Geomechanical study concerning Piast Ruch II Czeczott mine part II

Transcription

1 Geomechanical study concerning Piast Ruch II Czeczott mine part II Prof. Antoni Tajduś, PhD, DSc. Eng. Prof. Edward Popiołek, PhD, DSc. Eng. Prof. Janusz Ostrowski, PhD, DSc. Eng. Dr. Grzegorz Mutke, PhD, DSc., Eng. Dr. Marek Cała, PhD, DSc., Eng. Dr. Adam Lurka, PhD, Eng. Dr. Krystyna Stec, PhD, Eng. M.Sc. Eng. Artur Wójcik Kraków, January 009 1

2 CONTENTS 1. Introduction Assessment of the chance and range of the effect of the dog headings on the area surface within the planned investment limits The condition of the rock mass underneath the planned investment Theoretical basis for modelling and forecasting of area surface deformation resulted from underground mining exploitation Discussion on the forecast calculation results Assessment of the chance of discontinuous deformations to develop within the limits of the planned investment area The effect of mining coal-seams in the Wola I and Międzyrzecze mining areas Analysis of the results of geodetic observations in the context of surface discontinuous deformations Assessment of the risk of discontinuous surface deformations to occur within the limits of the planned investment area Estimation of the influence of seismic effects on the planned investment area Rules for determining of Parameters of ground surface vibration resulting from rock mass tremors in the Upper Silesia coal basin Theoretical and experimental base Empirical relationships used in determining maximum vibration accelerations of hard rock in the Upper Silesia Coal Basine (GZW) Effects of soft soil overburden on changing the value of vibration Characteristics of natural seismic activity in Poland Statistical evaluation of repeatability of strong rock mass tremors Geological characteristics of the overburden and tectonics Current effects of tremors on the surface Prognosis maps of maximum vibration accelerations The influence of re-waterlogging the rock mass and underground workings with water and the establishment of the retention-dosing reservoir on surface deformation area Conclusions References and documentation List of figures... 71

3 1. INTRODUCTION RWE Power AG and Kompania Węglowa (KW) have signed a Memorandum of Understanding to examine the possibility to build a new power plant on the site of the Piast Ruch II Czeczott mine belonging to (KW). In January 008 a team of scientists from AGH University of Science & Technology realised part I of the Geomechanical Study Concerning Piast Ruch II Czeczott Mine. The effects of subsidence from past and future mining activities on the location of a power plant (from now called planned investment for the purpose of this study just like in the part I) in the area of the former Czeczott mine was analysed. The effect of the water reservoir filling on the site of planned investment was also considered. The location of planned investment is signed (with yellow colour) on the fig The second part of the study covers several other issues already pointed out in the first part. These problems may be generally collected in three groups of issues. The first group of issues covers the problems with possible influence of existing underground mining openings on the stability of planned investment area. The identified problems are as follows: determination of possible influence of a network of dog headings on the investment area stability, estimation of the necessity for stowing/refilling of dog headings, evaluation of possible surface deformations in the light of a functioning of underground water reservoir. The second group of issues considers the analysis of ground movements in the past. The identified problems are as follows: investigation of possible discontinuous deformations (close to the faults) due to the former mining excavation, detailed analysis of measured ground movements (subsidence, strains etc.), evaluation of any mining induced breakings edges (especially in fault areas) and whether it is documented in mining maps. The third group of issues estimates the possible future hazard from seismic effects (events from neighbouring mines) on the planned investment area. The identified problems are as follows: collecting the information about past natural earthquakes and mining-induced seismic events in the area close to the planned investment, estimation of frequency and possible energy of seismic events from planned mining 3

4 excavation around investment area, forecast of surface accelerations from planned mining excavation around investment area, evaluation of functioning underground water reservoir on seismic hazard around investment area. The detailed analyse of issues mentioned above allowed to formulate the final conclusions about the stability of the rock mass in the vicinity of planned investment. Figure 1.1. Map of the mining areas of the Piast Ruch II Czeczott Coal mine, the BrzeszczeSilesia Ruch I Brzeszcze Coal Mine and planned investment area 4

5 . ASSESSMENT OF THE CHANCE AND RANGE OF THE EFFECT OF THE DOG HEADINGS ON THE AREA SURFACE WITHIN THE PLANNED INVESTMENT LIMITS.1. The condition of the rock mass underneath the planned investment On the grounds of the closed down main plant of the Piast - Ruch II mine, within the planned investment limits, there were 3 mine-shafts opening of the coal-seams. Shafts 1 and 3 were m deep, and shaft m. The shafts opened of the extraction levels of 500 m and 650 m. On the 500 m level, coal-seams 07 and 09/-3 were mined. On the 650 m level, coalseam 09/1 was mined. The mining was done at a considerable distance from the shafts, beyond the zone of faulting with a throw of h = m and a SWW-NEE orientation. At coal-seam 07, closest to the shafts, seam panels about 1100 m away from the shafts were mined in the years 1988 to At seam 09/1 the shortest distance from the mining panels to the shafts was about 100 m (mining years ). However, at seam 09/-3, closest to the shafts, the seam panels were mined in the years 1985 to The range of the effect of the mining did not cover the protected area of the Main Plant of the Piast - Ruch II mine, that is, the investment area. The extraction fields and the shafts were communicated through a network of dog headings made in extraction levels of 500 m (- 50 m above sea level /masl/) and 650 m (- 400 masl). On level 500 m, the access to the coal-seams was through the following drifts: stone drift R-/500, stone drift R-3/500 (main and parallel), stone drift R-4/500 (main and parallel), field drift (main and parallel). On level 650 m, the access to the coal-seams was through the following drifts: stone drift R-3/650 (main and parallel), stone drift R-4/650 (main and parallel). The dog headings on the 500 m and 650 m levels made up relatively thick network. Their layout is shown on the map of the 07 seam on the 500 m level (Fig..1) and on the map of the 09/1 seam on the 650 m level (Fig..). 5

6 Fig..1. Map of dog headings in the extraction level of 500 m The majority of the workings were supported with flexible arch timbering (ŁP9, ŁP10 and ŁP11). Based on technical data for this lining it was established that the dimensions of the headings were, depending on the lining type, as follows: for ŁP9 lining: width: 5100 mm; height: 3570 mm; sectional area: 14.8 m, for ŁP10 lining: width: 5600 mm; height: 3880 mm; sectional area: 18.5 m, for ŁP11 lining: width: 5900 mm; height: 4100 mm; sectional area: 3. m. 6

7 Fig... Map of dog headings in the extraction level of 650 m The above data was used to assess the chance of the rock mass over the workings giving access to both levels to become active in the case of unquestionable convergence of these workings, and also as a result of the outwashing of soluble rock particles during and after the filling of the retention-dosing reservoir, as well as to assess the range of such rock mass activity. 7

8 According to the memo taken by Jacek Matuszek, Senior Mining Hydrogeologist (authorized Mining Geologist), the main drainage at the Ruch II Piast Coal Mine was suspended on 9 July 007. Since then the post-mining workings and the rock mass are being filled with waters from a natural inflow. In addition, from August on, part of the most saline waters acquired as a result of the outwashing of the coal mines Ruch I Piast and Ziemowit is periodically thrown down to Ruch II mining pits. Till Oct the reservoir has been filled up to the elevation of 55. masl. It is planned that the Ruch II workings will be permanently filled up to the elevation of 50 masl. A retention-dosing reservoir (Fig..3). is intended to be made within the elevation range of - 50 masl to + 0 masl. The water level will be the outcome of the quantities of waters supplied to and discharged from the reservoir, with the natural inflow to be about.57 m 3 /min. From the present data it appears that the 650 m level in the Ruch II Piast Coal Mine has been already filled up with water completely. Whereas, the 500 m level is currently begun to be filled with water. The condition of the workings giving access to seams on both the levels is not known due to lack of possibilities to enter them. But it is to assume with great probability that the lining will be destructed over a longer period of time, especially if the workings are filled up with water, and following that, dog headings will also be destructed. The collapse chance of the headings giving access to coal-seams on both the levels and the related risk for the area surface over the workings bring about a need to approximately assess the risk. Possible surface deformation was assessed with consideration of the buildings and landscaping to be developed in the area of the closed down Main Plant of the Ruch II Piast Coal Mine, and with an assumption that the most unfavourable scenario, that is, convergence deformation of all the dog headings with the 500 m and 650 m levels would come true. In the first analysis stage, a total volume of dog headings, with their dimensions following from the ŁP linings (see above) taken into account was estimated, and the lengths of the workings were determined based on mining maps of seams 07 and 09/1. However, at present it is not possible to determine the ŁP lining type for individual headings. Therefore, for the estimation of dimensions of the workings, mean values of the parameters of individual lining types, with the space between the lining and the working rock surface taken into account were adopted. These dimensions are: 1 The newest data 30 Dec. 008 the reservoir has been filled up to the elevation of 43. masl. That means that the water level increases of average 5 m per month. 8

9 mean width of the working wm = 5.5 m mean height of the working hm = 4.0 m mean vertical section of the working sm = 0.0 m. In the second stage, areas with essential concentrations of headings on both the levels were outlined based on coal-seam maps. On the 500 m level the area with the greatest concentration of workings (hereinafter called a working block ) encompasses about km. On the 650 m level, this area covers about 0.5 km (Fig..4). Table.1 presents the estimation results for the sizes and volumes of the workings giving access to coal-seams on the 500 m and 650 m levels in some of the working blocks, and also the percentage of working space in the total area as well as the space volume for the greatest concentration of workings. Table.1 Estimation of the degree of disturbance in the rock mass caused by dog headings on the 500 m and 650 m levels Level Total length of Mean width of heading Mean height of Mean surface area of headings [m] [m] heading [m] the section of a heading [m ] 500 m m Total surface area of Total volume of headings [m ] headings [m 3 ] 500 m m Total Surface area of a working block [m ] The ratio of the surface area of the headings to the surface area of the working block, [%] The ratio of the volume Volume of the of the headings to the working block volume of the [m 3 ] working block, [%] 500 m m Mean

10 Fig..3. Filling of Ruch II workings (as of Oct. 008) compared to the retention-dosing reservoir operation 10

11 From the estimation presented in Table.1 it appears that in comparison both to the surface area and the volume, the mining workings make voids spanning over 15 % of the rock mass area where they are located. That area was not disturbed by extraction, which currently guarantees that the rock mass and the area surface over the 500 m level are stable. In the third analysis stage it was assumed that the effect of the dog headings from the 500 m and 650 m levels will be similar to the effect of mining carried out using the heading extraction method. This results from the concentration of workings, the real chance of them to be subject to destruction as well as from the effect of the retention-dosing reservoir which causes water flow and leaching of rock particles. In that situation, the filling of the voids created by the network of headings may be regarded as an effect of mining a hypothetical coal seam with an adequately adopted exploitation factor. The value of this factor was determined based on the experience acquired from mining with the heading method and from exploitation of ore using the room and pillar method with full hydraulic filling. It was regarded as reasonable to adopt the hypothetic exploitation factor for longwall workings equal to a = The forecast for displacement and deformation of the area surface in relation to the above described hypothetical mining through the network of headings was made according to the Knothe theory of mining impact that describes post-mining deformation of the surface area in conditions of Polish coal mines very well. Based on simulation calculations it was established that equivalent voids in areas highlighted in Figure.4 as working blocks with heights of g = 4.0 m are to be interpreted as extraction fields closed down using a method corresponding to the exploitation factor a = 0.01, as given above. The forecast deformation values for the surface area were determined ones by limiting the calculations to subsidence, inclinations and horizontal strain of the terrain surface... Theoretical basis for modelling and forecasting of area surface deformation resulted from underground mining exploitation The modelling of area surface deformation resulted from mining exploitation are made with the use of the Knothe-Budryk theory ((Knothe S ). This theory is implementation of a theoretical model of area surface deformation caused by the effect of underground exploitation of a deposit of, e.g., of a coal-seam. The fundamental assumption for this model is the so called influences function as follows: w max π g( x, y) = exp ( x + y ) r r (.1) 11

12 Fig..4. Overall sketch of the dog headings network in the extraction levels of 500 m and 650 m with workings blocks highlighted 1

13 where: w max = a g - maximum final subsidence [m], a - exploitation coefficient, dependent on the method for filling the post-exploitation void, g - thickness of the seam bed intended for exploitation [m]. The r value is a parameter for dispersion of influences, called also a radius of sphere of principal influences. The value of the r parameter depends on the depth of the deposition and on the angle of dispersion of influences: where: profile: H - depth of deposition [m], H r = (.) tgβ β - angle of dispersion of influences, dependent on the physical and mechanical properties of the rock mass, on which the mining exploitation is carried out. Assuming that the influence function is (1), we obtain equation for a subsiding trough where: w w λ + ξ r r 0 (.3) max ( x =, y = 0) = exp π dλdξ P - rectangle-shaped area of the exploited deposit. P The inclination along the axis of the coordination system accepted here is calculated as partial derivatives for subsidence defined by formula (3): T T x y y w max x x = exp( π ) exp( π ) d r r r r exp( π η 1 1 ) η y r x w max y y = exp( π ) exp( π ) d r r r r exp( π λ 1 1 ) λ x r where: x 1, x, y 1, y - coordinates of the edge of the rectangular exploitation field. The maximum inclination in a given point of the area surface is: 1 1 (.4) (.5) max = T x Ty (.6) T + The rate of horizontal strain along the x and y axes of the coordination system taken is calculated according the following formulae: 13

14 y w x x x x ξ ε x = max 1 1 π 1.0 exp exp π π dξ r r r r r r exp (.7) r y x w y y y y η ε y = max 1 1 π 1.0 exp exp π π dη r r r r r r exp (.8) r x Horizontal displacements within the subsiding trough is determined based on subsidence according to the following relationships: where: 1 u = B dw ; v = B dw (.9) dx dy u, v - horizontal displacement along the x and y axes respectively in the taken rectangular coordination system, B - factor of proportionality; its value under coal-seam exploitation conditions is (Popiołek E., Ostrowski J. 1978): 1 B = 0. 3 r (.10) The extreme (primary) values for horizontal strain in a given point of the area surface are: where: ε x, y - non-dilatational strain: ( ε + ε ) ± 0. ( ε ε ) ε ε, = (.11) g1 g x y x y x, y w = max x 1 x y 1 y ε x, y exp π exp π exp π exp π (.1) r r r r r The following values of parameters were taken according to the Knothe-Budryk theory: parameter of rock mass tgβ = 1.9 and exploitation coefficient a = Discussion on the forecast calculation results Figure.5 presents forecast values for subsidence in the form of an isolines and forecast values for horizontal strains in point locations of a computational grid. The forecast for displacement and deformations of a surface area showed that subsidence with a maximum value of v max = 0.07 m may appear, and the subsidence process would be gentle and long-lasting. It may be forecast that the target values for subsidence will be obtained even in ten or fifteen years. There is a real chance that the subsidence will be lower than the forecast ones, e.g. in the case when the changes in the water level in the underground retention-dosing reservoir will not be very great. 14

15 Fig..5. Forecast subsidence and horizontal strains within the planned investment area 15

16 The maximum horizontal strains will not exceed the value of ε max = ± 0. mm/m. The maximum values for inclinations will not exceed T max = 0. mm/m, either. The forecast values for deformation indices are very low and even lower than those adopted as limits of harmful influences (ε hrm < 0.3 mm/m, T hrm < 0.5 mm/m). In the conclusion it should be said that the area surface, its buildings and land development within the limits of the planned investment area will not be threatened with underground dog headings, even in the case of their complete convergence. Apart from the very low deformation indices, the favourable circumstance is also the anticipated very slow and gentle course of the ground lowering process. The experience from post-mining areas shows that the predicted values for displacement and surface deformation will be harmless to building and road structures. 3. ASSESSMENT OF THE CHANCE OF DISCONTINUOUS DEFORMATIONS TO DEVELOP WITHIN THE LIMITS OF THE PLANNED INVESTMENT AREA 3.1. The effect of mining coal-seams in the Wola I and Międzyrzecze mining areas The mining activity carried out in throughout 0 years within the limits of the mining area of the Piast Ruch II Coal Mine has brought about changes in the landform features over the mined coal-seams. Those changes covered about 80 % of the mining area located beyond the limits of the planned investment area. As it was demonstrated in the Geomechanical study concerning Piast Ruch II Czeczott Mine (AGH, Krakow 008), the mining range did not cover the investment area, which, during the operation of the mine, was protected with a pillar established for the Main Plant of the Piast Ruch II Coal Mine. As a result of extraction performed last years, an extensive subsidence trough appeared within the limits of the Piast Ruch II Coal Mine. This trough is formed as a combination of 10 local troughs with maximum subsidence of.0 to 4.5 m. The maximum subsidence developed within an unbuilt area in the central part of the mining area between the Main Plant of the Piast Ruch II Coal Mine in the south and the Bojszowy Górne estate in the north. Intensive extraction was carried out at seams 07, 09/1 and 09/-3. The aim of the analysis made in this chapter is characterization of surface deformation in the mining area of the Piast Ruch II Coal Mine in the context of a risk of continuous deformations. Due to the ceasing of coal extraction, and in connection with that, due to the

17 creation of anthropogenic voids inside the rock mass, the only hazard resulting in discontinuous deformations on the surface may be constituted by zones of faulting. According to the study titled Ocena wpływu likwidacji KWK Piast Ruch II na środowisko oraz znajdujące się na powierzchni obiekty i urządzenia ( Assessment of the impact of the liquidation of the Piast Ruch II Coal Mine on the environment and the building structures and installations ) - Zespół Biuro Projektów Górniczych w Krakowie Sp. z o.o. under direction of Z. Chmura, M.Sc. and T. Cichy, M.Sc; July 004 [quotation:]...the extraction of coal-seams has caused no discontinuous deformations in the form of sinks, steps and crevices.... Also according to the statement by Dariusz Bieroński, M.Sc., Manager of the Piast Coal Mine Department for Surveying and Geology at Kompania Węglowa S.A., no discontinuous deformations, either linear or surface, were found in the Wola I and Międzyrzecze mining areas. The above statements result from the fact that no phenomena of this type are reported when examining damage to built features. Since considerable areas of the Piast Ruch II Coal Mine grounds are open spaces, non-recorded discontinuous deformations in the form of crevices and ground sills might develop in the past. The reason for this assumption is that there are numerous tectonic faults in that area. 3.. Analysis of the results of geodetic observations in the context of surface discontinuous deformations Potential threat to the surface from the impact of the zones of faulting was analysed as a part of this study. The results of geodetic observations on stabilized lines in the Wola I and Międzyrzecze mining areas were the data used for this analysis. In the mining area of the Piast Ruch II Coal Mine there were 13 observation lines distributed in several areas (Fig. 3.1): area 1: Bojszowy Górne (lines no. 1 and ), area : Jedlina (lines no. 1, and 3), area 3: Międzyrzecze (lines no. 1, and 3), area 4: Bojszowy Stare (lines no. 1,, 3, 4 and 5). In the Jedlina area 3 survey cycles were carried out in the period between 1 June 1993 and 0 Aug At the lines in the Bojszowy Górne area, observations were taken in 17 survey cycles, between 30 July 1997 and 4 May

18 Fig Location of geodetic observation lines within the Wola I and Międzyrzecze mining areas 18

19 Surveys on the lines in the Międzyrzecze area were carried out in the period from 8 Oct to 15 July 003, survey cycles were made. Surveys on the lines in the Bojszowy Stare area were carried out in the period from 18 Oct to 30 Mar. 1999, survey cycles were made. Time intervals between the cycles were 1 month to 1 year, depending on the mining intensity below the observation line area. The observation results for the last course of surveys show that the surface deformation process has ended. On account of the aim of the discussion as well as the very extensive documentation, the analysis covered the sections of observation lines that were located below the zones of faulting. The analysis of the charts for subsidence and horizontal strains in the surface along those lines should show whether any phenomena proving the faults to influence the surface and the latter to be threatened with discontinuous deformations did occur there. The results of such an analysis will be a basis for the assessment of a chance of discontinuous deformations to appear within the limits of the project in the conditions of the rock mass not to be disturbed by mining works any longer. In Figure 3.1, presenting the situation of the geodesic observation lines, also the location of geological sections used in the analysis was marked. These are: the IX IX section with the SSW-N orientation, approximately parallel to the orientation of observation line no. 1 at Międzyrzecze and line no. 1 at Bojszowy Stare, and also the III III section related to observation line no. at Jedlina. As it appears from the sections, the observation lines cross the projections of lines of outcrop faults (in the Carboniferous roof) on the surface of the area. In the case of activation of the fault planes, disturbances in the regularity in the profiles of deformation indices in the area of the observation lines should be recorded by surveying. Observation line no. 1 at Międzyrzecze is situated on a part of the geological section no. IX, between boreholes MB-U(1968), MB-91(1985) and JG-(1960) marked in Figure 3.. Two faults with throws of h = 10 0 m (in the southern part of the section) and h = 8 10 m (in the northern part of the section) were found in coal formations in this area. The thickness of the Tertiary and Quaternary overlayers in this rock mass area is, above the throws of both the faults, m n = 100 m. Figure 3.3 presents charts for subsidence of observation line no. 1 at Międzyrzecze for selected courses of observations. Surface subsidence result here from the impact of coal extraction conducted under these grounds in the 1990s. The maximum, final subsidence v max = 1.50 m (in point 16) was observed on line no. 1 at Międzyrzecze. 19

20 Fig. 3.. IX-IX geological section for the MB-U(1968) JG-(1960) length with the projection of the Międzyrzecze observation line no. 1 0

21 Geodetic point number Observation line no. 1 "Międzyrzecze" 0.4 Subsidence [m] Subsidence [m] Outcrop of the fault h = 10 m Overlayer m n = 100 m Outcrop of the fault h = 8 m Overlayer m n = 100 m Fig Charts for subsidences of the points of the Międzyrzecze observation line in the period from Oct to Oct

22 Figure 3.4 contains charts for over-time development of subsidence in points located closest to the fault outcrop occurring under the analysed surface area. The analysis of the distributions of subsidence recorded along line no. 1 at Międzyrzecze in individual courses of surveying shows that there are irregularities in these distributions on the section from point 11 to point 14. However, these disturbances, observed since 1999, are not caused by the impact of the faults, because these are located under marginal sections of the subsidence trough profile. The reason for these irregularities is, most probably, a local change in the properties of roof rocks above the mined seam. Since the subsidence in points 5, 6 and 7 of observation line no. 1, located above the outcrop of fault h = 10 m were close to zero (the projection of the fault throw onto the surface is practically beyond of the range of the mining impact), Figure 3.4 presents charts for overtime subsidence of adjacent points (8, 9 and 10). The distributions of over-time subsidence in points located above the zones of faulting (Fig. 3.4) prove that these points sank evenly with the extraction progress. The slight irregularities visible in the charts result from the properties of the deformation process and possible surveying errors as well as from the scale of these charts, appropriate for the very slight subsidence. From the analysis it appears that, in the conditions of extraction of coal from seams of group 00 within the limits of the Międzyrzecze mining area no impact of faulting zones with their outcrops in the Carboniferous roof became apparent. It results from the very low projection values for these faults (h = 8 10 m) and from the considerable thickness of the Tertiary and Quaternary overlayers (m n = 100 m). Observation line no. at Jedlina (Wola I mining area) can be related to the III - III section, between boreholes MB-58(1973) and MB-95(1985), and further, up to the boundary of the mining area. The projection of line no. onto the section line makes it possible to determine locations of the faulting zones perpendicular to the section, in relation to the observation line (Fig 3.5). Below the specified part of the III III section there are three faults: fault h = 30 m, covered with an overlayer m n = 10 m, fault h = m covered with an overlayer m n = 00 m and fault h = 40 m covered with an overlayer m n = 195 m. Figure 3.6 presents charts for subsidence of observation line no. at Jedlina in selected courses of observation. The surface subsidence result from the impact of coal extraction conducted under this area in the 1990s. The maximum, final subsidence v max =.444 m was recorded on line no. at Jedlina (in point 44).

23 Subsidence w [m] Subsidence w [m] Point 7 Point 8 Point 9 Point 19 Point 0 Point 1 Fig Charts for over-time subsidences of selected points of the Międzyrzecze observation line no. 1

24 Fig III-III geological section for the MB-58(1973) Cz.-4(1986) length with projection of the Jedlina observation line no. 4

25 Geodetic point number Observation line no. "Jedlina" Subsidence [m] Subsidence [m] Outcrop of the fault h = m Overlayer m n = 00 m Fig Charts for subsidences of the points of the Jedlina observation line no. in the period from Mar to Aug

26 Figure 3.7 presents a chart for over-time subsidence in points closest to the outcrop of the fault occurring under the analysed surface. The regularity of depression trough profiles along line no. at Jedlina, recorded in individual courses of surveying (Fig. 3.6) proves that there is no impact of the zone of faulting on the formation of ground subsidence above its outcrop. This conclusion is confirmed also by an analysis of the courses of over-time subsidence of the line points situated above the fault outcrop area (Fig. 3.7). The fault with thrust h = m (greater than that of the case described above), does not influence the condition of the area surface because its outcrop is covered with a Tertiary and Quaternary overlay with high thickness of m n = 00 m. Observation line no. 1 at Bojszowy Stare (Wola I mining area) is approximately parallel to the line of the IX IX section on the section between boreholes JG-(1960) and MB- 46(1971). Below the specified part of the section there are two zones of faulting (Fig. 3.8). In the vicinity of borehole MB-11(197) there is a fault outcrop with thrust h = 80 m, covered with an overlayer with the thickness of m n = 300 m, and to the north of the borehole there is a fault with thrust h = 30 m covered with an overlayer with the thickness of m n = 80 m. Figure 3.9 present charts for subsidence of observation line no. 1 at Bojszowy Stare in selected courses of observation. Surface subsidence result from the impact of coal extraction conducted under this area in the 1980s and 1990s. The maximal, final subsidence v max = m was recorded on line no. 1 at Bojszowy Stare (in point 98). Figure 3.10 presents charts for over-time subsidence in points closest to the outcrop of the fault occurring under the analysed surface. The analysis of distributions of subsidences in points on line no. 1 in individual courses of surveying shows that there is no influence of the zones of faulting on the surface of the area. The slight disturbance in the trough profiles in point 10 results from its movement caused by reasons different than the influence of coal extraction. It is worth noting the final section (from point 7 to point 78) of the profile of the basin in the course of 30 March The initial influence of the extraction conducted to the north of the observation line was recorded in that course. The charts for over-time subsidence of points of observation line no. 1 situated above the zones of faulting (Fig. 3.10) do not show any features of influence of the faults. The small irregularity in subsidence of points 60, 6 and 64 (with the value of about 5 cm) should be interpreted as a systematic measurement or adjustment error made in the course of 4 Oct

27 Point 56 Point 57 Point 58 Point Subsidence w [m] Subsidence w [m] Fig Charts for over-time subsidences of selected points of the Jedlina observation line no.

28 Fig IX-IX geological section for the JG-(1960) MB-46(1971) length with projection of the Bojszowy Stare observation line no. 1 8

29 Geodetic point number Observation line no. 1 "Bojszowy Stare" Subsidence [m] Subsidence [m] Outcrop of the fault h = 80 m Overlayer m n = 300 m Outcrop of the fault h = 30 m Overlayer m n = 80 m Fig Charts for subsidence of the points of the Bojszowy Stare observation line no. 1 in the period from Jan to Mar

30 ObniŜenie w [m] ObniŜenie w [m] Point 60 Point 6 Point 64 Point 74 Point 75 Point 76 Fig Charts for over-time subsidence of selected points of the Bojszowy Stare observation line no. 1

31 Form the above it appears that coal extraction in the vicinity of the faulting zone with high throws (h = 80 m) did not brought a risk of discontinuous deformations to occur on the surface, most probably due to the considerable thickness of the Tertiary and Quaternary layers (m n = 300 m). The examples presented above illustrate the problem of the influence of faults on the surface of the area in a situation where the observation line is situated directly above fault outcrops in a Carboniferous roof. The recorded irregularities in depression trough profiles, as well as the lines of over-time subsidence in points are specific in character, which proves that there are causes other than influence of the faults. Such effects appear also on the lines that do not run above the zones of faulting or are parallel to them (for example, see Figs. 3.11; 3.1). Most often, the cause for such irregularities in subsidence profiles is diversity of physical and mechanical properties of rocks deposited above the extraction field, peripheral influence of neighbouring extractions, restabilisation and damage of measuring points as well as measurement errors. In order to confirm that there are no effects of the faults on the surface of the area in the conditions of coal extraction in the Międzyrzecze and Wola I mining areas, the distributions of the horizontal strains observed along line no. 1 at Międzyrzecze and line no. at Jedlina were analysed. The effect of a fault appearing on the surface in the form of a crevice should be registered on the observation line as a stretching strain zone with the value considerably exceeding the deformation values for the adjacent sections of the line. Due to high random distribution of horizontal strains it is assumed that if the values for horizontal deformation in a particular section are three times higher in relation to the other, the area is proved to be influenced by the zone of faulting. Figure 3.13 presents distributions of horizontal strains along the above mentioned observation lines, recorded in the last surveying cycles. From the charts in Figure 3.13 it appears that the line of the horizontal strains does not show any influence of the Carboniferous faults on the surface. 31

32 Geodetic point number Subsidence [m] Subsidence [m] Observation line no. 1 "Bojszowy Górne" Fig Charts for subsidence of the points of the Bojszowy Górne observation line no. 1 in the period from Oct to Sep. 001

33 Geodetic point number Observation line no. 1 "Jedlina" Subsidence [m] Subsidence [m] Fig Charts for subsidence of the points of the Jedlina observation line no. 1 in the period from Mar to Aug

34 Geodetic point number Horizontal strain [mm/m] Horizontal strain [mm/m] Observation line no. 1 "Międzyrzecze" Geodetic point number Horizontal strain [mm/m] Horizontal strain [mm/m] Observation line no. "Jedlina" -3.0 Fig Charts for horizontal strains along the observation lines: Międzyrzecze no. 1, Jedlina no. 34

35 3.3. Assessment of the risk of discontinuous surface deformations to occur within the limits of the planned investment area From the discussion above its appears that coal extraction conducted in the Wola I and Międzyrzecze mining areas did not generate any phenomena with additional influence of their faulting zones on the surface. The geological composition of the rock mass, the considerable depth of the mining and the thickness of the Tertiary and Quaternary overlayer covering the fault outcrops do not create favourable conditions for development of discontinuous deformations or re-activation of old workings to an extent posing a threat to the surface. The above conclusions result from the analysis of the phenomenon in the conditions of extraction. In the case of no extraction, or an impulse which might activate the zones of faulting, the chance of discontinuous deformations to occur on the surface is insignificant and limited only to small crevices which appear also because for reasons other than mining. Below the grounds within the limits of the planned investment area there is one zone of faulting with the orientation from the west to the east, covered with a Tertiary and Quaternary overlayer with the thickness of m n = 00 m (Fig. 3.14). The thrust of the fault is h = 10 m. Taking into account the conclusions from the analysis, it is to note that in the future the planned investment area will not be threatened with discontinuous deformations. It is worth noting the broad faulting zone occurring in the investment area, with throw h = m (MB-6(1974) borehole area in Figure 3.14). However, this zone will not pose any threat to the area because it runs from the north-east to the south-west, at the closest distance of about 00 m from the boundaries of this area. In the conclusion it should be noted that in the future the area within the limits of the planned investment will not be threatened with discontinuous deformations in the form of crevices, much less of steps and surface discontinuous deformations. 35

36 Fig XI-XI geological section for the MB-74(1973) MB-93(1985) length with boundary of the planned investment area 36

37 4. ESTIMATION OF THE INFLUENCE OF SEISMIC EFFECTS ON THE PLANNED INVESTMENT AREA Mining tremors are dynamic phenomena, resulting in consequence either of sudden displacement or fracturing and breaking of rock mass strata. In the conditions of the Upper Silesia Coal Basin (GZW), the rock mass tremors are consequences of underground mining, and for this reason are called mining tremors. Hence, in the GZW one distinguishes the miningrelated tremors being directly connected with the advance of mining in a given mining lot, and, usually, stronger regional ones being connected with the existence of large fault zones and related tectonic stress in the rock mass, and advances of mining operations in the scale of a larger area (e.g. mining area of several mines). In Polish hard coal mines the energy level of the mine tremors induced by mining operations and regional tectonics is in the range of Joule s (local magnitude M L is in the range of 0 4,5), (Mutke & Stec 1997). Each year in Upper Silesia, 1000 to 000 seismic events with the energy in excess of 10 5 J (M L > 1.5) are recorded. The rock mass tremor is always connected with the release of a defined amount of seismic energy, and is always a source of emission of elastic vibration. These vibrations propagate in the rock mass from the place of their origin that is from their focus, in all directions, in the form of seismic waves. Outside the close surroundings of the tremor focal point, a relation is observed that the amplitude of the rock mass vibration is directly proportional to the seismic energy of the event, and inversely proportional to the distance of the location of vibration detection from the focal point of the tremor. It has been also observed that the vibration amplitude of the surface-adjacent ground layer is strictly dependent on the structure of loose overburden strata. In the analyzed area of mine Piast Ruch II in which the exploitation was finished, seismic events can be induced after being formed of water reservoir, as the effect of sandstone saturation. In such a case zones of high concentration of stresses can arise. In these zones seismic events can induced themselves. The second source of vibration can be a week earthquakes from southern and central part of Poland. Analysis of historical data is pointing, that in period of 100 years, it is possible to expect weak intensity vibration in the area of analyzed mines. All these kinds of probable seismic activity are analyzed in this study. 37

38 4.1. Rules for determining of Parameters of ground surface vibration resulting from rock mass tremors in the Upper Silesia coal basin Theoretical and experimental base The principal characteristics of rock mass tremors are: coordinates of locations of tremor focal points and measures of intensity of tremors. A frequently applied measure of tremor intensity is its magnitude. However, in the GZW conditions, characterizing the intensity of tremors through determining their seismic energy is most commonly used. Continuous seismographic recording conducted by the regional and mine-located networks enable to observe of all rock mass tremors occurring in the GZW, to determine locations of their focal points and their energy value. From the point of view of the issues of protection of building development on the surface, of vital importance are the characteristics of vibrations of the base of building structures produced by the tremors. The fact is such that the intensity of vibrations and their frequency range are decisive for the degree of harmfulness of those vibrations to building structures. The principal parameters used in determining the intensity of vibrations of bases of building structures are: maximal amplitudes of acceleration or velocities and corresponding vibration frequencies. From measuring sites located both on the rock base and Quaternary overburden, more than 700 records have been collected for mining tremors from the range of energy spectrum from 10 3 J to 10 9 J. The collection of seismograms obtained from measuring stations on the rock base was used to develop empirical relationships connecting maximum resultant amplitudes of vibration acceleration with the seismic energy and epicentral distance. In turn, the records obtained in Quaternary overburden were compared with the records from the rock base (for tremors with the same seismic energy and identical epicentral distance). This provided a basis to perform quantitative determination of the effects of Quaternary and Tertiary overburden of different thickness on the variation of vibration acceleration amplitudes. The results obtained were used to verify analytical computations of the base factor (also called amplification factor ), resulting from the theory of propagation of seismic waves, and from solving the wale equation for defined conditions of lithological structure of the overburden strata in the locations of installed measuring sites. The results of analyses performed and examples of vibration records, coming from the mentioned measuring sites have been presented in detail in the subsequent chapters of this paper. 38

39 4.1.. Empirical relationships used in determining maximum vibration accelerations of hard rock in the Upper Silesia Coal Basine (GZW) Maximum amplitude of vibration acceleration on the hard rock When considering the physical side of the phenomenon, one can distinguish the near and far wave fields. In such a presentation, the area around the epicenter, being, on the surface, a limit of occurrence of the near wave field, can be considered to be an epicentral zone. For strong mining tremors from the Upper Silesia Coal Basin area (events with magnitude M 1. 7 or seismic energy E 10 5 J), the radius of such a zone is ca.1000 m. A database that had provided base for developing empirical relations between seismic energy of the tremor was the selected set of accelerograms recorded on a rock base in the epicentral zone. The bank of data contained ca. 700 events from the energy range from 10 3 to 10 9 J. A preliminary analysis did not point at the statistical normality of the distribution. For this reason, they were subjected to non-parametric tests with the use of Spearman s and Kendall s rank correlation method (Table 1). Two functional relationships have been determined using the non-linear regression method, for weaker tremors (E < 10 5 J or M L < 1.7), and for strong tremors ( 10 5 J E < 10 9 J or 1,7 M < 4. 0). The practice points out that, for the GZW area, of significant importance, when evaluating the harmfulness of mining tremors to building structures, is only the group of strong events. For this reason, given below is the regression curve only for his group of tremors (Mutke 1991; Mutke & Stec 1997): 3,66 a M = (log E) (4.1) where: E - seismic energy of tremor, J, a M - maximum amplitudes of vibration acceleration on hard rock base, m/s. Table 1. Test results of Spearman s and Kendall s rank correlation for variables a M and E Variable Spearman test Kendall test E a M (E x 10 5 J) From records of vibration due to mining tremors in the GZW area, it follows that for the strongest tremors occurring in this area (seismic energy of the order of 10 9 J), the maximum 39

40 vibration accelerations reach the values of ca mm/s for hard rock base, without amplification. Standardized function of decrease of maximum vibration acceleration amplitudes of hard rock base with increasing epicentral distance From the measurements performed on the rock base, at different distances from tremor focal point sit follows that along with the increasing epicentral distance, the acceleration amplitudes of base vibration decrease. In the epicentral zone, this phenomenon is less distinct, while out of the epicentral zone it is evident. This can be physically explained by the fact that in the near wave field absorption damping dominates, while in the far wave field, there is much stronger effect of geometrical damping. This practically means that the total damping function in the epicentral zone reveals an effect of flattening, in turn outside the epicentral zone, it is a monotonically decreasing function. On assumption that only strong mining tremors can, practically, be harmful to building structures, a standardized vibration damping function has been elaborated for high-energy events. The results of non-parametric tests using Spearman s and Kendall s rank correlation of the analyzed set of data are presented in Table. Table. Test results of Spearman s and Kendall s rank correlation for variables a D and D Variable Spearman test Kendall test D a D The relationship describing standardized decrease of horizontal components of acceleration a D of rock base vibration in the GZW for tremors with energy E x 10 5 J vs. epicentral distance D (up to 10 km from epicenter) is the following (Mutke 1991): a D = 1.53R exp( 0.65R) (4.) where: R = D + 0.5, D epicentral distance, km. The coefficient 0.5 in the formula for R corresponds with assuming an average depth of tremor focal points in the GZW being 0.5 km. By combining formulas 1 and, one obtains a relationship enabling to determine the maximum values of acceleration of rock base vibration in any location in the GZW, as a 40

41 function of seismic energy of mining tremors and epicentral distance. For the events that are important from the point of view of their harmful impact on buildings (i.e. for tremors with energy E 10 5 J), the functional relationship takes the following form: a MD = [ (log E) 0.089][1.53R exp( 0.65R) ] (4.3) Empirical functions for determining dominating frequencies of vibration of hard rock base in GZW Apart from maximum amplitudes of acceleration or frequencies of vibration of building structure base, an important parameter is also the dominating frequency of vibration having maximum amplitudes. This is so because dynamic responses of building structures are not uniform in individual vibration frequency ranges. With the aim to determine the range of dominating frequencies, a directional analysis of accelerograms in the frequency domain has been made, the essence of which was their transformation using Fourier transform (FFT), and determining, on that basis, minimum F min and maximum F max, values, for which the level of spectral amplitude decreased twice in relation to its maximum value. The results of these computations, providing an initial set of data was subjected to analyses using non-parametric Spearman s and Kendall s rank correlation tests (Table 3.). Table 3. Test results of Spearman s and Kendall s rank correlation for variables F max, F min and E Variable Spearman test Kendall test E F max F min The determined equations of regression curves, bounding from bottom and top the range of occurrence of dominating frequencies, are the following (Mutke 1991; Mutke & Stec 1997): F = [99exp( 0.49log E)] 1.7 (4.4) max + F = [91exp( 0.78log E)] 1.79 (4.5) min + 41

42 where: E seismic energy, J. The frequency range defined by the equations is valid for accelerations of vibration of rock base in the epicenter zone. Worth mentioning is the fact that dominating frequencies that are characteristic for highenergy tremors from the GZW area, are contained in the range 6 Hz, that is overlap with the range of the first harmonic frequency of natural vibration of typical building structures in the GZW (residential and public buildings of traditional, traditional-improved, large-block and large-panel constructions). This is one of the causes of damages to buildings in the GZW area at lower values of resultant acceleration, than in the case of earthquakes. The frequency range of dominating vibrations with maximum accelerations is also important when determining the base factor (so called vibration amplification), for it substantially depends on the frequency of dominating vibrations. On the other hand, numerous tremors with short time duration (e.g. one peak), do not constitute a high hazard to buildings, even at a high value of acceleration represented by this peak Effects of soft soil overburden on changing the value of vibration Both the measuring premises and theoretical solutions for propagation of a seismic wave through the multi-strata rock mass and soft soils reveal the change of vibration amplitudes in the course of passage of the wave across individual layers. Generally, one can say that the change of vibration amplitudes is influenced by the following factors: wavelength λ (or frequency f ) of incident wave, thickness of stratum H, density of stratum ρ, wave velocity ν, type of wave, angle of incidence of wave. The practice indicates that the less rigid, more cracked stratum, and velocity in it lower, the easier substantial increase of vibration amplitude may occur at a proper wavelength of incident wave. The structure of surface-adjacent strata reveals high variability across the GZW area. In some fragments of this area, the rock base approaches the surface, while in most part of it there is an overburden of soft soil, changing its thickness in the range from 0 to 300 meters. This overburden is made chiefly of Quaternary and tertiary formations (sands, clays, gravels, silts, 4

43 etc). The measuring data indicate that, depending on the structure of surface-adjacent strata, the maximum values of acceleration of ground surface vibration due to mining may undergo both partial attenuation and amplification. The values of this factor may vary in the GZW in a wide range, from 0.5 to 4.0 (from empirical data). In such a presentation, the maximum values of amplitudes of acceleration of ground surface vibration are determined from the formula : a MSK = a S (4.6) MD where: S - soil factor (amplification of vibration ). To calculate the ground factor, it is essential to know the dominating frequencies of vibration reaching the overburden strata. If a proper database of records of tremors on the rock station in a given area is not available, then this frequency can be determined from relationships 4. and 5. Many cases of damage to building structure, often at large distances from the epicenter of tremor have their causes In the effect of local amplification of vibration, that is their amplification by the overburden composed of loose strata (with low velocities of propagation of seismic waves).an example of stratified medium (and in particular soft soil overburden of this medium) on the change of value of vibration acceleration amplitudes in the GZW is given in Fig.4.1 below G-EW m/sek G-Z m/sek G-N S m/sek D -EW m/sek D -Z m/sek D -N S m/sek czas [sek] Fig Seismogram of the tremor of with energy E = 10 7 J from an epicentral distance r = 874 m. Soil factor, S (vibration amplification factor) at the surface station is S =

44 The highest values of vibration acceleration in the GZW are related to direct waves. In particular, they relate to transverse horizontally polarized waves, that is SH waves. Such a wave, when passing through horizontal boundaries, does not create other types of waves. The computation algorithm of the ground factor has been based on the solution within an elastic medium with damping, exactly on the rock mass described by the Kelvin model. For the SH waves falling on the surface at any angle, and for vertical longitudinal waves P, the amplitude of vibration is doubled (this results from boundary conditions that are met by the wave at the stratum-air boundary). If these waves propagate through a horizontal strata interface (z=0), they undergo partial refraction. The amplitudes of the waves falling on the interface, for comparison with other waves are taken as 1. The reflection of waves from the interface is described with the coefficient k o, and refraction with coefficient k 1. The value of the amplitude on the surface of overburden strata is (Savarienski 1956): S = (m 1 κ) / (Acos κb + i Bsin κb ) (4.7) where: m 1 =α 1 ρ 1, m 0 =α 0 ρ 0, κ = κ 1 - iκ (κ amplitude attenuation, κ 1 - dispersion), b = ωh / α 1. The equation above makes it possible to determine the ground factor W f, when such parameters as: m 0, m 1, α 0,1, κ 1,, b and H are known. An important characteristic resulting from this equation is also the possibility of attenuation of vibration amplitude after the wave has passed trough an overburden stratum, which is frequently observed in practice. All the equations described are valid also for a transverse SH wave falling at any angle e o, in the direction of the lower boundary of the stratum. For the SH wave, one should take into account: m 1 = b 1 ρ sin e 1, m 0 = b o ρ o sin e 0, b = π(h/λ) sin e 1 44

45 In practice, the records taken on the same overburden stratum will differ considerably between them, and only a part of them will show a maximum value of the ground factor (socalled amplification of vibrations). This is caused by simultaneous influence of several seismo-geological factors on the surface effect. In reality, the seismic waves are nonharmonic, and the resultant amplitude is conditioned by superposition of waves with all harmonics. The frequency range of maximum vibrations is different for tremors with various seismic energies, epicentral distances, etc. In consequence, the recorded result of amplification refers to certain averaged amplification values in the frequency range of dominating vibrations. Moreover, a minor change of the incident wavelength or of the angle of incidence is sufficient to cause a change of the amplification S. In the assessment of the effects of vibration on the building development on the surface, and on the safety of people, it is always necessary to take into account the possibility of occurrence of the strongest vibrations that is manifestation of the highest value of the ground factor for vibrations recorded in a given area. The presented solution refers to the case when the wave falls perpendicularly to the stratum. This gives maximum amplifications. In reality, in many situations, the wave falls at an angle, and the amplifications recorded will smaller than the maximum values. However, it should be assumed that, in practice, it is necessary to consider that the least favorable situation may occur. The conformity of the ground factor (amplification) values, for the Upper Silesia conditions, measured empirically with those computed analytically is presented in the references (Mutke, Dworak 199). In practice, one has, most frequently, to do with an overburden composed of several strata. For the multi-strata medium, his factor depends on the thickness of individual overburden strata, modulus of rigidity, density, viscosity, and frequency of input vibrations. Results of analyses of the effect of local structure of the soft soil overburden on the change of vibration acceleration amplitudes. For to compute the maximum accelerations of ground base vibrations for the current and predicted seismicity, the methodology described in this chapter has been used. The data relating to the overburden structure are based on the maps of thickness from the areas of the Piast Ruch I, Piast Ruch II and Brzeszcze mines. The resulting map of the ground factor refers to seismic events with energy of ca J, that is the strongest energies of 45

46 predicted mining-generated tremors. For weaker tremors, the distribution of the ground factor may be different because of the other frequency range of input vibrations characteristic of such energies. Fig. 4. presents the distribution of the amplification factor. The amplification factor varies, over the mining areas of mentioned mines, from 1 to Characteristics of natural seismic activity in Poland The earthquakes in Poland are connected with young tectonic movements, occurring in consequence of vanishing of the activity of the last great Alpine orogeny (Badura, Zuchiewicz 007). During its greatest activity, before ca. 3-1 million years (Oligocene, Miocene), new tectonic structures began to exist. Slowly, the Sudetes and their foreland block started to separate. In the area of the Sudetes foreland, rift valleys and horst mountains were formed. Tectonic movements were accompanied by volcanic eruptions. The young and present tectonic movements are investigated using many methods, from classical observations of deposits in the sites, through measurements of the stress field in deep boreholes, recording of earthquakes, to analyses of topographic maps, satellite photographs, morphometric analyses of drainage basins of streams and tectonic slopes, as well as measurements of water and volumes of liberated radon. About the present movements, we obtain information through seismic and surveying measurements. On the basis of a number of investigations and analyses, several regions are distinguished in the area of Poland, revealing a permanent tendency of vertical movements. Slow subsidence is recorded mainly in north-west Poland and in Silesia Lowland. The uplifted areas are the Carpathians and Sudetes. Less uplifted regions are those of Roztocze and Cracow-Częstochowa Upland. However, these displacements are imperceptible in a human life scale. Only after the lapse of hundreds of years, these changes will be measurable, and after thousands noticeable. The Sudetes foreland, including the Sudetes foreland block and monocline had gradually lowered in the Permian, Mesozoic and Kenozoic and had been the sedimentation area of deposits, and the Sudetes had been lifted. On the basis of geological studies performed in the zone of the Sudetes edge, it has been shown that during the last 500 thousand years there were vertical movements reaching the maximum amplitude in the Bardo region. The highest terrace of the Nysa Kłodzka river was lifted by ca 35 m. 46

47 Coal mine Piast Ruch I Coal mine Piast Ruch II Coal mine Brzeszcze Fig. 4. Map of distribution of soil factor S (vibration amplification factor) in the analyzed mining areas of Piast Ruch I, Piast Ruch II and Brzeszcze mines Fig. 4.3 presents currently observed tendencies of vertical movements in the area of Poland (Badura, Zuchiewicz 007). The GZW region, schematically marked in the figure, lies in the subsidence area. The earthquakes in Poland occur chiefly in the locations where there were the largest vertical displacements of tectonic blocks that are in the regions with not fully relaxed seismic stress. Such a situation in south Poland took place, among others, in the area of the Sudetes foreland block and the Carpathians foreland, as well as in central Poland, along Teisseyre and Tornquist s zone. The location of earthquake epicenters in Poland, based on macroseismic impressions since the 15th century has been presented in Fig. 4.4 (Guterch & Lewandowska- Marciniak 00). 47

48 KWK Piast Ruch I KWK Piast Ruch II Brzeszcze Fig.4.3. Currently observed tendencies of vertical movements in the area of Poland (Obszary o tendencji do Areas prone to; podnoszenia uplift, obniŝania subsidence, stabilne stable),(badura & Zuchiewicz 007) In the Sudetes and their foreland area, as mentioned in the previous chapter, several earthquakes have been documented over 1000 year period. These earthquakes were sensed over a large area, with some of them causing damages to buildings (Pagaczewski 197). The strongest of them was the tremor with magnitude 4.5, which corresponds with the degree 6.5 in accordance with the MSK scale, the one that occurred on 11 th June, 1895 in the Sudetes foreland block. The macroseismic information about this tremor was collected from 360 localities in which it was either sensed or caused damage to buildings. There were observed collapse of chimneys, fall of bricks from roofs and chimneys, origination of fissures in the 48

49 walls and cracks, even 1cm wide, of a building, rise of dust from walls and ceilings, and the inhabitants leaving quickly their houses. KWK Piast Ruch I KWK Piast Ruch II Brzeszcze Fig.4.4. Geotectonic structures (according to Guterch & Grad 1996) and location of epicentres of earthquakes in Poland since 15 th century (Guterch and Lewandowska-Marciniak 00) 1 depth of Moho discontinuity, km; Teisseyrea Tornquist s (T-T) zone; 3 boundary of crystalline base blocks; 4 main fault of the Świętokrzyskie Mts.; 5 transversal structures in the T-T zone; 6 Anomaly zone in the crystalline structure of the T-T zone in south-east Poland (Vp = 7.4 kms -1 ); 7 Epicenters of tremors; 8 mining tremors 49

50 Based on the catalogue of earthquakes in Poland, Czech Republic and Slovakia, and on records ground vibration acceleration, a map has been prepared of the distribution of intensity isolines of seismic tremors (Fig. 4.5.). The area of Piast Ruch I, Piast Ruch II and Brzeszcze is marked in the map. It follows from this map that in this region, in a several tens or hundreds of years, one should expect the intensity 5-6 in the MSK scale. The description of V and VI intensity degrees is the following (Medvedev et al. 1964): Degree V Relatively strong vibrations a) Effects on humans the tremors are sensed by most persons inside the buildings and by many outsiders. Many of those who sleep wake up. Whole buildings slightly shudder. Freely hanging objects sway distinctly, less stable object may move. Open windows and doors may shut and open. b) Effects on buildings - minor non-constructional damages possible (slight fissures in plaster, detachment of its small pieces), but only in the least resistant group of buildings. c) Effects on the natural environment occasionally the efficiency of water sources may change. Degree VI Strong vibrations (minor damages) a) Effects on humans the tremor are distinctly sensed by most people in and outside the buildings. Frightened persons may run out of buildings. In some cases the dishes and glass may get broken, books may fall from the shelves. Movement of furniture is also observed. b) Effects on buildings in few objects with medium resistance (group B), and in many from the weakest group (group A) minor damages may occur; in addition, in the objects of group A few medium damages may occur (minor cracks in walls, pieces of plaster coming off, falling roof tiles, fissures in chimneys). c) Effects on the nature environment - in few cases, cracks in wet ground may appear, and in mountain areas landslides are possible. Changes in the efficiency of water sources and changes of water level in wells are observed. 50

51 KWK Piast Ruch I KWK Piast Ruch II Brzeszcze Fig.4.5. Map of distribution of intensity isolines of seismic tremors in the areas of Poland, Czech Republic and Slovakia, with 90% probability of occurrence over 105 years (period of return 1000 years) (Schenk et al. 001)of seismic tremors in the territory of Poland 4.3. Statistical evaluation of repeatability of strong rock mass tremors The temporal distribution of magnitudes of strong seismic tremors can be determined using statistical extreme distributions, by reason of very high numerical strength and temporal character of the set of data on the tremors occurred. If one assumes that the seismic processes proceed in similar conditions (are characterized by a common distribution) and are stochastically independent, then to assess the extreme magnitude values it is possible to utilize probabilistic distributions. A mathematical model initially used was that based on asymptotic extreme distributions known as Gumbel s distributions type one and three. A great advantage of these distributions lies in taking general assumptions on the type of data which can be described this way. The Gumbel s distribution function type I is given by formula: [ Exp( B( x ))] G( x) Exp u = (4.8) 51