Vegetation is responsible for imposing relatively severe suctions on the foundation soils beneath lightly loaded structures. If the role of trees, shrubs and even grasses in causing movements in expansive clay foundation soils is to be predicted and understood, their influence must be accurately included in foundation soil moisture models. Surprisingly, little is known about the spatial extent of root masses, and consequently, their effect on foundation behaviour. This paper presents a series of observations of root architecture. The observations are drawn from a variety of sources within the literature and the authors' collective experience. From these observations, a set of guiding principles are established. The most fundamental of these, are that the roots of trees and shrubs are not limited to particular distances or depths, but rather, they will extend as far as they need to, or as far as they can, depending upon the type and species. Subject to their physical limitations, roots will extend as far as they must in order to ensure survival and/or proliferation, if water is readily available, or if root extension is prevented, the extent of root masses may be relatively small. If water is scarce and conditions allow, roots will extend to many meters in depth, and tens of metres laterally, provided they can penetrate the soil/rock profile and have access to oxygen. Hence, tree root architecture should be modelled according to the hydro-geological constraints of the soil profile being studied.
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