Investigation of seasonal soil moisture and temperature variations underneath a waffle raft foundation built on reactive soil

Lightweight structures, such as residential foundations, built on reactive soils experience stability issues due to the soil’s high shrink-swell movements. This study investigated the performance of a full-scale waffle raft foundation (~ 8 m x ~ 10 m) built on a highly reactive site. Spatiotemporal variations of the soil moisture content and temperature outside and underneath the foundation were monitored for more than a year. The shallow soil temperature beneath the foundation fluctuated in response to seasonal weather changes, but no significant spatial variations were observed. Seasonal weather conditions showed a dominant effect on the soil temperature variation outside and beneath the foundation at any depth, and intermittent rainfall events outside the wet/winter season displayed no significant impact on this. The fluctuations in soil moisture content diminish towards the centre of the foundation, although a notable fluctuation is still evident beneath its centre. At varying depths beneath the middle of the foundation, seasonal fluctuations in soil moisture content were observed up to a depth of 2.3 m, but the initial moisture content differences at different depths remained consistent. Overall, this study recommends revisiting the standard design procedure for waffle raft systems in terms of edge distance and depth of design soil suction change determinations.