Rocks & Minerals

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Rocks  &  Minerals

INTRODUCTION 

The construction of a borehole requires the removal of soil or rock from the crust of the earth so that a stable or stabilized void remains. Many different methods of drilling and several different techniques are used in the construction of boreholes and a sound understanding of the principals of operation of all methods, is important to the driller.

THE  REASONS  FOR DRI LLI NG   HOLES

Boreholes  are  drilled  for  a  number  of  purposes,  these  can  be summarized as follows

Geotechnical holes
Soil sampling
Cavity investigation
Permeability testing
Post holes for electricity and telephone poles
Foundation holes
Anchor holes
Blast holes surface
Blast holes underground
Drainage holes
Ventilation holes
Holes for pipes and cables
Raisebore holes – for shafts, ore passes etc
Tunnel boring

 

ROCKS   AND   MINERALS

Rocks are aggregates of minerals and are constantly changing through a host of geological processes. The rocks in the earth’s crust, fall into three main categories according to the way in which they were formed. The three main rock types are as follows:

IGNEOUS

Molten magma from the earth’s core rises through fissures in the crust and  cools  and  crystallizes  into  igneous  rocks.

If  these  are  deep underground, they are called plutonic and the resulting rock is usually medium to course grained. The rising magma forces its way into horizontal layers in the rock and forms silIs or into vertical fissures where it solidifies into dykes. Very often, the molten rock already contains largish crystals and the remaining rock solidifies quickly in the dyke  or  sill  and  the  resulting  fine  grained  ground  mass  with  large crystals is called a porphyry.

In cases such as these, the flows of magma are called intrusive

In cases where the magma reaches the earth’s surface and flows onto surface, such flows are called extrusive.

These extrusions generally cool very rapidly and so are fine grained.

Plutonic rocks are generally medium hard to hard and are relatively easy to drill with percussive methods.

HARDNESS AND ABRASIVENESS OF SOME COMMON IGNEOUS ROCKS- Table 3.1

SEDIMENTARY   ROCKS

Sedimentary rocks are made up of other rocks that have been broken down by weathering. Weathering of rocks can take place by mechanical or chemical means.  Mechanical weathering results in fragmental or clastic rocks, and the chemical composition of the minerals in the rocks does not alter.  Chemical weathering on the other hand, alters the rock chemistry and often masks identification of the mother rock. Coals and shales are formed organically and therefore, strictly speaking,  are not rocks.  However, they are very important parts of the earth’s crust and are usually bedded in strict layers like sedimentary rocks .Rock  fragments  formed  by  mechanical  weathering  are  transported away and deposited, usually by water, as horizontal layers.  Sorting in this deposition process is usually by size and weight of the particles. Heavier or larger fragments are dumped first and form conglomerates or breccia. Lighter sediments are carried further and are sorted according to grain size, with the finest deposits forming sandstone, fine quartz particles and shale, fine clay particles.  Table 3.2 below shows the classification of Sedimentary rocks

Sedimentary rocks are generally weak and are easily drilled with percussive or rotary drills. Grain size is a fundamental feature of sedimentary rocks and a primary sub-division is made between coarse grained, rudaceous, medium  grained,  arenaceous  and  fine  grained, argillaceous, rocks. Table 3.3 shows the relative hardness and abrasiveness of some common sedimentary rocks.

TABLE 3.2

CLASSIFICATION OF SEDIMENTARY ROCKS

RELATIVE HARDNESS & ABRASIVENESS OF SOME SEDIMENTARY ROCKS , Table 3.3

METAMORPHIC

Metamorphic rocks are igneous or sedimentary rocks which are transformed, or metamorphosed into new rocks by the action of temperature, pressure or chemical  activity.     Metamorphic rocks  are classified into three broad categories; thermal (contact), dynamic and regional metamorphism

Contact metamorphism occurs when hot magma intrudes into cold country rock.  At the contact zone, new minerals are formed by rapid heating and cooling. Sedimentary rocks can re-crystallize eg. pure limestone becomes marble and sandstone becomes quartzite

Dynamic metamorphism causes mechanical changes to rocks through pressure, eg. mudstone and shale is transformed into slate. Growth of new minerals is relatively insignificant.

Regional metamorphism occurs over wide areas and is caused by intense pressures and high temperatures. This results in banded or laminated rocks of which gneiss and schists are typical.

Metamorphic rocks are generally medium hard to hard or very hard and are often very abrasive, Table 3.4

RELATIVE HARDNESS AND ABRASIVENESS OF COMMON METAMORPHIC ROCKS

TECHNICAL PROPERTIES  OF  ROCK MATERIAL

The important engineering properties of rock material, which have an overall effect on rock drilling are:

hardness

abrasiveness texture

structure

breaking characteristics

HARDNESS

Hardness is the resistance of a smooth plane surface to abrasion.  It is often used as a measure of the engineering properties of rock material, and can be classified in several ways.  To the engineer who is trying to quantify rock behavior, a rock’s hardness indicates how much stress is necessary to cause failure within the rock  (i.e. the rock breaks).  Table3.5 shows degree of hardness as a function of Moh’s hardness and uniaxial  compressive  strength.       test  assigns  numbers  to  different minerals to indicate their relative hardness.  In moh’s scale, a mineral will scratch all those with a lower rating

ABRASIVNESS

Abrasiveness  is  a  time  dependent  parameter  for  drill  bit  wear;  it depends on the mineral composition of  the rock. The quartz content of the rock is usually considered as a reliable indicator of drill steel wear. (see table 3.6

TEXTURE

Texture refers to the grain structure of the rock and can be classified by such properties as porosity, looseness, density and grain size.Al of these have a definite relationship to driling speed

STRUCTURE

Structural properties sich as faults, joints, bedding planes and rock type contacts, dip and strike all influence the structural strength of the rock material and therefore affect drill hole straightness and drill bit penetration

BREAKING CHARACTERISTICS

Breaking characteristics describe rock behavior when struck with a hammer: each rock type has a typical manner and degre of breakage related to its texture, mineral composition and structure.

Breaking characteristivcs of different rock types can be described in terms of the “Los Angeles” co- efficient, which is a relative measure for determining the resistance of rock to crushing

STRUCTURAL GEOLOGY

Due to Deposition

With the formation of sedimentary layers, material can be differentiated one from another both laterally and vertically. Layering is characteristic of sedimentary rocks and the surface between any two strata is known as a bedding plane

Due to Displacement

A rock mass is seldom seen to be lying horizontally. Invariably it has been displaced and come to form a variety of structures. Here are some terms to describe them

Dip and Strike

Dip is the maximum inclination of the structure, measured from the horizontal in degrees. If some other inclination is measured, then this is termed the apparent dip. Strike is the direction at right angles to the direction of the dip and joins points of equal elevation

Folds

There  are  two  types  of  folds,  anticlines  and  synclines.  An anticline arcs upwards while a syncline is trough shaped

Faults

If  sufficiently    high    pressures      and/or suddenly       applied pressures, then the rock fractures. These fractures are referred to as faults. The simplest form of a fault is termed the normal or tensional fault. This fault is formed under tension which causes one side of the block of ground to subside under gravity. With a normal fault, there is a loss of ground. A fault that is formed due to compressional forces is called a reverse or thrust fault and is characterised by a gain of ground

ROCK   PROPERTIES

It is essential that certain fundamental physical characteristics are understood in order to distinguish between different rock masses and their effect on blasting.

Rock is generally considered to have elastically i.e. if the load is gradually increased, the rock will deform in a linear manner. If the load is removed, it will return to it’s original dimension

The terms used to describe the loading and deformation characteristics of  different  rock  types  are  stress,  strain,  Young’s  modulus  and Poisson’s ratio. Other properties are the rock’s uniaxial compressive strength (UCS) and uniaxial tensile strength (UTS).

STRESS

When a force is applied over an area of a material then the stress is defined as:-

Stress = Force / Area  (Pascal)

The units are Newton’s per square metre (N/m2 = 1 Pa)

STRAIN

When subjected to stress, a body becomes deformed. The deformation is  quantified  by  the  term  strain  which  is  calculated  by  dividing  the change in length, in a particular direction, by the original length in that direction. Two particular terms used to refer to strain is longitudinal and transverse strain. Longitudinal is the change in length and transverse is the change in width. Note, strain is a dimensionless quantity, it has no units.

 

POISSON’S RATIO

Poisson’s ratio is defined as the ratio of lateral strain to axial strain, for an  elastic  material  in  uniaxial  compression  or  tension. The  static Poisson’s Ratio for any rock type can be determined from unconfined compressive strength tests.

Poisson’s ratio is calculated as follows:-

Poisson’s Ratio = Transverse Strain/Longitudinal Strain

Poisson’s ratio values normally lie between 0.15 and 0.35 and can never exceed 0.5.  Lower values are usually associated with the higher strength, stiffer rocks and vice versa. High values indicate “rubbery” rock, which can be very difficult to break with explosives.

 

YOUNG’S MODULUS

Young’s modulus is the ratio of the vertical stress to the longitudinal strain and is a measure of the stiffness of a rock.  It is thus a measure of the deformation which rock can sustain before failure.  For a given applied stress, rocks with a low Young’s Modulus will deform more than a “stiffer” rock with a high Young’s Modulus.

The static Young’s Modulus for any rock type can be determined from a static uniaxial unconfined compressive strength test on a sample of the rock.

The dynamic Young’s Modulus can be determined from a dynamic sonic velocity measurement on a rock sample, or a seismic survey on the rock mass in-situ.  These sonic velocity measurements determine longitudinal (P-wave) and shear (S-Wave) velocities, which enables the dynamic Young’s Modulus to be calculated.

Young’s Modulus = Stress/Strain (Pascal)

UNIAXIAL COMPRESSIVE STRENGTH (UCS)

The UCS of a particular rock is the stress that causes an unconfined rock to fail in compression when loaded longitudinally.  These units of UCS are Pascal’s.

When   explosives   are    detonated   inside   a    blasthole,   an   intense compressive stress wave is transmitted into the surrounding rock mass. If the peak stress of this wave exceeds the rock’s dynamic compressive strength, a relatively small zone of intensity crushed rock is formed. This crushing is caused by collapse of intercrystalline or intergrain structure within the intact rock substance.

Intense crushing of rock around the blasthole will cause rapid attenuation of energy in the compressive stress wave.  In porous rocks, up to one third of the energy in the initial stress wave can be absorbed by work done within a distance of two blasthole diameters from the original blasthole wall.  Thus it is possible to waste effective energy by using explosives which can generate peak compressive stresses much greater than the strength of the rock to be blasted.  This indicates the importance of selecting explosives with suitable energy characteristics.

The static compressive breaking strength of intact rock can be determined from unconfined uniaxial compressive strength tests.  The dynamic compressive strength, which is relevant when stresses are applied very rapidly, can be up to ten times the static compressive strength of intact rock, but the static value is much easier to determine.

In rocks with a low compressive strength (e.g. 40 MPa for a weakly cemented sandstone) a low density explosive with a low VOD (and hence a lower peak pressure) may produce effective results, In these weak rocks, a high density explosive with a high VOD would be inefficient, as its high peak pressure would tend to waste energy by intense crushing of the rock immediately around the blasthole.                                              Fully coupled charges of dense explosives with a high VOD are best suited to rocks with high compressive strength.  Thus stronger rocks generally require explosives which produce more shock energy.

UNIAXIAL TENSILE STRENGTH (UTS)

The UTS of a rock is the tensile stress that causes a rock to fail in tension.  The units of UTS are Pascal’s.  The compressive strength of intact rock is typically 10 to 12 times greater than the tensile strength. Tensile failure of rock is thus the mechanism by which most new effective fractures are created during blasting.

After explosives are detonated in a blasthole, tensile stresses are produced in the rock as the compressive stress wave expands.  When these tangential tensile stresses (hoop stress waves) exceed the dynamic tensile strength of the rock, radial tensile cracks are created. A zone of dense radial fractures is thus formed immediately around the blasthole.  The number and length of radial cracks will increase as peak blasthole pressures increase  and     tensile  strength of the  rock decreases.

The inner zone of intense radial fractures is surrounded by an outer zone  of  more  widely  spaced  radial  cracks. These  cracks  are extensions of some of the inner fractures.  Extension of these cracks occurs because of penetration of explosion gases, after the tensile stresses in the rock have fallen below the tensile strength of the rock.

Tensile cracks may be terminated if they intersect pre-existing cracks or discontinuities.

When the compressive stress wave strikes a free face, or an open discontinuity within the rock, a reflected tensile wave is generated.  If the peak tensile stress of this reflected wave exceeds the tensile strength of the rock, “spalling” of the rock occurs progressively from the free face back towards the blasthole

The UTS of a rock is determined in a laboratory by means of the Brazilian (disc) test, where a disc of the rock is compressed diametrically.  This induces a tensile stress between the loading points.

It must be realised that the rock properties determined in the laboratory are the static properties whereas, in blasting it is the dynamic properties that are important and these can be higher than the static properties.  In order to take this into account, an Ultra Sonic Rock Tester can be used to determine Young’s Modulus and Poisson’s Ratio.           This piece of equipment measures the velocity of the P and S waves, where P and S are  the  compressional  and  shear  waves  respectively,  in  the  rock

sample   from    which   Young’s   Modulus   and   Poisson’s   Ratio   are determined using the following equations:-

Where:-

E   =    Young’s Modules n =          Poisson’s Ration Vp  =        P wave velocity Vs   =  S wave velocity

r     =    density

To approximate the UCS of a rock sample the Schmidt Hammer can be used.  With this instrument 20 readings are taken and using a graph supplied by the manufacture an estimate of the Young’s Modulus, Poisson’s Ration and the UCS can be obtained.

With both the Ultra Sonic Rock Tester and the Schmidt Hammer the density of the rock is an important factor and should be determined with care

Determining the rock properties in a laboratory is time consuming and expensive,  whereas  the  Ultra  Sonic  rock  tester  and  the  Schmidt hammer are quick and inexpensive, mainly due to the fact that the testing can be carried out on unprepared pieces of rock

INTERNAL FRICTION AND POROSITY

Strain waves travelling through  intact  rock    are      attenuated     as mechanical energy is converted to heat.   This occurs as a result of several mechanisms, which are collectively called internal friction.  This internal damping capacity is a relative measure of a rock’s ability to attenuate stress waves.  Internal friction varies considerably with rock type, and sedimentary strata generally have higher internal damping than igneous and metamorphic rocks.      Internal friction increases with porosity,  permeability  and  jointing  of  the  rock  mass.      Attenuation increases considerably across a fault or shear zone. Internal friction values in the direction parallel to bedding may be only 50% of those normal bedding planes.

Internal friction is usually determined by testing intact rock samples in the  laboratory.     In-situ  measurements  provide  a  more  accurate indication of a rock’s response to blasting, but are not frequently carried out.

Calculations indicate that the lengths of stress wave-induced cracks in a highly-porous rock are only about 25% of those in a non-porous rock of identical mineralogy.  The work of fragmenting highly porous rocks is thus performed almost entirely  by  gas  energy. Consequently,  it is particularly important to retain explosion gases at high pressures until they have completed all work of which they are capable.

Soft, porous rocks have high internal friction and tend to result in plastic deformation of the blasthole wall.    This results in a very high rate of dissipation of stress wave energy.  Little energy is carried beyond the immediate vicinity of the blasthole, and breakage within the bulk of the rock is limited.     In dry, coarse grained sandstones which are weakly cemented, blastholes tend to chamber because of the very high dissipation of stress wave energy in expanding the borehole wall and crushing the rock in the immediate vicinity of the charge

DENSITY

The  density and the strength  of  a  rock  are  normally  correlated.  In general, low density rocks can be most easily deformed and require relatively low explosives energy input to fragment. On the other hand, dense rocks   require           high    energy            input   to         create the      required fragmentation.  This  would  also  correlate  with  heave/displacement where the type of explosives used will have varied effects

WATER CONTENT

The water present in rock masses influences the type of explosives required as well as the number and size of initiators required per blasthole.   Multiple priming may be required to ensure reliable performance if ground water is flowing dynamically

In most rocks, water saturation increases the velocity of propagation and reduces the attenuation of blast-generated stress waves, because the pores and structures are filled with water.  As water is essentially incompressible,            its        presence       promotes        heave by        reducing        the expansion of gases into pre-existing cracks and voids in the rock mass. The advantage is partially offset because of the slight increase in net density resulting from water saturation

These considerations demonstrate the importance of groundwater, and indicate that it may have a significant effect on blasting performance and costs

Rock Testing Methods

The physical properties of a particular rock are generally determined in a laboratory using rock core which is loaded in a press.  At the start of the test the length and diameter of the core are measured.  The core is then placed in the loading machine and the load is gradually increased. As the load is increased the load and the corresponding deformation are recorded

GENERAL

The properties of rocks, unlike man made materials, can vary considerably. It must also be realised that when the properties of a particular rock type, from a particular location, have been determined in a laboratory, using a number of small samples, the effect of jointing, bedding planes, fractures etc have not been taken into account. As such the properties that are determined in the laboratory are those of the specific sample of rock and not of the rock mass as a whole.  This is why it is important when conducting an on site investigation that the frequency and relative direction of jointing should be observed.

ROCK   MASS

INTRODUCTION

The nature and diversity of the rock mass significantly influence the design and results of blasts.  Layers of different materials, weathering, bedding plane partings, joints, faults, voids and other discontinuities can present considerable challenges in blast design.  In some cases, such features allow much of the explosion energy to be wastefully dissipated rather than performing the work intended

DIVERSITY

Layers of different rock types can vary in thickness from millimetres to tens of metres vertically, and from a few tens of metres to hundreds of metres  along  strike  and  down  dip.                                                                        Even  within  a  small  area,  a particular rock, despite mineralogical similarities, may have widely different physical properties depending on the degree of weathering, depth of burial, and extent of deformation.  Relevant rock properties may be determined from laboratory tests, but samples may be required from several locations in a given area if meaningful results are to be obtained.

DISCONTINUITIES

All rocks contain geological discontinuities such as bedding plane partings and joints which are opened up either directly by the explosion gases or indirectly by ground movement during a blast.

These discontinuities can also prevent the propagation of blast generated fractures.Blast-induced cracks will only effectively cross discontinuities which are close to the explosive charge and virtually closed or filled.  The propagation of blast-generated cracks is usually arrested at discontinuities which are more distant or open.

Pre-existing structures such as joints and bedding plane partings tend to dominate fragmentation mechanisms.  Blast-induced overbreak and any related increase in instability potential are also governed largely by rock structure present.

JOINT SPACING

For more massive rock formations with few natural fractures, a larger percentage of the surface area of the fragmented rock needs to be created  by  explosion-generated  strain  waves. This  requires  an increase in explosives shock energy, which can be provided by dense explosives with a high VOD, fully coupled to the blasthole.

In rock that consists of large pre-formed blocks defined by joints, satisfactory breakage may be achieved if most of the blocks are intersected by a charged section of blasthole.  The mean size of blocks tends to dictate the dimensions of the blasthole pattern and, hence, the blasthole diameter.

Where the face is at right angles to well-defined joints, good fragmentation may require that a high percentage of blocks have a blasthole  within  them.   Because  stress  wave  energy  may  not  be effectively transmitted across the joints, and each block can move forward independently, blocks which do not contain a blasthole tend to be poorly fragmented and may remain in their original positions.  If the blocks of rock between adjacent joints are too large to be handled by the digging or hauling equipment, blasthole spacing may need to be reduced to less than the average spacing between joints

In highly structured rock, good fragmentation is controlled almost totally by the numerous discontinuities.         Gas energy is important in highly structured rocks as the explosion gases jet into, wedge open and extends pre-existing cracks.  Overall the degree of fragmentation tends to be structurally controlled, as closely spaced joints and bedding plane partings promote fragmentation.  As  the  rock  mass  becomes  more highly fissured, longer stemming columns and correspondingly lower powder factors can be used.      Explosives with a high VOD are not needed to initiate new fractures, and satisfactory results can be achieved as long as charges provide sufficient gas energy to displace the rock into a loose diggable muckpile

At air-filled cracks, water-filled cracks and zones of moderate density change in the burden rock, some stress wave energy is reflected and attenuated.  The aperture of tight joints parallel to a blasthole may be too small to cause appreciable reflection of the stress wave.  Where an open air-filled joint is approximately parallel to a blasthole, it does not close fully on passage of the compressive stress wave.  Thus, the joint partially  reflects  the  energy  as  a  tensile  stress  wave. If  this  is sufficiently strong, internal spalling occurs.  The radial cracks which the strain wave would have formed in a massive rock are prematurely interrupted by the joint.  Attenuation of the outgoing compressive stress wave may be so great that, after reflection at the free face, it fails to extend suitably oriented radial cracks.  This gives better fragmentation between the blasthole and the joint,  but tends to reduce breakage beyond the joint

The same effect is also observed where the joint is filled with a material which is less effective at transmitting energy than the intact rock

JOINT ORIENTATION

The orientation of discontinuities will determine their effect on blast performance. Whenever  an  open  discontinuity  is  intersected  by  a blasthole, some explosion gases escape through the structure without performing useful work.  If a major joint intersects the charged section of a blasthole, it permits high pressure gas venting which causes the joint to expand preferentially.  The loss of gas into the joint causes a rapid fall in blasthole pressure, and thus a reduction in rock breakage and heave.  This effect is most critical where persistent open joints or bedding planes extend from the blasthole wall to the face and/or top of the bench, providing a path for explosion gases to vent directly to atmosphere.

References:

  1. South African  Geology  for  Mining,  Metallurgical,  Hydrological and  Civil  Engineering,  Jos  Lurie,  Lupon  Publishing  ISBN-0-620277-97-1

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