THE DESIGN OF TUNNEL SUPPORT IN DEEP HARD-ROCK MINES UNDER QUASI-STATIC CONDITIONS

The induced elastic stresses in the sidewalls of gold-mining tunnels a t a depth of 4000 m in the Witwatersrand exceed the peak compressive s trength of even the best-quality quartzite rocks. Laboratory rests mea suring post-yield stress versus strain response show that brittle frac ture occurs, followed by a loss in strength associated with strain-sof tening behaviour, Numerical modelling based on an assumed Mohr-Coulomb criterion fitted to the test results shows that large ongoing deforma tions of the sidewall occur, leading to total tunnel failure without a dequate support, A methodology is presented to enable a quick assessme nt To be : made of support requirements in terms of ground characteris tic curves for numerically derived pressure versus displacement. From these curves, estimates can be made of the force levels for cable bolt ing and of the thickness and strength requirements for shotcrete linin gs, As an example of its application, the methodology is used in a com parison of the relative stability efficiencies of a square-shaped tunn el and a horizontal ellipse-shaped tunnel.