Migrating from the Simplified Method to the Stiffness Method for internal stability design of MSE walls

The 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 stabilized earth (MSE) walls. The differences between the legacy Simplified Method (which appears in earlier editions of the AASHTO code) 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 parameter determining the loads developed in reinforcement layers under operational conditions. The Simplified Method is a strength-based 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 the key features of the Stiffness Method and how the method was calibrated. Practical impacts on design outcomes using the Simplified Method and Stiffness Method approaches are identified.

Speaker:

Dr. Bathurst is Professor Emeritus of Civil Engineering at the Royal Military College of Canada, where he has taught since 1980. He also holds a cross-appointment with the Civil Engineering Department at Queen's University and an appointment as a Visiting Research Fellow at the Port and Airport Research Institute in Japan (2016-2026). He was the President of the Engineering Institute of Canada (2016-2018), Past-President of the Canadian Geotechnical Society (CGS) (2013-2014), Past-President of the International Geosynthetics Society (1998-2002), and Past-President of the North American Geosynthetics Society (1998)

Dr. Bathurst has authored or co-authored 227 papers in referred journals and made over 300 other scholarly contributions. He has made contributions in the areas of micromechanics of granular soils, railway ballast and track dynamics, pavement drainage, unsaturated soil-geotextile behavior, constitutive modeling of geosynthetic soil reinforcement materials, new test methods and the development of transparent granular soil surrogates for geotechnical laboratory-scale testing, and prediction of tsunami run-up. Dr. Bathurst’s current research activities are focused on the use of geosynthetic and metallic reinforcement in earth retaining wall systems including environmental impact assessment and sustainability, soil nailing, numerical modeling, seismic performance and design of these systems, probabilistic design of reinforced and unreinforced soil structures, reliability-based design, and load and resistance factor design (LRFD) calibration of soil-structures. He has received national and international awards for his technical contributions and has been an invited speaker and keynote lecturer at learned society conferences and symposia on many occasions. Dr. Bathurst was the 2021 recipient of the ASCE Geo-Institute Thomas A. Middlebrooks Award for the best paper published in the ASCE Journal of Geotechnical and Geoenvironmental Engineering in 2019. Dr. Bathurst has served on committees of the Canadian Highway Bridge Design Code, the National Building Code of Canada, and the American Association of State Highway and Transportation Officials (AASHTO) Committee T-15 Substructures and Retaining Walls. He is the lead author of two chapters of the 5th edition of the CGS Canadian Foundation Engineering Manual and was the technical lead for the Transportation Association of Canada “Guide to Design, Construction, Maintenance and Inspection of Mechanically Stabilized Earth (MSE) Walls.” Dr. Bathurst is the Editor-in-Chief of the peer-reviewed technical journal Geosynthetics International, which is an official journal of the International Geosynthetics Society and published by the Institution of Civil Engineers in the UK, and Editor-in-Chief of the International Journal of Geomechanics published by the ASCE, and Associate Editor of Geotextiles and Geomembranes (Elsevier). He has served on the editorial board of eight other technical journals. Dr. Bathurst was elected Fellow of the Royal Society of Canada in 2017, which is the highest academic honor in Canada. In 2021, he was awarded the “Bathurst Reinforcement Lecture of the International Geosynthetics Society” to be delivered at the International Conference on Geosynthetics held every four years. He was recently awarded the 2024 Julian C. Smith Medal of the Engineering Institute of Canada (EIC) "For achievement in the development of Canada"