考虑冰晶胶结特性的冰-冻土界面力学特性

Mechanical properties of ice-frozen soil interface under cementation of ice crystals

寒区多年冻土斜坡内部存在的下卧冰层对多年冻土斜坡稳定有着重要影响。为探究冰-冻土界面力学特性对斜坡稳定的影响,该研究以冰-冻土界面为研究对象,在低温环境(−3℃)下开展了不同冻土初始孔隙比和含水率下的冰-冻土界面及对应的冰-土颗粒界面直接剪切试验,建立了考虑冰晶胶结特性的冰-冻土界面非线性弹性损伤模型,结合试验结果对模型进行了验证,并分析了界面在剪切过程中切线刚度的变化规律。结果表明:冰-土颗粒界面的剪切应力表现为应变硬化特性,而冰-冻土界面的剪应力在峰值强度之后迅速下降,表现为应变软化特性。冰-冻土界面结构系数峰值点对应的剪切位移随着含水率的增大而增大,当冻土初始孔隙比为1.0、0.8和0.6,冻土初始含水率从14%增大至18%时剪切位移分别增大了32.7%、41.3%和52.1%。在−3℃下,冻土的初始含水率越大,则界面处的胶结冰越多,黏聚强度对界面的抗剪强度贡献也就越大。当结构系数达峰值点后,界面开始产生损伤,并随着剪切位移的增加进入加速损伤阶段。剪切过程中界面的切线刚度呈先增大后减小的趋势,在结构系数峰值点处为0,之后切线刚度随着剪切位移的增加逐渐减小。此外,界面的峰值切线刚度随着法向应力的增大而增大,当冻土初始孔隙比1.0、初始含水率为18%,法向应力从50 kPa增大至200 kPa时,则界面的峰值切线刚度增加了63.7%。在冰-冻土界面力学模型中考虑冰晶胶结特性更加合理,研究结果可为寒区斜坡稳定性分析提供参考。

Water migration often occurs in the permafrost slopes during seasonal variations.The water can be accumulated in the vicinity of the freezing front,and then congealed into ice,due to the influence of gravitational and temperature fields.The underlying ice layer was gradually formed with the increase of ice thickness and area.The interface between the ice layer and the overlying frozen soil can play a crucial role in the stability of the permafrost slope.The mechanical behaviors of ice-frozen soil interface can provide the theoretical support for the stability assessment of permafrost slopes over the long term.This research aims to investigate the mechanical properties of the ice-frozen soil interface.A series of direct shear tests were conducted under the temperature of−3℃.The shear stresses of the ice-frozen soil interface and the ice-soil particles interface were investigated under different normal stresses,initial water contents,and initial void ratios of the frozen soil.The experimental results showed that the shear stress of the ice-soil particles interface exhibited the hardening behavior,and then increased with the decreasing void ratio of the frozen soil.Softening behavior was found in the shear stress of the ice-frozen soil interface,particularly under lower normal stresses.The shear stress also increased with the increasing initial water content of the frozen soil.The structural coefficient,damage variable,and structural strength of the ice-frozen soil interface were defined,according to the mechanical properties and the failure evolution of the ice-soil particles interface and ice-frozen soil interface.There was an increase in the shear displacement corresponding to the structural coefficient peak point of the icefrozen soil interface with the increase of initial water content.Furthermore,the initial water content of frozen soil increased from 14%to 18%,while the shear displacement increased by 32.7%,41.3%,and 52.1%,respectively,when the initial void ratios of frozen soil was 1.0,0.8,and 0.6.A nonlinear elastic damage model was established to consider the bonding characteristics of ice crystals at the ice-frozen soil interface using the Duncan-Chang model,where the tangent stiffness was obtained.The calculation results demonstrate that the shear stress curve suddenly decreased after reaching the peak strength under low normal stress,when the ice content at the interface was high,indicating the noticeable brittleness interface.The mechanical properties of the ice–frozen soil interface shifted gradually from brittleness to plasticity,as the normal stress increased.There was the significant decrease in the magnitude of the shear stress after the peak strength.Therefore,the inhibitory effect of the increasing normal stress on the brittleness failure of the ice-frozen soil interface.The calculation was matched better with the experimental before the peak shear stress.By contrast,the decrease rate of the calculated shear stress curve was significantly lower than the experimental after the peak strength.The tangent stiffness of the interface increased with the increase of normal stress under the same initial void ratio and water content of the frozen soil.The normal stress increased from 50 to 200 kPa,while the tangential stiffness peak point of the interface increased by 63.7%,when the initial void ratio and water content of frozen soil were 1.0,and 18%,respectively.The increasing normal stress was induced a shear movement of soil particles at the ice-frozen soil interface,thus promoting the pore healing and strengthening the interface.Conversely,once the normal stresses were the same,the tangent stiffness of the interface increased with the increase of initial water content of the frozen soil,as the shear displacement was smaller than u0.While the shear displacement was larger than u0,the tangent stiffness gradually decreased with the increase of shear displacement and approaches zero.