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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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

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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

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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

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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