Calibration of Resistance Factors Needed in the LRFD Design of Drilled Shafts

The first report on Load and Resistance Factor Design (LRFD) calibration of driven piles in Louisiana (LTRC Final Report 449) was completed in May 2009. As a continuing effort to implement the LRFD design methodology for deep foundations in Louisiana, this report will present the reliability based analyses for the calibration of the resistance factor for LRFD design of axially loaded drilled shafts. A total of 16 cases of drilled shaft load tests were available to authors from Louisiana Department of Transportation and Development (LADOTD) archives. Out of those, only 11 met the Federal Highway Administration (FHWA) "5%B" settlement criterion. Due to the limited number of available drilled shaft cases in Louisiana, additional drilled shaft cases were collected from state of Mississippi that has subsurface soil conditions similar to Louisiana soils. A total of 15 drilled shafts from Mississippi were finally selected from 50 available cases, based on selection criteria of subsurface soil conditions and final settlement. As a result, a database of 26 drilled shaft tests representing the typical design practice in Louisiana was created for statistical reliability analyses. The predictions of total, side, and tip resistance versus settlement behavior of drilled shafts were established from soil borings using the FHWA O’Neill and Reese design method via the SHAFT computer program. The measured drilled shaft axial nominal resistance was determined from either the Osterberg cell (O-cell) test or the conventional top-down static load test. For the 22 drilled shafts that were tested using O-cells, the tip and side resistances were deduced separately from test results. Statistical analyses were performed to compare the predicted total, tip, and side drilled shaft nominal axial resistance with the corresponding measured nominal resistance. Results of this showed that the selected FHWA design method significantly underestimates measured drilled shaft resistance. The Monte Carlo simulation method was selected to perform the LRFD calibration of resistance factors of drilled shaft under strength I limit state. The total resistance factors obtained at different reliability index were determined and compared with those available in literature. Results of reliability analysis, corresponding to a target reliability index of 3.0, reveals resistance factors for side, tip, and total resistance factor are 0.20, 0.75, and 0.5, respectively.

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