Recent LRFD Calibration for Internal Stability Limit States for MSE Wall Structures

The next edition of the Canadian Highway Bridge Design Code (CHBDC) to appear in 2025 includes guidance on load and resistance factor design (LRFD) for internal stability limit states for mechanically stabilized earth (MSE) walls constructed with extensible (i.e., geogrid and PET strap) and relatively inextensible (i.e., steel strip and steel grid) reinforcement layers. The simplified, stiffness and coherent gravity methods appear in the main code and in the commentary to calculate the tensile loads in the reinforcement layers under operational conditions. This paper describes how the resistance factors for tensile rupture, pullout and soil failure (stiffness method) were computed for each of the load methods and reinforcement-type combinations. For steel reinforcement cases, potential loss of section due to corrosion is included in the calculations for resistance factors. Calibration incorporates the concept of "level of understanding" that appears in the CHBDC to reward designers with greater resistance factors for projects with greater quality and amount of input data, and greater confidence and experience in the design approach adopted for the project. The computations include the load and resistance bias statistics for each limit state, where bias is the ratio of measured value to predicted model value using an analytical equation for load or resistance, or a code-specified value. Updated resistance factors were also computed for possible inclusion in the next revision of the AASHTO LRFD Bridge Design Specifications in the US. Resistance factor calculation outcomes for polyester (PET) strap MSE walls are provided to demonstrate the general approach using a convenient closed-form solution.

Keywords: Mechanically Stabilized Earth (MSE) walls, Internal stability limit states, LRFD calibration, Canadian Highway Bridge Design Code, AASHTO LRFD Bridge design specifications.