A slope movement in a complex rock formation: Deformation measurements and DEM modelling

The case history of a deep-seated slope movement in a complex rock formation (Marly-Arenaceous Formation) is analyzed. The movement, monitored for more than 20 years, was recognized after the discovery of intense cracking in the concrete lining of a hydraulic tunnel running across the slope. The time history of displacements shows that the ongoing deformation process is essentially a stationary creep phenomenon, also influenced by transient variations in pore pressure distribution. The shearing zone is mainly formed by tectonized clay gouge and is characterized by a mobilized strength close to residual. The slope has been modelled (by DEM approach) as a complex blocky structure defined by several joint sets: bedding planes, inclined and sub-vertical joints. Different hypotheses about the geometry of the slip surface, compatible with field evidences, are discussed as well as their influence on the critical friction angle of the slope. The type of model adopted for rock mass, with continuous or staggered joints, influences the pre-failure deformation mode, but the failure conditions are not as much affected. Finally, the response of DEM models to the increase in water level at the toe of the slope and to the rise of the groundwater table inside the slope has been analyzed. The results of simplified models, without flow calculation, and coupled hydro-mechanical analyses are compared, finding some relevant differences.