The site investigation and construction records for the 1.5 km long Cardiff cable tunnel have been examined to see whether they can give lessons of good practice for tunnel site investigation. The tunnels were constructed wholly within the Keuper marl; excavation was mainly by full-face tunnelling machine and lining was with pre-cast concrete bolted segments. The report describes the site investigation and construction of the works in some detail with special emphasis on ground conditions. On this site preliminary information proved most useful. The site investigation gave a generally satisfactory account of the site geology. However, points to emerge from the study were that if mechanised tunnel excavation is a possibility and the ground is predominately soft, then bands of harder material at tunnel depth take on extra significance and must be carefully recorded and tested. Rock strength test results in the site investigation report should be carefully considered when making a choice of method. If the operating conditions call for a watertight tunnel close attention should be given to permeability results in the site investigation report which show the possibility of high ground water flows. (Author/publisher)
Abstract