Joint stiffness is extremely hard to judge, and getting these parameters is difficult. Because of the lack of reliable data, a good place to start could be to set the joint stiffness to be approximately 10 times the minimum modulus of the two materials on each side of the joint. The shear stiffness can be set to be approximately the same as the minimum modulus. This generally gives results that are fairly insensitive to the stiffness values.
A sensitivity analysis can then be performed for these parameters. In all cases where you are initially estimating properties, it is good practice to run sensitivity analyses on these parameters. To look at this topic in more detail, see the following references:
- Kulhawy, F. H. 1975. Stress Deformation Properties of Rock and Rock Discontinuities. Engineering Geology, Vol 9, pp. 327-350.
- Bandis, S. C., Lumsden, A. C. and Barton, N. 1983. Fundamentals of Rock Joint Deformation. Int. J. Rock Mech. Min. Sci. & Geomech. Abstr., Vol 20, No 6, pp. 249-268
- Barton, N, Bandis, S and Bakhtar, K. 1985. Strength, Deformation and Conductivity Coupling of Rock Joints. Int. J. Rock Mech. Min. Sci. & Geomech. Abstr., Vol 22, No 3, pp. 121-140.
- Barton, N.: "A model study of rock joint deformation", IJRM, #9 pp 579-602, (1972).
- chapter 12 of the recently published 4th edition of Fundamentals of Rock Mechanics by J. C. Jaeger, N. G. W. Cook and R. W. Zimmerman
See the Estimating Joint Stiffness section of the RS2 Online Help for a discussion on how to estimate stiffness from rock mass properties and joint infill properties.