Variations of Na+ and K+ concentrations in the pore fluid of a clayey soil affected by landslides: effects on shear strength and creep behaviour

Natural or anthropic variations of pore fluid composition can influence the mechanical behaviour of clayey soils. Samples from the Costa della Gaveta slope (Southern Apennines, Italy), affected by complex, slow-moving landslide systems, were reconstituted with distilled water, NaCl, and KCl solutions. Several specimens of soil (mainly non expansive illite-muscovite and kaolinite) were tested in direct shear apparatuses by controlling either the rate of displacement or the driving shear stress (creep tests). During some of the tests, the specimens were exposed to a fluid different from their pore fluid. This triggered water/ion diffusion processes that altered the pore fluid composition. A progressive decrease of shear strength (or an increase of shear displacement rates in creep tests) was observed with Na+ concentration decreasing in the pore fluid. Con-versely, with K+ concentration increasing, a rapid increase of shear strength was observed, up to values higher than those obtained in Na+-rich solutions. The performance of KCl-filled wells to produce a shear strength in-crease and limit landslide displacements is currently under investigation in a portion of an active landslide body.