Drilled Shaft Load Tests in Weakly Cemented Sandstone-Video

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Video Summary

David Graham presented bidirectional load tests on drilled shafts at three Midwestern bridge sites in weakly cemented sandstone: Minneapolis, the St. Croix Crossing, and Iowa’s Black Hawk Bridge. Despite poor core recovery and low RQD at some sites—and refusal SPT blow counts at the Iowa site—the shafts demonstrated high side and base resistance. Testing was generally limited by jack capacity rather than geotechnical failure. Measured resistance often exceeded predictions from conventional empirical equations, especially those intended for unstable rock. The results show that weak-looking sandstone can perform strongly in situ, though construction quality is critical: fluid management, groundwater control, and a clean shaft base all affect performance. Load tests also enabled shorter production shafts, higher resistance factors, and validation of construction methods. In discussion, Graham noted that polymer slurry was used partly to protect interbedded shale, and that pressuremeter testing might help where rock samples are unavailable. He cautioned that the limited data do not justify broadly revising empirical design equations.

Keywords

bidirectional load tests

drilled shafts

weakly cemented sandstone

bridge foundations

side and base resistance

empirical design equations