DEEP JET GROUTING GRIDS FOR SUPPORT OF A 42-STORY TOWER DESIGNED USING PERFORMANCE-BASED SEISMIC DESIGN

摘要

500 Folsom Street is part of the new Transbay neighborhood in San Francisco, which is beingredeveloped after the 1989 Loma Prieta Earthquake damaged a freeway viaduct; removal of the viaductcreated parcels of land available for development. The development consists of a 42-story residentialtower with a six-level basement. The slender, core-only structure was designed using performance-basedseismic design.Subsurface conditions include undocumented fill, Dune sand, marsh deposits, sand, and clay, alloverlying sloping bedrock of the Franciscan Formation. The building is supported on a 10-foot-thick matfoundation approximately 62 to 72 feet below street grades; clay, sand, and bedrock are all present at matsubgrade. Under a portion of the mat, sand susceptible to liquefaction was present. The variable subgradeconditions and potential for ground failure, along with the depth of mat subgrade below street grade,created a challenge for the design and construction team to solve. The ground below mat subgrade neededto be improved to mitigate the liquefaction potential and provide more uniform support.The areal extent and depth of ground improvement necessary to support the mat with an acceptableamount of deformation was determined. Settlement was analyzed and subgrade moduli were provided fordesign of the mat, with multiple iterations performed to converge between the geotechnical and structuralmodels.Based on the project schedule and logistics, it was beneficial to work from street grade to improve theground. In order to target the depths below the mat and minimize wasted effort and materials within thedepth of excavation, grids of jet grouting were determined to be the most economical groundimprovement method; however, it was challenging to install the columns from 65 feet above the top of thetarget zone. During installation, QA/QC was performed to check that the jet-grouted columns wereinstalled to the design depth and diameter and developed the design strength.