INCLUSION OF OVERBURDEN FRICTIONAL RESISTANCE INTO SHAFT NOMINAL CAPACITY FOR SIGNIFICANT ECONOMIC SAVINGS

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The paper presents an instrumented Osterberg cell (O-cell) load test used to reassess drilled-shaft design for Iowa DOT’s I-29 bridge replacement over the Floyd River in Sioux City, Iowa. Initial designs assumed bedrock and relied solely on rock-socket friction, requiring long shafts and sockets to meet a maximum factored axial load of 3,200 kips. Confirmation borings during test planning instead identified the supposed rock as dense, clayey, silty sand residuum.<br /><br />A 78-inch-diameter test shaft was instrumented with O-cells and strain gauges. The test mobilized substantial resistance in the soil, while the large downward movement below the loading assembly showed that the socket material was not true rock. At about one inch of displacement, the upper shaft developed an estimated 5,565 kips of resistance. Applying the LRFD resistance factor of 0.7 produced a factored capacity of 3,895 kips—695 kips above the required design load.<br /><br />The results demonstrated that overburden-soil friction, typically excluded from rock-socketed shaft designs, could safely contribute to capacity at this site. Iowa DOT therefore allowed it in production-shaft design, enabling construction to proceed at the planned dimensions without deeper shafts or design changes. The test helped avoid estimated added costs of at least $1.5 million, as well as the construction challenges associated with shafts exceeding 200 feet. The case illustrates how full-scale O-cell testing can verify ground conditions, calibrate design parameters, and support more economical foundation designs.

Keywords

O-cell load test

drilled-shaft design

Iowa DOT

rock-socket friction

axial load capacity

instrumented shaft

confirmation borings

residuum soil

LRFD resistance factor

bridge foundation