Rapid Drawdown Analysis Using Pore Pressures from BBM

Accurate pore pressure prediction under transient seepage conditions following the removal of external water loads, such as rapid drawdown (RDD), requires coupling the effect of changes in both hydraulic boundary conditions and stresses. The Barcelona basic model (BBM), which has been used as a coupled soil constitutive model to predict pore pressures, is used in this paper to predict pore water pressure for RDD stability analysis. RDD stability is evaluated using predicted pore pressures from the BBM for a hypothetical geometry and for the failed Sparmos Dam. The paper compares the pore pressure assumptions for the BBM to other methods of RDD and highlights the limitations of using the BBM in effective stress RDD stability analysis. Sensitivity analysis revealed that pore pressures following RDD are not affected by the values of the BBM parameters, if varied in reasonable bounds. During drawdown, the BBM simulates the influence of shear stresses on predicted pore pressures only related to an elastic response of the soil, which is not realistic for most soils. Despite these limitations, the pore pressures predicted by BBM calculated the most reasonable factor of safety close to unity using the most likely strength parameters for the failed Sparmos Dam when compared to other common RDD procedures.