冻融条件下基于核磁共振技术的膨胀土孔隙及抗剪强度研究

Study of pore and shear strength of expansive soils based on nuclear magnetic resonance technology under freeze-thaw conditions

为了探究季节冻土区膨胀土渠道边坡浅层滑动机理,与多向冻结条件对比,对单向冻结方式的土样开展核磁共振(NMR)试验、磁共振成像(MRI)试验、室内直剪试验。研究了单向开放补水冻结条件下膨胀土含水率和孔隙率变化与力学特性之间的联系。结果表明:通过磁共振成像试验可以得到水分迁移后更准确的含水率分布曲线。多向冻结条件下,冻融作用使土体内大孔隙数量减少,中孔数量增多,且孔径普遍减小。而单向开放补水冻结条件下,由于冰透镜体特殊的生长方式,导致土体中大、中孔隙数量均有所增加,且集中在冰透镜体发育位置。单向冻结土体原位冻结区的抗剪强度与多向冻结土体抗剪强度大致相等,分凝冻结区土体抗剪强度低于原位冻结区,对内摩擦角影响较大。并结合黑龙江北部引嫩干渠滑坡的调研情况,得到春融期微薄层状构造冰透镜体发育导致土体抗剪强度降低是季节冻土区膨胀土渠道边坡失稳的主要原因。

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.