TBM Q-logging and tunnel scale effects

The attractiveness of the TBM method of excavation besides high excavation rates (when conditions are favourable) is that the reduced level of disturbance compared to drill-and-blast gives a reduced need of rock reinforcement (and an apparently higher ( Q -value) over a significant central range of rock qualities. When, however, the rock is of very poor quality, almost equal stability conditions (overbreak and other problems) will be experienced by the two methods of excavation and a common, lower Q -value will tend to be in operation. When the rock is of very good quality, no problems with rock support arise in either the TBM or drill-and-blast tunnel, and the ( Q -values can be considered the same in each case.