EFFECT OF CAPILLARITY ON COEFFICIENT OF ELASTIC UNIFORM COMPRESSION OF SILTY SAND

Abstract

Knowledge of the dynamic soil properties are essential for analysing the behaviour of machine foundations and also substructures subjected to dynamic forces such as due to earthquakes. Coefficient of elastic uniform compression Cu, is one of the most important and basic dynamic soil parameters which is determined frequently, owing to the simplicity with which it can be determined by conducting either a plate load test or block vibration test. It is also related to dynamic shear modulus, G and Elastic modulus, E. For the determination of Cu, plate load test is invariably carried out these days, owing to its simplicity. It is well known that a plate load Lest when conducted on a sand/silty sand bed affected by capillary rise of water results in the unsafe prediction of both bearing capacity and settlement. Soil in the Indo-Gangetic plains, being fine silty sand generally is affected by large heights of capillary rise, many times upto 2 m. Though the effect of capillary rise on bearing capacity and settlement of a sand bed is fairly well established, no information is available to quantify the effect of capillary rise on the coefficient of elastic uniform compression, Cu. It is pertinent to mention that some investigators viz., Wu et. al., (1984, 85) have tried to quantify the effect of capillary rise on small strain dynamic modulus. But as is known, plate load test induces very large strains of the order of 5% to 10%, and no tests are available to quantify the effect of (i) capillary rise on very large strain dynamic modulus i.e. Cu. The present, investigation attempts to bridge this gap. Plate load tests were carried out on model footing of 15 cm dia, and 1.3 cm thick. A tank of diameter 59 cm and height 89 cm was used to contain the soil bed. Teats were carried out on a fine silty sand bed. Soil bed was prepared by using the rainfall technique. A density of 1.5 glee was maintained in all the tests. Tests were carried out under dry, submerged and capillary conditions with height of capillary rise varying from 15 cm. to 70 cm. An attempt has also been made to reinforce the information available on the effect of capillary rise on bearing capacity and settlement. An analytical solution to quantify the effect of capillary rise on the coefficient of elastic uniform compression, Cu has also been presented. Based upon experimental and analytical results, the following conclusions have been drawn : (1) Cu value is greatly influenced by capillarity. The value of Cu under capillary bed condition is substantially higher than that under submerged bed condition. The ratio of Cu under capillary bed condition to Cu under submerged bed condition ( Cuc/Cus) varies between 1 to 3.4 for a height of capillary rise h varying from 15 cm to 70 cm. (2) Bearing capacity and settlement are also greatly affected by capillarity. (3) Overburden pressure correction can be readily applied to the values of Cu affected by capillarity to reduce them to their corresponding values under submerged bed condition.