Effect of particle characteristics of sand bearing capacity

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.