Kestabilan Tambang Bawah Tanah 1

(1)

The Stability of

Underground

Ope

_Ope

ning

_ning

(2)

Review from Last Week

Insitu Stress (gravitational, tectonic, residual stresses)

An underground opening changes the stress condition Æ Induced Stress

Induced Stress Æ could triger unstability

(3)

Review from Last Week

Empirical equation to estimate insitu stresses e.g. Shoerey

) z 1 ( 0.001 E

7 0.25

(4)

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

shapes (circle, horseshoe, square, ellipse)

Review from Last Week

(5)

Case 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

Surface

7 km 1.4

k

(6)

Case Study A

ACCESS

Orebody XYZ

Surface _shaft

decline

(7)

Case Study A

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

(8)

Case Study A

For the design purpose, how far apart should these two adits be?

Access Surface

?

Orebody XYZ

Access Adits Plan View

A A’

(9)

Case Study A

Assuming the simplest condition, the

axisymmetric stress distribution could be used.

0.00

Jarak dari batas terowongan, r/R

Te

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

(10)

Insitu Stress

For a depth of 1,400 m, the equation gives the vertical stress σ

v

= 38 MPa , the

ratio

k

= 0.5 (for

Eh

= 25 GPa) and hence the average horizontal stress σ

h

= 19 MPa

During preliminary design, the empirical

(11)

Stress Distribution around

(12)

Insitu 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

(13)

Insitu Stress Measurement

Various ways to measure insitu stress

Overcoring - Triaxial Strain Cell

Hydraulic Fracturing

Flatjack Measurement

Borehole Breakout

Acoustic Emission

The most common set of procedures is based on the determination of strains in the wall of a borehole, induced by

(14)

Overcoring

(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

strain gage rosettes, embedded within the

epoxy.

(15)

Overcoring methods are measuring in situ stress Overcoring methods are measuring in situ stress

based on the stress relief around the borehole.

The relief of external forces by overcoring

causes the changes in strain on the borehole

wall.

Overcoring

(CSIRO Cell)

The field procedures consist of drilling a The field procedures consist of drilling a

concentric EX

concentric 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.

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

be converted to in situ stress in the rock.

(16)

The CSIRO cell is designed to measure diametral The CSIRO cell is designed to measure diametral

deformations of an EX

deformations 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

deformations are measured in three directions

(60 degree apart) in the same diametral plane.

(17)

Overcoring

(CSIRO Cell)

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

opening

Overcoring Cost Overcoring Cost –– CSIRO Cells (2 sites)CSIRO Cells (2 sites)

NIRM US$ 61KNIRM US$ 61K

ES&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 / m

(18)

Hydraulic Fracturing

Typically hydraulic fracturing is conducted in Typically hydraulic fracturing is conducted in

vertical boreholes. A short segment of the hole

is sealed off using an straddle packer. This is

followed by the pressurization of the fracture

followed by the pressurization of the fracture- -free segment of the hole by pumping in water.

(19)

The pressure is raised until The pressure is raised until the rock surrounding the

the rock surrounding the

hole fails in tension at a

critical pressure.

Hydraulic Fracturing

Following breakdown, the Following breakdown, the shut

shut--in pressure, the in pressure, the lowest test

lowest test--interval interval pressure at which the

pressure at which the

hydrofrac closes

completely under the

action of the stress acting

normal to the

hydrofracture

(20)

Hydraulic Fracturing

Limited to drill/pump equipment and ground Limited to drill/pump equipment and ground

conditions

conditions –– Max range 300m Max range 300m –– 1000m1000m

““QualitativeQualitative””

AssumptionsAssumptions

S1 Maximum Principle Stress is Vertical or S1 Maximum Principle Stress is Vertical or

aligned with

aligned with holehole

HydofracingHydofracing

NIRMNIRM US$ 87KUS$ 87K

(21)

Borehole Breakout

Extensive field evidence and laboratory Extensive field evidence and laboratory

experiments suggest that borehole breakouts,

defined as borehole cross

defined 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

rock.

(22)

Borehole Breakout

One of the early observations of breakouts was One of the early observations of breakouts was

in the quartzite and conglomerates of the

Witwatersrand gold mine in South Africa

(Leeman, 1964). The spalling was observed to

occur at diametrically opposed points on the

borehole wall perpendicular to the direction of

the maximum principal stress.

(23)

Borehole Breakout

The most publicized The most publicized

observation of breakouts

was in the 3 m diameter

drift at 420 m level in the

Underground Research

Laboratory (URL), Canada.

Two diametrically opposed

breakouts were

approximately aligned with

the vertical stress, which is

the overall least principal

stress at URL.

(24)

Case Study A

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

Orebody XYZ

Orebody XYZ Panel/ Drill Drift

(25)

Case Study A

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 abutment ahead of the undercut will be high and will increase as the undercut advances

(26)

Stress Induced in the

Extraction and Undercut Level

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

(27)

Stress Induced in the

Extraction and Undercut Level

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 σ

v

= 38 Mpa. The abutment stress = 76 - 114

(28)

Failure of yielding arch support

El Salvador Mine, Chile

(29)

Rockburst at Extraction Level,

DOZ Mine, Indonesia

(30)

Collapse of an extraction level drift,

El Teniente Mine, Chile, 1989

CONCRETE DAMAGE

(31)

Panel 15, 28 June 2003

(32)

Stress Induced in the

Extraction and Undercut Level

Several factors have the potential to

influence the levels of stress induced in the extraction level excavation:

Cave Hydraulic Radius

Undercut direction

In situ Stress regime

The timing of undercut relative to the extraction level development

(33)

Case Study A

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

(34)

Terminology

Drill Drift - Undercut

Fan Drilling

Draw Bell

Major Apex

(35)

Conventional

Panel

Caving

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

(36)

Advance

Undercut

Panel

Caving

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

(37)

Advance

Undercut

Panel

Caving

Drawbells are always prepared in the de-stressed zone behind the undercut,

(38)

Comparing 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

(39)

Comparing Abutment Stress Impact

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

deformation indicator.

(40)

3

(41)

Case Study A

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

(42)

Undercut Trial at KLM Mine

Panel 15 Post

(43)

43

Undercut Trial at KLM Mine

(44)

Result of KLM Mine Trial

Advanced Undercut vs Post Undercut

Stable after Cave Front

Passing

Post Undercut

Anomaly

(45)

Result of KLM Mine Trial

Advanced Undercut vs Post Undercut

Anomaly

Cave Advance Stable after

(46)

Anomaly

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 Remnant _Pillar

(47)

Examples

(48)

Case Study A

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

(49)

Stress Induced in the

Extraction and Undercut Level

Several factors have the potential to

influence the levels of stress induced in the extraction level excavation:

Distance between Undercut and Extraction

Cave Hydraulic Radius

Undercut direction

In situ Stress regime

The timing of undercut relative to the

(50)

Case Study A

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

(51)

Undercutting Sequence

(52)

Lead and Lag

Lead and Lag: the

distance between the

caving front on adjacent panels

Cave Front

(53)

Undercutting Sequence

(54)

Undercutting Sequence

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

(55)

Undercutting Sequence

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,

(56)

Undercut Opening Sequence

From modeling result, a wedge type sequence

appears preferable. Mining in weak ground

should be over a short front, and bordered by

(57)

Undercut Opening Sequence

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

(58)

Lead and Lag

To evaluate the lead To evaluate the lead

and lag, convergence

information from

KLM mine is used.

Cave Front

Displacement ( mm/ day)

Convergence data is Convergence data is

presented in velocity

(mm/day) contour

(59)

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

No Advanced

horizontal vertical

070501 140501

0.0 mm/day

070501 140501

-1.3

(60)

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

140501 290501

-1.3 mm/day

140501 290501

-0.74

mm/day -1.12 _mm/day -0.1 _mm/day

(61)

horizontal vertical

290501 120601

-0.74 mm/day

290501 120601

-0.5 mm/day

-0.1 mm/day

0.3 mm/day

(62)

Increasing of horizontal velocity due to no advanced of lead and lag distance (45 meter)

120601 260601

-0.5 mm/day

120601 260601

-0.65 mm/day

0.3 mm/day

-0.2 mm/day

(63)

Decreasing of horizontal and vertical velocity

(64)

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

130701 070801

-0.4 mm/day

130701 070801

-0.8 mm/day

0.0 mm/day

-0.1 mm/day

(65)

horizontal vertical

070801 230801

-0.8 mm/day

070801 230801

-0.1 mm/day

0.0 mm/day

(66)

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

230801 150901

-0.1 mm/day

230801 150901

-0.75 mm/day

0.0 mm/day

-0.4 mm/day

(67)

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

0.1 mm/day

(68)

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

260901 091001

-0.4 mm/day

260901 091001

-0.1 mm/day

0.1 mm/day

0.0 mm/day

(69)

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

(70)

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

261001 071101

0.0 mm/day

261001 071101

0.0 mm/day

(71)

Lead & Lag Issue

Reading Displacement Velocity (mm/day)

Vertical Displacement Velocity (mm/day)

(72)

Lead & Lag Issue

From the convergence measurement, From the convergence measurement, the ideal the ideal

lead and lag is between 5 to 8 meters

lead and lag is between 5 to 8 meters, cave , cave front can be stopped without any significant

front can be stopped without any significant

displacement

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

because excessive damage will occur in the

panels

(73)

Case Study A

XYZ Mine

Plan View Access

Adits

Undercut Sequence and

Direction