24722484 Education Drilling and Blasting Docs143

7/28/2019 24722484 Education Drilling and Blasting Docs143

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Drillability of a rock depends on

hardness of its constituentminerals.

grain sizecoarse-grained structure is easier todrill

crystal form. minerals with highsymmetry

abrasive


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Rocks are classified into threemain groups

Igneous or magmatic rocks(formed from solidified lava or "magma").

Sedimentary rocks (formed by

deposition of broken material or bychemical precipitation).

Metamorphic rocks (formed by thetransformation of igneous or sedimentary

rocks, in most cases by an increase inpressure andheat).


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Igneous rock

Depending on whether the magmasolidifies at depth, or as a dyke rock,or on the surface, the rock is givendifferent names even if thecomposition is the same.

plutonic rock: magma solidifies deep

down in the earth's crust. dyke rock: magma rises towards thesurface.

volcanic rock: on the surface.


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Sedimentary rocks

deposition of material bymechanical or chemical actionand a consolidation of this

material under the pressure ofoverlying layers.

also arise as a result of the

chemical precipitation ofminerals or by proliferation oforganic organisms


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Rock classification


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Rock Strength


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Fracture Mechanics

strength of brittle materials is theirability to break by crackpropagation.

tensile strength of these materials isso much lower than the compressivestrength.

stress concentration in front of the

crack tip stress intensity factor KI is a

function of the crack length and the

load o


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The critical value of KI when the crack just startsmoving is amaterial constant KIC.

The work done per unit new cracksurface is GIC which is coupled to KIC through therelation:


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Abrasiveness

The factors that enhance abrasive capacities ofrocks are the following: The hardness of the grains of the rock. The rocks

that contain quartz grains are highly abrasive. The shape of the grains. Those that are angular are

more abrasive

than the round ones. The size of the grains. The porosity of the rock. It gives rough contact

surfaces with localstress concentrations.

The heterogeneity. Poly mineral rocks, althoughthese are equally hard,are more abrasive because they leave rough surfaces

with hard grainsas, for example, quartz grains in a granite.

R k M St th d


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Rock Mass Strength andStructure


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Rock BreakingProcesses

Primary

b k ld


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Primary breakage processes wouldinclude the following

Impact or hammering.

Percussive drilling.

Button type cutters for raiseand tunnel borers

Disc type cutters for raiseand tunnel borers.

Drag bit.

Diamond bits.


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Theory


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Tertiary breakage processesinclude:

1. Breaking boulders by impactor mud blasting

2. Crushing 3. Milling


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sce aneous


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sce aneous

Thermal spalling

Water jets.


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24722484 Education Drilling and Blasting Docs143

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