Geological Hazard Report
MINE2106
Damien Shepherd
S42934048
Geological Hazards of mining the Goonyella Middle Seam
Summary
To the east of the current coal operations conducted in Goonyella, Queensland are areas that
have been proposed for potential mining. The area that has been defined and focused on is the
Goonyella Middle seam. It is apart of 3 major seams that exist in the region. The provided
datasets and maps allowed for the determination of major hazards that could be exposed when
attempting to mine the coal. The brightness profile of the coal increases with depth showing that
the better quality is lower in the ground. Open-pit mining was ruled out due to the depth and
underground mining was determined to be the best alternative. The use of longwall mining
could be used in the beginning stages but may need to be re-evaluated based on the conditions
of the roof and walls. Changing the mining method may be necessary as per the data given.
The position of the longwall would be as close to the bottom of the seam as possible where the
highest quality would be extracted and the lower to be left behind as roofing. Close monitoring
of geological structures and measuring stresses throughout the mining process would be
needed to ensure minimal risk. The area exhibits thrust faults, normal faults and slickensides.
All of these structural features would have to be routinely measured with respect to stress and
pressure on the given rocks being removed or displaced. Though longwall mining is proposed
based on the given interpretation and data it may be under varying stresses and the extent of
plane alteration is not definable within the strata. Furthermore, the impacts of a new coal mine
having on the environment need to be considered when implementing development.
Geological Hazard Report
MINE2106
Damien Shepherd
S42934048
Introduction
The Goonyella Middle seam has sedimentary features and structures that may pose potential
risks when managing the extraction of its coal. The coal deposits seen within the region are
specifically known as the Late Permian Moranbah Coal Measures. (Johnston and Kelleher,
2005). Interpreting the depositional setting and its respective stratigraphic units helped to further
define geological hazards. The properties of the coal found at the seam provided an indication
of structural integrity and stability when attempting mining processes. Data from drill cores and
subsequent analysis of units contributed to the overall interpretation of its environmental setting.
The implications of these mining processes having on the environment were also identified and
discussed. The variations seen in the seam and overlying intervals poses relationships that
conclude geological processes. Provident hazards were determined using geological
interpretation of the Goonyella Middle seam.
Locality and Intervals
The area is located in Queensland, Australia about 20-30km north of the town Moranbah. The
region/suburb is called Goonyella and extends into North Goonyella. The nearest accessible
port is around 220km’s East at Mackay. (Google Earth, 2013). An example of a major open-cut
mine that ‘exists’ today is run by the company BHP (subsidiary to BMA). They extract coking
grade coal from the mine located 30km north of Moranbah. The mine, also known as, the
‘Goonyella Riverside Mine’ extracts its coal from three major seams. (Johnston and Kelleher,
2005). The plotted area to be examined has three major seams present. These have been
labeled ‘Upper’, ‘Middle’ and ‘Lower’ Goonyella seams with five minor seams between these
Geological Hazard Report
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Damien Shepherd
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layers. A volcanic ash sedimentary unit known as a ‘tuff’ has also been identified and used to
correlate vertical movement of the layers during deposition. The presence of this tuff can also
be indicative of the type of environment. The sedimentalogical models devised in 1993 by
Falkner and Fielding outlines the specific depositional settings of the stratigraphic units. They
further concluded that the features found attribute to an Upper delta plain environment with tidal
inundations occurring on occasion. (Esterle and Fielding, 1996) (Falkner and Fielding, 1993).
The major trunks of the delta flowed south into the Bowen basin where axial drainage systems
were eroded and established. The tuff and ‘volcanolithic’ sandstones confirm the presence of
resurgent volcanic activity to the east of the basin. (Esterle and Fielding, 1996).
Data and Methods
The datasets were acquired for analysis of the Goonyella Middle (GM) seam. These included
wireline logs with gamma, brightness, density and sound variables and maps of borehole
locations. Table 1 below outlines the data obtained from the readings of five of the boreholes.
The identification of sedimentary units became apparent with further interpretation. Contouring
maps showing the varying structures with depths and faults were devised to further understand
sedimentary thicknesses and structural components. Isopachs were correlated manually and
provided insight into unit boundaries. Cross-sections were then used to combine the information
gathered and represent the findings proportionally with respect to different datum. Potential
hazards of units and features were proposed and negotiated. The result of the latter data
analysis provided a 3-dimensional structure that was easier to visualize.
Geological Hazard Report
MINE2106
Damien Shepherd
S42934048
Gamma
Brightness
Average Low-
Density
Interpretation
Low
The low density and average gamma content would
Average
indicate a coal type.
Low
Average
High
The presence of moisture and/or water in the layers
Low
Average-
Low
Indicative of large sandstone units that may be porous
Average
Gamma is a good indicator for lithic types will usually
High
High
Low
have a low affinity whereas igneous rocks will have high
readings.
Table 1: Approximation based on visual analysis of wireline logs GY38122 to GY38155.
Results
Figure 1 was devised from the datasets and includes unit thicknesses, sedimentary contacts,
faults and areas of potential risk. It has been composed to provide an overview of the available
areas for mining with minimal hazards. Using the Geological Hazard Map it can be determined
that the thickness of the roof is varying. Areas that are viable for extraction and the coal
thickness remains fairly high have been appropriately marked or indicated. This includes
positions where the left over coal would exceed 1m in either the roof or the floor of the deposit
post-mining. The presence of dykes was also plotted as the density of these mineralized rocks
can be detrimental to drilling processes and equipment. Hazards that cannot be marked relate
to other properties related to coal mining. This includes interbedded water within the
sedimentary units and identifying proximal aquifers and groundwater reservoirs. Additionally,
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gas and the combustible characteristic of coal have to be managed and monitored when mining
processes begin.
The properties of the coal found at drill sites were analysed physically through hand samples.
Drill cores were also examined and the coal layers were broken down into intervals of
deposition along with brightness comparison. The location of borehole is given in Appendix A.
This assisted in visualizing wireline datasets and provided further information on structural
controls. It was concluded that the coal is of Late-Permian age with about 10-15% moisture and
ash. The texture of the sub-bituminous ranked coal was then described as rough, brown in
colour, greasy in lustre and has a black/brown streak based on observations made. When
analyzing the GM seam on the drill core the brightness profile can be seen to increase with
depth. The presence of slickensides was noted on the underside of the GM seam and at the
bottom of the core. These properties were then collaborated and considered to determine
potential hazards - see Figure 2.
Discussion
Hazards were defined based on the interpretations made and the datasets given. The
geological risks that are determined are best defined as any type of lithological ambiguity that
could pose dangerous stresses on the structural integrity of the rock. It has been determined
that an open-cut mining would be deemed economically insufficient due to the lack of total coal
thickness, overburdened Upper seam and depth of coal layers. This varies throughout the
boreholes but shows a maximum thickness of around 7-8 meters. The brightness profile of the
GM seam drill core also indicated higher quality coal at greater depths perhaps due to less
exposure to compaction fracturing and oxygen. The GM seam also exhibited no major
fracturing/jointing. Underground mining would be effectively suited to mine the coal at the given
Geological Hazard Report
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S42934048
depth. The hazards of such an operation have been outlined in relation to sedimentary features
and units seen.
The height of the coal seam is an important factor when implementing mining operations. It
shows areas that are viable for removal. The Geological Hazard map deduces three major
areas considered for underground processing. A punch longwall mine is probably best suited to
mine the given coal. At the thickest part of the coal seam the height of the longwall could reach
a conservative 5meters. Wastage from the drilling operations could result in 1-2meters of
material. The longwall would be positioned as low as possible in the seam so that the lower
grade roofing can be left behind. Roof supports will have to be determined based on the
consistency of the roofing material. Volatile matter and gas concentration would need to be
closely monitored. In addition to this, any major fracturing or jointing/cleating that is found needs
to be assessed immediately. If deemed necessary due to possible caving other methods can be
implemented ie Board and Pillar mining that leaves sufficient support while extracting higher
grade coal from retreats.
One of the three major areas defined in the Geological Hazard map is a shallow region of the
GM seam. When mining this region the stratigraphic units above and below need to be
considered when choosing drill equipment. The density profile of these layers (including dykes
and intrusions) is substantially different allowing for sparks to occur while using lower density
drill pieces. Gas would also need to be closely monitored and possible vented/bled. Mudstone
units would need to be carefully mined to reduce powder/dust exposure and may require water
saturation. The thin mudstone layers can also attribute to a roof collapse if stresses applied from
the longwall structure and weight. If any lower density units are found in the floor (ie mudstone
or coal) they need to be assessed immediately as crushing can occur along with localized
subsidence. Mudstone can also become muddy when exposed to water saturation methods.
The region of the GM seam to be mined is under the influence of tectonic factors that influence
north-east localized faulting. The structural units seen are thrust faults, normal faulting and
Geological Hazard Report
MINE2106
Damien Shepherd
S42934048
slickensides. The Geological Hazard map presents the array of structures present including
sandstone bodies for roof support. The thrust faults seen have relatively low throw with around
1-7 metres of movement. These three smaller faults (north, south and at the centre of the
bottom) can be responsible for hazardous wedge failures. These hazards can be avoided or
managed when confronted but can displace substantial amounts of rock. However, the normal
faults seen – even though it may have a higher throw – do not create many hazards when being
confronted underground. This could be due to the stresses of normal faults propagating
laterally, parallel to the horizon. When other stresses of rock facies present are calculated the
normal faults will need to be considered for overall integrity of the longwall structure. Finally, the
slickensides seen occurred at the time of depositon when displacement of the layers may have
occurred due to a change in elevation or burial. The structural unit is formed when these layers
are subject to stresses that cause weaknesses across the planes. When pressure is applied or
removed the slickensides can fall easily due to the already-present weaknesses. These
features have been plotted on the map and define the hazards present.
Analyses of the depositional environment may have assisted in the original determination of the
drill core locations. It could be interpreted that the locations picked were indicative of an
underlying sedimentary structure. The areas chosen coincidently show the varying depths which
could have been determined by an interpretation of its depositional environment. The presence
of paleochannels and massive sandstone bodies could confirm the presence of this river
system/deltaic environment. “…Depositional modeling can be used to predict large-scale trends
in coal deposits on a regional scale and are therefore useful in the initial phases of coal
exploration. Further, small-scale variations in coal thickness, quality,and lateral continuity
frequently can be predicted, providing data that can be extremely valuable in mine planning and
development…” (Ward, n.d).
Geological Hazard Report
MINE2106
Damien Shepherd
S42934048
Environmental Impacts
When a new mine is setup to extract coal it has to adhere to regulations imposed by the
Environmental Protection Agency (EPA). Waste and tailings have to be managed and
contained to prevent contamination of the environment and food sources. Geological features
like Aquifers need to be located in relation to prospective sites. Depending on the type of mine
and location other factors like transport-related dust pollution need to be considered.
“…Australia’s international obligations under the agreement reached at the United Nations
Conference on Environment and Development (UNCED June 1992) give EPAs permission to
use the precautionary principle — that an action should not be taken if the consequences are
uncertain and likely to be dangerous to the public or the environment — in their assessments.
This is rarely, if ever, invoked in the case of approving new coalmines….”(Castleden, 2011).
This is indicative of the scale of environmental impact of new coal mines in Australia in 2011.
Conclusions and Recommendations
The hazards posed in this report were identified by building a foundation of information
interpreted and analysed to predict the outcome of differing mining operations. The location of
the GM seam under examination is due east of the present mining operations conducted at the
Goonyella Riverside Mine. The Geological Hazard map reveals the areas that were primarily
focused on and were deduced through an understanding of the underlying geology. The
potential hazards defined mostly relate to roof conditions and the physical/chemical properties
of coal. Though the variables are dependent a good indication of where and how to mine could
be determined with confidence. The cross-section used shows the presence of paleochannels
and large sandstone bodies which assisted in determining the depositional environment as
Geological Hazard Report
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Damien Shepherd
S42934048
deltaic. Additionally, effective roof support mechanisms could be placed on the marked areas to
ensure stable longwall conditions.
Open-pit mining was ruled out due to the depth and quality of coal. A longwall was
propositioned on the basis of stable roof/floor conditions. The three areas defined depict both
shallow and thick coal deposit mining. During mining operations close monitoring of gas
concentration, jointing/cleating, fracturing and faulting of the strata layers needs to be carried
out. With the proposed positions of the longwall, roof conditions and wedge failures would have
to be taken into consideration. Environmental impacts of a new coal mine would need to be
managed and implemented to prevent any contamination. The locations deemed sufficient in
regards to the hazards of the geological processes have been presented and concluded.
Further research into the behavior of the roof mechanisms in response to Normal fault related
stress could provide further information not discussed in this report. Furthermore other samples
and structures only become available during the mining processes so alterations could be
needed to the overall composition. Stresses and other factors could resort to the need of a
drastic change in the method of mining. The data and interpretations provided cannot conclude
that longwall mining will be guaranteed successful.