There has been some controversy in the Australian
ground control community over the importance of the
installed tension. There are devices available that allow
torque-tension bolts to be installed with more than 10 tons
of load, twice the usual amount.

The bolt density varied considerably in the mines
studied. Six mines used four bolts on 1.2-1.5 m centers, a
pattern that would be considered typical in the U.S.
Another four mines used more intensive patterns, with six
bolts on 1-1.2 m centers. Figure 6 compares Australian
with U.S. bolt densities.

The trend towards reduced roof bolt densities seems to
be relatively recent. At one mine I visited near Newcastle,
the bolt density has been decreased by 56% (from 6 per
meter to 4 per 1.5 meter) over the last three panels,
apparently without negative effect.

Rib bolting is almost universal in Australia, due to the
high seams. Two or four rib bolts are usually installed per
row of roof bolts, and additional rib bolts may be installed
later. Several mines expressed disappointment in
fiberglass bolts, which often fail at low load or even during
installation. Some plastic bolts are currently being trialed.

In the U.S., secondary tailgate support is usually
installed nearly simultaneously with the headgate pass. In
Australia, most mines try to leave the tailgate open for
access and ventilation. About half the longwalls visited
used no regular tailgate secondary support, and most of

the rest installed it only within 100 m of the tailgate corner.
Burrell cans seemed to be the most common standing
support, followed by Link-N-Lock cribs. No tailgate |
observed was supported by wood cribs (or "chocks," to use
the Australian terminology). Cable bolts and flexi-bolts
were also used for supplemental support.

In general, it seems that Australian longwails may have
traditionally relied upon primary roof bolts for tailgate
support, which may partially explain their higher bolt
density. The disadvantage is that intensive primary
support patterns slow down gate road drivage rates. The
current trend towards reducing the number of primary bolts
speeds up drivage, but requires that mines make more use
of cable bolts and standing supports in tailgates.

One notable contrast with U.S. longwalls is that more
than 60% of Australian faces are using four-leg supports.
Many of these are relatively new supports, with capacities
of up to 900 tonnes. In the U.S., only 2 of 65 longwails use
four-legged supports.

Retreat Mining: Pillar recovery operations have
historically accounted for the lion's share of ground contro!
fatalities in Australian coal mines. In 1991, seven miners
were killed on NSW pillar lines. Following a major
research, education, and regulatory effort undertaken by
the entire mining community (School of Mines, University
of New South Wales, 1994; Coal Mining Inspectorate and
Engineering Branch, 1992; Shepherd, 1997), no fatalities
have occurred during the last five years.

PRIMARY SUPPORT vs CMRR
g 100 :
= 97 2
Q 804 o0°
PB 70 og a
60 + 5 8 0
f 50 | "85080 em Fo
3 Gb g
g@ 404 D ae oe o's:
P .
2 305
= 204
@ 104
° oO T T
0.00 0.20 0.40 0.60 0.80 1,00
Primary Support Factor (PSUP)
0c Unsatisfactory (U.S.) © Satisfactory(U.S.) TM Australian

Figure 6. Primary bolt densities observed in U.S. and Australian coal mines. PSUP = (Bolt
length * bolt diameter * number of balts/row)/(Entry width * Spacing between rows of bolts) (see

Mark et al., 1994),


is the biggest single research area, with an annual funding
in excess of $500,000. Current research areas include rib
support techniques, roof monitoring, windblasts, and
replacing primary support with secondary support.

One criticism of ACARP could be that the miner's union
and the regulatory agencies do not have direct input into
the project selection process. However, it seems that
safety is still the primary research driver. Perhaps the
reliance on self-policing through “manager's rules” has
carried over into the industry-sponsored research program.

CONCLUSIONS

The first, obvious conclusion is that there are no unique
“Australian” ground conditions, any more than there are
unique “U.S.” conditions. A wide variety of conditions are
found in both countries. The mines under deep cover and
high horizontal stress in the Southern NSW coalfield do
‘seem to suffer more severe problems than are typical in the
U.S., but U.S. counterparts come to mind for most of the
other Australian mines.

The typical Australian mine uses a higher density of
primary support than is common in the U.S. Three factors
may explain this:

e Australian mines are less tolerant of roof falls;

Australian mines have traditionally relied less on
supplemental support in the tailgate; and,

@ in Southern NSW, lower pillar stability factors may
require more entry support.

It seems that there is currently some trend in Australia
towards reducing primary support in favor of secondary
support.

Another trend in Australia is towards experimentation
with place-changing systems. However, the combination of
low CMRR roof, two-entry mine layouts, inappropriate
equipment, and a low tolerance for roof falls may limit the
applicability of place changing. in many instances, the
miner-bolter development system used in a number of
Pittsburgh seam longwall mines may be a better U.S.
benchmark.

There is also quite a bit that we in the U.S. might learn
from Australia. Roof monitoring technology is far more
advanced in Australia, for example. The Australian
approach to safety management and integrating mining
research into the industry also have a lot to commend
themselves.

Finally, there are many areas in which we can learn
from each other. Both countries have recently made
valuable contributions in roof support technology, pillar
design, and numerical modeling that are readily
transferable across the Pacific. More interactions like this
Conference could greatly benefit both countries.

REFERENCES

Byres, R. and R. Doyle (1997). The Strata Management
Plan at Dartbrook Coal. Symp. on Safety in Mines,
Coalfield Geology Council of NSW, pp. 41-55.

Coal Mining Inspectorate and Engineering Branch (1992).

Manual on Pillar Extraction in New South Wales Coal
Mines. NSW Dept. Mineral Resources, 47 pp.

Colwell, M. and R. Frith (1997). The design of chain pillars
based on gateroad serviceability. Symp. on Safety in
Mines, Coalfield Geology Council of NSW, pp. 143-151.

Doyle, R. (1997). History of fatalities in New South Wales
coal and shale mines. Symp. on Safety in Mines, Coalfield
Geology Council of NSW, pp. 27-38.

Gale, W. (1998). Coal Pillar Design Issues in Longwall
Mining. Proc. 1st Australasian Coal Operators Conference,
Univ. Wollongong, pp. 133-146.

Graham, R. L. (1998). ACARP - Research that makes a
Difference. Proc. 1st Australasian Coal Operators
Conference, Univ. Wollongong, pp. 536-548.

Hayward, J. (1998). Dartbrook Mine - A Case Study. Proc.
1st Australasian Coal Operators Conference, Univ.
Wollongong, pp. 224-238.

Hebblewhite, B., and J. M. Galvin (1996). Issues affecting
longwall chain pillar design. Proc. Symp. On Geology in
Longwall Mining, Coalfield Geology Council of NSW, pp.
193-198.

Kelly, M., W. Gale, P. Hatherly, R. Balusu, and X. Luo
(1998). Combining Modern Assessment Methods to
Improve Understanding of Longwall Geomechanics. Proc.
1st Australasian Coal Operators Conference, Univ.
Wollongong, pp. 523-535.

Mark, C. (1998). Application of the Coal Mine Roof Rating
(CMRR) to Extended Cuts. SME preprint 98-231, 6pp.

Mark, C. and F. Chase. (1994). Design of Longwall Gate
Entry Systems Using Roof Classification. Proc., New
Technology for Longwall Ground Control, BuMines SP 01-
94, pp. 5-18.

Mark, C., R. Watts, T. E. Marshall, M.O. Serbousek, and F.
Carr (1994). Using Computerized Mine-wide Monitoring for
Ground Control. SME Preprint 94-168, 8 pp.

Molinda, G. and C. Mark (1994). Coal Mine Roof Rating
(CMRR): a Practical Rock Mass Classification for Coal
Mines. BuMines IC 9387, 83 pp.


only two of them. In the U.S., it has been found that
"extended cuts" are more likely to be successful when the
CMRR exceeds 40-50 (Mark, 1998), and the Australian
experience seems to fit the same pattern. Also, the large
Australian mining machines (like the ABM20) are usually
slow to tram, and two-entry gates with long crosscuts are
far from optimal for rapid place-change cycle times.

ALPS Stability Factors (SF) have been determined at a
number of mines that are involved in the ACARP project.
The preliminary data show similar trends to those found in
U.S. studies. Figure 5 shows of that when the case
histories are plotted against the U.S. design equation
(Mark et al., 1994), there are only four misclassifications.
Three of the misclassifications fall near the line, and the
fourth may be successful because of an extremely high
density of primary support. The very worst tailgate
conditions in the data base were encountered at a mine
where the ALPS SF was 0.8, well below the suggested
value for a CMRR of 35. In contrast, at other operations
where the ALPS SF was well above the suggested value,
it was possible to drive a vehicle all the way up to the
tailgate corner of the longwall.

A variety of techniques are used to design pillars in
Australia. The University of New South Wales (UNSW)
recently developed a pillar strength formula from back-
analysis of room-and-pillar case histories (School of Mines,
1994; Hebblewhite and Galvin, 1996) which has been
widely applied. Numerous mines have employed stress

measurements to evaluate pillar behavior, and
sophisticated computer simulations are available (Gale,
1998). However, the relative lack of variation in longwall
pillar size indicates that there may be room for more
innovation.

ROOF SUPPORT

One immediate and obvious difference between the two
industries is their tolerance for roof falls. In the U.S.,
approximately 2000 falls of supported roof occur each year,
which is approximately one for every 120,000 tons mined
underground on advance. In Australia, none of the large
mines | visited had experienced more than a handful of roof
falls. Indeed, | was told on several occasions that their
unwritten goal was “no outby roof falls.” The intolerance of
roof falls even extends to inby roof failures during mining.

The universal primary support in Australia is what would
be called a torque-tension bolt in the U.S. These are fully-
grouted with a two stage resin (a fast set for the anchor at
the back of the hole, and a slow set around the tensioned
portion of the bolt). Australian roof bolts tend to be of
larger diameter and made from higher strength steel, with
a typical yield load of about 20-25 tonnes. The bolt length
was 1.8-2.4 m almost everywhere. At the mines | visitied,
all the bolts were installed with drill rigs mounted on the
continuous miner, or with hand-held drills. It seems that it
might be difficult to obtain consistent quality installations
with the hand-held units.

Australian
ALPS SF vs CMRR
90
80 o oo
70
60
& 60 2°
540 °
30
20 « Unsatisfactory
10 0 Satisfactory
0 T T T T
i) 0.5 1 15 2 25
ALPS SF

Figure 5. Preliminary data showing Analysis of Longwall Pillar Stability (ALPS) stability

factors from Australian mines.


GS

xy
2
ry
&

I\

Figure 7. Modified Wonga-willi pillar
extraction sequence used at Brimstone
Colliery.

One reason for the improvement is better support.
Breaker line supports (BLS) were introduced in the mid-
80's, and are now standard. One typical application is at
Brimstone Colliery, under 400 m of cover. Here pillars are
developed on 40 m centers. On retreat, the pillars are split,
and then lifted using a Christmas-tree sequence (figure 7).
Three BLS are walked down the split as it is recovered.
Large "stooks” (pushouts) are left to support the
intersections, but they are engineered to crush out at this
depth of cover.

~
N
SS
>

centers), but then only Christmas tree down the entries,
leaving 20x40 m pillars for permanent overburden support
(figure 8). Again, three BLS are used in the lift. The major
change here has been to support the roof with a standard
bolt pattern, together with a scaling program. Previously,
much of the mine was only spot-bolted.

Roof Monitoring: Australian coal mines make far more
extensive use of roof monitoring than U.S. mines. Indeed,
it seemed that several individual mines probably had more
geotechnical monitoring installed than the entire U.S.
industry. For example, one of the Queensland mines |
visited installs a two-point tell-tale in every intersection.
The stations are initially read daily by a shift boss, and
plotted up in the mine office. If a station is observed to be
still moving rapidly when the deformation reaches 40 mm,
then cables or trusses will be installed. Reportedly,
approximately 25% of all intersections at this mine require
supplemental support. The same mine currently has two
pillar stress measurement sites installed. On the other
hand, about half of the mines visited only use geotechnical
monitoring for "special projects" like longwall recovery
rooms or driving through faults.

Table 2 lists roof monitoring "triggers" used by different
mines. It is interesting that three of these triggers are
based on the rate of roof sag, while the other two use a
total sag criterion. Moreover, the trigger rates vary by an
order of magnitude. Clearly, there is room for research to
relate critical roof movements to geology and other factors.
In the U.S., a typical trigger rate is about 5 mm/wk (Mark et.
al., 1994).

Figure 8. Partial pillar extraction sequence used beneath
massive conglomerate roof at Cooranbong Colliery.

At Cooranbong Colliery, the roof is a massive
conglomerate under only 100 m of cover beneath a
residential area. They also develop large pillars (45 m

Table 2

Roof Monitoring Action Points for Five Australian Mines

Coalfield CMRR | Type of Criterion | Magnitude
Queensiand 40 Total movement 40 mm
Queensland 50 Rate of movement | 5 mmiwk
NSW- Northern 40 Rate of movement | 10 mmivk
NSW- Northern 40 Rate of movement | 1 mmiwk
NSW- Westem 35 Total movement | 25-50 mm!
‘Additional actions at 100 mm and 150 mm of total movement.

A number of different devices were used for roof
monitoring, including an electrical system that uses a
theostats and was read by a tiny, hand-held LCD. Sonic
probes are losing favor in some circles because of their
price ($12,000 US), but they are still widely used for
research and to determine trigger levels for tell-tales.

A most impressive monitoring system is being
developed by Strata Control Technologies and Angus
Place Collieries. It is an entirely permissible system that
can be easily installed by the bolting crew. Each new
station "daisy chains" to the previous one, and
automatically identifies and initializes itself. The entire


Munckton, C. (1995). Safety Performance Improvements.
AusIMM Bulletin, No. 3, May, pp. 38-41

Reece, D. (1998). Implementation of the Moura
Recommendations-A Manager's Perspective. Proc. 1st
Australasian Coal Operators Conference, Univ.
Wollongong, pp. 668-671.

School of Mines, University of New South Wales (1994),
Strata Control for Mine Design Newsletter, No. 5, August,
8pp.

Shepherd, J. (1997). Safer Pillar Extraction: A Function of
Geology and Design. Symp. on Safety in Mines, Coalfield
Geology Council of NSW, pp. 133-142.


Comparison of Ground Conditions and Ground Control Practices in
the United States and Australia

Christopher Mark, Acting Chief, Disaster Prevention and Response Branch

National Institute for Occupational Safety and Health
Pittsburgh Research Laboratory
Pittsburgh, PA

ABSTRACT

Australia and the United States both have large,
modern underground coal mining industries. | Many
companies have invested in both countries, and there is
growing awareness that technological developments in one
country can rapidly affect the other. Yet the ability of
technology to pass between the countries depends on
comparability of ground conditions. Many Australian
observers believe that their ground conditions are
significantly poorer than typical U.S. conditions, but some
U.S. professionals are not convinced.

The author recently visited 10 Australian longwail mines
and two room-and-pillar mines. Each mine visit included
determination of the Coal Mine Roof Rating, roof support
installed, pillar design, and an in-depth discussion of
ground control experience. The Australian conditions
observed are compared with an extensive data base from
U.S. mines. The paper also offers observations on
geotechnical data collection, monitoring, and the roles of
the mine inspectorate and research community.

Acknowledgments

I would like to express my sincere appreciation to Mark
Colwell and Russell Frith for arranging all the longwall
mine visits, and to lan Anderson, Steve Mathews, and John
Shepherd for arranging other portions of the trip.

INTRODUCTION

Early in 1998 the author visited Australia to gather
information on the Australian state-of-the-art in mine safety
technology. The focus was on ground control and disaster
prevention and response. A secondary goal of the trip was
to assist an Austratian Coal Association Research Program
(ACARP) project that is evaluating the applicability of the

Analysis of Longwall Pillar Stability (ALPS) and the Coal
Mine Roof Rating (CMRR) to Australian mines (Colwell and
Frith, 1997).

A total of 12 mines were studied in the course of the
trip. On week was devoted to five longwall mines in the
newest Australian coalfield, central Queensland (figure 1).

50 mes

ma cmes

Figure 1. Coal mines visited in Queensland

Two of the mines were only on their second longwall panel.
The second week of the trip included visits to Mining
Engineering departments at three universities and
meetings with other ground control specialists. Two room-
and-pillar mines were also studied, one a deep-cover, full-
extraction operation, and the other a shallow, partial-
extraction operation beneath massive cong!omerate roof,
During the final week of the trip, five longwail mines were
studied in New South Wales (NSW). Two were in the Lake
Macquarie/Newcastle district, and one each in the Hunter
Valley, Western (Lithgow), and Southern (Wollongong)
coalfields (figure 2)


system can be tied into a computerized mine-wide
monitoring system. The goal is to eliminate the need for
specialists to install, read, and plot the data.

Microseismic Monitoring: Microseismics are being used
for a number of issues in Australia. In the Lake Macquarie
area, massive conglomerate main roof is a major concern
for at least three longwall mines. At Newstan Colliery,
extreme periodic weighting two years ago caused face falls
that cost more than 6 months of production. Longwall
panels at Newstan have since been limited to less than
150 m in width. The problems with periodic weighting at
Newstan have already been mentioned. At the nearby
Moonie Colliery, wind velocities of 40 m/sec were
measured during a windblast event. Both mines are now
using a South African microseismic monitoring system to
wam workers of impending airblasts.

A 1998 paper by Kelly, et. al., describes microseismic
studies at Appin and Gordonstone Collieries aimed at
predicting water and gas inflows resulting from overburden
response to longwall mining.

MINES INSPECTORATE AND SAFETY MANAGEMENT

According to lan Anderson, Senior Mine Inspector in
NSW, the basic qualification for an Inspector is actual
experience as a Mine Manager. A degree in Mining
Engineering is preferred, as is experience in mine rescue.
The high level of training is necessary because Australian
mining law gives the Inspectorate much more discretion
than in the U.S.

There are few statutory safety requirements in either
Queensland or NSW. Instead, the Coal Mines Regulation
Act requires each Mine Manager to develop a set of rules
that are appropriate for his operation, For example,
Manager's Rules on roof bolting would include criteria as to
when and where different bolt patterns are required. The
Manager's Rules must be approved by the District Coal
Mines Inspector. In addition, the plans for each new panel
are submitted for approval.

In recent years, the trend has been to upgrade the
Manager's Rules into Safety Management Plans (SMPs).
SMPs were one of the recommendations that came out of
the Moura disaster, The SMPs are Quality Assurance-type
documents that are typically prepared for Spontaneous
Combustion, Gas, Ventilation, Strata Control, Evacuation,
Outburst, and Water (Reece, 1998). The SMPs define the
range of hazards associated with each issue, and the
necessary controls and procedures for management of the
hazards. SMPs are very detailed. For example, the Strata
Hazard Management Plan for the North Goonyella mine is
44 pages long. A typical format is:

- Hazard Identification and Control Procedure
- Monitoring Scheme
~ Roles and Responsibilities

- Action Response Planning
- Training Requirements

- Corrective Action

~ Review Procedures

The SMPs provide auditable procedures that allow
stakeholders (workforce, mine management, shareholders,
and Inspectorate) to scrutinize whether the objectives are
being met. Of particular interest is that the SMP clearly
defines the roles of every individual at the mine, when
monitoring is to be used, and what the action triggers will
be (Bymes and Doyle, 1997).

MINING RESEARCH

Australia has an extremely vibrant, successful, and
confident mining research community. In part, this may
derive from the rapid growth of the industry over the past
30 years. The structure of mining research funding also
facilitates the success of mining research.

The primary source of research funding since 1978 has
been a $0.05/tonne levy on all black coal production.
Under a Memorandum of Understanding with the Australian
Commonwealth, the levy is administered by the Australian
Coal Association through ACARP. ACARP identifies high-
priority problems, and solicits research proposals in those
areas. The proposals may come from government
agencies, consulting groups, Universities, or mining
companies. Cooperative efforts, with cost sharing from the
private sector, are looked upon most favorably. Cost-
sharing leverages the funding for a typical research project
by a factor of 2.5 (Graham, 1998).

The ACARP approach has a number of significant
advantages. The research is customer driven, not
researcher driven. It helps researchers stay focused on
real problems, and it helps keep consultants and company
staff abreast of the latest technology. Currently, 68 mine
sites are involved in research projects, and 114 company
staff are involved in ACARP, either by participating in the
project selection process, or as Project Monitors. It has
been highly successful in breaking down barriers between
government researchers, academics, and mining staff. The
breadth of knowledge about mining research at all levels in
the industry is truly admirable.

Probably the most significant advantage of the ACARP
system is technology transfer. Because the ultimate
customers are intimately involved with the entire research
process, technology transfer begins almost with the
inception of the project. Technology transfer is also built
into every project. The result is a large number of
successful products, and a dramatically reduced time lag
between research and implementation

Approximately 40% of ACARP’s research budget is
spent on underground mining (the rest is split between
surface mining, preparation, and utilization). Strata control


BRIMSTONE
x

mi cmes

Figure 2. Coal mines visited in New South Wales.

BACKGROUND

Australia is the world's leading exporter of coal.
Although Australia's total production of about 250 million
raw tonnes of bituminous coal is considerably less than
China, Russia, or India, Australia's mining technology is
extremely modem. It is fair to say that the Australian coal
industry has more in common with the U.S. coal industry
than any other in the world. Indeed, in recent years the
same coal producing companies have made large
investments in both countries.

Underground mining currently accounts for about 28%
of Australian bituminous coal production. Almost all
underground production (and 96% of all coal production of
black coal) comes from the two states of Queensland (Qid)
and New South Wales (NSW). The last decade has seen
rapid growth and technological change in the Australian
industry. Underground production in NSW expanded by
nearly 20% (to 55 million tonnes) between 1986 and 1996,
and it nearly tripled during the same period in Queensland
(from 6 to 16 million tonnes). The decade also saw a
pronounced shift to longwall technology. Longwall mines
produced about 13 million tons in 1986, and that figure has
grown to 48 million tons a decade later. Longwalls now
account for nearly 80% of underground production in NSW.
and almost 100% in Queensland (figure 3).

On the whole, Australia maintains a mine safety record
that is similar to the U.S. The Australian lost time injury
frequency rate (LTIFR) in 1994 was 16 per 200,000 hours
worked, compared to an LTIFR of 11 for the U.S. (Muncton,
14995). There have been two major mine disasters in
Australia during the last five years. Eleven miners
perished in the Moura mine explosion in 1994, and four
miners were killed at Gretly Colliery by an inundation in
1996. There were a total of 12 other fatalities during the
period, 4 of which were attributed to roof or rib falls (Doyle,
1997).

2 ra ongwet
360

i. aRoom 8
£ NSW Longwall mer
px

bn

°
1906 1967 1968 1989 1990 1991 1992 1992 1994 1998 1996
Year

Figure 3. Trends in Australian underground coal
production.

Today, the Australian coal mining industry seems to be
at acrossroads. The optimism based on several decades
of sustained growth seems to have given way to
uncertainty. Just a year ago the trade magazines were
filled with plans for major new projects, but now layoffs and
mine rationalizations seem to be the order of the day.
Labor strife has also increased, with major confrontations
at Arco's Gordonstone mine and the Rio Tinto operations.
Low coal prices, competition from Indonesia and South
Africa, and concern about the economic slowdown in Asia
(the primary market for Australian coal) underlie the
uncertainty.

The Australian coal industry is now struggling to
increase productivity, while continuing to improve its safety
record. Ground control is a serious hazard throughout the
industry, along with spontaneous combustion and control
of mine gasses. The Australian mining community is
meeting the challenge with a vibrant, cooperative research
effort that is on the cutting edge of many mine safety
technologies. The remainder of this paper will describe the
state-of-the-art in Australian ground control, and compare
it with the U.S.

ROOF GEOLOGY

The Coal Mine Roof Rating (CMRR; Molinda and Mark,
1994) was determined at all 12 underground operations
that were visited. The range of CMRR values is shown in
figure 4a. The two strongest roofs were a massive
conglomerate (CMRR=90) and an interbedded shale and
sandstone (CMRR=60). The weakest top (CMRR<45)
included four mines with thick coal roofs and a sandstone
laminated with igneous tuff layers. The intermediate roofs
included one thick coal roof, two Bulli seam “laminites”
{interbedded shales and sandstones), a sandy shale, and
a stackrock sandstone. On the whole, the roof in
Queensland seemed somewhat more competent than that
in NSW.


AUSTRALIAN HISTOGRAM OF CMRR.

onsw cae {7
|

ig eae

"36-46 45-54 55-64 75-04 | 85-95
Coal Mine Roof Rating (CMRR)

Figure 4a. Range of Coal Mine Roof Rating (CMRR)
values observed in Australia.

{quar
Alabama

JmColorado, Wyoming,
‘New Mexico
linola

Jw Southern Appalachia|
JwNorth Appalachia

Wo. of Observations
pBPRRSSSSERS

The presence of horizontal stress was not very
noticeable at most of the mines visited. Active mines in the
Queensland coalfields particularly seem to have low stress
levels, though some deeper operations there have had
problems in the past. Horizontal stress appears to be most
important in the Western and Southern NSW coalfields. In
the Western coalfields, where the coal seams are only a
few meters above the basement rock, "structural zones”
have been associated with some of the most difficult
ground conditions in the country.

It seemed that the Australian mines were well supplied
with geotechnical information from core logs and laboratory
testing. For example, Dartbrook mine has a coverage of 18
holes per square km, or one hole every 235 m. About half
these holes were fully cored, and many samples were
tested for uniaxial, triaxial, and bedding plane shear
strength (Hayward, 1998). Many mines estimate rock
strength from geophysical logs, often using relationships
that have been derived from their own testing.
Measurement of horizontal stress also seems routine and
nearly universal.

The geologic characteristics of the visited mines are
summarized in Table 1.

Table 4
Geologic Characteristics of 12 Representative
Australian Coal Mines

‘0-45 (Weak) "45-45 (Moderate) "65-100 (Strong)
‘Coal Mino Roof Rating (CMRR)

Figure 4b. Range of Coal Mine Roof Rating (CMRR)
values observed in the U.S.

Figure 4b shows the range of CMRRs found in U.S. coal
mines. On average, it appears that the roof in NSW may
be roughly equivalent to that found in the Northern
Appalachian coalfields, while Queensland roof may be
closer to that in the Central Appalachians, Overall, the
range of roof quality was similar to that found in the U.S.

Large-scale geologic features had an important effect
‘on longwall performance at a number of mines, Igneous
dikes seemed to be the most common feature, followed by
faults. Massive roof, as discussed below, was another
important issue.

The depth of cover varies significantly from coalfield to
coalfield. Most of the Queensland mines gain access to
seam from the highwalls of surface mines, and the cover is
relatively light. Mines in Southern NSW have the deepest
cover because the coalfields are relatively mature and the
terrain is mountainous. Cover is moderate in the Northern
and Western NSW coalfields.

Horizontal
Coalfield CMRR Depth (m) Stress
Queensland 40-60 <260 5-15 MPa
NSW- Northem 40-90" 100-300 5-10 MPa
NSW- Western 35 100-350 20MPa
NSW- Southern 45-55 300-500 20-35 MPa

‘CMRR=90 with massive conglomerate roof.

LONGWALL LAYOUT AND MINING METHODS

All of the longwail mines employed two-entry gates.
The crosscut spacing was almost universally 100 m. Pillar
widths varied between 28 and 40 m, and entry widths were
4.5 to 5.3 m.’ One striking feature was the thickness of the
coal seams. tn the mines visited, the pillar heights ranged
from 2.5 to 4m.

Most of the gates were driven by a single machine,
typically an ABM20 with mounted roof drills. Often only a
single shuttle car was used. The typical procedure was to
mine 1-1.5 m, and then install a row of bolts. The machine
will drive crosscut-to-crosscut before moving to the other
heading. There is currently quite a bit of interest in
American-style "place-changing,” using a separate roof
bolting machine. At least four of the mines that | visited
have tried place changing, with satisfactory roof stability at