Reliability-Based Internal Limit State Analysis and Design of Soil Nails Using Different Load and Resistance Models

A general approach for reliability-based analysis and design for pullout and tensile failure internal limit states of soil nail walls is presented. Reliability index values are computed using a closed-form solution that captures the influence of nominal load and nominal resistance model type and accuracy (method bias), bias dependencies, uncertainty in nominal load and resistance values, and possible cross-correlation (dependency) between nominal nail load and resistance terms. Maximum nail loads under operational conditions for the two limit states are calculated using the current Federal Highway Administration (FHWA) simplified method and an improved version recently published by the authors. Nail pullout capacity is calculated using the effective stress method used in Hong Kong and a modified version that has been empirically adjusted to improve model accuracy for soil nails installed in two different Hong Kong soils. Example designs with three different nail length patterns are used to illustrate the design approach and the assessment of margins of safety using factor of safety and reliability index for the pullout limit state. The results of parametric analyses and design examples demonstrate that for the same target reliability index, the combination of improved load and resistance models gave better solutions based on total length of soil nails.