Effect of stress change on the bond strength of fully grouted cables

The bond strength of fully grouted cable bolts is primarily frictional and depends on the pressure at the cable-grout interface. Thus, any change in this interface pressure must affect the bond strength. The interface pressure increases with (load induced) dilation and may increase or decrease with the stress change due to nearby mining activities (excavation induced). In this paper, the effect of stress change on the bond strength is studied in the context of the bond strength model (BSM) previously established in a companion paper. A laboratory testing program was executed to evaluate the stress change effect on the frictional resistance of the steel-grout interface of a smooth bar and the laboratory results confirm the theory. It is concluded that mining induced stress change is one of the most important parameters controlling the cable bond strength. Theoretical results indicate that cables should be most affected by stress change in relatively soft rocks. Stress reductions of −40 MPa, often measured in deep mines, can cause the cables to lose most of their bond strength in low to medium stiff rocks. This study provides a new perspective on cable design and provides a methodology to increase the stability of openings.