Influence of pore fluid composition on the residual strength of a clayey landslide

This paper reports the results of some laboratory tests aimed to evaluate the influence of pore solution composition on the residual shear strength of a clayey landslide (Costa della Gaveta —Southern Italy). The natural pore fluid of several samples extracted from the subsoil was analysed. Direct shear tests were carried out on specimens reconstituted with—and/or exposed to—salt solutions at various concentrations. A solution reproducing the undisturbed in situ one at about 26 m depth was also used. The results of shear tests, carried out at constant displacement rate, show that the residual shear strength greatly increases with the pore solution concentration. On the other hand, depending on the composition of both solid skeleton and pore fluid, exposure to distilled water of solution saturated specimens can cause a noticeable decrease of the residual shear strength. Exposure to distilled water of the solution saturated material during a shear test at constant shear stresses lower than the residual strength evaluated for the solution-clay system can cause a creep-like process. Such behaviour is reasonably due to strength decrease caused by ion diffusion outward from the pores. An outward ion diffusion is probably among the causes of the current ion concentration profiles in the landslide, with the values decreasing dramatically from the deeper soil to the ground surface.