Engineering Rock Mechanics

Questions and answers: rock masses 133

140-

3 120--

5 loo--

= 2 80

% 60

40

w

x E^20

80

-- = (90 + $w)/2 = 60'

--

--

--

I

ing diagram, with the heavier line indicating the composite rock mass

strength.

140.0 7 -- .... - .. -

120.0

([I 100.0

h

80.0

2 60.0

5

G- m

c

c m

40.0

20.0

0.0 71- - -

0 30 60 90 120 150 180 210 240 270 300 330 360

orientation, degrees

The increasing influence of 1,2,3 or 4 such fracture sets being present
in the rock mass can be studied by introducing the sets one by one,
i.e. as in the polar diagrams on the next page, with fracture sets being
introduced progressively with offsets of o", 45", 90" and 135".
Note that the rock mass strength tends to become more isotropic and
that the mean rock mass strength decreases with an increasing number
of fracture sets. Here we have assumed that the four fracture sets all have
the same values of cohesion and friction angle, but the same tendencies
occur for the composite effect of fracture sets with different geometrical
and mechanical properties.

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?

A8.8 Rock is a strong solid material containing fractures which are often
of a similar size to the engineering structure. Soil is a weak particulate
material with grain sizes much smaller than the engineering structure.
Rock failure is often initiated by weaknesses in the rock mass and, as
has been seen by the answers to previous questions in this section, the