*3.4. Direct Shear Test*

3.4.1. Shear Strength Characteristics of Silt

Figure 15 shows the shear strength–displacement relation curves of pure silt with water contents of 2%, 4%, 8%, 16%, and 22%. The peak strength and residual strength of the silt specimens gradually increased with increasing vertical stress.

**Figure 15.** Shear strength of the silt samples with different water contents (*mbp*/*ms* = 0%): (**a**) *w* = 2%; (**b**) *w* = 4%; (**c**) *w* = 8%; (**d**) *w* = 16%; (**e**) *w* = 22%.

When the water content was 2% and 4%, the shear strength–displacement relation curves of pure silt showed a stress-softening phenomenon, and the peak strength was higher than the residual strength. When the water content was 16% and 22%, the shear strength–displacement relation curves of pure silt showed a stress-hardening phenomenon. When the water content was 8%, and the vertical stress was 50 kPa and 100 kPa, the shear strength–displacement relation curves of pure silt showed a stress-softening phenomenon, and when the vertical stress was 200 kPa, the shear strength–displacement relation curve displayed a stress-hardening phenomenon. The results show that with the increase in vertical pressure, the soil began to show a stress-hardening phenomenon [53,54].

Figure 15c,d show the stress softening into stress hardening of pure silt with water content increasing from 8% to 16%.
