zone of very dense sand and gravel. Less than 0.3 m (1 ft) of highly weathered rock overlay the bedrock at a depth of about 14.5 m (48 ft). The thin weathered zone suggests that the residual Wissahickon soils normally present were eroded by the Schuylkill River and replaced with alluvial deposits. The top of rock surface is gently undulating with an average top of rock elevation of -2.3 m (-7.5 ft). The groundwater table was measured in the piezometers at elevation 1.3 m (4.3 ft). The Geotechnical Challenge Numerous investigations have studied the settlement of buildings and there is documentation of the effectiveness of shallow foundations with tolerable settlement for tall structures in Florida. In Philadelphia, however, it has been customary to support high-rise buildings on deep foundations, mostly caissons. For this project, a mat was selected as the optimal foundation system because the cost of a deep foundation alternative was significantly higher. More importantly, that would have taken nearly two months longer to construct. The middle 19.8 m (65 ft) portion of the mat is 4.5 m (15 ft) thick with the bottom at elevation 2 m (7.0 ft) sloping up to elevation 4.5 m (15.0 ft) on both sides, making the mat 2 m (6.6 ft) thick. Constructing the mat foundation required excavating 8.5 m (28 ft) below the historic building. The proposed underpinning system for this project comprised intermittent, 1 m (3.3 ft) thick concrete piers with multiple levels of tiebacks. The piers were placed below the historic building wall at 1.1 m (3.6 ft) on center. Grade beams supported the building wall between the piers and timber lagging was placed below it. The unique issue for this project stems from the interaction between the underpinning of the historic building and the heavily loaded mat foundation. Basically, when the high-rise is fully loaded, it will settle causing the surrounding soil mass to settle as well. The abutting underpinning of the 108 * DEEP FOUNDATIONS * JULY/AUG 2024 Plan view of the high-rise The unique issue for this project stems from the interaction between the underpinning of the historic building and the heavily loaded mat foundation. Geotechnical section A