IMPROVEMENT IN BEARING CAPACITY OF SOIL
BY GEOTEXTILES -AN EXPERIMENTAL APPROACH
M. S. Ranadive (MIE)
Assistant Professor
Deptt. of Civil Engineering,
Govt. College of Engineering, Pune (India).
N. N. Jadhav
Master of Engineering Research Scholar
Deptt. of Civil Engineering,
Govt. College of Engineering, Pune (India).
ABSTRACT
The construction of reinforced soil foundation to support a shallow spread footing
has considerable potential as a cost-effective alternative to conventional methods of
foundation support. In this technique one or more layers of geo-synthetic reinforcement
are placed beneath the footing to create a composite material with improved performance
characteristics. Most of the studies reported in literature by Andrawes and others (1989)
and Fragaszy and others (1983), have been small-scale model tests to evaluate the
potential benefits of a reinforced soil mass below shallow foundations. Since the ability of
geotextile to reinforce such systems derives from friction at the soil-geotextile interface,
the tests were conducted using sand as the fill material.
Here an attempt is made to present the details of investigation on performance of
geotextile reinforcement in soil other than sand. For this purpose, model strip footing load
tests are conducted on soil with and without single and multi-layers of geotextile at
different depths below the footing. The load settlement characteristic for each soilgeotextile configuration has been observed. The influence of various selected parameters
on the load settlement behaviour are studied and critically appraised for their practical
significance.
MATERIAL PROPERTIES
The material properties of soil and geotextile material used for the test are given below.
a) Soil
The optimum moisture content and maximum dry density by standard Proctor test was
17.7% and 1.652gm/cm3 respectively. The California Bearing Ratio (CBR) at 2.5mm
penetration was 29.56%.
b) Geotextile
Non-woven geotextiles have high extensions, high creep and low strength, which
make them unsuitable for application as soil reinforcements. Hence woven geotextiles are
considered for the experiments. In woven geotextile the multifilament type was selected
due to its high tensile strength.
For experimental work the geotextile strips were made available from the local
manufacturer, Garware Wall-Ropes Ltd., Pune. The properties of the geotextile are as
given below.

Type of geotextile: Multifilament Woven.

Type of fiber
: Polypropylene.

Trade name
: GWF (T) 52 – 240.

Weight
: 240 gm/ m2.

Pore size
: less than 75 microns.

Maximum extension: 27 %.

Tensile strength (Wide strip method) : Warp 55 KN/m
:Weft 43 KN/m

Bursting strength : 5500 Kpa.

Permeability
: 31 Lit/ m2/ sec .
EXPERIMENTAL PROGRAM
The aim of this experimental work was to study the influence of selected parameters
on the behaviour of geotextile-reinforced soil supporting strip footing. The dimensionless
parameters chosen for the purpose are as given below.
1) Number of reinforcing layers ‘N’ which varied from 1 to 4 (Fig. 4). Figures listed on
page 8.
2) d/B ratio varied from 0.25 to1.0, where, d = depth of single reinforcing layer below
footing, and B = width of footing. (Fig. 3).
3) S/B ratio varied from 0.25 to 1.0 where, S = spacing between subsequent geotextile
reinforcing layers when depth of top layer below footing was kept constant equal to
0.25B. (Fig. 4).
Table 1 gives details regarding number and spacing of geotextile reinforcement layers ‘d’
and ‘S’.
Table 1: Details of Spacing of geotextile reinforcing layers
No. of reinforcing
Depth of top reinforcing
Spacing of subsequent
layers ‘N’
layer below footing ‘d’
reinforcing layer ‘S’
1
0.25B, 0.5B, 0.75B,1.0B
-
2
0.25B
0.25B, 0.5B, 0.75B,1.0B
3
0.25B
0.25B, 0.5B, 0.75B,1.0B
4
0.25B
0.25B, 0.5B, 0.75B,1.0B
The test was conducted for following system of geotextile layers.
a. Single layer test system.
b. Two layer test system.
c. Three layer test system.
d. Four layer test system.
A bearing capacity test on unreinforced soil for comparison of results was conducted.
Load verses settlement curves are produced and presented for each test. The improvement
in bearing capacity of soil due to introduction of geotextile is characterised by a
dimensionless number called Bearing Capacity Ratio (BCR).
BCR = Load taken by reinforced soil / Load taken by unreinforced soil
The variation in BCR over increase in settlement is represented by BCR verses settlement
curves.
Experimental Set up
The experimental set up was developed with reference to the model tests
performed by Mandal and Manjunath (1990) on bearing capacity of single layer of
geosynthetic sand subgrade.
The tests are performed in a mild steel rectangular box of size 60cm x 11cm in
section and 50 cm deep (Fig. 1). The box was well stiffened by welding mild steel angles
as stiffeners. The size of the footing under consideration was 10cm x10cm made of mild
steel plate 6mm thick stiffened by another plate of width 8cm and thickness 6mm (Fig. 2).
Testing was carried out on Universal Testing Machine. For accurate measurement of load,
it was applied through a proving ring of capacity 25 KN with least count 12.5 N.
Settlement was recorded using dial gauge of least count 0.1 mm.
Experimental procedure
The water content and dry density of soil was kept constant in all tests and equal
to 15% and 1.50 gm/cm3 respectively. To achieve this, the existing water content of soil
was determined before each test and the water content was adjusted accordingly. For
uniform compaction of soil, it was filled in minimum three or more lifts depending on the
spacing of geotextile. The weight of soil for each lift for desired dry density (1.50
gm/cm3) was worked out and that much quantity of soil was filled in the mould and
lightly compacted to achieve desired height of the lift. Rectangular geotextile strips of
length 60 cm and width 10.5 cm were cut and placed on compacted and leveled soil mass
at the desired spacings. Thus the soil was filled upto height 45 cm in each test. Model
strip footing was kept centrally on this mass and loaded. Loads required for settlements in
the increments of 5 mm were recorded till the total settlement reaches to 50mm.
INTERPRETATION OF RESULTS AND DISCUSSION
Various useful graphs have been prepared based on the observations of the
experiment. Following interpretations have been drawn.
A) General
1. Graph of load verses settlement of unreinforced soil indicates failure of the soil for a
settlement of 45mm (Fig. 9).
2. Improvement in bearing capacity of reinforced soil over that of unreinforced soil is
observed for all positions of reinforcing layers in all tests.
3. The load settlement curves for reinforced soil in all tests continues to rise beyond the
failure point of unreinforced soil at settlement of 45mm. This indicates the
contribution of reinforcement in resisting bearing pressure.
B) Layered System
a) Single layer system
1. Improvement in bearing capacity was very low (BCR=1.20 at settlement 50mm) when
the reinforcement was at depth d = 1.0B. The reason may be that reinforcement being
comparatively at greater depth, shear failure of soil has occurred in the zone above the
layer (Fig. 3).
2. Improvement in bearing capacity is maximum when the reinforcement is at depth
0.25B with BCR=1.39mm (Fig. 5).
b) Two layered system
1. Maximum improvement in bearing capacity is observed for S/B ratio equal to 0.25
(BCR=1.53 at settlement 50mm) whereas, minimum improvement for S/B ratio equal
to 1.0 with BCR=1.30 at settlement of 50mm (Fig. 6).
c) Three layered system
1. Maximum improvement in bearing capacity is observed for spacing S/B ratio equal to
0.25 with BCR=1.58 at settlement 50mm (Fig. 7).
2. The differences in BCR of S/B=0.5 at settlement of 50mm (BCR=1.46) and BCR of
S/B=0.75 at settlement of 50mm (BCR=1.42) is very low.
d) Four layered system
1. Maximum improvement in bearing capacity is observed at S/B ratio equal to 0.25
(BCR=1.60 at settlement 50mm) whereas minimum improvement for S/B ratio equal
to 1.0 with BCR=1.39 at settlement of 50mm (Fig. 8).
C) BCR and Settlement
1. In general the BCR verses settlement graphs show linear variations. The graphs
show increase in BCR during initial settlement but as the footing settles more
(about 20-30mm) it declines. This may be due to non-development of proper
friction mechanism between soil and reinforcement in the initial stages. As further
settlement takes place, the soil gets consolidated and friction gets developed and the
BCR starts increasing. Provision of end anchorage beyond the failure zone (at the
end of reinforcing strips) or provision of geogrids suitably spaced and tied at the
ends beyond the failure zone improves the bearing capacity.
2. In single layer system, the BCR verses settlement graph (Fig. 10) for D/B=1.0 is
almost a straight line, which indicates the failure of soil in the zone above the layer
of reinforcement. The increase in BCR is only due to the frictional resistance to
shear displacement at the interface of soil and reinforcement. The reinforcement is
not contributing any more in the improvement of BCR. Similar mechanism has
been observed previously by Andrawes2.
3. In two layer system, all the BCR verses settlement curves starts declining after
settlement of 20mm (Fig. 11). This is due to slippage of upper reinforcing layer.
The curves again start rising after some settlement because of consolidation of soil
or increase in stresses mobilize the friction and both the reinforcing layers gets
activated. For S/B =1, as the second layer is far below, its activation needs more
settlement, hence the graph continues to decline upto settlement of 40mm and then
rises.
4. In four layer system, the BCR verses settlement graph shows a declining trend due
to more number of closely spaced layers affecting the homogeneity of soil mass and
ultimately the bond between soil and reinforcement (Fig. 13). Hence the provision
of 4 or more number of layers could not be justified.
CONCLUSIONS
1. Improvement in bearing capacity was observed considerable in reinforced soil over
the unreinforced soil. For single layer system, BCR for depth of layer below footing
equal to 0.25B is maximum and BCR decreases as the depth of layer increases.
2. For multilayer system, BCR for a constant d/B ratio and S/B ratio increases with
increasing the number of reinforcing layer. The BCR is maximum for N=4 but the
percentage increase in BCR for N=4 over BCR for N=3 is very low. Thus N=3 is
recommended as optimum value.
3. For constant number of layers, and constant S/B ratio, BCR decreases with increase in
spacing of layers. Maximum BCR is observed at spacing of 0.25B. If the spacing of
layers is more than 0.1B, the lower layer does not contribute more in improving the
bearing capacity. Because the lower layers are out of tension zone and carry only
compression hence do not serve the function of reinforcement.
4. In multi-layer reinforcing system, if the subsequent layers below top layer are
provided at greater distance (greater than 2/3B), their contribution in improvement of
bearing capacity reduces significantly. At spacing of 0.25B maximum improvement in
bearing capacity was observed.
5. More number of closely spaced layers affect the homogeneity of soil mass and
ultimately the bond between the soil and reinforcement. This results in decreasing the
rate of improvement of bearing capacity. The test was conducted for 4 layers and
optimum value of number of layers observed was 3.
REFERENCES
1. Adams, M.T. and Collin J.G. (1997), “Large Model Spread Footing Load Tests on
Geosynthetics”, Geo-environmental Engineering, January 1997, pp 66-72.
2. Andrawes, K.Z., McGown, A and Wilson-Fahmy, R.F. (1989), “The Behaviour of a
Geotextile Reinforced Sand Loaded by Strip Footing”, Proceeding of the 8th European
conference on Soil Mechanics and Foundation Engineering, Helsinki, pp 329-334.
3. Ayyar,
T.S.R.,
Krishnaswamy,
N.R.
and
Viswanadham,
B.V.S.
(1989),
“Geosynthetics for Foundations on Swelling Clay”, Proceedings of the International
Workshop on Geotextiles, Nov.1989, Bangalore, pp 176-180.
4. Ayyar, T.S.R., Krishnaswamy, N.R., Ravishankar, S and Parashar, S.P. (1990),
“Bearing Capacity of Kaolinitic Clay Reinforced with Geosynthetics”, Proceedings of
the Indian Geotechnical Conference, Mumbai, pp 11-14.
5. Fragaszy, R., Lawton, E. and Asgharzadeh-Fozi, Z. (1983), “Bearing Capacity of
Reinforced Sand”, Proceeding of the 8th European conference on Soil Mechanics and
Foundation Engineering, Helsinki, pp 357--360.
6. Guido, V.A., Biesiadecki, G.L. and others (1989), “Behaviour of Geosynthetically
Reinforced Earth Slabs”, Proceeding of the International Workshop on Geotextiles,
Nov. 1989, Bangalore, pp 170-175.
7. Mandal, J.N. and Manjunath, V.R. (1990), “Bearing Capacity of Single Layer of
Geosynthetic Sand Subgrade”, Proceedings of the Indian Geotechnical Conference,
Mumbai, pp 7-10.
11
0m
m
m
600
500mm
m
M.S. ANGLE STIFFNERS
6mm
FIG.1: MILD STEEL MOULD
100mm
6mm
80mm
FIG.2: MODEL STRIP FOOTING
LOAD
d
PROVING RING
B
GEOTEXTILE LAYER
'd' VARIED FROM 0.25B TO 1.0B
FIG.3: SINGLE LAYERED TEST SERIES
LOAD
PROVING RING
S
S
d
B
GEOTEXTILE LAYERS
'd' KEPT CONSTANT IN ALL TESTS = 0.25B
'N' VARIED FROM 2 TO 4
'S' VARIED FROM 0.25B TO 1.0B
FIG.4: MULTI-LAYERED REINFORCEMENT TEST SERIES
1.4
BCR
1.3
1.2
FOR D/B =0.25
1.1
FOR D/B =0.50
SETTLEMENT (mm)
1
0
10
20
30
FOR D/B =0.75
40
FIG.10 BCR VS SETTLEMENT FOR SINGLE LAYER SYSTEM
50
FOR D/B =1.0
60
1.52
1.47
BCR
1.42
1.37
1.32
FOR S/B =0.25
1.27
FOR S/B =0.50
FOR S/B =0.75
SETTLEMENT (mm)
1.22
0
5
10
15
20
25
30
35
40
45
50
55
FOR
=1.0
60 S/B 65
FIG.11: BCR VS SETTLEMENT FOR TWO LAYER SYSTEM
1.6
1.55
BCR
1.5
1.45
FOR S/B =0.25
1.4
FOR S/B =0.50
1.35
FOR S/B =0.75
1.3
FOR S/B =1.0
SETTLEMENT (mm)
1.25
0
10
20
30
40
50
60
FIG.12: BCR VS SETTLEMENT FOR THREE LAYER SYSTEM
1.65
BCR
1.55
1.45
FOR S/B =0.25
FOR S/B =0.50
1.35
FOR S/B =0.75
SETTLEMENT (mm)
1.25
0
10
20
30
40
FOR S/B =1.0
50
FIG.13: BCR VS SETTLEMENT FOR FOUR LAYER SYSTEM
60