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Resistance factors for 100% dynamic testing, with and without static load tests.
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    Final report; 09/24/09-06/30/11.
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  • Abstract:
    Current department of transportation (DOT) and Federal Highway Administration (FHWA) practice has highly

    variable load and resistance factor design (LRFD) resistance factors, Φ, for driven piles from design (e.g., Standard

    Penetration Tests (SPT), Cone Penetrometer Test (CPT)) to construction (e.g., pile monitoring). Complicating the

    construction effort, are the number of piles monitored (e.g., 10% versus 100%), as well as the type of monitoring (e.g., high

    strain rate: Embedded Data Collector (EDC), Pile Driving Analyzer (PDA), static load test, etc.). Of great interest are

    quantifying the influence of number of piles within a group, number of piles monitored, as well as spatial variability on a

    pile group’s uncertainty and associated LRFD Φ factors.

    The work started with an investigation of probability of failure (POF) of a bridge in terms of its piers and underlying

    piles. It was discovered that the number of piles in a pier may have a large impact on POF of a pier, which is why the

    development of LRFD Φ should occur with respect to pier (i.e., pile group) level and include the total number of piles

    within the group as well as the distribution of monitored and unmonitored piles within the group. Next, the total

    uncertainty of the pier including spatial variability and error of the method (e.g., SPT, EDC/PDA, etc.) was investigated. The work started with spatial uncertainty of single pile resistance (side plus tip) from SPT data and then extended through

    kriging (considering different weights for individual borings) to group layouts (e.g., double, triple, quads, etc.) for assessing

    group resistance uncertainty, CVR. Subsequently, the kriging group work was carried over to assessing uncertainty, i.e.,

    spatial and method error (predicted versus static load test) for high strain rate field measurements. Equations and charts

    were developed to quantify group uncertainty, CVR, and LRFD Φ for typical group layouts and monitoring. The latter

    approach was considered to be inflexible, and the spatial uncertainty (i.e., kriging) was replaced with hammer monitoring

    in conjunction with high strain rate monitoring. Using the uncertainty of monitoring method (CV εm) and a measured

    uncertainty of blow count regression (CVεh) versus high strain rate monitoring, an LRFD Φ equation was developed for pile

    groups considering the numbers of monitored and unmonitored piles. The developed expression was evaluated at two sites

    and gave reasonable predictions compared to current practice.

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