Rock Properties

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Rock Properties Session Headings:

Introduction

(See Summary for main points)

Blasting results are greatly influenced by properties of the intact rock and the rock mass. Fragmentation, displacement and blasting damage can, in some instances, be more influenced by the rock than by the blast design.

Intact Rock Properties

The primary intact rock properties commonly applied in blast design are:

� Elastic constants (Young's Modulus, E and Poisson's Ratio, ν)

� Strength (uniaxial compressive σc and tensile, σ

t)

� Density (ρ)

� P-Wave Velocity (Vp)

Definitions


� Young's Modulus is the ratio of axial stress to axial strain in uniaxial compression (typically expressed in GPa).

� Poisson's Ratio is the ratio of lateral to axial strain magnitudes (dimensionless).

� Compressive strength is the ratio of peak load to the cross-sectional area of a test sample in uniaxial compression (typically expressed in MPa).

� Tensile strength is the peak load per cross section area at the point of rupture in tension (typically expressed in kPa).

� The density of a rock is the specific weight (typically expressed in g/cc).

� The P-wave velocity of a rock is a measure of the compressive wave transmission velocity (typically expressed in m/s).

Rock Mass Properties

The primary rock mass properties commonly applied in blast design are:

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� the number of fractures (density);

� the orientation of fracturing (absolute and relative to the free face).

Application of Rock Properties


Rock and rock mass properties can be used to assess the following:

� blastability

� fragmentation

� blast damage

Blastability Indices

Rock and rock mass properties can be used in the development of blastability indices. A popular method was developed by Lilly (1986) for use in the northwest iron ore range in Australia. This Blastability Index uses the following variables in the calculation of blasting energy requirements:

� rock mass description (fracture density);

� joint plane spacing and orientation;

� specific gravity; and,

� hardness.

Other indices have been developed that use similar data.

Fragmentation Prediction

The Kuz-Ram fragmentation (?) equations were developed by Cunningham (1983), Cunningham (1987) to estimate fragmentation from a blast with given geologic and design variables. The Kuz-Ram calculation uses a "Rock Factor" that is based on the blastability calculation described by Lilly (1986).

This method is widely accepted and easily adaptable to calculation within a spreadsheet.

Blast Damage

The determination of strain (vibration) based damage criteria uses the relationship between induced strain and peak vibration as shown in the following equation:

where:

ε = induced strain in the rock



PPV = peak particle velocity at the point of interest (mm/s) V

p = P-wave velocity of rock (mm/s)

From Hooke's law and assuming a brittle failure mode for rock, the maximum particle velocity can be calculated.

where:

PPVmax

= maximum particle velocity before tensile failure (mm/s)

σt = uniaxial tensile strength of rock (Pa) - 1/10 to 1/15 the UCS

Vp = P-wave velocity of rock (mm/s)

E = Young's Modulus of the rock (Pa)

Many other damage predictions methods have been published that use essentially the same information to assess damage potential from blasting.

Summary


It is important to remember that blasting is rarely undertaken in a homogeneous, isotropic rock mass. In terms of achieving successful blasting, the properties of the intact rock and the rock mass may be as important as the selection of an explosive and blasting pattern.

Examples of the influence of geology on blasting results is shown in the following figures:



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Blast Design and Assessment for Surface Mines and Quarries - April 14 2002



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Rock Properties