2018 Volume 67 Issue 1 Pages 28-33
Frost heave occurs in clayey soil and the coefficient of permeability increases after thawing. It has been believed that trace of ice lens in the freeze-thawed soil formed by frost heave is a major factor to the phenomena. The purpose of this study is to identify mechanism of the permeability that is increased in the freeze-thawed saturated fine-grained soil. In our previous studies, a 1-D freeze-thaw permeability test was conducted with a ramp-type freezing, which allowed the freezing front to proceed at a constant speed. The test result revealed that the permeability was affected by overburden pressure rather than cooling rate and temperature gradient. However, it was not clear whether the permeability change was caused by a disturbance of soil structure by the ice lens (IL) or other factors. In order to clarify influences of the trace of IL, a step-type freezing was carried out to form a thick IL which occurs in soil with the freezing front fixed. In addition, the permeabilities of freeze-thawed soils of IL and IL orthogonal directions were compared by a consolidation permeability test, and no difference was recognized for both directions. However, in this laboratory finding, it was pointed out that we could not ignore the influence when a test specimen was taken out of the 1-D acrylic cylinder. Therefore, a tri-axial freeze-thaw test was conducted in which a test specimen was wrapped with a soft membrane, and the test specimens of the consolidation permeability test were easily taken. The result of the freeze-thaw experiments has led us to the conclusion that there is no difference in permeability of freeze-thawed soils between ramp-type and step-type freezing. In the consolidation permeability test, the permeabilities in the IL and IL orthogonal directions were almost equal. However, the permeability on the unfrozen side in the IL orthogonal direction which was obtained from a step-type freezing was significantly greater than other freeze-thawed soils. It has been suggested that the shrinkage cracks formed by IL formation remained as traces even after thawing and might become water passageways in the case of extremely small overburden pressure.
