The extent of soil desiccation near trees in a semi-arid environment

Abstract

Australian standards provide guidance on classification of sites for ground movement and the subsequent design of shallow footings for residential buildings on expansive soils. It does not however provide any guidance on how the drying effects of vegetation might be incorporated in the footing design. Despite there not being a prescriptive Standard for designing for the effects of trees, engineers have been pressed by the community to allow for the extra soil drying. Some rules of thumb have been promoted and empirical design suction changes have been adopted. The design suction changes depend on the relative proximity of the tree to the building, as well as the site classification level and whether or not a single tree or group of trees is to be considered. Although it is understood that different species develop different root systems and so affect the soil differently, tree species is not a parameter that is required in the models.

The primary purpose of this paper is to present measured suction profiles pertaining to urban sites in a semi-arid climate (Adelaide, South Australia). All sites contained Australian native eucalypts or 'gum' trees. Generally the data were gathered as part of site investigation for footing failures – excessive movement and cracking, rather than structural failure. At least two suction profiles were determined for each site, close to and well away from the vegetation. A preliminary model is proposed to account for the drying settlement caused by trees.