Experimental study of deicing chemical redistribution and moisture mass transfer in highway subsoils during the unidirectional freezing

Abstract

Deicing chemicals, when applied to pavement surfaces, occasionally deposited on the ground beyond the road pavement, thus lead to their accumulation in roadside soils and producing secondary salinisation of the ground water. In order to explore moisture and chemical mass transfer in highway subsoils during the winter period, 9 non-saline soil samples of 1 m length were subject to simultaneously freeze–thaw cycles with freezing rate 2 °C/day. To monitor the dependence of mass transfer from the soil density, each further column was packed with increased dry density than the previous one and supplied with 22 g/L of NaCl solution from the base.The results show that soil columns with loose density were consolidated throughout the sample length with the maximum in its lower part. Whilst in the dense soil samples, only the top 10 cm was loosened and attracted a large amount of water to form ice lenses. Chemical mass transfer to the freezing front was observed along the entire length of the samples regardless of density and void ratio. The concentration of de-icing chemicals in the pore water exceeded the supplied solution by 2–2.5 times at the capillary raise zone. In this study, chemical mass transfer is considered in terms of the driving forces and the system equilibrium. The driving force was explained as a cryosuction force, effected to the hygroscopic water in a liquid state with dissolved de-icing chemical ions in it, which migrated towards the freezing front. The continuous secondary salinisation, lasting in the highway subsoils for years, requires an accurate protection as the use of plastic membrane or silicone gel insulation.