Calibration of Resistance Factors of Soil-Nails for the LBJ Express Project

摘要

This paper presents a reliability-based analysis for the calibration of pullout resistance factors of soil nails for the Lyndon B. Johnson (LBJ) Express construction project. LBJ Express is located on I-35E and I-635 in North Dallas Texas and consists of numerous temporary and permanent soil nail walls (SNWs) to be constructed throughout the project's completion. A total of 47 verification tests have been performed in cohesive soil, with 25 meeting one or more failure criteria. Prediction of pullout resistance was 47.9 kPa (1,000 psf) for all soil nails, with soil strength parameters remaining reasonably constant throughout the construction project. Survival Analysis was used to incorporate non-failed tests into the calibration of pullout resistance factors. Results show that the predicted pullout resistance generally underestimates the measured by a substantial amount, leading to an overly-conservative design. Monte Carlo simulations were utilized to perform load and resistance factor design (LRFD) calibrations of soil nail pullout resistance factors. Load factors (γ) between 1.0 and 1.75 were selected and as a result of the high structural redundancy of SNWs, a target reliability index (β_T) of 2.33 was chosen. LRFD calibrations with Monte Carlo simulations resulted in pullout resistance factors (Φ_(PO)) of between 0.61 and 1.49.