The British Geotechnical Association (BGA) is the principal
association for geotechnical engineers in the United Kingdom.
As part of the series of Departmental Seminars in the Department of Earth Science and Engineering at Imperial College, Nick Barton will give a talk on Failure modes in rock masses as evidenced by failure in deep tunnels, cliffs, mountain walls and the highest mountains.
Failure modes in tunnels in massive rock, where jointing may be limited, depend traditionally on the ratio of stress (tangential) and rock compressive strength. (The '0.4' mystery). But are we sure about this? A recent simple finding by Baotang Shen (developer of FRACOD - a fracture mechanics code) suggests that tensile strength divided by Poisson's ratio gives us the initial extension failure mode, perhaps at 1,000m depths in hard rock. This failure in tension ('slabbing') may even occur when all principal stresses are compressive. But there is strong stress anisotropy. At greater depths propagation may be in shear, where log-spiral failures may be seen. We now move outdoors and start thinking only in the vertical direction, about cliffs, mountain walls (rock-climbing territory), and the highest mountains. These occupy the remainder of the talk. A new formula is presented - embarrassingly simple - that does a better job than Mohr-Coulomb, since extension failure is a weaker link than shear failure. Finally, a classic formula for mountain walls from Terzaghi 1962, sometimes incorrectly applied to high (8-9km) mountains, is questioned (as also by Terzaghi at that time), and is replaced by critical state rock strength concepts, with possible limits at the slightly lower brittle-ductile transition.
About the speaker
International consultant, Rock Engineering
Barton has 45 years experience from rock engineering projects in 38 countries, mostly concerning road, rail and metro projects, also tunnels and caverns for hydropower, rock slope stability, major dam abutment and foundation stability, and nuclear waste disposal studies in various countries.
He is frequently involved in TBM project trouble-shooting. In 2000 he started his own consultancy in Norway: Nick Barton & Associates.
He obtained a Ph.D. on rock slope stability from Imperial College, London in 1971. He worked for 25 years in the Norwegian Geotechnical Institute, part of the time as division director, and later as technical adviser.
He is author/co-author of more than 300 papers in technical journals and conference proceedings, and authorof a book on TBM Tunnelling in Jointed and Faulted Rock in 2000, and of a text book on Rock Quality, Seismic Velocity, Attenuation and Anisotropy in 2006. He is currently writing a rock engineering textbook with Prof. Stavros Bandis.
He developed the well-known Q-system of rock mass characterization in 1974, and a non-linear shear strength criterion for rock joints in 1973/1982, now known as the Barton-Bandis criterion. Further empirical methods: Q TBM prognosis in 1999 and Q SLOPE for safe rock slope angles in 2015.
He received the 6th ISRM Müller Award, given only once every four years for distinguished contributions to rock mechanics and rock engineering. He has an Honoris Causa (Honorary Doctorate) award from the University of Cordoba, Argentina. He received ten international awards between 1975 and 2015, and is an ISRM Fellow.
Date: 24th January 2019
Location: Room G41, Royal School of Mines Building, Imperial College London