Effect of the soil unloading path on the stress-strain state of a rockfill dam

Introduction. Soil deformation exhibits a non-linear elastic-plastic behaviour. Consequently, modelling of the stress-strain state (SSS) of high embankment dams is necessarily carried out using non-linear rock mechanics models (Mohr – Coulomb, soil hardening and others). One of the key manifestations of the non-linearity of soil deformation is the different nature of deformations under two loading scenarios: during unloading, typically only elastic deformations occur, whereas during active loading, elastic-plastic deformations take place. During loading, the area of elastic deformation increases; this effect is commonly referred to as hardening. However, not all non-linear models, specifically the Mohr – Coulomb model, take this effect into account. This paper presents studies on estimating the extent to which the hardening effect influences the results of numerical modelling of the SSS of embankment dams.

Materials and methods. The analysis was carried out using the PLAXIS software package, using a homogeneous 100-high rockfill dam as an example. A linear model of ideal plasticity with the Mohr – Coulomb strength state condition was employed. To determine the extent of the effect, the results of SSS analysis were compared for two scenarios: one taking into account the existing unloading trajectory and one without it. The existing area of elastic deformation was accounted for using a plasticity surface.

Results. The investigation showed that taking the hardening effect into account radically alters the results of SSS numerical modelling of the rockfill dam. When subjected to hydrostatic pressure, part of the dam soil is not under active loading but is instead unloaded. Taking this hardening effect into account led to an approximately twofold reduction in the horizontal displacements of the dam caused by hydrostatic pressure. Furthermore, when the hardening effect is taken into account, the predicted stability of the dam with a diaphragm increases significantly; the risk of loss of shear strength and the formation of a collapse mass in its upstream face decreases considerably.

Conclusions. The use of models that do not account for the soil hardening, such as the Mohr – Coulomb model, in SSS analyses of embankment dams is not permitted. Such models not only underestimate the dam’s load-bearing capacity margin but also significantly distort the numerical simulation results.

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