Sometime next month we're going to be pouring a large concrete transfer slab at the second floor of One Delisle. Its function is to take the loads coming down from the entire tower above it and "transfer" them onto new structural elements, before being brought down to our mat foundation at the bottom of the parking garage.

Put differently -- and, as always, I should warn you that I'm not a structural engineer -- a transfer is used whenever you have a change in your structural grid and the loads don't have a straight path down to your foundations. Because whenever this happens, you're now introducing moment forces and those need to be dealt with structurally.

Transfer slabs are relatively common here in Toronto (which isn't the case in every market), but they are expensive and they consume a lot of depth. In the case of One Delisle, our level two transfer slab is 1.8 meters deep and it's going to contain about 1,200 m3 of concrete. (Some of you might also recall that One Delisle's mat foundation is over 4m deep.)

We were reviewing this with the team today and we think that we'll be able to pour about 100 m3 of concrete per hour. That means that this slab will take about 12 hours to pour! This requires a lot of coordination. Neighbors need to be notified, pumps need to be on standby in case of a breakdown, and so on.

Another major consideration is heat. When concrete cures it generates a lot of it. And with a thick slab like this one, I am told that we run the risk of the middle starting to overheat (especially with the hot weather that we've been having lately). The guideline limit is 60 degrees Celsius, so we'll be monitoring it for probably about 1-2 weeks following the pour.

I find these details fascinating. Maybe some of you do too. So once it's poured, I'll share a few photos.

This past week we poured the first bit of concrete in the giant rat slab foundation (or mat foundation) that sits, or will sit, at the bottom of One Delisle. At its deepest point, below the building's core, it will be over 4m tall. Meaning, the area occupied by the gentlemen in the above photo will be fully covered in concrete when it's complete. Note the height of the rebar in the middle of the photo. This picture doesn't even do it justice, though. You need to be on site and down in the bottom of the hole to really feel it. There's a lot of bar, and it's going to be very deep.

In our case, this raft slab foundation will, as it sounds, serve as the building's foundation. This is what the tower will rest on. However, raft slabs can also serve the function of withstanding hydrostatic pressures from below (groundwater). That is the case with the raft slab foundation at Junction House given that we have a watertight "bathtub" design for the underground. However, that's not the case here at One Delisle, as the groundwater levels aren't as high and this will not be a "bathtubbed" underground. So the job of this giant slab is as a mat foundation.

Disclaimer: I am not a structural engineer or a hydrogeological engineer. What do I know?

Many of you are probably aware of the 58-storey Millennium Tower in San Francisco which is estimated to have sunk about 17 inches and to have tilted about 14 inches to the west since it was built.

Well today it was announced that they may have a fix. Here is what is apparently being proposed as a retrofit (image from SFGate):

The tower was originally built on top of a 10 foot thick raft or mat foundation, which was then supported by concrete piles that went down 60-90 feet into soft clay. Notably, the piles didn’t reach bedrock.

The proposed solution involves drilling 275-300 new micropiles into the bedrock below. But here’s where things get really interesting: The plan is to stabilize the west side of the building first and allow the east side of the building to continue sinking. In theory, this will give the building an opportunity to level out before they fully stabilize it. 

According to SFGate, the entire retrofit is expected to take anywhere from 2 to 5 years, and cost somewhere in the range of $200 to $500 million. The original tower cost $350 million to build. (I’m assuming that’s just the hard cost number.)