Influence of grouting pressure on the behavior of an unsaturated soil-cement interface

The strength of soil-nails depends on the behavior of the soil-cement grout interface at saturated and unsaturated conditions. Nowadays, pressure grouted soil-nails are considered to provide better interface strength than gravity grouted soil-nails. Soil-nail pullout tests have limitations to control some boundary conditions. To overcome these limitations, direct shear tests can be used to determine the actual soil-cement grout interface behavior. In the present study, a series of interface direct shear tests are performed between compacted completely decomposed granite (CDG) soil and cement grout at both saturated and unsaturated conditions under different grouting pressures. The interface shear stress increases with matric suction for different grouting pressures, and a strain softening behavior is observed for different suctions except at saturated conditions. A dilative behavior is obvious for interface as the suction value is increased from a saturated condition. However, the dilation values of soil-cement interface for different grouting pressures are less than that of CDG soil under different suctions. The interface shear strength increases with grouting pressure at saturated conditions, whereas, a downward trend is obvious as the suction value is increased from saturated conditions. The rate of increase of shear strength with matric suction is greater for CDG soil compared to soil-cement interface. Interface shear strengths for different grouting pressures are greater than CDG soil at a lower suction range but become less than CDG soil at a higher suction range.