428 Questions 8.1-8.10: rock masses
48.5 To study the effect of a fracture on the rock strength, plot a graph
of (01 - 03) vs. BW using the single plane of weakness formula included
in Section 8.1. Assume u3 = 10 MPa, c = 0 and & = 35", and that the
intact rock strength is given by o1 = 75 + 5.2903. Also, explain the form
of the resulting graph.
48.6 If a rock mass contains more than one fracture set, we can apply
the single plane of weakness theory to each set, and superimpose the
results to find a lowest-bound envelope of strength.
(a) Plot the 2-D variation in strength for a rock mass containing
two orthogonal sets of fractures, A and B, the strengths of which are
CA = 100 kPa, @A = 20" and CB = 0", = 35", when the minor principal
stress has the value 10 MPa. The intact rock strength is again given by
(b) How would this strength variation change if the minor principal01 = 75 + 5.29aj.
stress were reduced to zero?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 45", the shear strengths of which are given
by a linear Mohr-Coulomb criterion with c =^100 kPa and 4w = 30". The
intact rock strength is given by 01 = 75 + 5.2903. Assume that the minor
principal stress, 03, is 10 MPa.48.8 How does the significant effect of fractures on the rock mass
strength indicate some of the differences between rock mechanics and
soil mechanics?48.9 The peak strength of a poor-quality, closely jointed and weathered
granite may be represented by the Hoek-Brown criterion,01 = a3 + ,/rnoca3 + so:, with m = 1.3 and s = 0.00001.
The uniaxial compressive strength of the intact rock material is estim-
ated as o, =^40 MPa, and the residual strength of the fractures is given(a) Plot, in t-a space, the expected peak shear strength envelopes for
the intact rock material, a fracture at residual strength, and the jointed
rock mass for normal stresses up to 10 MPa.
(b) The Hoek-Brown criterion is to be used for the analysis of potential
circular slopes cut in the rock mass. Determine values of instantaneous
cohesion and friction angle for normal stresses up to 10 MPa.48.10 To study the influence of fracture persistence, consider a rock
mass containing two fracture sets, A and B, mutually inclined at 45",
as shown to the right. Fracture set A is continuous and contains clay
infilling, while fracture set B is rough, clean and intermittent with a 'two-by C, = 0, +r = 15".