Effect of vertical cross sectional shape of foundation and soil reinforcement

INTERNATIONAL JOURNAL and Technology (IJCIET), ISSN 0976 – 6308
International Journal of Civil Engineering OF CIVIL ENGINEERING AND
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ISSN 0976 – 6316(Online)
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EFFECT OF VERTICAL CROSS-SECTIONAL SHAPE OF
FOUNDATION AND SOIL REINFORCEMENT ON SETTLEMENT
AND BEARING CAPACITY OF SOILS

M. Alhassan‫ ٭‬and I. L. Boiko
Department of Geotechnics and Ecology in Civil Engineering, Faculty of Civil Engineering,
Belorussian National Technical University, Minsk, Belarus

ABSTRACT

Bearing capacity and settlement of soils are both function of dimension and shape of
foundation, embedment depth, physico-mechanical properties of soil and load geometry. Soil
reinforcement is one of the methods of improving the engineering properties of soils that has
gained acceptance in geotechnical engineering practices. In this paper, patterns of load-
settlement characteristic of statically loaded shallow foundation models with different vertical
cross-sectional shapes on both unreinforced and reinforced soft clay soils are presented.
Models of shallow foundations with rectangular, wedge and T-shape vertical cross-sections
were studied. The study generally shows that reinforcement of soil under shallow foundations
with deferent vertical cross-sectional shapes increases bearing capacity and reduces
settlement of the subsoil base. Evaluation of Bearing Capacity Ratio (BCR) shows that
foundations with rectangular vertical cross-sectional shapes have higher BCR values than
those foundations with T and wedge vertical cross-sectional shapes.

Keywords: Bearing capacity, Bearing capacity ratio, Foundation shape, Settlement, Soil
reinforcement.

I. INTRODUCTION

The stability of civil engineering structures founded on soils depends on the ability of
their foundations to effectively and safely transmit the resulting loads to the soil or rock
below. By inference, it means that the stability of these structures depends on the ability of
the foundation soil to safely carry the structural loads without failure due to shear or
excessive settlement. The ability of soil to effectively perform this function under a
foundation is a function of dimension and shape of the foundation, embedment depth,

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International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308

physico-mechanical properties of the soil and load geometry. Foundations are generally
classified into shallow and deep foundations. Shallow foundations are considered those types
of foundations that transmit structural loads to the soil strata at a relatively small depth.
However, research studies have shown that, for shallow foundations, Df/B can be as large as 3
to 4[1-3].
Various types (shapes) of shallow foundations are known, with strip, square,
rectangular and circular being the most widely used. These types of shallow foundations have
different shapes which only vary from each other plan-wise or by horizontal cross-section.
The vertical cross-sections (depending on the design thickness) of these foundations are
basically the same. Their (mostly) rectangular vertical cross-sectional shapes make their
mode of interaction with the soil bases trunk-wise (vertically) basically the same. The
interaction of foundations with soil bases is mostly studied using load-settlement relationship.
Recent studies by Alhassan and Boiko [4,5], on shallow foundations with different vertical
cross-sectional shapes, have shown that soil above the bases (i.e. along the vertical trunk) of
foundations with T and wedge vertical cross-sectional shapes, is usually mobilized to
function not only as surcharge to the soil below, but also in actively and vertically resisting
structural loads.
Soil reinforcement is one of the methods of improving the engineering properties of
soils that has gained acceptance in geotechnical engineering practices. A lot of studies have
been carried out over the years on the interaction of foundations with reinforced subsoil bases
[6-26]. Effect of vertical cross-sectional shape of foundations on the settlement and bearing
capacity of reinforced soil has not been given attention in these past studies. The present
study experimentally investigates the effect of vertical cross-sectional shape of foundation
and soil reinforcement on load-settlement characteristic of soils. The study presents pattern of
load-settlement relationship of foundations with rectangular, T and wedge vertical cross-
sectional shapes on unreinforced and reinforced clay subsoil bases. This study is based on the
fact that, it is commonly believed that, for design of shallow foundations, settlement criterion
is more critical than the bearing capacity one [27]. Generally the settlements of shallow
foundations such as pad or strip footings are limited to 25 mm [28]. Studies on (especially
small scale) shallow foundations have shown that allowable bearing capacity occurs at
settlement of between 5 to 10 % of foundation width. In line with the reasons advanced by
Cerato and Lutenegger [29], for this study, bearing capacity at settlement of 10 % of
foundation width (i.e., s/B=0.1) was adopted as allowable.

II. EXPERIMENTAL METHODOLOGY

Four wooden models of shallow foundations were used for the study: the first model
was a rectangular shaped block (marked rectangular shape 1) with dimension of 30x60x60
mm for width, length and height respectively; the second model was a rectangular shaped
block (marked rectangular shape 2) with dimension of 50x60x60 mm for width, length and
height respectively; the third model was a wedge-shaped block of 60 mm height with width
and length for top and lower sides as 60x60 mm and 30x60 mm respectively; and the fourth
model was a T-shaped block of 60 mm height with width and length for top and lower parts
as 60x60 mm and 30x60 mm respectively (fig. 1). The dimensions of the models were so
chosen so as to be within Df/B≤2 (Dfand B are depth of foundation embedment and width
respectively). Two subsoil conditions were also modeled in the geotechnical laboratory of the
Department of Geotechnics and Ecology in Civil Engineering of Belorussian National

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Technical University, Minsk, Belarus. The experimental stand used for the study was a
The
rectangular container of dimension 1100х600х250 mmforlength, height and width
length,
respectively, with a transparent front side.

a b c d

Fig. 1: Foundation model a & b- rectangular shapes; c- wedge-shape; d T-shape.
models: d-

Soft clay soil having relative consistency of 0.67 and liquidity index of 0.33, with
cohesion c and angle of internal friction , at 17 kN/m3 unit weight and 20 % moisture
content as 0 and 33° respectively, was used in modeling the subsoil bases. The properties of
the soft clay are typical for normally consolidated (soft) clay soil found in Sokoto
(Northwestern) region of Nigeria, as reported by Ola [30]. The modeled subsoil conditions
b .
were homogeneous unreinforced soft clay soil (fig. 2) and reinforced clay soil (fig. 3). The
reinforcement material used was galvanized steel pipes of relatively small diameters In
diameters.
accordance with the works by Binquet and Lee [6], Guido et al.[8], Khinget al.[11] and
et al.
Puriet al. [19], u/B and h/B (u is depth of the first layer of reinforcement from the foundation
base, h is the vertical spacing of the reinforcement layers and B is the foundation width) were
both kept below 0.65 for the arrangement of reinforcement layers under all the respective
foundation models, and three (N=3) layers of reinforcement were used in accordance with
Akinmusuru and Akinbolade[7] and Demiröz and Tan [20]. For the T and wedge shape
[7] .
foundation models, the top widths of the foundations were used for the determination of u/B
and h/B.
The experimental stand was filled with the soil in layers of 25 and 50 mm, with each
layer compacted to unit weight of 17 kN/m3at moisture content of 20 %. To easily achieve
kN/
this, the weights of the wet soil, required to fill the resulting (from 25 and 50mm layers)
soil
volumes were measured and compacted to fit into the respective layers. The foundation
models were placed during placement and compaction of the last two upper layers as shown
placement
in figs. 2 and 3. Using 1:10 loading lever, loads were statically, vertically, centrally and
uniaxially applied to the foundations in an incremental manner, recording corresponding
settlement for each load increment, using dial gauges of 1/100 mm division. Subsequent load
ch
increments were done when the rate of settlement from the previous loads becomes less than
0.02 mm/min.
On the first modeled subsoil condition i.e unreinforced soil, static loads were applied
incrementally to maximum loads of 201, 170, 168 and 168 kN/m2 to rectangular rectangular-1,
rectangular-2, wedge and T-shaped foundation models respectively. While on the second
shaped
subsoil condition i.e reinforced soil, maximum loads of 450, 400, 278 and 278 kN/m2 were
applied to rectangular-1, rectangular-2, wedge and T-shaped foundation models respectively.
1, rectangular s

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The results for the foundation models on unreinforced and reinforced subsoil
conditions are graphically presented as load-settlement curves in figs 4 and 5 respectively.

Fig. 2:Unreinforced subsoil condition

Fig. 3:Reinforced subsoil condition

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III. RESULTS AND DISCUSSION

Results of load-settlement relationship of foundations models on the unreinforced and
reinforced subsoil conditions are shown in fig. 4 and 5. From fig. 4, it is observed that T-
shape foundation recorded the highest bearing capacities at corresponding settlements on the
unreinforced soil. This can be attributed to its shape, which gives it a relatively kind of
‘floating balance’, on the soft clay base, when compared with the rest shapes. The least
bearing capacity was observed with wedge shape foundation. This can be attributed to the
width of its lower part, which impact high settlement under the same loads magnitudes, when
compare with other shapes.
Observation of results on the unreinforced (fig. 4) and reinforced (fig. 5) subsoil
conditions shows that significant difference in load-settlement relationships were recorded
with all the foundation models on the reinforced subsoil conditions. Higher bearing capacity
values at lower settlements were generally recorded on the reinforced subsoil. The initial
sudden settlement exhibited by all the foundation models, on application of the first load, on
the reinforced soil, is attributed to the settlement of the soil layer in between the foundation
bases and the first layer of the reinforcement. With subsequent load application, the
reinforced soil base act as a single unit in resisting the loads. This phenomenon accounts for
the pattern of the curves henceforth. On this condition, rectangular shape foundation models
recorded the highest bearing capacity, while the least bearing capacity was also recorded
from wedge shape foundation model. The higher bearing capacity values recorded from
rectangular shape foundation models were as a result of the relatively wider widths of the
foundation models on the reinforced soil in comparison with lower parts of T and wedge
shape foundation models.

Load (kPa)
0 50 100 150 200 250
0.00
2.00
4.00
6.00
Settlement (mm)

8.00
10.00
12.00
14.00
16.00
18.00
20.00

Rectangular Shape-1 Rectangular Shape-2 Wedge Shape T-Shape

Fig. 4: Load-settlement curves for foundations models on unreinforced soil

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Load (kPa)
0 100 200 300 400 500
0.00

2.00

4.00
Settlement (mm)

6.00

8.00

10.00

12.00

14.00

16.00

Rectangular Shape-1 Rectangular Shape-2 Wedge Shape T-Shape

Fig. 5: Load-settlement curves for foundations models on reinforced soil

From the graphs, it is possible to evaluate the effect of the shapes of the foundations
on the bearing capacity and settlement of the soils. Studies have shown that for shallow
foundations on clay soils, the maximum settlement at which the bearing capacity is
considered allowable, is taken as 10 % of foundation width [29, 31-34]or 25 mm, whichever
is less from these values. Thus, the maximum permissible settlement of the studied
foundation models is taken as 10 % of the width of the foundations models, i.e. 3 mm, 5 mm,
6 mm, and 6 mm for rectangular-1, rectangular-2, wedge and T-shape foundation prototypes
respectively. Therefore, from the graphs (figs4 and 5), the allowable bearing capacity and
consequently, the bearing capacity ratio of each of the foundation model at the given
settlement is presented in Table 1.

Table 1: Bearing capacity of foundation models
Allowable Bearing capacity (kPa) Bearing capacity
Foundation model
Unreinforced soil Reinforced soil ratio
Rectangular shape –1 45 100 2.2
Rectangular shape –2 58 140 2.4
wedge-shape 56 120 2.1
T-shape 98 135 1.4

From table 1, it can be seen that on the unreinforced subsoil condition, the highest
allowable bearing capacity of 98 kPawas recorded with T-shape foundation model. The least
allowable bearing capacity of 45kPa was recorded from rectangular-1 foundation model. On
the reinforced subsoil condition, the highest allowable bearing capacity was observed with

85


the rectangular-2, while the least was observed with rectangular-1 foundation models. This
can be attributed to the wider width of the rectangular-2 on reinforced soil in comparison
with the rest of the foundation models. Considering the Bearing Capacity Ratio (BCR), a
nondimensional quantity, expressed as:
௤ೌ೗೗೚ೢሺೃሻ
‫ ܴܥܤ‬ൌ ௤ೌ೗೗೚ೢ
(1)

where ‫ݍ‬௔௟௟௢௪ሺோሻ and ‫ݍ‬௔௟௟௢௪ is the allowable bearing capacities on reinforced and unreinforced
soil, respectively, although the highest bearing capacity on the unreinforced soil, was
recorded from T-shape foundation, this shape of foundation recorded the least value of 1.4 for
BCR, wedge shape has BCR of 2.1. The highest BCR value of 2.4 was recorded from
rectangular shape-2, while rectangular shape-1 has 2.2 as BCR value. This implies that the
use of foundations with wedge and T-shape vertical cross-sections on reinforced soils,
especially when only the soil below the foundation bases is reinforced, will have relatively
less positive effect on the bearing capacity when compare with those of rectangular shapes.
This results conformed with findings by Alhassan and Boiko [4, 5], that “bulk of the load
resistance of subsoil bases at the instance of shallow foundations with rectangular vertical
cross-sectional shape is mostly associated with the soil beneath the foundation base, while at
the instances of those with wedge and T-shape vertical cross-sectional shapes, both soil
beneath the foundations’ bases and along their vertical stems, actively participates in
resistance of structural loads”. Since the soil above the foundation bases is unreinforced, this
account for the recorded values in the case of wedge and T-shapes foundations.

IV. CONCLUSION

The study generally showed that vertical cross-sectional shape of foundation affects
the bearing capacity and settlement of both unreinforced and reinforced soil bases. The use of
foundations with wedge and T-shape vertical cross-sections on reinforced soils, especially
when only the soil below the foundation bases is reinforced, have relatively less positive
effect on the bearing capacity when compare to those of rectangular shapes.

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