We asked our geotechnical expert Richard Rodgers:
What is the most challenging project you are currently involved in?
We are working on several challenging and interesting geotechnical projects. Near the top of the list has to be Phase III of the Trinity Place development under construction at 8th and Market Streets in San Francisco. The site encompasses a footprint of about 75,000 square feet and is underlain by variable soil conditions and groundwater is at a depth of about 20 feet. Cary Ronan (Senior Project Manager) and I have been involved in the development since 2007. Phase I and II buildings have been completed and are fully occupied; both buildings are in excess of 20 stories and pile supported. Phase III is being constructed adjacent to these existing buildings, will have six levels of basement parking, and will require an excavation of 65 feet. The primary challenges are to protect and support the two existing structures and adjacent streets and provide resistance to hydrostatic uplift due to groundwater.
The permanent structure will be supported on a reinforced concrete mat with approximately 700 tie-downs to resist hydrostatic uplift. Design considerations for the permanent structure also include the eventual construction of Phase IV immediately north of Phase III and extending to Market Street. Current plans are for Phase IV to also have six basement levels.
A soil-cement mixed-in-place wall with lateral support provided by a combination of tie-backs and internal braces has been selected as shoring for the excavation. Internal bracing will be used where tie-backs are impractical because of the pile foundations of the existing Phase I and II buildings. The internal bracing affects the design and sequencing of wall and floor slab construction of the basement, which along with planning for Phase IV, required careful coordination with the structural engineer (Magnusson Klemencic Associates), general contractor (Swinerton Builders), and shoring contractor (Malcolm Drilling).
Because of the variability of the soil profile around the site perimeter, and the surcharges imposed by the two adjacent buildings, several lateral earth pressure conditions had to be considered in design of the shoring.
Langan Treadwell Rollo performed the exploration necessary and developed the pressure diagrams in conjunction with the shoring designer Brierley Associates.
Construction of the shoring began in late December and the excavation is planned to be completed by June 2014.