T. H. Douglas writes: Graham West has provided a useful summary of data obtained from the case studies he has used to illustrate the effectiveness of site investigations for tunnels. However, due no doubt to the limited space allocated within the text of a single paper, there are a number of further comments which should be noted in relation to the comparison between predicted and actual geology of the Diversion Tunnel at Dinorwic.
Construction of the Diversion Tunnel started in November 1975 and was completed in July 1978. Taking into account the final cost and comparing the site investigation costs, updated to the same base date, indicates a figure of 1.4 for the site investigation as a percentage of the capital cost of these work. The figure includes for the special investigations associated with the old copper workings and the cost of additional precautions while tunnelling advanced through the critical zone.
Reference is included in the text to a comparison between predicted and actual lithology—this is shown on fig. 6 and listed in table 3. While the different lithologies were of some interest and the variation in hardness had a slight effect on rates of drilling, the principal influence on rate of advance was the amount of lining required. While a comparison between the amount included in the Bill of Quantities (651 lin. m) to that installed (455 lin. m) may be of interest a further comparison is shown in the Table 1.
It should be noted that the basis
- © The Geological Society 1984