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

Abstract

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.