Research on critical test conditions and causes for the transformation of loess shear curves

The experimental investigation revealed that morphological transformations in the shear stress-displacement curve of undisturbed loess significantly influence mechanical parameter determinations, particularly manifesting as anomalous negative cohesion values. To elucidate the triggering mechanisms and critical thresholds of this transformation phenomenon, systematic direct shear tests were conducted on regionally diverse undisturbed loess specimens under controlled laboratory conditions. This study quantitatively examines the interdependent relationships between moisture content, vertical loading, and characteristic curve patterns through comprehensive parameter analysis. Key findings demonstrate: An inverse correlation exists between critical transformation moisture content and applied vertical stress; Based on the transformation boundaries, there is an optimal range for the vertical load. The fundamental mechanism is attributed to the competitive activation of cementation strength versus frictional resistance-cementation-dominated systems display strain-softening curves with measurable cohesion, while moisture/load-induced structural collapse shifts dominance to frictional components, producing strain-hardening patterns and apparent cohesion anomalies. These findings provide theoretical basis for standardized testing protocols and numerical model calibration in loess mechanics research.