Microstructural Analysis on Shear Behavior of New-fill and Silt Interface in Check Dam.

Purpose: The interface between newly-filled soil and silted soil in a check dam is commonly acknowledged as the most vulnerable plane in the upstream heightening technique, posing a significant threat to dam stability. The objective was to investigate the microscopic shear failure mechanism in " soft–soft (or soil-soil) " interlayers under different working conditions, aiming to establish a theoretical foundation for dam hazard mitigation and reinforcement. Materials and methods: Direct shear tests were initially conducted on the interface between new-fill and silt soils, considering different water contents and compactness coefficients in the silt soil. Subsequently, scanning electron microscopy (SEM) technique and Image J software were employed to extract microscopic parameters from the sheared samples. By integrating macroscopic failure patterns with microscopic parameters, a qualitative analysis was established to investigate how water content and compaction degree influence the shear properties and deformations, ultimately revealing the failure mechanism of interface shearing. Results and discussion: A typical triple-stage (fluctuation, climbing, stable) hardening phenomenon occurred in the stress-displacement curve during direct shear test. The shear strength showed a positive correlation with compaction coefficient but an inverse relationship with water content. Cohesion of the contact surface increased initially and decreased as water content raised, whereas it changed slightly with the increase of compaction. The internal friction angle increased with the development of compactness degree but decreased with increasing water content. SEM images illustrated a progressive morphology transformation in the contact surface, from granular to laminar to an aggregated pattern. Microscopic parameters such as pore morphology properties, pore size distribution as well as pore spatial distributions were directly bonded with macro properties. Conclusions: The compaction of silt plays a crucial role in adhesive force and structure of particles at soil-soil interface, thereby leading to the occurrence of "optimal bonding" and the variation of shear strength. The shear failure of contact surface is characterized by progressive failure. Besides, the water content and compaction degree of soft soil has a significant influence on the progressive failure. [ABSTRACT FROM AUTHOR]