258 Excavation principlesAIS7 The table is as follows.
Intact Rock Properties Fracture Properties Rock Mass Properties
(1) Strength (UCS)
(2) Resistance to fracturing (ot)
(3) Sturdiness (p)
(4) EIasticity (E)
(5) Resistance to dynamic
loading (V,)
(6) Hardness (SHV)
(7) Deformability (u)
(8) Resistance to breaking (IC1,)(12) Strength (c, 4) (9) In situ block size (2)
(10) Fragility (D)
(11) Integrity (R,)In their article, Latham and Lu Ping (1999) point out that their work
is related to “uncontrollable factors governed by in situ geological con-
ditions and the term ’blastability’ has been deliberately restricted to
quantify this intrinsic resistance of the rock mass.” In this scheme,
the emphasis is on the intact rock properties, which we would expect
because the purpose of blasting is to reduce the natural block size
distribution to the required fragment size distribution.
Note that parameters 3, 4, 5 and 7 are intact rock properties not
directly related to failure, but are included because they characterize the
quality of the rock. Parameter 6 is more strongly correlated with the
failure properties, and parameters 1,2 and 8 are direct measurements of
rock strength.
In terms of the pre-existing fracturing in the rock mass, parameters 9,
10 and 11 are indicators of the degree of fracturing present in the rock
mass, but the only parameter explicitly representing the fracture failure
properties is parameter 12, the Mohr-Coulomb values.
In their article, Latham and Lu mention that ”one can include
more, such as the discontinuity orientation related to the face to be
blasted.. .water content in the rock mass, joint aperture and various
other parameters”. Let us consider then whether the following subjects- already discussed in this book - should be included in the BD index:
the in situ rock stress, the nature of intact rock failure (as brittle or
ductile), the number of joint sets, rock mass permeability, anisotropy and
inhomogeneity, other rock mass classifications and specific energy. The
rock stress is not usually a factor on exposed rock faces. The nature of
rock failure is indirectly included because it is correlated with the other
properties. Perhaps the number of joint sets could have been included,
but the information is indirectly included in parameters 9-11. Permeab-
ility is relevant because it is related to the openness of the rock structure
and the way in which the gas travels through the rock mass during
blasting, but is difficult to include. Anisotropy and inhomogeneity are
probably second-order factors here. The rock mass classification values,
RMR and Q, could be included but the classifications were developed for
tunnel support. And, finally, the specific energy is not an intrinsic prop-
erty of the rock mass. Thus, it would not be easy to add any of these to
the blastability index, except possibly the RMR and Q values, but much