Freezing front evolution in chloride soils under unidirectional freezing conditions and the characterization via electrical conductivity

Abstract

Frozen wall thickness (FWT) is an essential index to determine the freezing process in artificial ground freezing (AGF) project. Empirical equations based on soil temperature are typically used to estimate FWT. However, the error resulting from estimation can be excessively large, partly due to a lack of consideration of soluble salt content effects. In this paper, open-system unidirectional freezing tests were conducted on silty clay with various chloride salt contents (0%, 0.5%, 1%, and 2%) to measure the freezing front propagation and the variation of bulk electrical conductivity (σb) simultaneously. The results indicate that the freezing front develops fast in high salt content specimens and stabilizes within a short time, because the thermal conductivity of silty clay with higher salt content is larger at the initial freezing temperature. The freezing front evolution was modeled considering the effect of salt content. The results predicted via the proposed model fit well with the measured ones, and the estimation of the freezing front locations at the steady state is at the safe side for specimens with various salt contents. Moreover, σb measured by 5TE sensors exhibits staged variations during freezing as the dominant ion conductivity pathway changes. The σb reaches a steady state once the freezing front arrives at the location of the 5TE sensor. Based on the experimental results, a novel method was proposed to improve the accuracy of estimating the frozen wall evolution via the in-situ σb measurement, and two critical locations with different depths are recommended to place the sensors.