Study of pore and shear strength of expansive soils based on nuclear magnetic resonance technology under freeze-thaw conditions
In order to investigate the shallow sliding mechanism of swelling soil channel slopes in the seasonal permafrost zone,nuclear magnetic resonance (NMR) tests,magnetic resonance imaging (MRI) tests,and in-door direct shear tests were conducted on soil samples with unidirectional freezing mode in comparison with mul-tidirectional freezing conditions. The connection between the changes of water content and porosity and mechani-cal properties of expansive soils under unidirectional open freezing conditions was investigated. The results show that more accurate water content distribution curves after water migration can be obtained by MRI tests. Under the multidirectional freezing condition,the freeze-thaw action reduces the number of large pores and in-creases the number of medium pores in the soil,and the pore size generally decreases. In contrast,under one-way open freezing conditions,the special growth mode of the ice lens body leads to an increase in the number of both large and medium pores in the soil,and they are concentrated in the location where the ice lens body is de-veloped. The shear strength of unidirectional frozen soil in situ freezing zone is approximately equal to that of multidirectional frozen soil,and the shear strength of the soil in the sub-consolidation freezing zone is lower than that of the in situ freezing zone,which has a greater influence on the internal friction angle. And combined with the investigation of the landslide in the diversion dry canal of the northern Heilongjiang Province,we get that the decrease of soil shear strength due to the development of micro-thin lamellar structure ice lens during spring thaw is the main reason for the slope instability of the swollen soil channel in the seasonal permafrost region.
expansive soilsfreeze-thawnuclear magnetic resonanceshear strength
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