上荆江河段河岸土体组成分析及岸坡稳定性计算

Soil composition analysis and slope stability calculation for riverbanks in the upper Jingjiang reach

崩岸是上荆江河段河床演变的一个重要方面。崩岸过程不仅与近岸水流动力作用有关,而且还与河岸土体组成及力学特性密切相关。本文对上荆江4个典型断面的崩岸土体进行了现场取样与室内土工试验,较全面地分析了崩岸土体组成及力学特性。试验结果表明:上荆江河岸为上部黏性土和下部非黏性土组成的二元结构,且在大部分河岸上部黏性土层厚度超过其下部非黏性土层厚度;黏性土层的抗剪强度随含水率变化明显,随含水率增加,黏聚力先增大后减小最后趋于稳定,而内摩擦角呈减小趋势。根据上荆江崩岸发生形式及过程,结合崩岸土体的土工试验结果,综合考虑水流冲刷作用和水位变化等因素,计算了两个典型河岸在枯水期、高水期和退水期的岸坡稳定性,得到了不同水位及不同坡脚横向冲刷距离下河岸稳定的安全系数。计算结果表明:当河岸坡脚冲刷达到一定距离时,高水期安全系数将明显低于枯水期,说明崩岸在汛期比在枯水期更容易发生;退水期安全系数明显低于相同条件下枯水期和高水期;涨水期河岸安全系数随水位升高呈先减小后增大趋势,说明水位变化对河岸稳定性有重要影响。

Bank failure is one of important fluvial processes in the upper Jingjiang. It is closely related to soil composition and mechanical properties of riverbanks. This study conducted field sampling and laboratory tests of soil samples for four typical riverbanks in this reach, and obtained the composition and mechanical properties of these samples through a comprehensive analysis of the test data. Results show that the soil composition in the reach is characterized by a double-layer texture structure, i.e. upper cohesive soil and lower non-cohesive soil and that for most riverbanksthe upper layer＇s thickness is greater than that of the lower. The experiment revealed a close relationship between water content and shear strength indicators.With the increase of water content, cohesion first increases and then decreases, finally approaches a constant,while internal friction angel decreases significantly. For the periods of low and high water levels and the recession stage,we studied riverbank stability considering the type and process of riverbank failure and using the experimental results of mechanical properties of typical composite riverbank. The stability safety factors for different periods were calculated, and the effects of near-bank flow erosion, water level change and other factors on the bank stability were examined. Calculations give higher stability of riverbank at low water level and lower stability at high level, but the safety factor is much lower at the recession stage. The trend of decrease first and then increase in the safety factor during water level rising, indicates an important effect of water level variation on riverbank stability.