Efficiency of the combined pile-raft foundation

It is well known that a raft foundation is, in most cases, a more economical alternative to a conventional pile foundation. The concept of incorporating a raft into a piled system is not new and has been implemented in foundation design in several Western countries. However, the practical implementation of this concept remains limited and largely absent. Ukrainian design codes only mention the existence of combined pile-raft foundations (CPRFs) and the possibility of their application. In CPRFs, both components—the raft and the piles—act as load-bearing elements, and their design must follow one of two approaches, depending on which component serves as the primary structural element. The first approach treats the raft on natural soil as the main foundation system, with piles added to improve its functional performance. In this case, piles are used to reduce and equalize the raft’s settlement and to lower the reactive pressure beneath the raft base. The second approach considers the piles as the main load-bearing elements, with the raft incorporated to reduce the total number of piles. This type of CPRF can be particularly effective in cases of complex soil stratification, where at depths of 10–15 meters, there are thick lenses of weak soil, making traditional pile foundations require excessively long piles. This paper demonstrates, using a specific design case of a pile foundation, the feasibility of implementing an alternative solution in the form of a combined pile-raft foundation. The structural analysis model of the building was developed as a spatial multi-element system. The raft was modeled as a slab on an elastic foundation with a variable modulus of subgrade reaction across the plan. The design parameters of the model were determined analytically and through software, using the GRUNT subroutine of the LIRA-SAPPHIRE 2024 software package. The implemented alternative CPRF solution proved to be both feasible and efficient, satisfying all deformation and stability requirements for both the soil base and the piles. With minor modifications to the raft design, the number of piles was reduced from 290 to 190.