Kestabilan Tambang Bawah Tanah

The Stability of The Stability of UndergroundUnderground OpeOpeningning

Lufi Rachmad

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Review from Last WeekReview from Last Week

Insitu Stress (gravitational, tectonic, residual

stresses)

An underground opening changes the stress

condition Induced Stress

Induced Stress could triger unstability

Understanding stresses is an important part

in designing underground opening

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Empirical equation to estimate insitu stresses

e.g. Shoerey

)z1

(0.001E 70.25k h ++=

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Stress distribution around various opening Stress distribution around various opening shapes (circle, horseshoe, square, ellipse)shapes (circle, horseshoe, square, ellipse)


Underground opening design methodologyUnderground opening design methodology

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Case Study ACase Study A

An orebody XYZ has been defined as a block caving deposit. What we should design first?

Plan View Section A-A’

A A’Orebody

XYZ

Orebody XYZ

Surface

7 km 1.4

km

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ACCESS

Orebody XYZ

Surface shaft

decline

adit

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The access for the orebody are decided to be twin adits, 6.8 m wide and 6.0 m high.

The opening size considers the following factors:Biggest dimension

Effective size after ground support

Drainage pipe & trench

Intake airways

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Case Study ACase Study AFor the design purpose, how far apart should these two adits be?

Access Adits

(A-A’)

Surface

?

Orebody XYZ

Access Adits Plan View

A A’

The farther the more

ineffective

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Assuming the simplest condition, the axisymmetric stress distribution could be used.

0.00

0.50

1.00

1.50

2.00

0 2 4 6 8 10

Jarak dari batas terowongan, r/R

Teg

anga

n In

duks

i/T

egan

gan

Aw

al

Tegangan radial

Tegangan tangensial

r = 5R, the pre-mining stress would not be significantly different from the virgin stress field.

r = 17 meter as an early indication.

Might be further analyzed using pillar stability calc and numerical modeling

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Insitu StressInsitu Stress

For a depth of 1,400 m, the equation gives the vertical stress σv = 38 MPa , the ratio k = 0.5 (for Eh = 25 GPa) and hence the average horizontal stress σh= 19 MPa

During preliminary design, the empirical stress equation can be used to obtain a first rough estimate of the vertical and average horizontal stress in the vicinity of the tunnel

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Stress Distribution around Stress Distribution around ““HorseHorse--ShoeShoe”” TunnelTunnel

σσhh = = σσvvσσθθAA = 2.2 = 2.2 σσvv

σσθθBB = 1.3 = 1.3 σσvv

σσhh = 0.5 = 0.5 σσvvσσθθAA = 0.6 = 0.6 σσvv


σσhh = 0.33 = 0.33 σσvvσσθθAA = 0.1 = 0.1 σσvv


A

B B

σv

σh

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Insitu StressInsitu Stress

Given the rock mass strength is around 70-80 MPa, a preliminary analysis of the stresses induced around the proposed tunnel shows that these induced stresses are likely to exceed the strength of the rock and that the question of stress measurement must be considered in more detail

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Insitu Stress MeasurementInsitu Stress Measurement

Various ways to measure insitu stress Overcoring - Triaxial Strain CellHydraulic FracturingFlatjack MeasurementBorehole BreakoutAcoustic Emission

The most common set of procedures is based on the determination of strains in the wall of a borehole, induced by overcoring that part of the hole containing the measurement device.

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OvercoringOvercoring (CSIRO Cell)(CSIRO Cell)

The CSIRO cell, referred to as a hollow inclusion The CSIRO cell, referred to as a hollow inclusion cell. It consists of a thin epoxy tube, with three cell. It consists of a thin epoxy tube, with three strain gage rosettes, embedded within the strain gage rosettes, embedded within the epoxy.epoxy.

Strain Gages

Epoxy

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Overcoring methods are measuring in situ stress Overcoring methods are measuring in situ stress based on the stress relief around the borehole. based on the stress relief around the borehole. The relief of external forces by overcoring The relief of external forces by overcoring causes the changes in strain on the borehole causes the changes in strain on the borehole wall.wall.


The field procedures consist of drilling a The field procedures consist of drilling a concentric EXconcentric EX--size borehole, installation of the size borehole, installation of the deformation gage, and overcoring a stress relief deformation gage, and overcoring a stress relief borehole.borehole.

If the elastic properties of the rock are known, If the elastic properties of the rock are known, the changes in borehole diameter or strains can the changes in borehole diameter or strains can be converted to in situ stress in the rock.be converted to in situ stress in the rock.

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The CSIRO cell is designed to measure diametral The CSIRO cell is designed to measure diametral deformations of an EXdeformations of an EX--size (1.5" in diameter) size (1.5" in diameter) borehole during overcoring a concentric borehole during overcoring a concentric borehole (6" in diameter). The diametral borehole (6" in diameter). The diametral deformations are measured in three directions deformations are measured in three directions (60 degree apart) in the same diametral plane.(60 degree apart) in the same diametral plane.

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Need Need YoungYoung’’ss modulus and Poissonmodulus and Poisson’’s inputss inputs

LimitedLimited to within to within 1010--3030 meters of existing meters of existing openingopening

Overcoring Cost Overcoring Cost –– CSIRO Cells (2 sites)CSIRO Cells (2 sites)NIRM US$ 61KNIRM US$ 61KES&S US$ 44K approx. 20K per siteES&S US$ 44K approx. 20K per site

Price does not include drilling which will be Price does not include drilling which will be around US$ 120K / maround US$ 120K / m

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Hydraulic FracturingHydraulic Fracturing

Typically hydraulic fracturing is conducted in Typically hydraulic fracturing is conducted in vertical boreholes. A short segment of the hole vertical boreholes. A short segment of the hole is sealed off using an straddle packer. This is is sealed off using an straddle packer. This is followed by the pressurization of the fracturefollowed by the pressurization of the fracture--free segment of the hole by pumping in water. free segment of the hole by pumping in water.

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The pressure is raised until The pressure is raised until the rock surrounding the the rock surrounding the hole fails in tension at a hole fails in tension at a critical pressure. critical pressure.


Following breakdown, the Following breakdown, the shutshut--in pressure, the in pressure, the lowest testlowest test--interval interval pressure at which the pressure at which the hydrofrac closes hydrofrac closes completely under the completely under the action of the stress acting action of the stress acting normal to the normal to the hydrofracturehydrofracture

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Limited to drill/pump equipment and ground Limited to drill/pump equipment and ground conditions conditions –– Max range 300m Max range 300m –– 1000m1000m

““QualitativeQualitative””

AssumptionsAssumptionsS1 Maximum Principle Stress is Vertical or S1 Maximum Principle Stress is Vertical or aligned with aligned with holehole

HydofracingHydofracingNIRMNIRM US$ 87KUS$ 87KGolder US$ 188KGolder US$ 188K

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Borehole BreakoutBorehole Breakout

Extensive field evidence and laboratory Extensive field evidence and laboratory experiments suggest that borehole breakouts, experiments suggest that borehole breakouts, defined as borehole crossdefined as borehole cross--section elongations section elongations resulting from preferential rock failure, is a resulting from preferential rock failure, is a direct consequence of the in situ stress in the direct consequence of the in situ stress in the rock. rock.

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One of the early observations of breakouts was One of the early observations of breakouts was in the quartzite and conglomerates of the in the quartzite and conglomerates of the Witwatersrand gold mine in South Africa Witwatersrand gold mine in South Africa (Leeman, 1964). The spalling was observed to (Leeman, 1964). The spalling was observed to occur at diametrically opposed points on the occur at diametrically opposed points on the borehole wall perpendicular to the direction of borehole wall perpendicular to the direction of the maximum principal stress. the maximum principal stress.

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The most publicized The most publicized observation of breakouts observation of breakouts was in the 3 m diameter was in the 3 m diameter drift at 420 m level in the drift at 420 m level in the Underground Research Underground Research Laboratory (URL), Canada. Laboratory (URL), Canada. Two diametrically opposed Two diametrically opposed breakouts were breakouts were approximately aligned with approximately aligned with the vertical stress, which is the vertical stress, which is the overall least principal the overall least principal stress at URL. stress at URL.

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From insitu stress measurement, the bearing of the major principal stress is around 38-40 degree. What is the preferable panel/undercut drift orientation?

Plan View

Orebody XYZ

Plan View

Orebody XYZ

Panel/Drill Drift

σ1 σ1

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Ideally, the panel/undercut drift and the direction of cave advance are aligned with the principal horizontal in situ stresses.

If the direction of advance is perpendicular, the levels of stress in the abutmentahead of the undercut will be high and will increase as the undercut advances

Undercut Advance Direction

Plan View

Orebody XYZ

σ1

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Stress Induced in the Stress Induced in the Extraction and Undercut LevelExtraction and Undercut Level

High abutment stresses induced in the vicinity of an advancing undercut front is resulted from undercutting activity.

Abutment stress

Cave Advance

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The magnitude of abutment stresses in the cave vicinity could reach up 2 to 3 times the insitu stress magnitude.

This abutment stress could devastate development drifts if does not maintain properly

For XYZ Mine, the vertical stress σv = 38 Mpa. The abutment stress = 76 - 114 MPa

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Failure of yielding arch support Failure of yielding arch support El Salvador Mine, Chile El Salvador Mine, Chile

Photo: M. L. Van Sint Jan

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Rockburst at Extraction Level, Rockburst at Extraction Level, DOZ Mine, IndonesiaDOZ Mine, Indonesia

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Collapse of an extraction level drift, Collapse of an extraction level drift, El Teniente Mine, Chile, 1989El Teniente Mine, Chile, 1989

CONCRETEDAMAGE

CONCRETEDAMAGE

1.5 m

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Panel 15, 28 June 2003

Panel 15, 23 August 2003Panel 15, 7 August 2003

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Several factors have the potential to influence the levels of stress induced in the extraction level excavation:

Distance between Undercut and ExtractionCave Hydraulic Radius

Undercut directionIn situ Stress regime

The timing of undercut relative to the extraction level developmentUndercut face shape

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The timing of undercut relative to the extraction level development relates to the selected undercutting method. In general, there are three main undercutting strategies:

1.Post Undercutting

2.Pre Undercutting

3.Advanced Undercutting

For XYZ BC Mine, an undercutting method should be selected.

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TerminologyTerminologyDrill Drift - Undercut

Fan Drilling

Draw Bell Drift

Draw Bell

Major Apex

Panel Drift -Extraction

Minor Apex

Draw PointOrepass

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ConventionalConventional PanelPanel CavingCaving

Undercutting and drilling takes place after development of the underlying extraction level has been completed.

Drawbells and DB drifts are prepared ahead of the undercut and are ready to receive the ore blasted from the undercut level

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AdvanceAdvance UndercutUndercut PanelPanel CavingCaving

Undercutting and drilling takes place above a partially developed extraction level.

The partial development on the extraction level can consist of either extraction drift only or extraction drift and drawpoint drift

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AdvanceAdvance UndercutUndercut PanelPanel CavingCaving

Drawbells are always prepared in the de-stressed zone behind the undercut, usually adhering to the 45 degree rule.

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Comparing Abutment Stress ImpactComparing Abutment Stress Impact

Measuring abutment stress changes could be done indirectly by monitoring its impact.

The stress impact reflects in displacement / deformation occurred in the underground opening.

There are many different methods for monitoring displacement. The simplest and most common among them is a convergence gage

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Comparing Abutment Stress ImpactComparing Abutment Stress Impact

A convergence gage usually consists of a tape, wire, rod, or tub in series with a deformation indicator.

Precision is typically around 0.005 in (0.13 mm)

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33--Point ConvergencePoint Convergence

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Near XYZ BC Mine, there is an active BC mine, called KLM Mine, where the trial between Post Undercut and Advanced Undercut will take place.

Plan View

Orebody XYZ

KLM Mine

4 km

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Undercut Trial at KLM MineUndercut Trial at KLM Mine

Panel 15Post

Undercut

Panel 16 Advanced Undercut

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Undercut Trial at KLM MineUndercut Trial at KLM Mine

CaveUC Lvl

Extr Lvl

Cave Advance

Convergence Station

Last Blasting

Row

Abutment

18 m

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Result of KLM Mine TrialResult of KLM Mine TrialAdvanced Undercut vs Post UndercutAdvanced Undercut vs Post Undercut

Stable after Cave Front

PassingPost Undercut

Anomaly

Cave Advance

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Result of KLM Mine TrialResult of KLM Mine TrialAdvanced Undercut vs Post UndercutAdvanced Undercut vs Post Undercut

Anomaly

Cave AdvanceStable after Cave Front

Passing

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AnomalyAnomaly

The anomaly from KLM Mine Trial could be explained as the result of remnant undercut pillar or stump.

Stump is created when the undercut blasting fails to break the rock completely.

Cave

Cave Advance

Last Blasting Row

Remnant Pillar

Abutment Stress

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ExamplesExamples of of RemnantRemnant Pillars / StumpPillars / Stump

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The KLM Mine trial shows that the advanced undercut has the advantage to reduce the stress induced impact to undercut and extraction level.

Considering the KLM Mine trial result, XYZ BC Mine will implement the advanced undercutting method.

A note has been made that XYZ BC should establish undercut blasting control such that a remnant pillar will be avoided.

49


Several factors have the potential to influence the levels of stress induced in the extraction level excavation:

Distance between Undercut and ExtractionCave Hydraulic Radius

Undercut directionIn situ Stress regime

The timing of undercut relative to the extraction level developmentUndercut face shape

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The undercut face shape is controlled by the undercut opening sequence and the lead and lag among drill drift cave front

Irregularities of cave front could create unfavorable conditions in term of stress concentration in the production level

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Undercutting SequenceUndercutting Sequence

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Lead and LagLead and Lag

Lead and Lag: the distance between the caving front on adjacent panels

Cave Front

Lead and Lag

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Since trial with different undercut sequence is quite impossible, a numerical modeling will be used to evaluate the most preferable sequence for XYZ BC Mine.

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When comparing the results of the undercut sequence models, the main useful criteria to examine have proven to be:

1.Peak stress levels (in the stronger ground) induced on the production level elevation.

2. Average and maximum values of strain (as a measure of the severity of damage and deformation) induced on the production level elevation.

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3. Areas of damage on the production level elevation, measured in terms of areas where shear strains exceed a set limit of 2 x 10-3 (2 millistrains). This value was chosen because it includes damage in the stronger ground and not just the weaker ground areas, which are known to become extensively damaged, whatever undercut sequence is chosen.

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Undercut Opening SequenceUndercut Opening Sequence

From modeling result, a wedge type sequenceappears preferable. Mining in weak ground should be over a short front, and bordered by panels that are mining in stronger ground, which bears load and limits rock mass deformation in the weak ground area.

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Undercut Opening SequenceUndercut Opening Sequence

The undercut wedge apex should advance into the weaker ground, close to the boundary with stronger ground, with the apex angle broad rather than narrow.

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Lead and LagLead and Lag

To evaluate the lead To evaluate the lead and lag, convergence and lag, convergence information from information from KLM mine is used.KLM mine is used.

Cave Front

Displacement (mm/day)

Convergence data is Convergence data is presented in velocity presented in velocity (mm/day) contour(mm/day) contour

Displc. = LtDisplc. = Lt--L0L0

Increasing of horizontal and vertical velocity due to lead and lag (60 meter)

No Advanced

horizontalvertical

070501 140501

0.0 mm/day

070501 140501

-1.3 mm/day

-0.2 mm/day

-1.12 mm/day

Decreasing of horizontal and vertical velocity after reducing lead and lag distance (54 meter)

horizontal vertical

140501 290501

-1.3 mm/day

140501 290501

-0.74 mm/day

-1.12 mm/day

-0.1 mm/day

Advance 6 m


horizontal vertical

290501 120601

-0.74 mm/day

290501 120601

-0.5 mm/day

-0.1 mm/day

0.3 mm/day

Advance 9 m

Increasing of horizontal velocity due to noadvanced of lead and lag distance (45 meter)

horizontal vertical

120601 260601

-0.5 mm/day

120601 260601

-0.65 mm/day

0.3 mm/day

-0.2 mm/day

No Advance

Decreasing of horizontal and vertical velocity after reducing lead and lag distance

(30 meter)

horizontal vertical

260601 130701

-0.65 mm/day

260601 130701

-0.4 mm/day

-0.2 mm/day

0.0 mm/day

Advance 15 m

Increasing of horizontal velocity due to no advanced of lead and lag distance (30 meter)

horizontal vertical

130701 070801

-0.4 mm/day

130701 070801

-0.8 mm/day

0.0 mm/day

-0.1 mm/day

No Advance


horizontal vertical

070801 230801

-0.8 mm/day

070801 230801

-0.1 mm/day

-0.1 mm/day

0.0 mm/day

Advance 5 m

Increasing of horizontal and vertical velocity due to no advanced of lead and lag distance (25 meter)

horizontal vertical

230801 150901

-0.1 mm/day

230801 150901

-0.75 mm/day

0.0 mm/day

-0.4 mm/day

No Advance

Decreasing of horizontal and vertical velocity after reduce lead and lag distance (8 meter)

horizontal vertical

150901 260901

-0.75 mm/day

150901 260901

-0.4 mm/day

-0.4 mm/day

0.1 mm/day

Advance 17 m

Decreasing of horizontal and vertical velocity in the same of lead and lag distance (8 meter)

horizontal vertical

260901 091001

-0.4 mm/day

260901 091001

-0.1 mm/day

0.1 mm/day

0.0 mm/day

No Advance

Decreasing of horizontal and vertical velocity below 8 meter of lead and lag distance (5 meter)

horizontal vertical

091001 261001

-0.1 mm/day

091001 261001

0.0 mm/day

0.0 mm/day

0.0 mm/day

Advance 3 m

Constant stable of horizontal and vertical velocity below 8 meter of lead and lag distance (5 meter)

horizontal vertical

261001 071101

0.0 mm/day

261001 071101

0.0 mm/day

0.0 mm/day

0.0 mm/day

No Advance

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Lead & Lag IssueLead & Lag IssueReading

DateLead

and Lag Distance (m)

CaveAdvanced (m)

Horizontal Displacement

Velocity (mm/day)

Vertical Displacement

Velocity (mm/day)

07-May-01 Cave not started

0 0.0 0.2

14-May-01 60 0 1.3 (↑) 1.12 (↑)

29-May-01 54 6 0.74 (↓) 0.1 (↓)

12-Jun-01 45 9 0.5 (↓) -0.3 (↓)

26-Jun-01 45 0 0.65 (↑) 0.2 (↑)

13-Jul-01 30 15 0.4 (↓) 0.0 (↓)

07-Aug-01 30 0 0.8 (↑) 0.1 (↑)

23-Aug-01 25 5 0.1 (↓) 0.0 (↓)

15-Sept-01 25 0 0.75 (↑) 0.4 (↑)

26-Sept-01 8 17 0.4 (↓) -0.1 (↓)

09-Oct-01 8 0 0.1 (↓) 0.0 (↓)

29-Oct-01 5 3 0.0 (↓) 0.0≈

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Lead & Lag IssueLead & Lag Issue

From the convergence measurement, From the convergence measurement, the ideal the ideal lead and lag is between 5 to 8 meterslead and lag is between 5 to 8 meters, cave , cave front can be stopped without any significant front can be stopped without any significant displacementdisplacement

If the lead and lag is over the 12 m, the cave If the lead and lag is over the 12 m, the cave face cannot be stopped for more than one week face cannot be stopped for more than one week because excessive damage will occur in the because excessive damage will occur in the panelspanels

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XYZ Mine

Plan ViewAccess Adits

Undercut Sequence and

DirectionExtraction Drift Orientation

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