The deep-seated slope deformation at Encampadana, Andorra: Representation of morphologic features by numerical modelling

The present study is intended to improve some aspects referring to the general understanding of deep-seated gravitational slope deformations (DSGSD) by the description of field data from the Encampadana slope in Andorra, and using a simplified numerical modelling.The site investigation of the Encampadana DSGSD revealed two major morphologic features: a prominent “graben” close to the summit and counterscarps at different altitudes of the slope. The geological data showed that the slope is affected by an antiform consisting of hard resistant rocks in its outer parts and a more ductile material in its inner part, subsequently called core.Currently available information does not allow the appraisal of the failure mechanism affecting the slope. In order to improve the knowledge of both the pattern of the slope deformation and the influence of the different geologic, structural and geomorphologic features, 2D finite element modelling has been carried out simulating the joint pattern of the rock slope by interface elements. The Encampadana slope is located at a river valley that was subjected to erosion by Pleistocene glaciers plus the subsequent decompression of the valley walls after their retreat. These stages were implemented into the model to account for the evolving stress history. During modelling, focus was set on the effect of three different factors: (1) geological discontinuities, (2) glacier erosion, and (3) a ductile core in the slope consisting of black carbonaceous shales.The calculations showed that a staged approach for simulating the stress history of the site was very appropriate, although the presence of tectonic stresses was not incorporated. The results indicated that the discontinuity set and the glacier erosion generate an opening of the joints, some toppling-effects and the development of counterscarps. Moreover, the incorporation of a ductile core produced a “graben”-like structure. All these features are consistent with the morphology observed in the field.In conclusion, the numerical modelling indicates that some realistic results representing the general deformation of the slope and the observed field features can be achieved even with very limited input data.