Pore fluid salinity effects on sedimentation and geotechnical properties of fine-grained soils

Abstract

The objectives of this study were to evaluate the effects of soluble salt concentration (i.e., salinity) on geotechnical characteristics and sedimentation behavior of fine-grained soils (e.g., mine tailings) and identify test methods applicable for characterizing high-saline soils. Three fine-grained soils were used in this study: soda ash mine tailings, kaolin clay, and bentonite clay. The soda ash mine tailings (sodium carbonate) contained high-saline pore fluid and predominantly sodium on the exchange complex, whereas commercially-available kaolin and bentonite clay were used for comparison with the soda ash tailings. Salinity was controlled in the natural clays via adding salts with different valence (NaCl, CaCl₂, and FeCl₃) at concentrations ranging between 1 and 1000 mM. Salinity in the soda ash tailings was altered via extracting salts from solution using dialysis to create materials with different soluble salt concentrations. Sedimentation experiments were conducted in 63.5-mm-diameter by 457-mm-tall glass cylinders to evaluate the sedimentation rate and final solids content. The effects of pore fluid salinity on geotechnical characteristics of soda ash mine tailings and laboratory-prepared, sedimented soils were evaluated via measuring Atterberg limits, specific gravity, and particle-size distribution via hydrometer tests. Overall, an increase in ionic strength of the sedimentation fluid (i.e., increase in salt concentration) yielded higher sedimentation rates and larger volumes of released water for experiments on bentonite. In contrast, the sedimentation rate of kaolin was constant for salt concentrations between 1 and 100 mM, and the sedimentation rate decreased at higher salt concentrations. This behavior was attributed to an increase in fluid density and viscosity at high salt concentrations that reduced sedimentation. Soda ash sedimentation behavior was similar to kaolin and characterized by a decrease in sedimentation rate with increase in salt concentration. Geotechnical characterization of all materials indicated that liquid limit, plastic limit, and clay content decreased with increasing pore fluid salinity. Temporal evaluations of soil plasticity suggest that hydration times of at least two days are required to solubilize salts and capture salinity effects on soil plasticity. Additionally, experimental methods were developed and evaluated for correcting errors in hydrometer and specific gravity tests that may originate in the presence of soluble salts.