Influence of dry-wet alternation condition on mechanical properties of riverbank soil for typical sections in Jingjiang Reach of Yangtze River

The dynamic change of water level in a hydrological year can keep riverbank soil in the state of alternating dry and wet. The variety of water content of riverbank soil can change the mechanical properties of soil to a certain extent, which can affect riverbank stability. In order to investigate quantitatively the influence of dry-wet alternation condition on mechanical properties of riverbank clay soil, a field observation and sampling were first conducted at eight typical riverbanks in upper and lower Jingjiang Reach, which the clay soil of the typical riverbanks in Jingjiang Reach of the Yangtze River was taken as the study area. Through a comprehensive analysis of measured data, indoor soil tests and BSTEM simulation, the composition and mechanical properties of these samples were obtained, which indicated that the vertical soil composition of riverbank was characterized by a typical composite structure of non-cohesive lower bank and cohesive upper bank. The indoor soil test results revealed the quantitative change of soil mechanics with dry-wet alternation condition, and a close relationship between water content and shear strength indicators was obtained. With an increase of water content, the cohesion first increased and then decreased, with the peak values of 21 kPa and 34 kPa for the critical water content of 16.0% and 22.8% at Jing 61 and Beimenkou sections, and eventually reached a constant, while internal friction angel decreased significantly. Considering the dry-wet alternation condition of riverbank soil and the change of river water level in a hydrological year, the degrees of riverbank stability at Jing 61 and Beimenkou sections were analyzed during four different water level periods using BSTEM, and the process of bank failure of two sections were simulated in 2013. The results indicated that: the model-predicted results of the total bank retreat width were in close agreement with the measured data with the relative errors 1.69% and 3.74%, respectively. At the same time, the safety factors under different dry and wet conditions were calculated based on the BSTEM simulated results of two typical sections. The relationship between safety factor and water content was obtained, which indicated the safety factor first increased and then decreased with an increase of water content. It was consistent with the relationship between soil cohesive and water content which proved that the soil cohesive has an important influence on the stability of river bank. Quantitative relationships between safety factors and cohesive and internal friction angle of two typical sections were identified, respectively, and the correlation coefficients were 0.980 and 0.876 for Jing 61 section, and 0.992 and 0.986 for Beimenkou section, respectively. The relationships between them were linear function from which the safety factors increased with the increase of the cohesive and internal friction angle, respectively. Thus, a conclusion can be drawn that the safety factor was mainly affected by soil cohesive under different dry and wet conditions. For actual project, when water content of the riverbank soil was obtained, the cohesive and internal friction angle of soil can be calculated by proposed formulas, thereby the safety factor of riverbank can be calculated and the stability of river bank can be identified.