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Oil Shale, 2008, Vol. 25, No. 2 Special, pp. 145–152 ISSN 0208-189X
doi: 10.3176/oil.2008.2S.06 © 2008 Estonian Academy Publishers
GEOLOGICAL ASPECTS OF RISK MANAGEMENT
IN OIL SHALE MINING
S. SABANOV, T. TOHVER, E. VÄLI, O. NIKITIN,
J.-R. PASTARUS
*
Department of Mining, Tallinn University of Technology
5, Ehitajate Rd., 19086 Tallinn, Estonia
The paper deals with risk management problems in Estonian oil shale mines.
Investigations are focused on application of the method to determination of
the quality of geological data. Various factors relevant to mining technology
in Estonian oil shale deposit have been determined. For risk estimation, the
empirical and judgmental approaches and the event tree were used. They
allow determining the probability of the occurrence of geological features
and its influence on the mining process. Analysis of obtained results showed
that it is necessary to elaborate special methods for determination of the geo-
logical conditions in the mining area. The obtained information affords
specialists to improve the quality of geological information and consequently
the mine work efficiency. The analysis shows that the used method is applic-
able in conditions of Estonian oil shale industry. The results of the investiga-
tion are of particular interest for practical purposes.
Introduction
In Estonia the most important mineral resource is oil shale. Oil shale
industry of Estonia provides a significant contribution to the country’s
economy. Underground and surface mining in the Estonian oil shale deposit
causes a large number of technical, economical, geological, ecological and
juridical problems, which cannot be solved on conventional theoretical basis.
Risk management is a most powerful tool to solve complicated mining
problems. The data, which have been accumulating in the last 40–50 years,
concern the experience obtained by oil shale excavating and provide a good
basis for investigations.
This study addresses risks associated with stability of the immediate roof
in the mines Estonia and Viru, depending mostly on the geological feature.
The primary interest of this study concerns evaluating the usability of the
*
Corresponding author: e-mail pastarus@cc.ttu.ee
S. Sabanov et al.
146
method and estimating the probability of failure without a detailed assess-
ment of its consequences. The study is based on the world’s and Estonian
experience. As an example of application, the risk analysis of Estonian oil
shale mines has been conducted.
Risk management involves making a judgment about taking a risk, and
all parties must recognize the possibility of adverse consequences which
might materialize [1–4]. Therefore, owners will be required to deal effectively
with the consequences of a failure event. Prevention of the hazardous situa-
tion is more moral, ethical and economic than facing the adverse con-
sequences. Having received the information, the management of a mine or
open cast can come to adequate political and strategic decisions. The mitiga-
tion process will reduce the adverse consequences [1, 5]. Investigations have
shown that the share of risk relevant to geological data in mining and
environmental protection is very large. It is known that rock mass properties
vary and depend on its location. It is impossible to determine exactly all the
geological features. The reliability of geological data determines the
efficiency and safety of mining and environmental impact. It includes bed-
ding, underground and surface water conditions, existence of karst, joint
systems, etc.
Some of the various geological factors relevant to Estonian oil shale
mines have been determined. For risk estimation, the judgmental and
empirical approaches and event tree have been used. The risk management
method allows predicting the probability of failure of the immediate roof in
the location of interest. Getting the information allows specialists to mitigate
negative influence of risks on the excavation process and environment.
Analysis showed that the risk management method used is applicable to
Estonian oil shale mines, which are of particular interest for practical
purposes.
Theoretical background
In the world, risk management methods are used in different branches of
industry and for many different technical systems. In Estonia, including
Eesti Põlevkivi Ltd, risk management methods are focused on health safety
problems. There is less information about the application of risk manage-
ment methods to geological conditions and technological processes. In spite
of the varied terminology, there is general agreement on the basic require-
ments [1, 3, 5, 6]. The terminology and risk management/assessment
methodology used in the frame of this project are presented below.
Risk can be defined as the likelihood or expected frequency of a specified
adverse consequences [1, 4]. Risk management is the systematic application
of management policies, procedures and practices to the task of identifying,
analyzing, assessing, treating and monitoring risk [1, 3, 4]. Having obtained
the risk information, a decision-maker must come to a decision.
Geological Aspects of Risk Management in Oil Shale Mining
147
Risk assessment is the process of deciding whether existing risks are
tolerable [1, 3, 4, 7–10]. It involves making judgments about taking the risk
(whether the object or process is assessed as safe enough). Risk assessment
incorporates the risk analysis and risk evaluation phases. Schematically the
process of risk management/assessment is presented in Fig. 1.
Risk analysis is the process of determining what can go wrong, why and
how. It entails the assignment of probabilities to the events. This is one of
the most difficult tasks of the entire process. Probability estimation depends
on the type and quality of the available data: analytical, empirical or judg-
mental approaches [1, 3, 7]. Component event probabilities may be assessed
using a subjective degree-of-belief approach [2, 4].
Attaining an exact value of probability for technical systems and pro-
cesses is not a realistic expectation. Tools that are often used to help in risk
estimation are fault/event trees [1, 4, 11].
Risk evaluation is the process of examining and judging the significance
of risk. It is based on the available information and the associated social,
environmental and economic consequences.
Risk acceptance is an informed decision to accept the likelihood and the
consequences of a particular risk. In some countries, there is a certain risk
level that is defined as the limit of unacceptable risk. For failure events with
no potential fatalities or irreparable damage to the environment, the target
failure probability may be decided exclusively basing on economic condi-
Scope and risk analysis
plan definition
Risk identification
Risk estimation
Safety mana
g
ement
principles
Guidance on ris
k
-based
decisions
Is the ob
j
ect or process assessed as
safe enou
g
h?
How safe is
the ob
j
ect or process ?
How safe should
the ob
j
ect or process be?
Risk miti
g
ation
RISK ANALYSIS RISK EVALUATION
NO
Y
ES
Risk acceptance
Fig. 1. Risk management/assessment process.
S. Sabanov et al.
148
tions and corresponding risk analysis. A target level of 10
–3
to 10
–2
for life-
time risk of the object or process may be a reasonable criterion [1, 2].
Risk mitigation is a selective application of appropriate techniques and
management principles to reduce either likelihood of an occurrence or its
consequences, or both [1, 3–5, 12].
Contributing geological factors
Geological and technological aspects of underground mining can influence
the efficiency of mine works and environment protection. The share of geo-
logical information in these processes is large enough. Some of various
factors which are relevant to Estonian oil shale mines and open casts are
presented in Fig. 2.
In the first stage of investigations, the contributing factors are divided
into two groups: geological and technological factors. Main technological
aspects influencing the stability of a mining block (block of rooms at under-
ground mining) concern the quality of mining and blasting works. Feedback
control and adaptive design methods guarantee the stability of a mining
block [13].
The influence of geological parameters and features on the mining
efficiency and environment protection is significant. Stability of an
immediate roof in face is determined by geological features. The presence or
vicinity of karst, joints and fissures, and aquifer in the overburden rocks in
Fig. 2. Factors contributing to the mining process.
Geological Aspects of Risk Management in Oil Shale Mining
149
face of the mines Estonia and Viru determines the stability of the immediate
roof. These factors, in general, have been determined for the Estonia oil
shale deposit and are presented in a map. A great deal of the karst and joints
inside a mining block area is undetermined, as they are practically impossible
to determine. Risk management/assessment methods allow solving these
complicated problems.
Seismic activity in Estonia is at such a low level, practically negligible,
that it has been considered in this study only to a limited extent.
Immediate roof collapse risk in face, Estonia mine
In the Estonia mine, mining blocks are in different geological conditions. In
the southern area the geological conditions are complicated due to the
presence of karst, joints, aquifer in the overburden rocks. They influence the
stability of the immediate roof. The roof fall risk increases. Figure 3 presents
the event tree for immediate roof stability.
Investigation of in situ conditions has shown that immediate roof stability
depends on two factors: mine work quality (influence 70%) and geological
conditions (influence 30%). Investigations have shown that owing to high
quality of mine works the probability of roof stability is 90%.
In the Estonia mine the room height is 2.8 m. In normal geological condi-
tions it guarantees the stability of the immediate roof in face. Room height of
2.8 m in complicated geological conditions does not guarantee the stability
of the immediate roof. In this case the room height must be increased up to
3.8 m. Investigations showed that the probability of immediate roof collapse
in the Estonia mine is 5% (Fig. 3). It is evident that the estimated probability
exceeds the limit (10
–3
–10
–2
). On the other hand, it is known that determina-
Fig. 3. Event tree for immediate roof stability in face, Estonia mine.
S. Sabanov et al.
150
tion of the geological features inside a mining block is practically im-
possible. It is necessary to elaborate special methods to determine a geo-
logical feature inside a mining block. This complicated problem demands
additional investigations.
Immediate roof collapse risk in face, Viru mine
The geological structure and features of the immediate roof in stop determines
the number and sizes of potential dangerous blocks. Prediction of these factors
is practically impossible. Risk management methods allow solution of this
problem basing on the experimental data of in situ conditions.
The investigation was conducted at the Viru mine in the mining block
No. 184 (right wing). 33 collapses of the immediate roof in stop were
registered. Caving size ranged from 0.001 m
2
(0.1 by 0.1 m) to 6.0 m
2
(3.0
by 2.0 m). The height of the collapses in the roof varied from 0.05 m to 3 m.
Stability of the immediate roof in stop has been controlled after blasting
works. The visible potentially dangerous roof blocks were removed
immediately (enforced collapse). Long-term mining experience has shown
the efficiency of this method. After that the spontaneous collapses may
appear in stop, caused by rheological processes.
For probability estimation an empirical approach was used. All the
statistical calculations were based on the actual data of in situ conditions.
The event tree is presented in Fig. 4.
Analysis of the event tree showed that the probability of spontaneous
collapses, which appear during mine works, is negligible (0.015%). The
probability of enforced collapses remains below 0.5%. Such collapses are
not dangerous because during face inspection the potential dangerous blocks
will be removed.
In summary, collapses in stop are not dangerous for workers and equip-
ment. The probability of the collapses is below the limit – 10
–3
–10
–2
.
Fig. 4. Event tree for immediate roof stability in face, Viru mine.
Geological Aspects of Risk Management in Oil Shale Mining
151
Discussion
Risk management/assessment methods allow determining the probability of
the immediate roof collapse using the event tree. Having got this informa-
tion, the mine management may decide about taking risks: are they accept-
able or not; are they dangerous for workers and/or for the environment? If
this risk is not acceptable, the mine management must preview the risk
mitigations methods: use of appropriate techniques or/and management
principles to reduce either likelihood of an occurrence or its consequences,
or both. In the Estonia mine the room height of 3.8 m reduces the probability
of an immediate roof collapse and its negative consequences, being the only
true solution.
On the other hand, information about the probability of an immediate
roof collapse offers the scientists objects for future investigations.
Conclusions
As a result of this study, the following conclusions and recommendations
can be made:
1. Geological and technological factors relevant to immediate roof
stability have been determined. The share of geological factors, such
as karst, joints, fissures, aquifer, etc. in this process is large.
2. Geological risks by underground mining are estimated by empirical
and judgmental approaches. In the investigations the event tree was
used.
3. The influence of the quality of geological data on the mining process
is significant. It is necessary to elaborate special methods to determine
the geological features inside a mining block.
4. The risk management method is a powerful tool to solve complicated
mining problems. The analysis showed that the method is applicable
in conditions of Estonia oil shale deposit. The results of the investi-
gation are of particular interest for practical purposes.
Acknowledgements
Estonian Science Foundation (Grant No. 6558, “Concept and methods of
risk management in mining”) supported the research. It is also a part of the
project No SF0140093s08 of the Estonian Ministry of Education and
Research.
S. Sabanov et al.
152
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Received September 20, 2007
