Tree-induced deformation in unsaturated soils

Abstract

Settlements of lightly loaded structures are frequently caused by changes in matric suction causing shrinkage or swelling of soils. These structures are often not designed to deal with influences of these changes. This study explores issues related to ground displacement due to changes in matric suctions induced by root wateruptake by tree. Numerical simulations of moisture migration patterns in the unsaturated zone and in the vicinity of established tree were performed. The moisture flow model used is based on Richard’s equation with incorporated sink term and integrated with appropriate water-uptake models in 2D and 2D axi-symmetric form. The numerical solution was derived from a finite element approach for spatial discretisation along with a finite difference time-marching scheme. A series of numerical simulations have been performed by the model to assess functionality of stress-deformation analysis partially coupled to flow equation. The model is capable of estimating matric suction changes and accompanying volume change profiles due to water-uptake over a full-annual cycle. Time dependent boundary conditions based on rainfall data effects have also been explored. Results of sensitivity and parametric analyses shows that the predicted ground displacements are sensitive to all the parameters tested. On the other hand, all initial time step sizes used for testing the convergence criterion on the simulation works was found to converge and the simulated results shows difference of not more than ±5 %, which is considered satisfactory. The research also provides an assessment of the significance of trees induced suction changes on unsaturated soil slopes. The model is capable of estimating ground displacements due to matric suction changes on sloping ground. The magnitude of the ground displacement on the sloping surface depends on the relative position of trees on the slope. A relatively straight forward simple approach to model the matric suction changes and accompanying volume change profiles beneath tree as a result of water-uptake has been developed. The resulting model is potentially valuable for a range of geotechnical engineering problems situated on the vadose zones containing trees.