The latest 2020 edition of the authoritative AASHTO LRFD Bridge Design Specifications in the USA has adopted the “Stiffness Method” for the internal stability design of geosynthetic mechanically stabilised earth (MSE) walls. The differences between the previous 2017 AASHTO code that uses the Simplified Method (which is also the current Canadian Highway Bridge Design Code approach) and the Stiffness Method are explained in the lecture. The Stiffness Method approach represents a paradigm shift on how reinforcement loads are calculated for internal stability design of MSE walls under operational conditions. The Stiffness Method, as the name implies, includes the tensile stiffness of the reinforcement as a key factor determining the loads developed in reinforcement layers under operational conditions. The Simplified Method is a strengthbased approach which cannot distinguish between reinforcement materials falling within the same classification. The Stiffness Method has the additional benefit of being seamless across relatively extensible (geosynthetic reinforcement materials) and relatively inextensible materials (steel grids and steel strips). The lecture describes how and why the Stiffness Method was developed. Practical impacts on design outcomes
using the Stiffness Method are identified.