Room and Pillar Mining Method

Hassan Z. Harraz
hharraz2006@yahoo.com
2014- 2015
This material is intended for use in lectures, presentations and as
handouts to students, and is provided in Power point format so as to
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Permission of the author and publisher is required for any other usage.
Please see hharraz2006@yahoo.com for contact details.
Topic 3: Underground Mining Methods
 Room and Pillar method

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:
Room and Pillar (R & P): means a method in which a set of rooms is developed,
leaving pillars rock, usually of uniform size to support the roof. The pillars may
or may not be removed after the removal of the ore.

Characteristic of R & P
 It is the most common supported pillar method, designed and used primarily for mining
flat-lying seams, or tabular orebodies, or gently dipping bedded ore deposits of limited
thickness (like coal, oil shale, limestone, phosphate, salt, trona, potash, and bedded
uranium ores,).
 Room and pillar methods are well adapted to mechanization and are preferred to apply
for sedimentary deposits (such as shales, limestone, dolomite or sandstone) containing
copper, lead, coal seams, phosphate layers, and evaporate (salt and potash) layers.
 Pillars are left in place in a regular pattern while the rooms are mined out.
 Support of the roof is provided by natural pillars of the mineral that are left standing in a
systematic pattern.
 The mining cavity is supported (kept open) by the strength of remnants (pillars) of the
orebody that are left un-mined.
 Room-and-pillar mining method has a low recovery rate (a large percentage of ore
remains in place underground).
 In many room and pillar mines, the pillars are taken out starting at the farthest point from
the stope access, allowing the roof to collapse and fill in the stope. This allows for greater
recovery as less ore is left behind in pillars.
 It is an advantageous mining method for shallow orebodies –as a means of preventing
surface subsidence. Historic, ultra-shallow underground coal mines (<30 m) nevertheless
are characterized by surface subsidence in the areas between pillars (e.g., Witbank coal
field, South Africa).
 Pillars are sometimes mined on retreat from a working area, inducing closure and caving
of these working panels, and raising the risk of surface subsidence.

Figure from Hartman and Mutmansky, 2002.Note the control of ventilation, i.e., the
separation of contaminated (used) and
uncontaminated (fresh) air using a variety
of devices.
Room (Bord)-and-Pillar Layout
Figure shows Room and Pillar Mining

Underground mining: room-and-pillar mining of thick seams –“Benching”
Different approaches allow either the top or
bottom part of the seam to be mined out first.
Note: the “hangingwall” is
above the mining cavity, and
the “footwall” is below it.
Figure shows Room and Pillar is designed for mining
flat, bedded deposits of limited thickness.
Figures from Hartman and Mutmansky, 2002.
Front benching
Vertical
benching
Benching of
thicker parts of
orebody

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
17

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.

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
Prof. Dr. H.Z. Harraz Presentation Room and Pillar method
21

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)
recuperação
 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 withProf. Dr. H.Z. Harraz Presentation Room and Pillar method 38

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 39

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)
1 2
3
4 5
9
6 8
7ARLIMPO

Equipment operating with continuous miner :
Shuttle-car
Roof bolter

Room and Pillar Mining Machine

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 49

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

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.

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.

Room and Pillar Mining Method

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