A case study on pile relaxation in dilative silts

Brent D Richardson, University of Rhode Island

Abstract

In the mid-1980s the Jamestown Verrazzano Bridge was built in Rhode Island spanning the west passage of Narragansett Bay. The bridge was to be founded primarily on pre-stressed concrete piles acting as friction piles. A test pile program was conducted at the beginning of construction and the measured capacity of the piles was significantly lower than predicted values. This led to significant delays in construction, cost overruns, and ultimately led to a change in the design of the foundations. ^ The overall objective of this thesis is to evaluate the results of the test pile program and attempt to understand why the measured capacities were so much lower than design values. ^ The region in which the pre-stressed concrete test piles were driven is known to contain sands, non-plastic silts, and organic silts of varying densities. Pile relaxation is known to occur in dilative sands and silts, and it has been hypothesized that this occurred at this site. However, no one has been able to quantify how relaxation caused such a significant reduction in capacity. There are very few studies on the effects of cyclic pile driving in dilative silts, none of which provide correlations to observed pile relaxation and cyclic loading. Because of this and the fact that dilative sands and silts exist at other potential bridge sites in Rhode Island, this is an important case study to document. ^ Site characterization, geotechnical properties, and load test data was compiled from a large quantity of construction reports and correspondence from the project. Static capacity analysis was performed for each test pile at design depth and at the depth in which the static load tests were actually conducted. The analyses indicated the design depths should have been of sufficient depth to provide enough resistance for the design capacities based upon provided boring logs and lab data; which was not the case. The analyses also significantly over-predicted the ultimate capacities of the three test piles driven well beyond the design depths. ^ The disagreement between the static capacity analysis, CAPWAP and static load tests may have been a result of either one of the following reasons: arching, friction fatigue, post liquefaction behavior or dilation. It was concluded that a combination of the effects leading to real or apparent pile relaxation may have caused the significant difference between measured and predicted ultimate capacities, however, none of these effects can fully explain the large differences between predicted capacities and the capacities measured in the test pile program.^

Subject Area

Engineering, Marine and Ocean

Recommended Citation

Brent D Richardson, "A case study on pile relaxation in dilative silts" (2011). Dissertations and Master's Theses (Campus Access). Paper AAI1503337.
http://digitalcommons.uri.edu/dissertations/AAI1503337

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