Interaction Matrix Method in Hydrogeological Analyses at Coal Mines

Transcription

1 International Symposium on Water Management and Hydraulic Engineering Ohrid/Macedonia, 1-5 September 2009 Paper: A76 Interaction Matrix Method in Hydrogeological Analyses at Coal Mines Milorad Jovanovski, Cvetanka Popovska, Katerina Donevska, Igor Peševski University Ss. Cyril and Methodius, Faculty of Civil Engineering, Skopje, Macedonia, Abstract. Groundwater has an important practical influence which must be analyzed in protection measures design in coal mining. For an instance, protection from groundwater aggressive chemical and physical action, hydrostatic and hydro-dynamical forces in stability and drainage analyses, groundwater inflow etc, have significant impact on designed structures and finally on the exploitation costs. Coal mine exploitation has also important ecological influences, which requires a specific approach during hydrogeological analyses. The estimation of input parameters in such analyses is a basic prerequisite for adequate modeling of the media. So, the objective of this paper is to present possibilities of so-called interaction matrix method, applied in hydrogeological modeling. Case study is the coal mine "Suvodol" near city of Bitola in Macedonia. The basic elements of developing of Hydrogeological Engineering Systems (HES) are also given. The paper can serve as an example for further analyses in this field for similar cases. Keywords: Coal mine, groundwater, hydrogeology, interaction matrix method, Hydrogeological Engineering System 1 Introduction It is well known that rational and efficient engineering design activities in coal mines is not possible without knowing in detail the groundwater conditions. It is especially emphasized in cases of complex hydrogeological conditions in the environment, as they are defined at some zones in the area of coal mine "Suvodol". Here, we also introduce an interaction matrix method in ways in which some of the hydrogeological parameters and variables can affect one another in processes of mechanical and hydrogeological interactions.

2 194 The method is presented within the wider context of an approach to integrate all the relevant information in applied hydrogeology, in a similar way as it is given in rock engineering design and construction. Namely, the methodology of developing of so-called Rock Engineering Systems (RES) is firstly introduced by Hudson [3]. The methodology is based on the demonstrating the links between necessary parameters for engineering analyses, especially the pre-construction and post-construction interactions. Here, throughout a case of coal mine "Suvodol" will be explained possible application in applied hydrogeology, demonstrating the possible way of developing of Hydrogeological Engineering Systems (HES). The HES concept is an approach which aims to identify the parameters relevant to a problem, and their possible interactions. The whole concept providing overall coherency in approaching engineering problems at coal mines, where the need to study the interactions has always been present. In fact, the interaction matrix method can be shortly explained as an intelligent method for solving complex problems, using appropriate conceptual, mathematical, numerical or mechanical models. This is very important, having in mind that the hydrogeological processes are usually time-dependent, which means the parameters are usually not constant- changeable in the time. The paper deals with the importance of correctly set and carried out investigations of the groundwater conditions, as well as the need of appreciation of the principle of equal grade and whole grade of investigation for the expected zone of interaction between the natural environment and the engineering activities. A framework for this concept is earlier given in [5]. 2 Analysed Area and Basic Geological and Hydro-Geological Conditions The surface coal mine Suvodol is placed in the southwestern part of the Republic of Macedonia. The coal mass and the unproductive layers have been formed with a process of sedimentation in lake conditions during upper Pliocene. Mainly, there are layers at the bottom of the coal (layers of silty sands), productive series of coal and coal-like clay, and layers on the upper part of coal of volcanic material (so-called trepel). This coal mine is a main source for thermal-electricity plans in the country, with a production of about , 00 tons per year. The mine area is investigated in several phases before and after opening of the mine. These investigations have been preformed to get knowledge of the entire geological, geotechnical and hydrogeological terrain characteristics. For an instance, mapping of the wider area, investigation drillings, installing of group piezometers, investigations of the chemical composition of groundwater s, field investigation of filtration coefficient, as well as laboratory analyses of physical and mechanical properties is applied. This paper presents only the part dealing with the groundwater conditions, while the other results are shown in appropriate reports [1], [2], [6], and [7] and on conferences [4], [5]. The obtained data from investigations were basis for preparing reliable

3 195 physical models (profiles, maps, geotechnical models) used in further stability, dewatering and drainage analyses [6], [7]. The main idea of the investigation methodology is to include evenly and wholly the space in which it is expected mutual influence of the engineering activities and the natural environment. Because of large scale of excavations, it should be mentioned that, a wide area of the environment can be affected, including the earth fill dam upstream of the zone of excavation, zone of active landslide bellow the dam etc., Fig. 3 and 4. Taking into consideration the fact that groundwater has the greatest influence on the stability of the terrain; this was a problem of primary interest in this coal mine. The most important data were obtained by installing of so-called group piezometers. More than 60 piezometers groups are installed in several investigation stages. For each group piezometer special borehole was made which is the most safety way to isolate every aquifer zone. An illustration of the geological and hydrogeological conditions, as well as the scheme of installed triple piezometer is shown in Fig. 1 and 2. Fig. 1. Scheme of triple piezometer installed in a borehole B 0/56: al-alluvial sediments; OHcoal-like clay; 1-interstratified aquifer zone; 2-aquifer zone at the bottom of clay Fig. 2. Detail of geological and hydrogeological composition at one zone of coal mine Suvodol : 1-Aquifer zone with free water level; 2-Interstratified aquifer zone under pressure; 3-Aquifer zone at the bottom of the coal layer; 4-Designed cut; Q-Quarterian silty sand layer; TR-trepel (aquiclude); C-coal; OH-coal-like clay (aquiclude); S-silty sands (aquifer); Gngneiss; I-free water table; II-piezometric level for the aquifer zone at the bottom of the coal layer; III-piezometric level for the interstratified aquifer zone under pressure

4 196 Some of the zones at the coal mine are with high lithological heterogeneity which is the reason why there is high heterogeneity of hydrogeological characteristics. So, by the help of installed piezometers, the presence of several physically separated aquifer zones is found. Fig. 3A shows a model of ground water movement for the aquifer zone with free water level. Fig. 3B shows a model of groundwater movement for so called interstratified aquifer zone under pressure, placed between two layers of coal-like clay. In fact, the Fig. 3 is given as an example that that separate aquifer zones can exist on a same part of the investigated area. Fig. 4 gives a model of groundwater movement for the aquifer zone at the bottom of the coal layer which can be found in the whole mine Suvodol. Here, the aquifer zone under artesian conditions with very high values of pressures is found. Fig. 3A. Engineering geological map of the NE part of coal mine, legend: - groundwater flow pats for the aquifer zones; 670 contour lines of groundwater level; al-alluvial sediments; dl-deluvial sediments; TR-trepel; Gn-gneiss; Fig 3B. Model of the groundwater movement for the interstratified aquifer zone under pressure at the NE part of the coal mine: 670 contour line of groundwater level for the aquifer zone under pressure; 2 contour line of equal artesian pressures (in bars); Cocolluvial material (active landslide) From the aspect of chemical interaction of groundwater, the chemical composition is very important, because it influenced the installed equipment (pumps) for dewatering (Table 1). It can be noticed that there are aggressive groundwater components with presence of gas (CO 2, Ra and others), which is important from ecological aspect and working conditions at the mine.

5 197 Table 1. Typical chemical composition for the aquifer zones Content of ions in mg/l Aquifer zone with free water table Interstratified aquifer zone under pressure Aquifer zone at the bottom of the coal layer ph Ca 2+ Mg 2+ Fe 2+ Cl - SO 4 2- HCO 3 - free CO 2 rest 6,8 20,1 12,5 0, ,5-15,5 6, ,8 2, ,5 70 6,8 5, ,3 4, ,6 549, ,3 Countour map of groundwater table for the SE part of coal mine 'SUVODOL'- BITOLA, R.Macedonia B-6 B-7 B-5 B-3 B-1 B-4 B-2 Temporary water collector B-9 B-8 31 Temporary water collector LEGEND SCALE m 620 Countour line of groundwater table under artesian conditions Zone of aquifer under artesian conditions Groundwater flowpats B-1 Borehole with instaled piezometer Extraction well Borehole with artesian effects Fig. 4. Model of the groundwater movement for the bottom-coal aquifer zone under artesian pressure

6 198 All geological, hydrogeological, chemical and other characteristics of the natural environment are the basis for forming of conceptual matrices and they are treated as possible impact factors that should be analyzed in the presented methodology bellow. 3 Presentation of Interaction Matrix Method in HES Methodology As it is mentioned earlier, the interaction matrix method is the basic device used in rock-engineering analyses. For a coal mine engineering projects, the most important step in the HES methodology is to establish the objectives of the project and the analysis. This statement is also valid for the engineering problems in general. The relevant 'state variables' must be chosen in a first place and they are placed along the leading diagonal of the interaction matrix. In some problems, these variables have to be more conceptual in nature. Sometimes, there may be enough information to use well-defined physical properties with definite units. Then, all the interactions are established so that the problem structure is developed. If the state variables are conceptual in nature, the off-diagonal interactions can be assessed using a semi-quantitative method of coding [3]. One example of conceptual matrix with four elements in a leading diagonal is given on Fig. 5. These factors are involved mutually, in 12 basic kinds of interaction. Fig. 5. Conceptual matrix of interaction between four basic factors which have influence on the stability of the coal mine: F1-Structural physical and mechanical characteristic of the sediments; F2-In situ stress; F3-Groundwater condition; F4-Characteristics of the engineering activities F1 group of factors is related to the sum of properties such as: unit weight, porosity, moisture content, strength properties, discontinuities, plasticity of clay, thickness of layers and so on. F2 group is related to the in situ stress. F3 group is related to the velocity of ground water movement, hydraulic gradients, hydrostatic and hydrody-

7 199 namic pressures, filtration characteristics and aggressiveness of the waters etc. At the end, group of factors F4 is related to the characteristics of the structures in the mine (wells, slopes etc), such as its depth, wide, dips and heights, technology of excavation and so on. All possible interactions of these factors have influenced the efficiency of working and safety of the dewatering structures and mine slopes. It is obvious that not only the group of geological, geotechnical and hydrogeological elements influences the characteristics in the designing of the object, but also the construction of the structures contributes to the change of series of properties and conditions. Qualitative expression of these basic influences is given bellow. Table 2 Qualitative explanation of the interactions Interaction 1,2 Layers thickness and their weight influence the in situ stress. Changes in lithological composition, effective porosity influence the Interaction 1,3 filtration, groundwater movement velocity Physical and mechanical properties are the base for designing stable dips Interaction 1,4 and highs of the slopes In situ stress influences degree of compaction of sediments, opening of Interaction 2,1 the joints and cracks in the field In situ stress influences the groundwater condition, pore water pressure, Interaction 2,3 decreasing of the filtration in higher stressed zones In situ stress determines the point up to which excavation could be made Interaction 2,4 without penetration of groundwater on the floor of the cut (due to hydrostatic pressure values) Groundwater condition influences the decreasing of the strength properties Interaction 3,1 of sediments, increasing of porosity as a result of pumping Groundwater condition influences the decreasing of the total stress Interaction 3,2 (the concept of effective stresses) Artesian pressures, inflow of ground waters, chemical interaction, Interaction 3,4 influence on safety measures for protection. Excavation influences the change in natural moisture content, unit Interaction 4,1 weight, decreasing of the strength properties Interaction 4,2 Excavation leads to the changes in situ stress, stress concentration Excavation influences the groundwater condition through velocity Interaction 4,3 movement s changes, possibility of creating critical hydraulic gradients, appearance of underground erosion It is possible to analyze in detail the mutual influence between each physical, mechanical, structural and other characteristic, which on the other hand lead to the increasing of dimension of matrix of interaction.

8 200 4 Practical Implementation of Interaction Matrix Method at the zone of Coal Mine "Suvodol" The interaction aspects, besides qualitatively could be encompassed quantitatively by using analytical methods, monitoring during investigation phase and during construction. For illustration, a method used in design of dewatering wells in one zone of coal mine is given bellow (Fig. 6). For other zones and dewatering wells, similar procedures are applied, but this overcomes the frame of this article. Fig. 6. Hydrogeological profile 60, with data about groundwater levels before and after installation of dewatering well W-2: 3/60 and 0/60-single piezometers along profile line; P duoble piezometer along profile line for control of drawdown In every case, the first step in analyses is to prepare the conceptual matrices as it is shown in Fig. 7. One form of such matrix is presented in Fig. 8. Fig. 7. Main steps in defining of HES methodology starting from conceptual, analytical to matrices defined with direct observations during exploitation phase

9 201 Second phase in application of described procedure is usual use of known analytical solutions in order to calculate possible cone depression radius (R), the expected level of drawdown (S), groundwater inflow into the well (Q) etc. Finally, most appropriate quantitative definition of interactions, earlier predicted with known analytical methods, is with direct observations during longer time. Illustration of direct measured data during exploitation phase for the profile 60, are given in Fig. 9, 10 and 11. The data from measurements of drawdown (S), pressures (P) at the bottom of the aquiclude at different distance from observation points (L), are mutually interconnected and analyzed with regression analyses. F-1 Hydrogeological conditions (number of aquifer zones, groundwater levels, permeability, gradients etc.) Hydrogeological conditions has an influence on the in situ stress conditions expressed in a value of normal effective pressures Hydrogeological conditions are the basis for choosing of the position of the dewatering wells, their depth and type In situ stress has an influence on possible groundwater inflow in excavation, terrain stability etc, which cal lead in a changes of hydro geological conditions A construction of well has an influence on the establishing of new hydrogeological conditions, new gradients, velocity of groundwater flow around well, settlements etc. F-2 In situ stress (hydro-static, artesian pressure at the bottom of the aquiclude layers etc. ) An adequate chosen place of drainage and dewatering structures has a direct influence on the decreasing of stress and improvement of the stability of terrain The level of stress is one of the basic elements in choosing of the position of the wells (maximal effects when the construction is placed at zone of highest pressures) F-3 Position of the well, depth of pump installation ant other constructional characteristics Fig. 8. Conceptual interaction matrix of between tree basic factors in design of dewatering wells In Fig. 9 is presented the single regression line between observed drawdown (S) and distance of the analyzed point from the well (L). Further ways of connection is illustrated in Fig. 10. This is a case of so-called 2x2 interaction matrix in which the parameters drawdown (S) and distance (L) appear on the leading diagonal. In calculating the regression line linking these two parameters, it could be considered one independent variable as it is illustrated in the off-diagonal

10 202 boxes. Data points are the same in each case: the axes have simply been exchanged, and a different regression line is obtained in each of the two cases. Fig. 9. Single regression line showing influences between drawdown (S) and distance from the well (L) for the zones along profile 60 Fig. 10. Drawdown-distance from the well relations, illustrating the 2*2 interaction matrix In Fig. 11 it is shown 3x3 interactions matrix in which the parameters drawdown (S), distance (L) and artesian pressure at the bottom of the coal layer (P) appear on the leading diagonal. Further interactions with increasing of factors in leading diagonal can be defined with multi-regression analyses. A way of linking between tree parameters is given with following expression P= S L, where P is the pressure at the bottom of aquiclude layer (in bars), S is the value of level of cone depression at certain point (in meters), and L is distance from the extraction well (in meters). All this data are very important during the defining of the distance between wells, expected drawdown, inflow of groundwater into the well etc. The formula is estab-

11 203 lished with multi-regression analyze for one quasihomogenous zone at the NE part for coal mine Suvodol : It is obvious that even matrix of interaction of lowest range, shows several complex mutual influences between the natural environment and the engineering activities. Here, known method of simple regression and multiple regression analyses are very useful to analyze observed data, and to incorporate the results in interaction matrices of most important parameters. Defined parameters with such approach are a good basis for complex analytical and numerical analyses, where the interactions can be defined with all necessary outputs (safety factors, stress-strain conditions, groundwater quantities etc). Fig. 11. Drawdown-Distance-Pressure relations, illustrating 3x3 interaction matrix 5 Conclusions It is fundamental for successful design of each engineering activity to get acquainted in detail with the properties and conditions of natural environment. Among the factors that have the greatest influence on the excavation conditions, protection of environment, stability of terrain and so on, the groundwater conditions are specially emphasized. Without an adequate methodological approach in investigating, which will be completely adapted to the characteristics of the natural environment, it is not possible to define the physical model of the terrain. The physical model of the terrain must be the base for all mathematical models. It can be noted that this are only part of

12 204 analyses used in design of dewatering and stability analyses of "Suvodol" coal mine. More details overcome the frame of this paper, but it is clear that similar methodology can be applied for all engineering problems in coal mining, as well as for a similar hydrogeological and geotechnical problems. The authors recommend using separately for each object the concept of matrix of interaction about all influential factors and this will be the subject of our further analyses. References 1. Gapkovski, N., Jovanovski, M., Manasiev, J., Martinovic, A.: Report from the additional geotechnical investigations on the N.E. part of the coal mine Suvodol -Bitola, Faculty of Civil Engineering, Skopje, Gapkovski, N., Jovanovski, M., Manasiev, J., Petrevski, Lj., Martinovic, A.: Report from the geotechnical investigations on the landslide in the N.E. part of the coal mine Suvodol (in Macedonian), Faculty of Civil Engineering, Skopje, Hudson, J. A.: Comprehensive Rock Engineering, Jovanovski, M., Gapkovski, N.: An example of the extrapolation of the investigation data in a specific type of soft rocks, International Conference on the Trends of the development in Geotechnics, Beograd, Jovanovski, M., Gapkovski, N.: Methodology and results from investigation of groundwater condition on the N.E. part of coal mine Suvodol -R.Macedonia, 6 th International Mine water Association Congress, Bled, Slovenia, Jovanovski, M., Donevska, K., Peshevski I.: Design of the dewatering at a coal mine Suvodol, Faculty of Civil Engineering, Skopje, Jovanovski, M., Papic, J., Peshevski, I.: Design of the stability of the slopes at the coal mine Suvodol, Faculty of Civil Engineering, Skopje, 2008