Protection of thermal springs in a region of brown-coal opencast mining

Transcription

1 Future Groundwater Resources at Risk (Proceedings of the Helsinki Conference, June 1994). IAHS Publ. no. 222, Protection of thermal springs in a region of brown-coal opencast mining J. SKOREPOVÂ Institute of Geotechnics, Academy of Sciences of the Czech Republic, V Holesovickâch 41, Praha-8, Czech Republic Abstract The experimental modelling method is dealt with, which is used for prediction of deformations and possibilities of outflows occurring at the bottom of an open-pit mine during the excavation of brown coal. Thermal gas-bearing artesian water exerts an uplift pressure on the impervious subsoil of the coal seam and threatens the stability of the mine bottom. The breakthrough of the mine bottom would seriously affect the groundwater regime within the wide neighbourhood. Due to the fact that the mining is carried out within the protection zone of spa springs, these might be endangered by mining activities. INTRODUCTION The opencast mining of brown coal is taking place within the protection zones of natural thermal healing springs of the Karlovy Vary (Carlsbad) spa in an area with very complicated geological and hydrogeological conditions. There exists, in the basin, a wide horizon of thermal waters with confined groundwater level. Their delivery head exceeds highly the level of the excavated seam. The considerable transfer of the excavated materials affects not only the geological structure of the region, but also the regime of both the surface and subsurface waters. Complex geological, hydrogeological, geomechanical and geophysical research activities are being undertaken in this area. Geodetical measurements are being made on the open-pit bottom and at exploratory drifts, and observation and drainage wells have been established. A part of this research consists also in the evaluation of the stability and prediction of bottom deformations by mathematical and physical modelling methods. All these activities point to safeguarding progressively the natural healing springs against their possible exposure to dangers due to mining activities. CHARACTERISTICS OF THE BROWN-COAL BASIN To the palaeogenic sedimentation stage of the basin belongs the basal strata series of Stare Sed, composed of siltstones and sandstones, which form, together with crystalline rocks, the seat rock of the basin (Fig. 1). The sedimentation in the basin was affected by tectonic movements occurring during the entire sedimentation period. This tectonic activity made itself expressively felt on the geological structure of the basin. The oldest representative of the organogenic sedimentation (the coal seam Josef with thickness of 4-10 m) is deposited on the morphologically considerably articulated ground. After deposition of the Josef seam, the intensive volcanic activity on border

2 98 J. Skorepovd 500(m) above sea level (m) [ overlying clays ^% tuffitic impermeable layers ly;"."» internal dump p v ) permeable bedrock L LJ,J of krysfalline rock jg^jjgjl brown coal seam Fig. 1 Vertical section of the coal basin. repressed the formation of the coal seam. Volcanodetritic layers formed by tuffitic claystones, tuffitic sandstones and tuffs with entirely clayey pyroclastic portion started to settle in the basin. Volcanodetritic layers are practically impervious. Over this strata series with thickness of m, the zone of the seam Antom'n developed, attaining the thickness of m. The most recent representative of the Tertiary of basin is a strata series formed by cypress clays and claystones. The thickness of these sediments attains up to 150m(Fraus, 1984). The coal basin graben extends between two important faults. Both of them are broad fault zones with numerous fault branches and offshoots. Numerous dislocation are situated in the space of basin, which weaken the strength of tuffitic layers. There exists, in the basin, an extensive horizon of gas-bearing water with confined groundwater level, on a base of Tertiary basin fill, bound to the basal strata series of Stare Sedlo and the upper part of crystalline rocks. The water temperature varies within C, mineralization 6-8 g 1" 1, and its chemical composition is close to Carlsbad Spa springs. As far as the gas content is concerned, these artesian waters are unsaturated, containing up to 4500 mg l" 1 of carbon dioxide and up to 135 mg 1"! of nitrogen. The saturation pressure varies within MPa. Although the connection between these artesian waters and the Carlsbad springs has never been proved, there exist facts, which support this hypothesis. A case at the turn of this century may be quoted: there occurred outflows of gas-bearing thermal waters in the southern part of the basin, where subsurface mining of the coal seam has been carried out. After a certain time, the yield of the spa springs decreased. After flooding the lower horizons of the mine workings, the original yield of the springs has been restored. The most complicated situation occurs, above all, in the northern part of the basin. Owing to the dip of tuffitic layers and their varying thickness, the mining of the coal seam westwards creates situation, when the open-pit bottom gets well deep below the confined thermal groundwater level. The uncovered overpressure of artesian waters attains, at the mine bottom, up to 0.6 MPa (Fig. 2). In addition to that, the space of the mine field is permeated by numerous dislocations, which disrupt the continuity of tuffitic layers. The most unfavourable situation is encountered at intersections of tectonic lines. Due to these very complex geological condition, it is necessary to predict, for a safe excavation and for the safety of spa springs,the deformation of tuffitic layers after their

3 Protection of thermal springs in a brown-coal opencast mining region 99 Fig. 2 Situation of geological dislocations, isolines of uncovered overpressure of artesian waters and the extraction progress. exposition on the mine bottom, and control the deformation movements taking place on dangerously situated tectonic lines. The possibility of local breakage and failure of the water tightness of tectonic faults due to their separation should be also evaluated. MODELLING IN GEOTECHNICS Modelling in geotechnics enables the mechanisms of geomechanical phenomena to be investigated, the stress state changes to be predicted together with their manifestations during the progress of mining as well as the effect of preventive measures to be evaluated. In modelling, the rockmass is substituted by a model, which must satisfy all determining properties of the real system. Actually, there exist several numerical methods, which are successfully used to that purpose assuming, however, anisotropic medium. They are not always successful in cases of geologically complicate conditions, i.e. nonhomogeneous and anisotropic medium, complex geometry of the area, etc. We are therefore witnessing, at the actual time, a world-wide revival of physical experimental methods. The advantage of physical models consists, above all,in the complexity of study of the modelled problem, especially if 3-D models are considered, which represent the investigated area reduced in adequate scale. When formulating the boundary modelling conditions, the laws of geometrical and physical similarity (Stillborg, 1979) are applied. For possibly perfect imitation of processes taking place in the rock mass, the rock medium is substituted by equivalent materials, whose mechanical and deformational properties correspond, according to similarity laws, with properties of rocks.

4 100 J. Skofepovâ PROBLEM SOLUTION BY THE METHOD OF PHYSICAL MODELLING Global 3-D physical models For a long-term prediction of open-pit bottom deformations occurring due to extraction of the coal seam, three-dimensional physical models (Fig. 3) are used. The stripping, coal-seam mining and filling of the internal spoil banks are simulated, on these models, according to scheduled mining operations. Vertical deformations of the seat rock of the coal seam and eventual deformations of the entire model surface are measured after each change of the deformational conditions. The deformation of the seat rock of the coal seam is measured trigonometrically. The method of analytical stereophotogrammetry (Vencovskij, 1989) is most frequently used for the determination of the surface deformations of 3-D models. Fig. 3 Photographs of the model surface for two period of coal extraction. Detail evaluation of deformation behaviour on the open-pit bottom Detail models of the planar type (Fig. 4) are used for the evaluation of the stability of sites, where numerous dislocations run through the mine field and where local failures of the water tightness of tectonic faults may occur due to their separation. On these models, not only surface deformations of the tuffitic strata series in close neighbourhood of geological faults, but also the stress redistribution within the model body are followed up. Methods of geodesy are used for the measurement of surface deformations of tuffitic layers. Measuring marks are installed, to this purpose, on the models surface. After each change of strain conditions within the model, the position changes of these measuring marks are determined by the forward intersection method. Stress state alterations within the model body are measured by tensiometric measurements. Special semiconductor sensors, situated on exposed sites of the model during its construction (Fig. 5), are used for these measurements.

5 Protection of thermal springs in a brown-coal opencast mining region 101 Fig. 5 Semiconductor sensors. RESULTS OF MODEL SOLUTIONS AND THEIR APPLICATION TO THE PROTECTION OF THERMAL SPRINGS During the time that the method of physical modelling has been used for deformation predictions of tuffitic intermediate beds, the method proved well to be functional. A very good agreement has been found between deformation values, predicted on models, and those measured later on the open-pit bottom and in exploration drifts (Skorepovâ, 1992). A success was met also in predetermination of sites, where pronounced increases of

6 102 J. Skofepovd vertical deformations combined with eventual occurrence of groundwater outflows could be expected. This holds true, for example, for the model constructed in 1985, on which the assumed deformations for the time period were determined (Skofepovâ, 1987). Basing upon this model experiment, the site coal be located, where unacceptable deformations were discovered after removal of tuffitic layers on the mine bottom. Conclusions from this model were used, together with other research results on this open-pit mine, to apply first protective measures. These consisted, above all, in the modification of the schedule of mining operations (restriction of the width of mine bottom, increased angle of the interval spoil dump) and, since 1989, the confined groundwater level was locally reduced (by 17 m) by its pumping off. A subsequent model experiment has already been carried out at the confined water level within the model, corresponding the reduced water level in the mine. Deformations for the years were predicted on this model. In spite of the reduced confined water level, uncovered overpressure on the open-pit mine amounted to <0.35 MPa (Fig. 6). At sites with maximum uncovered overpressure on the modelled mine bottom, the vertical deformations exceeded 3 m (converted from the model to real pit bottom scale). Therefore, an additional reduction of the delivery head of water, which would correspond of practical water level in the mine by further 10 or 20 m, respectively, has been carried out on the model. Model measurements also proved the possibility of water outflows along the tectoniclines, due to fault separation by non-uniform deformations before and after the fault lines. These sites were therefore subjected to further detailed studies (Skofepovâ, 1983). Several model experiments were carried out, from which it resulted that the water outflows cannot be prevented by the suggested reduction of the confined water level and that it would be adequate to protect these sites by leaving coal SH"""^ l^r^h geological dislocations Fig. 6 Situation of isolines of uncovered overpressure of artesian waters for the reduced confined level.

7 Protection of thermal springs in a brown-coal opencast mining region 103 pillars, which would reduce locally, by their own weight, the uncovered overpressure of artesian waters, thus preventing the formation of communication paths for water outflows. Actually, as these dangerous sites are successively exposed, the suggested protective measures are being realized. Further research by laboratory model experiments and directly in the mine, is in due progress. CONCLUSIONS The scope of these research activities is to enable the coal seam to be safely mined and the natural healing springs to be maximally protected. Deformation processes connected with the mining activities, depend on many factors. They depend not only on the value of uncovered overpressure of artesian water but, to the same degree, on the deposition depth of the seam, its thickness, form and hydraulic properties of underlying tuffitic layers and, last but not least, also on the geometry of geological dislocations. It could be proved that physical models respect very well the effect of these factors. That is the reason, why suggestions of safety measures, resulting from the model experiments, are taken in account seriously during the realization of an effective protection of spa springs. REFERENCES Fraus, F. (1984) Têzba uhli a ochrana karlovarskijch termâlnîch pramenû ve vychodnî casti sokolovské pânve. Uhli 3, Rocnik32, Skofepovâ, J. (1987) Deformation of an open-pit bottom subjected to upward stressing by groundwater. In: Groundwater Effects in Geotechn. Engineering (Proc. 9th Euro. Conf., 1987), Balkema, Rotterdam. Skofepovâ, J. (1992) Analysis of deformations of the bottom of an open-pit mine predicted by physical models. In: Proc. 30th Conf. on Experimental Stress Analysis (Prague 1992), Skofepovâ, J. (1993) Study of the conditions of water tightness and creation of local failures on tectonic lines. Proc. 10th Danubia-Adria Symp. (Mêffn). Stillborg,B., Stephanson, O. & Swan, G. (1979) Tree dimensional physical model technology applicable to the scaling of underground structures. In: Proc. 4th Cong, of the Int. Soc. for Rock Mech., Balkema, Rotterdam. Vencovskij, M. (1989) Methods of measurement and graphic representation of physically rock mass model deformation (in Czech). Dr Se. Thesis, Prague.