Tom Odell spends a lot of time thinking about how to keep his basement dry.

It’s no ordinary basement: It’s 14 metres deep, 100 metres long, 15 metres wide and will hold tens of millions of dollars’ worth of electrical equipment to power Toronto’s downtown core.

Right now, it doesn’t look like much more than a giant hole in the ground just south of the Rogers Centre, beside the Roundhouse.

But it’s a high-tech hole in the ground, worth $195 million when it’s all done.

Dubbed the Clare Copeland Transformer Station, after Toronto Hydro’s former chairman, it’s one of the biggest infrastructure projects underway in the city.

Excavators have chewed through 52,000 cubic metres of soil, and another 17,000 cubic metres of underlying shale to scoop out the hole.

Now comes the tricky part: How to keep the water out.

Odell, who is managing the project for Toronto Hydro, and Ron Hicks, who’s in charge of construction for Carillion Canada, say they had a couple of things going for them to start with.

In the first place, says Hicks, the thicket of condominiums surrounding the site, plus the CN Tower and Rogers Centre, all have deep foundations that act as dams to keep water from flowing easily underground.

And when excavation started, the teams discovered there wasn’t that much water down there anyway.

The foundation walls are built by drilling an overlapping row of holes into the ground and filling each with concrete, so it becomes a column, or caisson. Every fourth caisson is reinforced with a steel pile.

Of the 425 concrete columns, or caissons, that form the walls, only six hit water, says Hicks.

The walls are braced by supporting tiebacks — bundled steel cables — drilled through the concrete at a 45-degree angle, down into the shale bedrock.

Since the columns are tubular, like the drill holes, a shovel scrapes the rounded side flat on the inside to give a conventional straight wall.

The floor is a concrete slab topped with a waterproof membrane: To the inexpert eye, it looks like high-tech linoleum. The linoleum also lines the concrete walls for extra protection.

The concrete is mixed with a substance that expands and seals the walls if water does start to penetrate.

“The design here is basically building a boat, so water’s not going to get into the boat,” says Odell.

Topping the base slab will be a heavy concrete floor, 1.5 metres thick, to anchor the station and form the floor for the bottom level.

It’s designed to withstand earthquakes. A quake can shift even something as massive as the station, so to increase stability, the floor is anchored by 345 bolts driven deep into the shale below the concrete floor.

“This isn’t going to budge at all,” says Odell.

Although the walls should keep groundwater from seeping in, there’s also the potential for surface water to flood down from above.

Sunk into the floor are two sumps equipped with powerful pumps, so that if water pours in, it can quickly be pumped back out.

The lowest level of the station will contain only cable that can stand being submersed in water.

Water-tolerant transformers and switchgear occupy the next floor up.

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The third, and highest underground level, will house the ventilation equipment for the station.

The sensitive control equipment — which could be vulnerable to flooding — will be above ground in the old machine shop of the adjacent roundhouse building that now houses a Leon’s store.

The machine shop has been dismantled, brick by brick, during the excavation, and will be reconstructed once the underground work is completed.

But the station isn’t much use if it isn’t connected to the rest of the hydro grid.

That’s where a second crew is busy, just on the other side of the newly built basement wall.

The second crew, from Dibco Underground, has sunk its own shaft outside the wall, where early this month they’ll lower a tunnel-boring machine. It will drive a tunnel east beneath Bremner Blvd., then north on Simcoe, to hook up to a main line running under Front St. W.

The excavation for the tunnel boring machine is a ticklish job, since crews are digging out the ground on both sides of the newly poured caisson wall simultaneously; they have to be careful not to collapse it one way or the other.

At $195 million, it’s an expensive project. Some groups argued before the Ontario Energy Board that aggressive conservation measures and small-scale power plants in the downtown core could have done the job more cheaply; but the board sided with Toronto Hydro.

One of Toronto Hydro’s key arguments at the board was that it’s needed to ease the load on the aging Windsor transformer station, just north of the Rogers Centre on Wellington St. W.

The Windsor station’s equipment is aging, but there’s no ready back-up for it, which means it can’t be shut down and modernized.

It also means that a major failure at Windsor — which feeds much of the downtown core — could be more than an inconvenience.

Navigant Consulting, which analyzed the need for the new station for Toronto Hydro, said that a major equipment failure at Windsor couldn’t be fixed in a hurry.

“The time for removal, transport and reconnection of an extremely large and heavy transformer would be up to 90 days or longer,” says the report from Navigant.

That would leave parts of the city core short of power for weeks or months.

A new transformer station would also allow Toronto Hydro to shift power between the east and west ends of the city, Toronto Hydro officials told the board. That would increase reliability for everyone.

The new station is due to be in service by the end of 2014.