Abstract:
Located in Philadelphia's Rittenhouse Square section, The Laurel is a 48-story, mixed use, high-rise residential project with 3 levels of below grade parking. To construct the reinforced concrete residential tower on a 15 feet thick concrete foundation mat, Peirce Engineering, Inc. designed excavation support and underpinning systems. This deep excavation fronts Walnut, Sansom, Moravian, and 20th Streets. The excavation support design was challenging due to the busy city location; a partial restriction on using tieback anchors; an L-shaped footprint crossing Moravian Street; construction surcharge loads; a 700 foot tower crane foundation; and a phased work plan. Phase 1 included the wall line excavation support system for the tower along Walnut and Moravian Streets. Tiedback, double, wide flange, soldier beams and timber lagging with multiple levels of tieback anchors, approximately 41 feet deep, supported the adjacent streets and sidewalks.The Phase 1 excavation required underpinning for 1905 and 1917 Walnut Street, The United Lodge of Theosophists, up to 29 feet deep. Underpinning design was challenging and unique due to the depth to bedrock and ground water and to the need for micropiles to support the underpinning and to prevent the underpinned Theosophists building from settling with the foundation mat's expected 1.5 inches of settlement. Tiedback underpinning with inter-pier beams and eccentric micropile connections required coordination with the project team for load requirements and a foundation mat design revision. Phase 2 work extended across and along Moravian Street to Sansom and 20th Streets to construct a 2-story retail building. The Phase 2 excavation support system included tiedback, double wide flange soldier beams and timber lagging with multiple levels of tieback anchors and raker braces. This presentation will discuss the design challenges and solutions for the deep excavation.When finished in 2021, The Laurel will be the tallest, residential building in Philadelphia.