A three-story Double Tree hotel was built in the early 1970’s over a former landfill at Berkeley Marina as close as 20 feet from the bay. The brick and concrete structure with an approximate footprint of 61 feet by 260 feet was supported by a 12-inch thick reinforced concrete mat foundation. Over the years, differential settlement caused cracking of doors and window frames, cracking of floor tiles, separation between cabinets and walls and roof runoff to enter the building. Up to 21 inches of settlement was observed in some areas of the building. Subsurface explorations performed at the site confirmed the presence of landfill material with significant amounts of organic matter, trash and construction debris from approximately 5 to 33 feet below the ground surface. The landfill material was underlain by very soft to stiff bay mud. Helical piles were originally proposed to stabilize the foundation. Two full-scale load tests were performed on two helical test piles driven to 85 feet below grade. However, due to the relative expense of deep helical piles and the limited access for interior work, push piers were chosen as a more cost-effective and practical alternative for the project. The foundation repair included two hundred fifty-two (252) four-inch OD push piers installed at a four-foot spacing. Access holes for installing piers through the 12-inch thick mat foundation were made with a ten-inch core bit. The push piers were advanced on average to depths of 120 to 130 feet to bear on competent material. The piers were driven to hydraulic pressures of at least 9,400 psi to correlate to drive loads of at least 90.5 kips. This provided a factor of safety of at least 2.0 with the 45 kip design working load. Hydraulic cylinders were used to stabilize and then uniformly lift the building, with a rate of approximately one to two inches per day for a total of up to 18 inches, back toward its original elevation. The lifting process included simultaneous polyurethane injection below the mat foundation. Fast-acting polyurethane was used to assist with lifting the structure. Polyurethane with slow reaction and with better penetration properties was used to fill the remaining voids. The installation of the 252 push piers was completed in 50 working days. Steve Helfrich: Double Tree Hotel
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Friday, March 15th, 2013 by Dave Weekley
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