Engineering Rock Mechanics

Questions and answers: excavation principles 257

be the most practical engineering method. The disudvuntuge of this spe-
cific index is that it is a rather coarse index. The Rock Mass Description
and Joint Plane Spacing could have an increased number of categories:
there is a ratings jump from 20 to 50 for both these properties, which
is unnecessarily abrupt. The Hardness does not seem to have a high
enough rating (assuming that this is the parameter used to represent the
rock strength).
However, this BI may well be a useful tool even though many rock
properties have been omitted in the index. Rock properties tend to be
related and so the effects of omitted properties may be included to some
degree, albeit indirectly. If the essence of the mechanical problem is
captured (as with the rock mass classifications discussed in Chapter 12),
the index will help with blasting assessments in rock engineering.

415.7 The rock engineering systems (RES) approach has been used
to develop a comprehensive Blastability Designation (BD) index for
rock masses (Latham and lu Ping, 19994), the purpose of which
is to provide a quantitative assessment of the resistance of a rock
mass to blasting. The index value is given as BD = Cy', WlRi where
Wi is a parameter weighting value, Ri is the parameter rating, with
the summation over the i parameters. Both the Wi and Ri values
are evaluated for the rock mass under consideration. The greater
the value of BO, the more difficult the rock is to blast. The twelve
parameters used in the BD index are:

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

strength (uniaxial compressive strength, UCS, MPa,

also via Point load Index);

resistance to fracturing (uniaxial tensile strength, et, MPa);

sturdiness of the rock (density, p, t/m3);

elasticity of rock (Young's modulus, E, GPa);

resistance of rock to dynamic loading

(P-wave velocity, V,,, km/s);

hardness of rock

(Schmidt rebound hardness value, SHV, rebound height scale);

deformability (Poisson's ratio, u, dimensionless);

resistance of rock to breaking

(fracture toughness, KlC, MPa-m'/2);

in situ block sizes (mean of block size distribution, mean);

fragility of rock mass (fractal dimension of rock block sizes, D);

integrity of rock mass

(ratio of field: lab P-wave velocities, R,, dimensionless);

fracture nlane's strength

(cohesion, c, MPa andfriction angle, 4, degrees).

Compile a table of these twelve parameters in three columns with

headings Intact Rock Properties, Fracture Properties and Rock Mass

Properties, and then comment on how well the rock mass has been

cha racterized.

4Latham J.-P. and Lu Ping (1999) Development of an assessment system for the

blastability of rock masses. Int. J. Rock Mech. Min. Sn'., 36,41-55.