Load Tests on Drilled Shaft Foundations in Moderately Strong to Strong Limestone-Article

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Three drilled shaft foundations were constructed and bidirectionally load-tested in moderately strong to strong limestone at the Siloam Springs Arkansas Test Site. The shafts had diameters of 1.2, 1.8 and 1.2 m, with rock sockets of 3.0, 1.5 and 2.1 m, respectively. Site testing indicated high-quality limestone overall (average rock quality designation of 70% and unconfined compressive strength of about 100 MPa), but a roughly 1.5 m weathered layer was present near the soil–rock interface.<br /><br />Cross-hole sonic logging found the concrete to be of good quality. Load-test results were affected by construction and instrumentation issues, including possible debris beneath a load cell, retrofitted telltales with poor contact, and delays between excavation and concrete placement. Shorter sockets provided less side resistance and could not adequately balance the upward and downward forces during bidirectional testing.<br /><br />Measured unit side resistance in rock ranged from 0.4 to 1.5 MPa. The longer-socket shaft mobilized greater resistance, while the weathered layer reduced resistance in the shorter-socket shafts. The empirical FHWA parameter, C, was 0.43 in competent limestone and 0.11–0.17 in weathered limestone. Downward movements were generally less than 0.1% of shaft diameter, so ultimate end-bearing resistance and the full load–movement response could not be established.<br /><br />The authors recommend designing for small movements in strong limestone rather than assuming that conventional 4–5%-of-diameter movements will occur. For movements up to 0.1% of diameter (or less than 0.2 cm), they suggest using unit side resistance equal to 0.3 of the predicted maximum. Weathered-limestone t–z behavior can be modeled using existing procedures, but the full response in competent limestone remains uncertain. For reliable bidirectional testing, provide sufficient socket length—about 3.0 m or 2.5 shaft diameters—and place concrete within a day of excavation.

Keywords

drilled shaft foundations

bidirectional load test

limestone

rock socket

Siloam Springs Arkansas

side resistance

weathered limestone

cross-hole sonic logging

FHWA parameter C

end-bearing resistance