Dr. Mccreath et Ms. Diederichs, ASSESSMENT OF NEAR-FIELD ROCK MASS FRACTURING AROUND A POTENTIAL NUCLEAR-FUEL WASTE REPOSITORY IN THE CANADIAN SHIELD, International journal of rock mechanics and mining sciences & geomechanics abstracts, 31(5), 1994, pp. 457-470
Atomic Energy of Canada Limited (AECL) is developing a concept for the
permanent storage of Canada's nuclear fuel wastes. As of 1992, the co
ncept was based on indoor emplacement of waste canisters within an eng
ineer ed repository constructed at depths of 500-1000m in plutonic roc
ks of the Canadian Shield. Canadian law requires that the Federal Envi
ronmental Assessment Review Office (FEARO) undertake a formal review o
f the concept before AECL proceed with site selection and design. In p
reparation for this upcoming reuiew, the federal ministry which is res
ponsible for the environment, Environment Canada, commissioned a small
panel to undertake an overview of the concept. The sole purpose of th
e panel's work was to identify issues of potential importance to the s
afety and acceptability of the storage concept, so that the future rev
iew process could be appropriately focused on these key issues. Althou
gh fracture mechanics research has made much progress in under standin
g rock mass failure in terms of fundamental fracture initiation, propa
gation and interaction processes, there is still no generally accepted
method by which to evaluate the actual fracturing response of a real
rock mass in situ. Nevertheless, public review of the repository conce
pt must address the issue of potential rock mass fracturing around the
underground openings. The challenge is to do so despite the lack of a
generally agreed methodology, in a manner which is practical within t
ight budgetary limits, acceptable in the context of normal engineering
practice, and sensible from a fracture mechanics perspective. The key
question is whether simple rock engineering analyses, based on conven
tional stress-strength comparisons, can be used to assess the potentia
l for near-field fracturing of the rock mass. The goal of the work rep
orted in this paper was to undertake a realistic but limited pie-feasi
bility level engineering evaluation of the potential for near-field fr
acturing of the rock mass, particularly in the vicinity of the caniste
r emplacement holes. The approach chosen was based on simple parametri
c modelling using boundary element analyses. Input parameters were sel
ected to conditions appropriate to batholiths of the Canadian Shield,
The results, based on conventional rock engineering stress-strength an
alyses, were evaluated within the context of current fracture mechanic
s research views and of engineering judgement derived fi om experience
with rock mass fracturing around deep underground openings in the min
es of the Canadian Shield. Results indicate that stress concentration
effects will cause fracturing beneath the room floors, and experience
suggests that rock damage will be mainly in the form of horizontal sla
bbing. With time, rock damage may extend over the full 5m depth of the
emplacement holes. These findings suggest that the concept of in-floo
r storage of the waste canisters must undergo detailed critical review
before being accepted as an inherent part of the AECL disposal concep
t.