Nonlinear response of dams founded on alluvial deposits in narrow canyons

Abstract

This paper presents a simplified model for the response of earth dams founded on alluvial deposits at the base of canyons. The goal of the model is to capture the effect of the most important factors influencing the response and to provide a realistic and efficient tool for preliminary safety evaluations. The developed theoretical closed-form solution combines the effects on the response of: (a) the foundation layer, (b) the canyon narrowness, (c) the variation of soil stiffness within the dam versus depth, and (d) soil nonlinearity. The latter is incorporated in a simple piece-wise linear scheme, in which the soil shear moduli and damping ratios within the dam and the layer are continuously updated at small time intervals during the analysis. The presence of a soft foundation layer is found to have a dramatic effect on the natural periods of the dam. A series of parametric studies is undertaken to investigate the effect on the response of the fundamental geometric and material characteristics of the dam-layer-canyon system subjected to transient and harmonic motion. The results show that a foundation layer with an average S -wave velocity similar or less than that of the dam under moderate or strong base excitation may experience significant shear strains. Such strains are capable of inducing high excess pore water pressures and perhaps liquefaction within contractive saturated cohesionless soils in the foundation. The cost of the simplified analysis is about three orders of magnitude less than that of an elastoplastic FE analysis.