根据参数反演结果分析非饱和黄土边坡破坏机理

ANALYSIS ON FAILURE MECHANISM OF UNSATURATED LOESS SLOPE WITH INVERSE RESULTS OF SOIL STRENGTH PARAMETERS

边坡的变形破坏本质上有两方面的因素控制,(1)开挖或堆载引起斜坡内应力的重分布;(2)地表水渗入或地下水变动引起其抗剪强度的变化。大部分黄土路堑边坡位于地下水位以上,斜坡开挖以后,应力不再改变。如果不是在人工灌溉区,其破坏大多可归结为降雨入渗引起的黄土抗剪强度的降低。对于非饱和黄土来讲,其孔隙水压力为负值,传统的Terzaghi有效应力原理不适用于Mohr-Coulomb抗剪强度公式,而Bishop的非饱和土有效应力公式中参数测定周期很长,技术还不是很成熟,目前在实际工程中应用还不现实。如何基于目前常规的测试方法,合理地确定边坡抗剪强度参数,是值得研究的问题。本文对局部发生滑坡的山西吉家塬黄土路堑51.6m高边坡,采用Bishop法反演边坡c、φ值;取边坡上的黄土样,测了不同含水率试样的直接快剪强度参数。将试验结果与反演结果对比发现,含水量在接近土样塑限时的直接快剪试验结果与反演结果较为接近。塑限是土的一个状态界限指标,即固态到塑态含水量的界限值。土由固态进入塑态,土强度显著降低,此时最容易发生变形破坏,这符合土体稠度状态和强度变化的本质。因此建议在黄土边坡设计中,一般可取塑限含水量下的直接快剪强度指标作为边坡稳定性评价的指标。

The are two factors controlling the deformation and failure of slope.One factor is the stress redistribution caused by excavation or loading;the other is variation of shear strength caused by surface water infiltration or groundwater level.Most of the cut loess slopes are above the groundwater level.After excavation,the stress condition does not change much.If no irrigation exsits,the failure of slope is attributed to reduction of shear strength caused by rainfall.The traditional principle of effective stress is not suitable for the strength formula of Mohr-Coulomb because of the negative pore water pressure in unsaturated soils.The Bishop's effective stress principle is also useless for its hard-measuring parameters and immature technology.It is worth discussing to determine the shear strength parameters based on the traditional test.The Jijiayuan loess landslide beside the G206 highway as a cut slope is applied to determine the strength parameters c and φusing the Bishop inverse analysis. Quick direct shear test is used to get the parameters under different moisture contents.Comparing the test result with the inverse result,it is found that they are almost the same when the moisture content is close to the plastic limit. Given that the plastic limit expresses the states of the soil,and when the moisture content exceeds the value to the plastic state,the strength of the soil decreases obviously,leading to failure,which conforms to the consistency and strength principle of the soil.So it is suggested that the shear strength parameters measured by quick direct shear under plastic limit can be applied to the slope stability assessment.