Experimental study on the mechanical behavior of sand and silty clay under vertical unloading

Ultimate unloading ratio and critical unloading ratio are crucial parameters for determining the depth of disturbance below excavation base. The vertical unloading tests were performed on Fujian sand and Fuzhou silty clay, with characteristic unloading ratios determined based on normalized resilience value. Results indicate that the soil's unloading response changes in three phases. When the loading ratio is less than the critical unloading ratio, the soil's response can be ignored. Meanwhile, as the loading ratio exceeds the ultimate unloading ratio, the soil's normalized resilience value and lateral pressure coefficient increase rapidly, and the resilience modulus and shear modulus weaken significantly. Additionally, at this point, the horizontal residual stress ratio displays an inflection point and begins to decline. Furthermore, the characteristic unloading ratios of clay, silt and sand in unloading tests are summarized. The experimental methods and the judgment criteria of characteristic unloading ratios are analyzed in this paper, while the test results from different soil layers are compared.