USE OF HIGH CAPACITY MICROPILES TO RESIST SEISMIC OVERTURNING OF COLUMNS-Paper

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Sharp Memorial Hospital’s 1972 Central Tower needed seismic upgrades to meet California requirements for continued operation. Its spread footings could support normal building loads but could not resist the overturning forces expected during a major earthquake. A proposed mat foundation was considered risky and difficult to construct beneath the operating hospital, so the project team developed an alternative using high-capacity micropiles.<br /><br />A total of 76 micropiles were designed to support ten core columns. Rather than connecting them to the existing footings, the piles were anchored directly to specially retrofitted column base plates. Collaboration among the specialty contractor, geotechnical and structural engineers, owner, and manufacturers helped shape the design and construction approach.<br /><br />Before final approval, a test pile was loaded in tension and compression. Strain-gauge data showed that nearly all test loads transferred through the pile’s upper 12.2 metres. This allowed the production piles to be shortened to 18.3 metres, reducing cost and simplifying installation.<br /><br />Construction took place in four phases while the hospital remained fully operational. Compact, electrically powered equipment accessed the work areas through standard doorways. Temporary containment, negative air pressure, vacuum collection of drill spoils, and careful routing of service lines helped meet infection-control requirements. Tight pile-position tolerances were managed with templates, and four production piles were load-tested.<br /><br />The micropiles and retrofitted base plates provided the required seismic resistance, allowing the Central Tower to remain in service through at least 2030. The project demonstrates how early contractor involvement, testing, and close coordination can produce a safer and more cost-effective solution for complex foundation work in an occupied hospital.

Keywords

Sharp Memorial Hospital

Central Tower

seismic retrofit

micropiles

foundation strengthening

earthquake resistance

hospital construction

load testing

geotechnical engineering

retrofitted base plates