冻融过程中未冻水含量对非饱和粉土抗剪强度的影响

Influence of the unfrozen water content on the shear strength of unsaturated silt during freezing and thawing

对含水率为18.3%的非饱和粉土进行了冻融过程中不同温度下的直剪试验,用核磁共振测定了冻融过程中孔隙水的相变过程,并分析了未冻水、孔隙冰对其力学性质的影响机制。试验结果表明:(1)非饱和粉土冻结可分为过冷段(>-1.15℃)、快速冻结阶段(-1.15℃^-2℃)和稳定冻结阶段(<-2℃),快速冻结阶段76%的孔隙水冻结,而稳定冻结阶段未冻水含量只减少7%;(2)冻融过程中黏聚力随温度发生显著变化,内摩擦角变化幅度很小;(3)冻结过程中抗剪强度的变化主要发生在稳定冻结阶段,快速冻结阶段黏聚力仅增大38.5%,内摩擦角基本无变化,而稳定冻结阶段黏聚力增大123.5%,内摩擦角降低12%。得到以下结论:(1)快速冻结阶段黏聚力增大主要是由于孔隙水冻结导致基质吸力增大,毛细黏聚作用增强;稳定冻结阶段黏聚力增大主要是由于冰对土颗粒胶结强度增大;(2)含冰量变化不大时,冻土抗剪强度主要受冰对土颗粒胶结强度的控制,而此胶结强度决定于未冻水膜的厚度;(3)稳定冻结阶段内摩擦角降低主要由孔隙中水冰相变发生体积膨胀时对土颗粒骨架的作用力导致。

A series of direct shear tests on unsaturated silt with a water content of 18.3% during freezing and thawing were conducted,and the phase composition of pore water was measured by NMR during the shear process. Effects of the unfrozen water content and the ice content on mechanical properties of the tested silt were analyzed. The results indicate that the freezing process of unsaturated silt can be divided into super-cooling stage >-1.15 ℃,no phase change),rapid freezing stage(-1.15 ℃–-2 ℃) where 76% of the pore water freezes,and stable freezing stage(<-2 ℃) where the unfrozen water content only decreases by 7%. The cohesion changes significantly with the temperature during the process of freezing and thawing while the friction angle varies slightly. The change of the shear strength of silt occurs primarily within the stable freezing stage,where the cohesion increases by 123.5% and the friction angle decreases by 12%. For the rapid freezing stage,the cohesion only ascends by 38.5% and the friction angle has no obvious change. It can be concluded that,within the rapid freezing phase,the freezing of the pore water drives the matrix suction to increase,which results in increasing of the cohesion,while that,within the stable freezing phase,the increase of the cementation strength between ice and soil particles leads to a rise of the cohesion. When the ice content changes slightly,the shear strength of frozen silt is mainly controlled by the cementation strength between ice and soil particles which depends on the thickness of the unfrozen water film. The decrease of the friction angle within the stable freezing phase is mainly caused by the force acting on the particle skeleton generated by volumetric expansion of the pore water upon freezing.