Predicting Evaporative Fluxes in Saline Soil and Surface-deposited Thickened Mine Tailings.

摘要

Salinity lowers the rate of evaporation-driven densification and shear strength gain of surface-deposited thickened tailings (SDTT) via three mechanisms: albedo; salt precipitation, and; osmotic suction. Albedo is significant but not sufficient to explain the scale of reduction in desiccating SDTT. The relative contribution of the last two mechanisms is unclear. Better understanding of the relative contributions of these mechanisms is important for accurate predictions of evaporative densification of saline soil and SDTT.;By conducting a series of column drying studies, the 1D solute transport in salinized soil and acid-generating SDTT was characterized in detail and related it to evaporation. The relative contribution of salt precipitation and osmotic suction to observed salinity-induced reduction in evaporation was assessed. Based on experimental findings, a numerical framework for predicting evaporation in salinized soil and tailings that accounts for 1D solute transport was proposed and validated.;Salt accumulation was observed in the first 1-2cm of the soil and tailings, causing a decline in evaporation; the magnitude of the decline increases with initial pore-water salinity and evaporative demand. Osmotic suction alone was sufficient to explain the salinity-induced reduction in evaporation from soil and tailings columns up till the solubility limit. Further reduction in evaporation was attributed to salt precipitation restricting water flow. Notwithstanding the role of salt precipitates, numerical predictions of evaporation using a commercial unsaturated flow code generally agreed well with experimental values for a variety of simulated weather conditions and deposition scenarios, only considering the temporal increase in osmotic suction at the surface. Using this modelling approach, a number of hypothetical deposition scenarios for SDTT were explored, including using a solute transport code coupled to the unsaturated flow code to assess the effectiveness of sand as a capillary barrier against evaporation-driven surface salt accumulation.;A new prototype matric suction sensor that offers several advantages over some of the current devices for suction measurement was designed and tested. The prototype sensor correlates the volume change (strain) of its porous linearly-elastic material to a test material's negative pore-water pressure. The sensor compares favourably with tensiometer, axis-translation technique, heat dissipation sensor and relative humidity sensor.