Questions and answers: rock masses 127Diagram 1 - one fracture set.
There will be a variation in the modulus of fractured rock masses
as a function of the loading direction (except for the case of a rock
mass with two orthogonal sets with equal normal and shear fracture
stiffnesses, which has the same modulus regardless of the loading
direction, the k = 1 plot in Diagram 2 overleaf).
The directions associated with maximal and minimal rock mass mod-
ulus are unlikely to be perpendicular, especially when there is more
than one fracture set.
These principles should be remembered when estimating the deform-
ation modulus- by any method, or attempting to understand the likely
deformation behaviour of a fractured rock mass48.4 A rock mass has the following characteristics: the compressive
strength of the intact rock is 80 MPa, the RMR (Rock Mass Rating) is
62Y0, and the GSI (Geological Strength Index) is 50. Estimate the in
situ deformation modulus, E,.
A8.4 Empirical formulae for estimating E, are required to answer this
question (as found in the references in Footnotes 2 and 4 in this chapter).