for its spas and artesian thermal springs. A pressurized aquifer underlies the mine. The
mining advance is limited by the hazard of potential hydraulic fracturing and resultant flooding of the mine, as well as by
serious changes to the hydrogeological conditions of the whole region. To prevent this hazard, the aquifer pressure has
been reduced by drainage wells. Between 1976 and the present time, estimates were made of the feasibility of the mining
advance at a minimum pressure head reduction, i.e. for the minimum environmental impact. An interactive procedure has
been applied using numerical models calibrated according to field measurements, and a monitoring system updated
according to the numerical models. The paper describes the predicted and observed performance of the mine in safe and
critical conditions, the estimated failure mechanisms and the hydraulic fracturing incidents that have occurred, as well as
different approaches used to assess the hydraulic fracturing hazard in various mining conditions.

KEYWORDS: Open-pit coalmine, pressurized aquifer, hydraulic fracturing hazard, safety assessment, interactive use of
monitoring and modeling, finite element application

INTRODUCTION
The paper describes the solution to the complex problems of environmental geotechnics, hydrogeology, and mining of the
Sokolovsko Coalfield. This coalfield, with its highly productive coal measure, is located near Karlovy Vary, a well-known
spa region of the Czech Republic famous for artesian thermal springs (Figure 1). A pressurized aquifer underlies the open-
pit coalmines, which limits mining advances because of the potential for hydraulic fracturing. Should hydraulic fracturing
occur, mines could flood, with serious consequences to the hydrogeological conditions of the whole region.

The paper focuses on the case history of the Jiri Mine (Figure 2), which is noted for a highly productive, good-quality coal
measure which is planned to be continuously operated up to 2030. Due to overburden removal (70-170 m), there is
substantial unloading of the 50 m to 60 m thick protective rock barrier composed of practically impervious tuffaceous
claystone (Figure 3). If the aquifer water pressure were to exceed the minor principal stress and tensile strength of the rock
barrier along the confining boundary, hydraulic fracturing could occur. To prevent the hydraulic fracturing hazard, the
aquifer pressure head has been reduced by drainage wells. Between 1976 and the present time, estimates were made of the
feasibility of the mining advance at a minimum pressure head reduction, i.e. for the minimum environmental impact. A
long-term interactive procedure involving numerical models calibrated according to field measurements and a monitoring
system updated according to the numerical models was applied. The procedure is an application of the observational
method that was introduced by Terzaghi and Peck (Peck, 1969). It provides authorities with reliable input for decision-
making, which results in a more effective control of mining advance.

The feasibility of open-pit mining in given conditions depends on a number of factors (depth of the mine bottom, thickness
of the rock barrier, occurrence and character of tectonic faults, and permeability of aquifer strata), which vary as the mine
advances. Accordingly, three characteristic periods of the mining advance are distinguished: a relatively safe period up to
1990, a critical period with maximum depth of mining up to 2000, and the current stage, which is influenced by a major
tectonic fault, the Grasset Fault.

During the initial period of relatively safe mining (1976-1990), the measurement results were back analyzed in order to
select a proper constitutive model and parameters for materials. Using a 2-D Finite Element Model (FEM), the possible
failure mode of the rock barrier was predicted. Due to large horizontal stresses, there is not much probability of the barrier
Dolezalova, M., Hladik, I., Zemanova, V. (2006), An open-pit coalmine surcharged by artesian water pressure,
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cracking by bending, as it has previously been assumed. Instead, opening of flow paths along the faults and seams reducing
the impermeability of the rock barrier could occur. Such critical situations occurred during the second period of mining
(1991-1999) when the mine reached the maximum depth. Occasional rupture of casings of wells crossing the rock barrier
took place, which caused temporary pressure drop in the aquifer. The reasons for these incidents and the related failure
mechanism were later identified by coupled 2-D UDEC (ITASCA, 1993) simulations, which matched well with the
observations.

Monitoring of the hydrogeological conditions of the rock barrier has been proposed and large coupled 3-D FEM models,
which conformed to the geometry of the mine and configuration of the faults, were performed. The 3-D models predicted
hydraulic fracturing hazard along a large tectonic fault (the Grasset Fault) being approached by the mine (Figure 4). Local
water pressure head reduction and other measures were recommended, but unfortunately, they were not fully implemented.
In 2000 the mine reached the Grasset Fault and due to local imbalance of the fluid pressure and rock pressure, some
accidents occurred. In order to cope with this new situation, a refined approach to the safety assessment was elaborated
using 2-D parametric studies to reflect the varying aquifer permeability and to evaluate the influence of this factor. To gain
a better insight into the problem, an accident bringing about leakage of thermal water from the aquifer was carefully
simulated. Based on this analysis and a series of new 2-D models, monitoring of the fault and protective measures ensuring
safe mining along the Grasset Fault were recommended. All these activities and the current situation at the mine are
outlined in the paper.


Figure 1
.
Layout of Jiri Mine with monitoring adits excavated along the bottom of coal seam.




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Figure 2. View of the Jiri Mine in 2004 (Vobornikova, 2004).




Figure 3. Cross section of Jiri Open-Pit Coalmine surcharged by artesian water pressure.





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Figure 4. View of the upper part of the Grasset Fault (Vobornikova, 2002).

BACKGROUND INFORMATION

Description of the Mine

The Jiri Open-Pit Coalmine has operated since 1960. Owing to a thick coal seam of good quality (sulphate content below
1%, thickness up to 50 m) the mine is one of the most economically attractive facilities of Sokolovsko Coalfield (Figure 5).
The layout of the mine and the measuring adits is shown in Figure 1. Mining advances in the eastwest direction with a rate
of about 100 m in a year. The excavation front is about 2 km and the width of the mine bottom, kept as small as possible,
does not exceed 100-150 m (Figure 3).

Figure 3 shows a typical cross section displaying the mining advance and the characteristic geological and hydrogeological
conditions around the mine. Crystalline rock, sandstone and a thin coal seam form the artesian water-bearing stratum. The
pressure aquifer is confined by tuffaceous claystone, which combined with a clayey coal stratum form an impervious rock
barrier of 50 m to 60 m thick. This stratum is overlain by 70 m - 170 m of overburden, which includes a basal 50 m thick
upper coal seam covered by stiff fissured clays.

Site Characterization

In situ tests of rock mass in an exploratory adit and laboratory tests of rock material were carried out (Hudek et al. 1986) to
investigate the deformation, strength characteristics, and other properties of the volcanic-detrital tuffaceous claystone (35 in
situ tests and 187 laboratory tests) and xylitol-detrital coal seams (17 in situ tests and 49 laboratory tests). Large-scale
samples (0.5 m x 0.5 m x 1.0 m) of the tuffaceous claystone and coal before and after testing are shown in Figures 6-9. The
main results of the tests are summarized in Table 1. The symbols denote unit weight , initial moduli at loading
in
and



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unloading E
unl
, Poissons ratio
in
, cohesion c, friction angle
, and tensile strength,
t
. The term Faults (1) refers to the
faults in the tuffaceous claystone and Faults (2) refers to those in the coal seam.

The hydraulic transmissivity and storativity of the upper part of the aquifer strata (granite, sandstone and the lower coal
seam) were determined by pumping tests (Pazdera and Vobornikova, 1992) and used to evaluate coefficients of
permeability. A summary of hydraulic properties of the aquifer strata, including crystalline rocks of varying permeability
(permeable granite and almost impermeable metamorphic granite) is given in Table 2.

According to the observational well records, the aquifer piezometric head before mining was at El. 367 m above the sea
level. To make the mining advance possible, since 1989