STRAIN PARAMETERS OF DISCONTINUITIES IN ROCK FOR FINITE-ELEMENT CALCULATION

The bulk strain induced by excavation of rooms for storage of highly r adioactive waste is usually unimportant, but the change in aperture an d shear displacement of permeable fractures may be of practical import ance. Hence, the normal and shear strain of such discontinuities have to be considered and are commonly predicted by using special deformati on moduli termed ''normal'' and ''shear'' stiffnesses in numerical cal culations. For the use of some finite element methods, these have to b e converted to compression and shear moduli, which, in turn, requires that the thickness of the discontinuity is known. The normal and shear strain can be expressed in terms of soil mechanical parameters, yield ing the compression modulus M and the shear modulus G, with the requir ed form for finite element calculations. By definition they are functi ons of the normal and shear stiffness K-n and K-s. It is concluded tha t calculation of the normal and shear strain of discontinuities with e lastic fillings by the use of finite element methods and deformation m oduli derived from stiffness numbers is very uncertain except when the geometry of the weaknesses can be accurately defined. (C) 1998 Elsevi er Science B.V. All rights reserved.