Settlement Based Load-Bearing in a Combined Pile-Raft Foundation

The understanding of the complex soil-pile-raft interaction and their mobilization with the level of settlement is an important aspect for the performance-based design of the combined pile-raft foundation (CPRF). This paper investigates the influence of various interactions governing the load-bearing mechanism of a vertically loaded CPRF using a three-dimensional finite element-based computer program. The soil was modeled using the conventional Mohr-Coulomb elastic-perfectly plastic constitutive relationship and the drained condition was adopted in the analysis. Piles and raft were modeled using embedded beam elements and plate elements respectively. After successful validation of the developed numerical model, extensive numerical simulations were carried out considering a CPRF of different geometries and configurations where the foundation was subjected to incremental settlement boundary conditions. The variation of pile-pile, pile-raft and raft-pile interactions with settlement and their influence in dictating the loads induced within the foundation components were obtained. The analysis results indicated that the settlement levels played an important role in the mobilization of loads and dictating the efficiency of CPRF. Piles within a CPRF were able to mobilize their full capacity at the settlement of 1.5-2% of the raft width. The level of settlement and the spacing between the piles dictated the transition from negative interaction to positive interaction indicating a higher capacity of a CPRF compared to the combined capacities of piles and raft at higher settlement levels. Results suggested the operating settlement of CPRF which may be useful for geotechnical practitioners in making this foundation a cost-effective foundation choice.