Rocks & Minerals
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
Post holes for electricity and telephone poles
Blast holes surface
Blast holes underground
Holes for pipes and cables
Raisebore holes – for shafts, ore passes etc
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:
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 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.
CLASSIFICATION OF SEDIMENTARY ROCKS
RELATIVE HARDNESS & ABRASIVENESS OF SOME SEDIMENTARY ROCKS , Table 3.3
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 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
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 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
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 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
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
There are two types of folds, anticlines and synclines. An anticline arcs upwards while a syncline is trough shaped
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
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).
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)
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 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 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:-
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
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
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
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.
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
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.
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.
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
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.
- South African Geology for Mining, Metallurgical, Hydrological and Civil Engineering, Jos Lurie, Lupon Publishing ISBN-0-620277-97-1
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