24686499 education mine support system docs03

7/28/2019 24686499 Education Mine Support System Docs03

1/49

Unit 15Mining

Support Systems


2/49

In this unit, you will learn about mine support systems,

such as the use of wood, concrete, and steel.


3/49

After completing this unit, you should be able to:

List the major support systems

Explain the properties of timber, concrete, steel

List the methods of mine construction using wood:

List the uses of concrete in mine construction

List the uses of steel in mine construction

Explain the use of natural pillars

Explain the use of hydraulic support


4/49

Mining Support Systems

In underground mines, the

walls and the roof or back of

stopes are not usually self-

supporting.

Below shallow depths, the

pressure of the rock abovemay cause surrounding rock

to move into excavations.

Some kind of support for the

surrounding rock is often

required for underground

excavations which are to bekept open to allow mining

operations to continue.


5/49


Underground openings

will attempt to assume a

dome or arch shape by a

progressive breaking off of

the rock in the roof.

If support is placed

immediately after the

excavation is made, the

breaking down of the roof

may be greatly prevented.


6/49


The materials used for

supporting mine openings are

wood, concrete, steel, brick,

and stone. The first material

used was probably wood

since it could be readilyavailable and was easily

framed. However, because of

its short life under conditions

of mining and its increasing

cost, several substitutes are

now in use. Reinforcedconcrete and steel are both

now commonly used .


7/49

Physical Properties of Wood

Moisture Content

Moisture content of wood is "the ratio of the weight of water in the

wood to a reference weight"

Stored wood or chips are about 15-25% moisture while "air-dry" chips

are about 10% moisture

Freshly cut or "green" logs will typically be 40-45% water on a wet

basis

wood contains 1/2-2/3 of the maximum amount of water it

could hold if all the cell lumens and other wood voids were

completely filled with water

As moisture content goes up, the strength of wood decreases

The strength of wood in the longitudinal direction is much greater

than in the radial or tangential directions


8/49

The timber industry prefers dry basis while the pulp and paper industry

prefers wet basis because wet basis calculations are more convenient to

calculate the required pulping chemicals

Distribution of Water

Water is held within the cell at hydrogen bonding sites, removal of this

water causes cell shrinkage

Fiber Saturation Point (FSP)

just enough water to completely saturate the cell wall substance, but with no

liquid water present in the lumen

typically 20-23% MCw

maximum swollen volume at FSP

increasing the water content above the FSP results in no further changes in

wood dimensions


9/49

Equilibrium Moisture Content

moisture content achieved by wood or paper upon prolonged

exposure to the environment

depends on the relative humidity, temperature and drying

history

Shrinking and Swelling

Changes in wood dimensions occur only below the FSPwith a change in content of bound water

these changes are anisotropic with respect to the

tangential, radial, and axial directions

Fibers (Tracheids)


10/49

expand 2-3% in radial direction

expand only 0.1-0.3% in the axial direction

Pits cause fiber flexibility in radial direction

angle, the smaller the dimensional change

Why more shrinkage in radial direction?

latewood cells have more cell mass

for a given moisture content change, latewood changes more than

earlywood

Tangential direction

latewood changes dimensions, which causes a large expansion

expansion in the tangential direction

Causes a T/R ratio of 1.4-2.0When a dimensional change is negative (shrinking)

T/R differential causes fissures to occur along the radial plane

results in wood splitting


11/49

Specific Gravity

density of a substance relative to a reference density Specific gravity gives a general idea of how much wood fiber or wood

substance can be obtained per unit volume of a given type of wood

one problem with wood density - both wood mass and wood volume

change with moisture content

Basic Specific Gravity

based on dry wood weight and water swollen volume

obtained by measuring the wood sample's water-swollen volume and

the OD weight of the same sample

the volume is known as the "green volume"


12/49

the higher the density, the more energy that is required toprepare the pulp

Wood Strength

Ability of wood to resist tensile, compressive, and shear forces

Influences wood grinding, chipping, and chip refining

General Nature

wood behaves as an elastic material for small strains of

short durationlarger strains or extended time of applicationresults in plastic deformation


13/49

Wood Failure

permanent deformation

Due to structural complexity, wood doesn't have the same

strength characteristics in all three planes, i.e., it is

anisotropic

for tensile strength

Axial > Radial > TangentialMoisture Effect

Below the FSP, increase in moisture content results in

lower wood strength

Temperature Effect

causes decrease in wood strength at a given moisture

content

interactions among temperature, time, pH determine

overall wood strength


14/49

Wood No substitutes, however, have

the peculiar advantage of wood in

failing gradually when loaded

beyond its strength and giving

warning of approaching failure by

audibly cracking.

A mine timber, due to the cellularnature of wood, may be

considered as a bundle of parallel

tubes. It resists pressure against

the ends much better than

pressure from the sides.

End pressure will split the timberlengthwise. Side pressure will

squeeze the cells together,

compressing the timber.


15/49

Wood

The density of wood is often a

good indicator of its strength,

for it represents the actual

amount of wood substance in

a unit volume.

Strength is affected by such

defects as decay, knots,

shakes, checks, splits, and

crossgrain.

Moisture affects the strength

of timber. Drying stiffens andstrengthens the wood fibers.


16/49

Types of Mine Timbering

a) One Piece Set

This term applies to a single

stick of timber, called a post,

stull, or prop. Post and prop

are applied to vertical

timbers, and stull is appliedto horizontal or inclined

timbers.

b) Two Piece Set

A typical 2 piece set consists

of a cap and a single post.


17/49


18/49


d) Blocking & Wedging

Sets are held in place by

blocking and wedging. Blocks

are placed at the ends of thecaps and on the caps over the

plate. When blocks take the

pressure on the side instead

of on the ends of the fibers,

they yield under pressure,

saving the set for a time.


19/49


e) Square Sets

For stopes, narrow veins and

small ore bodies, timbering with

stulls may provide temporary

support. In large ore bodies stulls

can not be used. Timbering in

square units or hollow cubes can

used instead.

The 4 vertical timbers of a square

set are called posts. Caps and

girts are placed on top of theposts, a line of caps being at right

angles to a line of girts.


20/49


f) Chutes

Chutes for ore or waste rock

are made by lining a set on

the inside with lagging and

carrying this lining up floor

by floor as the stoping

progresses. An inclined

bottom and some form of

wooden or steel gate are

placed in on the sill floor for

drawing off the rock.


21/49


g) Shaft Sets

Timber shaft sets are similiar to

square sets. Shaft sets provide

compartments for hoisting, a

manway for ladders and for water

compressed-air pipes and

electric cables.

Since shaft sets are placed from

the surface downward, each new

set must be supposed from the

set above until it is blocked in

place. Blocking, wedging and

lagging complete the work oftimbering.


22/49


h) Bearers

At 50 to 100 ft. intervals long end

plates are used, extending into

hitches cut into the wall rock,

their purpose being to carry the

weight of timbering up to the next

bearers above in case the shafttimbering becomes loosened

from its blocking against the

rock.

i) Guides

Wood guides for cages or skipsin vertical shafts are commonly

fastened to the dividers and end

plates


23/49


j) Cribbing

Cribbing offers a strong

method of supporting the

back and may take various

forms. The simplest cribbing

is made by the laying oftimbers on top of each other

at right angles. This forms a

hollow pen which can be built

up to any desired height. The

pen can filled with waste rock

for greater support.


24/49

Concrete

Permanent openings are

often sprayed with a

concrete lining to prevent

oxidation of the walls.

Reinforced concrete is

used as a permanent shaft

lining at many mines, both

as a continuous lining and

as rings spaced severalfeet apart, where the shaft

is in solid rock.


25/49

What is concrete?


26/49

Concrete is not found in nature the way we would find

aluminium, nickel or iron. Concrete is formed from

combining water, a special cement and rock:

PORTLAND CEMENT + H2O + ROCK = HARDENED

CONCRETE + ENERGY(HEAT)

A common mistake people make is to use the words

cement and concrete interchangably. It is important toremember that cement is only a component of concrete and

concrete is the structural material. The cement used in

concrete is not used as a building material because it would

be too expensive and not as strong as concrete.


27/49

Cement is a general name for a material that

binds other materials together. Yes, it isanother name for glue. There are many

materials which we would classify as

cements and they are usually identified with

certain uses, and can produce different

types of "concrete".

The structural concrete used in bridges and damsand other types of road surfaces is made from

Portland cement (#). This cement binds the rock

(also called aggregate) together to form concrete.

Portland cement is a mixture of processed

limestone, shales, and clays which contain the

following compounds: CaO (lime), Al2O

3(Alumina),

SiO2 (silica) and iron oxides. Properties of the

cement will vary depending on the relative

amounts of these compounds.


28/49

Adding water to the dry cement starts a chemical reaction (hydration).

While the mixture of cement, water, and rock is fluid, it can be poured into

molds (called formwork) of arbitrary shape. This is a valuable property ofconcrete which allows us to build structures with the many different

shapes. The compound gradually hardens into the desired final shape.

The water/cement ratio (w/c) of the mixture has the most control over the

final properties of the concrete. The water/cement ratio is the relative

weight of the water to the cement in the mixture. The water/cement ratio is

a design criterion for the engineer. Selection of a w/c ratio gives theengineer control over two opposing, yet desirable properties: strength and

workability. A mixture with a high w/c will be more workable than a mixture

with a low w/c: it will flow easier. But the less workable the mixture, the

stronger the concrete will be. The engineer must decide what ratio will

give the best result for the given situation. This is not an entirely free

choice because the water/cement ratio needs to be about 0.25 to completethe hydration reaction. Typical values of w/c are between 0.35 and 0.40

because they give a good amount of workability without sacrificing a lot of

strength.


29/49

The other important

component for strength is the

aggregate, the rock that isbeing bound by the hardened

cement. Aggregate is what

makes the difference between

hardened cement and the

structural material, concrete.

Aggregate increases thestrength of concrete and is a

fundamental economical

factor because it takes up a

large volume of the concrete

and is much less expensive

than an equivlent volume ofcement. To make very strong

concrete requires a low w/c

and strong aggregate.


30/49

Concrete

a) Guniting

Dry concrete mix containing

certain additives can be moved

with compressed air to a

nozzle. It can then be mixed

with the proper amounts of

water in the nozzle and blownagainst a rock face. This shell

is called gunite or shotcrete,

depending on the aggregate

size.


31/49

b) Cemented Rockfill (CRF)

Generally consist of waste rock mixed with a cement slurry to

improve the bond strength between the rock fragments. Methods

of placement involve mixing the rock and cement slurry in a

hopper before placing in stopes, or percolating a slurry over the

rock after it has been placed. The waste rock can be classified or

unclassified (sorted)

CRF contains a mixture of coarse aggregate (


32/49

Concrete

Hydraulic sandfill can consist either of

classified mill tailings or naturallyoccurring sand deposits mined on

surface . Hydraulic sandfill is prepared

by dewatering the mill tailings stream to

a pulp density of approximately 65-70%

solids (depending on S.G) and then

passing it through hydrocyclones to

remove the "slimes" and retain the sandfraction for backfill. Slimes are removed

to improve the percolation rate of the

backfill.

The backfill mixture is hydraulically

pumped from surface through a network

of pipes and boreholes to the stope.


33/49

Successful sandfills have permeability

coefficients in the range of 7x10-8 m/s to

7.8x10-5 m/s corresponding to a medium

silt to coarse sand. To overcome the lack of

true cohesion in the sandfill, cement and

other binders are added.

Note that backfill strength decreases with

water content and the water content needed

to transport sandfill is far in excess of what

is required for cement hydration.

Hence, mine operators are moving towards

less water in the fill to decrease cement and

binder consumption

Flow velocities in excess of 2 m/s are

required to maintain a homogeneousdispersion of the fill components in the

slurry


34/49

Sandfill is a mixture of concrete, water, and sand. This system is popular in

cut and fill mining methods. As the ore is removed, the opening is filled with

sandfill to provide support and a strong floor for the next slice

Timber Structures Required for Sandfilling


35/49

Timber Structures Required for Sandfilling


36/49

Mine Backfill

Filling stopes with waste material is a common procedure in cut

and fill mining and square-set mining for ground support. Filling

serves to minimize or control subsidence and to make it possible

to extract pillars of ore left behind in earlier stages of mining.

There are 4 types of mine backfill:

1. Dry Fills

2. Cemented Rockfill

3. Hydraulic Sandfill

4. Paste Backfill


37/49

Mine Backfill

1) Dry Fills:

Dry fill generally consists of surface sand, gravel, open pit waste

rock, underground waste rock, smelter slag. Material is generally

unclassified except to remove large boulders. The dry fill is

usually transported underground by dropping down a raise from

surface directly into a stope or to a level where it is hauled to a

stope with an LHD or trucks.

The fill usually contains some adsorbed surface moisture.

Suitable for mechanized cut and fill or avoca or other method

where structural backfill is not required.


38/49

Mine Backfill

2) Cemented Rockfill

Generally consist of waste rock mixed with a cement slurry to

improve the bond strength between the rock fragments.

3) Hydraulic Sandfill

Generally consists of cement and classified mill tailings and ishydraulically pumped from surface through a network of pipes

and boreholes to the stope


39/49

Mine Backfill

4) Paste Backfill

Paste backfill is a high density backfill (>70% solids depending on SG). In

order to pump material at this density, a component of fines is required.

As a general rule, the fines content (


40/49

Mine Backfill

Backfill Costs and Benefits

Benefits Costs

ECONOMIC:

Backfill allows higher extraction ratio

Backfill reduces dilution

Aids in pillar recovery SAFETY:

Backfill improves regional stability in the

mine

Backfill can reduce risk of rockbursting

ENVIRONMENT:

Backfill provides a means of disposing of

waste rock and mill tailings, thereby

minimizing surface disturbance

ECONOMIC:

Backfill costs money particularly if

binders are used

Backfill introduces delays in the mining

cycle

Additional manpower and infrastructure

Additional dewatering costs

Dilution due to backfill

SAFETY:

Risk due to bulkhead failure and

liquefaction of tailings Risk due to collapse of consolidated

backfill walls ENVIRONMENT:

Groundwater contamination


41/49

Mine Backfill

Mining Methods Requiring Backfill Conventional Cut and Fill

Mechanized Cut and Fill

Captive Cut and Fill

Drift and Fill

Mining Methods Employed with or

without Backfill

Longhole

Longwall Mining Room and Pillar

Open Pit

Mining Methods Employedwithout Backfill

Shrinkage

Block Caving

Sub-Level Caving

Square Set Resuing

Undercut and Fill

Avoca


42/49

Steel sets are used in themine entries of a number of

coal mines and in shafts of

metal mines. In most cases,

the load is taken as a factor

multiplied into the sum of the

width and height of thetunnel.

Steel can be erected in a

shorter time and by fewer

men than timber. Timber rots

and decays behind concretelining, which results in an

uneven load on the framing or

lining.

Steel


43/49

Steel - Roof Bolts

The support tool most employed in

hard rock mines is the rock bolt. The

traditional mechanical rock bolt

("point anchor" with an expansion

shell) has been largely replaced in

hard rock mines with resin bonded

steel bolts for permanent headings

and friction (split-set) bolts for

temporary (6+ months) support.

Other types of rock bolts see

occasional application and cement

grouts or cartridges have been used

instead of resin cartridges. The traditional mechanical bolt was

torqued at installation to 50-60% of

the tensile strength of the bolt and it

was considered that this tension had

to be maintained for the bolt to

remain effective.

These bolts were sometimes very difficult to

install properly. Over a period of time, the

mechanical anchor was subject to creep in

the hole resulting in loss of tension and amaintenance chore to retorque or replace

bolts.


44/49

Steel - Rock Bolts

The process of rock bolting consistsof 3 steps:

1) anchoring the bolt in a hole,

2) applying tension to the bolt to

place the rock under

compression parallel to the bolt

and

3) placing the bolt in such a

pattern that they will properly

support the rock structure.

The basic principle of bolting is that

it should make the bolted rock anintegral part of the supporting

structure.


45/49

Steel Arches

Tunnel support systems made of

steel are roughly in 5 types:

continuous rib

rib and post

rib and wall plate

rib, wall plate and post

full circle rib

The factors which must be

considered in choosing a support

system are method of excavation,

rock behaviour and the size and

shape of the tunnel cross-section.


46/49

Steel Yielding Arches

A Yielding Arch are employed tosupport loads caused by changing

ground movement or faulted and

fractured rock. Steel arches are

made of segments which overlap at

the joints. When the ground load

exceeds the design load of thearch, yielding takes place at the

joint.

During yielding, as the overburden

settles into a natural arch of its

own in order to bring all forces intoequilibrium, the shortened arch

increases strength.


47/49

Hydraulic Support

Shortwall and Longwall miningutilize hydraulic supports These

supports protect the miners and

equipment working in the face.

They move up as the face

advances, and the roof behind

them is allowed to cave. Thesupport, called a Shield, and two

large hydraulic cylinders hold

the canopy against the roof. The

end has a hinge arrangement so

the support will resist lateral

movement of the roof.


48/49

Natural Pillars

Small ore bodies may be

mined from wall to wall

without any pillars being left.

Where ore bodies are larger,

pillars or ore are left to keep

the roof span to a safe

dimension. Pillars may beregularly or randomly spaced

and the method may be

applied to either horizontal or

inclined deposits. It finds its

greatest application in flat-

lying, bedded - type deposits.


49/49

You have reached the end of Unit 15

24686499 education mine support system docs03

Documents