Engineering Rock Mechanics

256 Excavation principles

the table below, with the various ratings related to the specific rock
mass properties

Rock Mass Description (RMD)

Powdery or Friable ................ 10

Blocky .......................... 20

Totally massive ................... 50

Close (<0.1 m) .................... 10

Intermediate (0.1 to 1 m) ........... 20

Wide(>0.1 m). ................... 50

Joint Plane Orientation (NO)

Horizontal. ...................... 10

Dip out of face.. .................. 20

Strike normal to face. .............. 30

Dip into face ..................... 40

SGI = (2556 - 50) where SO is in tonnes/m3

H = hardness on scale 1-10

Joint Plane Spacing (JPS)

Specific Gravity Influence (SGI)

Hardness (H)

The value of the Blastability Index is given as

61 = O.S(RMD +IPS + JPO + SCI + H)

and the amount of explosive required is 0.004 BI in kg ANFO/t,

which is therefore a measure of specific energy.

(a) Calculate the specific energy that this index predicts for the

following conditions: a highly laminated, soft ferruginous shale

with horizontal to sub-horizontal bedding and BI rating values of

RMD = 15, JPS = 10, JPO = 10, SGI = 10, H = 1. With reference to

the values discussed in Q15.4, comment on the value obtained here.

(b) Comment on the advantages and disadvantages of this index.

A756 (a) The sum of the ratings is 46, so the Blastability Index BI = 23.

The explosive requirement is then 0.004 x 23 = 0.092 kg ANFO/t. We

know that 1 kg of ANFO releases 3.92 MJ, so the specific energy is

0.092 x 3.92 = 0.361 MJ/t.

However, we need this energy value in terms of cubic metres of rock,

rather than the unit mass of rock. Because the SGI value is 10 and we

know that XI = (25SG-50), the SG is 2.4 t/m3. Thus, the specific energy

= 0.361 x 2.4 = 0.867 MJ/m3.

This value is much lower than the 12.2 MJ/m3 for the blasting example

presented in Q and A15.2 and discussed in A15.4, probably because here

the intact rock (a highly laminated, soft ferruginous shale) is weak and

the rock mass is well fractured.

(b) The advantage of a blastability index of this type is that it provides

a rapid and cost-effective guide to the amount of explosive required

for excavation in a given rock mass. Blasting is a complex mechanical

process, and so some type of empirical classification approach is likely to