SETTLEMENT OF FOOTINGS ON PRELOADED SAND DEPOSIT

Abstract

The effect of pre3.oadjng on a sand deposit is to increase its stiffness. A a consequence, a tooting resting on a prei.oaded sand deposit will undergo smaller settlement as compared to a tooting resting on normally loaded sand deposit of the same relative density. 'Though to this fact is known/acme 'research workers, no att&t has so far been made to quantity the effect of preloa ding on the settlement of footings on sand. Hence an analytical as well as an experimental investigation has been carried out to study the effect of preloading on the settlement of a footing resting on sand. Assuming the modulus of compressibility as a func-tion of relative density (SPT value) and in situ vertical stress an analytical solution is presented. The analysis takes into account the change in in situ vertical stress and hence the reduced COMPtessIbility due to prelong of the sand deposLt., The results of the analysis are presented in the form of settlement per unit pressure vs width and settlement ratio vs width ratio to enable esti- or metion Of settlement Of a footing directly/from the results of a plate load test conducted on a preloaded sand deposit, contd.., Load tests on modal footings of 12 cm diameter were conducted on preloaded test beds prepared at a relative density of 85.% • The tests were conducted for various conditions of test bed, via., dry bed, submerged bed, moist/wet bed and with different preloa ding pressures.. The results of load tests are presented in the form of load-settlement curves. The results of analytical and experimental inves-tigations indicate that the preloading of sand deposit reduces the settlement of footings substantially. The magnitude of reduction is dependent on the magnitude of preloling pressure experienced b 8and deposit. If the effect of praload•ng on settlement is. disregarded in the estimation of the settlement of a footing using plate load test data, it may lead toe serious underestimation of the settlement of the footing.,