Water extraction by isolated trees and its possible impact on building foundations on clay soils

Spatial and temporal variation in soil water content and bulk density was measured near Melaleuca and Ulmus trees, with neutron and gamma gauges, on a Duplex soil in Melbourne, in order to examine water extraction patterns of these trees and foundation movement arising from water extraction. The experiment simulated the situation of isolated trees growing on city pavements. Results indicated that both trees extracted similar amounts of water. However, Ulmus, the exotic, deciduous species, extracted water from a greater depth near the tree than Melaleuca, the Australian native, evergreen species. Melaleuca extracted water from a greater distance than Ulmus. Data on bulk density indicated differential swelling and shrinkage of greater magnitude near Ulmus than Melaleuca, although seasonal shrinkage adjacent to Melaleuca was higher than Ulmus. The results are discussed in relation to variation in compressibility factor of overburden and damage to building foundations.