A quantitative framework for tree–soil interaction mechanisms in expansive clay: Field investigation and empirical modeling

The complex behaviors of expansive soils, particularly their volumetric changes driven by moisture variations, pose significant challenges in urban geotechnical engineering. Although vegetation-induced moisture changes are known to affect ground movement, quantitative characterization of tree–soil interactions remains limited due to insufficient field data and unclear relationships between tree water uptake and soil response. This study investigates the mechanical behavior of expansive clay soils influenced by two Lophostemon confertus samples during a 14-month field monitoring program in Melbourne, Australia. The research methodology integrates measurements of soil displacement, total soil suction, moisture content, and tree water consumption through instrumentation and monitoring systems. Field measurements suggest that tree roots reached the limits of their water extraction capacity when total soil suction exceeded 2880 kPa within the active root zone. The spatial extent of tree-induced soil desiccation reached 0.6–0.7 times the tree height laterally and penetrated to depths of 2.5–3.3 m vertically. The mature sample, with an 86% greater crown area and a threefold larger sapwood area, exhibited 142% higher water consumption (35 kL), demonstrating the scalability of tree–soil interaction mechanisms. A multiple linear regression model was developed to quantify the coupled relationships between soil movement and key variables, achieving a high adjusted R² value of 0.97, which provides engineers and practitioners with a practical tool for estimating ground movement near trees. These findings offer valuable insights for infrastructure design in tree-adjacent environments and can inform computational models and design codes to enable more accurate site assessments and sustainable urban development.