Studies of Failure Initiation in Unsaturated Slopes

Mass movements, such as landslides and debris flows, are usually triggered by hydrologically driven instability of partially saturated earthen slopes and pose grave threats to life and property. Deep insight into the triggering mechanisms is vital for the prevention and mitigation of landslide hazard. This project will investigate the initiation of various failure phenomena in unsaturated earthen slopes induced by increased saturation and fluid flow through a combination of experimental modelling and numerical simulations. Small-scale slope models will be constructed, well instrumented and tested to failure in a geotechnical centrifuge. Increased saturation and fluid flow will be induced artificially by controlled sprinkling while the slope model is in flight. Numerical simulations of the centrifuge model tests will be conducted through finite element modeling using a coupled solid deformation-fluid flow formulation. To this end, a newly developed stabilized mixed hexahedral finite element employing equal order of interpolation for the displacement and pore pressure fields will be used, along with a critical-state plasticity model for the soil capable of representing compactive and dilatant plastic deformation behavior. Fluid mass is conserved globally in both the physical and numerical models because of the well defined boundary and initial conditions, thus ensuring a meaningful fluid flow analysis. The constitutive model will be calibrated with element tests on unsaturated soil used in centrifuge model. The numerical model will be validated with the centrifuge model tests.

Landslides, or fluid-like debris flows, are hazardous processes threaten lives and properties worldwide. Landslides occur when earth material moves rapidly downhill after failing along a shear zone. Their destructive nature demands for a physical-based characterization of the initiation process and triggering conditions of in the partial saturated soil. Soil can be simplified as three-phase porous material, with a solid skeleton of soil grains and voids filled with water and air. It is known that an increase in pore water pressure, or decrease in capillary forces, reduces the stability of the slope. However, this interplay is yet to be incorporated into existing slope stability models to allow the prediction of potential slope failure. In this project we go beyond traditional limit-equilibrium analysis by means of a 3-D numerical simulation able to account for the heterogeneous, hydro-mechanical coupled multi-phase nature of soil. The conducted experiments on model slopes with various initial conditions and rainfall intensities serve as a validation database for the numerical model. The centrifuge modeling technique allows studying the slope behavior on reduced-size model slopes maintaining the true stress conditions of the un-scaled slope. Furthermore, we are able to observe the failure mechanism including the initiation and development of the unstable shear zone. Triggered landslides in the model can help to estimate the potential energy of instable hillslopes. Finally, the developed numerical code enhances the evaluation of slope stability.