Kestabilan Tambang Bawah Tanah 1
The Stability of
Underground Opening
Lufi Rachmad
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
Understanding stresses is an important part
in designing underground opening
2
Review from Last Week
Empirical equation to estimate insitu stresses
e.g. Shoerey
1
k = 0.25 + 7 Eh ( 0.001 + )
z
3
Review from Last Week
Stress distribution around various opening
shapes (circle, horseshoe, square, ellipse)
Underground opening design methodology
4
Case Study A
An orebody XYZ has been defined as a block
caving deposit. What we should design first?
Surface
A’
7 km
Plan View
1.4 km
A
Orebody
XYZ
Orebody
XYZ
Section A-A’
5
Case Study A
ACCESS
Surface
decline
shaft
Orebody
XYZ
adit
6
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
Intake airways
7
Case Study A
For the design purpose, how far apart should
these two adits be?
Orebody
XYZ
Surface
The farther
the more
ineffective
A
A’
?
Access
Adits
( A-A’)
Access Adits
Plan View
8
Case Study A
Assuming the simplest condition, the
axisymmetric stress distribution could be used.
r = 5R, the pre-mining
stress would not be
significantly different
from the virgin stress
field.
Tegangan Induksi/Tegangan Awal
2.00
Tegangan radial
Tegangan tangensial
1.50
1.00
r = 17 meter as an
early indication.
0.50
0.00
0
2
4
6
Jarak dari batas terowongan, r/R
8
10
Might be further
analyzed using pillar
stability calc and
numerical modeling
9
Insitu Stress
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
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
10
Stress Distribution around
“Horse-Shoe” Tunnel
σv
σh = σv
A
B
B
σh
σh = 0.5 σv
σθA = 2.2 σv
σθB = 1.3 σv
σθA = 0.6 σv
σθB = 1.8 σv
σh = 0.33 σv
σθA = 0.1 σv
σθB = 1.9 σv
11
Insitu Stress
Given the rock mass strength is around 7080 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
12
Insitu Stress Measurement
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.
Various ways to measure insitu stress
Overcoring - Triaxial Strain Cell
Hydraulic Fracturing
Flatjack Measurement
Borehole Breakout
Acoustic Emission
13
Overcoring (CSIRO Cell)
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.
Epoxy
Strain
Gages
14
Overcoring (CSIRO Cell)
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.
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.
The field procedures consist of drilling a
concentric EX-size borehole, installation of the
deformation gage, and overcoring a stress relief
borehole.
15
The CSIRO cell is designed to measure diametral
deformations of an EX-size (1.5" in diameter)
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.
16
Overcoring (CSIRO Cell)
Need Young’s modulus and Poisson’s inputs
Limited to within 10-30 meters of existing
opening
Overcoring Cost – CSIRO Cells (2 sites)
NIRM
US$ 61K
US$ 44K approx. 20K per site
ES&S
Price does not include drilling which will be
around US$ 120K / m
17
Hydraulic Fracturing
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 fracturefree segment of the hole by pumping in water.
18
Hydraulic Fracturing
The pressure is raised until
the rock surrounding the
hole fails in tension at a
critical pressure.
Following breakdown, the
shut-in pressure, the
lowest test-interval
pressure at which the
hydrofrac closes
completely under the
action of the stress acting
normal to the
hydrofracture
19
Hydraulic Fracturing
Limited to drill/pump equipment and ground
conditions – Max range 300m – 1000m
“Qualitative”
Assumptions
S1 Maximum Principle Stress is Vertical or
aligned with hole
Hydofracing
NIRM US$ 87K
Golder US$ 188K
20
Borehole Breakout
Extensive field evidence and laboratory
experiments suggest that borehole breakouts,
defined as borehole cross-section elongations
resulting from preferential rock failure, is a
direct consequence of the in situ stress in the
rock.
21
Borehole Breakout
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.
22
Borehole Breakout
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.
23
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?
Panel/ Drill Drift
Orebody
XYZ
Orebody
XYZ
σ1
σ1
Plan View
Plan View
24
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
σ1
Orebody
XYZ
Undercut
Advance
Direction
Plan View
25
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.
Cave Advance
Abutment
stress
26
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.
For XYZ Mine, the vertical stress σv = 38
Mpa. The abutment stress = 76 - 114
MPa
This abutment stress could devastate
development drifts if does not maintain
properly
27
Failure of yielding arch support
El Salvador Mine, Chile
Photo: M. L. Van Sint Jan
28
Rockburst at Extraction Level,
DOZ Mine, Indonesia
29
Collapse of an extraction level drift,
El Teniente Mine, Chile, 1989
1.5 m
CONCRETE
DAMAGE
CONCRETE
DAMAGE
30
Panel 15, 28 June 2003
Panel 15, 7 August 2003
Panel 15, 23 August 2003
31
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:
In situ Stress regime
Undercut direction
The timing of undercut relative to the
extraction level development
Undercut face shape
Cave Hydraulic Radius
Distance between Undercut and Extraction
32
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
For XYZ BC Mine, an undercutting method
should be selected.
33
Terminology
Drill Drift - Undercut
Fan Drilling
Draw Bell
Minor Apex
Major Apex
Panel Drift Extraction
Orepass
Draw Bell Drift
Draw Point
34
Conventional Panel Caving
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
35
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
extraction drift and drawpoint drift
36
Advance Undercut Panel Caving
Drawbells are always prepared in the destressed zone behind the undercut,
usually adhering to the 45 degree rule.
37
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
convergence gage
38
Comparing 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)
39
3-Point Convergence
40
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.
Orebody
XYZ
Plan View
4 km
KLM
Mine
41
Undercut Trial at KLM Mine
Panel 15
Panel 16
Post
Undercut
Advanced
Undercut
42
Undercut Trial at KLM Mine
Cave
UC
Lvl
Cave Advance
Abutment
Extr
Lvl
Convergence
Station
18 m
Last
Blasting
Row
43
Result of KLM Mine Trial
Advanced Undercut vs Post Undercut
Stable after
Cave Front
Passing
Cave Advance
Post Undercut
Anomaly
44
Result of KLM Mine Trial
Advanced Undercut vs Post Undercut
Stable after
Cave Front
Passing
Cave Advance
Anomaly
45
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
Advance
Abutment Stress
Remnant
Pillar
Cave
Last Blasting Row
46
Examples of Remnant Pillars / Stump
47
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
that a remnant pillar will be avoided.
48
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:
In situ Stress regime
Undercut direction
The timing of undercut relative to the
extraction level development
Undercut face shape
Cave Hydraulic Radius
Distance between Undercut and Extraction
49
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
unfavorable conditions in term of stress
concentration in the production level
50
Undercutting Sequence
51
Lead and Lag
Cave
Front
Lead and Lag: the
distance between the
caving front on adjacent
panels
Lead
and
Lag
52
Undercutting Sequence
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.
53
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
and deformation) induced on the production
level elevation.
54
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,
whatever undercut sequence is chosen.
55
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
panels that are mining in stronger ground, which
bears load and limits rock mass deformation in
the weak ground area.
56
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
rather than narrow.
57
Lead and Lag
Cave
Front
Displacement
( mm/ day)
To evaluate the lead
and lag, convergence
information from
KLM mine is used.
Convergence data is
presented in velocity
(mm/day) contour
Displc. = Lt-L0
58
Increasing of horizontal and vertical
velocity due to lead and lag (60 meter)
No Advanced
070501
140501
0.0
mm/day
horizontal
070501
-1.3
mm/day
140501
-0.2
mm/day
vertical
-1.12
mm/day
Decreasing of horizontal and vertical velocity
after reducing lead and lag distance (54 meter)
Advance 6 m
290501
140501
140501
-1.3
mm/day
horizontal
-0.74
mm/day
290501
-1.12
mm/day
vertical
-0.1
mm/day
Decreasing of horizontal and vertical velocity
after reducing lead and lag distance (45 meter)
Advance 9 m
290501
120601
-0.74
mm/day
horizontal
290501
-0.5
mm/day
120601
-0.1
mm/day
vertical
0.3
mm/day
Increasing of horizontal velocity due to no
advanced of lead and lag distance (45 meter)
No Advance
120601
260601
-0.5
mm/day
horizontal
120601
-0.65
mm/day
260601
0.3
mm/day
vertical
-0.2
mm/day
Decreasing of horizontal and vertical velocity
after reducing lead and lag distance
(30 meter)
Advance 15 m
130701
260601
260601
-0.65
mm/day
horizontal
-0.4
mm/day
130701
-0.2
mm/day
vertical
0.0
mm/day
Increasing of horizontal velocity due to no advanced
of lead and lag distance (30 meter)
No Advance
070801
130701
130701
-0.4
mm/day
horizontal
-0.8
mm/day
070801
0.0
mm/day
vertical
-0.1
mm/day
Decreasing of horizontal and vertical velocity
after reducing lead and lag distance (25 meter)
Advance 5 m
230801
070801
070801
-0.8
mm/day
horizontal
-0.1
mm/day
230801
-0.1
mm/day
vertical
0.0
mm/day
Increasing of horizontal and vertical velocity due to
no advanced of lead and lag distance (25 meter)
No Advance
150901
230801
230801
-0.1
mm/day
horizontal
-0.75
mm/day
150901
0.0
mm/day
vertical
-0.4
mm/day
Decreasing of horizontal and vertical velocity after
reduce lead and lag distance (8 meter)
Advance 17 m
260901
150901
150901
-0.75
mm/day
horizontal
-0.4
mm/day
260901
-0.4
mm/day
vertical
0.1
mm/day
Decreasing of horizontal and vertical velocity in the
same of lead and lag distance (8 meter)
No Advance
091001
260901
260901
-0.4
mm/day
horizontal
-0.1
mm/day
091001
0.1
mm/day
vertical
0.0
mm/day
Decreasing of horizontal and vertical velocity below 8
meter of lead and lag distance (5 meter)
Advance 3 m
261001
091001
091001
-0.1
mm/day
horizontal
0.0
mm/day
261001
0.0
mm/day
vertical
0.0
mm/day
Constant stable of horizontal and vertical velocity
below 8 meter of lead and lag distance (5 meter)
No Advance
071101
261001
261001
0.0
mm/day
horizontal
0.0
mm/day
071101
0.0
mm/day
vertical
0.0
mm/day
Lead & Lag Issue
Reading
Date
Lead
and Lag
Distance (m)
Cave
Advanced (m)
Horizontal
Displacement
Velocity (mm/day)
Vertical
Displacement
Velocity (mm/day)
07-May-01
0
0.0
0.2
14-May-01
Cave not
started
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≈
71
Lead & Lag Issue
From the convergence measurement, the ideal
lead and lag is between 5 to 8 meters, cave
front can be stopped without any significant
displacement
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
72
Case Study A
XYZ Mine
Undercut
Sequence and
Direction
Extraction Drift
Orientation
Access
Adits
Plan View
73
Underground Opening
Lufi Rachmad
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
Understanding stresses is an important part
in designing underground opening
2
Review from Last Week
Empirical equation to estimate insitu stresses
e.g. Shoerey
1
k = 0.25 + 7 Eh ( 0.001 + )
z
3
Review from Last Week
Stress distribution around various opening
shapes (circle, horseshoe, square, ellipse)
Underground opening design methodology
4
Case Study A
An orebody XYZ has been defined as a block
caving deposit. What we should design first?
Surface
A’
7 km
Plan View
1.4 km
A
Orebody
XYZ
Orebody
XYZ
Section A-A’
5
Case Study A
ACCESS
Surface
decline
shaft
Orebody
XYZ
adit
6
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
Intake airways
7
Case Study A
For the design purpose, how far apart should
these two adits be?
Orebody
XYZ
Surface
The farther
the more
ineffective
A
A’
?
Access
Adits
( A-A’)
Access Adits
Plan View
8
Case Study A
Assuming the simplest condition, the
axisymmetric stress distribution could be used.
r = 5R, the pre-mining
stress would not be
significantly different
from the virgin stress
field.
Tegangan Induksi/Tegangan Awal
2.00
Tegangan radial
Tegangan tangensial
1.50
1.00
r = 17 meter as an
early indication.
0.50
0.00
0
2
4
6
Jarak dari batas terowongan, r/R
8
10
Might be further
analyzed using pillar
stability calc and
numerical modeling
9
Insitu Stress
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
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
10
Stress Distribution around
“Horse-Shoe” Tunnel
σv
σh = σv
A
B
B
σh
σh = 0.5 σv
σθA = 2.2 σv
σθB = 1.3 σv
σθA = 0.6 σv
σθB = 1.8 σv
σh = 0.33 σv
σθA = 0.1 σv
σθB = 1.9 σv
11
Insitu Stress
Given the rock mass strength is around 7080 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
12
Insitu Stress Measurement
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.
Various ways to measure insitu stress
Overcoring - Triaxial Strain Cell
Hydraulic Fracturing
Flatjack Measurement
Borehole Breakout
Acoustic Emission
13
Overcoring (CSIRO Cell)
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.
Epoxy
Strain
Gages
14
Overcoring (CSIRO Cell)
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.
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.
The field procedures consist of drilling a
concentric EX-size borehole, installation of the
deformation gage, and overcoring a stress relief
borehole.
15
The CSIRO cell is designed to measure diametral
deformations of an EX-size (1.5" in diameter)
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.
16
Overcoring (CSIRO Cell)
Need Young’s modulus and Poisson’s inputs
Limited to within 10-30 meters of existing
opening
Overcoring Cost – CSIRO Cells (2 sites)
NIRM
US$ 61K
US$ 44K approx. 20K per site
ES&S
Price does not include drilling which will be
around US$ 120K / m
17
Hydraulic Fracturing
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 fracturefree segment of the hole by pumping in water.
18
Hydraulic Fracturing
The pressure is raised until
the rock surrounding the
hole fails in tension at a
critical pressure.
Following breakdown, the
shut-in pressure, the
lowest test-interval
pressure at which the
hydrofrac closes
completely under the
action of the stress acting
normal to the
hydrofracture
19
Hydraulic Fracturing
Limited to drill/pump equipment and ground
conditions – Max range 300m – 1000m
“Qualitative”
Assumptions
S1 Maximum Principle Stress is Vertical or
aligned with hole
Hydofracing
NIRM US$ 87K
Golder US$ 188K
20
Borehole Breakout
Extensive field evidence and laboratory
experiments suggest that borehole breakouts,
defined as borehole cross-section elongations
resulting from preferential rock failure, is a
direct consequence of the in situ stress in the
rock.
21
Borehole Breakout
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.
22
Borehole Breakout
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.
23
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?
Panel/ Drill Drift
Orebody
XYZ
Orebody
XYZ
σ1
σ1
Plan View
Plan View
24
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
σ1
Orebody
XYZ
Undercut
Advance
Direction
Plan View
25
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.
Cave Advance
Abutment
stress
26
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.
For XYZ Mine, the vertical stress σv = 38
Mpa. The abutment stress = 76 - 114
MPa
This abutment stress could devastate
development drifts if does not maintain
properly
27
Failure of yielding arch support
El Salvador Mine, Chile
Photo: M. L. Van Sint Jan
28
Rockburst at Extraction Level,
DOZ Mine, Indonesia
29
Collapse of an extraction level drift,
El Teniente Mine, Chile, 1989
1.5 m
CONCRETE
DAMAGE
CONCRETE
DAMAGE
30
Panel 15, 28 June 2003
Panel 15, 7 August 2003
Panel 15, 23 August 2003
31
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:
In situ Stress regime
Undercut direction
The timing of undercut relative to the
extraction level development
Undercut face shape
Cave Hydraulic Radius
Distance between Undercut and Extraction
32
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
For XYZ BC Mine, an undercutting method
should be selected.
33
Terminology
Drill Drift - Undercut
Fan Drilling
Draw Bell
Minor Apex
Major Apex
Panel Drift Extraction
Orepass
Draw Bell Drift
Draw Point
34
Conventional Panel Caving
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
35
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
extraction drift and drawpoint drift
36
Advance Undercut Panel Caving
Drawbells are always prepared in the destressed zone behind the undercut,
usually adhering to the 45 degree rule.
37
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
convergence gage
38
Comparing 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)
39
3-Point Convergence
40
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.
Orebody
XYZ
Plan View
4 km
KLM
Mine
41
Undercut Trial at KLM Mine
Panel 15
Panel 16
Post
Undercut
Advanced
Undercut
42
Undercut Trial at KLM Mine
Cave
UC
Lvl
Cave Advance
Abutment
Extr
Lvl
Convergence
Station
18 m
Last
Blasting
Row
43
Result of KLM Mine Trial
Advanced Undercut vs Post Undercut
Stable after
Cave Front
Passing
Cave Advance
Post Undercut
Anomaly
44
Result of KLM Mine Trial
Advanced Undercut vs Post Undercut
Stable after
Cave Front
Passing
Cave Advance
Anomaly
45
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
Advance
Abutment Stress
Remnant
Pillar
Cave
Last Blasting Row
46
Examples of Remnant Pillars / Stump
47
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
that a remnant pillar will be avoided.
48
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:
In situ Stress regime
Undercut direction
The timing of undercut relative to the
extraction level development
Undercut face shape
Cave Hydraulic Radius
Distance between Undercut and Extraction
49
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
unfavorable conditions in term of stress
concentration in the production level
50
Undercutting Sequence
51
Lead and Lag
Cave
Front
Lead and Lag: the
distance between the
caving front on adjacent
panels
Lead
and
Lag
52
Undercutting Sequence
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.
53
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
and deformation) induced on the production
level elevation.
54
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,
whatever undercut sequence is chosen.
55
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
panels that are mining in stronger ground, which
bears load and limits rock mass deformation in
the weak ground area.
56
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
rather than narrow.
57
Lead and Lag
Cave
Front
Displacement
( mm/ day)
To evaluate the lead
and lag, convergence
information from
KLM mine is used.
Convergence data is
presented in velocity
(mm/day) contour
Displc. = Lt-L0
58
Increasing of horizontal and vertical
velocity due to lead and lag (60 meter)
No Advanced
070501
140501
0.0
mm/day
horizontal
070501
-1.3
mm/day
140501
-0.2
mm/day
vertical
-1.12
mm/day
Decreasing of horizontal and vertical velocity
after reducing lead and lag distance (54 meter)
Advance 6 m
290501
140501
140501
-1.3
mm/day
horizontal
-0.74
mm/day
290501
-1.12
mm/day
vertical
-0.1
mm/day
Decreasing of horizontal and vertical velocity
after reducing lead and lag distance (45 meter)
Advance 9 m
290501
120601
-0.74
mm/day
horizontal
290501
-0.5
mm/day
120601
-0.1
mm/day
vertical
0.3
mm/day
Increasing of horizontal velocity due to no
advanced of lead and lag distance (45 meter)
No Advance
120601
260601
-0.5
mm/day
horizontal
120601
-0.65
mm/day
260601
0.3
mm/day
vertical
-0.2
mm/day
Decreasing of horizontal and vertical velocity
after reducing lead and lag distance
(30 meter)
Advance 15 m
130701
260601
260601
-0.65
mm/day
horizontal
-0.4
mm/day
130701
-0.2
mm/day
vertical
0.0
mm/day
Increasing of horizontal velocity due to no advanced
of lead and lag distance (30 meter)
No Advance
070801
130701
130701
-0.4
mm/day
horizontal
-0.8
mm/day
070801
0.0
mm/day
vertical
-0.1
mm/day
Decreasing of horizontal and vertical velocity
after reducing lead and lag distance (25 meter)
Advance 5 m
230801
070801
070801
-0.8
mm/day
horizontal
-0.1
mm/day
230801
-0.1
mm/day
vertical
0.0
mm/day
Increasing of horizontal and vertical velocity due to
no advanced of lead and lag distance (25 meter)
No Advance
150901
230801
230801
-0.1
mm/day
horizontal
-0.75
mm/day
150901
0.0
mm/day
vertical
-0.4
mm/day
Decreasing of horizontal and vertical velocity after
reduce lead and lag distance (8 meter)
Advance 17 m
260901
150901
150901
-0.75
mm/day
horizontal
-0.4
mm/day
260901
-0.4
mm/day
vertical
0.1
mm/day
Decreasing of horizontal and vertical velocity in the
same of lead and lag distance (8 meter)
No Advance
091001
260901
260901
-0.4
mm/day
horizontal
-0.1
mm/day
091001
0.1
mm/day
vertical
0.0
mm/day
Decreasing of horizontal and vertical velocity below 8
meter of lead and lag distance (5 meter)
Advance 3 m
261001
091001
091001
-0.1
mm/day
horizontal
0.0
mm/day
261001
0.0
mm/day
vertical
0.0
mm/day
Constant stable of horizontal and vertical velocity
below 8 meter of lead and lag distance (5 meter)
No Advance
071101
261001
261001
0.0
mm/day
horizontal
0.0
mm/day
071101
0.0
mm/day
vertical
0.0
mm/day
Lead & Lag Issue
Reading
Date
Lead
and Lag
Distance (m)
Cave
Advanced (m)
Horizontal
Displacement
Velocity (mm/day)
Vertical
Displacement
Velocity (mm/day)
07-May-01
0
0.0
0.2
14-May-01
Cave not
started
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≈
71
Lead & Lag Issue
From the convergence measurement, the ideal
lead and lag is between 5 to 8 meters, cave
front can be stopped without any significant
displacement
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
72
Case Study A
XYZ Mine
Undercut
Sequence and
Direction
Extraction Drift
Orientation
Access
Adits
Plan View
73