Drilled Shaft Load Tests in Weakly Cemented Sandstone-Paper

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The paper evaluates three bi-directional load tests on large-diameter drilled shafts socketed into weakly cemented Paleozoic sandstone at bridge sites in Minneapolis and Oak Park Heights, Minnesota, and Lansing, Iowa. The sandstone was highly friable and difficult to sample; at the Black Hawk site, no representative core was recovered near the test shaft, illustrating the limits of conventional site characterization.<br /><br />Despite weak-looking drilling spoils and poor sample recovery, all three shafts developed substantial axial resistance. Measured maximum unit base resistance ranged from 389 to 444 ksf, averaging 410 ksf. Side resistance varied among shafts and socket segments; the average across the tests was about 25 ksf at roughly one-third inch of displacement. The tests showed that side and base resistance could mobilize together at compatible displacements. In most comparisons, standard empirical design equations underestimated measured resistance, although test displacement and available jack capacity limited how fully resistance could be mobilized.<br /><br />The authors emphasize that performance depends on construction quality, including groundwater and support-fluid management, socket cleanout, concrete mix and placement, and inspection. These challenges make load testing especially valuable: it directly assesses performance where representative rock data are difficult to obtain, can justify shorter production sockets, and can verify construction methods through a demonstration shaft. With suitable design, construction, and quality control, drilled shafts in comparable weakly cemented sandstone can provide high axial capacity.

Keywords

bi-directional load tests

drilled shafts

weakly cemented sandstone

axial resistance

base resistance

side resistance

load testing

socketed shafts

construction quality

bridge foundations