Geotech Geol Eng
https://doi.org/10.1007/s10706-018-0717-2
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ORIGINAL PAPER
Room and Pillar Design and Construction for Underground
Coal Mining in Greece
Michail Tzalamarias . Ioannis Tzalamarias . Andreas Benardos .
Vassilis Marinos
Received: 19 June 2018 / Accepted: 6 October 2018
Springer Nature Switzerland AG 2018
Abstract The underground mining is the only
potential way for the utilization of the lignite reserves
from an open pit exploitation which could remain
unexploited due to high stripping ratios. This paper is
dealing with the findings of a pilot scale underground
exploitation that was developed in the Prosilio open pit
coal mine in Northern Greece. The method used is the
room and pillar mining method where the initial entry
galleries are driven into the coal seam starting from the
surface excavation face, as used in the highwall
mining cases. The design of the mining scheme is
presented in detail along with the building of the 3D
numerical model which simulates the overall development of the pilot mine. The evaluation of the
stability conditions is further discussed and analysed
with the use of the results of the numerical model and
through their validation with the findings and observations of the actual excavation’s response. The mine
scheme selected exhibited its flexibility in coping with
the prevailing conditions and its performance, in terms
M. Tzalamarias (&)
METE S.A., Mining Company, 53100 Florina, Greece
e-mail: mtzalamarias@yahoo.com
I. Tzalamarias A. Benardos
School of Mining and Metallurgical Engineering,
National Technical University of Athens, Athens, Greece
V. Marinos
School of Geology, Faculty of Sciences, Aristotle
University of Thessaloniki, Thessaloniki, Greece
of the stability conditions attained, further supporting
the development of a large scale underground coal
excavation.
Keywords Underground coal mining Mine design
3D FEA Open pit to underground
1 Introduction
Greece is still heavily depended on its lignite recourses
for its electricity demands. According to Eurocoal
(2017) lignite production accounts approximately for
one-third of the electricity market (31% in 2016), with
37.0 Mt in 2017 making it Greece’s most important
indigenous energy resource. Lignite mining is mainly
located at the north-western Greece through massive
surface exploitations that are now taking place in the
greater Plotemais area. Nevertheless, large quantities
of coal reserves remain unexploited when the open pit
mines become marginal due to the constant increase of
the overburden volume that must be removed to
proceed with the coal extraction.
Those unexploited reserves are now in the spotlight
for their potential utilization through a subsequent
underground mining scheme. This is the first time after
almost 40 years where the underground exploitation
of coal is pursued in Greece, after the closure of the
underground Aliveri mine site in the early 1980’s
(PPC 2010). Former coal mines used to exist in the
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Athens greater metropolitan area, in Peristeri and in
Kalogreza, even under the current location of the
Athens’ Olympic complex.
Of course, the concept of establishing a direct
underground excavation through the coal face is not a
new one. It goes back to 1940 in the US where the first
such efforts were made and gradually evolved to a new
mining method; highwall mining that is capable of
facilitating the enhanced recovery of the coal seam
(Mo et al. 2016). This method is now used throughout
the world, an indication of its importance in the coal
mining business, mainly with the use of continuous
miner equipment (Luo 2014; Shimada et al. 2013), that
report capacities of more than 1 Mtpa. In Australia,
the peak production of CHM reached around
3–4 Mtpa in the late 90’s while in the US the total
run of mine production from highwall mining (CHM
and auger) was estimated to be 59 million tn in 2003
(Zipf and Bhatt 2004; Mo et al. 2016).
In the Prosilio mine case, near Kozani in Northern
Greece, that the underground exploitation is analysed
in this paper the mining is not made through the use of
continuous miner but rather with the development of a
room and pillar mining scheme though conventional
mechanized excavation. Nevertheless, this method
can also offer important potential economic advantages. Obviously, the feasibility of such underground
exploitations is something that is heavily depended on
the attained stability conditions of the underground
workings as well as on the productivity characteristics
as attained by the mining scheme selected. In this
direction, the research presented in the paper is dealing
with the findings of a pilot scale underground
exploitation that was developed in the Prosilio coal
mine. Due to higher stripping ratios, a large part of the
coal reserve is remaining unmined in an open pit
exploitation. In order to be utilized a much bigger part
of the coal reserve, an underground mining scheme is
designed to be developed underneath the slopes of the
existing open pit mine. The method utilises the room
and pillar mining method where the initial entry
galleries are driven into the coal seam starting from the
excavation face of the open pit, as used in the highwall
mining cases. This endeavor is undertaken with main
aims to analyze the rockmass response to the underground excavations and to test the general costefficiency of the followed mining method.
In the following sections the mine design is
analysed and the stability conditions are evaluated
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and discussed by using both the findings of the 3D
numerical analysis on the pillars’ performance and
ability to provide the required stability of the excavation and the actual on-site response of the mine
workings as recorded throughout the mining period.
2 Going Underground in the Prosilio Mine Site
The Prosilio mine site of METE S.A. is located in the
Nortwestern Greece at the Kozani prefecture, exploiting the brown coal deposit found there (Fig. 1). The
coal strata is found as a single, almost horizontal, layer
having a thickness of about 5.5 m. The coal is of good
calorific value (2000–2400 kcal/kg), superior to the
other lignite deposits of the area and is currently being
used as booster to the PPC power plants. The mine
started with the exploitation of the deposit using
typical open pit excavation, nevertheless in order to
maximize the recovery in the boundaries of the current
excavation the underground mining of the coal seam is
being investigated (Fig. 2). In those parts, the overburden is gradually increased up to around 150 m.
This renders that particular coal body section as nonmineable by surface excavation. Thus, the exploitation
of the coal deposit directly from the pit bottom seems
as a promising practice following the principles given
by the highwall mining system (Porathuir et al. 2017).
In this manner, the coal is accessed from galleries
starting at the base of the highwall and driving their
way through the deposit. Nevertheless, in the case
examined the exploitation is not being made by auger
systems but by using the room and pillar mining
scheme without any recovery of the pillars. The
thickness of the deposit enables such a scheme and
under this method a number of horizontal and
transverse galleries are excavated (rooms or entries),
each of which serves the multiple roles of ore source,
access opening, transport drift and airway. Pillars are
generated as coal remnants between entries, to control
both the local performance of immediate roof rock and
the global response of the host rock medium.
Combining those two mining methods, the efficient
recovery of the 5.5 m-thick deposit is achieved and the
flexibility in the mining excavations in terms of pillar
dimensioning and support requirements is attained to
cater for the different conditions encountered. At the
same time, limited capital expenditure is required for
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Fig. 1 Location and general plan of the Prosilio mine
Fig. 2 General view of the Prosilio coal open pit mine
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the excavation development, a key issue for the
successful development of the project.
METE S.A. mining company, commenced the pilot
scale exploitation to showcase the validity of the mine
design and to gather valuable information on actual
pillar performance and support requirements. Beyond
that, the data collected would assist in benchmarking
the cost-efficiency of the followed mining method
when applied under the real conditions of the Prosilio
mine site.
3 Design of the Pilot Scale Exploitation
Ensuring the stability of the excavations and the safety
of the personnel is of paramount importance to the
development of the mining plan. The key to this is the
dimensioning of the coal pillars that govern the overall
mine stability performance (Hustrulid 1976). For
almost a century now coal pillar design has attracted
the interest of many researchers (Du et al. 2008) trying
to capture the mechanics of pillar strength as Greenwald et al. (1941), Holland (1964), Salamon and
Munro (1967), Obert and Duvall (1967), Bieniawski
(1992), Madden (1996) or Mark (1999) just to name
some of the classics.
This has come a long way from the initial formulae
(Mark 2006) to modern approaches taking into
account probabilistic design (van der Merwe and
Mathey 2013). Of course now, apart from the empirical design the estimation of pillar strength is possible
using numerical modeling and it may provide a viable
and perhaps better alternative to earlier conventional
pillar strength approaches (Mohan et al. 2001), either
in the form of 3D numerical FEA or using 2D
approximations (Deliveris and Benardos 2017). In this
manner, numerical models can further assist in the
understanding of pillar performance, especially if
validated by the actual feedback gained from the
actual excavation’s response.
For the development and the numerical analysis of
the following 3D-model, the Rocscience’s RS3 software was used. The first step was to gather the
geotechnical properties of both the coal orebody and
the surrounding strata. Such data are based on
geotechnical studies made for this underground
exploitation, on field observations and on international
experience. The rock mass comprises of a flat-lying
and relative shallow brown coal orebody with an
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average thickness of 5.7 m and of marl formation
found both in the hanging wall and in the footwall. The
height of the overburden is taken at 65 m, the average
actual value of the overburden at the pilot mine area.
The general setting can be seen in Fig. 3 where the
cross-section of the lithological types is identified
along with the values of the main geotechnical
properties. The geotechnical properties of the formations that were selected and used for this analysis
(Table 1) are based on the in situ campaign, field
observations and the lab testing of the Prosilio mine
site marl and lignite deposits (Vardakastanis and
Pantekis 2014), as well as on international experience
in similar formations. This encompasses the intact
rock properties rci, Ei, and geotechnical classification
values with the GSI system (Marinos and Hoek 2000),
while mi parameters for the marl and lignite formation
were obtained from the literature (Hoek et al. 2002;
Shen and Karakus 2014). Especially for the assignment of the GSI values, field observation from the
stopes’ crown and sidewalls and bored sampling of the
formations were taken into account. It is witnessed
that the density of the discontinuities become gradually higher when moving from the upper marl to the
middle and lower strata, as shown in the core samples
of Fig. 4a, b and c (Vardakastanis and Pantekis 2014).
In this manner, a value of 70 selected for the GSI of the
upper marl while 60 was the GSI value selected for the
middle and lower marl, as it is also shown in Fig. 5.
The geological environment was modeled assuming elasto-plastic deformation using the Mohr–Coulomb failure criterion, as according to the ISRM
classification, both the lignite and marl can be
described as weak rocks; therefore, the Mohr–
Coulomb failure criterion was considered as the most
appropriate for the analysis.
It was decided to develop two main access tunnels
(X1, X2) from which the main part of the pilot mining
was developed (Fig. 6). This consisted of two additional galleries (Z1, Z2) parallel to the main access
tunnels and of four perpendicular cross-cuts (Z1, Z2,
Z3 and Z4) that eventually developed a total of nine
coal pillars, as it is shown in Fig. 7.
The width of the main entries was decided to be
7 m, while the span of all other openings was set to
6.4 m. The total height of the all galleries matched the
coal thickness at 5.7 m, with 4.5 m high vertical walls
followed by a curved roof design at their 1.2 m crown
area. The pillars have rectangular shape with
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Fig. 3 Cross section of
rock mass geotechnical
parameters
Table 1 Rock mass
geotechnical parameters
rci (MPa)
Upper marl
3.3
mi
7
Ei (MPa)
GSI
c (MPa)
/ ()
450
70
0.18
37
Middle marl
3.3
7
450
60
0.14
34
Lignite
5.3
20
380
75
0.25
53
Lower marl
3.3
7
450
60
0.14
34
dimensions 5.4 m by 7.4 m. The recovery rate reaches
74% at the main mine area of the pillars, while the total
attained recovery for the whole underground mine,
including the non-mined panel at the entrance section,
is 54%. For the FEA model the shape of the galleries
was taken as rectangular instead of the curved design
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capacity) at a grid of 2.0 m by 1.5 m at the stopes’ roof
(Fig. 9), followed by the application of steel mesh.
Furthermore, at the intersection areas the bolting was
decided to follow a more aggressive pattern to
minimize potential failures and thus the grid was
further reduced to 1.0 m by 1.5 m.
The finite element model tries to simulate the pilotscale room and pillar coal exploitation by taking into
consideration not only the above layout, dimensions
and roof reinforcement but also, as far as it was
possible, the excavation’s progression. In this manner,
in the model developed, the main entries X1 and X2
were first excavated simultaneously, followed by the
stopes Y1–Y4 and finally by the stopes Z1, Z2, making
up a total of 84 excavation stages (Fig. 10).
4 Results from the FEA Analysis and the On-Site
Observations
Fig. 4 Core samples from the a upper, b middle and c lower
marl formation
due to some aberrations of the software used that
didn’t make it possible to perform the computing
(Fig. 8).
The excavation uses a 2-m step length, and is made
with the use of mechanical excavator. The support of
the galleries is consisted by the application of Swellex
rock bolts (2.4 m length, 120 KN typical bearing
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The results of the FEA analysis and the actual
measurements made in the mining period were in
good agreement. Starting from the pillar loading in
Fig. 11 the redistribution of the sigma 1 effective
stress through the different phases of the excavations,
as they are shown in Fig. 10, are presented. This is
modelled at the roof section of the mine at the
intersection between coal and marl at the section Y–
Y0 . Stage 0 refers to the premining conditions (virgin
stress field), stage 66 refers to the stress values
corresponding to the development of the three main
pillars (29.0 m 9 7.4 m) whereas stage 84 refers to
the stress value at the end of mining activity when the
final 7.4 m 9 5.4 m pillars are formed (see Fig. 9). As
it shown, the value of sigma 1 effective stress at the
pillars at stage 66 is around 2.0–2.2 MPa while at
stage 84 the stress value reaches around 3.0–3.2 MPa
having an increase in the order of 45–50%.
The FE analysis shows that the modelled displacements appear to have their maximum values at the
middle of the excavated galleries (rooms), especially
at the center of the pilot mine area, around the central
pillar at the intersection of the Y2 and Y3 galleries
with the Z1 and Z2 ones. Those appear to be the most
critical sections with the convergence values ranging
from 50 mm to 70 mm (Fig. 12). At the main access
galleries (X1 and X2) as well as at the back end of the
mine (gallery Y4) the roof displacements seem to have
lower values of around 40 mm.
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Fig. 5 GSI values of
different marl layers
Fig. 6 View of the main mine entries
Apart from Fig. 12, in Fig. 13 the total displacements along the X–X0 cross section (see Fig. 7) are
presented. The maximum roof displacement is around
50 mm at the central rooms. As a result of the stress
redistribution and the increase in pillars’ loading due
to the mining activity, the pillars deform in all three
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Fig. 7 Layout of the pilot application of the room and pillar
mining at the Prosilio mine
main axes, as it is shown from the FEA where
displacements of approximately 15 mm are calculated
at the pillars’ sidewalls (Fig. 14) and validated by field
observations.
The actual convergence measurements were executed by topographical equipment with the assistance
of targets located at the central part of the roof and at
the middle section of the pillars. The closest point to
the face, where the targets were installed and displacements were measured was at approximately 6 m.
This monitoring campaign provided detailed data
about the actual performance of the mine in terms of
displacements. The measured displacements show
good agreement with the actual performance of the
mine. The deformations in the pillars’ sidewall were
between 10 mm and 20 mm. For the case of the roof’s
displacements the data from the main access galleries
that were measured throughout the pilot mining
application showed an average value of 30 mm, with
the maximum values reaching around 40 mm.
The following chart (Fig. 15) is refering to the
development of the convergence as measured at the
Fig. 8 Cross sections of the stopes used in the pilot mining scheme and in the numerical analysis model, respectively
Fig. 9 Main characteristics and layout of the roof bolting
applied at the Prosilio coal mine
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roof of the X2 entry stope. As it is shown, the tunnel
face progresses up to 25 m, from the control point, and
then stops, with the measured convergence to reach
18 mm at that point. The face of the X2 gallery doesn’t
progress any more but the convergence continue to
rise, until the end of the underground excavations, for
more than 100%, reaching the final value of 40 mm.
This is due to the excavation of the neighboring Y1,
Y2 and Z1, Z2 stopes which affect the overall stability
conditions of the mine and hence lead to the deformation of the X2 gallery’s roof.
The distribution of the Strength Factor and the
yielded elements as calculated by the numerical
analysis at the cross section X-X’ are shown in
Fig. 17. The analysis calculates a minimum Strength
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Fig. 10 Different phases of the stoping activity as it was modeled in the FEA
Fig. 11 Sigma 1 effective stress through the different phases of the stoping activity as it was modeled in the FEA
Factor of 1.16 at the core of the pillar, with the total
unyielded width to be about 2.70 m or 50% of the
pillar’s body.
Following this observation of the FEA analysis, it is
witnessed that the core of the pillar remains intact in
spite of the yielded area that is localized at the pillar’s
skin. This suggests that the pillar’s response to the load
imposed is adequate and consequently the pillar’s
overall stability in terms of performance is sufficient,
given the temporal character of the mining excavation.
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Fig. 12 Total
displacements contours at
the spatial model
Fig. 13 Displacement contours at the deformed X–X0 cross section
Fig. 14 Total displacements in the coal pillars as derived from the FEA model
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Fig. 15 Measured convergences related to the progress of underground works
This was verified by the actual performance of the
pillars not only during the mining period but also after
the end of the excavations. In the worst case scenario,
where the yielding area would progress though the
core of the pillar, the application of immediate support
measures (e.g. bolting, shotcreting or clamping of the
pillar’s sides) could be used to remediate the pillar’s
stability conditions.
The pillars’ deformation behaviour is also related to
the structure of its geology, as the foliation of the
lignite is perpendicular to the principal axis of loading
and this affects positively the pillars’ geomechanical
response against failure. During the underground
mining period, limited cracking was observed at the
excavation’s corners as a result of local shear failures
(Fig. 16). Furthermore, at the main body of two pillars
a limited surficial spalling was observed, as it was seen
in the 3D analysis at the yielded areas of the pillars
(see Fig. 17). Both of these two failure conditions
were described by Lunder and Pakalnis (1997)
(Fig. 18a, b, c, d). Nevertheless, such issues didn’t
result to any further degradation of the pillars’
performance, indicating that their core remained intact
and can withstand the loading imposed to them. This is
seen in the numerical model, as well.
The performance of the support measures in both
the numerical model and the actual mining can be
considered as satisfactory. The data from the
Fig. 16 View of lignite pillar edge. Detached lignite rocks from
the lower corner due to local cracking
numerical analysis shows that a small number of
rock-bolts has failed, mainly as the support load
exceeds its bearing capacity. Nevertheless, such
failures are only localized without affecting the overall
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Fig. 17 Strength Factor contours at the central pillar and yielded elements (A–A0 cross section)
Fig. 18 Schematic illustration of the evolution of fracture and
failure in a pillar in massive rock (after Lunder and Pakalnis
1997)
stability of the immediate roof. The sound rock mass
properties of the marl in the immediate roof and the
limited extend of the yielded zone of the roof (yielded
elements are highly concentrated in a zone up to 2 m
over the roof) are factors that make the applied
supporting system suitable for this pilot-scale underground lignite exploitation.
This is validated from the experience gained in the
pilot mining scheme. Apparently, no bolt failure was
witnessed (e.g. large deformations, possible bending
or rupture or plate failure) until the end of the mining
campaign. The observation of the stopes’ crown
during the excavations and the very good response
of the immediate roof to the stoping activity are
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indications that the actual yielded zone of the roof was
much more limited than modelled. This is also
partially due to the actual geometry of the tunnel
crown where the arching effect also develops more
stable conditions as the stress redistribution prevents
the development of a tension stress field.
The support system selected works not only in
suspension mode but also develops a support beam
throughout the bolt’s length capable of addressing
major failure threats. This system can be further
enhanced with the application of an additional
shotcrete layer in the latter stage when the underground mining will be fully deployed.
In addition to the generally good structural response
of the lignite pillars, factors such as the use of
mechanical excavation which reduced significantly
the disturbance of the surrounding rock mass in
conjunction with the temporal nature of this underground mine affected favorably the global stability
and allowed the completion of this pilot exploitation
without any significant difficulties (Fig. 19).
However, it should be noted that for the case of
higher overburden, a new pillar design would be
required. The main aim would be to provide design
guidelines that are compatible with the prevailing
geotechnical and stress conditions. In doing that, the
room-and-pillar method provides the required flexibility. The major change would be focused in the
dimension of the pillars. The room width could follow
the general guidelines of the pilot application (approx.
6.5 m); however, the width of the pillar would most
probably be increased to new design values that would
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roof strata. Although at these conditions the
selected support is sufficient, in cases where areas
of lower geotechnical properties are to be encountered more support options should be further
considered (e.g. shotcrete, lattice girders, etc.).
Fig. 19 View of the underground exploitation after the
completion of the stoping activity
ensure the stability of the excavation and the required
FoS of pillar.
Finally, the good agreement between the 3D numerical
model and the actual conditions is to be noted. This is a
big advantage to the exploitation engineers as they can
explore further options in the overall mine design as
the model develop seems to have the ability to
accurately capture and model the excavation’s
response.
The pilot mining application revealed interesting
features and provided useful experience. In the next
mining phase, which is now under planning a larger
area will be exploited by underground excavation that
will closely follow and sync with the open-pit mining
activities.
5 Conclusions
The development of underground exploitations can
provide viable solutions for the mining of reserves that
cannot be economically mined through classic surface
exploitation schemes. The stability conditions of the
pillar structures however are a key aspect that could
determine the feasibility of the endeavor. In the
Prosilio mine site the pilot mining was implemented to
test the implementation of the room and pillar mining
scheme in order to be further investigated the potential
of further utilization of the lignite reserve through an
underground exploitation.
Based on the results of the numerical analysis and
the field observations it can be deduced that:
•
•
•
The pillar design and dimensioning appear to be
capable of withstanding the loading from the
overburden at this level. Although minor failures
appeared in the pillars’ skin, the core remained
intact. Thus, in the case where a less aggressive
design is selected the problems can be further
minimized.
The most critical parts of the mine are located at
the central rooms. In there the maximum displacements are found to be around 60 mm. Displacements of lower magnitudes are experienced in the
pillars sidewalls.
The support system selected shows a very competent performance minimizing any failures of the
Acknowledgements The authors would like to acknowledge
the help of METE S.A. in the development of the paper and for
allowing the publication of the data.
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