Location: San Francisco, CA
Project Partner: Lendlease Construction
Challenge: Design build of a shoring system for a deep excavation at a narrow jobsite
Overview: Condon-Johnson’s package included the site demo, underpinning, dewatering, soil mix cut-off wall, excavation, internal bracing, and permanent tie downs. The site demo and excavation was subcontracted to AB Construction. The shoring and dewatering were design build, designed in house by Condon-Johnson. The tie downs were design assist where Condon-Johnson’s recommended design was included on the structural engineer’s building drawings.
Results: Condon-Johnson worked tenaciously to overcome the inherent difficulties associated with executing a 65 foot deep excavation with soldier piles up to 85 feet in length at a jobsite just 70 feet wide in the congested financial district of San Francisco. Drilling crews worked diligently to install a secant pile wall underpinning an adjacent structure, and utilized cutter soil mixing (CSM) to install a 42,500 ESF (55,600 GSF) impermeable shoring wall. A dewatering system utilizing eight deep well points was designed and installed to effectively dewater the excavation without depressing groundwater outside of the building footprint. The internally braced excavation required pin piles and two trestles to support the installation of three levels of bracing necessary to achieve an excavation depth of 65 feet. Support crane and off-haul excavator alternated trestles as the bracing was installed in six sections (three levels, each with two independent sections.) Upon reaching subgrade, eighty-nine permanent tie downs were installed to resist uplift of the structural mat slab. As the basement was constructed, Condon-Johnson crews worked closely with the General Contractor to remove the internal bracing and allow unimpeded construction of the permanent structure.