Temperature Effects on the Hydraulic Properties of Unsaturated Sand and Their Influences on Water-Vapor Heat Transport

Thermally driven moisture and vapor transport in unsaturated soil under a thermal loading applied by underground structures affects the magnitude and distribution of matric suction. There is a lack of data about the temperature dependency on the hydraulic conductivity function (HCF). This study (1) developed and used a temperature-controllable soil column for directly and simultaneously measuring the temperature dependency on both the soil-water retention curve (SWRC) and HCF using the instantaneous profile method, and (2) conducted water-vapor heat transport analyses to illustrate the effects of temperature dependency of SWRC and HCF on matric suction. The measurements showed that for sand there was a suction threshold of 2 kPa, below which the hydraulic conductivity of sand heated to 50°C became higher than that at 20°C due to decreased water viscosity. Above this threshold, the hydraulic conductivity at 50°C became lower, by as much as an order of magnitude at 10 kPa suction, because of temperature-induced reduction of water-retention ability. A parametric study revealed that using SWRC or/and HCF obtained at 20°C overestimated the magnitude of suction for soil heated to a temperature higher than 20°C. The overestimations were greater when heating took place in drier soil. The effects of vapor transport were negligible during the early stage of heating, and became prominent only after 7-8 days of continuous constant heating of the soil, regardless of the amount of initial soil suction considered.