A Parametric Study of the Effect of Trees on Residential Footing Design on Expansive Soils

Abstract

The influence of trees on residential footings constructed on expansive soils has long been a concern of geotechnical engineers since the invasive tree root could exert suction forces on the soil surrounding them and extract a considerable amount of water depending on seasonality, leading to significant shrinkage settlement and the subsequent foundation failure. In this study, a Eucalypt, Corymbia maculata, planted in an expansive clay site in Melbourne, Australia, was closely monitored for 44 months to assess tree-induced soil moisture patterns and ground movement. Monthly tree water use was measured using a sap flow sensor over 12 months. The results show that the tree consumed 53 kL of water annually, with the highest water demand of 5.9 kL in December and the lowest monthly water use of 2.7 kL in June. The soil suction and moisture content profiles reveal that the desiccating effect of tree roots extended to 3 m depth, while the ground movement data shows a significant shrinkage settlement at a distance equal to half the tree height (HT). A parametric study was also carried out to examine the influence of various parameters on footing design for the effect of trees and the consequent construction cost. It was found that homeowners could save a construction cost of about 32% if the footing is built at a distance of 0.6 HT from the tree compared to the cost for a footing constructed at a distance less than or equal to 0.5 HT.