Engineering Rock Mechanics

132 Rock mosses: deformobilw, strength ond failure

h^80

0

& 70-

60.-

5 50-

40.-

Ul

L L

u)

2 30

0 20-

10-

0-

Y

0

.-

-4

stress axis

Fracture set A Fracture set B

i

11 /

! 180

Strength locus of

fracture set B

1

j !I :.,

• 225 270 315 360
  / / ,/ / Inclination of discontinuity to major principal stress axis
  Locus of rock mass strength

48.7 When a rock mass contains a large number of fracture sets

and each set has similar strength properties, the rock mass strength

can tend to become isotropic, with the multiple plane of weakness

theory generating an approximately isotropic strength criterion.

Develop such a criterion for the 2-D case of a rock mass that

contains four sets of fractures mutually inclined at 459 the shear

strengths of which are given by a linear Mohr-Coulomb criterion

with c = 100 kPa and cpw = 30°. The intact rock strength is given

by (11 = 75 + 5.29~3. Assume that the minor principal stress, u3, is

10 MPa.

A8.7 Each fracture set has strength parameters of c = 100 kPa and
& = 30". The rock mass strength variation as a function of maximum
principal stress orientation for one such fracture set is shown below.
To investigate the strength of the rock mass, we need to examine
the case for the four fracture sets with relative orientations of O", f45"
and 90". Superimposing four of the curves while applying the necessary
offsets, and examining the range of /Iw for 0-360", we obtain the follow-