Impact of zero bond to rock on shotcrete lining design

Shotcrete linings usually achieve good bond to the underlying rock substrate when first applied during construction. This bond usually remains sound through many years of service. However, in some cases the bond may be inadequate or diminish with the passage of time causing the lining to detach from the substrate. To prevent detachment, and possible collapse in these circumstances, a shotcrete lining needs to be secured in place using a secondary means of attachment which, in the case of linings on hard rock, usually comprises rock bolts with external cover plates or spider plates on the ends of bolts. The potential causes of bond loss are many both prior to spraying and in service. Testing of bond strength frequently leads to apparent ‘zero bond’ results that may reveal an actual absence of bond or may be an outcome of the coring and testing process. If bond is relied upon in design it is important to consider the significance of zero bond results rather than simply dismiss them as artefacts of the testing process. This paper considers how partial or total bond loss can be considered in shotcrete lining design.