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Backfilling technologies for Estonian oil shale mines
I. Valgma, M. Kolats, A. Anepaio, V. Väizene, M. Saarnak and J.-R. Pastarus
Department of Mining, Tallinn University of Technology, Estonia
ABSTRACT
The oil shale deposit in Estonia is located partly
in a densely populated farming district of high
soil fertility. Underground oil shale production
is conducted using the room-and-pillar method
with drilling and blasting. A large amount of
neutral (limestone) and hazardous waste (ash) is
generated by the oil shale industry. The use of
ash and limestone as backfilling materials reduces the volume and area required for surface
disposal and consequently the environmental
taxes. In 1980, a preliminary investigation was
started with respect to the backfilling technology for the Estonian oil shale mines. Currently,
experiments continue with tests on new ashes
and waste rock aggregates. The main focus of
the current study is to clarify if backfilling in
given conditions would be technically possible.
The study includes underground and surface
mining space modelling, fill material supply and
sample and mixture tests, discussion of technological schemes for backfilling. Laboratory
UCS tests show, that lower ash content in the
mixture results in higher strength. Mine tests
show the warming effect of large scale mixtures
can have a positive influence to the hardening
process.
tion is about 7 million tonnes of oil shale, not
including separated limestone which amounts to
an additional 40% of the produced mass per
year. Currently oil shale is mined in 3 underground mines in addition to 7 surface mining
fields. The maximum number of underground
mines has been 13 with a total output of 17 million tonnes per year (Fig. 1, Valgma, 2000). Oil
shale bedding depth reaches 80 m, while seam
thickness is 2.8 m. The room and pillar mining
system with drilling and blasting is used today
with squared shape pillars left to support the
roof.
The losses in pillars increase up to 40 %. On
the other hand, a large amount of neutral (limestone) and hazardous waste (ash) is generated
by oil shale industry (Valgma, 2003). Using ash
and limestone as backfilling materials could reduce the volume and area required for surface
disposal and consequently the environmental
taxes (Adamson et al., 1998).
The main source of the backfill material today is Heavy Media Separation (HMS). The oil
shale seam consists of up to 50% of limestone
layers and pieces. This raises the question of
whether to utilising the waste rock or ash in the
1. INTRODUCTION
The oil shale deposit in Estonia is located partly
in a densely populated farming district of high
soil fertility. Oil shale is consumed by power
plants which produce about 90% of Estonian
electricity, oil and large part of thermal power.
Underground mining is performed for half of
the Estonian oil shale mining capacity. Produc-
Figure 1: Map of Estonian oil shale mining areas.

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Figure 3: Drum screener and hammer crusher for dry selective oil shale separation.
Figure 2: Spatial site model of oil shale deposit.
surface or underground mines or to dump them.
The main focus of the current study is to clarify if backfilling under given conditions could
be technically possible.
2. METHODS
The study includes the following steps:
1. Underground and surface mining space modelling.
2. Tests for the fill material.
3. Determination of technological schemes for
backfilling.
2.1 Underground and surface mining data collection and modeling
Site analyses have been carried out for determining potential backfilling conditions. For that
purpose a geological and technical model has
been created (Valgma, 2002). A quasi-stable area has been detected in large areas (Fig. 2). Areas of collapse, subsidence and zones of stability
have been determined.
2.2 Backfill mix components and origin
The components of the backfill mix could include water, limestone (waste rock from oil
shale mining), ash from the power plant, and in
addition sand, fibers or cement. Concerns are
related to haulage costs and other processes in
the mine. One of them is the origin of limestone
aggregate material. If limestone could be sepa-
rated from the oil shale in situ, then a reduction
in haulage costs could be achieved. For dry underground separation, tests with Bradford drums
have been carried out (Fig. 3). In addition crushing buckets have been tested in several sites.
The currently used impact crusher also partially
works like a selective crusher, but additional
HMS is needed on the surface.
Sizers or other types of crushers are needed
for generating the 0-15 mm oil shale fraction,
and 0-45 mm limestone fraction. Since fines are
difficult to handle both in the power plant and in
the oil generation process, the 0-5 mm fraction
should be minimal. The main problem in mines
is the high percentage of the 0-25 mm fine fraction, which amounts to 30% of the total production. However, there is a possibility to use it in
the power plant. If the separation process produces suitable material, the residue could be
used as backfilling material (Valgma, 2009).
Part of the analyses was focused towards optimizing the layout of rooms, pillars and workings. One of the options was to evaluate the
possibilities of shortwall extraction with roadheaders (Hungarian F2 road header and Russian
coal road header 4PP-3). This was based on the
hypothesis that roadheaders or continuous miners could be used for oil shale and phosphate
rock extraction (Fig. 4, Valgma et al., 2008a,b).
If this could be proved, the mechanical extraction could allow decreasing the size of pillars by
roughly 0.3 m from each currently blasted side
without decreasing the strength of the pillars
(Orru et al., 2013). In addition, further decreasing of the pillar side could be compensated with

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6th International Conference on Sustainable Development in the
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Figure 4: High-selective mining with continuous miner,
has not been tested for open cast.
side pressure by backfilled material (Zha et al.,
2011).
A similar method for stabilising pillar walls
could be by using cutting machines. In addition
to currently partially used horizontal cutters,
vertical cutters could be considered (Fig. 5).
2.3 Fill material tests
Industrial tests were performed with ash, aggregate and water mixture. A preliminary investigation has been started for selecting backfilling
technology in Estonian oil shale mines (Valgma
et al., 2012). Experiments in mines have been
performed (Väizene, 2009). The main methods
that have been tested were:
- dry casting of waste rock to the mined out
rooms and adding ash and water mixture,
- pumping wet mixture with piston pump to
the rooms.
Currently experiments have been continued
Figure 5: Cutting in layer A.
Figure 6: Hydraulic backfilling in an oil shale mine.
by testing new ashes (new burning and heating
technologies) and waste rock aggregates. Industrial tests and laboratory tests have been performed (Fig. 6).
Road stabilization tests were performed in a
mined out area using a hydraulic backfilling
technology (piston pump, slurry, frill hole,
pumping tube). The drift was stabilised and the
mixture reached stability within 2 days.
Laboratory tests were performed with different ash mixtures (Fig. 7). Ash mixtures were
formed in the standard concrete moulds and
kept in different conditions. For simulating the
mining environment, a refrigerator was built
with temperature and humidity monitoring. For
holding low temperature (8 degrees Celsius) an
air conditioner and an air humidifier were used.
In addition water circulation with wet textile
was applied. The air conditioner together with
the wet textile and the air humidifier guaranteed
Figure 7: Tested mixtures.

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4. RESULTS AND DISCUSSION
Figure 8: Open cast scheme with backfilling.
8 degrees temperature and 90% humidity. The
refrigerators stored samples for different periods
of time. After each period the sample was tested
for uniaxial compressive strength. In addition
the sample was kept in water and leached water
was analysed.
In case of testing backfill material, the temperature, humidity and size of the sample play an
important role. Several samples, that have been
kept in standard conditions show good compressive strength. On the contrary, samples kept in
the mine environment showed less compressive
strength (Fig. 12). Tests conducted in the mine
actually showed better compressive strength results: 10 MPa. This could be related to the
3. POTENTIAL TECHNOLOGIES
Different schemes for potential technologies
have been proposed and evaluated. The backfilling space between the spoils or trenches could
be used for depositing ash and at the same time
for stabilising spoils. Stabilised spoils could influence the maximum possible overburden
thickness in the open cast mines (Fig. 8). Peat
and quaternary sediments could be mixed with
ash. Tests have shown that trees grow faster on
this mixture than without the sediments.
Open pit mines has reached old underground
workings and cross sections of the subsidence
occurrence have been studied (Fig. 9).
Several underground schemes have been
tested, including hydraulic, pneumatic and mechanical methods (Figs. 10-12).
Figure 9: Subsided roof in the opened mine.
Figure 10: Partial backfilling with waste rock.
Figure 11: Combined room and pillar mining with partial
backfilling with hardening material.
Figure 12: Mining with combined pillars.

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2,5
UCS, Mpa
2
1,5
3
2
1
1
0,5
0
0
10
20
30
40
Days after mixing
50
60
Figure 12: UCS of the mixtures.
warming effect of the large scale mixture which
can have a positive influence to the hardening
process. In addition mixture composition had
influence. Highest ash content resulted in lowest
compressive strength (Fig. 13).
The study should be continued with larger
scale tests with low ash content.
ACKNOWLEDGEMENTS
This research is related to the project MINNOVATION
mi.ttu.ee/min-novation;
ETF8123 “Backfilling and waste management
in Estonian oil shale industry” - mi.ttu.ee/
ETF8123; Energy Technology Program Sustainable and environmentally acceptable Oil
shale mining No. 3.2.0501.11-0025 - mi.ttu.ee/
etp and Doctoral School of Energy and Geotechnology II, interdisciplinary research group
“Sustainable mining” DAR8130/ 1.2.0401.090082 - mi.ttu.ee/doktorikool
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