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Longwall Pre-driven Recovery Roads –
a US Case History
28th October 2014
CONTENT
1. General Overview on the Use of Pre-driven Recovery Rooms
2. Longwall Holing Database
3. Recent US Case History
1. General Overview on the Use of Pre-driven
Recovery Rooms
WHY?
1. Existing roadway in longwall panel
• Wongawilli Colliery
• Angus Place Colliery
• Wollemi Mine
• Jim Walters (US)
2. Poor ground conditions during conventional longwall recovery
• Bull Mountains Mine (US)
• West Cliff
• Crimun
• Hunter Valley mines
3. Speed up longwall recovery • Newstan Colliery
• Appin Mine
• US mines
• Recent interest in Australia
Why Bother?
• The geomechanical holing model is based on two principles:
1. The load concentrates in stiffer structures
2. The overburden behaves as a cantilever and is able to span and redistribute
load away from the fender
• Effectively, this means that as the fender reduces in width and therefore softens, the
overburden redistributes more and more load onto the surrounding pillars.
• Critically, if a sufficient amount of support is installed, the fender never yields in a
material sense or fails.
Geomechanical Holing Model
Critical to a successful design is a
fundamental understanding of the
geomechanics during holing
(i) Stage 1 - Onset of Ground Movement in Roadway (up to 30m Inbye of Holing)
(ii) Stage 2 - Accelerating Ground Movements in Roadway (20 to 2m Inbye of Holing)
(iii) Stage 3 - Onset of Fielder Yield and Transfer of Load to Outbye Pillar (11 to 3m Inbye of Holing)
(iv) Stage 4 - Maximum Likelihood of Fender Instability (2 to 0m Inbye of Holing)(i) Stage 1 - Onset of Ground Movement in Roadway (up to 30m Inbye of Holing)
(ii) Stage 2 - Accelerating Ground Movements in Roadway (20 to 2m Inbye of Holing)
(iii) Stage 3 - Onset of Fielder Yield and Transfer of Load to Outbye Pillar (11 to 3m Inbye of Holing)
(iv) Stage 4 - Maximum Likelihood of Fender Instability (2 to 0m Inbye of Holing)
Schematic of the 5 Stages During Longwall Holing
(i) Stage 1 - Onset of Ground Movement in Roadway - up to 30m Inbye of Holing
(ii) Stage 2 - Accelerating Ground Movements in Roadway – 20 to 2m Inbye of Holing
Results from:
- the rotation of the upper roof as a cantilever (or series of cantilevers) onto the fender
- the ongoing build-up in vertical stress on the fender
- floor heave as the fender punches into the floor
(iii) Stage 3 - Onset of Fielder Yield and Transfer of Load to Outbye Pillar – 11 to 3m Inbye of Holing
- as the fender effectively “sheds” load, the stiff nature of the loading environment ensures that the
pillar deforms at a controlled rate and retains a large portion of its structural integrity
(iv) Stage 4 - Maximum Likelihood of Fender Instability – <2m Inbye of Holing
The likelihood of failure is controlled by:
- the competency of the cantilever(s)
- the load bearing ability of the fender
- the ability of the standing supports to assist the fender in controlling the roof-to-floor convergence.
(v) Stage 5 - Secondary Surge in Roof Displacement - Removal of Fender and Hole Through Roadway
Results from:
- the removal of the fender (load bearing through final shear)
- the consequent increase in roof span ahead of the powered supports
(v) Stage 5 - Secondary Surge in Roof Displacement (Removal of Fender and Hole Through Roadway)
Roof-to-Floor and Rib-to-Rib Convergence on Holing
(i) Roof-to-floor convergence (ii) Rib-to-rib convergence
Stage 1
Stage 4
Stage 5
Stage 3
Stage 2
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500
12:00 14:24 16:48 19:12 21:36 0:00 2:24 4:48 7:12 9:36 12:00
TIME (March 4-5, 2006), hrs
LE
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bars
0
0.8
1.6
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E, in
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S152 S152 Site # 2 (Mid-Panel) - Panel
Site # 2 (Mid-Panel) - Center Row Site # 2 (Mid-Panel) - Pillar
Longwall Leg Pressure (Barczak et al, 2007)
2. Longwall Holing Database
Longwall Holing Database
• Database includes 162 examples from 34 mines from the US, Australia, South Africa,
UK and Norway
• Compared to the 1998 database (i) over 44 new cases have been added, (ii) roadways
of <50m in length have been removed and (iii) a number of the previous sites have
been updated
• Of the 162 examples, 17 are reported as failures and 2 as marginal cases
• Three types of failure mechanisms are evident from the database:
1. Weighting or fender failures – the roof fell and/or the longwall ended up in a near
iron-bound condition
2. Roof falls ahead of the face and/or in the roadway
3. Floor failure – limits the ability of the fender and/or standing supports to control
the roof
Schematic Example of Failed Pre-driven Recovery
Room
(i) Weighting Failure in a US Mine (Pulse, 1990)
Schematic Example of Floor Failure in Pre-driven
Recovery Room
(ii) Floor Heave and Associated Cantilever Failure in an Australian Mine
Main Parameters Separating the Successful and
Unsuccessful Cases
0.0
1.0
2.0
3.0
4.0
5.0
6.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
Sta
nd
ing
Su
pp
ort
Den
sit
y (
MP
a)
Roof Reinforcement Density (MN/m)
Successful case
Unsuccessful case - roof fall
Unsuccessful case - weighting
Unsuccessful case - floor failure
Marginal case - cantilever failure
- Standing support density
and roof reinforcement
density
- The database indicates
that the majority of the
failures relate to those
roadways where a standing
support density of <0.3 MPa
and a RDI of <0.6 MN/m was
used
- Exceptions include floor
failure and slow mining
Relationship between the Onset of Fender Yield and
Depth of Cover
- The onset of fender yield is
controlled by Depth of Cover and
as such, it is reasonable to assume
that with increasing depth:
1. The more the fender and
roadway will deform on holing and
2. The greater the required density
of roof, rib and standing support
Minimum Ground Support Guidelines
• The database indicates minimum ground support standards:
1. Minimum standing support density of >0.5 MPa
2. Minimum roof reinforcement density of >0.9 MN/m
3. If slow mining is a risk, a minimum standing support density of >1.2 MPa
• Before the above are finalised, consideration must however be given to:
1. Depth of Cover
2. Density of rib reinforcement
3. CMRR
4. Floor geology
5. Roadway width
3. Recent Case History in the US
Prior Pre-driven Recovery Road Experience at Mine
• Poor ground conditions encountered during the first conventional recovery of the longwall led to the
mine utilising a pre-driven recovery road for second recovery
• Roadway dimensions – 3m high x 9.1m wide x 380m long
• Depth of Cover – 60 to 80m
• The first 1m of roof was dominated by shale, which was overlain by 7 to 9m of sandstone
• Coal Mine Roof Rating (CMRR) of around 45 to 55
• Unfavourable regional joint orientation (sub-parallel to face)
• Periodic weighting
• Ground support type and density
• Roof was reinforced with a RDI of 0.47 MN/m – 3.7m long 40 tonne capacity point-anchored
cables
• Standing support capacity ranged between 0.32 and 0.35 MPa – 110 and 200 tonne capacity
pumpable cribs
Schematic of Inferred Failure Mechanism in the Pre-
driven Recovery Road
Stage 1 – 5m from Holing, Cantilever Rotates onto Fender and Supports
Stage 2 – 2 to 3m from Holing, Cantilever Continues to Displace and
Overloads Fender and Supports
• The standing support was inadequate and
as a result, provided little assistance to the
fender and longwall shields in controlling
the rotation of the overlying cantilever(s).
• 5m inbye of holing, the cantilever(s) started
to displace at an accelerating rate and in
doing so, loaded up the standing supports,
fender and longwall shields.
• At this point, the fender started to visually
deform and at 2 to 3m inbye of holing, the
fender had completely yielded.
• When the fender yielded, the standing
supports also started to yield and fail.
Schematic of Inferred Failure Mechanism in LW 2
Recovery Road
Stage 3 – Fender and Supports Continue to Yield, Allowing the
Cantilever to Displace at an Acceleration Rate and Eventually Fall
Stage 4 – Shields Become Iron-bound
• By this stage, the majority of the
overburden load was sitting on the
longwall shields.
• The cantilever displaced at an accelerating
rate and eventually failed (as evidenced by
the large goaf break).
• The cantilever was unable to redistribute
any load onto the outbye pillars.
• As a result of the associated dead weight:
(i) around 600mm of convergence
was observed in the recovery
room and
(ii) a large number of the longwall
shields became iron-bound
LW 3 Pre-driven Recovery Road
• Roadway dimensions – 2.9m high x 12.8m wide x 380m long – widest roadway in the database
• Maximum Depth of Cover – 60m
• The first 5m of roof was comprised of units of shale, siltstone
and sandstone, overlain by a 1m thick rider seam
• Coal Mine Roof Rating (CMRR) of around 45 to 55
• Roadways generally exhibit static behaviour at the mine on
development
• The first 0.3m of floor was comprised of shale and below
this, was dominated by a reasonably strong sandstone unit
• No significant geological structure
Ground Support Design in the Pre-driven Recovery Road
• Roadway was driven in 2 passes, and the following densities of support were installed:
1. 1.6 MN/m of roof support in the mid-face area
2. 1.0 MN/m of roof support in the protected ends
3. Fender was reinforced with 30 tonne capacity fibre-glass dowels
4. Mesh was pinned to the roof
5. Roadway was backfilled with a minimum of 5.5 MPa cellular concrete
• Secondary support consisted of 7.6m long 63 tonne capacity post-groutable cables – first know
example of Australian hardware used in the US coal industry
Ground Support Design in the Pre-driven Recovery Road
0.0
1.0
2.0
3.0
4.0
5.0
6.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
Sta
nd
ing
Su
pp
ort
Den
sit
y (
MP
a)
Roof Reinforcement Density (MN/m)
Successful case
Unsuccessful case - roof fall
Unsuccessful case - weighting
Unsuccessful case - floor failure
Marginal case - cantilever failure
Signal Peak - 3 Right Recovery Room Mid-face
Signal Peak - 3 Right Recovery Room Protected End
Ground Support Design in the Pre-driven Recovery Road
The Adequacy of the Backfill
• Compared to standing support:
1. The backfill would not only support the roof, but would also confine the ribs and
floor
2. The backfill would not be susceptible to sudden and uncontrolled failure.
• It was critical that the strength and stiffness of the backfill was similar to the surrounding
coal.
• Considering both the in situ strength of coal and the need to ensure the backfill was
cuttable, it was recommended that the backfill attained a nominal UCS 7 MPa and a minimum
of 5.5 MPa.
The Adequacy of the Backfill
• Modelling indicated that due to the
fact that the backfill would be
confined on three sides during
holing, the in situ strength of the
cellular concrete could be as much
as 30 MPa and as such, it was
reasonable to assume that:
1. The strength of the backfill
would far exceed 5.5 MPa
and
2. The backfill wold not fail
en masse on holing.
Installation of the Backfill
• It was imperative that the backfill was filled as tight as possible to the roof.
• Critical points to note in regard to tight filling are:
1. Unlike fly-ash / cement mixtures, cellular concrete is not susceptible to significant
amounts of shrinkage
2. Cellular concrete is self-levelling
3. The slight (i.e., 1%) tailgate to headgate cross grade meant that the fill would be
gravity-filled to the roof
4. The mine pumped the backfill into a series of bulkheads, and each bulkhead
compartment included an injection hole and a ventilation hole
5. Each bulkhead was surveyed with a borehole camera prior to the completion of
the backfilling
Monitoring Results
Monitoring Results
• Stress cells in fender first measured a build-up in load when the fender was between 53
and 25m wide
• Accelerating rate of loading when the fender was approximately 18m wide
• Measured an ongoing build-up in load until the fender was 1m wide
Monitoring Results
• Stress cells in the barrier pillar reached a peak load when the fender was 1m wide
Monitoring Results
• Convergence monitoring indicated that the roof in the roadway began to converge at an
significant rate when the fender was approximately 5m wide
• Indicates between 75 and 110mm of roof-to-floor convergence in the mid-face areas and
between 28 and 43mm in the protected ends
• A deceleration in convergence when the fender was between 1 and 0m wide
Summary of Monitoring Data
• The fender started to yield when it was around 1 to 2m wide
• The fender was still load bearing throughout the holing process.
• The above coupled with the limited magnitude of stress measured on the fender and
backfill suggest a controlled loading environment.
Closing Remarks
• Previous two longwall recoveries took around 3 months to complete
• The longwall holed into, cut out the backfill and recovered the shields in 14 days!
• Monitoring results during holing indicate room for optimisation in ground support
design
• One significant optimisation would be to use grout pillars as opposed to complete
backfill
• Experience at Wongawilli Colliery indicates favourable geotechnical and operational
results using grout pillars compared to conventional standing support such as Fibre-
cribs
Grout Filled Pillars at Angus Place Colliery
Thank you
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