Drilling Down on 1000M Construction
2 July 2019
Behind our dazzling Michigan Avenue barricade, construction trucks move in to the future site of 1000M. How, exactly, does a parking lot become a skyline-changing, ultraluxury 74-story skyscraper? It all starts with drills. We spoke with Ryan Rusk, a Foundation Engineer at GEI Consultants, to get all the dirty details.
What’s Below the Surface?
Before building above the surface, we have to start below. This means drilling into the earth, and drilling deep—100 feet below ground level.
To get a picture: there are several layers of what’s lying below the surface. The first 6-8 feet below ground level are man-made fill, placed there to “raise the grade,” or in layman’s terms, even out the surface. Because 1000M is located so close to the lake, beneath that lies the historic shoreline. On our site, beach sand goes down to 15 feet below ground level. Even deeper is clay, going until 70-80 feet below ground level, where it hardens. That hardened clay is called hardpan, and is what most building’s foundations are built into. Going deeper, at 90 feet is a solid layer of rock called bedrock, and this is what 1000M’s foundation is being built into.
Traditionally, any building that’s under 50 stories can have a foundation that only goes down to hardpan. For more significant structures like 1000M, it’s necessary to have a foundational drillshafts that go 90 feet into the bedrock below to securely support such a showstopping skyscraper.
Since June 27th, passerby are able to stop on Michigan Avenue and see drill rigs at 1000M. The drills are coring 100 feet down into the bedrock to extract samples of the rock.
Because of site variance, the bedrock at 1000M is between 89-94 feet below ground level, and the drillshaft has to go below that. The extracted rock is lab tested for durability; the results of these tests will determine exactly how deep beneath the bedrock level the drillshafts must go. This informs a document showing the depth for each drillshaft, given to the construction contractor and the city of Chicago for thorough review.
For 1000M, there are 32 total drillshafts. Each will be drilled, sampled and filled with concrete. The diameter of the drillshafts will vary all the way from three to eight feet—which is massive by any standard.
A Strong Foundation
The drillshafts will eventually be filled with concrete to become caissons — concrete pillars that span from ground level to 100 feet down and serve as foundational points of support to build upon.
With caissons in place, a load-bearing test is the next step, proving the capacity of that particular foundational element.
The construction team at 1000M is working tirelessly — according to Rusk, foundation contractors notoriously try to push days as long as possible, sometimes even up to 24 hours. It can take several days for each drillshaft, but with multiple drill rigs on site and a construction team powering through any weather condition, we’re already getting excited for the next steps. For any updates, continue to check back on our Seek Beauty blog and follow us on social!