The site investigation and construction records of the 2.2 km long Dinorwic diversion tunnel have been examined to see if they can provide lessons of good practice for tunnel site investigation. The tunnel was constructed entirely within hard Cambrian and Ordovician slates and grits and excavation was by the drill and blast method. The report describes the site investigation and the ground conditions encountered during tunnel driving in some detail. Because of the difficulty of vehicular access, borehole investigation was limited and most of the site investigation consisted of mapping the surface geology. Old copper mines were known to exist near the tunnel route and an underground survey was made of those workings that could be safely entered. Some geophysical surveying was also done in a region of low cover to the tunnel. During construction, ground conditions were, in general, much as expected from the site investigation, but extrapolation from surface outcrop to tunnel level was shown to be unreliable in detail. Probing ahead from the tunnel face was carried out when the heading passed by the region of the copper workings. It is suggested that inclined boreholes may have overcome the access problem and given more detailed information on the rock conditions at tunnel level. (Author/publisher)
Abstract