Implementation of LRFD of Drilled Shafts in Louisiana

Abstract

This paper presents reliability-based analyses for the calibration of resistance factor for axially loaded drilled shafts. A total of 16 cases of drilled-shaft load tests were collected from the Louisiana Department of Transportation and Development (LADOTD) archives. Only 11 out of the 16 cases met the Federal Highway Administration (FHwA) failure criterion. Because of the limited number of available drilled-shaft cases in Louisiana, additional 15 drilled-shaft cases were collected from a neighboring state, Mississippi, which has subsurface soil conditions similar to Louisiana soils. A database of 26 drilled shafts representing the typical design practice in Louisiana was created for a statistical reliability analysis. Predictions of load-settlement behavior of drilled shafts from soil borings were determined using the FHwA design method through the SHAFT computer program. Measured drilled-shaft axial nominal resistance was determined from either the Osterberg cell (O-cell) test or the conventional top-down static load test. Statistical analyses were performed to compare the predicted ultimate drilled-shaft nominal axial resistance and the measured nominal resistance. Results show that the selected design method underestimates the measured drilled-shaft resistance by an average of 17%. The Monte Carlo-simulation method was selected to perform the LRFD calibration under strength I limit state. Total resistance factors for different reliability indexes (