INNOVATIVE MICROPILE UNDERPINNING OF A HISTORIC STRUCTURE-Paper

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A 52-story residential tower in Philadelphia was built beside a three-story historic building supported on shallow rubble foundations. The tower’s heavily loaded mat foundation was expected to settle about 38 mm (1.5 in.). Modeling showed that this settlement could drag down the historic building’s adjacent underpinning while the opposite wall settled less, producing damaging differential movement.<br /><br />Geotechnical investigations included 17 borings, rock coring, groundwater monitoring, pumping tests, and pressuremeter tests. Finite element analyses of construction stages estimated approximately 25 mm (1 in.) of differential settlement across the historic building—an angular distortion of about 1/240.<br /><br />To protect the historic structure, engineers supported its underpinning piers on rock-socketed micropiles. The micropiles transferred loads to the underlying mica schist bedrock and limited underpinning-system settlement to about 6.5 mm (0.25 in.). Because the piles were installed beneath the tower mat, pile caps connected them to the underpinning. A 76 mm (3 in.) gap above each cap, along with separation details and a geomembrane bond breaker, allowed the mat to settle without loading or damaging the micropiles.<br /><br />The mat foundation was a more economical option than deep foundations and shortened the construction schedule by approximately two months. The tower was completed, and nearly two years later the historic building remained in service without signs of distress.

Keywords

Philadelphia tower

historic building protection

differential settlement

mat foundation

micropiles

rock-socketed micropiles

geotechnical investigation

finite element analysis

underpinning piers

mica schist bedrock