We would like to thank Dr Barton for his comments, which dealt principally with the material forming the Appendix to the paper. The basis for Dr Barton's hindsight ‘prediction’ for systematic support of 2.5 m long bolts at 1 m spacing plus 50 mm of reinforced shotcrete (with its similarity to the engineers' enquiry specification shown on pp. 291 and 292, and figs. 9 and 10) is interesting although inherently probably somewhat subjective.
It must be emphasised that the NGI system was not available to us at the time of the tunnel investigation and was only briefly appended to the case study in order to try to compare it, albeit rather superficially, with the Bieniawski (1973) system. We tried however to be as objective as possible in applying it on the basis of the site investigation results only.
The basic approach to design was that the rock, together with its primary support system, provided the main strength of the tunnel. Such primary system was a permanent part of the works. For a hydraulic tunnel such as this one a smooth circular concrete surface was required, Over the life of the structure the provision of this concrete lining, together with grouting measures, consolidates and completes the tunnel strength, durability and serviceability. From the results of the site investigation the recommended primary support for those lengths of the tunnel which were not highly weathered was based largely upon one of Bieniawski's
- © The Geological Society, London 1978