The Laurel - 1911 Walnut Street Case Study-PPT Presentation

PPT Presentation
Pdf Summary
The Laurel is a high-rise development at 1911 Walnut Street in Philadelphia’s Rittenhouse Square. Its L-shaped footprint crosses Moravian Street and includes two basement levels, requiring excavation beside historic buildings, restricted property easements, groundwater, and variable bedrock. Excavations reached approximately 40 feet, with strict movement limits to protect neighboring structures.<br /><br />The work was completed in two phases. Excavation support used drilled soldier beams and timber lagging, supported by tiebacks where easements were available and by internal raker braces where they were not. Shallow underpinning protected the Rittenhouse Annex, while deep underpinning protected the United Lodge of Theosophists. Along Moravian Street, lack of tieback permissions and a 0.5-inch deflection limit required heavy soldier beams and rakers, making the system costly and difficult to build around.<br /><br />Groundwater and deep bedrock ruled out underpinning the Theosophy building directly to rock. Instead, 17 concrete underpinning piers were supported by micropiles socketed into bedrock, designed to isolate the historic building from settlement of the tower’s thick foundation mat. During construction, the mat settled 0.5 inches while the tower reached 28 stories.<br /><br />A tower crane was added after the initial excavation-support design, requiring redesign to account for the crane foundation’s surcharge and loads. This highlighted the importance of coordinating crane locations, construction loads, easements, and the permanent building design early.<br /><br />The case study’s lessons emphasize avoiding wall-line excavation support when tiebacks or cantilevering are unavailable; recognizing that internal rakers add construction, removal, waterproofing, and repair costs; and coordinating excavation support with the full project team before design or construction. Rushed submissions and late design changes can lead to costly revisions, extra reviews, and increased risk.
Keywords
Laurel
Rittenhouse Square
excavation support
soldier beams
timber lagging
tiebacks
raker braces
underpinning
micropiles
bedrock