This report is one of a series of case studies of tunnel projects made from the point of view of the site investigation. The tunnel was constructed under the river Tyne through coal measures mudstone with seams of sandstone, shale and coal. Information was already available from geological studies and experience in nearby tunnels. Faults, rock fracture caused by mining subsidence, and ancient scouring of the river bed gave the possibility of connection with the overlying water-bearing sands and gravels, and the tunnel depth initially chosen was therefore increased. The site investigation established the levels and character of the deposits present, the distribution of faults and data for shaft and tunnel design. Boreholes were supplemented by over-water seismic surveys. A tunnelling shield and compressed air were specified, because of the possible danger of connection with the river above. Machine excavation was satisfactory in the mudstone but drilling and blasting was required for a full face of sandstone. Probing ahead from the face detected water-bearing and clay-filled fissures which were successfully negotiated using compressed air. The general implications of the site investigation experience are reviewed. (Author/publisher)
Abstract