Vertical Load Test on Drilled Shafts Installed in Glacially Derived Sediments-Paper

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The paper reports a static, bi-directional Osterberg-cell load test on a 29.5-inch-diameter drilled shaft installed about 47 feet into Vashon recessional outwash near Lakewood, Washington. These glacial deposits consist mainly of sands and gravels, with cobbles and boulders, and have limited published load-test data.<br /><br />The shaft was instrumented to measure displacement, load transfer, and strain. The O-cell applied up to 437 kips in both directions. At maximum load, upward and downward movements were each less than 0.1 inch. The test ended when the loading system reached its capacity, so ultimate resistance was not established.<br /><br />Results were compared with design estimates using FHWA and AASHTO methods. The equivalent top-load curve indicated approximately 550 kips of compression resistance and 400 kips of tension resistance at 0.1-inch settlement. The tension resistance was about 70% of the compression resistance, suggesting that the assumed tension-to-compression relationship may be conservative or need adjustment for similar deposits.<br /><br />Measured side friction showed a deflection-hardening response and approached the design limit of 4 ksf; the authors suggest that an ultimate unit side-friction value of about 5 ksf may be appropriate. Mobilized end bearing was about 21 ksf—roughly 65% greater than the corresponding FHWA-based estimate at the measured displacement. However, the test did not reach the larger displacements needed to determine ultimate end-bearing resistance.<br /><br />The authors conclude that the design assumptions were generally adequate, while emphasizing that service-load performance and displacement deserve greater attention. Additional, higher-capacity tests in similar glacial deposits are recommended to refine resistance estimates and support more efficient designs.

Keywords

Osterberg-cell load test

drilled shaft

bi-directional load testing

Vashon recessional outwash

glacial deposits

side friction

end bearing

FHWA design methods

AASHTO methods

tension resistance