Utilisation and its decay with time

The three numbered lines 1, 2 and 3 in Figure 34 have gradients ( m values in Equations 8 and 9 ) of –0.17, –0.19 and –0.21 respectively. A typical value of say m = –0.20 can be contrasted to m = –0.15 for ‘learning curve’ cases, where machine modifications and increased efficiencies are achieved. The lowest line in Figure 33 (‘extremely poor’) has a gradient m = 0.25. The pre-injection delays discussed above showed a mean gradient m = –0.45, which causes a dramatic loss of utilisation if greater efficiencies cannot be achieved. Figure 36 showed the direct effect of pre-grouting on utilisation very clearly. So far as it is understood, this was one of the very first TBM projects where systematic pre-injection was required to prevent settlement damage to buildings founded on nearby clay-filled depressions in the rock surface.