Authors: Wienand, GA; Ferreira, RIL

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Wienand, GA & Ferreira, RIL 2007, 'Planning and Practice of Remnant Pillar Mining - A Case Study', in Y Potvin (ed.), Proceedings of the Fourth International Seminar on Deep and High Stress Mining, Australian Centre for Geomechanics, Perth, pp. 141-162.

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Abstract:
The extension of the life of some shafts by mining extractable, accessible and economically viable remnant pillars, has become much of a necessity. The geotechnical investigation into the potential for successful extraction of these pillars is crucial, and is approached in a multi-disciplinary manner. This case study outlines the mining methodology, and relates the initial investigation to the practical experience of extracting a 8000 m2 remnant pillar in a tabular reef at 1600 m depth, at the Thabelang “Let’s be happy” Shaft of Gold Fields’ Driefontein mine.

References:
Code of Practice to Reduce Rockfall and Rockburst Accidents, Driefontein Gold Mine.
Driefontein Gold Mine Standard (2003) Std_RE1. Guidelines for the Rock Engineering Investigations for Pillar Mining.
Jager, A.J. and Ryder J.A. (1999) (eds) A Handbook on Rock Engineering Practice for Tabular Hard Rock Mines. The
Safety in Mines Research Advisory Committee (SIMRAC), Johannesburg, pp. 25-26, p. 30, pp. 33-34.
Van Aswegen, G. (2005). Routine seismic hazard assessment in some South African Mines. 6th International
Symposium Rockbursts and Seismicity in Mines, RaSiM6, Y. Potvin and M. Hudyma (eds) Australian Centre for
Geomechanics, Perth, Australia.
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Deep Mining 07, Perth, Australia 155
Appendix A - Glossary of Terms and Definitions
Abutment: The areas of un-mined rock at the edges of mining excavations that may carry
elevated loads resulting from re-distributions of stress.
Advanced heading: An excavation cut in the immediate footwall of the reef for the purpose of
enabling the removal of rock from the stope face and providing access to the face
for men and material.
Apparent volume: A measure of the volume of rock in which inelastic seismic deformation has
occurred.
Back: This is the ore-body between a level and the surface, or between two levels.
Bedding planes: Planes of weakness in the rock that usually occur at the interface of parallel beds
or lamina of material within the rock mass.
Brittle fracture: Sudden failure associated with the complete loss of cohesion across a plane.
Closure: Reduction in dimensions of an opening.
Convergence: Reduction of the distance between 2 basically parallel surfaces (usually
hangingwall and footwall).
Cribbing: Steel or wooden units used to form part of the structure on installing sets in an
underground excavation, providing passive areal support between the set legs and
the rock face.
Cross-cut: A horizontal opening, like a tunnel, that cuts the rock formation at an angle to the
strike in order to reach an orebody.
Deformation: A change in shape or size of a solid body.
Dip: Angle at which a stratum or other planar feature is inclined from the horizontal.
Discontinuity surface: Any surface across which some property of a rock mass is discontinuous (e.g.
bedding planes, fractures).
Drive: A horizontal opening, like a tunnel lying in or near the orebody, parallel to the
strike.
Earthquake: Groups of elastic waves propagating within the earth that cause local shaking/
trembling of ground. The seismic energy radiated during earthquakes is most
commonly caused by sudden fault slip, volcanic activity or other sudden stress
changes in the earth's crust.
Energy Release Rate (ERR): A theoretically calculated quantity related to the amount of elastic closure and
stress change associated with mining, which relates to the expected severity of
the seismicity that may be expected.
Face: The ‘face’ is the immediate area where mining operations take place, and is
typically between the solid rock to be blasted and the ‘old area’ demarcation.
Failure: Condition in which the maximum strength of a material is exceeded by an applied
load.
Footwall: Mass of rock beneath a discontinuity surface (in tabular mining, the rock below
the reef plane).
Footwall heave: The occurrence of heaving or doming of the footwall behind and between the first
or second rows of support, also caused by increased stress at the face, indicating
that lateral movement is being restricted on either the footwall, the hangingwall,
or both.
Planning and Practice of Remnant Pillar Mining — A Case Study G.A. Wienard, R.I.L. Ferreira
156 Deep Mining 07, Perth, Australia
Geological structure: A general term that describes the arrangement of rock formations. Also refers to
the folds, joints, faults, foliation, and schistosity, bedding planes and other planes
of weakness in rock.
Geophone: A seismometer that measures ground velocity. A ‘tri-axial geophone’ boat
contains three ground motion sensors, oriented orthogonally to each other, to
measure ground motions in three dimensions.
Gully: An excavation cut in the immediate footwall or hanging wall of the reef for the
purpose of enabling the removal of rock from the face or providing access to the
face for men or material.
Hangingwall: Mass of rock above a discontinuity surface (in tabular mining, the rock above the
reef plane).
Hazard: A source of, or exposure to, danger.
Hypocentre: Location in 3 dimensions of the source of a seismic event. Also known as the
focus (or source location).
Inelastic deformation: The portion of deformation under stress that is not annulled by the removal of the
stress.
Induced stress: This is the stress that is due to the presence of an excavation. The magnitude of
the induced stress developed depends on the magnitude and orientation of the in-
situ stress and the shape and size of the excavation.
Joint: A naturally occurring plane of weakness or break in the rock (generally aligned
sub vertical or transverse to bedding), along which there has been no visible
movement parallel to the plane.
Ledging: Is the process whereby stoping operations are established from a raise.
Level: All openings at a horizon from which the orebody is opened up and mining is
started.
Magnitude (seismic): Measure of the size of a seismic event. May encompass energy, moment, or both
in its calculation. Ml = 0,324 log E + 0,464 log M – 6,039; where
E = Seismic Energy and M = Seismic Moment.
Mining induced seismicity: The occurrence of seismic events in close proximity to mining operations. During
and following blast times, there is a significant increase in the amount of seismic
activity in a mine. Mining induced seismicity is commonly associated with
volumes of highly stressed rock, sudden movement on faults or intact failure of
the rock mass.
Overstoping: An extension of mining over some other excavation; an effective method of
protecting off-reef excavations from the effects of large mining-induced stress
changes.
Peak Particle Velocity: Maximum velocity of the rock mass measured directly at a geophone or
calculated from ground motion relations.
Permanent support: Support that, once installed, is not removed.
Pillar: Rock left in situ during the mining process to support the local hanging wall, roof
or to provide stability to the mine or portion thereof.
Plane of weakness: A naturally occurring crack or break in the rock mass along which movement can
occur.
Raise: Any tunnel having an inclination (above horizontal in the direction of the
working of more than 5 degrees but not included under the definition of a shaft).
Ravelling: This is the gradual failure of the rock mass by rock blocks falling / sliding from
pit or tunnel walls - usually under the action of gravity, blast vibrations or
Case Studies 1
Deep Mining 07, Perth, Australia 157
deterioration of rock mass strength. A gradual failure process that may go
unnoticed. The term unraveling is also used to mean the same thing.
Reef: A vein, bed or deposit (other than a surface alluvial deposit) that contains
minerals, except in the case of coal or diamondiferous formations.
Regular review: Assessment of the conditions of an area through discussions, plan critique,
planning meetings and/or underground visits.
Risk: The likelihood that occupational injury or harm to persons will occur.
Rockburst: Seismic event that causes damage to underground workings.
Rock Engineering: Is the engineering application of rock mechanics.
Rock fall (fall of ground): Fall of a rock fragment or a portion of fractured rock mass without the
simultaneous occurrence of a seismic event.
Rock fracture: Rock fracture is the failure of the rock as a result of the in-situ stress which
exceeds the strength of the rock.
Rock mass: The sum total of the rock as it exists in place, taking into account the intact rock
material, groundwater, as well as joints, faults and other natural planes of
weakness that can divide the rock into interlocking blocks of varying sizes and
shapes.
Rock mass instability: A softening within a critical volume of rock indicated by accelerating
deformation and a drop in stress.
Rock mechanics: The scientific study of the mechanical behaviour of rock and rock masses under
the influence of stress.
RQD: Referring to ‘rock quality designation’, the ratio of the length of core recovered
from drilling, counting those pieces of 100 mm or larger, to the total length of the
core.
Scattered mining layout: A layout where dip-pillars are left behind as ‘planned remnants’, acting as load-
carrying stability pillars, and permitting significant advance exploration to be
carried out.
Seismically active mine: A seismically active mine is a mine that sustains losses to persons and/or
property, underground or on surface, caused by the dynamic response to a
seismic event induced when creating or enlarging an excavation.
Seismic event: Transient earth motion caused by a sudden release of the strain energy stored in
the rock.
Seismicity: The geographic and historical distribution of earthquakes.
Seismology: The scientific study of earthquakes by the analysis of vibrations transmitted
through rock and soil materials. The study includes the dynamic analysis of
forces, energy, stress, duration, location, orientation, periodicity and other
characteristics.
Seismometer: A device (transducer) that converts ground motion into an electric signal.
Seismic moment (scalar): Measure of the strength of an earthquake or of a seismic event and an indication
of the amount of deformation (displacement) at a seismic source.
Sets: A structure or structural feature erected into an underground excavation to
maintain its stability.
Shaft: Means any tunnel having a cross-sectional dimension of 3.7 m or over and: (i)
Having an inclination to the horizontal of 15 degrees or over, or (ii) Having an
inclination to the horizontal of less than 15 degrees but more than 10 degrees
where the speed of traction exceeds 2 m/s.
Planning and Practice of Remnant Pillar Mining — A Case Study G.A. Wienard, R.I.L. Ferreira
158 Deep Mining 07, Perth, Australia
Snook: A snook is an unmined portion of a pillar that is left behind during the process of
pillar extraction. Snooks of predetermined size may be left intentionally to crush
once pillar extraction has progressed a certain distance, or they may be left
unintentionally due to adverse geology, etc.
Spalling: This the longitudinal splitting in uniaxial compression, or the breaking-off of
plate-like pieces from a free rock surface.
Special areas: During the course of routine mining an increased risk of rock falls or rock bursts
may develop. Such areas requiring additional attention and precautions must be
designated special areas.
Stope: An underground excavation made in removal of any ground or mineral, other
than coal, but does not apply to excavations made for engine rooms and pump
chambers or for development purposes such as shafts, drives, winzes and raises.
Stope width: Width of the tabular excavation made during stoping operations.
Strain: The change in length per unit length of a body resulting from an applied force.
Within the elastic limit, strain is proportional to stress.
Strain burst: Rock burst at the lower end of the spectrum of violent events occurring
essentially at the surface of an excavation.
Strength: The maximum stress that a material can resist without failing for any given
loading regime.
Stress: Force acting across a surface element divided by the area of the element.
Stress=force/area
Stress field: A descriptive term to indicate the pattern of the rock stress (magnitude and
orientation) in a particular area.
Stress shadow: An area of low stress levels due to the flow of stress around a nearby excavation,
e.g. a large stope. This may result in joints opening up, thereby causing rock falls.
Strike: Direction of the azimuth of a horizontal line in the plane of an inclined stratum
(or other planar feature) within a rock mass.
Support: A structure or a structural feature built into or around an underground excavation
to maintain its stability.
Thickness: Perpendicular distance between bounding surfaces (e.g. bedding planes).
Triaxial compression: Compression caused by the application of normal stress in 3 perpendicular
(orthogonal) directions.
Uniaxial (unconfined) compression: Compression caused by the application of normal stress in a single
direction. UCS refers to uniaxial compressive stress, a measure of rock strength.
Updip: Name given to a ‘wide raise’, where mining progresses in the up-dip direction.
Vamping: The removal of all redundant equipment, and remaining broken ore from a
depleted or abandoned working place, prior to sealing it off.
Vector: A quantity having magnitude and direction but no fixed position
Virgin stress: Are natural stresses which exist in the rock mass prior to any excavation.
Width-to-height ratio: An indicator of pillar stability; the strength of a pillar, assuming competent
foundations, increases significantly as a function of its w:h ratio.
Working place: This is the place where mine workers normally work or travel to.
Yield: Occurs in a sample when there is a departure from the elastic behaviour in the
material and some permanent deformation occurs
Case Studies 1
Deep Mining 07, Perth, Australia 159
Appendix B – Obligatory Strategies
The obligatory strategies state that: -
1 A notice displaying the words “RESTRICTED SPECIAL AREA” must be posted at the waiting
places or stope entrances.
2 Two separate access ways, independent of each other, shall be maintained, be clearly marked and
kept clean and free of obstructions, as far as is practicably possible.
3 The number of person in the crew assigned to a special area working place shall be kept minimum,
which shall be controlled by the responsible miner on each shift. All other person entering a special
area must sign on entry and exit from that special area in a book provided at each entrance and shall
report to the miner in charge. If a person exits on a different level then he should sign out in the book
at the exit, and should have indicated his intention to do so in the entry book.
4 Waiting places, explosives boxes, miners’ boxes and refuge chambers must, as far as practically
possible, be situated away from areas where they may be affected by ground movements.
5 Telephone communication to surface must be available at all waiting places. The waiting places and
stope entrance travelling ways must be adequately illuminated. A first aid bag and stretcher per
10 persons working in a restricted special area must by readily available at the closest refuge bay to
the working place or at the waiting place.
6 Support of all access ways, gullies, faces and headings in a special area should adhere to the
recommended and documented standard at all times. Special attention must be given to the support
and headboards/additional support units must be installed wherever necessary.
7 Where there is risk of damaging seismic activity, then all support types used should have the
capacity for stable yield under rockburst conditions.
8 Face advance should be continuous and regular, ideal face shapes should be maintained and lead/lags
controlled, as far as is practically possible.
9 The stoping width shall be kept to the agreed and documented minimum, which shall not be less than
1.2 m. Where the stoping width has to be increased, to comply, the footwall waste must be opened
up.
10 Broken rock must not be allowed to accumulate on the faces and in the gullies and access ways.
11 No development into a special area shall be allowed unless authorised by the responsible Operations
Manager and under the conditions laid down in writing and in consultation with the suitably
qualified rock engineering practitioner.
12 Any other development that passes over or under a special area, and whose continued use is
required, shall be adequately supported.
13 The latest Special Area Sheet shall be posted on the notice board at the waiting place. The above
precautions shall be implemented from the first day that is restricted special area declaration is made
and shall be enforced until the area has been vamped and stropped, unless written instruction to the
contrary has been issued and signed for by the responsible Operations Manager.
Planning and Practice of Remnant Pillar Mining — A Case Study G.A. Wienard, R.I.L. Ferreira
160 Deep Mining 07, Perth, Australia
Appendix C
Identify the Mineral Resource
Pillar
Reason for Mineral Resource being In-situ
Remnant RIH/ RIF Sweeping Reclamation
Ventilation Rock Mechanics Accessibility Valuation Geology Pillar Mining Safety
Is the Resource Block mineable?
YESNO
Classify Mineral Resource
Block/Pillar/ Remnant according to Code
as PS or PZ and assign to appropriate
category in the Mineral Resource
Inventory
Document reasons – create Permanent
record of investigation and conclusion.
MRM responsible to maintain file for
Mine Management and GME.
• Document evaluation
• Determine optimal Cut-off/ Pay Limit at which it can be mined
• Is shaft/ other infrastructure/ services available?
• Is block in a current mining district?
• Decision on block?
Determine In Situ value of Mineral Resource
Unpay Pay
Portions Pay
Re-block Pay area on Geological
parameters and evaluate
Examine seismic history & stress levels
Perform financial valuation:
Cost of access/ mining layout/ ventilation
Is a profit to be made?
Project Team recommendation
Survey/ Rock Engineering layout
Not approved Approved
Plan to Mine
Mining Schedule
Can the structures be negotiated?
Is there a Pay trend and channel?
Geological structures
NO YES
NO YES
YESNO
Feasibility
Underground by
Project Team
Mining
Project Team
Mining
Survey
Safety
Identification
Project Team
Unit Manager
Ventilation
Geologist
Rock Mechanics
General Miner
NO YES
Remnant Pillar – Decision Rights Framework
Shaft: _________________
Work Place: ____________
Size of Resource: ________
Value: _________________
Block Number: __________
Value Value
Value
Start Date
Case Studies 1
Deep Mining 07, Perth, Australia 161
Appendix D
Managerial support instruction for double wide raise updip mining:
Carbon Leader Reef – Updip Mining (Double Wide Raise)
Driefontein Consolidated
W
6.0m Maximum
Lead / Lag
1.8m
Not to Scale
W
Rig Holes
Between
15m to
20m
1m
SWEPT AREA
SWEPT AREA
1.8m
Gully
Gully Gully
1.8m
Minimum 12.6 m slusher center to center
Thabaleng Shaft MSI 03-2007
Manager Operations.
Snr Rock Eng.
Hydraulic Props are installed max. 1.0m
from the face before the blast.
Hydraulic Props installed at 1.0m on DIP
and 1.0m on STRIKE
1.8m
1.8m
3.5m
1.8m 1.8m
3.5m
1.0m
1m
1.8m1.8m 1.8m
Mine Overseer.
Rock Eng Manager.
Shaft Manager 10#.
Rig Holes
1.8m
1.8m 1.8m1.8m
Skin to Skin Packs Skin to Skin Packs
W
Rig Holes
3.5m
1m
NOTES: .Special Instructions
•Any dislodged support 7m from the face needs to be replaced.
•No person to work further than 1m from installed support.
•Gullies to be blasted to within 3.5m from the face. All gullies, dip and Abutment gullies to be supported with
1.2m tendons on diamond pattern 1:2 and 1m apart up to last line of packs.
•Maximum Support Distances to Face
Maximum distance, RYHP to face 1.0m before the blast.
Pack to face 3.5m after the blast. Pack size in stope 90 x 90( CLC) at a Stope width of min 1.2m
Maximum blasted gully width 1.6m
Maximum span across gully 1.8m Gully Pack Size 1.80 x 90( CLC)
Temporary Support-(when necessary ) Camlok Props
Maximum 1.0 m from the face and 1.5m on strike
Planning and Practice of Remnant Pillar Mining — A Case Study G.A. Wienard, R.I.L. Ferreira
162 Deep Mining 07, Perth, Australia




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