A. Thomas, University of Southampton
As the authors rightly note, settlement is a major design issue in tunnelling and has long been recognised as such.1 Volume loss—the volume of the settlement trough normalised by the excavated volume of the tunnel—is a parameter which is often used to compare the performance of tunnels. Given the nature of the paper, there was limited quantitative information on settlement and it would be useful if the authors could provide more information.
One value of volume loss was quoted, namely 2·5% for tunnels on the Los Angeles Metro in mixed ground. This seems quite high, especially considering the extensive ground treatment of the area. Taking examples of good performance, for sprayed concrete lined tunnels in soft ground, typically the volume loss ranges from 0·50 to 1·20% while for open-faced shields one would expect between 0·75 to 1·50%.2
With the soft rock/soft ground conditions of Los Angeles, Copenhagen and Toronto in mind, it would be interesting to hear the authors' comments on the efficacy in controlling ground deformations by using a lining of ribs and lagging (with rock bolts), which seems to be the preferred approach in North America for short tunnels, as compared to a primary lining of reinforced sprayed concrete (with rock bolts), which is the more common approach in Europe.
