Engineers Australia

2 Day Seminar: Developing Confidence in Critical State Soil Mechanics

Mike Jeffries

Abstract

About Critical state soil mechanics (CSSM) Critical state soil mechanics (CSSM) is the only framework that predicts/quantifies the effect of void ratio on the mechanical behaviour of soil. CSSM is built on a century of developments in theoretical soil mechanics, but these developments are posed in both unfamiliar form and require background in aspects of mechanics that are not commonly taught in civil engineering degrees. In addition, pre-1985 developments in CSSM were over-idealized to the extent that they were verging on useless for real soils. These issues appear to have constrained adoption of CSSM in engineering practice (and indeed limited even the teaching of CSSM). CSSM has its origins in static liquefaction failures (going back more than 100 years), and the recent history of large tailings dam failures involving liquefaction has renewed interest in CSSM. This renewed interest also builds on comprehensive further developments to CSSM during the past 25 years. CSSM has subsequently become the dominant framework for understanding, and quantitatively modelling soil behaviour from coarse sands through to clays, with particular relevance to liquefaction.

Workshop Outline

The aim of this workshop is for all attendees to become confident in using CSSM across a spectrum of applications, from assessing laboratory data through to developing design parameters and evaluating CPT data. A workshop setting is desired because most engineers will not develop true understanding in CSSM from studying papers or listening to lectures. The unfamiliar nature of work hardening plasticity requires people to implement CSSM from “first principles” to truly understand the theory. Such a ‘tutorial’ rather than ‘lecture’ setting was insisted on three decades ago by Peter Wroth as the only way to grasp CSSM – and is still valid today.

Part 1 – Theory and guided tutorials

The first part of the course is presented as a series of short lectures presenting aspects of the theory, each of which is then followed by a guided tutorial or implementation. The tutorials take the form of Excel spreadsheets that participants put together to implement CSSM models of progressively increasing generality and leading to the simulation of static liquefaction. Once the ideas have been grasped, the course will move on to applying the ideas.

Part 2 – Determination of soil properties

The second part of the course will introduce test data, on both a sand and a silt, for participants to determine the soil properties and to model the tests. The intention is for participants to convince themselves of the accuracy and power of CSSM.

Part 3 – Assessment of CPT data

The third part of the course looks to the practical question of determining the state parameter in-situ, which comes down to evaluating CPT data. The background to the CPT (and its stress normalization) will be covered. Participants will be introduced to the ‘widget’, which is an open-code executable that generates soil-specific CPT calibrations. This final part of the course will cover using the widget and processing CPT data to recover the in-situ state.

About the speaker

Mike JeffriesConsulting Engineer, Lincoln, UK

Mike is a registered Canadian professional engineer with some 40 years experience in offshore platforms, dams, ground improvement, and quantative risk assessment. Strongly influenced in his early years by Professors Bob Gibson, Alan Bishop, and Peter Wroth, Mike has pursued an interest in theoretical soil mechanics despite working as a consulting engineer around the globe. Mike is the originator of NorSand model, and proponent of the state parameter in the highly cited paper – A state parameter for sands – written with the late Dr. Ken Been. Mike has published some eighty papers (with more than 5000 citations) and is co-author of the influential book Soil Liquefaction: A Critical State Approach. An invited contributor to Geotechnique, the Canadian Geotechnical Society’s Fall/2012 Cross-Canada Lecturer, the 2014 Šuklje Lecturer, and the 2018 Jenning’s Lecturer, Mike was awarded a Telford Premium for geotechnical research in 2017.

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