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ORIGINAL ARTICLE

Improving the bearing capacity of footing on soft clay

with sand pile with/without skirts

Ashraf Kamal Nazir *, Wasim R. Azzam

Department of Structural Engineering, Faculty of Engineering, Tanta University, Tanta, Egypt

Received 24 October 2009; accepted 21 June 2010

Available online 26 January 2011

KEYWORDS

Soil improvement;

Bearing capacity;

Circular footing;

Soft clay;

Confinement;

Skirts and settlement;

Sand pile

Abstract This paper presents the results of laboratory model tests for studying the improvement of

soft clay layer by using both partially replaced sand piles with/without confinement. This research is

performed to study the effect of sand pile to improve the bearing capacity and to control the set-

tlement. Also the research aimed at investigating the variation of subgrade modulus, and the

induced failure mechanism of shallow circular footing on replaced soil with/without skirts. The

results show that the improvement of load bearing capacity is remarkable; using both partiallyreplaced sand pile with and without confinement by skirts. The adopted technique can substantially

modify the stress displacement curve of the footing rested on soft clay layer, significantly decreases

the settlement and the replaced soil block inside the skirts behave as deep foundation. Therefore, the

bearing capacity failure mechanism of a footing rested on soft clay can be modified from exclusive

settlement to general bearing capacity failure at the tip of confined replaced sand column.

ª 2010 Faculty of Engineering, Alexandria University. Production and hosting by Elsevier B.V.

All rights reserved.

1. Introduction

Soft clay deposits are extensively located in many costal areas

and they exhibit poor strength and compressibility. Utilization

of various improvement methods for soft soil particularly soft

clay is used in a wide range. These methods were based on

using lime, cement and flay ash stabilization as presented by

previous studies carried out by Ali et al. [1], Balasubramaniam

et al. [5], Muntohar and Hantoro [13], Muntohar and Hashim

[15], and Muntohar [14]. Further information has been re-

ported in order to improve the soft ground by using soil ce-

ment column method as reported by Hebib and Farrell [10]

and Porbaha and Bouassida [16]. In addition, there are many

investigations which were carried out to enhance the load bear-

ing capacity of soft soil by adopting the soil reinforcement

technique as presented by Hirao et al. [11] and El Sawwaf [8].

* Corresponding author. Mobile: +20 121704422.E-mail addresses: [email protected] (A.K. Nazir),

[email protected] (W.R. Azzam).

1110-0168 ª 2010 Faculty of Engineering, Alexandria University.

Production and hosting by Elsevier B.V. All rights reserved.

Peer review under responsibility of Faculty of Engineering, Alexandria

University.

doi:10.1016/j.aej.2010.06.002

Production and hosting by Elsevier

Alexandria Engineering Journal (2010) 49, 371 – 377

Alexandria University

Alexandria Engineering Journal

www.elsevier.com/locate/aejwww.sciencedirect.com

mailto:[email protected]


http://dx.doi.org/10.1016/j.aej.2010.06.002





http://www.sciencedirect.com/science/journal/11100168

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The bearing capacity of footings on soft clay can be also

improved considerably by placing a layer of compacted gran-

ular fill of limited thickness without/with geotextile or geogrid

reinforcement at sand clay interface Love et al. [12]. The depth

of the replaced fill depends on the required bearing capacity

and the allowable settlement. Sometimes this technique leads

to great heights of soil replacement and hence excessive cost

and effort. As a cost-effective alternative to this solution in this

investigation, a combination of both structural skirts that arefixed to footing edge and local replacement only inside the

skirted footing can be provided in stabilizing soft soil. Where

the technique of using vertical reinforcement in the form of

skirts that are fixed to foundation edges used to improve the

load bearing capacity, controls the horizontal movement of

soil under the footing and provided significant confinement

tools for the supporting soil. The soil confinement is one of

the used methods for improving soil capacity. The various

researchers have studied the effect of lateral confinement on

bearing capacity by both skirts system and separate vertical

cell. Al-Aghbari and Khan [2], Al-Aghbari [3], Gourvenec

[9], and Al-Aghbari [4] have provided a valuable detailed con-

sideration of skirts applications with footing. For instance, the

behavior of foundation resting on confined subgrade by verti-cal cell was investigated by El Sawwaf and Nazer [7] and Singh

and Prasad [17]. The previous investigation was carried out

only in sandy soil and cannot be thoroughly applied to cohe-

sive soil. The low cost workable ground improvement tech-

nique presented in this paper is a combination of using

skirted foundation and in site local replacement of soft soil

only inside skirts of the circular footing. This adopted tech-

nique is done by partial replacement of soft soil via well com-

pacted sand that confined with skirts of the adopted footing

(as a replaced sand column). This replacement system is only

used under the footing and inside the footing skirts without

whole site stabilization. This local stabilization is used only

for decreasing both the amount of replaced soil and compac-tion effort that is required for replacement process. Therefore,

the main objective of this research is to address the aspect of

both the bearing capacity improvement and settlement reduc-

tion through using both local soil replacement of soft soil over-

lying sand bed and the optimum skirts footing soil system.

2. Laboratory model tests

2.1. Model box

Fig. 1 shows a schematic view of the experimental model appa-

ratus used in this study. The test box is cylinder-shaped, having

inside diameter of 90 cm in plane and 120 cm in depth, the

walls of tank have thickness of 6 mm. The tank box was built

sufficiently rigid to maintain plain strain conditions by mini-mizing the out of plain displacement in all directions. The tank

walls were braced from the outer surface using horizontal steel

beam fitted at the mid depth of the tank. The inside walls of

the tank are polished smooth to reduce friction with the soil

as much as possible by using galvanized coat on the inside

wall.

The loading system consists of a hand-operated hydraulic

jack and pre-calibrated load ring to apply the load manually

to the footing soil system.

2.2. Model footing

The model footing was made of a steel circular plate with a

diameter of 100 mm, and 20 mm thickness. A rough base con-

dition was achieved by fixing a thin layer of sand onto the base

of the model footing with Epoxy glue. The load is transferred

to the footing through a ball bearing which was placed be-

tween the footing and the proving ring.

2.3. Structural skirts

A circular mild steel pipe with knife edge is used as structural

skirt with thickness 4 mm. with different heights. The internal

diameter of the skirts is 102 mm.

2.4. Test material

2.4.1. Soft clay

The normally consolidated soft clay bed was prepared by

pouring the clay in layers, at a water content equal to its liquid

(LL). The clay was identified as CL according to Unified Clas-

sification System. The shear strength for the soil was obtained

through Vane Shear Test. The soft clay bed was thoroughly

mixed and placed by hand into the model tank overlying sand

layer in various depths according to testing program. A uni-

form pressure of 40 kN/m2 was applied gradually to get the

normally consolidated bed of clay as presented by El Sawwaf

[8]. The pressure was kept for 48 h then gradually released in

2 h. The properties of the clay bed are given in Table 1.

120 cm

90 cm

1

2

3

4

57

8

9

6

10

1. loading frame

2. Hydraulic jack

3 . Proving ring

4. model footing

5. skirts

6 . Dia l g aug e

7. Replaced fil l

8 . Soft clay bed

9. Compacted sand

10. Test tank

B is footing diameter

D is compacted sand layer

thickness

h is thickness of clay bed

L is skirts depthB

L

D

h

Figure 1 Schematic view of the test setup.

Table 1 Properties of soft clay.

Specific gravity 2.63

Liquid limit (%) 40

Plastic limit (%) 20

Shrinkage limit (%) 13

Plasticity index (%) 20

Optimum moisture content (%) 15

Av erag e mo istu re co nte nt du rin g t esti ng (%) 3 6

Field unit weight during testing (kN/m3) 15.50

Untrained cohesion (kN/m2) 22

372 A.K. Nazir, W.R. Azzam

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2.4.2. Sand bed

The sand used in this research to perform the underlying strata

beneath the soft clay layer is medium to coarse rounded silica

sand. For each test a homogenous bed of dry silica sand was

formed. The mean grain size D50% = 0.33 mm and the unifor-

mity coefficient is 3.5. The physical and mechanical properties

of tested sand are shown in Table 2. To prepare the compacted

sand bed the Japanese method was adopted. The relative den-

sity and the sand depth were kept constant during the tests.This sand layer is acted as bearing layer for partially replaced

system.

3. Testing program and methodology

First, the sand bearing layer is placed at bottom of the tank in

layers where each has a thickness of 50 mm and compacted to

achieve the required relative density, Dr = 80%. This tech-

nique is continued to reach to total thickness of 40 cm (four-

times footing diameter, D/B = 4). This thickness is kept con-

stant during the test program. The top layer of soft clay is

placed carefully as described before and maintained at the

studied depth. After preparing the clay bed, the steel pipe

was pushed till the required depth, the soft layer is excavatedout side the pipe, then pipe was charged with sand and com-

pacted in layers 50 mm thick. The pipe was kept for studying

the skirted condition while in the case of non-skirted condition

the pipe was withdrawn to a certain level and charging of sand

for the next layer was continued. The process of charging the

sand, compaction and withdrawal of the pipe was carried

out simultaneously. The sand used for partial replacement is

similar to the sand layer beneath the soft clay. The replaced

sand column is compacted manually in layers at relative den-

sity of 80% using manual steel circular hammer having

70 mm diameter and 20 mm in thickness. The number of drop

passes is evaluated at the beginning of the study to establish

the desire sand density. Compaction was carried out very care-fully so that the prepared soft clay did not get disturbed. The

depth of replaced soil is equal to the skirts’ depth for all testing

program. After installation the soil bed, the footing is placed at

the pre-described location with skirts to predetermined depth,

the load was applied by a hydraulic jack. The load was applied

in small increments until reaching failure. The settlement of the

skirted footing on partially replaced soft clay was measured

using two dial gauges placed on the opposite side of the

footing.

3.1. Testing parameters

A series of loading tests were carried out for footing with and

without structural skirts resting on partially replaced soft clay.

An experimental program was carried out to investigate the

behavior of skirted footing on replaced sand column. Fig. 1

shows the definitions of the studied parameters. Table 3 shows

the details of studied parameters:

qult skirts: Ultimate bearing capacity in the case of skirted

footing.

qulto: Ultimate bearing capacity in the case of without

reinforcement.

In all testes the depth of replaced fill equals to skirts depth

(L).

4. Results and discussion

The load–settlement relationship and the ultimate bearing

capacity of the footing on replaced soil with/without skirts

were obtained. The ultimate bearing capacity of the footing/

soil system for each test was estimated from the load displace-

ment curves where slope of the load displacement curves either

first reaches to zero or becomes steady at minimum value.

4.1. Effect of partial replacement with/without skirts

In order to investigate the effect of partial replacement soil

with/without skirts Fig. 2 is presented, typical variation of

bearing pressure (q) and normalized settlement (S /B) for the

footing without skirts resting on partially replaced soft soil

as a replaced trench beneath the footing at different fill depth

(L/h). The figure shows that the partial replacement of soft clay

highly improves and modifies the load bearing capacity at the

same settlement ratio. Whereas, the stress–settlement relation-

ship of skirted footing on partially replaced soft clay for differ-

ent skirts heights is shown in Fig. 3. For the case of (h/D = 1).

It can be seen that the installation of skirts with the replaced

Table 2 Physical and mechanical properties of tested sand.

Specific gravity 2.65

Maximum dry density, cdmax (kN/m3) 17.96

Minimum dry density, cdmin (kN/m3) 15.6

Maximum shear angle / tri 40

Maximum voids ratio e max 0.699

Minimum voids ratio emin 0.472

Relative density D r during tests 80%

Table 3 Details of laboratory model tests.

Test series Type of test Details of test program

C ons tant par ame ters V aria ble pa ramete rs

1 Normal footing without replacement D = 4B = 40 cm h/D = 1, 1.5, and 2

2 Partial replacement

Without skirts

h/D = 1, D = 40 cm L/h = 0.25, 0.50, 0.75, and 1.00

3 h/D = 1.5, D = 40 cm L/h = 0.25, 0.50, 0.75, and 1.00

4 h/D = 2, D = 40 cm L/h = 0.25, 0.50, 0.75, and 1.00

5 Partial replacement

With skirts

h/D = 1, D = 40 cm L/h = 0.25, 0.50, 0.75, and 1.00

6 h/D = 1.5, D = 40 cm L/h = 0.25, 0.50, 0.75, and 1.00

7 h/D = 2, D = 40 cm L/h = 0.25, 0.50, 0.75, and 1.00

Improving the bearing capacity of footing on soft clay with sand pile with/without skirts 373

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fill noticeably improves the bearing capacity of the footing as

well as the stiffness of the foundation bed. The vertical skirt

is acting as a confining cell to the replaced sand column and

effectively decreases both the vertical and the horizontal strain

underneath the footing compared with the other case. It also

controls the settlement of the footing/soil system according

to the skirts depth.

The presence of skirts can extensively modify the stress set-

tlement curves and the ultimate bearing capacity is increased

with the increase of skirt depth. The improvement in the bear-

ing capacity due to soil confinement along with the partially re-

placed system is reported by many investigators when using

vertical confinement cell [7,17]. Therefore, the existence of such

skirts on replaced soft clay can totally eliminate and prevent

the bulging that occurred in the case of without skirts. These

figures also indicate that the ultimate bearing capacity is in-duced at settlement ratio (S /B) about 12% for the skirted foot-

ings while this value ranged between 14% and 16% for non-

skirted footings. Such increase in the settlement is due to bulg-

ing failure.

4.2. Effect of replaced depth and skirts installation

Fig. 4 shows the variation of bearing capacity ratio, BCR with

normalized fill depth (L/h) for different soft clay depth ratios

(h/D) for the footing with/without skirts on replaced soil. It

has been found that the bearing capacity ratio (BCR) is grad-

ually increased with the increase of the replaced depth. A sig-

nificant increase in the bearing capacity ratio is observed at(L/h > 0.5) until the replaced sand penetrating the soft clay

layer; the maximum increase in the bearing capacity ratio is

induced. The replaced sand under circular footing was consid-

ered as a single sand pile which enhances the load bearing

capacity as that recently published by Etezad et al. [6]. Also,

the degree of improvement on BCR mainly related to soft

clay depth or ratio (h/D), wherever as the depth of soft clay

increases the depth of partially replaced fill is increased as a

result the bearing capacity is increased. The skirts minimized

the horizontal movement and acted as a restraint for partial

replacement of sand column inside skirt accordingly no bulg-

ing can be appeared. As the failure approaches in tests carried

out on footing with skirts, the replaced sand inside the skirts

behaves as one unit and behaves similar to deep foundation(once the load was increased, the skirted footing and replaced

fill settled simultaneously).

It noticed that as the depth of clay increases the bearing

capacity ratio is increased. Also, the skirts have a great effect

in increasing the degree of improvement compared with case

of footing on sand column without confinement. The skirt in-

creases the bearing capacity to 3.1 times of initial ultimate

bearing capacity of soft clay at h/D = 1 and 4.2 times at

h/D = 2. Alternatively, at the case of no confinement, the

bearing capacity is increased by 2.6 times value at h/D = 1

and 3.3 times at h/D = 2. This variation in the degree of

improvement in the two cases was related to the bulging

H/D = 1.00

D/B = 4.00

0

2

4

6

8

10

12

14

16

18

0 50 100 150 200 250 300 350 400

Bearing pressure, q (kPa)

S e t t l e m e n t , s / B ( % )

no replcement

L/h=0.25

L/h=0.50

L/h=0.75

L/h=1.00

Figure 2 Variation of bearing stress, q versus normalized

settlement for different. Replaced depth for footing without skirts.

0

2

4

6

8

10

12

14

16

18

0 50 100 150 200 250 300 350 400

Bearing pressure (kPa)

S e t t l e m e n t , s / B ( % )

no replcement

L/h=0.25

L/h=0.50

L/h=0.75

L/h=1.00

H/D = 1.00D/B = 4.00

Figure 3 Variation of bearing stress, q versus normalized

settlement for different. Replaced depth for footing with skirts.

Without skirts

With skirts

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 0.25 0.5 0.75 1

L/h

B C R

h/D = 1.00

h/D = 1.50

h/D = 2.00

h/D = 1.00

h/D = 1.50

h/D = 2.00

Figure 4 Variation of the BCR with the normalized replaced

depth (L/h) for different soft clay depth and footing with/without

skirts.


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behavior of unconfined condition. This can also confirm that

the effectiveness of skirts as a confined tool.

4.3. Settlement reduction due to confinement effect

The extension of the investigation of using laboratory loading

tests for footing on improved soft clay with/without skirts was

carried out in order to examine their effects to reduce and con-

trol settlement under the footing. A parametric study bringsout to the effects of the fill depth and depth of soft clay for

both footing with and without skirts. The settlement reduction

is generally expressed according to values of the percentages

reduction in settlement (PRS) as:

PRS ¼ ðS o S rÞ=S o

where S o = settlement of footing on soft clay layer corre-

sponding to its ultimate bearing capacity. S r = settlement of

footing on replaced fill with/without skirts corresponding to

its ultimate bearing capacity of reinforced sand.

The relationships between the PRS in percentage and the

replaced depth for footing with and without skirts are pre-

sented in Figs. 5 and 6. It has been found that the existence

of sand column below the footing decreases the amount of set-tlement of the system. The percentage reduction in settlement

was affected by both replaced depth and soft clay thickness

as distinctly explained in Fig. 5. In this figure nonlinear corre-

lation is found exist because the replaced sand column when

subjected to footing load compressed until bulging induced.

Moreover, as the depth of fill increases the percentage reduc-

tion in settlement increases. This is due to the shear stress

mobilized at the perimeter of sand column. The PRS (%) is

reached to 62% for footing on replaced sand column and at

(h/D = 2). This value is decreased according to soft clay thick-

ness, wherever, at (h/D = 1) the PRS (%) was dropped to 55%

for the same replaced depth (L/h = 1) as distinctly shown in

Fig. 5. On the other hand, with the intention of study the con-

finement effect, the variation of PRS (%) with the skirts depth

for different soft clay depth is shown in Fig. 6. It also noticed

that the percentage reduction in settlement PRS (%) is greatly

increased with the existence of skirts with sufficient depth com-

pared to other case. Also, the skirts can significantly modify

the relationships to linear regression. This also, again justifying

that the use of confined partially replaced sand column withskirts is more effective in increasing the foundation stiffness,

controls the horizontal and vertical movement underneath

the footing, and demonstrates that the footing and sand col-

umn confined by skirts acted as a rigid one unit behave similar

to deep foundation. The PRS (%) reached to 62 and 75 for

0

10

20

30

40

50

60

70

80

0 0.2 0.4 0.6 0.8 1 1.2

L/h

P R S %

h/D= 1

h/D= 1.5

h/D= 2

Figure 5 The variation of PRS (%) and replaced depth L/h for

different soft clay thickness for footing without skirts.

0

10

20

30

40

50

60

70

80

0 0.2 0.4 0.6 0.8 1 1.2

L/h

P R S %

h/D= 1

h/D= 1.5

h/D= 2

Figure 6 The variation of PRS (%) and skirts depth L/h for

different soft clay thickness.

Figure 7 Variation of subgrade modulus for different settlement

ratio, D/h = 1.00.


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(L/h = 1 and 2, respectively). These values are more than

those of footing without confinement. For that reason, in cases

where structures are very sensitive to settlement, confined re-

placed sand column can be used to obtain the same allowable

bearing capacity at a much lower settlement.

4.4. Comparison of subgrade modulus of skirted and non-skirted

sand columns

The subgrade modulus of skirted and non-skirted sand column

for various floating ratios was calculated from load–displace-

ment relationships of the performed tests at D/h = 1.00.

Fig. 7(a) and (b) are presented for non-skirted and skirted sand

piles respectively for variation of the ratio of modified sub-

grade modulus, E s and original subgrade modulus, E i . From

these figures it is observed that ratio of E s/E i is increased from

1.80 up to 7.00 times at the earlier stages of loading. These ra-

tios are for L/h = 0.25 and 1.00, respectively, for non-skirted

piles. While these ratios were ranged between 1.80 and 9.50

for skirted conditions. Also it can be seen that for non-skirted

sand pile the subgrade modulus reduced rapidly especially for

the L/h ratio greater than 0.50. This reduction for the earlier

stages of loading than the relations is almost flat and thereduction rate is very small. The same trend is observed for

skirted sand piles, while the rate of reduction is greater than

that for the non-skirted condition. Also it is observed that

the rate of the reduction of subgrade modulus for different

embedment ratios for skirted condition is higher than that of

the non-skirted as the penetration ratio of pile is reduced.

5. Conclusions

From the present laboratory investigation on the behavior of

circular footing resting on partially replaced sand pile with/

without skirts, the following conclusions can be summarized:

(1) The improvement of load bearing capacity is remark-

ably increased using both partially replaced sand piles

with/without confinement by skirts.

(2) The use footing of both replaced sand pile with/without

confinement can substantially modify the stress displace-

ment relationship.

(3) The percentage of improvement of bearing capacity has

been gained in two ways, with the increase of replaced

depth, skirts depth as well as the increase of soft clay

thickness.

(4) Confined replaced sand pile with skirts has a consider-

able effect in decreasing the vertical settlement and elim-

inating the horizontal subgrade movement. It alsoprevents the bulging occurrence.

(5) The percentage reduction in settlement is highly

depended on both fill and skirts depth, and soft clay

thickness.

(6) In cases where structures are very sensitive to settlement,

confined replaced sand column can be used to obtain the

same allowable bearing capacity at a much lower

settlement.

(7) The replaced soil block inside the skirts behaves as a

deep foundation or one unit. Therefore, the bearing

capacity failure mechanism of normal footing on soft

clay is modified from excessive settlement pattern to gen-

eral bearing capacity failure at the end of confined

replaced sand column.

(8) For both skirted and non-skirted sand pile the ratio of

E s/E i is increased.

(9) For non-skirted sand pile there is a significant reduction

in the subgrade ratio E s/E i , especially for the L/h ratio

grater than 0.50 after which the reduction rate is very

minor. The same trend is observed for skirted sand piles,while the rate of reduction is grater than that for non-

skirted condition.

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