Keynote Lecture: Continuum or Discontinuum – That is the Question

Several decades ago there was a strong focus on the need for discontinuum modelling to improve upon the empirically based analysis of excavations in jointed rock. The remarkable codes developed by Peter Cundall: UDEC and 3DEC were put to full use in the nineteen eighties and nineties. For example, Q-system based cavern support could be verified or improved with such analyses. Of course, these codes preferably require knowledge of rock mechanics and rock joint behaviour, and perhaps familiarity with non-linear constitutive models as in UDEC-BB. Regrettably the classic textbooks of Hoek and Bray and Hoek and Brown in this period were subsequently followed by the suggestions for continuum modelling using a still not finalized GSI – there are many attempts at improved quantification. JRC now reaching 50 years is also the subject of improved quantification, but it is not followed by the extraordinary page-wide equations for ‘c’ and ‘φ’ so no software is needed. The incorrect addition of these components of shear strength (as indeed in Mohr-Coulomb) in commercial continuum codes is the final source of error of so many analyses. So-called plastic zones are exaggerated around tunnels, and rock slopes are given seldom observed deep spoon-shaped failure predictions, ignoring the frequent influence of major discontinuities, and the usual failures within the slope faces. Of course, lake-bed open-pit slope deposits or extremely weathered rock will give spoon-shaped failures as for rockfill and soil, but competent jointed rock will not fail like this: major discontinuities will usually be involved, and wedge or planar failures will be the usual reality.