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

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The St. Francis Hospital parking garage in Hartford, Connecticut, was rebuilt on its original footprint beside an active hospital. Soft, compressible clay, deep bedrock, high structural loads, limited space, and vibration concerns made conventional foundation replacement costly and challenging.<br /><br />The project reused 46 of the existing 1980s drilled-shaft caissons and added 13 micropiles and 12 composite foundations. Each composite foundation paired a 20-foot-long, 3-foot-diameter reinforced concrete shaft—providing lateral resistance—with a central steel-cased micropile socketed into bedrock to carry axial loads.<br /><br />Because original installation records were unavailable, 12 existing caissons were tested using high-strain dynamic load testing. Their measured resistances ranged from 2,000 to 4,500 kips, confirming their capacity for the new structure’s service loads. Two micropiles were also tested in tension to 900 kips. Testing established a bedrock bond strength of 65 psi, above the assumed 50 psi, allowing production rock sockets to be shortened by about 25%.<br /><br />The team developed an installation sequence that placed and grouted the micropile before completing the surrounding concrete shaft. Vibration monitoring found no detrimental effects on nearby hospital structures.<br /><br />Reusing existing caissons and using composite foundations saved more than $3 million and several weeks compared with full foundation replacement. The approach also produced roughly one-third of the carbon emissions of a complete drilled-shaft replacement, demonstrating how targeted testing and innovative foundation design can make reuse practical under demanding site conditions.

Keywords

St. Francis Hospital

Hartford Connecticut

parking garage

foundation reuse

composite foundations

micropiles

drilled-shaft caissons

bedrock socket

dynamic load testing

sustainable construction