3D Finite Element Analysis of Piled-Raft Foundation in Deep Clayey soils

Abstract

Abstract The concept of piled-raft foundation has received considerable attention in recent years. The piled-raft foundation combines the load-bearing elements of the piles, raft and soil in one composite structure. This concept provides an economical foundation alternative when the performance of the raft foundation does not satisfy the design requirements, especially in deep clayey soils. For this case, the addition of piles improves the load capacity, minimize the maximum and differential settlements, and reduce the bending moments within the raft. Several methods have been developed to simulate the behavior of piled-raft foundations including the simplified analysis, approximate numerical analysis and numerical analysis. In the current study, 3D Finite Element Modeling Technique is adopted to evaluate the performance of piled-raft foundation in deep clayey soil formations. Three case studies have been used to verify the modeling of piled-rafts and their outputs using field data measurements and former finite element analysis. Afterwards, a comprehensive study is conducted to investigate the effect of related parameters including clay properties, raft thickness, piled-raft configuration, pile length and pile diameter, and construction period. The performance of each component of the piled-raft foundation is assessed. The induced average and differential settlements in addition to the straining actions in raft are estimated. The maximum vertical and horizontal displacements of piles as well as pile load resisting components (skin friction and end-bearing) are evaluated. The load sharing ratio between the raft and piles for different piled-raft configurations are calculated. Moreover, the consolidation settlement of the piled-rafts and the dissipation of excess pore water pressure within soil mass are evaluated. The current research shows that 3D FEM modeling is a powerful technique for simulating the complex piled-raft foundation system. The study reveals that using piled-raft foundation in deep clayey soils leads to a reduction in the average settlement of the raft by values ranging between 25% and 70% and a reduction in differential settlement by values ranging between 45% and 90% compared to the unpiled-raft foundation. The piled-raft foundation system can be used to minimize the differential settlement using a piled group to raft width ratio equals to 0.4. In addition, the excess pore water pressure decreases by values ranging between 30% and 50% compared to unpiled-raft. It is also found that, depending on piled-raft configuration, the load carried by piles ranges between 15% and 75% of the total load applied on the foundation. Finally, the optimum piled-raft foundation system is achieved at piled group to raft width ratio (Bg/Br) equals 0.70 in stiff clay and equals 0.80 in medium stiff clay.