Large Diameter Shafts – Youghiogheny Reservoir Bridge-PDF

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The Youghiogheny Reservoir Bridge replacement in Pennsylvania used two 9-foot-diameter drilled shafts at each pier, socketed 8.5 feet into bedrock. The paper describes site geology, shaft design, and a full-scale axial load test, focusing on how laboratory rock-strength estimates compared with measured socket side resistance.<br /><br />Bedrock comprised several sandstone and siltstone units. Axial design followed AASHTO methods, estimating ultimate unit side resistance from unconfined compressive strength (UCS), with a PennDOT resistance factor applied. Soil resistance and rock zones with rock quality designation below 20% were excluded. Lateral capacity was assessed using COM624P. Lateral resistance governed socket length at most piers; recommended lengths were generally 20 feet, with a maximum of 28 feet.<br /><br />A 133-foot test shaft was loaded using Osterberg cells. At approximately 0.4 inch of displacement, measured ultimate unit side shear was about 10 tsf in the upper and lower siltstones (Units C1/C3), closely matching the 10.6 tsf design estimate. In the weaker argillaceous siltstone (Unit C2), measured resistance was only about 1.5 tsf, versus a 5.7 tsf design estimate.<br /><br />The discrepancy was attributed to high variability in Unit C2 laboratory UCS results: its standard deviation was nearly equal to its mean. The authors conclude that relying on the mean UCS can overestimate side resistance when test results vary widely, especially in weak rock. Design strength selection should account for the full distribution of laboratory results and how sensitive socket lengths are to those values. In this project, the mismatch had limited impact because Unit C2 was thin and lateral capacity generally controlled socket lengths.

Keywords

Youghiogheny Reservoir Bridge

drilled shafts

bedrock socket

axial load test

Osterberg cells

side resistance

unconfined compressive strength

lateral capacity

sandstone and siltstone

PennDOT design