EXPERIMENTAL AND MATHEMATICAL MODELLING OF PROGNOSES OF GEOHYDRAULIC CHANGES CAUSED BY COAL EXCAVATION

 

JIRINA TRCKOVÁ

 

Institute of Rock Structure and mechanics AS CR

V Holesovickach 41, 182 09 PRAHA 8, Czech Republic

Phone: +4202/3121748 Fax: +4202/6880649 E-mail:trckova@irsm.cas.cz

 

 

ABSTRACT

Open-pit brown-coal excavation is carried out in a coal basin with a large confined aquifer of thermal gas-bearing water. The methods of physical and mathematical modelling are utilized for assessment of open-pit bottom stability against its possible break-through owing to overpressure of this water and for prediction of geohydraulic changes in consequence of excavation. The results of model solutions are compared with the levelling measurements. Physical models proved the ability to predict deformation processes in the open-pit with sufficient accuracy. The results of interactive mathematical modelling brought new findings on stress state and deformation changes and on the possibility of rise of hydraulic failure of the open-pit bottom. The model solution have become a valuable source of information which are used in the designs of preventive measures designed to ensure geotechnical security of the open-pit excavation of the coal seam and the protection of thermal gas-bearing waters.

 

Keywords:Thermal water, confined level, open-pit excavation, physical and mathematical models.

 

INTRODUCTION

The Sokolov brown-coal basin is situated in the vicinity of the Karlovy Vary spa resort. In the recent years a significant development of open-pit excavation has been achieved especially in the central part of the basin. Considerable transfer of excavated masses affects changes into the geological structure of the territory and influences the regime of both surface and underground water. In the basin exists a large confined aquifer of artesian thermal gas-bearing water. Even though the hydrogeological continuity between Karlovy Vary thermal-spring structure and the thermal water of the Sokolov brown-coal basin has not been proved yet but not excluded either, it is impossible to neglect the possible danger for Karlovy Vary springs in consequence of mining activities. The illustrative vertical section through the basin is in Fig. 1.

 

Fig.1 Vertical section of the coal basin.

 

To ensure the geotechnical security of the open-pit excavation in the coal seam and the protection of thermal water wide research activities have been carried out in the open-pit mines. Most of these activities are directed towards the ensurance of open-pit bottoms against their possible failure owing to the overpressure of gas-bearing water of the basal aquifer. This failure would not only make the excavation impossible, but would also cause a considerable disturbance of hydrological conditions in the basin and in its surrounding.

On the open-pit bottoms and in drainage adits periodical levelling measurements have been carried out for more than 20 years. These measurements have provided information about the behaviour of stress removed underlying rock of the excavated coal seam during the mining process and about deformations caused by the overpressure of thermal gas-bearing water of the basal aquifer.

Observation wells have been set up in the open-pit mines and in their surrounding. In these wells the pressure changes of artesian water level (Pazdera 1989 - 1995) have been regularly observed. The drainage wells have been used for the local decrease of the confined level of this water under the open-pit mine bottom. Geotechnical measurements have been carried out in the open-pit mine, too.

For many years the modelling methods, the method of 3-D physical model and mathematical modelling, have been utilized for prediction of deformation of the open-pit mine bottom caused by the uncovered overpressure of thermal water and for assessment of artesian confining layer stability. For prognostic abilities of both methods we focused on the period of 1985 - 1995, when both ways of modelling were simultaneously utilized and their results were reciprocally compared.

 

PROGNOSES ACHIEVED FROM PHYSICAL MODEL EXPERIMENTS

In 1985 a 3-D physical model of open-pit mine Jirí was constructed (Fig.2.). In this model real excavation procedures which were carried out in 1980-1985 and planned for 1985-1990 were modelled (Skorepová 1985). In the points of the modelled equivalent of the drainage adit the deformations of the coal seam underlying bed were gradually measured after every change of situation in the model. The whole experiment was carried out at the height of the free water surface in the model, corresponding with the confined level in the open-pit mine. The maximum vertical displacement of the underlying rock was recorded in the model in measuring points situated into the place where in 1989-1990 the open-pit bottom was situated. In these places the bottom of the open-pit mine started to be stressed by the uncovered overpressure of artesian waters, which reached 0,3 MPa. For situations corresponding with the excavation state in 1980-1984 the deformations derived in the model were compared with the levelling measurement carried out in the drainage adit situated in front of the mine field. Despite different accuracy of both methods (mean error of levelling measurements is in milimetres, mean error of the deformation determined in the model reaches as many as 5 cm), the values of measured deformation for this period showed very good agreement (Fig.3.).

 

 

Fig.2 Situation of mine bottoms, geological dislocations and measuring points in the equivalent of drainage adit.

 

 

 

Fig. 3 Comparison of values of vertical deformations determined for various stages of model experiment and measured by levelling (model 1985).

 

In 1987 the prediction of the open-pit bottom deformation for the period of 1990-1995 were solved on physical model (Skorepová 1988). In these years the open-pit bottom arrived to the most endangered places. The uncovered overpressure of artesian water in this place reached as many as 0,6 MPa. Significant geological faults passed through the mine field. In the beginning of the experiment the free water surface in the model corresponded with the original unlowered confined level of the open-pit water. In the model there were large deformations owing to a higher acting overpressure and also to the existence of geological faults. That is why before every simulation of excavation activities on the model the height of the water level was lowered and then again gradually increased. The model did not sufficiently simulate the process which would go on in the open-pit in the case of lowering of the water confined level. Nevertheless the results of the model experiment have proved that a safe excavation in this area cannot be realized without lowering the confined level. On the base of the deformation values measured on the model it was recommended to bring down the level of artesian water for the year 1990 to 350m above the sea level (i.e. 17m lower in comparison with the initial level), in the year 1993 to 337m above the sea level.

In 1991 the mining organization presented its requirement to construct a model for lowered level of artesian water to 350m above the sea level. The new model experiment was designed (Skorepová 1991). The excavating process in the years 1984-1990 with following excavation according to projected process was simulated. The deformations determined on the model for planned advances till the end of 1992 did not exceed the value which would endanger the stability of the open-pit bottom.

In the next model the excavation process for the years 1989-1995 (Skorepová 1992) was simulated with respect to the lowered confined level of the thermal water to 350m above the sea level. After the simulated excavation on the projected state in the year 1995 significant deformations appeared on the open-pit bottom, especially in the surrounding of modelled faults. The confined level was further lowered to 340m, in fact to 330m above the sea level. Good agreement of deformations measured in the model and those determined by periodical levelling measurements was achieved again. By means of this, the assumption of the real prognosis for 1992-1995 was verified. On the basis of model test before reaching the most critical places by the open-pit bottom (i.e. places with the lowest location of the coal seam in the surrounding of faults) it was recommended to carry out further local lowering of the free water level (under 330m above the sea level). Larger protective pillars to leave in the surrounding of the faults to prevent not only large deformations, but also the possibility of local breakage and water outflows in consequence of the fault separation.

 

MATHEMATICAL MODELLING OF THE OPEN-PIT BOTTOM BEHAVIOUR

In the year 1985 the stability of the open-pit mine Jirí was solved by the finite elements method for the excavation process realized in the years 1981-1983 and planned for the years 1984-1991 (Dolezalová et al. 1985). Two alternatives were carried out. The first variant with the coefficient of the lateral pressure K_o=0,33 and with a more flexible contact between the vulcanodetritical layers and the basal strata and the second variant for K_o=0,9 and with a more rigid contact. Both alternatives were calculated for the initial level of artesian water and for 20m and 40m lower. The calculations showed that the danger of the bottom breaking first of all depends on the used value K_o. For K_o=0,33 growth of the stress and deformation appeared after the year 1985. For K_o=0,9 the open-pit bottom was stable during the all modelled period without the need of lowering the confined level. On the basis of comparison of the open-pit bottom deformations determined in the model with the levelling measurement in the open-pit (Fig.4.) the probable value of the lateral pressure coefficient K_o=0,55 was estimated.

 

 

Fig. 4 Comparison of results of levelling with FEM calculation.

 

In the next model for the excavation process in the years 1990-1995 (Dolezalová et al. 1989) the results of the first approaching calculations were taken into consideration. Partial improvement of the deformational and strength characteristics, modulus of deformation, the normal stiffness of the contact between the tuffitic layer and the underlying bed, was carried out. The generated coefficient of lateral pressures was K₀=0,5. The initial state in 1990 was modelled for the lowered water level to 350m above the sea level. On the basis of the model results it was observed that the excavation process until 1995 can be realized without the formation of larger tension zones and failure zones in the underlying bed of the excavated coal seam with the confined level of artesian waters 350m above the sea level. The model did not shown any impact of the faults on the underlying rock deformation. This model was supplemented by a parametrical study which confirmed the possibility of excavation in 1992 for lowered confined level of water to 360m above the sea level (Dolezalová 1990).

In 1991 further approching calculations were carried out, backdate for the years 1988 and 1989, for the initial state of K_o=0,425. These calculations showed the significance of the faults for the tuffitic layer deformation and the necessity of their modelling (Dolezalová et al. 1991a). The calculation was relized for several alternatives in dependence on the changing stiffness of the contact with the underlying bed, on new specification of the rock mass properties and in the last alternative even on lowering the water level to 355m above the sea level. This was the first case when the intensive deformation of the open-pit bottom near tectonic faults was successfully modelled. In the same year the approaching calculations for the open-pit state in March 1991 (Dolezalová et al. 1991b) were carried out. They were carried out on the basis of the measured vertical movements of stabilized points in the drainage adits and on the open-pit bottom. The results of models were taken into consideration, too. The K_o was considered equal to 0,9 for initial state. The fault zones were modelled by means of contact elements. Contact elements along the confining layer were also used for modelling of the mechanical behaviour of the fault system in the basal aquifer. The constitutive model was designed as non-linear and track-dependent. Altogether 9 alternatives were carried out (the changes: K_o, compaction parameters, the shear influence, coefficients for the residual modulus of failure and contacts in the disturbance zone). For the coefficient K_o=0,82 the results achieved in the model were comparable with the measurement in the open-pit mine. The open-pit bottom remained undisturbed, maximum deformation equal to 30-50 cm were related only to a fault block separated by two faults. In the alternative with K_o=0,425 the deformations on the bottom reached 60-130 cm and the open-pit bottom was disturbed including the break-through by the pressure water.

 

CONCLUSION

Both methods of modelling the situations which are taking place in the open-pit mine in relation to the coal seam excavation provided valuable knowledge. It is possible to state that physical models proved the ability to predict with sufficient accuracy the deformation processes which appeared in the open-pit mine in the years which were the subject of the model experiments. The failure possibility of the tuffitic underlying rock impermeability in the places of tectonic faults in consequence of the deformation recorded on the points situated into these places was proved. During last years in the open-pit mine there were found several outflows of thermal water in the area of the faults. Protective measures, first of all the necessity of considerable lowering of the confined level of artesian water, which had been suggested on the base of these experiments, was consequently applied on the open-pit mine.

The results of interactive mathematical modelling did not have the character of prognosis. They gave valuable geotechnical knowledge of the stress state and deformation changes and the possible rise of hydraulic fracturing of the open-pit bottom in consequence of the disturbance of the impermeability of the tuffitic underlying rocks. At present time new conception of the solution have been utilized. This solution provides information not only about the changes of stress state and deformation, but also about hydraulic conditions with the prediction possibility of the rise of spontaneous outflows with incontrollable leakages of thermal water in the vicinity of the boreholes and tectonic faults where big deformations are caused by the excavation.

The model solutions became a valuable source of information which are used in the proposals of preventive measures carried out in the open-pit. The preventive measures must ensure not only safe mining in the coal seam, but first of all to prevent the inadmissible changes of hydrogeological conditions in the area, which could endanger the thermal springs in Karlovy Vary.

 

ACKNOWLEDGEMENTS

This work was funded by the Grant Agency of the Czech Republic (Grant No. 205/97/0783 "Impacts of brown-coal mining on the water-bearing system of Sokolov coal basin with aspects of Karlovy Vary thermal springs protection"). The author gratefully acknowledges for this support.

 

REFERENCES

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