The action of rockbolts and cables installed in the rock mass surrounding a tunnel can be complex. For example, fully grouted rockbolts act as reinforcement of the rock in much the same way as reinforcing steel acts in concrete. As a result they change the shape of the characteristic curve rather than provide internal support equivalent to that given by steel sets or shotcrete linings. On the other hand, ungrouted anchored rockbolts can be considered to resist the inward displacement of the rock mass and this is equivalent to the application of an internal support pressure in the tunnel.