Investigating the Development of Pile Capacity over Time-Paper

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The paper examines why drilled shafts may gain capacity over time, potentially making deep-foundation designs more conservative than necessary. It identifies three possible mechanisms: soil relaxation as pore pressures dissipate after construction; improved concrete–soil adhesion as moisture changes the interface; and long-term settlement, which compresses and densifies soil beneath the shaft.<br /><br />A case study assessed the foundations of Cincinnati’s Anderson Building, originally built as a book depository and later renovated into offices with additional floors and parking. The existing 16-inch-diameter drilled shafts were tested about 70 years after construction. A load test established an allowable capacity of at least 33.25 tons, and the shafts were judged adequate for the renovated building. Previous building loads had also partially consolidated the foundation soils, helping limit settlement from the added floors.<br /><br />Because no capacity tests were conducted when the building was built, the authors compared the measured capacity with a computer model of hypothetical shafts in similar soils. The model estimated allowable capacities of 19–25 tons, versus 33.25 tons in the tested shaft. The authors interpret this difference as evidence of substantial long-term capacity gain, but acknowledge that the comparison does not directly measure how much capacity developed or isolate the contribution of each mechanism.<br /><br />The paper concludes that time-dependent strength gain may help reduce unnecessary reinforcement, shaft length, construction materials, and costs. However, evidence for drilled shafts remains limited, so the gains are not currently included in design practice. The authors call for further research and recommend that load tests, when appropriate, be carried through to failure to provide more useful design information.

Keywords

drilled shafts

time-dependent capacity gain

deep foundation design

soil relaxation

pore pressure dissipation

concrete-soil adhesion

long-term settlement

load test

Anderson Building

foundation capacity