mining engineering

Saturday, August 28, 2010

Mine Layout
1. Introduction
The classic procedure for designing a mine starts by determining the mining method(s) and
probable optimum mining rate (discussed in other chapters). This chapter is principally devoted
to the next step – determining initial mine layout or “conceptual mine design.” The procedure is
also considered initial mine planning.

If the mining method is open pit, the layout starts with the basic design of the open pit itself.
This includes pit layouts in intervals up to the final design (ultimate pit). With the pit established,
the infrastructure is planned, including surface haul roads, stockpiles, dumps, tailings
impoundment, utility corridors, and surface plant layout. The mine layout for an open pit mine
might have to be modified if underground mining is contemplated when the pit is exhausted.

If the plan includes underground mining, planning starts with locating and sizing pre-production
and on-going development requirements. The initial planning includes determining level
intervals, haulage ways, primary access (shaft, ramp or adit), and other major entries. The
design of major entries requires considering the requirements for ore handling, waste rock
handling, primary ventilation circuit, backfill, transfer, materials handling, access for personnel,
refuge stations, and escape route(s). Once the underground mine concept is established, the
surface infrastructure is designed, including access roads, dumps, tailings impoundment, utility
corridors, maintenance facilities, explosives storage, and surface plant layout.

While the procedures outlined above may appear to be sequential, they are actually iterative to
the extent that the process can become tedious. The practical solution for this dilemma is to
conduct the exercise employing short-cut methods based on the following activities.
• Comparisons
• Intuitive reasoning
• Rules of thumb
• Tricks of the trade

Comparisons
Comparisons refer to the study of comparable well-engineered projects. In some cases, the
layout of another mine may be accepted as a starting model.

Intuitive Reasoning
Intuitive reasoning by the team participants is knowledge-based and relies on rational
perception, first-hand mining experience, and good judgment.

Rules of Thumb
Rules of thumb may be applied to break circular references by providing benchmarks and
starting points. Rules are also useful in identifying significant planning problems at an early
stage.

Tricks of the Trade

Tricks of the trade are particular concepts and efficient procedures employed to save time and
effort.

2. Rules of Thumb
Pit Layout
• The overall slope (including berms, access roads, and haul roads) of large open pits in good
ground will eventually approach the natural angle of repose of broken wall rock (i.e. 38
degrees), except for the last few cuts, which may be steeper. Source: Jack de la Vergne
• When hard laterites are mined in an open pit, safe pit slopes may be steeper than calculated
by conventional practice (as steep as 50 degrees between haul roads). Source: Companhia
Vale do Rio Doce
• For haul roads in general, 10% is the maximum safe sustained grade. For particular
conditions found at larger operations, the grade has often been determined at 8%. It is usually
safe to exceed the maximum sustained grade over a short distance. Source: USBM
• The maximum safe grade over a short distance is generally accepted to be 15%. It may be
12% at larger operations. Source: Kaufman and Ault
• The maximum safe operating speed on a downhill grade is decreased by 2 km/h for each 1%
increase in gradient. Source: Jack de la Vergne
• Each lane of travel should be wide enough to provide clearance left and right of the widest
haul truck in use equal to half the width of the vehicle. For single lane traffic (one-way), the
travel portion of the haul road is twice the width of the design vehicle. For double lane (twoway),
the width of roadway required is 3½ times the width of the widest vehicle. Source: Association
of American State Highway Officials (AASHO)
• A crushed rock safety berm on a haulage road should be at least as high as the rolling radius
of the vehicle tire. A boulder-faced berm should be of height approximately equal to the height
of the tire of the haulage vehicle. Source: Kaufman and Ault

Crown Pillar
• A crown pillar of ore beneath the open pit is usually left in place while underground mining
proceeds. The height of the crown pillar in good ground is typically made equal to the maximum
width of stopes to be mined immediately beneath. When the overburden is too deep, the ore
body is not mined by open pit, but a crown pillar is left in place of height the same as if it were.
If the outcrop of the ore body is badly weathered (“oxidized”) or the ore body is cut by major
faults, under a body of water or a muskeg swamp - the height of the crown pillar is increased to
account for the increased risk. Source: Ron Haflidson and others

Mine Entries
• Small sized deposits may be most economically served by ramp and truck haulage to a
vertical depth of as much as 500m (1,600 feet). Source: Ernie Yuskiw
• A medium-sized deposit, say 4 million (short) tons, may be most economically served by ramp
and truck haulage to a vertical depth of 250m (800 feet). Source: Ernie Yuskiw
• In good ground, at production rates less than one million tons per year, truck haulage on a
decline (ramp) is a viable alternative to shaft hoisting to depths of at least 300m. Source: G.G.
Northcote
• Shallow ore bodies mined at over 5,000 tpd are more economically served by belt conveyor
transport in a decline entry than haul trucks in a ramp entry. Source: Al Fernie
• As a rule, a belt conveyor operation is more economical than rail or truck transport when the
conveying distance exceeds one kilometer (3,281 feet). Source: Heinz Altoff

Shafts
• The normal location of the production shaft is near the center of gravity of the shape (in plan
view) of the ore body, but offset by 200 feet or more. Source: Alan O’Hara
• The first lift for a near vertical ore body should be approximately 2,000 feet. If the ore body
outcrops, the shaft will then be approximately 2,500 feet deep to allow for gravity feed and
crown pillar. If the outcrop has been or is planned to be open cut, the measurement should be
made from the top of the crown pillar. If the ore body does not outcrop, the measurement is
taken from its apex. Source: Ron Haflidson
• The depth of shaft should allow access to 1,800 days mining of ore reserves. Source: Alan
O’Hara
• For a deep ore body, the production and ventilation shafts are sunk simultaneously and
positioned within 100m or so of each other. Source: D.F.H. Graves

Underground Layout
• Footwall drifts for blasthole mining should be offset from the ore by at least 15m (50 feet) in
good ground. Deeper in the mine, the offset should be increased to 23m (75 feet) and for
mining at great depth it should be not less than 30m (100 feet). Source: Jack de la Vergne
• Ore passes should be spaced at intervals not exceeding 500 feet (and waste passes not more
than 750 feet) along the footwall drift, when using LHD extraction. Source: Jack de la Vergne
• The maximum economical tramming distance for a 5 cubic yard capacity LHD is 500 feet, for
an 8 cubic yard LHD it is 800 feet. Source: Len Kitchener
• The amount of pre-production stope development required to bring a mine into production is
equal to that required for 125 days of mining. Source: Alan O’Hara

3. Tricks of the Trade

• Job one for mine layout of an open pit (and important for an underground mine) is to obtain
aerial photographs and a resulting contour map of the mine area. In the long run, one is better
off to get good topographical and survey controls right in the beginning. Source: Richard Call
• Outcrop ore bodies are traditionally open cut to the economic limit, after which mining may
take place underground. For a steeply dipping ore body of uniform width, the optimal depth of
the open cut is a function of the stripping ratio, which in turn approximates the ratio of
underground to surface mining costs. It may be economical to increase the stripping ratio where
waste rock is to be later employed underground as fill. Various Sources
• For an outcrop ore body (ore extends from surface of the bedrock), it is said to be best to
mine by open pit down to the point where the cost of mining the last ton is equal to the cost of
mining that ton from underground. This concept is not simple to apply. The last cut in the pit is
highly profitable and while the first production from underground is the most costly because it
will take months to develop the sequence of stoping required to meet full production capacity.
Moreover, the economical depth of an open pit is likely deeper if no underground mining is
contemplated. Source: Tim Koniaris
• Haulage costs for open pit are at least 40% of the total mining costs; therefore, proximity of
the waste dumps to the rim of the pit is of great importance. Source: Frank Kaeschager
• As a pit deepens, grade control and blending may become more difficult. This is one reason
that proposed underground operations should be phased in at an early date. Source: Unknown
• The old rule that says a vertical shaft should be located 200 feet from the crest of an open pit
has been proven invalid by sorry experience. The set back distance should be determined by
rock mechanics. Source: Jack de la Vergne
• To design the optimum layout for a new underground mine, it is important to first determine
the planned mining methods and stoping sequence. Conceptual engineering should be
referenced first to the ore body. Mine layout serves the miners; it is not the other way around.
Source: Jack de la Vergne
• Bad ground is traversed at less risk with a vertical shaft than a lateral or inclined heading.
Where a choice must be made, a shaft should be located in the bad ground and the lateral
access to the ore body in the good ground – not the other way around. Source: Jack de la
Vergne
• In the case of a deep ore body, it has already been well proven that a twin shaft layout can be
used to bring a new mine into a high rate of production at an early stage, which must be the aim
of every new mining venture. Sinking two shafts simultaneously also provides desirable
insurance against the possibility of one shaft encountering serious sinking difficulties. Source:
L.D. Browne
• For deep mines with a natural rock temperature exceeding 1000 F (380 C), the size and
location of shafts should be determined mainly by ventilation considerations. Source: Dr. J.T.
McIntyre
• A twin shaft layout (shafts close together) for a deep mine will require twin cross cuts to the
ore body for an efficient ventilation circuit. It may be better to set the shafts far apart. Source:
Jozef Stachulak
• Normally, the concentrator (mill) should be located close to the mine head. Pumping tailings
from the mill is less expensive than truck hauling ore over a similar distance. When pumping
water to the mill, hauling concentrate from the mill and use of a portion of the tailings for paste
fill is also considered, the argument is even stronger. Source: Edgar Köster
• The mine administration offices should be located as near as possible to the mine head to
reduce the area of disturbance, improve communications, and reduce transit time. Source:
Brian Calver
• When a mine has a camp incorporated into its infrastructure, the campsite should be as close
as practical to the mine to minimize the cost of service and utility lines, as well as to expedite
emergency call-outs. Source: George Greer
• It is normally false logic to consider particular items of used plant and equipment at the
conceptual design stage. The conceptual engineering should consider all new plant and
equipment sized and built to provide optimum extraction and recovery. In this manner, a
benchmark reference is provided against which opportunities to provide particular items of used
plant and equipment may be later evaluated. Source: Jack de la Vergne

4. Strategy for Underground Mines

Ramp Haulage
For small ore bodies, ramp haulage is the default selection because it normally provides the
most flexible and economical choice. (In a cordillera, the terrain may provide relief adequate for
a level entry or “adit.”) A ramp (or adit) drive can typically be oriented to provide an
underground diamond-drilling base and provide shorter crosscuts to the ore zone. The
crosscuts are provided rapidly and economically because they provide a second heading for
the main drive. It is possible to sink and develop from a shaft at the same time; however, this is
a difficult and expensive procedure.

Another advantage to the ramp or adit entry is direct access by mobile equipment when
trackless mining is to be employed. For a typical shaft, the equipment must be dismantled and
reassembled underground. The set-up time required to initiate ramp driving is usually shorter
than for a shaft. One to three months may be required to provide access and collar a ramp
portal, while the collar, hoist, and headframe required for a shaft may take six months of site
work.

For medium sized ore bodies, ramp haulage may still be the best choice where the ore body is
relatively flat lying. In this case, the ramp may have to be enlarged to accommodate larger
trucks. In some cases, it may be practical to provide twin ramp entries to handle two-way traffic.

Belt Conveyor
For large flat-lying ore bodies, a belt conveyor is typically the most economical method of
hoisting ore. The legs of the conveyor are put into a ramp that has been driven straight (i.e. a
“decline”) for each leg of the proposed conveyor way. If the soil overburden is very deep, or
deep and water bearing, a ramp or decline may not be a practical method due to the
extraordinary cost of excavating and constructing a portal. If the ground (rock) beneath the
overburden is not competent or is heavily water bearing, a ramp or decline access may be
impractical due to the driving time and cost.

Shaft System
For large steeply dipping ore bodies, a shaft system is usually best. In this scenario, it may be
advisable to have a ramp entry as well to accelerate the pre-production schedule and later to
provide service access to the mine.

Conventional Methods of Ore Transport

At the conceptual stage, it is normally better to consider only conventional methods for the
transport of ore and resort to the unusual methods only under unusual circumstances. A good
example of “unusual” is the aerial tramway installed across a fjord at the Black Angel Mine in
Greenland to access an ore body located high on a cliff face.
Table 4-1 lists methods employed at underground mines for ore transport.
Table 4-1 Ore Transport Methods Employed at Underground Mines

The following flow chart (Figure 4-1) summarizes rules of thumb for bringing ore to surface from
an underground mine. The flow chart is only a guideline. For example, one underground mine
employed truck haulage (for a very small rich ore body) to a vertical depth of 700m before the
operation was abandoned.
Figure 4-1 Moving Ore to Surface

Main Entry
The foregoing strategy determines a main entry to the underground on the basis of ore
transport. In many cases, this entry also serves for personnel and materials transfer, particularly
at small operations. Consideration should be given to a separate entry for man and materials
handling when it can be afforded. For example, some mines use the production shaft for
ore/waste hoisting, main exhaust, and alternate escape while a second shaft provides cage
service in the main fresh air entry.

If a shaft system is employed at an operation of substantial capacity, it is not uncommon to find

a ramp access from surface as a third entry. This is a logical progression when an internal ramp
system is required by the mining method to be employed. The access ramp has advantages
described in previous text of this chapter.

gambrenk at 12:10 PM

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