饱和黏性土不排水分析中总应力强度指标的选用

Discussion on the choice of strength parameters for the analysis of saturated clay under undrained conditions

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cu,并给出从固结不排水强度指标计算不固结不排水强度cu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。

With regard to the total stress method for undrained analysis of saturated clay,the Terzaghi’s principle of effective stress is interpreted firstly in depth,with emphasis on the distinction between steady pore pressure and excess pore pressure and the distinction between equilibrium total stress containing total pore pressure and constitutive total stress containing only excess pore pressure. When applying the total stress method to undrained analysis of saturated clay,the “total stress”must be the constitutive stress. Furthermore,it is clearly pointed out that the consolidated undrained strength parameters usually should not be used directly in the strength calculation of undrained soil,and the error induced thereby is related with the slope of the total stress path. It is proposed that the unconsolidated undrained strength cu shall be employed,and calculation formulas for determining the strength parameter cu from the consolidated undrained strength parameters are provided. Three typical types of geotechnical problems, namely ground bearing capacity,Rankine’s earth pressure and excavation stability,are calculated by using the two types of strength parameters,and the differences in the results are demonstrated and discussed.