A Decade of Experience with Drilled Foundations in Honolulu-Paper

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Honolulu’s high-rise buildings were traditionally supported on driven precast concrete piles extending more than 100 ft through shallow coral formations into deeper bearing layers. These piles were costly and inefficient, and their installation generated substantial noise and vibration.<br /><br />The introduction of continuous flight auger (CFA) piles in 2003 and subsequent use of drilled piers offered a more suitable alternative. By mobilizing side shear in the area’s coral, sands, gravels, and stiff alluvial soils, drilled foundations could be shorter and support higher loads. At the 40-story Hokua Tower, test piles sustained twice their design loads without failure, and CFA piles were estimated to reduce foundation costs by nearly 30 percent. Drilled shafts have since supported loads of up to 5,000 kips.<br /><br />Load-test data from projects across Honolulu support design methods for estimating side shear in local soils and porous coralline limestone. End bearing is generally excluded from design because it requires greater movement to mobilize and is difficult to verify below the groundwater table.<br /><br />A major construction challenge is the presence of voids in the coral, which can cause grout or concrete loss. CFA installation requires careful monitoring of grout pressure and levels; some piles may need redrilling and regrouting. Drilled piers generally require slurry methods, prompt concrete placement, and allowances for substantial concrete overage. Overall, drilled foundations are an efficient option in similar geological settings when subsurface conditions and void-related risks are properly addressed.

Keywords

Honolulu foundations

drilled piles

continuous flight auger

CFA piles

coral formations

side shear

load testing

drilled piers

voids in coral

foundation engineering