Quantifying the Influence of an Urban Tree Transpiration on Soil–Water Dynamics and Ground Settlement Using In Situ Monitoring

Reactive, or expansive, soil experiences considerable volume changes and movements as a consequence of variations in its moisture content due primarily to seasonal weather conditions. The movements could be significant if trees were present, extracting a substantial quantity of water in hotter months and resulting in shrinkage settlement. Lightly weighted structures founded on reactive soil may be subject to foundation failure if constructed too close to trees. This study evaluated the drying influence of an Australian native tree, Lophostemon confertus, grown in a clay site installed with various monitoring instruments in Melbourne, Australia. The variations in ground movement, volumetric soil moisture content, and total soil suction were closely monitored for 32 months. Tree transpiration was measured using sap flow sensors. The monitoring results revealed that the influence depth of the tree reached at least 3.8 m; the marked soil settlement that occurred at a lateral distance of 0.5 tree height away was attributed to the desiccating effect of the root systems of the tree. Tree root–soil interaction analysis showed that tree transpiration partially contributes to a reduction in soil moisture content and a rise in suction, resulting in soil-drying settlement.