This General Report, complemented by invited lectures by Dr. J. C. Roegiers and Dr. R. Hart, surveys some recent developments in analytical and numerical approaches to the study of the deformation and stability of deep excavations in rock, both small (boreholes) and large (tunnels, caverns). Emphasis is given to the classical closed-form analytical solution of elastic-plastic (inelastic) behaviour of rock around excavations to illustrate the influence of various factors that affect the constitutive behaviours, both elastic and inelastic, of the rock. Consideration of discrete discontinuities is essential to understanding of the stability of large excavations.
One of the goals of this Symposium is to strengthen the dialogue between petroleum engineers and mining or civil engineers concerned with the development of stable excavations in rock at great depth. An obvious difference between the two concerns is, of course, the cross-sectional area of the excavations; the small petroleum boreholes and the much larger tunnels, stopes and caverns of mining and civil engineering. In the former, deformation and stability of the excavation are dependent principally on the mechanical properties of the rock material itself, whereas in the latter, controlling influence. Significant progress is being made in studies of both small-scale (borehole) stability and large excavation stability, but each tends to have a different 'flavour'. Borehole stability is amenable to laboratory investigation, for example, whereas behaviour of the large excavations require direct observation in the field for confirmation of analysis. In considering how best to review the progress being made in each, whilst also examining points of common concern to both, it was decided to supplement this report with two special lectures. One, by Dr. J. C. Roegiers, reviews recent developments in excavation stability research, with emphasis on borehole stability and petroleum engineering. The second, by Dr. R. Hart, reviews recent developments in discontinuum studies, with applications to large excavations in mining and civil engineering. This report will focus on a general discussion of stability around excavations, broadly applicable to both small and largescale excavations.
Much of the discussion concerning stability of underground excavations, both small scale and large scale, starts from the classical problem of a circular hole in an homogeneous, isotropic, elasto-plastic medium loaded in plane strain by a uniform external compression P at infinity, and a uniform internal pressure Pi'. As P is increased, the strength of the rock at the wall of the excavation is eventually exceeded and an annulus of broken rock develops around the inner radius a, i.e. forming the 'plastic' [or 'inelastic' in Fig.1] zone. The thickness [(b - a) in Fig. l] of the inelastic zone to achieve this equilibrium at the interface will depend on the constitutive behaviour of the broken rock in this zone. A wide variety of modifications to this distribution (some for example showing discontinuities in the tangential stress at the elastic/inelastic interface), are possible, depending again on the constitutive behaviour of the broken rock [See, for example, Brady and Brown (1985)].
