Piled-Raft foundation behaviour on consolidating soft soil

Abstract

In areas where soft soil of low strength and high deformability extends over considerable depth from ground surface, deep foundation like piles are adopted by the designers for construction of high rise buildings, resulting high cost of construction. However, in such cases, a relatively cheaper foundation system like raft if adopted can counteract the possibility of shear failure due to huge super structural loads on the sub soil but the resulting settlement would be too large to be permitted. Thus, to have an economic foundation system, a raft is provided over the base of the proposed building and some piles are installed at specified location below the raft to increase the load carrying capacity of the combined system with reduced resulting settlement. But in a consolidating soft soil, the interaction between soil, raft and pile becomes time dependent. In this paper, a simple design methodology for pile raft system is proposed. An attempt has been made to incorporate the effect of time dependent consolidation settlement, calculated from the evaluated consolidation characteristics of the subsoil from soil exploration. Results of the pile load test data conducted at the site were considered for determination of pile capacity. It is shown that for a chosen piled-raft system in soft clayey subsoil, consideration of consolidation effect of the sub soil gives more rational evaluation of the interaction problem of a piled raft foundation.