Analysis of Raft Foundation on Expansive Soil.

Abstract

Load-bearing response of Raft or mat footing is affected by symmetry, load concentration and textural composition of the underlying or surrounding soil. Raft or mat foundation is a combined footing that covers the entire area beneath a structure and supports all walls and columns. This work presents Finite Element Analysis of raft foundation on expansive clay. Plaxis 3D computer software was used for the analysis. The result was compared with the classical Mohr-Coulomb analysis. The free swell index (FSI) of test clay samples collected from 0 – 1.5 metres depth ranged from 105.95 to 118.18%, which classified it under highly expansive clay. The deformation of model raft foundations were estimated at three stages namely; the initial stage, the excavation stage and the loading stage. The results revealed that the deformation of raft footing was higher at the excavation stage with a value of 4.55*10-3m, when compared with 615.15*10-6m recorded at the initial stage under the same load. With the introduction of model raft, the total deformation of the footing at this critical stage (excavation) reduced to 606.95*10-6m. Under the same threshold pressure and load-factor difference, the deformation obtained using the classical Mohr-Coulomb model is 18.442*10-3m, which is higher than the 601.01*10-6m obtained using the finite element analysis. Finally, the loading rate efficiency of modelled raft foundation using Finite Element Analysis is 10.3% higher than that of Classical Mohr-Coulomb model. Raft foundation analyzed using Finite Element Analysis is therefore recommended especially where the underlying strata is or has similar properties as that of expansive clay.