Authors: Eloranta, P. & Simonen,A. & Johansson,E.
Commisioned by: Nuclear Waste Commission of Finnish Power Companies.
ABSTRACT:
The time-dependent strength and deformation properties of hard crystalline rock are studied. Theoretical models defining the phenomena which can effect these properties are reviewed. The time-dependent deformation of the openings in the proposed nuclear waste repository is analyzed.
According to the literature, the most important time-dependent phenomena causing alteration of deformation properties in crystalline rock are creep and cyclic fatigue. These phenomena are caused by the subcritical crack growth especially under high stress conditions. The most important factors affecting the subcritical crack growth in crystalline rock are the stress state, the chemical environment, temperature and the microstructure of the rock.
There are several theoretical models for the analysis of creep and cyclic fatigue: deformation diagrams, rheological models, thermodynamic models, reaction rate models, stochastic models, damage models and a time-dependent safety factor model. They are defective in describing the three-axial stress condition and strength criteria. In addition, the required parameters are often too difficult to determine with adequate accuracy. Therefore these models are seldom applied in practice.
The effect of microcrack-driven creep on the stability of the work shaft, the emplacement tunnel and the capsulation hole of a proposed nuclear waste repository was studied using a numerical model developed by Atomic Energy of Canada Ltd. According to the model, the microcrack driven creep progresses very slowly in good quality rock. Poor rock quality may accelerate the creep rate. The model is very sensitive to the properties of the rock and the secondary stress state. The results show that creep causes no stability problems on excavations in good rock. The results overestimate the effect of the creep, because the analysis omitted the effect of support structures and backfilling.