SEISMIC SETTLEMENT OF SHALLOW FOUNDATIONS

Foundations failures have been observed during several earthquakes because of excessive settlement and tilt. The problem of settlement due to dynamic loads has attracted the attention of researchers since early sixties. Shallow foundations subjected to static and seismic loads have mostly been designed using pseudo-static approach, the settlement and tilt of the foundation was also calculated using the static methods. Empirical equations for calculation of settlement and tilt of foundations subjected to static loads and moments have also been used for estimating the settlement and tilt for the seismic case. Earlier studies have indicated that the cyclic shear stresses induced by the earthquake ground motion are the main cause of seismic settlements. Vertical accelerations generally do not result in significant settlements unless they exceed about 1g. The research effort has therefore been dedicated towards determining the shear strains induced in the soil deposits and correlating these strains with the settlements. Simplified procedures similar to ones used for liquefaction analysis have also been used for estimating shear strains in soil deposits due to earthquake which is assumed equal to volumetric strain. The settlement can then be estimated using the volumetric strain and the thickness of the deposit. For the case of a layered deposit, the strain in each layer may be calculated and total settlement obtained as the cumulative settlement. The settlement calculated in this manner generally represents the settlement of soil deposits in the free field i.e. as if the foundation was not there. The foundation may experience additional settlement due to the loads and moments imposed on the foundation by the earthquake. There has been significant research effort devoted to account for such additional settlements by considering the reduction in shear modulus of soil by induced shear strains. The total settlement of the structure due to earthquake shaking obtained as the sum of these two components of settlement. Charts based on simplifying assumptions are available to determine the post volumetric strain for soils dry and partially saturated also soils that are likely to liquefy, using the normalized standard penetration data or relative density of sand and the factor of safety against liquefaction.