Program > Distinguished Invited Talk

Thermal over-closure of rougher joint sets – consequences for HLW disposal strategies and HTM modelling
Nick Barton, PhD
NB & A
Biosketch

Nick Barton obtained a London University B.Sc. in Civil Engineering from King’s College in 1966, and a Ph.D. concerning shear strength and rock slope stability from Imperial College in 1971. He worked for two long periods at NGI in Oslo, and for four years at TerraTek in Salt Lake City. Since 2000 he has had his own international rock engineering one-man consultancy, Nick Barton & Associates, based in Oslo and São Paulo. He has consulted on several hundred rock engineering projects in a total of 38 countries during 45 years, has 260 publications as first or single author, and has written two books, one on TBM prognosis, the other linking rock quality and seismic attributes of rock masses at all scales. He is currently writing a book with Bandis: Engineering in jointed and faulted rock (expected 2020). He has thirteen international awards including the 6th Müller Lecture of ISRM. He developed the widely used Q-system for classifying rock masses, and for selecting rock tunnel and cavern single-shell support in 1974. He was originator of the rock joint shear strength parameters JRC and JCS and co-developer of the resulting Barton-Bandis constitutive laws for rock joint coupled M-H modelling in 1982, which was incorporated as a sub-routine in UDEC-BB in 1985. He has also developed the Qtbm prognosis method and Qslope for selecting maintenance-free rock cutting and bench-face angles. His chief areas of consulting activity have been in hydropower tunnelling and cavern construction and performance, nuclear waste disposal site characterization, metro tunnels and caverns, and site characterization at high dams. He has given more than forty keynote lectures in international conferences.

Abstract

Rough joints can be over-closed and remain over-closed by a previous application of a higher normal stress. This is an exaggerated form of hysteresis.  Rough joints in igneous and metamorphic rocks can over-close even due to temperature increase alone, due to better fit. Rock mass deformation moduli, thermal expansion coefficients, physical and hydraulic apertures, shear strength and seismic velocities may each be affected. Evidence for these effects comes from well-controlled laboratory HTM tests, in situ HTM block tests, and large-scale heated rock mass tests, lasting several years at Stripa, Climax and Yucca Mountain. Thermal-OC effect in HTM numerical modelling will require, as a minimum, thermal expansion coefficients that include rather than exclude relevant joint sets, if these have marked roughness and if they originated at elevated temperature. Subsequently elevated deformation moduli that attract higher stress must be expected due to the ‘softening’ of normal stiffness during the heating. During the cooling phase of an HLW repository, one may experience rougher joints (JRC > 8?) that have been thermally over-closed, and that may not open during cooling. Their effective JRC has been dramatically increased. These joints have increased cohesive, frictional and even tensile strength and reduced aperture. They may also be preferentially involved in chemical deposition and sealing. Smoother, planar, and maybe more continuous features formed in response to prior tectonic loading will tend to open to compensate for those that may remain closed during the cooling, thereby potentially losing strength and gaining permeability. This should alert designers to avoid the continuous features in e.g. their disposal canister deployments. Obviously done, but clearly extra important.