150315579 Surface Mine Design and Practice
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150315579 Surface Mine Design and Practice

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Engr. Izhar Mithal Jiskani

Department of Mining Engineering,
Mehran University of Engineering & Technology


Jamshoro, Sindh

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The ore deposits being mined by open pit techniques today vary considerably in size,

shape, orientation and depth below surface. The initial surface topographies can vary

from mountain tops to valley floor. In spite of this there are a number of geometry based

design and planning considerations fundamental to them all.

The ore body is mined from top to the down in the series of horizontal layers of uniform

thickness called benches and after a sufficient floor area has been exposed, mining to the

next layer can begin. The process continues until the bottom bench elevation is reached

and the final pit outline achieved. To access different bench a road or ramp must be

created. The width and steepness of this road or ramp depends upon the type of

equipment to be accommodated. Stable slopes must be created and maintained during the

creation and operation of the pit.

Slope angle is an important geometrical parameter which has a significant economic

impact. Open pit mining is very highly mechanized. Each piece of mining machinery has

an associated geometry both related to its own physical size, but also with the space it

requires to operate efficiently. There is a complementary set of drilling, loading and

hauling equipment which requires a certain amount of working space. This space

requirement is taken into account when dimensioning the so called working benches.

From both operating and economic view points certain volumes must or should at least

be removed before others. These volumes have a certain minimum size and an optimum


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The basic extraction component is an open pit mine is the “bench”. Bench –

nomenclature is shown in Figure 1.

Fig.1: Bench Nomenclature

FACE: It is an exposed area from where the overburden or mineral/ore is extracted.

CREST: highest point of the face.

TOE: lowest point the face.

BENCH: the step or floor accommodating the mine machinery.

BENCH HEIGHT: each bench has an upper and lower surface separated by a distance

“H” equal to the bench height. The bench height is determined by the size of mining

equipment and formation of the area.

• The loose/soft rocks allows, bench height up to shovel reach.

• In hard and very strong rock, bench height is usually 10-40 meters.

BENCH SLOPE: the bench slope or the bench face angle is the inclined plane of the

bench made an angle with the horizontal. Or the average angle that a face makes with the

horizontal. The exposed sub-vertical surfaces are known as bench faces. The bench faces

are described by the toe. The crest and he bench face angle ‘α ∝’. The bench face angle

can vary considerably with rock characteristics, face orientation and the blasting

practices. In most hard rock pits it varies from about 55O to 80O. A typical initial design

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value might be 65O. This should be used with care as the bench face angle can have a

major effect on the overall slope angle.

BENCH FLOOR: The exposed bench lower surface is called as the bench floor.

BENCH WIDTH: The bench width is the distance between the crest and toe measured

along the upper surface.

BANK WIDTH: It is the horizontal projection of the bench face.

There are several types of benches; a working bench is that one which is in process of

being mined. The width being extracted from the working bench is called the cut. The

width of working bench WB is defined as the distance from the crest of the bench floor to

the new toe position after the cut has been extracted as shown in figure 2. After the cut

has been removed, a safety bench or catch bench of width SB remains.

The purpose of leaving safety benches is to:

a) collect the material which slides down from the benches above; and to

b) stop the downward progress of the boulders.

During primary extraction, a safety bench is generally left on every level. The width of

safety bench SB varies with bench height “H”. Generally the width of the safety bench is

of the order 3
2 of the bench height. At the end of mine life, the safety benches are

sometimes reduced to a width of about 3
1 of the bench height.

In addition to leaving the safety benches berms (piles) of broken materials are often

constructed along the crest. These serve the function of forming a ditch between the berm

and the toe of the slope to catch falling rocks.

A safety berm is also left along the outer edge of the bench to prevent trucks and other

machines from backing over. It serves much the same function as a guard rail on bridges

and elevated highways. Normally the pile has a height greater than or equal to the tire

radius. The berm slope is taken to be about 35O, i.e. also called the angle of repose.

The steps which are followed when considering bench geometry are:

i) Deposit characteristics (total tonnage, grade distribution, value etc) dictate a

certain geometrical approach and production strategy.

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ii) The production strategy yields daily ore-waste production rates, selective

mining and blending requirements, numbers of working places.

iii) The production requirements leads to a certain equipment set (fleet type and


iv) Each equipment set has a certain optimum associated geometry.

v) Each piece of equipment in the set has an associated operating geometry.

vi) A range of suitable bench geometries results.

vii) Consequences regarding stripping ratios, operating v/s capital costs, slope

stability aspects etc are evaluated.

viii) The best of the various alternatives is selected.

In the past, when the rail bound equipments were being extensively used, great attention

was paid to bench geometry. Today highly mobile rubber tired/crawler mounted

equipments has reduced the detailed evaluation requirements some-what.

Fig. 2: Working Bench

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Fig. 2a: Functions of a catch bench

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In mining literature, going the initial knowledge about the physical access to the Ore

body is of great importance. For this, the question arises that: “How does one actually

begin the process of Mining?” Obliviously the approach depends upon the topography of

the surrounding ground. To introduce the topic, it is assumed that the ground surface is

flat. The overlying vegetation has been removed as has the soil/sand/gravel overburden.

In this case it will be assumed that the ore body is 700 feet in diameter, 40feet thick, flat

dipping and is exposed by removing the soil overburden. The ore is hard so that drilling

and blasting is required. The bench mining situation is shown in figure: 3.

Figure 3: Geometry of the ore body

A vertical digging face must be established in the ore body before major production can

begin. Further more a “ramp” must be created to allow truck and loader access. A drop

cut is used to create the vertical breaking face and the ramp access at the same time.

To access the Ore body, the “ramp” shown in figure: 4 will be driven it has an 8% grade

and a width of 65 feet.