Abandoned Coal Mine filling - Holz

Bill Holz, Mine Fill Manager, Keller Mine Fill, 2011
Summary
The upgrade of one of Queensland’s busiest road transport corridors, the Ipswich Motorway, between Dinmore and
Goodna, by the Queensland Department of Transport and Main Roads (DTMR) involved re-aligning the motorway
corridor over two separate abandoned coal mines and adjacent a third.
During the preliminary site investigation and through the early stages of design, abandoned coal mines were
discovered under sections of the footprint of the new motorway. Whilst there was some knowledge of the existence
of these mines that had been operational from the mid 19th Century through until the late 1980s, details as to size,
depth and accurate location were largely unknown. The DTMR formed Origin Alliance, an alliance made up of
designers and constructors, to construct the motorway and to find solutions to the partial filling of these old
abandoned coal mines where there was the potential to cause subsidence damage to the new motorway. Origin
Alliance together with Keller Mine Fill, a specialist mine fill contractor, formed a sub alliance to tackle the task of
accurately locating and target filling these mines so as to provide protection to the motorway asset against a future
subsidence event.
1.
INTRODUCTION
The three mines that required partial filling displayed very different characteristics requiring different approaches to
the drilling and filling methodology adopted and indeed the preparation of the mine before either of these activities
could begin.
Consideration of methodology varied with different levels of mine location knowledge, whether flooded or not,
whether mines were filled with methane, differing mine stability considerations with different associated safety
concerns. The methods needed to cater for a variety of collapse conditions from total collapse, to almost no collapse
at all. Methodologies also needed to incorporate drilling and filling operations under live motorway conditions. This
paper outlines some of those issues and some of the innovations undertaken to successfully complete the filling
operation.
2.
GENERAL HISTORY OF MINING IN THE IPSWICH COAL SEAMS
The earliest coal mining in Queensland occurred in the Ipswich area. The site of the first mine is generally believed
to be at Redbank where in 1843 early mining efforts were directed toward providing fuel to steam powered boats
plying the nearby Bremer and Brisbane Rivers.
With further discoveries of coal, the city of Ipswich grew as a hub supporting the growing number of coal mines in
the area.
3.
GOODNA MINE
3.1. HISTORY OF GOODNA MINE
Mine filling commenced on the Goodna Mine located below the existing live motorway and under the proposed
footprint of the new motorway. This mine was the least documented of all three mines, having no mapping details
available at all. Anecdotal evidence from texts and local knowledge was the only information that could be sourced
relating to depth of workings, location and mine geometry.
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FILLING OF ABANDONED COAL MINES
LOCATED UNDER THE IPSWICH MOTORWAY,
QUEENSLAND, AUSTRALIA
Goodna Mine itself was worked from the 1860’s through until 1910. The working heights in the mine ranged
between 1m and 1.4m and workings were located between 20m and 40m below ground level. Filling operations at
this mine were complicated by issues associated with the fact that the mine was completely flooded at the time of
filling.
3.2. DESIGN AND FILLING PROCESS
The mine fill design involved the construction of a down-dip barrier wall on the southern boundary of the fill zone.
The barrier wall was designed to span between the natural east and west boundaries of the mine. The barrier
design consisted of a series of drill holes spaced at 2.4m centres in a zigzag pattern along the line of the proposed
wall. This barrier wall was designed with a low slump paste mix (barrier mix) consisting of flyash, course sand,
cement and water. The paste was pumped down a drilled hole via a capped steel tremie tube to the base of the void
where the cap was blown off under paste pressure. The paste was then injected until a backpressure equivalent to
80% of the overburden pressure was reached. The concept is that the mounding paste rises to the roof level of the
mine, spreads out against the ceiling of the workings and seals off the roadway forming a barrier to the flow of future
infill slurry. Figure 1 below illustrates the injection of barrier paste to form the barrier wall. Note the plan of the barrier
wall pattern and infill layout pattern at the top of the diagram. The inset shows the fill activity depicted in the graphic.
Figure 1 Construction of the barrier wall down-dip of the fill
zone
Figure 2 Injection of infill mix up-dip of the barrier wall
To ensure that the intended barrier wall was mounding in a suitable fashion, above ground paste trials of the
proposed mix consistency were performed so as to view the mounding of the paste. Once complete these trials
were then repeated underwater in a dam, which when subsequently drained showed the successfully built barrier
mound.
As with the barrier mix, similar trials above ground and underwater were carried out with the infill slurry mix to
ensure the workability was suitable for the task.
The high slump slurry mix was then injected via a tremie pipe into the mine through a series of primary and
secondary injection holes in two 8m x 8m square grids offset by 4 m from each other. Figure 2 illustrates the
process.
The close 4m grid spacing was maintained as in the original design of this mine because of the erratic and
inconsistent geometry of the mine layout.
Considerable mapping of the voids was attempted with the use of a down-hole sonar device in this mine. Whilst this
provided some information on the extent of filling of the mapped void, the roadway location information was not
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1A bill introduced into the New South Wales parliament in 1854 (prior to Federation when modern day Queensland
was part of New South Wales) required that mining activities, including the details of shafts and tunnels be
recorded. However, there is no evidence that this was ever policed in Queensland. The few very early mine maps
that were available from the Queensland Department of Mines relating to the Goodna Mine were lost during
substantial flooding of their offices in 1974.
The sonar was useful in determining likely locations to insert mine water extraction pumps covered later in this
paper. It also assisted with confirmation of the extremities of the mine and gave some macro indication of the likely
fill volume at the location which assisted day to day planning of the works. In addition it was useful in determining
locations to direct verification core drilling to ensure the target was in fact a filled portion of the mine and not a pillar,
limiting wasted attempts at core drilling.
Despite the crude nature of the bord and pillar layout and the limited information available from the sonar, the extent
of the drill fill works to the west of the fill zone was able to be reduced from the original design. Areas where there
were no voids were able to be isolated thus confidently enabling the elimination of a number of holes at the
extremity of the original mine fill design position.
3.3. ADDRESSING THE MINE WATER ISSUES IN THE FLOODED MINE
The need to fill the flooded Goodna Mine led to the concern that filling could potentially displace contaminated mine
water both horizontally and vertically, resulting in adverse environmental effects. To prevent this, very tight limits
were placed on the allowable water level fluctuations. The limits were 1000mm upward and 300mm downward.
These limits were achieved by pumping water from the mine void whilst injecting a slurry mix into a linked section of
the same void, thus maintaining a balanced water table during the filling operation.
Other water level concerns related to the potential to trigger a subsidence event under the existing live motorway
carrying 80,000 vehicles per day. The concern was that a substantial drop in the watertable level would reduce the
buoyancy effect on the overburden and increase the load on the pillars resulting in a collapse. To this end the
maximum downward fluctuations (300mm at design), were reduced further by the onsite fill team, self imposing a
100mm downward limit.
The complicated part of the filling was to achieve a balance between filling the mine with slurry volume and
extracting water of a matched volume. The lack of any mapping of this mine, coupled with the non-geometric nature
of the old mine workings, made this exercise impossible to predict in advance of filling. In order to
balance the water extracted with the paste injected it is necessary to pump water from the same or a connected
void. The art was to develop a system to ensure that only when connectivity was clear, could the filling and
extraction process proceed. Of course this connectivity relationship needed to be re-established each time a new
drill hole was filled and so the “pump in and pump out” operation started over again at the next set of
injection/extraction locations.
To achieve this balance, accurate volume meters were fitted to the paste injection pumps to measure paste volume.
Accurate water volume meters were also fitted to the water extraction outlets. This allowed the knowledge that these
two volumes were measured and could be compared. Coupled with this, mobile hydrostatic pressure meters were
purpose built for this mine to measure small fluctuations in the water level in the mine as paste was injected. By
placing one of these meters in a drill hole (future paste injection hole) near the paste injection point, a small initial
burst of paste would trigger a slight rise at the nearby hydrostatic meter. By simultaneously placing another
hydrostatic meter nearby the water extraction point (in a different future paste injection hole) the same burst of paste
would also register a slight water table fluctuation at this location. By closely monitoring both hydrostatic meters
during pumping and filling, assurance was gained that water was extracted from the same or a connected void as
the injected paste was being placed, thus maintaining groundwater equilibrium.
As mine water was extracted it was pumped overland and treated at a purpose built Reverse Osmosis water
treatment facility then reused, either in the paste mix or for dust suppression.
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sufficiently complete as to be able to confidently map the mine and thus increase drill grid spacing and so reduce
the number of drill holes.
NEW REDBANK MINE
4.1. HISTORY OF NEW REDBANK MINE
The New Redbank Mine, partially located below the motorway footprint, was mined from 1916 through to 1933 when
it was closed following a fatal gas explosion. These workings were irregular but nonetheless mapped to a
reasonable level of accuracy.
Available maps of the mine workings indicated up to three layers of mines in some areas and as drilling and filling
for the works revealed, collapses had occurred to varying degrees throughout the explored sections of the mine
which was located at depths of up to 90m.
This mine contained high levels of methane gas, a factor for which multiple control measures were adopted to limit
the potential for explosion during the drilling and filling operations.
4.2. VARIED METHODOLOGY FOR DIFFERENT COLLAPSE SCENARIOS
In order to understand the workings it is important to be aware that there are different approaches to filling
dependant on the degree of collapse. In the case of the New Redbank mine these approaches are best explained
by splitting the mine into the two extremes of collapse that were observed.
4.2.1. OPEN ROADWAYS WITHOUT COLLAPSE
The first collapse type was where there was limited fall and open, albeit irregular, roadways that were
generally found to be in accord with the available mine maps.
Where roadways were open, a traditional barrier wall could be constructed in a similar fashion to that described for
Goodna Mine. The difference in this mine was that it was not flooded and as such video footage enabled clear, real
time images of the building of the barrier wall as filling was in progress. This not only showed the process was
working but gave added confidence that the potential to waste excessive infill mix was reduced by observing the
completed barrier.
Figure 3 Barrier mix paste extruding from the tremie pipe and
building toward void ceiling
Figure 4 Build up of barrier mix paste against void ceiling
The infill of these open roadways was usually straight forward as the infill slurry could be observed running up to
80m in extreme cases. The ability to observe the flow of the material using down hole video technology gave
confidence in departing from the 4m square drill grid to a drill hole targeting regime that ultimately led to the
elimination of up to 60% of drilled holes in some areas at a great saving to the project.
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4.
4.2.2. COLLAPSED ROADWAYS
Parts of the New Redbank Mine were so substantially collapsed that barrier wall construction was not possible. In
areas where this occurred, infill slurry was injected and the natural barriers of the mine, estimated from mine fill drill
records and limited accuracy maps, were used to contain the slurry. As infill slurry was injected cameras positioned
in nearby up- dip and cross-dip boreholes were used to observe collapsed material filling with infill slurry mix.
4.3. METHANE SAFETY ISSUES
New Redbank also tested the Mine Fill Team’s ingenuity in providing a safe method for the drilling and filling of a
methane filled mine.
Drilling and filling techniques were developed to limit the escape of methane to safe levels. Prior to commencement
of each shift, the work area was checked for inverted layers of methane using gas detectors. In addition the
following measures were put in place to promote a safe work environment:
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No mobile phones or electronic devices allowed inside of designated areas
All site personnel were to wear anti- static clothing
Gas detectors on drill rigs and all drillers were to carry one at all times with gas levels continuously monitored
in the work area
If methane levels reach 4%, an air blower was used to disperse the gas level around and away from the drill
annulus
Drilling techniques within 10m of the mine void or fractured rock zones prohibited the use of air as a drilling
medium
A diverter was used when drilling to divert any methane gas and drill cuttings to a water filled skip clear of
personnel and the diverter hose was maintained under water when drilling the last 10m above the void
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Figure 5 Infill slurry flowing from up- dip of camera inspection location
5.
WESTFALEN NO 3 MINE
The third mine filled as part of the motorway stabilisation programme, Westfalen No 3, was the most modern of the
three having been mined as recently as 1987. The new Motorway footprint was located adjacent and within the zone
of influence of this mine. The portion of the mine that was filled varied between 50m and 80m in depth and a
reasonably accurate map of the workings was available. The mine in the fill zone had been mined in two levels with
typical workings heights ranging between 7m and 7.5m on each level. This mine in particular contained high
concentrations of methane and was considered to be unstable. Mine filling works on this mine also involved purging
the mine of methane gas and replacing the mine atmosphere with nitrogen prior to drilling and filling works.
5.1.
NITROGEN INERTISATION
5.1.1. THE CONCEPT
Westfalen No 3 Mine contained methane gas concentrations of up to 74% within the mine voids. Stability
assessments of the mine pillars provided by eminent mine stability experts indicated that there was a risk of sudden
mine collapse during the drilling and filling operations. A collapse had the potential to generate a crater of up to 7m
in depth, accompanied by the rapid escape of substantial volumes of methane gas with the potential to endanger
those in the vicinity of the mine.
Having recognised the collapse potential as a substantial risk, the Mine Fill Team instigated a number of control
measures to address the issue. The most significant control was to inertise the mine through the injection of
nitrogen, an inert gas, to replace the methane gas.
Whilst the injection of the nitrogen was common to the two methods of methane removal, there were two ways of
disposing of the methane gas from the void. The first was by venting the gas into the atmosphere and the second
was by flaring the gas in a methane gas flare. Both means were available. However, for the majority of the time, the
flaring option was adopted converting the CH4 into CO2 minimising greenhouse gas emissions. Venting was only
used when low concentrations of methane, insufficient to operate the flare, were available toward the end of the
inertisation process.
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Figure 6 Drill rig discharging potentially methane laden cuttings underwater and away from immediate operations
5.1.2.1. AN OVERVIEW
Westfalen No 3 mine was flooded in the lower down-dip workings outside the design fill zone. Therefore, all voids
up-dip of the flooded section of the mine were methane rich and the entire volume, estimated at 350,000m3, was
required to be purged of methane prior to mine fill works being carried out there.
5.1.2.2. SAMPLING
Prior to commencement of inertisation samples of mine gases were taken and tested for content and concentrations
as a benchmark. Gas levels achieved in testing pre-inertisation were:
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CH4-50% in the lower levels of the mine to 74% in the upper levels
O2< 0.5%
CO2 = 2 to 3%
H2S < 0.5%
N2 = 24% to 47%
5.1.2.3. THE FLARE
2The
flare used for the process was designed and manufactured by LMS for flaring landfill gases. The certifications
available with the module were suitable for use in an industrial environment so very little modifications were needed
to suit the mine void extraction process.
Figure 7 Shielded methane flare
2The
flare is fitted with a detonation arrestor and a slam-shut valve to eliminate the possibility of a flashback. A
flashback is only possible with a flammable mix of methane, one containing a sufficient concentration of oxygen,
achievable if there was a leak in the pipe work leading to the flare unit.
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5.1.2. THE PROCESS
Figure 8 Flare burners inside flue
The flare is equipped with an inlet monitor, which monitors the methane levels and is designed to shut-off if methane
levels fall below 30%. Given that our target was to reduce the methane levels to 1% this posed a small problem.
With the addition of an oxygen monitor designed to shut off when oxygen levels reached 8%, the methane shut-off
was able to be eliminated and still allow safe operation of the flare.
One concern was that the coal gas may commence regeneration once methane extraction began causing ongoing
methane generation inside the mine. Westfalen No 3 mine was not considered a “gassy” mine during its operation
and as such the regenerated methane was expected to be insignificant. Before nitrogen injection began the
extraction fan on the flare was activated for a period to see if gas pressures dropped. The concept of this was that a
pressure drop in the mine gas meant that gases were not regenerating, at least at any substantial rate. This proved
to be the case as pressures did indeed drop prior to the injection of the nitrogen.
It is at this stage that nitrogen is to be injected at a rate to match as closely as possible the extraction rate. Note that
one of the considerations for the use of the flare type in question was its design to maintain the visible flame within
the flue and so not unduly raise community concerns.
5.1.2.4. THE NITROGEN GENERATOR
Nitrogen was generated on site by extracting it from the air (the air being a mixture of gases including 78%
nitrogen). The nitrogen was then injected into the down-dip end of the mine at a rate of 500m3 per hour via two
predrilled boreholes drilled into the mine void. At the same time, the flare, equipped with an extraction fan, was
connected via a gas pipeline to the methane outlet casing tapping the methane in the upper end of the mine. This
allowed the methane to move toward the ceiling of each of the two levels of mine workings at the mine’s uppermost
extremities.
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The flaring involved the need for a flue with an extraction fan at the inlet side of the flue to provide a negative
pressure to the top of the drilled hole. The extraction fan then draws the methane from the void at the upper end of
the mine as it is simultaneously being replaced at the lower end of the mine with nitrogen injected there via two
predrilled boreholes.
Two targeted cased boreholes had been predrilled at the up-dip end of the mine. These boreholes were drilled into
the mine void ceiling, one exiting at the lower level void ceiling, having passed through the upper void and one
exiting at the upper level void ceiling. These casings were then both connected to the shielded methane flare. The
methane was then extracted from
the ceiling of the mine via these drilled casings and into the flare where the methane was then burned off. Valves
located at the surface some 50m from the flare could be controlled to direct vented methane from either mine level
to the flare.
5.1.2.5. METHANE MONITORING
As methane was displaced, total methane levels could be monitored at the flare and directly within the mine at any
level of the void. The concentration of the methane dropped from a high of 74% to less than 1% upon completion of
inertisation.
Monitoring of gas concentrations was achieved using a down-hole meter that transferred gas concentration data in
real time to the site office for monitoring.
The inertisation process was completed ahead of the mine team gaining access to the area for full scale drill and fill
operations. The exercise of inertisation was not one that had originally been foreseen at the early concept stages of
the mine fill operation and came about when developing safe work methods as part of the pre- planning for drilling
and filling this mine.
5.1.3.
STUDIES AND APPROVALS
Many issues required addressed prior to inertisation to ensure the safety of the workforce, the public and the
environment. Approvals were sort from DERM, the environmental regulator, to carry out the flaring and venting
works. Guidance provided by CSIRO and SIMITARS (Safety in Mines Testing and Research Station) was invaluable
in planning the works.
SIMITARS were commissioned to conduct computer modelling of the dispersion of the methane during venting at
the proposed flare location. This modelling was to ensure that if methane was vented unflared, that there was no
potential for an explosion and dissipation levels were such that this was ensured. By modelling local atmospheric
conditions based on meteorological data from nearby Rocklea, SIMITARS were able to prove that safety of the
public and the environment would not be compromised.
Whilst advice was available from these organisations, the supplier of the flare facility was forthcoming in matters
related to the flaring. The supplier of the nitrogen generator was able to supply information about nitrogen
generation but the services of an expert in the inertisation control were also sort to tie these two operations together.
This provided confidence of having “covered all bases” in terms of ensuring the safety of the process and that of the
public. It should be noted that flaring technology of this type is quite common in landfill sites. However, the
inertisation of an abandoned mine coupled with the flaring operation in an urban environment is believed to be an
Australian first.
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Figure 9 Nitrogen generation unit and down-dip nitrogen injection point
6.
CONCLUSIONS
This paper outlines the very different issues presented during drilling and filling and the methodologies adopted to
address the challenges presented by each of the three mines. The methodologies need to be varied to account for
varying knowledge of mine location data, whether water or methane are present in the mine and they need to
address different filling approaches dependant on the degree of collapse observed during drilling works.
7.
ACKNOWLEDGEMENTS
The author wishes to acknowledge the assistance of CSIRO, SIMITARS, Staff of the Origin Alliance, the Origin
Alliance Mine Expert Panel, Staff of the Origin Keller Sub Alliance and Mr Geoff Bray of Bray Solutions Pty Ltd for
their assistance during the mine fill works on this project.
8.
REFERENCES
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Millar, D & Holz, B (2010), “Ipswich Motorway Upgrade-Filling of Abandoned Coal Mines”, Queensland Roads
(Edition No 9, September 2010), Department of Transport and Main Roads, Brisbane Australia, Pages 46-61.
Bray, G (2009), Westfalen Mine Degassing Report, Bray Solutions Pty Ltd.
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FILLING OF ABANDONED COAL MINES IPSWICH MOTORWAY – Bill Holz
A total of 550,000 m3 of nitrogen was manufactured from the air using a nitrogen generator running 24 hours a day,
7 days a week until the methane levels were reduced to 1%. Once methane extraction was complete and confirmed,
the drilling and filling works were able to commence.