Abstract:
The present study introduces the concept of bearing capacity factor, Ny as a
function of relative density and particle characteristics of sand, such as
sphericity, elongation, flakiness. Sand beds were formed in a model tank having
similar angle of internal friction and different particle characteristics and
ultimate bearing capacity was determined using rigid strip, square and circular
footings. Sand samples were collected from three different locations of Bangladesh
namely Gozaria, Sylhet and Mymensingh. TIle equal angle of internal mction for
sands with different particle characteristics were obtained by mixing or rejecting
different sieve fractions of sand samples and controlling the densities. Direct shear
tests were performed combining or rejecting different sieve fractions. It was
observed that if the fraction passing no. 100 sieve for all three sand samples and
retaining no.30 for Sylhet sand were rejected the angle of internal friction of the
prepared samples from Gozaria, Sylhet and Mymensingh sand become very close.
Thus three artificial sand samples, which were uscd in the investigation, were
obtained from the three parent sand samples. Direct shear tests were performed at
different densities of sand to attain a relationship between density and angle of
internal friction. Sand samples were deposited in a transparent glass sided model
tank from a particular height to attain the density required for the desired angle of
internal friction. The sand bed thickness was maintained to three times the least
dimension of the footing. The strip, square and circular shaped footings were used
for bearing capacity tests. Strain controlled loading was applied on the footings.
Load, settlement and rotation were measured using a proving ring and strain dial
gauges. A large number of sand particles from each sieve fraction were used to study the
particle characteristics like sphericity, elongation and flakiness of sand samples
using a sensitive balance and a microscope of high magnification.
The test results show that the bearing capacity of surface footing predicted by
Terzaghi (1943), Feda (1961), Meyerhof(1963), Hansen (1970), Vesic (1973) etc.
are all on conservative side. Significant variations in bearing capacity have been
observed for the sand samples of equal friction angle with varying particle
characteristics. A mathematical model has been developed to estimate the
bearing capacity factor, Ny using density and particle characteristics data.
Numerical analysis of the data suggests that the model can closely estimate
the bearing capacity factor. The density and sphelicity were found to have
significant effect on bearing capacity whereas, the effects of flakiness and
elongation were insignificant. Analysis of the experimental data indicates that
the shape factor of a footing is also a function of density and sphericity.
