EFFECT OF RAFT FLEXIBILTY ON ELASTIC SETTLEMENT OF PILED RAFT FOUNDATION

Abstract

Rapid growth of population and the advance way of technology in the recent years, the need for high-rise buildings and high-loaded superstructures has been increased rapidly. When considering foundations for high-rise buildings, a major task is the reduction of settlements and differential settlements to ensure safety and serviceability. Piled-raft foundation for high rise buildings have been proved to be an appropriate alternative instead of conventional pile or mat foundation. This thesis work concern the effects of raft stiffness on the load sharing and settlement analysis of piled raft foundations. For this analysis, the thickness of the raft varies between 0.5-3m with 0.5m interval. The relative soil/foundation stiffness index (Kr) values differentiate the raft as flexible and rigid. Two raft cases were considered; one case considered rigid raft behavior with Kr value of 3.75 and raft thickness of 2m and two cases considered flexible raft behavior with Kr values of 0.054 and with raft thickness of 0.5m. The built-in material models of the Finite Element (FE) program PLAXIS were used to capture the settlement and load carrying capacity of pile and raft at different spacing(3d-6d) and length(10m-40m) of the pile for the two raft cases. Static analysis carried out using Mohr Columb soil material model and for pile and raft linear-elastic model were used. The FE analysis results indicated that, increase in raft thickness in the piled raft foundation, the load shared by the raft decrease. Due to the decrease in contact with the subsoil. The load sharing percent achieved its 50-50 percent, nearly at 1.8m raft after that more loads are taken by piles. Due to contact pressure between raft and soil, the load carried by the raft was about 58.3% and 49.99% at the spacing of 4d at t= 0.5 &2 m. On this study, larger pile spacing is observed to be more efficient in reducing the average settlement and enhancing the load-sharing coefficient. For this particular study, the optimum spacing of piled raft to minimize differential settlement is 4.5d. The length of pile is observed to be efficient in reducing both differential and total settlement