room and pillar mining method

A short series of lectures prepared for the

Fourth year of Geology, Tanta University

2014- 2015

by

Hassan Z. Harraz

[email protected]

This material is intended for use in lectures, presentations and as handouts to students, and is provided in Power point format so as to allow customization for the individual needs of course instructors. Permission of the author and publisher is required for any other usage. Please see [email protected] for contact details.

Topic 3: Underground Mining Methods

Room and Pillar method

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Outline of Topic 3:

Practical importance of the Room and pillars method

Different applications of the R & P method

R & P in hard rocks:

Conditions of deposit for application of R & P in hard rock

R & P equipment in hard-rock

R & P in soft rocks:

Conditions of deposit for application of R & P in soft rock:

Characteristics of R & P method in non-coal applications

R & P classic

Step mining

Post-pillar mining

Configuring the R & P method in coal

Main design parameters of R & P in coal:

a) dimensions of the galleries

b) dimensions of the pillars

c) Mining with or without recovery of pillars

d) number of front panel

Advantages and Disadvantages

Appendix A: Screws Ceiling

Appendix B: Design of pillars in coal mine

Prof. Dr. H.Z. Harraz Presentation Room and Pillar method 2

We will explore all of the above in Topic 3.

Room and Pillar method:


Practical importance of the Room and pillars method:

Method widely used in coal mines and non-carbon

worldwide.

In the 70s in the USA, over 50% of production

(underground) coal came from this method.

Currently, most of the production is still done by R

& P.

It is the mining method used in underground coal

mining in southern Brazil.


Different applications of the R & P method:

In hard rock: for example, limestone,

dolomite, metals (lead, zinc, copper, gold,

etc.)

In friable rock (soft rock): (eg coal, potash,

salt).


R & P in Hard Rocks

Conditions of deposit for application of R & P in hard

rock:

ore resistance: moderate to high;

resistance of the host rocks: moderate to high;

Form: tabular lenticular (variable);

diving: generally <30° to the horizontal;

ore grade: low to moderate;

uniformity: variable;

Depth of deposit: shallow to moderate.


Recoveries of extraction (% typical extraction) in the R & P are variable depending on the type of ore, as shown in the table below.

From: Underground Min. Methods: Eng. Fundamentals and International Case Studies, 2001, W.A.Hustrulid & R.Bullock.

Compare coal and non-coal in USA!


Recoveries of extraction (% typical extraction) in

situations of hard rock :

can be as high as 85% and are generally obtained in mining in advance;

recoveries pillars are rare because pillars are small and removal is at risk;

sometimes the pillars are irregular and systematic removal is impossible.

R & P equipment in hard-rock: diesel in general (FEL-front end loaders, LHD's, jumbos, trucks); all

mobile equipment (on tires) due to significant horizontal extension of the ore bodies.


Configuration of R & P in hard rock ... (Mina Nova - AngloGold-GO/BRA, ouro)


R & P in Soft Rocks

Conditions of deposit for application of R & P in soft

rock:

resistance of the ore: mild to moderate;

resistance of the host rocks: moderate to high;

forms: tabular (in layers), large lateral extent;

diving: generally horizontal or <15o with the horizontal;

good uniformity of grades and thicknesses of ore;

depth of deposit: on carbon, preferably less than 600

meters.


Characteristics of R & P method in non-coal applications :

this method withdraws ore rooms (stopes) that remain open

during mining; pillars are left to support the roof;

the ceiling should remain intact (roof bolts are commonly

installed to reinforce the strata);

the rooms and the pillars are generally arranged in regular

patterns; pillars can present cross section circular, square or

rectangular;

to obtain maximum recovery of ore, the pillars are made with the

smallest possible dimensions.


Characteristics ... (cont):

- Geological variations originate from different

variations of the method :

R & P classic

Applies to horizontal ore deposits, with mineable strata ranging from moderate to very thick. The stopes, the floor plan is maintained, allowing the transit of vehicles on tires. Ore bodies are mined large vertical dimension in horizontal slices, starting at the top and ending with the floor dismount on countertops.


R & P classical (complete mechanization):


R & P classical (partial mechanization): Prof. Dr. H.Z. Harraz Presentation Room and Pillar method

14

Characteristics ... (cont): Step mining

It is an adaptation for the case where the dip of the ore body is too big (15o to 30o) to use equipment on tires at the same time that its thickness is relatively small (2 - 5 m). A special orientation of transit and stopes galleries creates areas with horizontal floor, allowing the use of equipment on tires. The mining progresses from top to bottom in mining panels.


Characteristics ... (cont):

Post-pillar mining

Applies to inclined deposits

with dip between 20o and 55o.

Have large vertical dimension,

and filling the mined space

suffers (backfilling). The filler

maintains the stable rock

(minimizes columns) and

serves as a working platform

for the next slice.


Configuring the R & P method in coal :

regularly spaced openings formed orthogonal arrangement of square or rectangular pillars;

Development (axle) and exploitation (panel) galleries usually have very similar characteristics; several parallel galleries and connected by dashes;

is a method for mining large-scale, with the various panels can be conducted simultaneously;

the basic unit of mining is the panel that defines the area to be worked and ventilated.


Configuration ... (cont.)

Main axis

Mining panel

Galleries return air

contaminated


Overview of Coal mine employing R & P (Mine Fontanella, Treviso-SC)

Inclined plane and

main access

Ventilation shaft

(exhaust)

Mining panel (project)

Axis development


Main design parameters of R & P in coal :

a) dimensions of the galleries

b) dimensions of the pillars

c) Mining with or without recovery of pillars

d) number of front panel


a) dimensions of the galleries:

widths of 5 to 6m are currently quite common;

height is limited by the thickness of mineable coal (in SC deposits, the section is rectangular with heights ranging from 2.0m to 3.5m approx.);

condition shoring ceiling;

interfere in operational and safety aspects;

dimensions are used to calculate the recovery of mining.


Shoring ceiling:

Currently, for the Boards and Pillars method to succeed, the deposits must have a particular geological condition of the roof. The overlying layer extracts the coal must be self-sustaining condition or be likely anchor (there are several types of anchoring screws ceiling, with different principles of operation - see Appendix A).

Due to costs, shoring bows with metal or wood are used only in

restricted areas of the mine.


b) dimensions of the pillar:

influence on operational aspects (eg transport

distances) and safety (risk of collapse of pillars);

influence the recovery of mining;

Types of pillars:

square and rectangular pillars

chain pillars (internal pillars of the mining panel)

barrier pillars (the pillars of safety)

Further reading on the pillars R & P: Underground Min. Methods: Eng. Fundamentals and International Case Studies, 2001, W.A.Hustrulid & R.Bullock; cap. 59.1 a 59.3.


Recovery of extraction:

portion of reserves mined in relation to the total

reserves.

Comparative Example Retrieval. the panel:

Panel with 14m square pillar aside and gallery 6m

wide ...

Panel with 11m square pillar aside and gallery 6m

wide


1) area of pillar Ap = 142

total area At = 202

Sum. = (1- Ap/At) x 100% = 51%

2) Ap = 112

At = 172

Sum. =

(1- Ap/At) x 100% = 58%


0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0.70

0.75

0.80

0 5 10 15 20 25

largura dos pilares quadrados (m);

(para galerias de 6m de largura)

recu

pera

ção

Comparative recoveries of mining panel with square pillars and galleries 6m wide :


C) Mining with or without recovery of pillars:

most common ways of mining ...

in advance and without recovery of pillars (as is done in Brazil);

with recovery of pillars in reverse (made in several countries, eg USA).


Recovery strategies pillars :


Equipment to aid

in the recovery

of pillars:


Example of mining panel:

Mina Leão (CRM-Rio Grande do Sul)


Mining with pillar recovery :


d) number of fronts for efficient operation of equipment in the panel:

The advancement of the panel is done through several parallel galleries (5 galleries or more, depending on the mine) connected by dashes. It takes more fronts in conventional mode (drill-blast-load-haul) and less fronts with continuous miner. The use of continuous miner least affects the roof and pillars and produces thin material.


Information necessary (essential) for planning of

a coal mine:

Map layer thickness;

depth map layer;

quality maps ...

ash content, could

heating value, sulfur, volatile;

geomechanical data from ore, floor and ceiling;

structural map (faults, dykes, ...)


Advantages of the method R&P: Moderate to high productivity (m3 / man-hour)

Moderate cost of mining (relative cost = 0.3)

Moderate to high production rate

High degree of flexibility (allows variable thickness in the ore); method easily

modifiable; may operate simultaneously on multiple levels

Allows high degree of mechanization

Selective method, lets leave waste material on site.

Does not require much anticipated development.

It can be operated on multiple fronts.

Reasonable recovery without extraction of pillars (50-65% on carbon), low

dilution (10-20%).

Comparing with Longwall: LW is Pratic. inflexible, requires greater investment,

there is subsidence on the surface, has high yields (must be a market for the

product!), LW global recoveries are possibly lower than R & P with pillar

recovery.

In direct comparison with Longwall: LW is Pratic. inflexible, requires greater

investment, there is subsidence on the surface, has high yields (must be a

market for the product!), LW global recoveries are often lower than R & P with

pillar recovery.


Disadvantages:

Requires ongoing maintenance of the roof and

eventually the pillars. The tension in the open

spaces increases with depth.

Significant capital investment for extensive

mechanization.

Loss of ore in pillars.

In the case of R & P in hard rock, it may be

difficult to achieve good ventilation to dilute

contaminants in due to low air velocity panel in

large open spaces.

Requires good technical and engineering support. Prof. Dr. H.Z. Harraz Presentation Room and Pillar method 36

Equipment used in the mining Room and

Pillar method for coal

Conventional mechanized sets;

Mechanized sets with LHD's;

Continuous miners ;

Drag chutes + Bob-cat loaders.


Conventional mechanized equipment set :


Example conj. conventional mechanized used in the coal Metropolitana SA (SC-BRA)

12

3

45

9

68

7AR LIMPO

AR SUJO

1 - LOADER

2 - SHUTTLE- CAR

3 - PERFURATRIZ DE TETO

4 - CORTADEIRA

5- PERFURATRIZ DE FRENTE

6 - ALIMENTADOR

7 - CORREIA TRANSPORTADORA

8 - CENTRO DE FORÇA

9 -VENTILADOR


Equipment operating with continuous miner :

Shuttle-car

Roof bolter


Configuration

panels:

Continuous miner.


panelsDrill & blast


Considerations for R & P applicability of the method on coal

characteristics layers

for continuous miners and conventional drill & blast equipment - layers between 1 and 4m;

structural characteristics have great influence on the success of the method;

Structural characteristics ...

presence of faults and dikes

can derail the sequence mining panel (for sufficiently large vertical displacements in the coal layer);

can cause change in the pattern of advancement and equipment from the mine;

require special care in shoring ceiling (ceiling reinforcement, screens, ...)

dipping mineralized layer

if severe can derail transportation on tires that work well with shuttle-car up to 14 or 15% (in hard rock the slopes used +/- go up 20%, with LHD's);

equip. crawler to work +/- 25%;

Thickness of overburden:

Vertical pressure resistance and limited exploration of sedimentary rocks coal no more than 1200m overburden;

in USA, Chambers and-pillars are commonly used overburden of up to 600m and 600m and 800m between longwall;

in southern Brazil, overburden to coal mines in activity is generally less than 300m.


Features floor and ceiling ...

R & P accepts ceiling varied conditions due to the

different alternatives shoring. The reduction of the

spans can rid the roof of disrepair, but results in

loss of recovery.

Longwall requires shales / siltstones (rocks with

low resistance as an immediate ceiling) in the first

10 or 20m ceiling.

soft weak / floor brings trouble pillars and

prevents good productivity of equipment on tires;

Longwall is more problematic in soft ground that

the R & P. Prof. Dr. H.Z. Harraz Presentation Room and Pillar method 44

Appendix A: Screws Ceiling


Screws Ceiling

This technique shoring is the introduction of a metal rod in

a hole, commiserating with her massive for anchoring,

sealing (resin or cement) or friction.

Advantages of this type of shoring:

low cost

can be mechanized and installation is relatively easy

allows more useful section to the gallery

can be combined with other methods of propping

strut openings allows great height.


The main differences with other types of bracing

the confinement force (F) exerted by the screw implies the

existence of an equivalent reaction (R) more or less split in

mass. In other props, the reaction is the opposite surface.

Action and reaction on the mass

of different types of shoring


Types of screws:

1) point anchoring

2) divided anchoring

3) screws to friction

Screws to chill expansion

1) Screws to anchor point

Consists in placing in the hole a stem

anchored in the bottom of the hole by

a permanent mold or expanding an

expanding wedge. The stems are

usually employed ductile steel or high-

strength steels.


Mooring system (the screw with jockstrap)

It consists of

a cone expansion sympathetic to the rod

which extends the jockstrap (similar to the bushing bolt system)

under the action of the expansion cone pressing her against the wall

of the hole.

System Operation cone-chill

When exerting a traction on the stem to establishing a balance between a

double cone-chill and permanent mold-wall hole.

Equilibrium of a system expansion Prof. Dr. H.Z. Harraz Presentation Room and Pillar method 49

2) Screws anchoring distributed:

The system consists of a stem integrally to the ground by means of a

chemical such as resin or cement. A plate, which in this case is not

essential to the operation of the bolt is used.

This type of shoring screw allows for confinement (the goal is to fight or

shear crack width). The screw, the anchor point acts as bearing shoring.

The rods used are of three types (see figure below).


The resin

Product compound (resin +

hardener).

It is a high strength material :

Rc = 120 a 140 Mpa;

t = 30 Mpa.

Placement Scheme


cement

The most commonly used products are:

pure cement + water

crushed concrete mix water in the proportions (by weight): 42%, 42%,

16%.

products sold by manufacturers screw

In a sealed rod length L in a hole

of diameter D, exerting an effort to

pull the F axis of the rod, there

may be:

a rupture of the sealer

slipping the rod.

Placement scheme


3) The screws friction (friction)

Split – set:

It comprises a slotted tube along the

generatrix.

The screw is inserted 36 mm in diameter

by a hammer drill.

The standard model has the following

characteristics:

• thickness of tube: 2.3 mm

• outer diameter: 38 to 39 mm


Swellex:

It is manufactured by Atlas-Copco.

It consists of a bent tube that injection of

water, deforms and occupies the hole.

Reaches up to 3.6m in length and can be

coupled with other pipes.

advantages:


Appendix B: Design of pillars in coal mine


Dimensioning of pillars in coal mine by the tributary

area method:

It is the simplest method of sizing and with some

adaptations, serves to coal (horizontal tabular

bodies) and for other geometrically regular

deposits (eg .: metalliferous stratiform and

lenticular bodies) both. In this method, only the

state of axial stress on the pillars is taken into

account.


Steps to scaling in coal pillars:

a) Determine σc (uniaxial compressive strength) of the rock that makes

up the pillars, obtained from drill cores, referring to the diameter D

of the testimonies (D in inches).

1 psi = 6,895 x 103 N/m2

b) determining k = σc D½ , where k is a constant that relates the uniaxial

compressive strength of the samples with small σ1, the uniaxial

compressive strength of rock volumes comparable to the in-situ.

pillars dimensions.

The parameters σ1 is given by:

applicable when the thickness h of the mined

layer is > 36 inches (0.9m);

applicable when the thickness h of the mined

layer is <36 inches. Prof. Dr. H.Z. Harraz Presentation Room and Pillar method 57

c) Determine the equation of σp column strength of mine.

There are several different equations obtained from different studies.

Among the most used (w = width of the pillar, h = thickness of the

pillar):


Note: if the pillar is not square, we make w = we in the

formula for σp, where we = 4 Ap / C. Ap = area of post (perpendicular to the vertical axis) that undergoes axial

loading due to H coverage;

C = outer perimeter of the pillar.

(Ref .: Brady & Brown, chap.13.)

d) Set the width of the gallery B.

e) Sp to determine the load on the pillar for a layer thickness H in the

storage area.

Sp = γ H [ (w+B)/w ] [ (L+B)/w ],

being γ = average specific weight of the cover;

L = length of the column.

f) Select the factor of safety F. Make σp / F = Sp and solve this equation

for w.

The range 1.5 ≤ F ≤ 2.0 is generally used, but each has the formula

recommendation to F to be used.


g) Check the recovery mining, assuming that the total

thickness of the layer will be mined:

Rec = 1- [ w/(w+B) ] [ L/(L+B) ] .

h) If recovery is not acceptable and needs to be increased,

decreasing w and / or L to meet the goal. Check if the new

combination w and L is acceptable from the point of view

of stability ( F = σp / Sp ). In Bieniawski formula, F = 1.5 is used for pillars of short duration (panels);

F = 2.0 for long term pillars (axes).


Example:

Check the following configuration of pillars of coal mine, in terms of safety and recovery of mining...

H = 500ft (152m);

B = 18ft (5.5m);

w = 60ft (18.3m);

L = 80ft (24.4m);

h = 7ft (2.1m).

The value k is 5580, based on σc = 3822 psi obtained from tests on cores diameter NX.

Using the formula for calculating the Bieniawski pillar strength.


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