Tunnelling with low ground cover under a water storage reservoir

An 1800 mm nominal diameter steel liner (casing) was installed by tunnelling under a 77 million litre capacity reservoir in British Columbia, Canada without causing any significant settlement of the reservoir’s floor and its roof columns despite a very low (600 mm) ground cover over the crown of the tunnel. The liner was required to carry a 1500 mm carrier pipe (water main) inside it. The horizontal distance between the roof column footings and the spring line of the tunnel was less than 1200 mm. This paper discusses the options considered for selecting the type of liner and describes its installation methodology including instrumentation, construction control and method of grouting the annulus between the liner and the tunnel bore. It concludes with some comments on the lessons learned.