Chicago typically doesn't have well accessible bedrock. I know of one civil engineer who unexpectedly ran into granite and was able to sell it for added profit on a project. Point being, CE's don't typically expect great bedrock yet have developed methods to build skyscrapers in a city that was formerly a swamp.
"He says even though new technology makes it easier to find solid bedrock beneath 100 feet of wet clay, it doesn’t always make sense to drill that deep. Modern engineers still use the same general principle Burnham & Root employed when they floated the foundations of the Monadnock Building on an even flimsier layer of soil known as desiccated crust: They just spread the load."[1]
Chicago typically doesn't have well accessible bedrock. I know of one civil engineer who unexpectedly ran into granite and was able to sell it for added profit on a project. Point being, CE's don't typically expect great bedrock yet have developed methods to build skyscrapers in a city that was formerly a swamp.
"He says even though new technology makes it easier to find solid bedrock beneath 100 feet of wet clay, it doesn’t always make sense to drill that deep. Modern engineers still use the same general principle Burnham & Root employed when they floated the foundations of the Monadnock Building on an even flimsier layer of soil known as desiccated crust: They just spread the load."[1]
[1] https://www.wbez.org/stories/building-skyscrapers-on-chicago...