If you’ve ever had water coming into your basement, deliberately flooding a construction project probably sounds like a fairly terrible idea. I’d want an explanation, too. But the crew building a gas pipeline tunnel near Squamish, British Columbia, will eventually be expected to do exactly that.<\/p>\n
Two tunnel boring machines have been working toward each other from opposite ends of a route about 5.5 miles (8.9 kilometers) long and roughly 13 feet (4 meters) across. FortisBC’s tunnel contractor, the Frontier-Kemper Michels joint venture, launched one from the Woodfibre LNG site and the other from the old BC Rail yard in Squamish. The engineers’ published design<\/a> calls for gas pipes to go in once the digging stops, followed by some of the rock the machines pulled out. Crews will then seal the tunnel and fill it with fresh water. And nobody’s going back in after that.<\/p>\n You won’t get a maintenance corridor out of this. Once the system’s up and running, the engineers rule out re-entering the tunnel to work on the pipelines. It’s a temporary workspace, even though the pipes are supposed to keep doing their job for decades.<\/p>\n The route links the BC Rail site in Squamish to Woodfibre LNG, where the gas will feed an export facility. The Squamish Nation made an underground route a condition of approval, to keep construction off the surface of the estuary. So the crews had to get through soft ground beneath that habitat first, then through hard rock toward Woodfibre. Those are two different problems.<\/p>\n The machine that started at Woodfibre tackles granite. Its rotating head presses disc cutters against the rock until pieces break away.<\/p>\n The BC Rail machine got the trickier job, at least to my eye. It’s had to hold up loose, wet deposits under the estuary while digging them out, so it uses slurry, which is basically a mixture of water and clay. Pumped in under pressure, the slurry pushes back against the ground at the front of the machine and keeps soil and groundwater from rushing into the excavation. FortisBC finished that stretch, about 2.2 miles (3.5 kilometers) of tunnel running some 82 feet (25 meters) below the estuary waterbed, in April 2026. The granite drive has been the slow part ever since.<\/p>\n You can see a related headache in Toronto’s tunnel through a buried prehistoric valley<\/a>: a route that looks continuous on a map can put very different material in front of the cutting head. Running two machines here looks pretty sensible once you get past their similar round faces.<\/p>\n The two drives are supposed to meet in an enlarged chamber cut in sound rock. They aren’t meant to settle it by running their cutting heads into each other.<\/p>\n Pipe installation strikes me as a fairly unforgiving job. Crews will weld sections of 24-inch pipe together outside the BC Rail entrance and check the welds. Then a hydraulic pushing system feeds each assembled length into the tunnel, where it rides on rollers with guides that hold it on course around the bends. You’re pushing miles of welded steel through a hole in a mountain, which is basically a friction problem, and that’s what the rollers are for. The plan calls for finishing the first line before starting the second.<\/p>\n FortisBC says the twin lines give the system redundancy. A backup’s a useful thing to have when nobody’s going back in for routine maintenance. It doesn’t mean you can read the pair as a promise of double the gas capacity.<\/p>\n Crews will check and pressure-test the pipes before backfilling. They’ll also fit leak detection, corrosion protection and buoyancy control, then pack excavated rock around the pipes before they close the tunnel off and flood it.<\/p>\n I haven’t found an official announcement that the two drives have met. A FortisBC spokesperson told the Squamish Chief<\/a> in July that less than a kilometer of tunneling was left and that excavation should finish this fall, so they’re somewhere near the end of it.<\/p>\n We know about the flooding plan because FortisBC and its tunnel contractor wrote it up in a technical paper for the World Tunnel Congress, held in Montr\u00e9al in May 2026. The engineers point to rock along the route that can generate acidic drainage. Certain sulfur-bearing minerals react with oxygen and water and produce acid, which can carry dissolved metals into the water around them. Keeping that rock submerged makes it harder for oxygen to reach it, and the reactions slow down.<\/p>\n But doesn’t water help cause the problem?<\/p>\n Yes, and that’s why wet rock and submerged rock aren’t the same situation. A wet surface that’s exposed to air keeps getting oxygen. A water cover makes that supply much harder to maintain, which is the principle behind the water covers studied by Canada’s Mine Environment Neutral Drainage program<\/a>. The gas stays inside the pipes, the tunnel around them gets the water, and the design calls for fresh water rather than seawater from the estuary.<\/p>\n I’d be a bit careful about turning that into a claim of zero environmental impact. The designers are describing a way to reduce one specific risk from the rock, and their paper doesn’t give us measured results from a finished, flooded tunnel. Nobody’s got one to measure yet.<\/p>\nSo why two different machines?<\/h2>\n
The gas pipes have to go in first<\/h2>\n
So why would you flood it?<\/h2>\n