Claire Lilienthal Elementary School, San Francisco, CA
The San Francisco Unified School District elected to perform a voluntary seismic retrofit at Claire Lilienthal Elementary School and engaged ENGEO to perform a geologic hazard assessment, prepare construction drawings and specification for strengthening the soil below existing and new footings and provide quality control and quality assurance (Qa/Qc) during construction. Ground improvement comprising compaction and permeation grouting were selected to limit differential settlement due to liquefaction under footings to 0.5 inches over a span of 30 feet; other project benefits included optimized construction costs and performance of the retrofit in a period of 9 months. The project schedule was rapidly accelerated as a result of the state mandated COVID-19 lock-down, transitioning from the field verification test section directly into production work. The ground improvement achieved the project performance criteria, reduced construction costs by 50% compared to micropiles and accelerated project completion to less than 5 months. The ground improvement work resulted in an increased level of safety for the students, teachers, and faculty while maintaining the historic and cultural significance of the building.
In December of 2017, ENGEO published the geologic hazard assessment report for the proposed seismic retrofit. The site is located in a mapped liquefaction hazard zone as shown in, Exhibit 5. In our analysis, we identified that the sandy soil at the site was susceptible to seismically induced liquefaction settlement. In addition, the site is underlain by compressible Young Bay Mud deposits that are susceptible to consolidation settlement under new building loads and/or new civil fill placed at the site.
ASCE 41-13 was used as the seismic design criteria for the retrofit design. For the Basic Safety Earthquake Level 1E (BSE-1E), we estimated up to 8 inches of total settlement due to liquefaction with up to 4 inches of differential settlement over a span of 30 feet could occur without mitigation. The options developed for the structural retrofit include supporting existing and new footings on micropiles embedded in the dense sand layer below the Young Bay Mud deposits, and supporting new and existing footings on soil improved with a combination of compaction and permeation grouting.
We performed a supplemental geotechnical study to develop soil densification performance criteria and prepare construction drawings and specifications for compaction and permeation grouting.
Throughout production, casing advancement for both compaction and permeation grouting encountered obstructions. Obstructions were typically caused by either debris in the loose sandy fill or undocumented work from the 1973 seismic retrofit. Adjustments to approved submittal injection geometry were required where injection casings achieved refusal. The field adjustments required coordination between the contractor and ENGEO inspectors to recommend geometries that were both constructional and provided adequate coverage of the grouting work performed.
With an exemplary effort from the entire project team, the ground improvement work was completed without delay. Production work totaled approximately 7,300 cubic feet of compaction grout and 120,000 gallons of permeation grout injected. ENGEO provided continuous inspection and construction consultation, helping to ensure the improvement followed the project plans and specifications for the various field conditions encountered. The results of the findings and recommendations from the field verification test section and production work were approved by CGS. Following the completion of the grouting, the structural improvements for the seismic retrofit
began.
