The square shaft helical screw pile is a deep foundation utilized to support new and underpin existing structures. Its performance in the expansive clay areas of Denver, Colorado, U.S.A., is studied. Since 1986, approximately 110,000 square shaft helical screw piles have been installed in the Denver region, about 10,000 of those into the region's most expansive clays with no cases of adverse performance. The reasons are: 1) Properly sized piles are installed to a minimum of 5.4 kN-m (4,000 ft-lbs) of installation torque, or the refusal condition, into the underlying stable claystones and sandstones, 2) Detrimental moisture migration along the sides of the pile shaft does not occur, 3) Lead sections use only one or two helices, 4) Dead load is concentrated on each pile, 5) Small shaft surface area substantially reduces uplift forces and 6) The steel pile shaft surface exhibits low skin friction. Two case histories are presented.
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