Assessment of contour profile quality in D&B tunnelling

Contour profile quality affects tunnel excavation costs, in terms of operational safety, support materials and construction time. In drill and blast tunnelling, under/over-excavation and rock mass damage arising from excavation phase can be evaluated by means of the elaboration of survey data and geophysical testing or coring, before and after the blast. As far as the quality of the profile is concerned, some indices can be used to define the contour and for the rock mass in the boundary as well. This paper proposes a methodology well applicable to rock tunnelling, and a case study based analysis to correlate the over-excavation and the rock mass conditions is discussed to validate the procedure. Profiles and geological parameters have been processed with automatic code specifically developed for the study. Over-excavation distance and Tunnel Contour Quality Index are evaluated and compared with Q-system values. The results have been discussed, compared with other literature cases and validated for engineering applications.