Built on swamp land, Chicago is a pretty unlikely spot for skyscrapers. While Manhattan has sturdy bedrock just below ground — or even poking out in places like Central Park — Chicago instead requires workers to dig down at least 80 feet through clay and hardpan soil to find rock. Since digging that deep wasn’t feasible in the late 19th century, the Windy City devised the caisson foundation system to allow the construction of taller buildings.
Essentially large subterranean concrete piers drilled down to hardpan, most Chicago caissons are belled out at the bottom to provide a wider area on which to sit. This prevents Chicago’s buildings from sinking into the soft clay. Though the majority of Chicago buildings incorporate this type of foundation, what if a construction site is too confined or perhaps too sensitive to vibration to use traditional caissons or piling? Enter the micro pile — a topic Curbed Chicago construction photographer Harry Carmichael chose to highlight for Micro Week.
As the name suggests, micro piles are much smaller in diameter compared to their caisson counterparts. While caisson drilling rigs typically take up a large area, pile installation equipment is small by comparison — roughly the same size of a pick up truck. This allows crews to work within a much more restricted job site, even inside pre-existing structures.
Unlike caissons, piles hold up the building with the friction created along their sides as opposed to "standing" on a more solid surface. To make up for their tiny size, a greater number of these piles are typically used. This in turn adds more lateral stability to the building being supported.
Downtown's yet-to-open 150 N. Riverside was erected using large diameter caissons, but crews turned to micro piles to support the skyscraper’s adjacent plaza and garage over active rail lines. The relatively tiny size of the equipment allowed workers to squeeze between tracks with a minimal impact on commuter train operations.
While traditional piles, like flat "sheet" piling, are violently — and very loudly — pounded or jiggled into the earth with enough force to rattle nearby properties, micro piles utilize lower vibration drills. Because of their low impact, micro piles are the obvious choice when building next to structurally fragile or noise sensitive sites. Even the pumps that fill the pile shafts with concrete are comparatively quiet.
Though micro piles may lack the absolute strength of say the 12-foot diameter caissons being drilled to support the supertall Vista Tower, they certainly have a number of applications. They are an important tool for building designers and engineers when it comes to keeping our beautiful skyline from sinking down into Chicago’s jelly-like ground.