Geotechnical Studies for Introducing High Capacity Longwalls and
Longwall Top Coal Caving Mining in SCCL
J.V. Dattatreyulu*, Manoj Khanal#, Deepak Adhikary#, Rao Balusu#
* The Singareni Collieries Company Limited, Kothagudem Collieries, Khammam District, Andhra
Pradesh, Pin: 507101, India
#
CSIRO Earth Science and Resource Engineering, 1 Technology Court, Pullenvale, 4069 QLD,
Australia
Abstract
Longwall Top Coal Caving (LTCC), a relatively newer technology in India, is an underground
mining method developed for thick seam extraction. Various factors and systematic
approaches are to be considered and properly evaluated in order to qualify and gain
confidence in the assessment of introducing LTCC mining method. The proposed paper
presents the various processes involved in designing a LTCC technology in one of the mine sites
at Singareni Coal Company Limited (SCCL), India. The project undertook a comprehensive
analysis of geological and geophysical data of the mine site and developed detailed
geotechnical frameworks for the assessment of LTCC technology.
Keywords: LTCC, coal, modeling, simulation
1. Introduction
Singareni Coal Company Limited (SCCL) is a Government coal mining company jointly owned by
the State Government of Andhra Pradesh and Government of India on a 51:49 equity basis.
The SCCL is located in the mid‐eastern region of India, Figure 1. The SCCL’s coal reserves
stretch across 350km of the Pranahita‐Godavari Valley of Andhra Pradesh with a proven coal
reserves aggregating to 8791Mt. SCCL currently operates 14 opencut and 36 underground
mines in Andhra Pradesh with manpower of around 67,000, and produced around 51 Mt of
coal in the fiscal year 2010/11.
SCCL’s projections of coal reserves and production level require a successful and rapid
introduction of new generation longwalls and advanced thick coal seam extraction techniques,
such as longwall top coal caving method. In Ramagundam region of SCCL the coal seams lie
within 170 m of metamorphic basement rocks and are overlain by approximately 500 m of
Barren Measure sediments. Four mineable seams named from top to bottom I, II, III and IV and
additional three thin seams, IA, IIIB and IIIA are regionally consistent over many kilometres.
The thickness of seams I, II and IV varies from 2.0 to 5.5m. III Seam is the thickest seam with an
average thickness of around 8 to 10m and hence requires advanced thick seam mining
methods for optimum extraction. Currently, SCCL uses bord and pillar method involving two
sections and Blasting Gallery (BG) method to extract thick seam (III Seam).
Figure 1 A map of India showing the location of coal regions and SCCL mines.
China uses the most advanced high capacity longwalls mining methods to extract the seams of
up to 5.5m thick and longwall top coal caving (LTCC) systems to extract thick seams up to 12m.
The LTCC technique is extensively used in China with over 100 faces producing over 200MtPa
in conditions ranging from soft (<10MPa UCS coal) to hard (>50MPa UCS coal) [1]. It is a cost
effective mechanism as the shearer slices only the bottom part of the seam and the top coal
fractures due to gravity [1–3]. The only additional cost will be added to the rear conveyor and
modification of the chocks of the normal longwall equipment.
Compared to conventional longwall mining method, LTCC has an additional rear conveyor belt
attached behind the chock/support, on which the caved coal from the top coal is drawn by a
moveable flipper attached to the canopy at the rear of the support. This method relies on the
fracturing of the top coal by front abutment pressure to achieve satisfactory caving into the
rear conveyor. Poor fracturing will cause larger blocks to form resulting in poor caving through
the rear AFC. Excessive fracturing will in turn cause roof control issues ahead of the face
supports. Thus the suitability of LTCC will depend upon the site specific geological
environment. A thorough site investigation is required in order to better understand the
geologic setting of the mining zone, including mechanical properties of both coal seams and
interburden rocks. Estimation through modelling of the degree of fracturing occurring during
this cycle is at the core of investigating the feasibility of introducing LTCC at any greenfield
thick seam sites. LTCC is considered ideal for thick seams (>4.5m) with moderate strength coals
at depths greater than 150m. In addition to proven safety advantages in the areas of face
stability, operational control and management of spontaneous combustion the LTCC method
allows for recovery of additional coal that is usually not minable using conventional longwall
extraction. Under favorable conditions, LTCC is an economical underground mining technique.
The top coal recovery depends on the top coal fragmentation mechanism and fragment size
distribution generated during normal longwall extraction of the bottom coal [4].
There are numerous intrinsic and non-intrinsic parameters which govern the feasibility of LTCC
in any mine, and have to be evaluated properly. The intrinsic parameters are thickness of the
coal seam, coal strength and deformation properties, inclination of the coal seam, roof
sandstone strength and deformation properties and coal geology. The non-intrinsic
parameters are existing equipment support for the normal longwall extraction, life of the
mine, financial health of the mine and a detailed geological study of the mine.
The efficient implementation of the LTCC may be achieved through past experience of mining
in identical geological and excavation situations or from a detailed numerical modeling of the
LTCC using comprehensive and accurate mine parameters. The past experience from a mine
can be a challenging tasks for a new mine as none of the mines are identical. Similarly a
detailed modeling of a mine is also a challenging work as mine as a whole can be visualized as
a heterogeneous structure with built-in imperfections. The paper focuses on the various steps
followed to design a LTCC mine in the Indian mining environment.
2. Mine site investigation, characterization and data collection
Introduction of advanced thick seam extraction methods and high capacity longwalls at SCCL
required detailed knowledge of coal seams and their partings, as well as thorough
understanding of interburden geotechnical characteristics. The first step involved in designing
LTCC was to perform a characterization of field site conditions, including determination of
geological, geotechnical, physiomechanical and hydrogeological parameters. The field site
selected for this project was SCCL’s 10 and 10A mines and their extension mining blocks,
known as Adriyala mining block. The layout of the field site including the existing mines and
proposed extension mining block is shown in Figure 2.
The main purpose of mapping and modelling coal seam interburden is to provide a geological
framework for detailed geotechnical characterisation and hazard prediction. With a well
developed interburden model, maps highlighting areas of potentially weak roof or weighting
sandstones can be quickly generated to aid underground mine design or to be used as hazard
maps during mining. A well defined interburden correlation can also show various rock units
with consistent strength and other geotechnical properties. However, in the absence of
extensive geophysical data the coal seams and interburden units have to be interpreted from
geological observations. In general, fine‐grained and thinly bedded units are weaker than thick
beds of moderate to coarsely bedded sandstone.
Figure 2 Field site layout showing both the existing mines and their extension mining blocks.
The geophysical logging data was collected from a number of new boreholes drilled at the field
site for detailed characterization of the interburden strata. Prior to recent implementation of
new digital logging system there were no digital geophysical logging data available for the
majority of the boreholes at SCCL. The data acquired earlier were provided as graphic files
scanned from the analogue logs. These logs were used for manual strata correlation during
construction of the geological models for these blocks.
In a first pass interpretation the major sandstone units between each of the already picked
seams down to seam IV were identified. In addition, the lowermost claystone of the Barren
Measures was correlated. During this interpretation the SCCL coal seam picks were honoured
wherever possible. Distribution of each sandstone unit is like sheet, and all units occur across
the entire model area. Through this process, thickness distribution maps of various sandstones
were derived, for example, Figure 3 shows the thickness distribution map of the sandstone
unit above coal III Seam. Seam III has a thickness range of 7 to 11m (average 9.4m) and is a
potential seam to consider for LTCC implementation.
Figure 3 Thickness distribution map of the sandstone unit above coal III Seam.
In addition to valuable geological core and chip observations, modern digital wireline log data
has the potential to add significant geological and geotechnical information to the mine model.
To investigate and characterise the number of clearly distinguishable rock types in the wireline
log data, the self‐organising map (SOM) method was applied to the data analysis. The results
from this analysis were then validated with the LogTrans analysis method using a reliable
automated interpretation system.
One of the main applications of the geological model is to provide reliable and predictable
geotechnical rock mass model. It was demonstrated that the geophysical wireline log data can
robustly define a series of clusters that are interpreted as consistent rock types [5]. Similarly,
the verification of the consistency in rock types for the consistent rock properties was
performed. A representative average of the uniaxial compressive strength and Young’s
modulus were calculated for each SOM cluster.
Once the initial geological model was developed, the next step was to add significant
geological and geotechnical information to the geological model. A more detailed rock mass
characterisation in the roof and interburden strata around the working coal seams, e.g. I Seam
and III Seam were performed. The challenge was to subdivide the interburden sandstone units
into coherent rock types that may be related to consistent geotechnical properties. Another
requirement was to identify major bedding planes that have the potential to shear or separate
during mining. The main tool for this analysis was a fence diagram that compiles all the
boreholes with wireline log data on a single section. A detailed integrated geological model
was then developed for Adriyala mining block and a typical screen snapshot is shown in Figure
4. A detailed correlation resulted in the subdivisions of three sandstone units, SS100, SS80 and
SS70.
Figure 4 A detailed integrated geological model was then developed for Adriyala mining block.
In order to gain understanding of the mine site rock mass parameters, in situ stress and
permeability tests were carried out by MeSy India Pvt. Ltd [6] at Adriyala site. Seventeen
successful hydrofracturing tests were conducted by MeSy India at Adriyala longwall block.
These tests yielded reliable determination of a stress‐depth profile for the depth range from
77 to 522m. The tests showed that the shut‐in pressures linearly increased with depth from
2.3 to 8MPa, the refract pressure values increased from about 2.7 to 11 MPa. The breakdown
pressures for fracture initiation ranged between 6 and 12 MPa. The tests indicated in situ
tensile strength of the strata of around 2.9 ± 1.2MPa. For all of the 17 hydrofracturing tests the
orientation of induced fractures was determined by impression packer tests. The mean
azimuth of the vertical fractures was determined as N (24 ± 14) degrees (NNE). The direction of
horizontal compression is NNE and is inclined to the Godavari graben axis.
In addition to 17 hydrofracturing tests, 13 packer tests were conducted and yielded a unique
data set for rock permeability within Adriyala block. The tests showed that the rock
permeability apparently decreases with depth and is in the order of some hundreds
micro‐Darcy (the permeability value ranging from 10 to 104μDarcy). There is no significant
difference in permeability of coal‐bearing rock and coal seams. The near‐wellbore permeability
for the sandstone section ranged from 0.6 mDarcy to about 40 μDarcy with a mean of (175 ±
1.82) μDarcy. The coal seam permeability ranged between 2 μDarcy and 16 or 32 mDarcy.
Coal strength and deformability are significantly affected by composition of the coal and
presence of flaws/defects such as cleats or fracture planes. The scale effect is a dominant
feature of rock and coal behaviour. As size/volume of the rock/coal increases its strength
decreases due to probability of presence of larger flaws and this effect is called ‘scale effect’.
National Institute of Rock Mechanics (NIRM) carried out a number of in situ coal strength tests
as part of this project. In situ strength tests were conducted in I Seam and III Seam at GDK‐10A
Incline [7]. The in situ strength tests were conducted on 30 cubic centimetre blocks and noted
minimum and maximum strengths of 4.74MPa (when corner failed) and 19.27MPa respectively
for III Seam, thus providing an average strength of 13.2MPa. Coal from III Seam was found to
be stiffer than I Seam, with stiffness value ranging between 50,000kg/cm and 70,000kg/cm. As
coal and rock exhibit extensive size and scale effects on their strengths, the in situ strength
test values cannot be directly used in the numerical simulations. The laboratory and in situ test
results conducted on relatively small specimens need to be scaled using empirical
relationships. The scale‐strength expression derived by Medhurst [8] was used in this project.
Laboratory tests on NX size core of I Seam conducted by Central Mining Research Institute
(CMRI), Dhanbad revealed compressive strength ranging from 21.5 to 23.4MPa [9]. These
strength ranges correspond to values obtained for mid‐bright coal of around 275 kg/cm2, as
reported in [8].
Various scale effect equations that are available in the literatures were assessed and shown in
Figure 5. The figure shows the percent reduction in strength with increase in size for coal and
coal measure rocks. The figure also shows the scale effect equation for hard rocks [10]. It is
seen that Medhurst [8] formulation significantly reduces strength of coal with increase in size
compared to Mc Nally’s formulation [11]. The hard rocks exhibit relatively less reduction in
strength when compared to coal measure rocks. These formulations were used to estimate the
mass scale strength values required for numerical simulations.
Figure 5 Predicted UCS strength reduction with increase in size using scaling laws developed by
various authors.
Figure 6 presents the distribution of UCS for various rock formation. The moisture content or
water saturation may influence the failure strength of sedimentary rocks by reducing the
surface energy between the molecular bonds or by weakening the cementing bond between
the rock grains. There were no wet test data available for Adriyala site. However, in order to
understand the effect of moisture on strength and deformation characteristics of rocks at SCCL
sites, CSIRO analysed the dry and wet laboratory test results available for various rock units at
another site located at about 100kM NE from Adriyala, and observed that the strength
reduction due to water saturation for the sandstone units overlying I Seam could be as much
as 50% to 65%.
Figure 6 Uniaxial strength distributions (with mean and standard deviation) across the
stratigraphic units of Adriyala site.
3. Empirical assessment of LTCC:
Once the site characterization study was completed, empirical assessment of LTCC was
performed. Two of the main requirements for a successful LTCC operation are easily caving top
coal and main roof strata immediately overlying the top coal. The top coal that does not cave
in properly during the extraction of bottom coal will render the LTCC method unviable.
Similarly, the immediate roof strata that do not cave during LTCC extraction may cause severe
geotechnical and safety problems such as face instability, roof guttering, windblasts. Thus it is
imperative that top coal and roof rock caveability is appropriately assessed.
A first pass assessment of LTCC in III seam of Adriyala mine was made using already
established caving indices (Chinese index and CSIRO index) for LTCC. The seam is about 10m
thick and is dipping at 8 to 10deg. The seam depth is ranging from 300 to 400 m.
According to Chinese experience with LTCC, depth of mining, coal strength, top coal thickness,
stone band thickness, degree of coal fracture and immediate roof thickness are the
parameters which influence the caveability of any LTCC operation [12]. Zhongming [12]
undertook numerical simulations to systematically analyse the effect of these parameters and
developed the following formula for the caveability and drawability of top coal using
regression analysis:
y = 0.704 + 0.0006338 H – 0.00786 Rc + 0.6264 C – 0.1797 Mj + 0.01434 Md -0.23056
Where H is depth of mining in meters, Rc is the UCS of coal in MPa, C coal fracture index, Mj is
stone band thickness in (m), Md is top coal thickness in (m).
For Singareni, Chinese index was evaluated by considering the above parameters including
average laboratory UCS of 25.5MPa C=0.3 and Mj=0.1 and Md = 7m. The caving index ranged
from 0.84 to 0.91 for depths between 300 m and 400 m; which could be categorised into
classification 2, yielding "good" rating for LTCC with predicted coal recovery of about 70 to 80
%.
In 2008, CSIRO conducted a parametric study [13] with COSFLOW and identified depth of
mining, coal strength and top coal thickness as the three most important parameters that
influence the degree of fracture in the top coal and hence the caveability of top coal, which
indirectly governs the success of LTCC. The caving index (CI) developed by CSIRO using multi
variant regression of the parametric study is shown below:
CI = ‐2.64 + 0.0395 H – 0.72 CS + 0.191 TC
Where H is depth of mining in meters, CS is coal strength at test scale in MPa, TC is top coal
thickness in meters
The CSIRO's caving index for the same material properties yielded a rating between ‐7.5 and
‐3.5 or depths between 300m and 400m. This yields a "good to moderate" rating for LTCC with
predicted coal recovery of about 56 and 67%.
However, it is important to note that both of the above methods are based on Chinese
geological conditions, thus it is necessary to investigate caveability of top coal at SCCL through
numerical modelling using site specific in situ stress conditions and rock mass geotechnical
parameters.
4. Numerical simulation of LTCC
A number of numerical experiments were performed to calibrate and validate the numerical
models based on the Adriayala data in order to construct a predictive numerical model which
can be used to assess the viability of introducing high capacity longwall at the Adriyala site.
Field studies to monitor caving conditions, stress changes, ground movement and stability in
existing thick seam mines of SCCL were also conducted. Then extensive numerical simulations
were performed to obtain a fundamental understanding of caving mechanics and the effect of
various mining and design parameters under different mining conditions. The in-house
continuum code called COSFLOW was selected to perform the numerical simulations.
Due to the data availability from the GDK10A mine, it was selected for the calibration study.
Panel 3A was simulated and the numerical results were compared with the field load cell and
Tell-Tale monitoring data. Thus calibrated numerical model parameters (shown in Table 1)
were used for a number of predictive models. The model construction details and validation
processes are available in [4,5]. I Seam and III Seam were divided into I_TOP and I_BOT, and
III_Top_coal and III_Bottom_coal respectively for modeling and simulations.
Table 1 Stratigraphy and material properties used in COSFLOW simulations
Stratigraphic Unit Density σT
UCS E (GPa) 
K (GPa) G (GPa) C (MPa) ψ

(kg/m3) (MPa) (MPa)
BMB_TOP
2045.0 0.25 2.51 3.70 31.42 0.2 2.06 1.54 0.7
5.0
BMB_BOT
2045.0 0.50 5.02 4.63 31.42 0.2 2.60 1.92 1.41
5.0
CL100
2206.0 1.47 14.67 8.24 35.00 0.38 11.44 2.99 3.82
5.0
SS100
2172.0 0.68 6.76 7.14 40.71 0.20 3.90 2.98 1.55
5.0
IAT
1701.0 1.46 5.84 3.00 40.00 0.15 1.43 1.30 1.36
7.50
IB90
2229.0 1.09 10.92 12.49 37.00 0.07 4.84 5.84 2.72
5.00
IA
1720.0 1.32 5.28 3.00 40.00 0.17 1.52 1.28 1.23
7.50
SS80
2125.0 0.61 6.5
6.07 41.56 0.19 3.31 2.54 1.38
5.00
Stratigraphic Unit Density σT
UCS E (GPa) 
K (GPa) G (GPa) C (MPa) ψ

(kg/m3) (MPa) (MPa)
IMM_ROOF
1778.6 0.84 12.90 3.04 31.82 0.19 2.55 1.28 3.37
5.00
I_TOP
1497.0 1.35 5.30 3.00 40.00 0.12 1.32 1.34 1.24
7.50
I_BOT
1497.0 1.35 5.30 3.00 40.00 0.12 1.32 1.34 1.24
7.50
SS70
2247.0 0.98 9.82 7.43 40.43 0.20 4.18 3.09 2.27
5.00
IIT
1721.0 1.11 5.25 3.00 40.00 0.15 1.43 1.30 1.22
7.50
II
1510.0 0.97 3.89 3.00 40.00 0.08 1.18 1.39 0.91
7.50
SS60
2192.0 0.75 7.55 7.17 35.54 0.14 3.33 3.14 1.94
5.00
IIIB
2080.0 0.48 1.92 3.00 40.00 0.15 1.43 1.30 0.45
7.50
SS50
2234.0 0.78 7.81 6.66 39.82 0.18 3.43 2.83 1.83
5.00
IIIA
2290.0 1.01 4.06 3.00 40.00 0.35 3.33 1.11 0.95
7.50
SS40
2187.0 0.58 6.5
5.70 39.35 0.19 3.09 2.39 1.37
5.00
III_Top_coal
1529.0 1.07 12.00 3.00 40.00 0.10 1.25 1.36 2.80
7.50
III_Bottom_coal
1529.0 1.07 12.00 3.00 40.00 0.10 1.25 1.36 2.80
7.50
SS30
2249.0 0.84 8.37 7.96 45.50 0.24 5.05 3.22 1.71
5.00
IV
1472.0 0.72 3.70 3.00 40.00 0.20 1.69 1.25 0.86
7.50
SS20
2197.0 0.81 8.08 8.96 42.79 0.22 5.36 3.67 1.76
5.00
V
1502.0 1.11 5.25 3.00 40.00 0.15 1.43 1.30 1.22
7.50
BASE
1957.0 0.94 20.00 5.35 39.48 0.18 3.17 2.26 4.72
6.19
Where, σT is tensile strength, UCS is Unconfined Compressive Stress, E is Elastic Modulus,  is
friction angle, K is shear modulus, G is bulk modulus, C is cohesive strength and ψ dilation
angle.
In COSFLOW the constitutive models used for the rock blocks were the elastic perfectly plastic
Mohr‐Coulomb model. The constitutive model used for the joints was the standard
Mohr‐Coulomb slip model. A unique feature of COSFLOW is the incorporation of Cosserat
continuum theory [14] in its formulation. In addition, a strain softening constitutive flow rule
was used in three of the models. The softening/hardening parameters which are a function of
plastic strain were assumed based on the experience.
Figure 7 shows the plan view of the predictive model, which encompasses 9km2 to minimise
the boundary effects. The 3D model was discretised with approximately 1.5 million finite
elements. The mine plan was rotated to align the model boundaries with the principal
horizontal stress directions. The chocks were also installed in the fine mesh region as shown in
the figure. The model was prescribed roller boundaries on the four sides and the base of the
model. The top surface was simulated as a free surface with zero stress. Initial stress field
equal to the in situ stress measured and reported by Messy India [6] was prescribed. 3.5 m
thick seam was extracted from the average seam thickness of 10m and the longwall panel was
extracted in steps to minimise the dynamic response of the model.
Figure 7 Plan and close up view of the mesh
A number of model variations were simulated to study the effects of variation on the strength
properties of the SS40 (main roof), SS50 and coal seams. Table 2 presents the variation of rock
mass strength properties used in the COSFLOW simulations. In the numerical models, explicit
planes of weaknesses were introduced in‐between SS40 and IIIA, and IIIA and SS50.
Table 2 Different cases of the model
#
Case
Property
1
Case1
Massive sandstones, I, III, IV
2
Case2
Case1 + double coal strength
3
Case3
Case1 + double coal strength and Elastic modulus
4
Case4
Case1 + layered SS40
5
Case5
Case1 + layered SS40 and SS50
6
Case6
Case1 + double strength of SS40
7
Case7
Case1 + strain softening
8
Case8
Case1 + double strength of all sandstones
9
Case10
Case1 + different strain softening and cut-off values
10
Case11
Case1 + different strain softening and cut-off values
III Seam of Adriyala mine was divided into bottom and top parts. The bottom part of the coal
was assigned a uniform height of 3.5 m and the top part of the coal was assigned the
remaining height of the coal seam (i.e. around 6.5 m). The bottom part was extracted using the
normal longwall extraction method as applied in I Seam extraction simulation in Adriyala [5]. In
the COSFLOW simulation both bottom coal and top coal were extracted in sequence. The top
coal extractions lagged behind the bottom coal extraction by one step to mimic the actual top
coal caving extraction process, i.e. “when the ‘n’ excavation step of the bottom coal was being
extracted the ‘n‐1’ excavation step of top coal was extracted as shown in Figure 8. Thus the
model assumes that with the advancement of chock, the broken top coal located just above
the chock falls onto the rare AFC and gets transported to the surface.
Figure 8 Top coal caving method used in COSFLOW simulation
The predictive model was simulated to anticipate the chock convergences, top coal caving
behavior, strata breakage behavior, abutment stresses and vertical stresses developed during
mining. From the longwall extraction of Adriayala it was found that 1100t chocks were
appropriate for the standard longwall mining [5], in the current simulations 1100t chocks were
also used in the simulation. The detailed results of all the model cases and parameters are
available in [5]. The following sub-sections show an idea on how each of the results was
analyzed.
Chock convergence: A comparison of chock convergences for a 250m wide panel for Case1 at
different locations along the mining width is shown in Figure 9. As expected, it can be seen
from the figure that due to a large width of the panel the middle part is sagging and yielding
higher convergence compared to the chocks located sideways. In average the chock on the
middle of the panel can be seen to converge more than twice the chocks on the edges. The
chocks located in the middle of the panel can be seen to undergo cycling loading with a cycle
of about 30 to 35m.
COSLFOW predicted the following observations in par with the view generally perceived in the
engineering community that the stronger the roof strata the more pronounced will be the
loading on the chocks. COSFLOW also predicted chock convergence will be higher for panels
with massive and stronger sandstone units, stronger SS40 yielded relatively higher chock
loading than the model with weaker SS40. COSFLOW also predicted that if the strength of each
rock unit in the geological model is identical then the presence of a massive unbedded rock
unit in the immediate roof will yield higher chock loading, and the variation in the strength of
the coal seam seems to have the least effect on chock loading.
80
Case1, 10m-20m from tailgate
70
Case1, 110m-120m from tail gate
Case1, 240m-250m from tailgate
Convergence, mm
60
50
40
30
20
10
0
0
20
40
60
80
100
120
140
160
180
Distance, m
Figure 9 Comparison of convergence at three different places along the mine width.
Top coal caving behavior: This fine scale discretisation was necessary to investigate the nature
and evolution of fracture/yield profile within the top coal, so the top part of III Seam was
discretised into seven elemental layers. For the considered geological condition, top coal,
SS40, SS50 and SS60 and other overlying rocks will be affected by the excavation of the bottom
coal. Figure 10 presents the yielding of the top coal at different excavation steps for Case8 (see
Table 2). These yield plots are taken along the vertical cross‐section passing through the
mining face (i.e. perpendicular to the mining direction). On the plot except for the case of zero
yield (i.e. coal never yielded and remained elastic), every other values indicate that the coal
have undergone one or another form of yielding (i.e. either tensile or shear or combination of
both). On the figure red colour shows the yielded/fractured coal and blue colour shows the
intact coal. It can be observed from these figures that all the seven elemental layers of the top
coal located above the chocks are fractured due to the extraction of the bottom coal.
598 m after retreat
614 m after retreat
Figure 10 Yield of the top coal represented by seven elemental layers at different excavation
steps as noted in the pictures for III Seam of Case8
Strata caving behavior: The model results were also analysed to better understand the
deformation behaviour of overlying strata specially the sandstone layers SS40, SS50 and SS60
during LTCC extraction. Figure 11 presents the pictures showing the yielding of different layers
of SS40, SS50 and SS60 units for a particular excavation step, at about 630 m longwall retreat,
for Case6 (see Table 2 i.e. the case with the highest SS40 strength) with 1100t chock and 250m
wide panel. Recalling the colour code used in the plots, except for the case of zero yield (i.e.
rock never yielded and remained elastic), every other values indicate that the rock has
undergone one or another form of yielding. In the figure, bottom row pictures are close to III
Seam and top row pictures are close to SS70. These plots clearly indicate that SS40 would cave
in without much difficulty. The lower part of the SS40 has failure contained within the
excavation region compared to the upper part where the failure is extended beyond the
mining face and side pillars. For SS50 and SS60 units, the failure is contained within the face.
SS50 and SS60 units are likely to cave in with some delays. The failure can be seen to lag
behind the face position suggesting fractures in these sandstone units would probably occur
after the fracturing in SS40 after some delays.
SS50-bottom
SS50-top
SS40-layer1
SS40-middle of the SS40
(adjacent to top coal of III seam)
SS60
SS40-top (layer5)
Figure 11 Plots showing the yield of different layers of SS40, SS50 and SS60 units at a longwall
retreat distance of, 630m after retreat, for Case6
Abutment stress: Figure 12 presents the distribution of abutment stress along the centre of
the 250m wide panel with 1100t chocks for Case8 (see Table 2) at different retreat distances
as noted in the pictures. The pictures on the left hand column show the abutment pressure
measured at the bottom coal and the pictures on the right hand column show the abutment
pressure measured at the top coal. The maximum abutment stress can be seen to be
approximately 65 MPa for the bottom coal and 52 MPa for the top coal, which is almost 80 %
of the bottom coal abutment pressure.
Bottom coal
Top coal
596m from the start line
695m from the start line
Figure 12 Distribution of abutment stress along the centre of the panel with 1100t chocks for
Case8 for 250m wide panel at different retreat distances as noted in the pictures
Vertical stress: Figure 13 presents a plot showing the distribution of vertical stress for 250m
wide panel for Case1 with 1100t chocks. The figure is plotted at the distance of 630m from the
start line. The plot indicates that the vertical stress in the chain pillar can be more than 40MPa
i.e. more than 4 times the in situ pre‐mining stress. This stress estimate can be used in the
design of chain pillars.
Figure 13 Vertical stress for 250m wide panel for Case1 and 1100t chocks at the middle of
bottom and top coal layers, 630m after retreat
5. Conclusions
The paper demonstrated a various steps involved in investigating the feasibility of LTCC mining
method in one of the mines at SCCL. Various factors affecting the LTCC behavior were
considered and evaluated in order to assess the feasibility of LTCC method. The project
undertook a comprehensive analysis of geological and geophysical data of the mine site and
developed detailed geotechnical frameworks for the assessment of LTCC technology. The
paper also showed various parameters which are to be evaluated in order to gain confidence
and implement LTCC at the SCCL mine site.
6. Acknowledgements
The authors are thankful to the Asia‐Pacific Partnership (APP) on Clean Development and
Climate Program and the Australian Government Department of Resources Energy and
Tourism (DRET) for funding this research project through their Coal Mining Task Force (CMTF).
The authors are also grateful to the SCCL for providing the mine data and field input during the
period of project execution.
7. References
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