Overcoming Limitations in Raft Footing Design: Alternative Approaches to Uniform Modulus of Subgrade Reaction Analysis

Abstract

Raft footing is a cost and time effective foundation solution for building and infrastructure development. By eliminating the need for deep piling, raft foundation significantly reduces embodied carbon, construction time, capital expenditure and disturbances to surroundings. In current industry practice, the Winkler Spring Model is commonly employed for shallow foundation design. This approach simulates the founding soil as a series of independent spring supports defined by the modulus of subgrade reaction (ks) for analysis using structural software such as SAFE. However, the simplified method for estimating ks and the assumption of uniform subgrade support across the entire raft footing have limitations, often producing unrealistic settlement and differential results. Misinterpreted subgrade values may lead to the rejection of the raft footing option and necessitating to opt for a more costly and time-consuming piling foundations. This paper explores an alternative approach to estimate the modulus of subgrade reaction (ks) in a more comprehensive manner using geotechnical finite element software PLAXIS 3D. This method can capture complex subsurface conditions and soil-structure interaction effects, allowing the distribution of ks to vary spatially across the raft. The outcome provides a more accurate and realistic ks estimation, ensuring better decision-making in the design of raft footings.