ANZGEO 2023 Conference Workshops

Topic 1: The new AASHTO stiffness method for internal stability design of MSE walls

Topic 2: Probabilistic reliability-based analysis and design of MSE walls

Time:  1300 – 1630
Speaker:  Prof Richard J. Bathurst of the Royal Military College of Canada

Overview – Topic 1:
In the most recent edition of the AASHTO LRFD Bridge Design Specifications published in 2020 in the US, the Stiffness Method is specified for the internal stability design of geosynthetic MSE walls. A key feature of this approach is the use of the creep-reduced tensile stiffness of the reinforcement as a key parameter to compute the magnitude of reinforcement loads under operational conditions. This is a paradigm shift from the AASHTO Simplified Method specified in earlier editions of the same code and based largely on the strength of the soil. The workshop describes the essential features of the Stiffness Method and how the method was calibrated against a large database of measurements taken from instrumented field walls. The Stiffness Method is demonstrated to be more accurate on average than the Simplified Method when compared to measured loads in instrumented walls, and leads to designs with more efficient placement of the reinforcement layers. The Stiffness Method includes a soil failure limit state which is used to keep the reinforced soil zone at working stress conditions assumed for geosynthetic MSE walls under operational conditions. The Stiffness Method has the advantage that it is seamless across MSE walls constructed with extensible (geosynthetic) reinforcement products and relatively inextensible steel grids and strips. The improved accuracy of the Stiffness Method has the corollary benefit of removing conservatism observed in Simplified Method design outcomes (particularly for geosynthetic MSE walls) and has facilitated load and resistance factor design (LRFD) calibration. Examples of design outcomes using the Stiffness Method, Simplified Method and the Coherent Gravity Method (for inextensible reinforcement) are presented.

Overview – Topic 2:
Reliability-based design of geotechnical structures is gaining traction over classical (deterministic) factor of safety, partial factor and LRFD approaches. The latter provide safe designs but margins of safety expressed probabilistically are not available. The workshop shows how traditional estimates of nominal factors of safety can be linked to the probability that a limit state is not satisfied and thus provide the designer with a more nuanced appreciation of the margins of safety for a particular limit state. In this workshop the general approach is applied to external and internal stability design of MSE walls. The presentation demonstrates deterministic and reliability-based assessment of strength limit states (tensile resistance and pullout) and the service limit state for soil failure for mechanically stabilized earth (MSE) walls using the AASHTO Stiffness Method. The general approach considers the accuracy of the load and resistance models that appear in each limit state equation plus uncertainty in the estimate of nominal load and resistance values at time of design. The presentation also shows how load measurements from large databases of instrumented MSE walls for load side models, and laboratory measurements compiled in databases for resistance side models can be used to quantify model accuracy (bias). A closed-form solution is presented that can be used in a spreadsheet to compute the reliability index for internal stability limit states with lognormal distributions of bias values and nominal values. As an example, the equation is used to compute the reliability index for the pullout internal stability limit state for an as-built MSE wall. The general approach provides a quantitative link between the classical factor of safety used in working stress design past practice, and margins of safety expressed within a probabilistic framework.

Time:  1300 – 1630

Convenors: Darren Paul, WSP / Tony Miner, A.S. Miner Geotechnical

Overview:

The last edition of the Australian Geomechanics Society Guidelines for Landslide Risk Management (AGS 2007) was published about 16 years ago in March 2007 and has been widely applied within Australia and New Zealand over the past 16 years. This workshop seeks to bring together those with experience using AGS2007, to share their experiences and to assess needs for future development of the guidelines. Feedback on the currency and effectiveness of the AGS 2007 guidelines will be sought and ideas gathered on how the guidelines could be improved. There will also be discussions on future challenges for landslide risk management in Australia including the effects of climate change and changes in the regulatory environment. If appropriate based on feedback, the setting of actions that can be taken to evolve guidelines for landslide risk management in Australia and New Zealand. There will be an opportunity for workshop attendees to express their interest in maintaining involvement in review and development of the guidelines.

Time:  1300 – 1630

Convenors: Burt Look, FSG Geotechnics and Foundations Specialists

Overview:

The workshop provides an overview of the 2022 book “Earthworks” for practicing engineers. This book was derived from “Earthworks” course notes compiled over 10 years of presenting the 2-day course for over 1,000 practicing engineers (mainly civil and project engineers) and benefited from their feedback and issues. A case study approach is adopted over the 14 chapters to examine everyday design and construction earthworks practices.

Compaction mechanics is one of the simplest engineering concepts. Success in any project requires getting the simple things right. Yet common misunderstandings are still occurring with the application of this simple concept – from theory to practice. Many measurements have become targets rather than part of a decision-making process. This targeting simplification can lead to either over conservatism or unstable works.

Case studies are used to show how earthworks theory is applied in day-to-day practice. Subgrade and expansive clay assessment and treatment will be covered. Useful design practices as well as the development and application of specifications are outlined. A specification, test, or design in one climatic condition or geology may not apply in another.

See the following link for those wishing to purchase the book which will have the course notes: https://www.routledge.com/Earthworks-Theory-to-Practice—Design-and-Construction/Look/p/book/9781032104706. The book is not provided for this workshop.