Engineering Rock Mechanics

462 Questions 15.1-75.10: excavation principles

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

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

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

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

SGI = (25SG - 50) where SG is in tonnes/m3

H = hardness on scale 1-10

Joint Plane Spacing UPS)

Joint Plane Orientation (JPO)

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

Specific Gravity Influence (SGI)

Hardness (H)

The value of the Blastability Index is given as

BI = 0.5 (RMD + JPS + JPO + SGI + 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 follow-

ing 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.

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, 1999), 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 = WjRj where Wj is a parameter

weighting value, Rj is the parameter rating, with the summation over

the j parameters. Both the Wj and Rj values are evaluated for the rock

mass under consideration. The greater the value of BD, the more difficult

the rock is to blast. The twelve parameters used in the BD index are:

(1) strength (uniaxial compressive strength, UCS, MPa, also via Point

(2) resistance to fracturing (uniaxial tensile strength, a,, ma);

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

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

(5) resistance of rock to dynamic loading (P-wave velocity, V,, km/s);

(6) hardness of rock (Schmidt rebound hardness value, SHV, rebound

(7) deformability (Poisson’s ratio, u , dimensionless);

Load Index);

height scale);