Pore Pressure Dissipation and Reconsolidation Observed in Large Scale Experiments of Liquefied Debris Flows

Abstract

Debris flows can have devastating consequences due to their high speeds, far distal reach, and sudden occurrence. The high mobility of debris flows is often attributed to liquefaction, which can reduce basal friction to near zero and cause fluid-like properties. Consolidation of liquefied materials occurs when the generated excess pore pressures dissipate, and grain contacts and effective stress are regained. The evolution of excess pore pressures and their persistence following deposition must be considered in numerical solutions to predict mobility and understand the risk of remobilization. The objectives of this research were twofold i) to develop a cost-effective and field-applicable test method for calculating the consolidation parameters of liquefied granular materials and ii) to investigate the evolution of pore pressures in debris flows and its effect on mobility. This thesis presents a novel test method to calculate a spatially and temporally variable coefficient of consolidation, the relationship between one-dimensional stiffness and effective stress at low effective stresses, and estimate surface settlement, using two analytical and one numerical model. Estimates of the coefficient of consolidation were similar across the three models and model estimates of settlement matched experimental results well. We also present results from twelve large-scale flume tests carried out at source volumes of 0.2, 0.4, 0.6, and 0.8 m3 of a well-graded granular material with a consistent water content. Excess pore pressures were observed throughout travel and dissipated following flow arrest. Estimates of the coefficient of consolidation matched well with results from the small-scale field test. Pore pressures were found to affect debris flow mobility, as mobility increased with source volume, which was not seen in previous dry tests, and increased spreading compared to previous tests where excess pore pressures did not develop. We anticipate the novel field-deployable test provides an accessible and valuable tool for investigating field events of debris flows, tailings dams and seismic areas, and the debris flow test series is an experimental dataset that can be used in calibration of both theoretical and numerical models of pore pressure evolution and mobility predictions.