An error in the equation for the support capacity of blocked steel sets resulted in an overestimate of the support capacity of sets with very small block spacings, used to estimate the capacity of steel sets backing onto shotcrete or embedded in shotcrete. In many current tunnelling operations, particularly in TBM bored tunnels, the steel sets are placed in direct contact with the rock or with shotcrete used to backfill overbreak. Consequently, the equation for estimating the support capacity of steel sets has been simplified to that for sets in direct contact with the rock as illustrated in Figure 9.
resulted in an overestimate