, Ds/B=0

Df/B=0.5, Ds/B=0.5Df/B=0.5, Ds/B=1

Df/B=1, Ds/B=0

Df/B=1, Ds/B=0.5

Df/B=1, Df/B=1

0.0010

0.0100

0.1000

1.0000

0.01 1.00

Bea

rin

g p

ress

ure

in N

/sq

.mm

Settlement in mm

Df/B=0, Ds/B=0

Df/B=0, Ds/B=0.5

Df/B=0, Ds/B=1

Df/B=0.5, Ds/B=0

Df/B=0.5, Ds/B=0.5

Df/B=0.5, Ds/B=1

Df/B=1, Ds/B=0

Df/B=1, Ds/B=0.5

Df/B=1, Ds/B=1



L/B=6 on loose sand


L/B=1 on medium sand





0.001

0.01

0.1

1

0.01 0.1 1 10

Bea

rin

g P

ress

ure

in N

/sq

.mm

Settlement in mm

Df/B=0, Ds/B = 0

Df/B=0, Ds/B=0.5

Df/B=0, Ds/B=1

Df/B=0.1, Ds/B=0

Df/B=0.5, Ds=0.5

Df/B=0.5, Ds/B=1

Df/B=1, Ds/B=0

Df/B=1, Ds/B=0.5

Df/B=1, Ds/B=1

0.001

0.01

0.1

1

0.01 0.1 1 10

Bea

rin

g P

ress

ure

in N

/sq

.mm

Settlement in mm

Df/B=0, Ds/B= 0

Df/B=0, Ds/B=0.5

Df/B=0, Ds?B=1

Df/B=0.5, Ds/B=0

Df/B=0.5, Ds/B=0.5

Df/B=0.5, Ds/B=1

Df/B=1, Ds/B=0

Df/B=1, Ds/B=0.5

Df/B=1, Ds/B=1

0.0010

0.0100

0.1000

1.0000

0.01 1.00

Bea

rin

g p

ress

ure

in N

/sq

.mm

Settlement in mm

Df/B=0, Ds/B=0

Df/B=0, Ds/B=0.5

Df/B=0, Ds/B=1

Df/B=0.5, Ds/B=0

Df/B=0.5, Ds/B=0.5

Df/B=0.5, Ds/B=1

Df/B=1, Ds/B=0

Df/B=1, Ds/B=0.5

Df/B=1, Ds/B=1

0.001

0.01

0.1

1

0.01 1

Bea

rin

g P

ress

ure

in N

/sq

.mm

Settlement in mm

Df/B=0, Ds/B=0

Df/B=0, Ds/B=0.5

Df/B=0, Ds/B=1

Df/B=0.5, Ds/B=0

Df/B=0.5, Ds/B=0.5

Df/B=0.5, Ds/B=1

Df/B=1, Ds/B=0

Df/B=1, Ds/B=0.5

Df/B=1, Ds/B=1

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Fig 9, 10 and 11 shows the results of the tests

performed on medium sand. In case of sand it is

difficult to get the definite break point in the

pressure settlement characteristics graph and

hence log-log plot is used.

DISCUSSION

From all graphs, it is apparent that improvement in

the performance was observed in case of MSF.

Improvement factor was calculated as ratio of

bearing capacity of MSF to that of MF for a

constant value of L/B and Df/B. Improvement

factors calculated for different MSF over MF has

been lised in Tables 6,7 and 8.

Table 6. Comparison of Improvement factors

w.r.to square MF at surface in soft clay

Df/

B

Ds/

B

Bearing

Pressure

(N/mm2)

IF for

(TD/

B)*

=0.5

IF for

(TD/

B)*

=1.0

IF for

(TD/

B)*

=1.5

0.0 0.0 0.0222

0.0 0.5 0.0311 1.4

0.0 1.0 0.0444 2.0

0.5 0.0 0.0267 1.2

0.5 0.5 0.0400 1.8

0.5 1.0 0.0489 2.19

1.0 0.0 0.0311 1.4

1.0 0.5 0.0489 2.19

1.0 1.0 0.0578

( TD/B)*= (Total depth of embedment/B)

=(Df +Ds)/B

It is observed that the Improvement factor

increases with increase in total depth of

embedment, ie (TD/B)*. Also observed that the

major contribution to improvement factor is

always from the skirt depth (Df ). Comparing the

tables 6, 7 and 8, skirts were found to be more

effective for square MF as IF (Improvement factor)

is maximum in square footing . The maximum

value of IF in clay was 2.19.


w.r.to Rectangular MF at surface in soft clay

Df/

B

Ds/

B

Bearing

Pressure

(N/mm2)

IF for

(TD/

B)*

=0.5

IF for

(TD/

B)*

=1.0

IF for

(TD/

B)*

=1.5

0.0 0.0 0.0200

0.0 0.5 0.0311 1.56

0.0 1.0 0.0378 1.89

0.5 0.0 0.0200 1.00

0.5 0.5 0.0244 1.22

0.5 1.0 0.0311 1.55

1.0 0.0 0.0222 1.10

1.0 0.5 0.0267 1.34

1.0 1.0 0.0311

Table 8. Comparison of Improvement factors w.r.to Strip MF at surface in soft clay

Df/

B

Ds/

B

Bearing

Pressure

(N/mm2)

IF for

(TD/

B)*

=0.5

IF for

(TD/

B)*

=1.0

IF for

(TD/

B)*

=1.5

0.0 0.0 0.0178

0.0 0.5 0.0222 1.25

0.0 1.0 0.0311 1.75

0.5 0.0 0.0178 1.00

0.5 0.5 0.0222 1.25

0.5 1.0 0.0289 1.63

1.0 0.0 0.0200 1.13

1.0 0.5 0.0244 1.37

1.0 1.0 0.0267

Improvement factors were also found out for other

two series of tests, i.e loose sand and medium sand

and listed in tables 9 and 10. For Loose sand the

maximum value of IF was 3.49 and for medium

sand the maximum value was 2.28. Significant

reduction in the settlement was also observed in all

cases. From the results of Table 9 the skirt was

found to be very effective when placed at very

shallow depth in loose sand (IF=3.49) as there is

3.49 fold increase in the bearing capacity.



w.r.to MF at surface in loose sand

L/B Df/B Ds/B Bearing

Capacity

(N/mm2)

Improvement

factor w.r.to

MF

(IF)

1 0.0 0.0 0.01778 1.00

1 0.0 0.5 0.03556 2.00

1 0.0 1.0 0.06222 3.49

1 0.5 0.0 0.04000 1.00

1 0.5 0.5 0.04444 1.11

1 0.5 1.0 0.07111 1.78

1 1.0 0.0 0.06222 1.00

1 1.0 0.5 0.07111 1.14

1 1.0 1.0 0.08000 1.29

2 0.0 0.0 0.01778 1.00

2 0.0 0.5 0.03556 2.00

2 0.0 1.0 0.05333 2.99

2 0.5 0.0 0.04444 1.00

2 0.5 0.5 0.05333 1.20

2 0.5 1.0 0.05333 1.20

2 1.0 0.0 0.06222 1.00

2 1.0 0.5 0.07111 1.14

2 1.0 1.0 0.08000 1.29

6 0.0 0.0 0.01778 1.00

6 0.0 0.5 0.03111 1.75

6 0.0 1.0 0.04000 2.25

6 0.5 0.0 0.04444 1.00

6 0.5 0.5 0.04888 1.09

6 0.5 1.0 0.06667 1.50

6 1.0 0.0 0.07111 1.00

6 1.0 0.5 0.07111 1.00

6 1.0 1.0 0.08000 1.13

From table 9 and 10 also it was observed that

contribution of skirt depth is more prominent than

the contribution of depth of footing.

Reduction in the settlement for providing

skirt was also noted. Settlement reduction factors

were 16%-68% in soft clay, 44%-98% in loose

sand and 37%-93% in medium sand. Settlement

reduction factors are observed to be mostly

dependent upon the skirt factor (Ds/B).

Table10. Comparison of Improvement factors

w.r.to MF at surface in medium sand

L/B Df/B Ds/B Bearing

Capacity

(N/mm2)

Improvement

factor w.r.to

MF

(IF)

1 0.0 0.0 0.0311 1.00

1 0.0 0.5 0.0444 1.42

1 0.0 1.0 0.0711 2.28

1 0.5 0.0 0.0533 1.00

1 0.5 0.5 0.0711 1.33

1 0.5 1.0 0.0888 1.67

1 1.0 0.0 0.0622 1.00

1 1.0 0.5 0.0711 1.14

1 1.0 1.0 0.0888 1.43

2 0.0 0.0 0.0355 1.00

2 0.0 0.5 0.0533 1.50

2 0.0 1.0 0.0889 2.50

2 0.5 0.0 0.0500 1.00

2 0.5 0.5 0.0800 1.66

2 0.5 1.0 0.0890 1.78

2 1.0 0.0 0.0622 1.00

2 1.0 0.5 0.0711 1.14

2 1.0 1.0 0.0800 1.29

6 0.0 0.0 0.0400 1.00

6 0.0 0.5 0.0533 1.33

6 0.0 1.0 0.0755 1.89

6 0.5 0.0 0.0666 1.00

6 0.5 0.5 0.0711 1.07

6 0.5 1.0 0.0933 1.40

6 1.0 0.0 0.0978 1.00

6 1.0 0.5 0.1067 1.09

6 1.0 1.0 0.1155 1.18

While studying the performance of skirt on

soft clay, the increase in ultimate bearing capacity

by providing skirt can be calculated in the

following way. The increased bearing capacity

may be considered as product of cohesion (C in

kN/m2), surface area of the skirt (As in m

2) and

skirt adhesion co-efficient α.

Mathematically, 𝑞𝑒𝑥𝑐𝑒𝑠𝑠 = 𝛼𝐶𝐴𝑠

Where qexcess is the bearing capacity contributed to

the MF by the skirt. The value of α has been

calculated and tabulated in table 11.

50

th

IG

C

50th


17th

– 19th



Table11. List of values of Co-Efficient α

Sl No Df/B L/B Ds/B α

(kN/m2)/m

2

1 0.0 1 0.5 0.870

2 0.0 1 1.0 1.090

3 0.0 2 0.5 1.450

4 0.0 2 1.0 1.160

5 0.0 6 0.5 0.670

6 0.0 6 1.0 0.920

7 0.5 1 0.5 1.300

8 0.5 1 1.0 1.090

9 0.5 2 0.5 0.520

10 0.5 2 1.0 0.717

11 0.5 6 0.5 0.737

12 0.5 6 1.0 0.933

13 1.0 1 0.5 1.740

14 1.0 1 1.0 1.300

15 1.0 2 0.5 0.588

16 1.0 2 1.0 0.725

17 1.0 6 0.5 0.403

18 1.0 6 1.0 0.380

CONCLUSION

1. Use of structural skirts in conjunction with

conventional shallow structural foundations

increase in the bearing capacity and reduce

settlement. The bearing capacity increases with the

increase in the length of the embedment.

2. Ultimate bearing capacity is increased by an

improvement factor of maximum 2.0,3.49 and

2.28 for soft clay, loose sand and medium sand

respectively.The experimental results of ultimate

bearing capacity for MF (Model Footing ) were compared to analytically found results of Terzaghi,

Meyerhof Hansen and Vesic equations. They are

in good agreement which proves the reliability of

the results.

3. Skirts work better in shallow depths and it is

clear that for the same total depth of embedment,

Skirt in conjunction with traditional footing

performs better than traditional footing alone.

4. The maximum IF is observed for all three cases

is corresponding to a total depth of penetration

1.5B.

5. Square skirts shows the best performance .For

clay the value of skirt adhesion factor is maximum

for L/B=1 and minimum for L/B=6.

REFERENCES

[1] Rao, B.G. and Jain,M.P (1993) “ Innovative

foundation in poor ground” Third International

Conference on case histories in geotechnical

engineering, St. Louis, Missouri, June 1-4 ,Paper

No.7.45

[2] AL-Aghbari, M.Y. and Mohamedzein,Y.E.A

(2004). “Bearing Capacity of Strip Foundations

with Structural Skirts,” Geotechnical and

Geologocal Engineering, 22, 43-57.

[3] AL-Aghbari, M.Y. and Mohamedzein,Y.E.A

(2006) “Improving the performance of circular

foundations,” Ground Improvement, 10(3), 125-

132

[4] Sireesh, S., Sitharam, T.G. and Dash, S.K.

(2009) “Bearingcapacity of circular footing on

geocel-sand mattress overlying clay bed with

void.” Geotextiles and Geomembranes, 27,

89- 98. [5] Wakil, A. (2010) “Horizontal Capacity of

Skirted circular Shallow Footing on Sand”

Alexandria University Journal,2010,49

(379-385).

[6] ] Rao, B. Gobind. (1982). “Behaviour of Skirted

Granular Pile Foundations.” Ph. D Thesis,

University of Roorkee.

[7] Das, Bibha (1983). “An Optimal Limit State

Design of Cylindrical skirted Foundation and

Circular footing structures and Limit analysis of

Cylindrical Silos”. Ph. D Thesis, IIT Kanpur.


Documents

Indian Geotechnical Conference (IGC-2010)igs/ldh/files/igc 2015 pune/THEME 3...5 0 th 50th INDIAN GEOTECHNICAL CONFERENCE 17 th–19 DECEMBER 2015, Pune, Maharashtra, India Venue: