EFFECTS OF LIQUEFACTION ON SHALLOW FOUNDATIONS

Abstract

Design of foundations in earthquake prone areas needs special considerations. Shallow foundations may experience a reduction in bearing capacity and increase in settlement and tilt due to seismic loading. The reduction in bearing capacity depends on the nature and type of soil and ground acceleration parameters. International Building Code generally permits an increase of 33 % in allowable bearing capacity when earthquake loads in addition to static loads are used in design of the foundation. However, according to Indian Standard (IS: 1893- 2002), when earthquake forces are considered, the allowable bearing pressure in soils shall be increased by a margin of 0 to 50%, depending upon type of soil. Amount of settlement of shallow foundation is much greater for the liquefaction condition compared to the non liquefied state. Construction of shallow foundation on liquefiable soils shall be allowed only after proper soil treatment. Construction of shallow foundation on liquefiable soils is possible in the presence of a sufficiently thick and shear resistant nonliquefiable soil crust. Due to earthquake, excess pore pressure build up and the associated shear strength degradation of liquefiable soils may result in reduction in bearing capacity and increase in settlement of shallow foundations. Cascone and Bouckovalas (1998) shown that shear strength of liquefiable soil is expressed in terms of a degraded friction angle. Since friction angle is used for determination of bearing capacity factor, therefore reduction in friction angle leads todecrease in bearing capacity and factor of safety. This degraded friction angle can be used for evaluating bearing capacity and degraded factor of safety of liquefiable soil by any analytical method such as Terzaghi, Meyerhof, Vesic and IS 6403:1981. Liquefaction induced settlement are correlated to the seismic excitation characteristics and the degraded factor of safety. The presence of unliquefiable layer may drastically reduce settlement and increase the bearing capacity of footing. Clay crust, dense sand and improved sand may be used as a unliquefiable layer. Thickness of unliquefiable layer affects significantly the performance of shallow foundation under liquefaction. The peak ground acceleration much affects the liquefaction induced settlement than the bearing capacity. The depth of liquefiable layer, also affects the liquefaction performance of shallow foundations