Discussion

MR D.R. CULVERWELL, Consulting Engineer

Reference is made in paragraph 4 of Paper 8 to the Owner Risk Assessment which was carried out by the employer to assist him in the comparison between the immersed tube and bored forms of tunnel. This comparison was important and posed some difficulty because of the disparate nature and magnitude of the respective risks inherent in each form. The tender price of the bored tunnel was expected to be lower (and was so) but the consequences of things going wrong during construction, mainly through adverse ground conditions, were likely to be more severe and, moreover, the opportunity to recover lost time would be small. By contrast, the tender price for the immersed tube was expected to be higher (and was so) and the likelihood of things going wrong was expected to be greater; however, the consequences of things going wrong were likely to be less severe and the chances of recovering lost time were greater.