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Van Tuyl Lecture: Mark Diederichs, Queens University
November 21, 2019 @ 4:00 pm - 5:00 pm
Mark Diederichs, Queens University
Berthoud 241, 4-5PM
Beyond the Equations: A Philosophical Discussion of Time Dependent Excavation Response in Rock
Abstract: In geological vocabulary, the term “brittle” is used to describe deformation behaviour in which the rock loses continuity or as the permanent change that occurs in a solid material due to the growth of fractures and/or due to sliding on fractures after the rock stresses exceed some critical value. Ductile behavior, conversely, is inelastic deformation without loss of material continuity. Processes of time dependent behaviour or creep (dislocation creep, pressure solution, mechanical twinning, grain boundary/volume diffusion) are well described in the geological literature for ductile deformation processes over geological time and a deep crustal temperature and pressure. In the low temperature and pressure realm of fracture and cataclasis, fracture mechanics gives us the concept of subcritical crack growth to explain time dependent behaviour. For some materials such as rock salt, all of these processes are possible over different but human time scales at standard temperature and pressure.
Engineers typically adopt a of suite simple mathematical/mechanical analogues to describe phases of creep in geomaterials. So called primary, secondary and tertiary creep are phases of time dependent deformation that can be included by chaining these analogues together to represent the different phases in analytical or numerical simulations. Engineers by nature seek simple analogues for complex processes, leading to the adoption of the creep formulations for behaviour ranging from salt cavern deformation, to progressive deformation and long term instability in rock tunnels (both weak rockmasses and strong, brittle rock), to large slope deformations and stability problems, to ultra-long term response of deep geological repositories. While reasonable phenomenological predictions are often obtained through this broad application of simple response equations, the lack of true understanding of process in these different scenarios limits confidence and can lead to erroneous assessment of long-term performance.
This discussion will be focused on the internal mechanics of different geomaterials, in different engineering settings, and at different scales, to examine the applicability of conventional tools for time dependent deformation and strength loss. The concept of progressive damage “speed-bumps” is introduced to provide a continuity of process for incremental damage and deformation processes in geomaterials from the micro- to mega-scale.