Seismic Retrofit of a Concrete Immersed Tube Tunnel with Reinforced Shotcrete: Fid-Bau Portal

Seismic Retrofit of a Concrete Immersed Tube Tunnel with Reinforced Shotcrete

Morgan, D. R. / Kazakoff, K. / Ibrahim, H.

The George Massey Tunnel is an immersed concrete tube tunnel under the South Arm of the Fraser River near Vancouver, BC, Canada. It was constructed over 45 years ago to take while traffic on Hwy 99 the main highway connecting Vancouver and the 15 interstate highway to Seattle and beyond. This 629 m long immersed tunnel carries four lanes of traffic in two split running tunnels, each 8.2 m wide and 5.2 m high. There are fresh air ducts 2.3 m wide by 5.4 m high on each side of the sunken box tubes. Structural analysis found that the tunnel was seismically deficient by current seismic standards and was vulnerable to breaching in the event of an earthquake of sufficient magnitude. The British Columbia Ministry of Transportation dictated that the tunnel be retrofitted to with-stand a nearby non-subduction earthquake of a magnitude of 7.0 and a distant subduction earthquake of magnitude 8.2. This paper describes the reinforced shotcrete seismic retrofit of the air ducts component of the immersed tube tunnel. It was determined that the form and pour method of reinforced concrete retrofit, while suitable for use in seismic strengthening of the floor of the air ducts and roadways, was not suitable for use in the air duct soffits (because of a requirement for full bond between the prepared duct soffit and repair material). It was thus decided to carry out the retrofit using reinforced shotcrete, with the reinforced shotcrete being applied in three successive 100 mm thick layers. Details of preparation of the concrete air duct soffit for shotcrete application, the shotcrete mixture design, batching and supply, as well as the shotcrete application, finishing and curing processes used are described. The results of quality control tests for compressive strength, boiled absorption and volume of permeable voids, as well as bond tests and finished shotcrete tolerance measurements are also presented. At the time of writing this paper work on the east air duct was nearing completion and work had yet to start on the west air duct. This paper demonstrates that reinforced shotcrete is a viable means of seismically strengthening an underwater tunnel under challenging working conditions.