Abstract:
A 52-story high rise building in Philadelphia is constructed adjacent to a 3-story historic structure, which is supported on a rubble wall foundation at a depth of 8 ft below grade. The high rise is supported on a mat foundation at a depth 30 ft below grade with an average service contact pressure of 14 ksf. Obviously, the existing foundation requires underpinning but extra care is necessary to safeguard the historic building. A detailed finite element analysis showed that the mat foundation would settle about 1.5 inches along the old building which would drag down the underpinning and cause intolerable differential settlement. This magnitude of settlement would impose a substantial risk of detrimental cracking to the historic structure. To minimize the settlement impact, micropiles are used to support the underpinning and transfer the existing building foundation load as well as the added downdrag from the new mat foundation into the underlying mica schist bedrock. Special details using an air gap on top and sand on the sides of the micropile caps are installed to prevent overstressing the piles by the mat foundation. The specifics of the micropile pile design and associated details are presented herein. The building has been topped off recently and the adjacent historic building has been function well without signs of distress.