FREEZING AND HEAVING OF SATURATED AND UNSATURATED SOILS
Analysis of the behavior of air water, ice water and air ice interfaces shows that the apparent contact angle, y, between an air ice interface and the wall of a soil pore ought to vary rapidly with changes in pore water pressure Pw and ice pressure, P 1 This paper gives the expected relationship and shows that cooling of fairly dry soil, with freezing nucleated at one spot, should cause certain pores to fill abruptly with ice, thus depleting the water content of the surrounding soil. This conclusion agrees with available data. It is suggested that the ice pressure is slightly greater than atmospheric pressure when pores fill. Limited data available in the literature indicate that a given moist soil may or may not heave; when it does, the ice lenses apparently form some distance behind the freezing front. To explain this, the concepts of primary and secondary heaving are proposed for saturated soil. Primary heaving occurs when the base of the growing ice lens coincides with the limit of freezing, and the rate of heaving is limited by the rate of heat extraction. Secondary heaving is believed to occur when the freezing extends below the nominal base of the (visible) ice lens. Ice in the frozen pores can move, relative to the particles, as an integral part of the growing lens. A solution model of secondary heaving is used to illustrate an increase of ice pressure from the freezing front to the base of the ice lens, which will occur at a point where P1 is equal to the overburden pressure. It is contended that secondary heaving produces larger heaving pressures than primary heaving and explains why previous theories underestimated the maximum heaving pressure of saturated soil. It is inferred that whenever heaving occurs as the freezing front is descending through the soil, the process must be secondary heaving. It is suggested that the development of significant heaving pressures by unsaturated soil will ordinarily involve secondary heaving and that the pressure developed is less than that developed by the same soil when it is saturated.