Coupled solid-deformation/fluid-flow simulation of failure initiation in variably saturated slopes

Principal Investigators: Ronaldo I. Borja, Keith Loague
Project Sponsor: National Science Foundation

Project Description

Landslides occur when earth material moves rapidly downhill after failing along a shear zone. Debris flows are differentiated from landslides by the pervasive, fluid-like deformation of the mobilized material. Landslides and debris flows threaten lives and property worldwide. Despite the fact that good progress has been made within the last two decades relative to understanding hydrologically-driven slope failure, important research has yet to be conducted in 3D physics-based fluid flow and hydrologically-driven slope instability in variably saturated soils.

This award funds interdisciplinary research focused on a physics-based characterization of coupled hydrologic response/slope stability processes for steep hillslopes at the catchment scale. The model will couple solid deformation with fluid flow processes in variably saturated soils, as well as quantify the exchange of water between the subsurface and surface continua. This allows us to better understand the effects of surface runoff, evapotranspiration, and percolation on the spatial and temporal variations of degree of saturation, effective stress, and deformation pattern within the variably saturated slope. The coupled model will be tested with comprehensive and exhaustive data from the Coos Bay experimental catchment (CB1), as well as with the numerical results of recently conducted simulations on the same catchment using an Integrated Hydrology Model (InHM). This research will also utilize a recently developed stabilized low-order finite element approximation scheme employing equal orders of interpolation for the solid displacement and pore pressure fields. The highly instrumented CB1 slope failed as a large debris flow in November 1996, thus providing large volumes of data with which to compare the model predictions.

The research team will combine expertise in geotechnical engineering, computational geomechanics, and quantitative hydrogeomorphology available at Stanford University to develop and test a physics-based model of slope failure initiation. To the knowledge of the PIs, no slope failure initiation model currently exists in the literature that addresses the effect of variable saturation in a quantitative way. We believe that the FE method has reached such an advanced stage that it can now handle not only complex geometry but also the effect of variable saturation. A further intellectual merit of this research lies in the tremendous opportunity for testing and validation of the proposed mathematical approaches with the available data set. The CB1 data set allows model testing and validation on a large-scale slope with complex topography and variable saturation. The availability of high-quality hydrological and geotechnical data for CB1 will help constrain the parameters of the problem, thus providing tremendous opportunity to gain a better understanding of the important processes controlling slope instability.

The study is a timely contribution towards an improved understanding of the processes that control slope instability in a system driven by a rigorous characterization of the near-surface hydrology and soil constitutive properties. The simulation effort will effectively demonstrate the utility and/or limits of physics-based slope stability models less comprehensive than the one to be developed here for field conditions similar to CB1. The proposed research will also utilize the advances in computational fluid dynamics for application to geotechnical and geosciences problems. Both PIs are seriously committed to ensuring full involvement of undergraduate and underrepresented students in this project. This can be gleaned from their proven track record of mentoring, advising, supervising, and graduating undergraduate and underrepresented students at Stanford.

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