不平衡推力法与Sarma法的讨论

不平衡推力法是我国独创的边坡稳定性分析方法,在滑坡的分析治理中得到了广泛应用,然而对其精度的分析却廖廖无几。目前在与其他方法的对比分析中,发现在某些情况下其误差非常大,如果不加限制地使用该方法,可能会给工程带来巨大的隐患。针对该方法存在的问题,通过理论分析和算例比较,认为折线形滑面的计算精度与滑面控制点处滑面倾角的变化密切相关,通过控制该变化角度可以控制其精度,工程中建议将滑面控制点处的倾角变化小于10°作为该方法的使用条件,超过该限制应对滑面进行处理使它满足使用条件或采用其他的分析方法。针对Sarma法所给条间剪力方程存在的问题,给出了一个新的条间剪力方程。新的条间剪力方程不但符合边坡稳定性问题的合理性要求,且能正确表示任意条块的条间剪力。基于新的条间剪力方程对Sarma法进行了改进。改进后的Sarma法认为条块界面和滑动面上的剪切强度具有不同折减系数。算例分析表明,改进后的Sarma法优于Sarma法,应用它能够得到更合理的安全系数值,即使条块形状是变化的,也能够给出一致的安全系数值,而且能够保证迭代过程稳定收敛。

DISCUSSION ON IMBALANCE THRUST FORCE METHOD AND SARMA'S METHOD

Imbalance thrust force method(ITFM) is originally developed approach for slope stability analysis in China, which has been widely used in the field of landslide stability analysis. But few papers are interested in the accuracy of results obtained by ITFM. It has been proved that the error by ITFM is high enough not to be negligible, and the hidden danger will take place in practice if misusing it. By theoretical analysis and comparison of examples, it is considered that the accuracy of ITFM is influenced by the increments of slip segment obliqueness, and the accuracy can be improved by adjusting the increments of slip segment obliqueness. The increments of slip segment obliqueness must be less than or equal to 10 degrees when using ITFM, otherwise the other slope stability analysis methods should be selected or the slip line should be improved. A new inter-slice shear force equation is presented in this paper to solve the existing problems in the inter-slice shear force equation of Sarma's method. The presented equation meets the demands of slope stability analysis, and can properly express inter-slice shear forces for general slices. Sarma's method is improved, based on the new inter-slice shear force equation. The improved Sarma's method considers that the reduction factor for shear strength along inter-slice boundary is different from that for shear strength along slip surface. Corresponding examples indicate that the improved Sarma' s method works better than Sarma's method and a more reasonable value of safety factor can be obtained by the improved Sarma's method. The consistent values of safety factor can be given and the iteration is stably convergent even if the shape of slices is varied.