Combined Piled Raft Foundation (CPRF) System for Polymerization Loop Reactor Structure

The current practice followed in the detailed engineering of foundations for critical industrial structures ignores the contribution of the raft (pile cap) and assumes that the loads are supported entirely by the piles. This approach would result in unduly conservative and uneconomical design where the settlement is reduced smaller than necessary with the use of significantly higher numbers of piles. In a combined piled raft foundation (CPRF), the pile cap provides a significant proportion of the required load capacity with the piles strategically placed to boost the performance of foundation by acting as settlement reducers. The paper presents a detailed step-by-step procedure for the implementation of a CPRF as a cost-effective and technically competent foundation system for a polymerization loop reactor structure which represents a critical component of polymerization plant in the refinery unit. The design process consisted of an initial stage of geotechnical site characterization and computation of required parameters based on the results of soil investigation report prepared for detailed engineering. The structural analysis was then undertaken for various code-prescribed critical load combinations to compute the support reactions for foundation analysis and design. The feasibility and further preliminary assessment of CPRF layout was done using Poulos-Davis-Randolph (PDR) method. The pile numbers, length and locations were then refined using finite element-based geotechnical program, PLAXIS 2D. It was found that the implementation of a CPRF versus a conventional piled only foundation provided the required strength and serviceability performance while delivering a cost saving in the order of 30–50%.