Performance of a Tunnel in Squeezing Rock and Rehabilitation Strategies

The phenomenon of swelling is one of the catalysts in the progressive deterioration of a tunnel section constructed between 1973 and 1975 in the Georgian Bay shale rock formation found in Mississauga, Canada. A 1.68 km 3 m diameter storm sewer tunnel was advanced using various techniques. Distress of the tunnel lining along the drill and blast section was observed as early as 1977. Convergence readings and inspection of the lining were carried out regularly between 1987 and 1991 at which time a 4 mm horizontal convergence of the tunnel was recorded. The tunnel section currently exhibits characteristic structural failure of the lining with tension cracks up to 20 mm in width at the spring lines. Rock cores extracted in 1977 and 2001 were tested for swelling potential. A general decrease in the swelling potential of the rock and a significant reduction in its strength and stiffness properties were observed over time. The results of the tests were used to estimate the current horizontal and vertical stresses on the tunnel lining in order to develop tunnel rehabilitation alternatives. This paper presents the results of the laboratory tests and monitoring data of the tunnel lining closure and discusses the strategies considered for rehabilitation of the tunnel.