||Laboratory study on mechanical properties of frozen clay through state concept
Towards development of a mechanical model that can be part of multi-physical analysis of frozen soils, a program of systematic frozen-unfrozen parallel triaxial tests at different temperatures and strain rates was conducted. The mechanical behavior of the reconstituted high-plasticity clay samples was investigated and interpreted through state concept based on Ladanyi and Morel’s (1990) postulate on the unique relationship between the inter-particle “effective” stress and the strain path.To begin with, constant-condition (i.e., constant strain rate and temperature) compression tests were conducted on frozen Kasaoka clay specimens with normal consolidation prior to freezing. With other conditions set identical, the shear strength linearly increased with a decrease in temperature for the range from -10oC to -2oC, and log-linearly increased with an increase in the strain rate for the range from 0.001%/min to 0.1%/min. Direct comparison of the strain-rate effects between frozen and unfrozen specimens with identical strain paths and states in the soil skeleton clearly indicates that the viscoplasticity derives from that of pore ice. The Critical State lines (CSLs) for clay specimens frozen undrained were mapped by referring to the shear behavior of unfrozen specimens sharing the same strain history. Moreover, reduction of confining pressure after stabilized freezing of frozen specimens turned out to have no significant effect on critical state strength of frozen clay.By varying-temperature and varying-strain rate compression tests, consistency of stress-strain curves was observed at large strains between varying- and constant-temperature conditions, and less clearly, between varying- and constant-strain rate conditions. The latter observation, if further confirmed, may lead to isotach formulation of strain rate effects for the investigated range of strain rate (i.e. up to 0.1%/min), where the behavior is largely ductile.Finally, a nondestructive testing method mainly focusing on measurements of stiffness is developed. Different temperatures and axial strain rates were applied in order each time after the specimen was loaded and unloaded in quasi-elastic strain ranges. In the investigated range of conditions, the shear stiffness, averaged from the multiple probes of the same condition, increased linearly with the decrease in temperature, and increased linearly with the logarithmic increase of strain rate.The above observations of behavioral features of frozen and unfrozen soils, with further experimental work, are expected to lead to construction of a unified framework for describing the behavior under both states and transition between them.
Hokkaido University（北海道大学）. 博士(工学)