The US$66.6m Sheppard Subway Twin Tunnels Project in Toronto is nearing completion after 33 months of construction. The two Lovat EPB machines broke through on the final two bored tunnel sections in May and June, having each completed 4km of single pass, segmentally lined tunnels.

The excavated tunnel profile is 5.9m diameter and the i.d is 5.2m. The two EPB machines excavated the tunnels through varied glacial deposits of till, clay, fill, sand and silt interstrewn with boulders up to 3m diameter.

Eighty per cent of the alignment is below the water table, with pressure up to 1.5 bar at tunnel crown and low cover ranges from only 4m to 15m. The EPB technique provided extremely tight control of surface settlement. Rick Collins, resident engineer for the contract, told T&T International, "Ninety-nine per cent of settlement readings fell below 15mm and 99% below the alert levels of typically 30mm. The TBMs are mothballed and up for sale."

The 60MPa, 225mm thick PCC lining has been erected almost completely within the 10mm stepping tolerance and has resisted construction loads of up to 2300 tonnes without producing any cracks. Leaks into the tunnel are negligible, with no drips above springline because the segment gaskets are performing effectively.

  The contract included risk sharing, with the owner, Toronto Transit Commission (TTC), providing TBMs, linings, geotechnical interpretation, quality control, monitoring, insurance and survey checks. Partnering was practised throughout construction. Tunnel design was performed by Hatch Mott MacDonald, and construction was undertaken by a Canadian JV comprising McNally/PCL/Foundation, with construction Management by Delcan Hatch Mott MacDonald.