OasisLMS

Abstract:

The history of Geotechnical work in Chicago is as storied as the political background that gives the town it’s nickname. In recent times though, the windy city deep foundation market has been a relatively stable mix of driven piles and belled caissons. When Auger Cast-In-Place (ACIP) pile technology started making its way around the states, various factors kept the budding technique from gaining a foothold in the market saturated will belling tools and hammers. This is despite the subsurface conditions being excellently suited for the single stroke staple.

Since that failure to launch over twenty years ago, some significant developments have occurred. Namely, top drive drill rigs have gained significant capabilities, data acquisition systems have become robust and commonplace, and developers are building further and further out from the Magnificent Mile. Malcolm is particularly well suited for the task because of it’s ever growing stable of Bauer crawler drills. The newest machines can reach depths that were only possible with crane mounted lead systems previously. Today’s standard automated monitoring systems tell a far more complete story of pile installations than hash marks on a mast and a foot on a grout line did in days past. As taller buildings get built further from downtown, the traditional bearing layers start presenting more risks than rewards for belled caissons.

160 N Elizabeth Street is a planned 27-story residential building that is located in the growing neighborhood of “West Loop.” At this location there is a 3 to 8 foot (0.9-2.4m) thick layer of water bearing silt that overlays the competent glacial hardpan bearing material typical to Chicago. The presence of this silt layer above the hardpan, in conjunction with water bearing sand below the hardpan, increases the risk that open holed caissons would experience excavation instability. The lacustrine deposits in the top half of the overburden profile do not contribute much to the support of the large structural building loads anticipated.

Malcolm approached the building contractor with an alternate design that replaced 70 each belled caissons with 132 each 30 inch (76.2cm) diameter ACIP piles. The piles would bypass the soft upper lacustrine material to transmit surface loading down to bear in the dense glacial gravel that overlies bedrock. The building contractor brought Malcolm onto the team to help facilitate the design process for this new approach. The final hurdle bringing the new technique to the market was to convince building officials that ACIP elements were suitable for the project. Performing extra O-cell tests confirmed geotechnical assumptions, but more importantly, confirmed Malcolm’s means and methods, and put the officials at ease.

Although 160 N Elizabeth was a relatively small project for Malcolm, it has the potential to be a huge project for the city of Chicago. The technique is well suited form foundations in areas with relatively thin hard pan and where water bearing lenses are present, and provides a huge value to developers looking to build the next piece of the city’s skyline. 

Keywords: Auger Cast-in-place, Value Engineering, Workpad Stability