冻结粉土强度准则探讨

冻土的强度随围压的增加呈先增大后减小的趋势，通过修正子午面内q-p曲线的斜率M*来探讨冻土强度变化规律。基于摩尔圆和包络线定理，推导了冻土内摩擦角随平均正应力的变化规律。假定偏应力的大小采用Mohr-Coulomb和Von-Mises强度准则的组合形式表示，将内摩擦角引入到偏平面中获得了偏平面内的强度准则。在不同温度（-6、-10、-15 ℃）和0.3～15.0 MPa围压下对冻结粉土进行了一系列低温三轴压缩试验，根据试验结果确定了子午面和偏平面的相关强度参数。结果表明：（1）子午面内的强度准则能较好模拟冻土在低围压下的强化效应和高围压下的弱化效应；（2）内摩擦角随平均正应力增大呈先增大后减小的趋势；（3）随控制参数s的增加，在偏平面内偏应力由Mohr-Coulomb包络线逐渐向Von-Mises包络线转化，其形状大小与极限强度值呈一一对应关系。最后通过试验结果与已有的强度准则对比表明：修正的强度准则具有一定的适用性。

Investigation on strength criterion for frozen silt soils

The strength for frozen silt soil increases with the increase of confining pressure at relatively low pressures, but decreases with the increase of confining pressure at relatively high pressures, which is different from unfrozen geomaterials such as metal, concrete, rock, sand and clay. To investigate the strength criterion for frozen soils and simulate the breaking process under different confining pressures, the strength criterion is derived from the modified slope M* of q-p strength curves in the meridian plane, and the formulation is deduced, which is related to internal friction angle and average normal stress on the basis of the Mohr’s circle and envelope theory. The strength criterion in the deviatoric plane is obtained from the assumption that the deviatoric stress is a combination of Mohr-Coulomb and Von-Mises strength criterion. A series of cryogenic triaxial compressive tests are conducted at three different temperatures, i.e. -6, -10 and -15 ℃, and different confining pressures of 0.3, 1.4, 3.0, 6.0, 10.0 and 15.0 MPa to determine the related strength parameters. Testing results are shown as follows: 1) the modified strength criterion in the meridian plane can simulate the strengthening effect at a relatively low confining pressure and the weakening effect at a high confining pressure; 2) the variation tendency of internal friction angle increases first and then decreases with the increase of normal stress; and 3) with the increase of parameters, the envelope shape of deviatoric stress is transformed from Mohr-Coulomb to Von-Mises criterion in the deviatoric plane, and its value corresponds to yield stress. Finally, the comparison between literature and experimental data demonstrates that the proposed strength criterion has a satisfactory applicability for frozen silt soils.