饱和黏性土不排水分析中总应力强度指标的选用

For the total stress analysis of saturated cohesive soil under undrained conditions, first, the Terzaghi effective stress principle is further elaborated in depth. It is pointed out that attention should be paid to distinguishing between hydrostatic pressure and excess hydrostatic pressure, and distinguishing between the equilibrium total stress containing total water pressure and the constitutive total stress containing only excess hydrostatic pressure. When using the total stress method for undrained analysis of saturated soil, this "total stress" can only be the constitutive total stress. Furthermore, it is pointed out that the consolidated undrained strength index is generally not suitable for direct use in undrained strength calculations, and the error situation brought about by direct calculation is related to the slope of the actual total stress path. It is recommended to use the unconsolidated undrained strength $c_u$ measured by the undrained shear test of undisturbed saturated soil samples after pre-consolidation under the actual effective self-weight stress at this time, and a formula for calculating the unconsolidated undrained strength $c_u$ from the consolidated undrained strength index is given. Finally, examples are given for three types of problems: bearing capacity of foundation, Rankine earth pressure and foundation pit stability analysis. The calculations are carried out using the unconsolidated undrained strength and directly using the consolidated undrained strength index respectively. Through comparison, the error situation of the latter is shown and the specific reasons are analyzed.

针对饱和黏性土在不排水条件下的总应力分析,首先进一步深入阐释泰沙基有效应力原理,指出应注意区分静水压和超静水压,区分含有总水压的平衡总应力和仅含有超静水压的本构总应力。采用总应力法进行饱和土不排水分析时,这"总应力"只能是本构总应力。进而指出固结不排水强度指标一般不宜直接用于不排水强度计算,直接计算所带来误差的情况与实际总应力路径的斜率有关。建议此时采用饱和土原状土样在实际有效自重应力下预固结后不排水剪切试验测得的不固结不排水强度c_u,并给出从固结不排水强度指标计算不固结不排水强度c_u的公式。最后就地基承载力、朗肯土压力和基坑稳定分析三类问题给出算例,分别采用不固结不排水强度和直接采用固结不排水强度指标进行计算,通过对比展示后者的误差情况并对其具体原因进行分析。