Mind the Gap: Micropile Underpinning of a Historic Structure

This article pertains to a 52-story residential high-rise building in the Rittenhouse Square section of Philadelphia, a densely developed area with mixed residential and commercial buildings. The tower is surrounded by Moravian Street along the north, Walnut Street to the south, a residential building to the east and a three-story historic building to the west. It measures 40 m (130 ft) long by 22.8 m (75 ft) wide with a finished floor elevation of 12 m (40 ft) and a full basement at elevation 7.5 m (24.5 ft). The historic building has a finished floor elevation of 13.7 m (45 ft) and its basement at elevation 10. 7 m (35.3 ft). It is supported on a 0.5 m (1.5 ft) thick rubble wall with a bottom at elevation 10.6 m (35.0 ft). The tower mat extends 36 ft (11 m) below grade or about 28 ft (8.S) below the existing historic build­ing, thereby requiring underpinning. When fully loaded, the settlement of the mat will cause the abut ting underpinning to also setUe, which would be detrimental to the historic building. Therefore, an innovative solution was necessary to isolate the underpinning from the stresses induced by the adjacent mat to safeguard the historic structure.