Shear strength of a pyroclastic soil measured in different testing devices

The paper deals with the shear strength of a Vesuvian pyroclastic soil (southern Italy), sampled in a district, where flowslides frequently occur. Direct shear tests, drained and undrained triaxial tests, as well as simple shear tests were carried out. The use of different testing devices was motivated by an extensive literature, which outlines some differences among the measured shear strength due to different failure mechanisms. The interpretation of the experimental results was made through distinct procedures, specific for each type of device. As main insight, some differences were pointed out for the effective friction angle (ϕ) and for the stress ratio (M) at critical state, as computed in the deviatoric (q–p) plane for triaxial conditions. Particularly, the results of the simple shear tests, whether interpreted in the (t–s) plane, well agree the direct shear and triaxial tests, and the differences were related to the normalized displacement of the specimen at failure.