Ongoing strong demand for clean, renewable electricity in Ontario, Canada resulted in the creation of the Niagara Tunnel Project. Due to be commissioned in 2013, the new 10.4 kilometre long, 14-metre diameter Niagara Tunnel was built with the help of the world's largest hard rock tunnel boring machine, "Big Becky". The tunnel was advanced from downstream to upstream. When Big Becky broke through in 2010 into the tunnel's 45-metre deep intake shaft in the middle of the river just upstream from Niagara Falls, conditions were suitably dry for its safe dismantling and extraction thanks to the presence of a grout curtain designed and constructed in 2007 by Geo-Foundations.
The intake shaft excavation is 16 metres x 26 metres in plan x 45 metres deep. Above the bedrock river bottom, the river is held back from occupying the shaft excavation by a multiple-cell cofferdam built of sheet piling and rock fill. Below the cofferdam, in the bedrock itself, the grout curtain constructed by Geo-Foundations works to seal the rock fissures - formed by cracks, open horizontal bedding planes and plunging joint networks - that would certainly, without treatment by grouting, let enough water pass into the shaft excavation to cause flooding detrimental enough to halt all further shaft construction. The grout curtain endured its sternest test in the interim between completion of the shaft excavation and the tunnel boring's eventual breakthrough - the grout curtain resisted 45 metres of hydrostatic head across its relatively thin width.
Grouting of the fissured bedrock included full depth, fourth-order split spaced holes and employed simultaneous grout injection at multiple holes. A sophisticated suite of drilling and grouting equipment was used, including water-hammer drilling, real-time, response-driven additive dosing to modify grout formulations during grout injection and an automated bulk grout batching plant capable of delivering more than 20 cubic metres of cement grout per hour.
Grout curtain construction required more than 13,000 lineal metres of drilling and consumed more than 541,000 kg (dry weight) of cement. Several verification holes were drilled and two holes were core sampled and tested for residual hydraulic conductivity as part of grout curtain quality assurance.
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