Experimental study on the triaxial static shear characteristics of Yellow River silt under different initial states

The middle and lower reaches of the Yellow River are rich in silt. The Yellow River silt can be utilized as a subgrade filling material along the Yellow River expressway to enhance its resource utilization potential. However, research on the geotechnical mechanical properties of the Yellow River silt is limited. In this study, a series of triaxial shear tests were conducted using the global digital systems triaxial apparatus to examine the effects of initial conditions (confining pressure and relative density) and test conditions (shear rate and drainage conditions) on the static strength and deformation characteristics of the Yellow River silt. The stress–strain curve, volumetric strain curve, envelope of shear strength, stress ratio curve, and internal friction angle distribution under different characteristic states were obtained. The test results showed that the shear strength of the Yellow River silt was more sensitive to confining pressure, relative density, and drainage conditions. The stress–strain curves of the Yellow River silt samples under drained conditions showed a slight strain-softening phenomenon; therefore, there were three characteristic states: peak state, phase transformation state, and critical state. Moreover, the stress–strain curves of the Yellow River silt samples under undrained conditions showed strain hardening characteristics, and there existed three characteristic states: the peak state, critical state, and peak pore pressure state. Additionally, the Yellow River silt samples simultaneously reached the peak and critical states at the end of the shear procedure. Specifically, the strength at the peak and critical states increased with increasing confining pressure and relative density. The shear strength under the undrained conditions was greater than that under the drained conditions. The development of pore pressure under the undrained conditions was in contrast with the dilatancy characteristics under the drained conditions; however, the pore pressure developed more rapidly than that depicted by the dilatancy characteristics. The distribution interval of the friction angle at the characteristic states of the Yellow River silt was between 22.6° and 38.1°. The initial shear modulus and ultimate deviator stress of the Yellow River silt increased with increasing confining pressure and relative density but were not sensitive to the shear rate. The ultimate deviator stress under undrained conditions was greater than that under drained conditions, while the initial shear modulus under drained conditions was smaller than that under undrained conditions under medium–low confining pressure. To strengthen the shear resistance of Yellow River silt, more attention should be paid to improving the compaction degree when the Yellow River silt is used as the filling material of expressway subgrades. This study can provide data and theoretical references for the resource utilization of Yellow River silt in subgrade engineering.