7. mechanical excavation

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7. Mechanical Excavation

7.1 Mechanisms of rock cutting

7.1.1 The main types of tool

Section 2 explained briefly that the basic method of mechanical rock breakage is impact andpenetration by a wedge shaped tool. Table 7.1 gives the various classifications of cutting tool.

Table 7.1: Classifications of cutting toolType of tool Mode of action Specific tool types Machines using tool

Drag tool Applies a force

roughly parallel to

the rock surface

Diamond drill bit Rotary drilling machine

Pick

(point attack and wedge)

Roadheader

Continuous miner

Shearer

Indenter Applies a force

normal to the rocksurface

Brazed drill bit

Button drill bit

Rotary percussive drilling

machine

Tricone drill bit Rotary drilling machine

Raise borer

Disc cutter Tunnel boring machine (TBM)

Mobile minerRaise borer

The load applied by an indenter is one of compression and that applied by a drag bit is one of

shear (Hood and Roxborough 1992). However, research has indicated that both types of tool cause

the formation of large rock chips by inducing tensile fractures. The impact of an indenter crushes asmall zone of rock beneath the tool. This crushed rock dilates and induces tensile stresses in the

rock immediately surrounding the point of impact. When a drag tool hits the rock surface at anoblique angle, there is a free face towards which material can be displaced. Hence, the tool can

more easily penetrate the rock, causing shear fractures that, in turn, cause the propagation oftensile fractures as rock chips are peeled off.

Most rocks are an order of magnitude (x 10) stronger in compression than in tension and hence

any action that involves the crushing of rock will consume a large amount of energy. Because thedrag tool initiates tensile fractures in a more direct manner, with less crushing, it is more efficient

than an indenter. However, indenters are by far the most widely used type of tool; why is this? Thereason lies in the strength of the tool itself. The materials used for the cutting edge must be hard

but, because of this property, they are also brittle. The mode of action of a drag tool inducesbending, or tensile, stresses in the tool cutting edge and makes catastrophic failure of the tool more

likely. An indenter, on the other hand, is loaded mainly by a compressive force along its main axisand the material of which it is made is inherently strong in compression. (Hood and Roxborough

1992.)

7.1.2 Drag picks

a) Cutting parameters

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Drag tools used in cutting machines are calledpicks.Figure 7.1 illustrates what happens when a

drag pick impacts a lump of rock in a direction parallel to the main rock surface. A major tensilecrack is generated in the form of a curved arc, originating at the point of penetration and emerging

at the main rock surface. This fracture produces a large chip. Evans (1962) was the first researcherto propose a rock cutting theory, developed from work conducted mainly with drag picks in

British coal mines. The important variables associated with rock cutting are shown in figure 7.2.

Where: F = Force applied by toolFC = Cutting force component

FN = Normal force component acts to either penetrate the rock surface ofmaintain it at the required penetration depth

d = Penetration depths = Tool spacing

= Breakout angle

Important measures of cutting performance

Yield (Q) The volume f rock produced by cutting depends on penetration depth (d),breakout angle ( ) and distance cut

Specific Energy (SE) The work done by the cutting force (FC) to excavate unit volume of yield.

Dependent on: Rock strength and toughness

Degree of fracturing

Machine type and method of operation

Tool type and condition

Available tool forces (machine size and power)

Penetration depth

b) Tool types

There are two main types of drag pick design, radial and point attack (figure 7.3).

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Figure 7.1: Cutting action of drag

pick (Speight 1997)

Figure 7.2: Drag pick cutting variables (Speight 1997)

Figure 7.3: Two maintypes of drag pick

(Speight 1997)

a) Point attack b) Radial


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Radial pick chisel tip and rectangular shank. Axis of tool shank passes through the rotational

axis of the machine cutting head. In abrasive rocks, radial picks quickly develop a wear flat andtool performance suffers.

Point attack pick conical tip and cylindrical shank. Shank axis is inclined relative to the rock

surface. The tool is designed to rotate by the action of cutting to produce even wear and is

therefore favoured in abrasive rocks. Often, however, dirt clogs the tool, so that it cannot rotate.(Speight 1997).

7.1.3 Indenters

a) General considerations

There are a number of theories that aim to describe the process of chip formation when an indentertool hits a rock surface. Some researchers postulate that the penetration of the wedge induces shear

stresses that cause a chip to break away. These shear stresses are generated by lateral pick forces,the magnitude and direction of which are influenced by the wedge angle of the tool. Hood and

Roxborough (1992) describe a model for disc cutters developed by Lindqvist and Ranman (1980)which assumes that most of the force is directed normally into the rock, similar to the action of a

flat bottomed punch. Induced tensile stresses generate cracks that run parallel to the rock surface

(figure 7.4) and if the spacing between the cutters is sufficiently small, tensile cracks propagatingfrom each groove will join up to form a rock chip.

There is no single universally accepted theory for the formation of rock chips by indenter tools.

The main cause of rock breakage is the propagation of tensile fractures, but it is the mechanism bywhich these fractures are first initiated that is uncertain. Rock breakage takes place because a

tensile fracture is initiated and propagated. This fracture is induced as a result of combinedloading: first, the tensile extension of pre existing flaws along the fracture plane and second, by

tensile stresses induced by the crushed zone beneath the wedge. (Hood and Roxborough 1992)

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Figure 7.4: Disc cutter and cutting action


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b) Disc cutters

Cutting machines designed for hard rock applications use indenters in the form of disc cutters.These machines are the tunnel boring machine (TBM), mobile miner and raise borer. For efficient

rock cutting to take place, the tensile fractures produced by tools cutting in adjacent grooves joinup to form rock chips. Hence, a crucial aspect of cutting machine design is to optimise the spacing

between adjacent grooves cut by tools mounted on the excavation machine.

7.1.4 Comparison of drag picks and disc cutters

Disc cutters mounted on a machine such as a TBM are capable of cutting rocks of uniaxial

compressive strength (UCS) greater than 150 MPa (Speight 1997). This performance is achieved byapplying a very high thrust force (equivalent to FN for the drag pick) and a lower rolling force

(equivalent to FC for the drag pick). The lower rolling force means that the torque that has to beapplied by the machine is reduced. However, because disc cutters break rock by indentation rather

than by cutting parallel to the rock surface, they are less efficient than drag picks. In a given rock

mass, the SE required for a disc cutter machine will be higher.

7.1.5 New developments

a) Oscillating disc cutter

A new method for rock excavation has been developed at the Centre for Mining Technology and

Equipment (CMTE), Brisbane, Australia. It uses a tool called the oscillating disc cutter (ODS),which attacks the rock by undercutting it so as to induce tensile failure (CMTE 2000). Because rockis much weaker in tension than compression, a machine fitted with the ODC system does not

require the high axial forces associated with indenter cutting discs. A conventional disc cutterworking in a hard rock such as granite requires a force of about 550 kN to machine a groove 25

mm wide and 10 mm deep. In the same rock the ODC cuts a groove twice as wide at a force ofonly 12 kN. The eventual aim to design a continuous excavation machine capable of working in

rocks with of UCS > 240 MPa.

b) Penetrating cone fracture technology

Penetrating cone fracture (PCF) involves the initiation of rock fractures within shallow holes

drilled in a rock face. The fractures result in the formation of large, roughly cone shaped, rockchips with the base of the cone at the rock face. Fracturing is produced either by an explosive

propellant such as the Sunburst system or by a high pressure water pulse, e.g.Hydrex system(Speight 1997). The borehole pressures generated are much lower than with industrial explosivessuch as ANFO, ranging from about 400 MPa to 700 MPa (Olson 1993). The low intensity of the

breaking action means that the system could be incorporated in a continuous hard rock excavationmachine. Typically, the type of concept being investigated involves a mobile vehicle equipped

with a drill boom. A series of short holes are drilled and, as each hole is completed, a propellantcharge and initiating device are inserted into the hole, which is then sealed. Tests carried out by

the US Bureau of Mines (Olson 1993) used a gas injector to initiate the charge and a stemmingsystem comprising a stemming bar, energy absorbing device and mechanical seal. Each hole is

fired to break off a series of rock chips, which are then collected by a debris clearing system suchas that installed on a roadheader.

c) Plasma blasting

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The plasma blasting concept is similar to PCF, except that an electrical discharge is used to

generate a rapidly expanding plasma, which induces rock fractures. An electrode is pushed into ashort drillhole and the space between this probe and the hole walls is then filled with an electrolyte

(water is suitable). The conceptual plasma blasting machine is similar to a drill jumbo, with boomscarrying either rock drills or electrodes. Electrical energy is stored in a capacitor bank and released

when a switching device is activated. The electrical power delivered power to the electrodeincreases at the rate of 200 MW per microsecond until a peak power of 3500 MW is attained, after

about 40 microseconds. The effect of this energy discharge is to convert the electrolyte into a hightemperature high pressure plasma, at explosive rates. Indirect measurements indicate that the

drillhole pressure exceeds 2000 MPa (Nantel and Kitzinger 1990).

7.2 Influence of rock strength

The application of mechanical excavation machines is limited by rock strength. Rock cutting

machines are used for both civil engineering (mainly tunneling) and mining purposes. Although

they posses many inherent advantages over drilling and blasting, the current level of technologicaldevelopment means that their use is still highly constrained.

7.2.1 Rock strength parameters

The principal intact rock properties influencing cuttability include: compressive strength (UCS),tensile strength, toughness, brittleness, elastic modulus, hard mineral content, grain size and

shape, porosity and origin (sedimentary, igneous, etc). Tables 7.2 and 7.3 give some typical rockstrength parameters.

Table 7.2 Typical values of UCS for various rock types (Hartman 1992)

Rock type UCS (MPa) Rock type UCS (MPa)Coal 2040 Dolomite 100

Shale 13110 Basalt 80120

Sandstone 24100 Quartzite 100150

Limestone 50100 Granite 100200

Table 7.3: Rock strength classifications for cutting (Atkinson et al 1986)Rock strength

assessment

UCS

c

Youngs

modulus

E

Modulus of

toughness

T

Toughness index

TE

c

i = x 100

2

2MPa GPa (lb/in3) Derived Calculated

High 108.3 40 72.72 45 40.0

Medium 116.0 29 44.05 25 23.8

Low 58.5 13.4 18.32 15 12.2

Very low 29.9 7.8 7.13 9 5.8

Because of complex mechanics of rock cutting and the large number of variables, there is nouniversally accepted system for predicting cutting machine performance from rock strength

parameters. Manufacturers and operators do use UCS as a guide, even though it is not a reliableindicator of cuttability (Speight 1997). Typical threshold values for defining whether a rock is

hard with respect to mechanical excavation are 103 MPa (14900 psi) (Forrester 1996) and 124 MPa

(18000 psi), (Bullock 1994).

7.2.2 Classifications of machine

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The tunnel boring machine (TBM) is afull face excavator which cuts rock by means of disc cutters

mounted on a circular revolving cutting head (figure 7.5). The following design features enable itto cut stronger rock than any other type of mechanical excavator.

Large mass

Hydraulic stabilising jacks

High cutting force provided by hydraulic thrust rams Cutting discs for rock breakage by indentation

Because of the size and weight of the typical TBM, they are suitable only for the excavation of longstraight drivages such as civil engineering tunnels.

Figure 7.5: Tunnel boring machine

a) Rock breakage system

A TBM breaks rock with disc cutters mounted on the rotating cutter head in such a pattern that

they roll against the rock of the tunnel face in a series of concentric circular grooves or kerfs. The

cutting force is produced by powerful hydraulic thrust rams. Each disc cutter is free to rotate

within its mounting. Figure 7.4 illustrates the mode of rock chip formation, previously described inSection 7.1.3. The high cutting forces required to break strong rock types produce equally highreaction forces on the machine. To maintain contact with the rock and to maintain the optimum

spacing between cutting grooves, the machine is held stable by a combination of its great mass and

by hydraulic jacks (grippers) acting against the side of the tunnel. Broken rock is gathered from the

floor by scoops mounted around the perimeter of the cutting head and discharged at the crown of

the cutter head onto a belt conveyor that runs through the center of the machine (figure 7.6).

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Figure 7.6: Assembly of Robbins TBM

A TBM system provides continuous excavation, supports being set and debris removed as cuttingtakes place. In civil tunnels, the most common type of support is a concrete lining. Pre cast

segments are transported along the length of the TBM, lifted into place and set by a hydraulicmechanism.

b) Example: Atlas Copco Jarva TBM

Figure 7.7 illustrates the basic design of the Atlas Copco Jarva range of TBMs. The main body withthe grippers and the thrust rams form the stationary part of the machine, which is anchored in the

tunnel bore. The moving part consists of: at the front cutterhead, bearing housing, roof and dust

shield and invert scraper; at the rear gear case, motors, planetary gearboxes and lift legs. Front

and rear parts are connected by the torque tube, which slides through the main body during the

boring stroke. A conveyor transports debris to the rear of the machine where it is discharged eitherinto a line of rail cars or a fixed belt conveyor. Figure 7.8 shows how the complete cutting cycle(boring and reset) is carried out. (Atlas Copco)

Figure 7.7: Atlas Copco Jarva TBM

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GrippersCutter head Conveyor

Invertscraper

Frontlift leg

Rearlift leg

Thrustcylinder

Mainmotors

Mainbody


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The multi national Atlas Copco group has in recent years taken over the Robbins Company, whichdeveloped the original concept for the TBM. Robbins produce two basic designs of TBM, an open

type for operation in hard rock and two shielded types for soft or broken ground or high water

inflow situations.

c) Performance

Advance rates are generally high; in a 3.6 m diameter tunnel, drill and blast techniques will give a

typical advance of 3 m/shift; a TBM will typically achieve some 10 m per shift.

Table7.6: TBM performance case studies

Machine Location Rock strength Performance

Robbins 193/214 Selby coalfield 240 MPa (34800 psi) 80m/week

Lovatt Cape Breton, Nova Scotia average 69 MPamaximum 140 MPa

Disadvantages of TBMs

high capital cost (several million dollars)

can only cut circular section

large turning radius (100 m)

time consuming to install

minimum tunnel length of 2 km required to justify installation

7.3.2 Robbins Mobile Miner

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1. Startboringstroke

2. Endboringstroke

3. Startresetstroke

4. Endresetstroke

Figure 7.8: Cutting cycle of Atlas Copco TBM


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The Robbins Mobile Miner is a relatively new development, a machine designed to apply theexcavation principles of a TBM to a practical mining situation (Figure 7.9). The cutting head is a 4.1

m diameter wheel with disc cutters mounted around the circumference. It rotates at 15 rpm in avertical plane and is supported by a boom which can swing the cutter head through a horizontal

arc. The tunnel cross section has a flat roof and floor with slightly elliptical walls, 4.1 m high and5.5 8.0 m wide (Forrester 1996).

Figure 7.9: Robbins Mobile Miner (Bullock 1994)

Only a portion of the face is contacted by the cutter wheel at any point in time so that the reactionforces are lower than for a TBM. This design feature allows for a smaller, more maneuverable

machine which can cut a curve of minimum radius 25 m. The main body of the Miner is mounted

on crawlers. To start cutting, the machine is anchored by means of hydraulic jacks. The wheel isthen rotated and the cutters sumped into the face by hydraulic rams to the require depth.Hydraulic swing cylinders then traverse the boom across the width of the drive. On completion of

the first pass cut, the staking jacks are retracted and the miner moved forward to a new cuttingposition.

The first production machine was tried at Mt Isa with a decline in strong abrasive quartzite.

Problems were experienced with differential wear of disc cutters, resulting in high cutter costs andinsufficient stiffness of the cutting mechanism (Bullock 1994). An improved version with an

advance rate capability of 1 m/hour is in use at Broken Hill. It weighs 265 tonnes, a cutter head

power of 500 kW and has performed successfully in rocks ranging in strength from 100 MPa to 300MPa (Forrester 1996). A further improved design has been considered for the new MIM HoldingsLtd. McArthur River lead zinc mine in the Northern Territory. The rock at this mine is a brittle

altered shale with a compressive strength of 130 MPa. The orebody will be cut parallel to thebedding planes, which will suit the cutting action of the Mobile Miner (Chadwick 1995). MIMs

web page states that underground mining is by conventional drill and blast techniques, so thecontinous hardrock miner is still experimental. ( www.mim.com.au/mcarthur.html )

7.3.3 Raise Borer

The raise borer is a machine designed to cut a circular excavation either between two levels in a

mine or from a level to the surface. The boring machine is set up on the upper level and a pilothole drilled down to the lower level (Figure 7.10). When the hole is complete, a reamer head is

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attached to the end of the drill string. The reamer head is a domed assembly, fitted with rollercutters, or disc cutters, depending on the rock strength. The raise boring machine on the upper

level provides rotational power to the drill rods and pulls the reamer up along the line of the pilothole. Raise borers are very widely used. In addition to often reducing the cost of constructing

raises, the method is virtually free from serious hazard. Conventional raising by drill and blast isnotoriously dangerous.

Figure 7.10: Operation of raise borer

Figure 7.12: Raise borer installation

Table 7.7: Raise borer performance figuresDiameter Performance Costs

(including rig set-up and removal)

Ventilation 1.8 m Pilot hole 0.9 m/hour 1.8 m dia. $1200/mBackfill 0.3 m Reaming 0.6 m/hour 2.4 m dia. $1500/m

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As with all tunneling machines, the advance rate of roadheaders depends on many other factors,such as ground conditions, time required to set supports, mineral clearance problems, team skill

and motivation, etc. Advance rates in British coal mines range from 3 7 m/shift.

7.3.5 Wirth Continuous Miner Machine

The Wirth Continuous Miner Machine (CMM) is equipped with four booms, attached on acommon rotating mounting. One boom is positioned in the center of the mounting and the other

three are spaced equally around the circumference. Each boom can be swung radially inwards andoutwards and is fitted with a disc cutter (figure 7.14). The whole assembly is fitted to an inner kelly

(slotted/geared shaft) which can be advanced or retracted a distance of 1.0 m within the stationarymain body of the machine. The cutting assembly includes stabilisers for positioning against the

roof, floor and sides and the main body of the machine is braced against the floor and roof bymeans of two TBM type gripper pads. The CMM weighs 143 tonnes and has a cutter head power

of 525 kW.

Figure 7.14: Wirth Continuous Miner Machine

The center cutter swings from the outside towards the center to cut the middle of the tunnel face.The outer booms cut the rest of the face, working from the inside towards the tunnel perimeter.

The three outer discs all cut at the same diameter in spiral cutting tracks. When they reach themaximum inner circular profile of the drive cross section, they begin to form the corners of a

square section, as required. For this purpose, the outer booms can be individually extended underhydraulic power. The CMM cuts a square shaped tunnel with rounded corners. Maximum

excavation height and width are 4.5 m and the minimum curve radius that can be excavated is 25m. The desired profile can be cut automatically, according to a pre set computer program.(Chadwick 1995)

7.4 Soft Rock Machines

7.4.1 Continuous Miner

A continuous miner (CM) is an excavating machine capable of cutting soft rocks such as coal,potash or trona at high rates whilst simultaneously gathering the cut mineral and loading it onto a

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transport system such as a shuttle car or a belt conveyor. CMs are used in virtually all mechanisedcoal mines in Australia, Britain, the USA and South Africa for either room & pillar mining or

longwall development.

The main body of the continuous miner (CM) is similar to that of the roadheader, being compact,crawler mounted and with the same type of debris clearance system. Cutting, track, conveyor and

gathering arm motors are electric. The cutting drum is much wider, however, at about 3.5 m andcan be moved only in the vertical plane (Figure 7.15) so that only a rectangular profile can be

formed. Modern continuous miners are all electric , multi motor machines. Electric power is

supplied by a reel operated trailing cable. Hydraulics, powered by an on board power pack areused for operations such as raising and lowering the cutting drum and stabilising jacks.

Figure 7.15: Continuous Miner

The normal operation of a CM is to sump in the cutting drum at the top of the face, then powerthe drum down, cutting from roof to floor. The CM is then trammed forward to take another slice.

This process is continued until a certain span of roof has been exposed, at which point the CMmust be withdrawn in order for roof bolts to be installed. This maximum cut distance depends on

the competency or stand up time of the roof. In Australia, the maximum cut distance is typically

4 6 m but can be as high as 15 m (e.g. Laleham Colliery). Where the roof is particularly strong, the

maximum permitted cut depth will correspond to the distance between the front of the machineand the operator position, i.e. the operator must always be under supported roof. Use of remote

control systems allows this distance to be further extended.

A typical heading width in coal mining is 6 m. Therefore, the CM must actually make two passesto take the full width. The first pass is taken at one side of the heading to the full width and depth

of the cut. The CM then backs out and takes the remaining fillet of coal. After one complete cut has

been taken, the roof is bolted.

CMs are suitable only for cutting soft rocks such as coal or potash. This is because the number ofpicks on the large drum is high and hence the individual pick forces are relatively low. For

maneuverability in a mining situation, the machine size and weight has to be limited. CMsproduce very high cutting rates, up to 650 tonnes per hour, or 10 m per shift . In room & pillar

mining operations, a single machine will operate in a mining panel of two to seven headings, being

moved between headings to advance the whole panel.

Production CMs in Australia discharge into shuttle cars. These machines are haulage vehicles,

capable of holding about 12 tonnes of coal, which transfer loads of coal from the CM at the face toa conveyor loading point located further back.

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Typical continuous miner design the Joy 12CM

Dimensions length 10 m, width 2.1 m, height 1.35 m

weight 36 41 tonnes (depending on type of cutting drum)Performance max cutting rate 12 tonnes per min

tramming speed 16 17 m/min

6 electric motors, total power 312 kWCutting head. Rotating drum fitted with cutter picks. Two variants: 2.6 m length powered by

two 60 kW electric motors or 3.2 m with two 90 kW motors.

Clean up system Two gathering arms mounted on a moveable front spade or apron. Powered

by 45 kW motor that also drives the conveyor. Apron operated by hydraulics.

Conveyor This runs through the centre of the machine from the gathering arm apron to a

discharge jib at the rear. It is a chain conveyor and the rear end can be swung

45o left or right to spot shuttle cars.Traction Crawler mounted, powered by DC motors.

In coal mining, it is important to ventilate the cutting drum and to control dust generation. A small

electric or hydraulic dust extraction fan or a compressed air operated air mover is mounted at therear of the machine. This system draws dust laden air via ducting from the cutting zone, through a

filter and discharges behind the operator. It also draws fresh air in to ventilate the cut area. Dustsuppression is achieved by means of high pressure sprays mounted behind the cutting drum.

These sprays also assist in ventilation of the cut area.

7.4.2 Bolter-miner

The bolter miner is basically a radically modified continuous miner that can cut and install roofbolts at the same time. They are becoming increasingly popular in Australian coal mines,

particularly for longwall development and the most common machines are manufactured by VoestAlpine (Figure 7.16). The main components of the machine are:

1. Crawler track assembly

2. Main frame3. Slide frame assembly, incorporating cutting unit and conveyor

4. Roofbolting module, incorporating four hydraulic roofbolting and two rib (side) bolting drills

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Figure 7.16: Voest Alpine ABM30 Bolter Miner

The slide frame is mounted on the main frame and can be moved 1.0 m forwards and backwards,

relative to the main frame, by means of hydraulic rams. In operation, the machine is driven up tothe coal face and stabilised with floor jacks. The slide frame is then pushed forward, the cutting

drum biting into the face to take a slice of coal in the normal manner. Since the chassis remainsstationary, bolting operations can be carried out simultaneously with cutting. When a cut of 1.0 m

has been taken, the slide frame is retracted back into the chassis and the whole machine moved

forward to commence the next cycle. The ABM 20 is a single pass machine i.e. the cutting drum is

the full width of the heading.

7.4.3 Longwall Shearer

A shearer is a cutting machine used on longwall coal faces. It forms part of a total mining systemof which the other major elements are; armoured flexible conveyor (AFC), hydraulic roof supports

(shields), stage loader and belt conveyor. The main body of the machine is designed to fit within theconfines of a coal face and is generally long and thin in aspect. Cutting is carried out by one or tworotating drums mounted on a ranging arm, which can be raised and lowered in the vertical plane

(Figure 7.17). Coal is cut by point attack drag picks mounted on a set of spiral vanes welded toeach drum. These vanes serve to guide cut coal onto the AFC which, in turn, loads onto the mine

belt conveyor system. The shearer is mounted on the AFC and hauls itself along the face by meansof sprockets that engage in a rack incorporated in the AFC assembly.

Figure 7.17: Longwall shearer

Shearers are classed as either single ended ranging drum shearer (SERDS) or double ended

ranging drum shearer (DERDS) and as either electro hydraulic or all electric. Electro hydraulicshearers have a single large electric motor that drives a hydraulic pump and the cutting drum

through a system of shafts and gears. The hydraulic pump provides power to hydraulic tractionmotors and the ranging arm cylinders. An all electric shearer comprises a number of separate

modules fitted into a rigid frame. Each module: traction unit, cutting drum drive, power pack hasits own electric motor. Total installed power is typically 1000 kW.

The typical peak cutting rate of a shearer on an Australian longwall is 17 18 tonnes per minute,with outputs of over 12,000 tonnes per day being common. High performance longwalls can

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achieve production rates of up to 26,000 tonnes per day. Modern shearers have sophisticatedelectronics and can be fitted with computer control systems to provide automatic drum guidance

and/or automatic initiation of the AFC and powered supports.

7.5 References

Bullock, R.L., 1994, Underground hard rock continuous mining,Mining Engineering, November

1994, pp. 1254 1258.

Chadwick, J., 1995, Mechanised drivage,Mining Magazine, April,1995, pp. 227 236.

CMTE, 2000, 'Oscillating disc cutter' [Online, accessed 4/5/00]. URL: http://www.cmte.org.au. Centre

for Mining Technology and Equipment, Brisbane, Australia.

Forrester, D. 1996, Underground continuous mining an overview, CIM Bulletin, vol. 89, no.

1000, pp32 37, May 1996.

Gertsch, R.E., 1994, Mechanical mining challenges and directions,Mining Engineering, November

1994, pp. 1250 1253.

Hood, M.C. and Roxborough, F.F., Rock breakage mechanical, SME Mining Engineering

Handbook, Chapt 9.1, vol. 1, pp. 680 721, Hartman, (Ed.), H.L., Society for Mining, Metallurgy and

Exploration, Inc., Littleton, Colorado.

Lindqvist, P.E. and Ranman, K.E., 1980, Mechanical rock fragmentation chipping under a disccutter, University of Lulea, Lulea, Sweden, Technical report 59T.

Nantel, JH and Kitzinger, F, 1990, Plasma blasting techniques, Proceedings of Third International

Symposium on Rock Fragmentation by Blasting, Brisbane, August 1990. AusIMM, Melbourne.

Olson, J., 1993, Rapid excavation research a retrospective view, Proceedings of MineMechanisation and Automation conference, Editors Algren et al, Balkema, Rotterdam.

Speight, H., 1997, 'Observations on drag tool excavation and the consequent performance of

roadheaders in strong rock', The AusIMM Proceedings, No. 1, 1997.

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7. mechanical excavation

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