Abstract:
By: Kelli McCurdy, Project Engineer and Serena Jang, Principal, John Gouchon, Principal
The 200 Park Avenue project has recently completed construction in the heart of downtown San Jose, California. The project comprises of a 19-story office tower with four levels of below-grade parking. The site is underlain by recent Holocene deposits that generally consist of medium stiff to hard clays that is interbedded with layers of loose to very dense sand and gravel. Depth to bedrock is estimated to be approximately 1, 000 feet below ground surface. Groundwater is at about 20 feet below grade; however, artesian pressures were encountered in a lower sand layer below the bottom of excavation. The potential for liquefaction, artesian pressures, and consolidation are all issues at the site. In addition, we performed Soil Structure Interaction (SSI) analysis to develop site-specific spectra.
Since it was critical not to lower the groundwater level outside the site during foundation and basement construction, a cutter soil mix (CSM) cutoff wall was utilized to retain the 54- to 58-foot-deep excavation and to cutoff groundwater. An extensive construction instrumentation and monitoring program was implemented to evaluate the performance of the tied-back cutoff wall during dewatering and excavation, including survey data points, six inclinometers, and four vibrating wire piezometers.
Predominantly stiff to very stiff clay and dense to very dense sands are present at the bottom of the excavation and are sufficiently strong to provide adequate capacity for building support; therefore, the tower is supported on a mat foundation. Additionally, hold down elements were constructed at the bottom of the excavation to resist the hydrostatic uplift pressures during a potential interim phase. The interim phase consisted of turning off the dewatering system and placing the construction of the proposed above-grade building on hold. The hold down elements consisted of augered, cast-in-place (ACIP) piles extending 75 feet below bottom of excavation.
This paper will present the subsurface conditions, geologic hazards, geotechnical issues, and the shoring wall and foundation solutions for support of the development. Results of the tied-back cutoff wall monitoring program will be discussed.