Clay Creep and Displacements: Influence of Pore Fluid Composition

Time dependent shear displacements in clay soils under constant effective stresses can also be induced by changes in pore fluid composition. This paper presents the results of laboratory tests carried out on a sodium bentonite and on samples of the Varicoloured Clays formation outcropping east of Potenza, Southern Italian Apennines. The soils reconstituted with a 1M NaCl solution were submitted to shear tests under constant shear stresses in two different conditions: i) after shearing to the residual state, and ii) intact at various OCR. The applied shear stresses were lower than the residual strength of the materials reconstituted with the salt solution (residual friction angle, φ'r ≈ 15°) and higher than that obtained with distilled water (φ'r ≈ 5°). While exposed to 1M NaCl solution, the specimens didn't experiment creep; on the contrary, exposure to distilled water made the displacement rate increase greatly. The decrease in pore ion concentration obliterated the over-consolidation effects.