Engineering and Technology Journal

Skirted foundations are one of the solutions proposed to increase the bearing capacity of the soil. They assist in increasing the load and depth of failure in weak ground or soils with low shear resistance and reducing the foundation settlement if a soil improvement method cannot be applied or the cost of implementing deep foundations increases. This study examined and investigated the extent of soil bearing of skirted foundations on sandy soils and studied the effect of soil saturation cases and three cases of water content reduction to measure the matric suction value of unsaturated soil. A physical model was created to simulate the strip foundation and compare these cases (dry-fully saturated-partially saturated). It was found that the soil load carrying capacity in the case of unsaturated soil is the highest, where matric suction is at a depth of 450 mm, followed by the dry case and then the saturated case as it represents the weakest state of the soil.

In this work, the comparison between the values of bearing capacity of soil
determined by two different methods, the first method is field plate load test and the
second method is several laboratory tests. The study find out that the allowable bearing
capacity estimated from the first method widely near from its value computed by second
method. Also this study confirm the probability of using Plate load test for estimating
soil bearing capacity for small jobs and shallow layer depth to avoid using
comprehensive soil investigation and economizing both cost and time. Empirical
relation ship for bearing capacity estimation achieved from the results of the two
methods by using stiff brown silty clay soil at Erbil governorate - north of Iraq.

The classical bearing capacity theories rely on the superposition of three separate
bearing capacities – a technique that is inherently conservative – but they also rely on
tabulated or curve-fitted values of the bearing capacity factor, Nγ, which may be
unconservative. Further approximations are introduced if the footing is circular
(multiplicative shape factors are used to modify the plane strain values of , Nc, Nq and Nγ) or
if the soil is non-homogeneous (calculations must then be based on some representative
strength). By contrast, the method of stress characteristics constructs a numerical solution
from first principles, without resorting to superposition, shape factors or any other form of
approximation.
In this paper, the validation of the method of stress characteristics is tested by solving
a wide range of bearing capacity problems. The results are compared with classical bearing
capacity theories; namely, Terzaghi, Myerhof, Hansen and Vesic methods.
It was concluded that the bearing capacity predicted by the method of stress
characteristics for the case of a circular footing in clay ranges between (3.7 – 4.0) greater than
Terzaghi, Meyerhof, and Vesic methods. This means that the method is not conservative for
this case and can be dependent for economic design of foundations. The bearing capacity
predicted by this method increases linearly with (D/B).
For all values of the angle of friction, φ, the method reveals bearing capacity values for
smooth footings greater than Terzaghi and Hansen and smaller than Meyerhof and Vesic
theories. Considering the foundation to be rough, the method gives bearing capacity values
greater than all other methods. The difference increases as the angle of internal friction
(φ) increases. This makes the method unreliable for rough foundations.