Precast Concrete Tunnel Linings for Toronto Subway

Precast concrete tunnel linings have been used in the Toronto subway system in a wide range of soil conditions. This has resulted in a significant cost saving compared with the traditional cast iron lining. The linings are 16-ft. ID and are used without secondary lining. They are interchangeable with cast iron linings which are also used on the project. Eight segments plus a key are bolted with ¾-in. bolts to make a complete ring. Segments were produced at the rate of 200 per day in a precast concrete plant using steel forms. Specified concrete strength was 6,500 psi and over 7,000 psi was achieved. Erection was with the aid of shields of typical design. Some spalling and cracking occurred during shield shoving, but this type of damage was mostly superficial; replacement of damage segments was rare. Grouting outside the lining was carried out immediately after shield shoving using neat cement grout. The cost saving compared with cast iron lining is over $300 per ring.