Dynamic performance of an embankement using linear and non linear approach

The paper presents a comparison between linear and non-linear dynamic analyses performed to examine the stability of a tailing dam located near the village of Cave del Predil (Italy) where, during the years of major mining activities, more than 2x106m3 of tailings were accumulated into four basins built on the left bank of the Rio del Lago stream. The slags present a grain size distribution typical of silty sands and have been confined by levees made by the coarse soil fraction deriving by mining activity. As the collapse of the levees could cause the obstruction of the stream and the release of pollutants and heavy metals in the ecosystem, a first study was performed at the beginning of 90’ to evaluate the stability conditions of mine tailings deposits under seismic conditions using a pseudostatic approach implemented into limit equilibrium methods. Starting from these previous analyses, the knowledge of geotechnical properties on both materials have been completed and integrated recently with new in situ and laboratory tests with the aim to create a finite element model and investigate the dynamic behaviour of basins subjected to a seismic action during a period of heavy rainfall. The study was developed in sequential steps: after a seepage analysis performed to evaluate pore pressure distribution inside the basin, FEM method was used to investigate its dynamic behaviour and post seismic stability conditions. The initial seepage analysis highlights the high permeability contrast between fine tailings and coarse levees materials and that the water pressures inside the basins are not particularly influenced by intensity and frequency of rainfall. The results of this initial seepage analysis, in terms of major water pressures and water table increments, were assigned to the following pre-seismic stability analysis. The technical performance of tailings during an earthquake may be defined with reference to serviceability and ultimate limit states. The first ones are represented by the development of differential displacements that may cause localized failures inside the levees through which the polluted sediments may come out and arrive at the biosphere; the second ones are related to the formation of shallow and deep slidings of the levees induced also by liquefaction phenomena. The dynamic analysis of the basins has been performed using different accelerograms changing the severity of seismic action depending from the hypothesized return period. For return periods associated to the serviceability limit state, the shear strains computed with a linear elastic model resulted small and lower than the threshold generally adopted in this kind of analysis. Moreover, the instantaneous displacements and excess pore water pressures were small while the maximum ground acceleration, lower than critical one, was not sufficient to develop permanent displacements assuring the serviceability of the earth structure. The analysis performed for long return periods associated to ultimate limit states, at present in progress, show shear strains much more high and sufficient to adopt a non-linear constitutive soil model even if permanent displacements remain anyway admissible for an earth levee. The development of higher excess water pressures highlights the possibility of liquefaction phenomena in the saturated zones inside fine tailings but the location and the thickness of these zones are not sufficient to trigger ultimate limit states like the loss of embankment global stability.