RESPONSE OF FOOTING RESTING ON SAND UNDER DYNAMIC LOADS

Abstract

The structures resting on soil may fail due to excessive settlement, tilt and sinking into the sub-soil despite being structurally safe during earthquakes. This devastating effect on civil engineering structures has been observed during several past earthquakes. The considerable deformation of the foundation often endangers safety and stability of the whole structure, otherwise safe. These considerable settlements caused by a joint action of static loads and dynamic loads apppear often when foundation rests on non-cohesive soils, particularly on saturated sands. It has been observed that sands with low porosity values possess high value of resistance to static loads even larger than the corresponding values for cohesive soils, such as that of plastic clays. Hence, sands having medium and higher than medium densities are considered good natural base for foundations. However, it is true only for the case of static loads. Under dynamic load conditions; sand or other non-cohesive soils exhibit considerable decrease in their strength. This has been verified experimentally in laboratories and also observed in fields. The quantitative effect of vibrations on cohesive soils have been seen to be much smaller than that on non-cohesive soils, especially sands. Experiments show that vibrations cause changes in the dissipative properties of soil,