An empirical approach for estimating shear strength of decomposed granites in Galicia, Spain

Decomposed granites are often found in Galicia, Spain, where they are referred to as jabres. Similar decomposed granites are also found in many other parts of the world. Granite weathering rarely produces an homogeneous altered rock mass with all material decomposed to the same degree – or even a simple weathering profile where the degree of alteration decreases progressively with depth. This paper presents an empirically based approach that allows estimation of the shear strength of this type of materials in terms of Mohr–Coulomb cohesion and internal friction angle values. The approach is based on back analysis of failed slopes in decomposed granites, and it is implemented through simple in-situ testing and classification of the weathered granite, treating it as a soil material. An advantage of the used approach is that it allows prediction of shear strength values at different locations within the deposit in a quick and economical way, from which average values of shear strength parameters can be computed. This is particularly important when considering the heterogeneous nature of decomposed granite formations in relation to the scale of the engineering work involved. Analysis of three slopes instability cases in jabres in Galicia, illustrating the approach are described in detail. These correspond to a road cut, a natural slope and the wall of an open pit mine.

Research highlights
► Case history of shear strength characterization of decomposed granites (DGs) in Spain.
► Based on field testing (vane tester and pocket penetrometer) and empirical relationships.
► The approach allows estimation of values of cohesion and friction angle of DGs.
► Three cases of strength characterization: a road slope, a natural slope and a mine bench.
► Back analysis of slope failures to determine an average value of shear strength.