Chicago Method tests

Project Location: Alabama, Georgia, Texas

Deep-dish pizza isn’t the only good idea to come out of Chicago that’s now to be found all over the USA.  The “Chicago Method” bi-directional deep foundation load test is finding application far away from the Windy City.  Wherever there is a hard rock formation overlain by relatively weak overburden, and the plan for deep foundations is to just scratch the top of rock, the Chicago Method comes into its own.

“Chicago Method” isn’t just some marketing gimmick; it is the name used for the technique in the FHWA GEC-10 (2018) manual.  In this specific type of test, the bi-directional hydraulic jack with a relatively small base plate is installed as close as possible to the base of an oversized shaft, to develop the necessary skin friction reaction.  The test result can be utilized simply as a plate load test, with the oversized shaft friction column serving as reaction.  Unit end bearing values in the hundreds of ksf have been proven with this technique.  Or, the full equivalent to load curve can be derived, if the unit end bearing is scaled properly.  Just don’t forget Boussinesq’s stress bulb from your undergraduate Soil Mechanics class – the displacement has to be scaled also!

In the past year, Russo Corporation along with LTC has been utilizing the Chicago Method across the Southeast, with successful tests in Alabama, Georgia and Texas verifying unit end bearing values to over 600 ksf.  The tests resulted in significant savings by shortening or completely eliminating the need for rock sockets in competent rock just below the overburden interface.

Installing Chicago Method assembly
Original (solid) and scaled (dashed) unit end bearing curves

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