Characterizing unfrozen water content of saline silty clay during freezing and thawing based on superposition of freezing point reduction

Soil freezing point and unfrozen water content are essential parameters for the design of an artificial ground freezing (AGF) system. Previous studies on AGF technology mainly focused on soils in non-saline areas and unfrozen water content characteristic of soils in saline areas with different initial water contents was rarely investigated. Through a series of freezing point and nuclear magnetic resonance (NMR) tests, this paper investigated the influences of salt type (i.e. NaCl and Na2SO4), salt content, and initial water content on the freezing point of saline soils during freezing, unfrozen water content and pore size distribution during freezing and thawing. NaCl saline soils showed a salt content-dependent freezing point, deviating from Na2SO4 saline soils. Hysteresis effects were evident in the phase transition point, unfrozen water content, and pore size distribution during the full freeze-thaw cycle. NaCl saline soils exhibited a gradual reduction in dominant pore size with decreasing temperature during freezing and thawing, while Na2SO4 saline soils was associated with wider range of pore size. The Clapeyron equation was adopted and modified to accurately predict the freezing point and unfrozen water content of saline soils with different salt and initial water contents under different temperatures with help of newly introduced correction coefficients (i.e. a, b, k3, k4, and kt). Furthermore, the accuracy of the existing model in predicting cumulative pore size distribution was improved with help of a newly correction coefficient (i.e. β) through an regression analysis.