The stability of most earth embankments is strongly influenced by the water content of the soil. The water content directly influences the suction or pore pressure in the soil, as well as the mass of material, thereby affecting the stress state and strength, and leading to changes in the stability. These aspects are coupled by the so-called soil water retention behaviour, which is observed to be a hysteretic phenomenon. Moreover, soils are known to be spatially variable or heterogeneous in nature, which can lead to preferential flow paths and stronger or weaker zones. In this paper the behaviour of a heterogeneous earth embankment subjected to cyclic water level fluctuation, including the impact of hysteresis, is investigated. The soil property values governing the unsaturated hydraulic response of the embankment are considered as spatially random variables, with the mechanical property values considered deterministic in order to isolate the impact of the hydraulic behaviour. The Monte Carlo Method (MCM) is used to conduct probabilistic analyses and an assessment of the relative influence of material properties illustrates that the saturated hydraulic conductivity, ksat, plays a dominant role in the slope stability. Moreover, in the initially drying condition, the average factor of safety (FOS) and the 95th percentile FOS of the slope considering hysteresis are smaller than those without considering hysteresis, at all times, while the variability of the FOS considering hysteresis is larger than that when not considering hysteresis. In practice, this means that slopes under seepage conditions, which are assessed to have a low FOS, should be assessed including the hysteretic behaviour to ensure stability.

OPERA-PU-TUD311
The Onderzoeks Programma Eindberging Radioactief Afval (OPERA) is the third national research programme for the geological disposal of radioactive waste in the Netherlands, operating during the period 2011 to 2016. This document is the final report of Work Package 3.1, where the principal technical feasibility of the current OPERA reference design of a radioactive waste disposal facility, in Boom Clay at a depth of about 500m was investigated, during the period 12-2012 till 12-2014.