Questions and answers: rock mosses 1 3170
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IOthe heavier line in the 0" to 180" range in the plot below.
h Fracture set A Fracture set B-I, ,-7
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0 45 90 135 180 225 270 315 360
Inclination of discontinuity to major principal stress axis.-
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0-4(b) To assess the effect of the minor principal stress, which can be
considered as a confining pressure, we repeat the procedure in (a) but
with the minor principal stress reduced from 10 h4Pa to 0 MPa.
Forfracture set A, the strength locus (see below) is obtained. Note the
dramatic reduction in strength in the region of Bw = (90 + 4w)/2 = 55".
This is because the rock mass, acting as two blocks and without any
confining stress, is much more susceptible to the effect of the major
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IOprincipal
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cn cn
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Ofstress.
80 TI
0 30 60 90
inclination of dlscontlnuity normal to major principal Strength locus of
stress axis fracture set A
Forfracture set B, the strength locus is shown on the next page. The lack
of the confining stress and the zero cohesion cause the strength locus to
manifest a sudden change as &, passes the cPw value of 35".
The superimposition of the two strength loci for fracture sets A and
B, together with the lower strength envelope is shown in the plot below.
Comparison with the equivalent curves for part (a) of the answer shows
that the confining stress has had a significant effect in strengthening the
rock mass to being always above 20 MPa strength.
Note that, with the exception of Bw angles close to 0", 90", 180" and 270",
the rock mass strength (the heavier line) is virtually zero. Since the peaks
at these orientations are too narrow to have any engineering significance,
the rock mass strength has been reduced to almost zero by the presence of
the two sets of fractures. This example demonstrates the sigruficant loss
of strength that occurs when fractures dissect the rock mass.