MINISTRY OF SCIENCE AND TECHNOLOGY DEPARTMENT OF TECHNICAL AND VOCATIONAL EDUCATION

MINISTRY OF SCIENCE AND TECHNOLOGY
DEPARTMENT OF
TECHNICAL AND VOCATIONAL EDUCATION
Sample Questions & Worked Out Examples
For
Min 05066
COAL MINING
B.E.
Mining Engineering
1
YANGON TECHNOLOGICAL UNIVERSITY
DEPARTMENT OF MINING ENGINEERING
COAL MINING
(Part I)
(MIN- 05066)
BE (Mining)
QUESTIONS AND ANSWERS
16 SEPTEMBER, 2003
YANGON
2
CHAPTER 1
STRIP MINING (OPEN-CAST MINING) OF COAL
1.* Discuss the following factors to be considered in planning and design of strip
mining operation. (10 marks each)
(i) Stripping ratio
(ii) Physical and mechanical properties of overburden
(iii) Surface topography of mine area
2.* Explain and discuss the three stripping system of coal extraction with necessary
sketch. (10 marks)
3.* Explain the concept of Maximum Usefulness Factor (MUF) to determine the
ability of a machine. (10 marks)
4.* Describe the stripping operation or stripping sequence by shovel-dragline
combination. (20 marks)
5. ** Explain the operational function of wheel excavator in strip mining operation
with sketch. (20 marks)
6. ** Explain the application of bulldozer at shallow stripping operation. (10 marks)
7. *** Discuss in detail the use of Maximum Usefulness Factor (MUF) to select the
proper size of equipments for the most economical stripping operations. (20 marks)
8. *** Discuss the factors to be considered in the selection of equipments for coal
loading from the coal seam and hauling to obtain the efficient extraction. (20
marks)
9. *** Discuss as much as you know the procedure of strip mining operation prior to
planning and design of a coal deposit. (20 marks)
3
CHAPTER 2
REMOTELY CONTROLLED MINING SYSTEMS
FOR STRIP COAL MINING
10.* Discuss the importance of selection of drilling machine, which affects on the
drill performance, depending on the working condition. (20 marks)
11.* Discuss the blast-hole pattern and spacing for overburden removal of strip coal
mining with necessary sketch. (20 marks)
12.* Discuss the casting overburden with explosive for shallow coal deposits. (20
marks)
13.* Discuss in detail the characteristics and performance of ammonium-nitrate
explosive. (20 marks)
14. ** Discuss the effectiveness of drill-hole directions in the coal extraction of strip
mining operation by drill-blast method. (20 marks)
15.*** Discuss as much as you known what types of explosive (high explosive, low
explosive or blasting agents) should be used in coal strip coal mining operations.
Give a reason for using selected explosive. (20 marks)
16. *** Design the blast-hole pattern and spacing for the following figure (1). The
necessary data can be assumed as you like. The physical and mechanical properties
of overburden rocks are varied. (20 marks)
Run direction
Free face
Shale
Sand Stone
Mine direction
Soil
Hard limestone
Coal
Figure (1) Cross section and Plan for a section of coal deposit
4
CHAPTER 3
OPEN-PIT MINING
17.* Explain in detail the breaking mechanism of rock in the field of blasting
operation. Discuss the effects of stress pulse on rock structure generating from
explosion. (20 marks)
18.* As a blasting engineer, it is necessary to design for controlling the blasting
effects and to obtain the efficient blasting operation. Explain and discuss the
formula for the calculation of blasting pattern, charge amount and control of blast
vibration. (20 marks)
19. ** Explain and discuss, with necessary data and sketch, the effects of drill-hole
diameter, drill-hole size, the nature of bench and bench height on the breaking
costs. (20 marks)
20. ** Discuss the effects of drilling pattern, inclination of drill-hole and firing
sequence on the fragmentation of rock. (20 marks)
21. *** Based on the reflected blasting theory, discuss with sketch the advantages of
inclined drill-hole compared to other to obtain the efficient and good blasting
results. (20 marks)
22. *** For bench blasting operation, it is necessary to choose the proper drill-hole.
As a drilling and blasting engineer, discuss what factors are considered in the
selection of drill-hole diameter to obtain the optimum blasting results. (20 marks)
CHAPTER 4
LOADING ORE AND WASTE
23.* Explain the guidelines for the selection of shovel and dragline to meet a given
production. (10 marks)
24.* Explain in detail the influences of factors in determining the capacity of haulage
capacity for haulage system analysis. (20 marks)
5
25. ** Discuss briefly the factors to be considered for the selection of truck haulage.
(10 marks)
26. *** Discuss briefly the factors to be considered for the selection of haulage
systems. (Compare track and rail haulage systems). (10 marks)
27. *** Discuss briefly the factors affecting on the choice of loader and truck to
match the effective production. (10 marks)
6
SAMPLE QUESTIONS AND ANSWERS
1. Question
No (1) * Discuss the following factors to be considered in planning and design of strip
mining operation. (Chapter 1)
(i) Stripping ratio
(ii) Physical and mechanical properties of overburden
(iii) Surface topography of mine area
Answer:
(i) Stripping ratio
The universal factor most used to determine the economics of strip mining is
called the stripping ratio. This refers to the cubic yards of overburden which must be
dug to uncover 1 ton of coal. Ratios are as high as 20:1 at some stripping operations.
At one Illinois mine as much as 80 ft of overburden is stripped to recover 28 in. of
coal.
Many other factors must also be considered, such as the proportion and
hardness of the rock in the overburden, the thickness and quality of the coal seam,
costs of labor and materials, sale price of the coal. But, basically, the stripping ratio is
of the most importance.
(ii) Physical and mechanical properties of overburden
Topography, coal reserves, expected selling price of the coal, physical and
mechanical properties of overburden to be removed, spoil area, and tonnage of coal
desired per shift are some of the major factors influencing the selection of equipment.
Once the capacity of a mine has been decided and the major stripping machine
chosen, other units such as drills, power shovels for loading coal, and trucks should be
selected to build a balanced production cycle.
The equipment must be in balance ratio-wise with the maximum output of the
mine and at the same time capable of handling the maximum depth of overburden. In
other words, maximum yardage determines size of machine and maximum depth and
properties of strata to be excavated determines the range of the machine.
7
Coal is a seasonal product and it is necessary to choose equipment which
meets maximum production periods; this equipment is usually larger than would be
required to maintain a steady average production rate.
Assuming that a block of land has been acquired which contains sufficient
coal reserves to warrant the investment in stripping equipment and cleaning plant then
the next step is to accumulate information on the surface topography, and the depth
and thickness and characteristics of the coal seam as well as the nature and thickness
of the overburden overlying the coal seam.
(iii) Surface topography of mine area
Surface topography is one of the major considerations for coal extraction,
which affects on the extraction costs per of coal. Aerial topomaps make good base
maps on which to plot coal outcrops, and on which to lay out existing property lines,
and to locate proposed roads, plant, spoil areas, etc. Data from the results of
preliminary to detail investigation are provided on this map. Locations for exploratory
bore holes may also be spotted on these maps.
The physical properties, mechanical properties of overburden, nature and
thickness of the various strata, which must be stripped from the coal seam, may be
determined by means of diamond drill cores and the physical characteristics and
composition of the coal seam may be determined by examination and analysis of
diamond drill cores.
............................................................................................
2. Question
No (2)* Explain and discuss the three stripping system of coal extraction with
necessary sketch. (Chapter 1)
Answer:
The three basic stripping systems are the following:
(1) A single stripping shovel traveling on the exposed coal seam digs and
removes the overburden ahead of it and piles it in the cut from which coal has
previously been removed.
8
(2) A single dragline travelling on a bench above the coal strips overburden to
widen the bench for its travel way for the next cut and also removes the high-wall
bench over which it has just travelled to expose the coal seam.
(3) A shovel and a dragline are used in tandem with both travelling on exposed
coal. The shovel works ahead of the dragline removing the lower bench to expose the
coal seam and piling the spoil in the cut from which coal has previously been
removed. The dragline removes the upper portion of the overburden to form another
bench and casts the spoil over and behind the spoil piled by the shovel.
Numerous combinations of shovels, draglines, bulldozers, scrapers, and wheel
excavators are possible.
............................................................................................
3. Question
No (3)* Explain the concept of Maximum Usefulness Factor (MUF) to determine the
ability of a machine. (Chapter 1)
Answer:
Maximum Usefulness Factor (MUF) determines the ability of a machine. This
objective is to find a method of providing a preliminary simple overcastting analysis
for a stripping prospect. The approach to the problem employs indicated trends in the
relationship of the weight of the machine to its ability to do stripping work. The
ability to do work is established through “MUF” numbers.
In this method, the approach employs situation where the geometry of each cut
and spoils section is assumed to take certain defined relationships for varying
overburden depths. Slopes are considered to be stable which means such practical
factors as the mechanics of soils are neglected for the sake of convenience.
A value concept of a machine is arbitrarily established, consisting of the
product of the nominal dipper size of the shovel times a functional dumping reach.
A shovel’s capability to negotiate cuts in deep overburden is limited by its
ability to dispose of the spoil in most instances. Thus the dumping reach, along with
its respective dumping height, is significant.
9
The angle of repose of the spoil material must be taken into consideration. It
will vary in actual practice among different mines, jobs, and materials. However, a
slope frequently found in practice and used in planning of 1.25/1 is used.
Pits that follow a straight line when projected on a horizontal plane are
referred to as “straightaway”. It is assumed that all sections pertain to straightaway
cuts, which in turn means that areas can be treated relatively as volumes.
Volumes of overburden are expressed in virgin cubic yards, when material is
displaced from its virgin state to a spoil section it usually occupies a larger volume
than in situ. The difference is called “swell” and it is expressed in percentages of the
original volume.
............................................................................................
4. Question
No (11)* Discuss the blast-hole pattern and spacing for overburden removal of strip
coal mining with necessary sketch. (Chapter 2)
Answer:
(i) Blast-hole pattern and spacing for overburden removal of strip coal mining
The blast-hole pattern used and the spacing of holes depend upon a number of
factors including;
(i)
thickness of overburden,
(ii)
type of rock in overburden, and
(iii)
Size of the blast holes.
Spacing between the rows of holes which parallel the high-wall may be
anywhere from 15 to 25ft and spacing between individual holes in a row may be from
above 20 to 30 ft.
The following Figures show typical drill-hole patterns and spacing in use at
several operating mines.
10
Figure 1: Blast-hole patterns and spacing
(a) Deck loading concentrates part of the charge in the upper portion of thick overburden for
better fragmentation.
(b) Deck loading is also advantageous in overburden consisting of rocks with varying harnesses.
Millisecond delays increase the effectiveness of explosive.
(c) Buffer shooting possible a uniform drilling pattern, eliminates large chunks which are
sometimes produced when shooting against an open face.
............................................................................................
11
5. Question
No (12)* Discuss the casting overburden with explosive for shallow coal deposits.
(Chapter 2)
Answer:
In explosives casting large amounts of low-cost ammonium nitrate mixtures
are loaded into medium sized drill holes in a usual ratio of more than 1 lb. of powder
per cubic yard of overburden. The explosive charges are detonated through
millisecond delay electric blasting caps. When the shot is fired, a large part of the
overburden is blasted into the pit away from the high-wall and up on the spoil pile
where it attains a favorable angle of repose.
One stripping operation with a 50-ft sandstone overburden prepares its bank
for shooting by drilling five rows of holes on 12 ft burdens with 15 ft spacing.
Four rows are 6 ¼ in. diameter holes while on the outside row 7 ⅜ in. diameter
holes are used to insure the desired movement of the toe. Hole depths average 45 ft.
The powder factor averages 1.2 lb./yd3.
This blasting operation moves a minimum of 35 percent of the overburden
without the use of equipment and uncovers 35-40 percent more coal in the same time.
Even thought drilling and blasting costs are higher, mining profits are greater because
of the increased production.
Factors Favoring the Use of Explosives Casting
(1) Deep, hard overburden requiring extensive shooting.
(2) Dumping radius of primary stripping unit less than 150 ft.
(3) Narrow, steep cuts, 60-100 ft wide.
(4) Under-capacity of primary stripping unit.
(5) Overcapacity of coal mining unit.
(6) Ability to use least expensive AN-FO explosive.
Factor Unfavorable for Explosive Casting
(1) Overburden is shallow and easily excavated.
(2) Cuts are more than 100 ft wide.
(3) Condition are poor (water, etc.) for the use of bulk AN-FO.
The haulage road may have to be run past the stripping pit.
12
Figure 2: Overburden casting with explosives. Plan and profile of a typical application of
explosive to overburden casting shows a minimum of material to be handled
by excavating machines.
.....................................................................................................
6. Question
No (13)* Discuss in detail the characteristics and performance of ammonium-nitrate
explosive. (Chapter 2)
Answer:
Ammonium nitrate (with carbonaceous material added) is the most commonly
used blasting agent in strip mining operations. A mixture of granular ammonium
nitrate and fuel oil is presently in extensive use in strip mining operations. These
ingredients are usually mixed in the proportion of 100 lb. of ammonium nitrate to 5 or
6 lb. of fuel oil.
AN-FO mixture may be bagged for transportation to the drill holes or it may
be transported in bulk and poured or blown into the holes by means of compressed air.
Ammonium nitrate is a relatively slow velocity explosive and explosion must
be initiated with high explosive primers. Ammonium nitrate also detonates better
when it is loaded in large diameter drill holes and difficulties are sometimes
experienced in getting complete detonation in small diameter and relatively long holes
(better for 2.0 inches diameter).
13
Many of the explosive manufacturers are producing variations of the “AN”
mixtures. These may contain technical grade ammonium nitrate, mixed with a
carbonaceous dust and a sensitizer, such as nitromethane.
(a)
(b)
(c)
(d)
Figure 1: Characteristics of ammonium-nitrate explosives
The detonation velocity is increased as the size of prill is decreased.
Water has a marked effect on velocity; failure occurring at 8 percent moisture.
Velocity falls faster with fuel-lean than with fuel-rich mixtures.
Above 4 percent diatomaceous earth, the detonation velocity falls rapidly.
14
Ammonium nitrate is a relatively slow velocity explosive and explosion must
be initiated with high explosive primers. Ammonium nitrate also detonates better
when it is loaded in large diameter drill holes and difficulties are sometimes
experienced in getting complete detonation in small diameter and relatively long holes
(better 2.0 inches diameter).
Many of the explosive manufacturers are producing variations of the “AN”
mixtures. These may contain technical grade ammonium nitrate, mixed with a
carbonaceous dust and a sensitizer, such as nitromethane.
Mixing ANFO
Most companies making their own AN-FO mixes use either a “prilled” or
grained ammonium nitrate with No. 2 fuel oil. Approximately 4 quarts of oil are
added to each 100lb. of ammonium nitrate. (Note: “prills” are pallets.)
Many of the larger companies have their own mixing plants where the
ingredients can be metered as they flow to a mechanical mixer. Some companies use
the product immediately after it is packaged while others prefer to let the mixture
season before using it. The seasoning period varies from several hours to several days.
Other producers prefer to mix they nitrate and oil at the hole site. Oil is poured
over the opened bags at each hole then left to percolate down through the ammonium
nitrate for a short period before holes are charged.
The most recent development is a slurry type blasting agent. New slurry
consists of a mixture of ammonium nitrate, sodium nitrate, high-explosive sensitizer,
and water. It has a density of about 1.5 and a rate of detonation of about17000 ft/sec.
.........................................................................................................
7. Question
No (15)*** Discuss as much as you known what types of explosive (high explosive,
low explosive or blasting agents) should be used in coal strip coal mining operations.
Give a reason for using selected explosive. (Chapter 2)
Answer:
The cost-effectiveness for overburden stripping is the primary important. Most
of the overburden stripping operations of coal mining are usually excavated in the
softer shale and sandstone. Thus, the numbers and diameter of holes to be drilled is
15
also considered in blasting operation. Since larger holes can hold more explosive,
fewer holes are required to break a given tonnage of rock.
Therefore, larger diameter of drill hole with low explosive or blasting agent
should be used for soft rock excavation to distribute the blasting effect on entire rock
structure.
Ammonium nitrate is a relatively slow velocity explosive and explosion must
be initiated with high explosive primers. Therefore, ammonium nitrate is extensively
used in strip mining operation.
The most recent development is a slurry type blasting agent. One of new slurry
consists of a mixture of ammonium nitrate, sodium nitrate, high-explosive sensitizer,
and water. It has a density of about 1.5 and a rate of detonation of about17,000 ft/sec.
Where blast holes pass through strata of varying degrees of hardness an effort
is usually made to obtain the greatest concentration of explosive in the toughest strata.
A denser mixture may be formulated for placement in such rock, with increased
density being obtained by proper gradation of the grain size of the mixture of
ammonium nitrate granules and prills.
.........................................................................................................
8. Question
No (18)* As a blasting engineer, it is necessary to design for controlling the blasting
effects and to obtain the efficient blasting operation. Explain and discuss the formula
for the calculation of blasting pattern, charge amount and control of blast vibration.
(Chapter 3)
Answer:
Blasting engineer must consider the blasting effect on environment (vibration,
throw, fly rock and air blast) prior to planning of blasting. Blasting operation should
take within the permissible vibration. Rock structure is very complex and also
blasting effect on environment is difficult to determine, but most of effect on
environment is controllable.
Calculation of permissible burdens and the required amount of explosive for
an efficient blast is still not a simple matter of inserting values in a standard universal
formula. In designing the blasting operation, blasting theory, rock failure theory and
16
may be applied. Moreover, several empirical guides should be used to verify or justify
with each other in planning and design of blasting operation.
A number of simple formulae have been based on the relationship that the
quantity of explosive is directly proportional to the quantity of rock broken. Quantity
is generally measured by weight, which is a function of volume, and volume, in turn,
varies as the cube of a length. Thus the required weight, Q, of a single concentrated
(i.e. crater) charge breaking to a plane surface will vary as the cube of the burden
distance,
B : Q = KB3
Where, K is an experimentally determined numerical factor that takes care of
variations in rock and explosive characteristics. For a column charge in a drill hole,
the formula could be Q = K B2 per ft and, knowing the density of the explosive, the
size of hole required could be calculated. With such formulae it has been found that K
for soft, tough rock will be about twice the K for hard, brittle rock. Likewise, an
increase in the number of faces to which the charge can break will reduce K; thus K
for a cube (six faces) is only one-fourth the K for a bench blast (two faces).
Other formulae have been based on incorrect but useful assumptions. Some
are based on assuming that the blast will shear at a constant angle with the face –
usually 45° - and give an explosive factor of 0.5 – 1.0 lb./ton. One bench blasting
formula assumes that the blast must overcome the tensile strength of rock over the
new face area plus the shear strength of the rock over the new floor area plus the
frictional resistance of the whole block moving across the new floor area. And there is
a coyote tunnel blasting formula based simply on shearing the new floor area using 2
– 2 ½ lb. of powder per square foot of floor area.
A great number of underground and surface practices use in a wide variety of
ground all over the world. It is a very simple formula of O. Anderson (Blast hole
burden design, Proc. Australian IMM, No. 166-167, Melbourne, 1952)
B = dL ,
where B is burden (ft), d is diameter of blast hole (in.), and L is length of hole (ft) on
single hole shots or when holes are at the recommended maximum spacing S = 1.5 B.
If a smaller spacing, S2, is used then the new burden
2
− S 2 + S 2 + 4A
B2 =
2
17
Where A = B2 + SB. If free face is less than 2B then the new burden should also be
used: B2 = Bx , where x is half the length of the free face. To improve fragmentation
– which is adversely affected by deep holes, wide spacing and large burden may be
reduced to two-thirds of that calculated.
A markedly similar formula can be derived from one which is used to
establish a rock characteristic called resistance to blasting by K. H. Frankel (Factors
influencing blasting results, Manual on Rock Blasting, Section 6:02, Stockholm,
1952).
Solved for maximum burden it gives:
RL0.3 l 0.3 d 0.8
B=
50
Where B is burden in meters, R is resistance to blasting (experimental factor, one for
difficult-to-break to six for very-easily-broken rock with two being a rough mean of
frequent figures), L is length of hole in meters, l is length of charge in meters and d is
diameter of hole (mm). It appears even more similar when the following “practical”
values are used: reduce actual B to 0.8B and limit it to loss than 2/3 L; use l = 0.75 L;
and keep spacing under 1.5B. Frankel says this has been used since 1944 with 35
percent LFB explosive, common Swedish ammonia gelatin dynamite that is widely
used in mining
A burden formula based on physical characteristics of rock and explosive is:
B = Kd
Pa
T
Where B is critical radius (or maximum burden) in inches, K is a constant based on
rock characteristics and varies from 0.7 to 1.0 with an average of 0.8 (it can be
calculated using Poison’s ratio and the damping characteristic of the rock), d is the
cartridge diameter (or hole diameter with proper confinement by tamping and
stemming) in inches, Ps is reaction stability pressure of explosive (in psi) and T is
ultimate tensile strength of rock (in psi).
Other formulae are worthy of review in a thorough study, but many become highly
involved. One that has received some recognition was developed by U. Langefors.
Vibration produced by a blast has come in for considerable study and a simple
formula for planning purposes has been suggested by R. West water (Heading
blasting, Mine & Quarry Engineering, London, July 1957). It is:
18
A=
K Q
D
where A is amplitude of vibration in 0.001 in., K is a ground factor varying
from 100 in hard rock to 300 in wet rock or clay, Q is pounds of explosive and D is
distance between blast and point of interest in feet. He mentions that amplitudes of
0.008 in. will not damage normal structures but that it should be kept below 0.003 to
prevent human sensing and complaints. Other studies have emphasized the critical
effect of acceleration rather than just amplitude.
.........................................................................................................
9. Question
No (19) ** Explain and discuss, with necessary data and sketch, the effects of drillhole diameter, drill-hole size, the nature of bench and bench height on the breaking
costs. (Chapter 3)
Answer:
Figure (1) shows the effects of varying blast hole sizes and bench heights on
breakage costs. In this case, the drilling cost per unit volume of hole is taken to be
proportional to 1/ d . Costs are shown from holes with diameters from 2 to 12 in. and
3
Cost ($/m )
for bench heights of 33 and 50 ft.
Figure 11: An example of total cost of drill-hole and explosive as a function of drill hole
diameter, d, at different bench heights, K, and a drilling cost per volume of hole
proportional to 1/d1/2 One row hole assumed. Width of bench taken a B=40 m
19
Laboratory experiments have demonstrated that an increased amount of rock
can be broken with less explosive by using inclined rather than vertical blast holes.
(B. J. Kochanowsky)
With the inclined holes less explosive energy is wasted in the rock under the
toe and more energy is used in breaking rock above the level of the floor.
.........................................................................................................
10. Question
No (26) *** Discuss briefly the factors to be considered for the selection of truck
haulage. (Chapter 3)
Answer:
The followings are some of the guidelines for the truck selection, which may
be affected by many factors.
(i)
The final delivery point of excavated material
(ii)
Ground condition for haul road
(iii)
The distance and grade of haul roads
(iv)
The required production
(v)
Expected life of operation
(i) The final delivery point of excavated material
The on-off high way truck has an advantage when the product has to be won
on site and delivered over public roads. For this application it can not be carried out
by either off high way rigid or articulated truck.
(ii) Ground condition for haul road
The types of ground conditions found on a particular job will obviously
influence the type of truck chosen. Ground conditions, especially, bad ground
conditions may be considered.
(1) Soft or boggy, where excellent traction and flotation are essential just
to keep moving.
(2) Bumpy and undulating, the truck will require excellent suspension or
haul speeds will be reduced
20
(3) Rocky, loose lying rocks can lead to excessively high tire costs
(4) Steep grades, these increase the problems faced by machine, especially
in soft ground.
(iii) Distance and grade of haul roads
The choice of haul units depends upon the type of material handled and
distance of haul road.
-
For short distance (up to 100 yards), a dozer or tractor would usually be used
-
For middle distance (up to 2 mile), scraper would be used
-
For longer distance (more than 2 mile) truck would be used
(iv) The required production
The size and number of truck required will depend on the haul distance and
grade, the production required, the loader used and number of faces. The truck size
for most economic working should not exceed four times the bucket capacity
selected.
(V) Expected life of operation
The final selection should aim for maximum machine utilization at the lowest
possible cost per ton. If the truck is correctly matched to the loader, if the power
train is matched, if the operator is in safe comfortable environment with suspension
equal to speed potential, it should then meet this goal of the lowest possible cost
per ton. However, it should be realized that the cheapest truck to purchase may
well not provide the lowest production cost. These require the economic
comparison.
................................................END......................................................
YANGON TECHNOLOGICAL UNIVERSITY
DEPARTMENT OF MINING ENGINEERING
COAL MINING
(Part II)
(MIN- 05066)
BE (Mining)
QUESTIONS AND ANSWERS
16 SEPTEMBER, 2003
YANGON
1
(QUESTIONS)
CHAPTER 1
UNDERGROUND MINING SYSTEMS
1.* Write down the short note on the followings.
(i) The ratio of development to production. (5 marks)
(ii) Cyclic (or conventional) coal extraction. (15 marks)
(iii) Coal haulage (10 marks)
2.* From the geomechanics point of view, discuss the comparison of the percentage
recovery of rib pillar and square pillar for a following typical room and pillar
mining system. (20 marks)
Rib pillar
Square pillar
Rib pillar
Figure (1) comparison of the percentage recovery of rib pillar and square pillar
3.* Explain the general procedure for the determination of maximum permissible roof
span for extraction of coal mining systems. (20 marks)
4.* Describe and discuss the basic underground mining systems of coal extraction
with necessary sketches. (20 marks)
5. ** Describe the general procedure of mine development for room-and-pillar
mining method. (10 marks)
6. ** Derive the general formula for determination of dimensions of the pillar in
designing for room and pillar mining systems. (10 marks)
Assume that: bedded rock condition, which is thin compared with the span of
excavated opening and bond strength between beds are weak.
2
7. *** Derive the formula of the following pillar designs for determination of
dimensions of the pillar in designing for the room and pillar mining systems. (10
marks each)
Assume that: bedded rock condition, which is thin compared with the span of
excavated opening and bond strength between beds are weak.
(1) Regular spaced rib pillar
(2) Regular spaced rectangular pillar or square pillar
(3) Random spaced pillar with irregular shape
CHAPTER 2
PILLAR MINING SYSTEMS
CONVENTIONAL (CYCLIC) MINING
8.* Explain the sequence of cyclic operation for room and pillar mining system. (10
marks)
9.* Calculate the dimension of pillar for the following conditions: (10 marks)
- Depth of opening = 305.0 m
- Height of opening = 10 m
- Safe roof span = 10 m
- Average uniaxial compressive strength of rock mass = 80 MPa
- Unit weight of rock mass = 0.026 MN/cu-m
10. ** Write the short note with necessary sketches on the following coal cutting
machines used in room and pillar mining operations. (10 marks each)
(i) Short-wall coal cutter
(ii) Arc-wall cutter
11. ** Write short note for the functions of coal loaders with sketches on the
followings. (10 marks each)
(i) Gathering-arm loaders
(ii) Duckbill loader
(iii) Conveyor loader
3
12. *** Design and sketch the mine layout plan and the operating sequence of mine
development work for coal extraction using the mobile loading equipments with
belt conveyor. (20 marks)
Given: Coal deposit; flat lying deposit, underlying the shale and overlying the
sandy shale, Thickness of deposit = 1.20 m, Average overburden depth = 130.0 m, the
following conditions are satisfied by the designed data.
Coal pillar width = 8.0 m, Main entries 7.0 m will be driven in sets of 4 on 15.0 m
center, with cross cut 6.0 m wide on 20.0 m centers.
Machine; Short wall cutter, Mobile loading machine, Belt conveyor, Three hand held
drills, Power to operate at 250 V d. c. Required data can be assumed as you like.
CHAPTER 3
PILLAR MINING SYSTEMS
CONTINUOUS (NON-CYCLIC) MINING
13.* Explain, with sketches, the basic types and operating functions of continuous
mining machines used in coal mining operation. (20 marks)
14.* Explain the operating procedure of ripper type Joy continuous machine and
continuous boring machine. (10 marks each)
15.* Explain and discuss the methods used in pillar extraction in long wall continuous
mining system. (20 marks)
16. ** Explain and discuss the general requirements for pillaring operations (20
marks)
17. *** The nature of coal deposit and machinery used are as follows.
Given: Coal deposit; flat lying deposit, underlying the shale and overlying the
sandy shale, Thickness of deposit = 0.95 m, Average overburden depth = 100.0 m,
the following conditions are satisfied by the designed data. Required data can be
assumed as you like.
Design and sketch the layout plan of mine for coal extraction using continuous
boring machine. (20 marks)
4
18. *** Discuss the semi-cyclic operation of long wall mining system. Explain the
advantages of the objectives of semi-cyclic operation depending on the nature of
coal deposit. (20 marks)
CHAPTER 4
LONG-WALL MINING – CYCLIC OPERATIONS
19.* Discuss the general procedure of mine development for long wall mining system.
(10 marks)
20.* Discuss the long wall mining operation of coal extraction. Give an example. (20
marks)
21.* Explain the cyclic mining operation of long wall mining system. (10 marks)
22.* Write the short notes on the followings. (10 marks each)
(1) The operational functions of long wall chain coal cutter
(2) Jib coal cutter
23.* Describe the various types of loaders and discuss the operations of loaders for
loading in cyclic long wall mining system. (20 marks)
24. ** Discuss briefly the advantages and disadvantages of Curved Jib coal cutter and
some of these applications in practice (20 marks)
25. ** Discuss the factors to be predetermined for the development and equipment
selection prior to planning and design of long wall mining operation. (20 marks)
26. *** Describe briefly the advantages and disadvantages of advance/retreat system
of long wall mining operation. (10 marks each)
5
CHAPTER 5
LONG-WALL MINING – NON-CYCLIC OPERATIONS
27.* The nature of coal deposit is as follows.
Coal deposit; flat lying deposit, underlying the shale and overlying the sandy
shale, Thickness of deposit = 1.35 m, Average overburden depth = 85.0 m.
Required data can be assumed as you like.
Discuss the detailed development work procedure and sketch the layout plan
for extraction by long wall mining system. (20 marks)
28.* Describe the supporting systems for trepanner and discuss briefly support
methods fro the operation of long wall mining. (10 marks each)
29.* Discuss the operational functions of continuous ripping machine used in coal
extraction and road way excavations. (20 marks)
30. ** Describe the various methods of stone disposal and discuss operation of these
methods in coal mining. (20 marks)
31. *** Application of geomechanics is primary concerned in the selection of
underground coal mining systems. Therefore, it is necessary to investigate the
required data since in planning stage of mining operation. As a mining engineer,
discuss what data are required and define the importance of data in the
determination continuous long wall mining system.
6
SAMPLE QUESTIONS AND ANSWERS
1. Question
No (2) * From the geomechanics point of view, discuss the comparison of the
percentage recovery of rib pillar and square pillar for a following typical room and
pillar mining system. (Chapter 1)
Square
pillar
Rib pillar
Figure (1) comparison of the percentage recovery of rib pillar and square pillar
Answer:
In mine planning, it is desirable to be able to determine how much coal may
safely be extracted during first mining without danger of extensive roof collapse. In
order to increase the maximum amount of coal extraction, the roof span will be
maximized while still leaving enough in the pillars to safely support the roof or
hanging wall. On the other hand, to increase the coal extraction, it is necessary to
make the ratio of Ao/Ap (Area of opening/Area of pillar) larger than unity.
The average unit stress in pillar can be calculated by:
Mined Area + Pillar Area
……..
σp = σv
Pillar Area
(a)
From geomechanics point of view, designer can afford that, for a mined area.
As shown in figure (1), for a case the area is supported by square pillar, the square
pillar support gives the smaller opening width and the pillar width is larger than for
rib pillar. The square pillar is more resistant to destruction by shear stresses than is an
equal area of rib pillar because its dimensions allow the existence of larger lateral
confining pressures (Theory of triaxial state stress) in the interior of the pillar. The
lateral pressures reduce the magnitude of the effective shear stress induced by vertical
compressive stress.
7
Equation (a) can be rewritten for square pillar:
 W 
σ p = σ v 1 + o 
 W 
p 

Wo  σ p 
= 
Wp  σ v 
1
2
2
−1
…….. (a)
…….. (b)
By substituting the uniaxial compressive strength of pillar of coal, (or, rock),
Equation (b) becomes,
Wo  σ cp
=
Wp  σ v



1
2
−1
………
(b')
Where, Wo = width of opening
Wp = width of pillar
σcp = uniaxial compressive strength of pillar of coal
σv = vertical earth stress
...................................................................................
2. Question
No (3)* Explain the general procedure for the determination of maximum permissible
roof span for extraction of coal mining systems. (Chapter 1)
Answer:
Case (1)
In most of the stratified rocks, the strata are relatively sound and posses lateral
continuity. If the roof is intact stratified rock, the stability of the roof may be analyzed
by considering the roof a beam with a thickness.
Figure (1) Beam action of layer rock
By considering only a beam of unit width, the maximum tensile stress is:
γL2
Tmax =
……..
(a)
2t
8
For a design purposes, equation (a) can be rewritten as follow:
L=
2Rt
γ × SF
Where, L = maximum permissible roof span
R = modulus of rupture of the rock layer
t = thickness of the roof beam
γ = unit weight of rock layer
SF = safety factor
Case (2)
If the surrounding rock mass around the opening is moderately fractured, the stability
of opening is analyzed by the concept of arching action.
Reaction
W/2
Lateral Component
A
B
B
A
W/2
W/2
Figure (2) Arching action in fractured rock
Z = 2/3×t
Pressure diagram
t
L
Figure (3) Linear arch action
From the maximum resisting moment to the maximum bending moment of a
gravity loaded arch, Evans derived the equation for the maximum length of a self
supporting linear arch is: (Woodruff)
5Qt
3γ
Where, L = maximum length of self supporting linear arch
L=
Q = maximum compressive strength of the rock
t = thickness or vertical depth of the arch
γ = unit weight of rock
........................................................................................................
9
3. Question
No. (4)* Describe and discuss the underground mining systems of coal extraction
with necessary sketches. (Chapter 1)
Answer:
Practically all coal produced by underground mining is mined by one of the
following systems:
1. Pillar-mining systems. These include the room-and-pillar system and the block
system as well as the bord-and-pillar system.
2. Long-wall systems. These include the long-wall advancing and the long-wall
retreating systems.
(1) Pillar-mining Systems
Entries, cross entries, panel entries, and cross cuts, or rooms are driven
through the coal bed to divide it into pillars or blocks which may then be extracted on
retreat. Figure 1 shows the general layout for a room-and-pillar system. At present,
such hand loading methods have now been almost entirely replaced by mechanical
loaders and 90 percent of the coal produced by underground mines is loaded
mechanically.
With the room-and-pillar system entries, cross-entries, and panel entries are
driven to “block out “large panels of coal and rooms are turned off (usually at right
angles) from the entries. Rooms are driven as wide, or wider, than the entries. Pillars
left between rooms may, or may not, be extracted. Different mine employ different
sizes of pillar. Room pillars may commonly be from 20 to 40 ft wide and from 40 to
90 ft long. When room pillars are not to be recovered they are made as narrow as is
feasible while still leaving enough coal to support the roof.
(2) Long-wall Systems
“Main roads” or “mother gates” are driven through the coal bed and are semipermanent in nature and “gate roads” or “gate” leading to the long-wall face are
maintained.
A long face or “long-wall” is usually several hundred feet long and is served
by two or more “gate roads”; the long dimension of the working face being at right
angles to the direction of the gate roads. Some long-wall faces are several thousand
feet long while others are operated as a series of stepped faces, each several hundred
feet long.
10
The long-wall is advanced continuously by extracting slices of coal from the
face and transporting the broken coal to the gates from which it is transported to the
main roads or mother gates and thence to the main shaft. “Cross gates” are maintained
as angular cross-connections between gates in order to shorten haulage and ventilation
distances.
Figure 1: Room-and-Pillar System Mining Method
Figure 2: Typical layout for a long-wall operation
Figure 2 shows a layout for a typical long wall mining system as employed in
European mining practice.
............................................................................................................
11
4. Question
No (5) ** Describe the general procedure of mine development for room-and-pillar
mining method. (Chapter 1)
Answer
Development of Room-and-Pillar-mining
Development work comprises driving entries, entry cross cuts, room necks, or
other openings necessary to open up access to production places from which coal will
be produced later.
Development openings formerly were driven narrower than production places
and only a few men could work as a group in a development heading. Because the
working places were so narrow the rate of coal production was low and the cost of
development coal was high.
As mines became more mechanized and yielded larger quantities of coal at a
faster rate per shift it became necessary to increase the quantity of ventilating air. In
order to provide adequate airways it was necessary to increase the number of
development headings in a group and /or to make the development headings wider. It
is common practice now to drive development headings ten or twelve in a group and,
where roof conditions will allow it, the headings are made wider. As the width of the
development approaches that of the production place the productivity form
development approaches that from production places.
With the block system of room-and-pillar-mining, a series of entries, panel
entries, rooms, and cross cuts is driven to divide the coal into a series of blocks of
approximately equal size which are then extracted on retreat. Development openings
are most commonly driven between 15 and 20 ft wide. Pillars are most commonly
from 40 to 60 ft wide and from 60 to 100 ft long.
Generally, the room-and-pillar system is favored for the thinner coal beds (ie.,
less than 3-4 ft thick) while the block system is used more often in the thicker coal
beds where equipment can move about freely without the necessity of “brushing”
roadways to obtain headroom.
The bord-and-pillar system divides the coal into very large pillars or blocks by
means of very narrow openings. “Bords” approximately 12-15 ft wide are driven at
approximately right angles to the main coal cleat, and narrow openings (“wall”) about
6 ft wide are driven parallel to the main cleat. With the bord-and-pillar system pillars
12
as large as 130-200 ft square are blocked out and extracted on retreat. This system
may be considered a compromise between the block system and the retreating longwall.
..........................................................................................................
5. Question
No (6) ** Derive the general formula for determination of dimensions of the pillar in
designing for room and pillar mining system. (Chapter 1)
Assume that: bedded rock condition, which is thin compared with the span of
excavated opening and bond strength between beds are weak.
Answer:
For relatively large area with room and pillar mining, the average pillar stress can be
calculated by:
σp =
Atσv
Ap
(or)
σp = (
1
)σ v
1- R e
Where, σp = average pillar stress
At = total mined area
σv = vertical earth stress
Ap = area of pillar to be designed
Re = extraction ratio
(A - A p )
Am
Re = t
=
At
Am + Ap
Am = mined area
Let safety factor, SF = 4, the required pillar area is:
SF × σ v × A t
SF × σ v
Ap =
, and
Re = 1−
σ cp
σ cp
Where, σcp = uniaxial compressive strength of pillar
...................................................................................................
13
6. Question
No (7) *** Derive the formula of the following pillar designs for determination of
dimensions of the pillar in designing for the room and pillar mining system.
(Chapter 1)
Assume that: bedded rock condition, which is thin compared with the span of
excavated opening and bond strength between beds are weak.
(1) Regular spaced rib pillar
(2) Regular spaced rectangular pillar or square pillar
(3) Random spaced pillar with irregular shape
Answer:
Case (1)
Regular spaced rib pillar
Rib pillar
Where the roof and pillar are the same length,
Extraction ratio is:
Am
Re =
Am + Ap
Am
Ap
(or)
Re =
Unit length
L
L + Wp
Therefore, the width of rib pillar is:
SF × σ v × L
Wp =
σ cp − (SF × σ v )
Case (2)
Regular spaced rectangular pillar or square pillar
Square pillar
Ap = WpLp
Lp = length of pillar
Wp
L = span between pillars in both directions
L
Wp
Therefore,
Wp =
L
 kσ cp

 SF × σ v
  k −1 1+ k 
 + 
−

  2   2 
Where, k = Lp/Wp
For square pillar, k = 1
Lp
Wp
L
14
Case (3) Random spaced pillar with irregular shape
Ap
Am
=
SF × σ v
σ cp − SF × σ v
Ap
Am

 d 
Where, σ cp = σ c 0.778 + 0.222 
 h 

σc = uniaxial compressive strength of specimen for d/h = 1
d = diameter of specimen
h = height of specimen
.................................................................................................................
7. Question
No (9)* Calculate the dimension of pillar for the following condition: (Chapter 2)
- Depth of opening = 305.0 m
- Height of opening = 10 m
- Safe roof span = 10 m
- Average uniaxial compressive strength of rock mass = 80 MPa
- Unit weight of rock mass = 0.026 MN/cu-m
Solution:
Vertical earth stress = 305.0 × 0.026 = 7.93 MPa
Average uniaxial compressive strength, σcp = 80 MPa
For safety factor, SF = 4, the extraction ratio Re = 1 – (SF×σv/σcp)
Re = (80 – (4×7.93))/80 = 0.6035
Therefore, for regular spaced pillar, the pillar width is:
Wp = SF × σv × L / (σcp – SF × σv)
Wp = 4.0 × 7.93 × 10 / (80 – (4 × 7.93))
Wp = 317.2/48.28 = 6.57 m
(Answer)
Therefore, for regular spaced square pillar and k =1, the pillar width is:
Wp = L/√[(kσcp/SF×σv) + ((k-1)/2) – ((1 + k)/2)]
= L/√[(kσcp/SF×σv) – 1]
15
= 10//√[(80/(4×7.93)) – 1]
= 10/√1.5221 = 8.1055 m
(Answer)
.....................................................................................................
8. Question
No (19)* Discuss the general procedure of mine development for long wall mining
system. (Chapter 4)
Answer:
The development of long wall mining to its present state of the art has mainly
taken place with substantial gains in the last ten years. Mechanized long wall has been
the result of several related major innovations:
(1) Development of the flexible armored face conveyor which can be installed
along the face and moved forward without disassembly.
(2) Development of face machines such as single and double drum Shearer and
coal plows which operate in conjunction with the armored conveyors.
(3) Development of self-advancing hydraulic roof supports which now include
chocks, chock shields, and shields.
(4) Reliance of caving of the immediate roof based on proper planning and
development.
(5) Use of pneumatic stowing to enable extraction below overbuilt areas.
The most recent developments in the mining of coal by long wall include the
introduction of shield supports, single-entry long wall development.
..........................................................................................................
9. Question
No (25) ** Discuss the factors to be predetermined for the development and
equipment selection prior to planning and design of long wall mining operation.
(Chapter 4)
16
Answer:
The following factors should be determined before development and
equipment selection for long wall mining,
(1) Deposit size
(2) Deposit thickness
(3) Deposit dip
(4) Overburden and immediate roof
(5) Face slabbing
(6) Rock burst potential
(7) Support stability
(8) Artificial roof
(9) Surface subsidence
(10) Regulations and recovery
(11) Manpower
(12) Ventilation
(13) Water
(14) Materials handling, and
(15) Spontaneous combustion.
(1) Deposit Size
The size of a deposit to be considered for long wall mining should be
large enough to justify capital outlay for equipment and development. In the US, this
would be 283 hectares (700 acres) or more. The most successful operations are
located in areas where several long wall panels can be located adjacent to each other.
A deposit and operation of this type will result in continuous caving and conditions
that can be predicted with a reasonable of accuracy.
(2) Deposit thickness
The thicknesses of deposit which have been successfully mined by long wall
methods have ranged from 0.61 m (2 ft) to more than 61 m (200 ft). The most
common thickness for each method is: single-lift mining, 1.22 to 4.57 m (4 to 15 ft);
multi-lift mining, 4.57 to 9.14 m (15 to 30 ft); and sublevel caving 9.14 to 61 m (30 to
200 ft) or more.
17
(3) Deposit dip
Deposits have been with dips of up to 1.22 rad (70°). The most
common dip for long wall mining with out major equipment modification is the 0 to
0.26 rad (50°) range. Under special conditions vertical seams have been mined in
Germany, Spain, and Scotland.
....................................................................................................................
10. Question
No (20)* Discuss the long wall mining operation of coal and give an example with
necessary sketches. (Chapter 4)
Answer:
Long wall Mining Operation of Coal
After the panel has been developed, long wall equipment is installed and
mining begins (See figure 1). As the face advances, overlying strata is allowed to
cave behind the support units (See figure 2). In those areas where caving must be
minimized, the area behind the supports can be filled with waste.
In general, long wall mining areas for coal are developed in blocks which
range in width from 76.2 to 365 m with a length of 304 to 1829 m (See figure 3).
The size of the block is generally based on equipment life, geologic conditions, and
previously projected mine layout.
In most countries, long wall development utilizes single-entry head and tailgates (See figure 4). In the US, multiple-entry head and tail-gates are required by
law (See figure 3). Long wall blocks are generally developed by continuous miners
for retreat panels and continuous miners and/or the long wall Shearer for advancing
panels.
Mining methods most commonly used in coal seams up to 4.57 m thick
include advancing, retreating, and advance-retreat. Several variations and
combinations of these methods will be discussed later.
In coal seams over 4.57 m thick, mining methods include simultaneous liftretreat, non-simultaneous lift-retreat, sublevel long wall caving-retreat, and nonsimultaneous lift-advance/retreat. Although there are no thick seam operations in
18
the US at this time, some are in the planning stage and many in Europe and Japan
have been in operation for a number of years.
Face equipment
Support
Shearer or plow
Face conveyor
Stage
loader
Return air
GOB
Intake air
Belt conveyor
Face
DIRECTION
OF MINING
Long wall support
MAIN LINE BELT
STORAGE UNIT
Belt storage unit
Figure 1: Typical long wall panel prepared for
Figure 2: Typical long wall panel during mining
19
TAILGATE
ENTRIES
FACE WIDTH
HEADGATE
ENTRIES
Face width
START
ENTRIES
Bleeder entry
PANNEL LENGTH
Panel length
Head gate entry
DIRECTION
OF MINING
Tailgate entry
DIRECTION
OF MINING
Starter entry
BARRIER PILLAR
BARRIER PILLAR
MAIN
ENTRIES
Figure 3: Typical multiple-entry long wall panel
Mains
Figure 4: Typical single-entry long wall panel
.................................................................................................................
20
11. Question
No. (26)** Discuss briefly the advantages and disadvantages of advance/retreat
system of long wall mining operation. (Chapter 4)
Answer:
Advance/Retreat System of Long Wall Mining:
This system is closely allied to the multi-entry method. Generally two entries
are driven on each side of the face and about 61.0 to 91.4 m in advance of the face.
The two inner gates are allowed to close behind the face, and coal transport is from
the inner gate through a stenton (narrow road) and down the main outer gate (See
Figure 1). The two inner gates are usually at seam height, 1.22 to 1.37 m, whereas the
two other gates are 3.7 to 2.5 m. Although capital-intensive, this system is practiced
by several single face mines in Great Britain.
Its advantages are:
(i) Dirt disposal can be separated from coal production.
(ii) Production from the face can begin much sooner than with normal retreating
methods.
(iii) Careful planning of layout will allow the outer roads to be used for subsequent
faces.
(iv) Where conditions are suitable, it allows both arched and square sets to be used.
The disadvantages are:
(i) In some instances, the face has caught up with the headings and in this
respect the development is not entirely divorced from the face. Failure or
delays in any one head can affect the entire panel.
(ii) Three of the four gates have a conveyor running their entire length and
mechanized equipment in each head “move-up” of equipment occurs fairly
frequently.
(iii) Ventilation of the headings has to be provided, and there could be
difficulty in ventilating and/or stone disposal in the utilization will be low.
21
Advance
Retreat
Stop line
Start line
Advancing
Pannel
(worked out)
Retreating
pannel
Advance
Stop line
Figure 1: Advance/Retreat mining system
............................................................................................................
12. Question
No (27)* The nature of coal deposit is as follows.
Given: Coal deposit; flat lying deposit, underlying the shale and overlying the
sandy shale, Thickness of deposit = 1.35 m, Average overburden depth = 85.0 m.
Required data can be assumed as you like.
Discuss the detailed development work procedure and sketch the layout plan
for extraction by long wall mining system. (Chapter 5)
Answer:
If the mine area is most suitable for long wall application, it is decided to
develop in initial stage. Development for long wall panels will depend on the type of
deposit to be mined and the method of long wall to be used. A typical multi-entry
layouts for single lift advancing or retreating long wall is shown in Figure (1)
22
H
B
C
N
M
L
J
G
F
Coal
Thickness = 1.35 m
R
A
Geological section of coal deposit
Figure (1) Typical long wall development
Retreat system:
For a retreat system, headgate (F) and tailgate (G) entries are driven the length
(J) of the block to be mined perpendicular to the mains (A). The bleeder entries (B)
and the starter entry (C) are then connected at the designed width of the panel (H).
The material mined during development is transported by conveyor belts installed in
entries (L) and (M) of the head and tailgates as development advances.
Up on the completion of development, the conveyor belt located in the
headgate entry (L) may be left if longwall mining is soon to start or it may be
removed and restalled when long wall mining begins at a much later date. The tailgate
conveyor belt (M) is removed upon completion of development.
Advancing System:
an advancing longwall system is developed by driving the head (F) and
tailgate (g) entries perpendicular to the mains (A) until they reach a projection of the
inside face line of the barrier pillar distance (R). At that point, two or more entries are
driven the width of the panel (H) to connect the head and tailgates. As in the retreat
system, roof bolts or steel sets are used for support.
.................................................END.......................................................