LRFD Calibration of Internal Limit States for Polymer Strap MSE Walls

Abstract

The paper demonstrates load and resistance factor design (LRFD) calibration for tensile strength, connection strength, pullout, and soil failure internal stability limit states for polyester strap mechanically stabilized earth (MSE) walls using a reliability theory-based approach. The calibrations are carried out using the simplified and stiffness methods to compute tensile loads and the pullout and connection models found in current LRFD specifications for MSE walls. LRFD calibration is also carried out for the soil failure limit, which is unique to the stiffness method. Computed resistance factors in combination with code-specified load factors are compiled in tables based on US and Canadian LRFD practice. Recommended resistance factors are provided for both jurisdictions. Actual margins of safety expressed as a reliability index are calculated using current and proposed new resistance factors for the case when each limit state is just satisfied. Example designs are provided to show the impact on reinforcement demand when using different load models in combination with current and proposed new resistance factors. Mechanically stabilized earth walls are a proven technology to perform the soil retaining wall function and are ubiquitous on the civil engineering earthworks landscape. These systems can be constructed for much less cost than competing concrete wall solutions. The signature feature of these wall structures is the use of horizontal layers of geosynthetic (polymeric) reinforcement, steel straps, and steel grids to stiffen and strengthen the backfill soil behind the wall facing. The authoritative US and Canadian sources for the design of these systems adopt the load and resistance factor design approach for design. Mechanically stabilized earth walls constructed with polyester strap reinforcement are now gaining traction as another type of these systems. At the time the work in this paper began, the calculation of the resistance factors for the internal stability design of these systems within the North American load and resistance factor design framework had not been carried out. This paper fills that gap. In addition, this paper provides guidance and examples to produce designs that satisfy a minimum acceptable level of safety.