A 100 m high gravity dam (Castrovido dam, Burgos, Spain) is founded on a soft Weald red siltstone with interbedded sandstone strata. Dam stability is essentially controlled by the available shear strength of sedimentation planes. Block samples were recovered for the different types of contact identified. They were tested in high capacity shear box machine (30x30 cm) and in large scale shear tests. The roughness of the sheared plane was determined after each one of the tests. Additional information includes the results of residual strength tests performed on clay fillings on a ring shear apparatus. The results of all the tests performed are compared in the paper showing the effect of field surface roughness. The paper also discusses the procedure followed to select an appropriate strength envelope for design purposes on the basis of all the available information. The case points out the difficulty to select unambiguously a suitable design strength for the critical sedimentation surfaces despite the efforts displayed in the field and in the laboratory.