Investigation of the pore water pressures of coarse-grained sandy soil during open-system step-freezing and thawing tests

•We investigated the pore water pressures of sandy soil during step-freezing.•Freeze-thaw history, water content and temperature affected pore water pressure.•Pore water pressures first decreased and then increased slightly during freezing.•Pore water pressures increased as the temperatures rose during soil thawing.•Pore water pressure mode before freeze-thaw differed from that after freeze-thaw.

An investigation of the pore water pressure in freezing soil has been essential for exploring frost heave mechanisms such as water migration and the initiation of ice lenses. In this article, the pore water pressures of a coarse-grained sandy soil were measured during open-system laboratory testing. A total of three soil samples were tested, and the degrees of saturation were 34.78%, 61.40% and 100.00%, respectively. Each experienced a first step-freezing stage, a thawing stage and a second step-freezing stage. The results showed that changes in the pore water pressure depended on the freeze–thaw history, degree of saturation and temperature. Under the same temperature conditions, the change pattern of the pore water pressure in the first step-freezing stage differed from that in the second step-freezing stage. The pore water pressures in samples with high water content first decreased and then increased slightly as the temperature had a sudden drop in the step-freezing stage; the pore water pressures in samples with low water content were less sensitive to the temperature change. During thawing, the pore water pressures increased as the soil sample temperatures rose. In terms of the variations in the pore water pressure, we further proposed that the phase change of sandy soil occurred in a temperature range and that there was water migration when sandy soil freezes. Furthermore, the permeability between every two pore water probes was calculated, using Darcy's law, with the pore water pressures and water volumes absorbed into the soil sample.