When the soft soil freezes accompanying with the water suction, the effective stress increases with the lowering of pore water pressure that takes place near the freezing front, and thereby the unfrozen soil adjacent to it is subjected to a fairly strong dewatering consolidation. In this paper pore water pressure distribution in front of unidirectional freezing front penetrating at a constant rate is analyzed theoretically, and it is found that the lowering of pore water pressure is larger than expected. Therefore the lowering of the pressure may explain a soft soil subsidence after thawing and sludge dewatering by freezing. Influence region of dewatering consolidation, δ_s, is given by C_υ/U, and transitional time attaining to a steady state, τ_t, is given by A·C_υ/U². Here C_υ is coefficient of consolidation, U is frost penetration rate and A is a positive constant.
In the case of soil which is relatively low in the degree of consolidation, the freezing expansion and dewatering consolidation take place simultaneously, and at far from δ_s in unfrozen soil the result of their mutually subtracted effect is observed as the amount of frost heave. Then, frost heave ratio, ξ, is given approximately by ξ₀+n_fΓ. Here ξ₀is a characteristic constant of the soil, n_f is free water content of soil at freezing temperature andΓ is nondimensional volumetric expansion ratio of water at freezing ≅ 0. 09.