Numerical modelling techniques for studying longwall geotechnical problems under realistic geological structures
Journal of Mining and Earth Sciences Vol. 62, Issue 3 (2021) 87 - 96
87
Numerical modelling techniques for studying
longwall geotechnical problems under realistic
geological structures
Dung Tien Le *, Tung Manh Bui
Faculty of Mining, Hanoi University of Mining and Geology
ARTICLE INFO
ABSTRACT
Article history:
Longwall - associated geotechnical problems have been extensively
studied by using numerical modelling methods. However, proper
representation of its geological structures remains a challenging task.
This paper presents a systematic understanding of numerical modelling
techniques for studying longwall coal mining with geological structures.
The modelling techniques derived from conventional and advanced
continuum and discontinuum methods were reviewed in detail with
emphasiz on their mechanic's formulation and applications. This study
suggests that the successful selection of a proper modelling technique
should be based on the physical principle of longwall problem, texture and
shape of materials, and mechanics formulation of the numerical program
used for modelling. The paper’s conclusions assist numerical modellers in
quickly and properly selecting modellingtechnique for investigating a site
- specific longwall problem.
Received 12th Feb 2021
Accepted 23rd April 2021
Available online 30th June 2021
Keywords:
Discrete Fracture
Grain - Based Model (GBM),
Longwall,
Network(DFN),
Trigon logic.
Copyright © 2021 Hanoi University of Mining and Geology. All rights reserved.
stability, face/rib spalling, pillar yielding, roof
fall/caving, and floor heave are the objects of
1. Introduction
Longwall is one of the most productive
underground coal mining methods, and has been
widely applied in many coal industries. The
method’s application has been extended to
various geo - mining conditions due to the
significant improvement in face equipment (i.e.,
shield support) and deeper insight into rock mass
behaviour around the mined - out area. Longwall
geotechnical problems such as entry/roadway
many investigations. Depending on the research
methods used, those studies shed different lights
on the mechanisms driving the problems. For
example, according to Galvin (2016), empirical
methods (which are based on experiment, field
data and observations) may produce an actual
response of rock mass around the longwall face. A
clear understanding of the underlying physical
phenomenon anddatabase forthedevelopmentis
a key for the method's success, as seen in Yu et al.
(2015; 2017). Due to the high cost in setting
experiment and the long time for implementation
at longwall panel, empirical methods should be
_____________________
*Corresponding author
E - mail: [email protected].vn
DOI: 10.46326/JMES.2021.62(3).10
88
Dung Tien Le, Tung Manh Bui/Journal of Mining and Earth Sciences 62(3), 87 - 96
carefully designed before their application.
Analytical methods, on the other hand, may reveal
fundamental physical principles of longwall
behaviours, as can be seen in Wang and Wang
(2019) and Le (2021). However, the methods
require significant time and effort for solving
equations when the problem involves many
detailed structures that typically occur in coal
measure rocks. On laboratory scale, physical
modelling methods can obtain similar results to
field experiment at a more reasonable cost, as
seen in Sui et al. (2015) and Zhang et al. (2019).
The methods still require significant time and
effort for constructing models compared to the
project time frame.
Compared to the above methods, numerical
modelling methods, as based on mathematical
equations founded on analytically derived
formulae/algorithms (Galvin, 2016), are
particularly suitable for studying longwall
problems. In particular, the methods can
investigate detailed micro - mechanisms of large -
scale longwall problems within a reasonable time.
Compared to physical modelling and analytical
methods, numerical methods are more efficient in
incorporating many geological structures.
Nevertheless, compared to empirical methods,
proper modelling of geological structures is more
challenging due to the complex geometrical
distribution of structures in the field. While the
derivation of rock material properties at
laboratory scale is reliable through different
testing techniques, the derivation at field scale for
large - scale modelling such as longwall is limited
due to available apparatus and inhomogeneity of
geological structures. Although some numerical
modelling techniques have been developed for
the realistic representation of longwall geological
structures, they can be very different in the
mechanic’s formulation and may be misused if
insufficiently understood.
site - specific longwall problem.
2. Continuum modelling techniques
Finite Difference Methods (FDM) are widely
used continuum techniques for studying longwall
coal mining. By using the methods, a longwall
domain is discretised into many sub - domains
whose connectivity between them remains
unchanged during the modelling (Kelly et al.,
2002). The methods - with representative
programs such as FLAC and FLAC3D - have
interface elements or slide lines that allow the
modelling of discontinuum domain such as
longwall coal seam/rock strata to some extent
(Itasca Consulting Group, 2019). For example, Bai
et al. (2016a) used FLAC2D to study coal wall
micro - responses caused by longwall retreat.
Although different geometrical configurations of
geological structures exist in the stratigraphic
columnofthearea, itseemsthatthestructures are
implicitly represented in the model (Figure 1).
This representationtechniques canalso be seenin
Li et al. (2018) and Zhang et al. (2019), in which
FLAC3D was used to investigate rockburst and
roof fracture caused by longwall top coal caving
respectively. A longwall panel containing sub -
horizontal parting planes was explicitly modelled
by FLAC3D using interface elements in Behera et
al. (2020). The modelling technique did not
consider other typical structures such as joints
within coal seam (Figure 2). To realistically
represent both parting/bedding planes and joints
within thick coal seam, Wang et al. (2020) used
FLAC3D coupled with a Discrete Fracture
Network (DFN) for the modelling (Figure 3). This
modelling technique accordingly captured well
failure mechanisms caused by longwall mining.
However, due to the continuum nature of the
program, the technique could not consider
realistic caving development in LTCC, which is a
key phenomenon to the success of this mining
method. Further details on DFN are described in
Section 3.2.
This paper presents
a
systematic
understandingof numericalmodelling techniques
for studying longwall geotechnical problems
under realistic geological structures. Different
techniques derived from continuum and
discontinuum codes are carefully reviewed. The
paper’s conclusions and suggestion assist
numerical modellers in quickly and properly
selecting modelling technique for investigating a
3. Discontinuum modelling techniques
Distinct Element Methods (DEM) are widely
used discontinuum techniques for studying
longwall mining. By using the methods, a longwall
domain is divided into an assemblage of discrete
Dung Tien Le, Tung Manh Bui/Journal of Mining and Earth Sciences 62(3), 87 - 96
89
Figure 2. FLAC3D model of longwall with parting planes (Behera et al., 2020).
90
Dung Tien Le, Tung Manh Bui/Journal of Mining and Earth Sciences 62(3), 87 - 96
Figure 3. Longwall model with DFN - FDM (Wang et al., 2020).
a) pre - existing fracture orientations; (b) pre - existing fracture lengths; (c) DFN realization; (d) FDM
model; (e) FDM - DFN model; (f) fracture influenced zones; and (g) geometrical size of model.
blocks formed by connected fractures (Le et al.,
2017). The methods - with representative
programs such as UDEC, 3DEC, PFC2D, PFC3D -
are most suitable for moderately fractured rock
mass as in longwall coal mining. The use of DEM
in representing geological structures in longwall
model can be classified into conventional,
Discrete Fracture Network (DFN), and Grain -
Based Model (GBM) technique as follows.
of structures are defined by statistical geometrical
parameters such as dip angle, trace length, gap
length, spacing, and spatial location. This
modelling technique has been widely used to
study coal caving/spalling under typical
structures such as horizontal bedding planes and
vertical joints (Wang et al., 2016;Kong et al., 2019;
Le and Bui, 2020). Note that in these studies, the
structures are persistent throughout the strata. At
a smaller longwall - scale problem (i.e., longwall
entry), the conventional joint generator was also
used to model non - persistent vertical joint sets,
which can be considered as DFNs, as seen in
Abousleiman et al. (2020) (Figure 4).
3.1. Conventional technique
A conventional joint generator built - in DEM
based programs (e.g., UDEC) is used to create sets
of geological structures in a longwall model. A set
Dung Tien Le, Tung Manh Bui/Journal of Mining and Earth Sciences 62(3), 87 - 96
91
because the field measurement of fracture
characteristics can only be implemented at
limited space (e.g., outcrop, underground
exposure, drill core), the reliability of the
modelled DFN is highly dependent on the
number, availability, and quality of field
measurement. Application of DFN in mining and
civil geomechanics along with challenges in
building conceptual model, collecting field data,
and simplifying, calibrating, and validating DFN
are described in detail in Lorig et al. (2015).
For stratified rock mass as in underground
coal mine, DFN has been used mostly in the
“Synthetic Rock Mass - SRM” technique (Pierce et
al., 2007) for studying the effect of scale and
anisotropy on coal mechanical properties
(Deisman et al., 2010; Scholtès et al., 2011; Gao et
al., 2014b; Wang et al., 2019a; Wang et al., 2019b;
Karimi Sharif et al., 2019). In such studies, the
dimension of the SRM is commonly about several
meters, which is much less than a typical longwall
dimension. The use of DFN in the 3D longwall
problem remains, however, very limited. This is
because when the problem scale increases, the
detail level of the DFN model significantly
increases that requires excessive solution time.
The detail level may decrease in large - scale 2D
longwall problems, as for longwall entry stability
(Abousleiman et al., 2020; Zhu et al., 2020)
(Figure 5). It should be noted that in the
mentioned entry study, the DFN was created by
using a built - in conventional joint generator,
while in the roof study, a third - party DFN
Figure 4. Longwall entry model with DFN
(Abousleiman et al., 2020).
3.2. DFN technique
The basic concept, generation, advantage,
and disadvantage of DFN technique can be found
in many papers and textbooks. According to Jing
(2003), Elmo and Stead (2010), Mas Ivars et al.
(2011), and Hadjigeorgiou (2012), DFN was
developed since the 1980s to study fluid flow and
transport process within jointed rock masses
through a network of connected fractures. A DFN
is generated based on the geometryof the fracture
systemandthepersistence ofindividualfractures.
The fracture system geometry is established
based on the stochastic distribution of
characteristics such as density, orientation, size,
aperture, and persistence. One major advantage
of this technique is the realistic simulation of
fracture distribution at the field. However,
Figure 5. Longwall gob - size entry model with DFN - DEM (Zhu et al., 2020).
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Dung Tien Le, Tung Manh Bui/Journal of Mining and Earth Sciences 62(3), 87 - 96
generator named GEOFRAC (Ivanova et al., 2014)
was used. Other DFN generators can also be listed,
such as FracMan (Golder Associates, 2020) used
in Wang et al. (2019b), or MoFrac
Farahmand et al. (2018) and Wang and Cai
(2019).
Cundall, 1987; Trueman et al., 1996) or contact
stiffness (Nicksiar, 2013). Since 2010, a few
standard calibration processes were established
for the derivation of a unique set of contact micro
- properties for matching on - site rock mass
response (Kazerani and Zhao, 2010; Kazerani,
2013; Gao and Stead, 2014). Due to the standard
process in calibration and advantage in modelling
failure development, GBM has been widely used
in UDEC for studying longwall stability such as
roadway (Coggan et al., 2012; Gao et al., 2014c;
Bai et al., 2016b), roof strata (Gao et al., 2014a; Li
et al., 2016), surface subsidence (Zhang et al.,
2017), coal face (Yao et al., 2017), and coal pillar
(Wu et al., 2019) (Figures 6 - 7). The use of GBM,
however, must take into consideration the
disadvantages and uncertainties of the technique.
For example, according to Gao and Stead (2014),
micro - properties of contacts created by Voronoi
logic seem to be overestimated after calibration
and the solution time increases significantly.
Mayer and Stead (2017) and Zhang and Wong
(2018) emphasise that Voronoi logic reduces
kinematic freedom of polygonal blocks that
facilitates tensile failure. Meanwhile, Trigon logic
increases the kinematic freedom of triangular
blocks that facilitates shear failure. The two GBM
logics may misrepresent realistic failure
mechanisms if they do not sufficiently represent
the texture and shape of minerals constituting
rock mass.
3.3. GBM technique
GBM reflects the use of polygonal structures
(e.g, grain mesh, Voronoi, and Trigon
tessellations) to model grain - shaped material
mostly in DEM based numerical programs
(Damjanac et al., 2007; Potyondy, 2010; Lan et al.,
2010; Gao and Stead, 2014). GBM was first used in
UDEC by Lorig and Cundall (1987) to overcome
the intrinsic limitation of UDEC that this program
cannot explicitly model intact block failure. GBM
generation is done by subdividing an intact block
into smaller polygonal blocks through Voronoi
logic or triangular blocks through Trigon logic in
arbitrary sizes (Itasca Consulting Group, 2019).
The contacts (or boundaries) between polygons
are not representative of natural fractures but
play potential paths for failuredevelopment in the
rock mass. The micromechanical properties of
contacts and polygons must be calibrated to
match the modelled rock mass’s behavior against
reality. The calibration process can be simple by,
for example, adjusting contact strength (Lorig and
Figure 6. Longwall roof stability model with Trigon logic (Gao et al., 2014a).
Dung Tien Le, Tung Manh Bui/Journal of Mining and Earth Sciences 62(3), 87 - 96
93
Figure 7. Longwall surface subsidence model with Voronoi logic (Zhang et al., 2017).
Note that due to the 3D nature of geological
structure distribution, the 2D modelling must be
implemented with care for retaining the
representativeness of the problem’s structure. At
the same time, GBM logics are suited for the
representation of explicit failure and caving
development along well - calibrated fictitious
fractures. This study suggests that the successful
selection of a proper modelling technique should
be based on the physical principles of longwall
problems, textures and shapes of materials
4. Conclusions
This paper presents
a
systematic
understandingof numericalmodelling techniques
for studying longwall geotechnical problems
under realistic geological structures. The
modelling techniques derived from conventional
and advanced continuum and discontinuum
methods were reviewed in detail with emphasis
on their mechanics formulation and applications.
For FDM - based techniques, the current study
confirms that they well represent the micro -
mechanics of rock mass failure rather than
explicit caving caused by unjointed or moderately
bedded coal seam/roof strata in longwall. The
continuum formulation of the programs, on the
one hand, limits the complete and large
detachment of elements in the domain. On the
other hand, it may break down the calculation
process when many discontinuities (e.g.,
geological structures) are incorporated. For DEM
- based techniques, the study finds a wide use
from longwall entry stability to coal wall spalling,
roof caving, and roof fracture. Conventional joint
generator built in the programs is found to be
limited to modelling deterministic fractures
which are persistent. DFN logics are seen to be
suited for explicit representation of stochastic
fractures as in practice, but their use in longwall
problems is currently limited and mainly in 2D.
constituting
problems,
and
mechanics
formulation of the numerical program used for
modelling.
Author contributions
Dung Tien Le reviewed and wrote the
introduction and discussion; Tung Manh Bui
collected documents and wrote the conclusions.
Acknowledgements
This research is funded by Hanoi University
of Mining and Geology, Vietnam.
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