Parking Garage Supported on Re-Used Caissons and Composite Caisson Micropiles-PPT Presentation

PPT Presentation

Pdf Summary

The presentation describes replacing the deteriorated four-level Collins Garage at Saint Francis Hospital in Hartford, Connecticut. Because the new garage would occupy nearly the same footprint, and nearby buildings and hospital operations limited construction options, the project team considered reusing the garage’s existing bedrock-socketed caissons. Building-code requirements, limited original records, and changes in garage geometry meant the foundations needed verification and supplemental support.<br /><br />Installing new, full-depth caissons—about 120–150 feet long—was over budget. Instead, the team developed composite piles combining a short, cased drilled shaft at the top with a micropile socketed into bedrock. This approach cost approximately 30% as much as new full-depth caissons. The design also addressed combined axial and lateral stresses, strain compatibility, existing reinforcement, and the use of high-strength steel.<br /><br />Testing was central to verifying the foundations. Existing caissons underwent high-strain dynamic load tests using a 32-ton drop hammer; measured compressive resistances of 2,000–4,500 kips were verified. New micropiles were also tested, and tension tests on shortened rock sockets demonstrated higher ultimate bedrock bond strength than originally assumed. The team recommended duplicate strain gauges because installation can damage instrumentation.<br /><br />The composite-pile solution saved at least $3 million and several weeks of construction. The presentation emphasizes that successful reuse depended on careful testing, specialized installation and interpretation methods, and close coordination among the owner’s engineer, contractor, and contractor’s engineer.

Keywords

Collins Garage

Saint Francis Hospital

Hartford Connecticut

foundation reuse

bedrock-socketed caissons

composite piles

micropiles

high-strain dynamic load testing

bedrock bond strength

construction cost savings