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

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