Improved pressure tunnel lining methods, a case study of the Niagara Tunnel Facility Project

Abstract

The Niagara Tunnel Facility Project in Ontario, Canada, is one of the most outstanding hydropower projects recently completed. The pressure tunnel with a nominal internal diameter of 12'800 mm is constructed with a final unreinforced concrete lining of 600 mm. The long-term stability of the lining is ensured with a passive pre-stressed concrete lining, facing an operational pressure of maximum 13 bars at the low point of the tunnel. Upon installation of the final concrete lining, interface grout is injected at high pressure between the waterproofing layer and the initial shotcrete lining. Both the final lining and the rock mass are compressed by the grouting pressure applied. The deformation of the lining needed to withstand the operational water pressure depends on the strength, the deformability and the creeping parameters of the concrete lining and the surrounding ground as well as the shrinkage of the concrete lining and strain losses caused by temperature changes during watering-up. The pre-stressing process is a rather sensitive process in capturing the small strain rates of the lining in the various stages prior to watering up. A sophisticated monitoring method was established to fulfil the required measuring accuracy in the range of 2/10 of a millimetre.