UCLA

Slope failures cause severe human and economic losses worldwide, yet conventional stability methods describe them only up to the onset of failure. Limit equilibrium methods treat the failing mass as rigid and perfectly plastic, and the finite element method is restricted to small deformations, so neither follows the slide once it forms. The Material Point Method (MPM) avoids this limitation by discretizing soil into Lagrangian material points that move through a fixed background mesh, so no mesh distortion develops under large deformation. This dissertation applies MPM to the analysis of slope failures in two parts, the assessment of slope stability and the influence of vegetation on rainfall-induced failures. The one-phase and coupled two-phase formulations are first derived and verified against classical benchmark problems in poromechanics. Next, MPM is applied to evaluate the stability of slopes using the strength reduction technique. For dry and unsaturated slopes, slip surfaces agree with finite element solutions and factors of safety differ by less than about 10%, reflecting the different criteria by which each method identifies failure. Unlike mesh-based methods, MPM continues past the onset of failure and captures the post-failure runout. The effect of root reinforcement is then investigated. A multiple-hardening constitutive model for root-reinforced soil is implemented within the coupled formulation and validated against triaxial tests. Comparative analyses of bare and vegetated slopes under rainfall show that roots delay the initiation of failure, roughly halve the displacement, and change a retrogressive slide into the movement of a single block, although they do not prevent failure under continued rainfall. Finally, the method is validated against a documented centrifuge slope test. The simulation reproduces the observed retrogressive block failure and the measured pore pressures at failure, and a rooted counterpart delays failure and reduces movement, consistent with reinforced tests. These results establish MPM as a practical tool for assessing both the stability of vegetated slopes and the consequences of their failure.