Consolidation, Swelling and Swelling Pressure Induced by Exposure of Clay Soils to Fluids Different from the Pore Fluid

Abstract

This paper reports experimental results relative to volume changes caused by exposure of clay soils to fluids different from the pore fluid. Furthermore it reports some results relative to the influence of pore fluid composition on the swelling pressure of an active smectitic clay. The experimentation on volume change behaviour was carried out on several clay soils with different grain size distribution and mineral composition. The results show that exposure of water saturated Ponza bentonite to NaCl, KCI and CaC1₂ solutions causes consolidation. At high void ratio, pores and fissures form. On re-exposure to water, Na⁺ effects are reversible and the material swells noticeably. On the contrary, Ca²⁺ and K⁺ make swelling potential decrease greatly because they substitute Na⁺ as counterions. Under low values of axial stress and high void ratio, potassium and calcium effects can be reverted by exposing the material to concentrated NaCl solutions. The effects of exposure to electrolytes of soils with a smectite content between 10% and 30% — such as Bisaccia, Gela and Marino clays — are lower than those on the Ponza bentonite, however they are noticeable. Furthermore, depending on axial stresses, type of solution and on types of exchangeable cations, swelling caused by subsequent exposure to distilled water can be lower or higher than previous consolidation. The commercial kaolin, the Potenza and the Villa d’Agri clays undergo small consolidation as an effect of exposure to the salt solution and further large consolidation because of subsequent exposure to water. The Milazzo clay undergoes swelling because of exposure to NaCl solution. The results relative to volume change can be interpreted qualitatively in terms of double layer processes, ion exchange and initial fabric. Results relative to swelling pressure show that Bolt’s model interprets satisfactory the behaviour of the Bisaccia clay reconstituted with a concentrated salt solution and exposed to distilled water. As expected, the model interprets only qualitatively the behaviour of the material exposed to concentrated salt solutions.