Piotr STRZAŁKOWSKI1, Roman ŚCIGAŁA2 Abstract Introduction

Transcription

1 Piotr STRZAŁKOWSKI 1, Roman ŚCIGAŁA 2 THE EXAMPLE OF LINEAR DISCONTINUOUS DEFORMATIONS CAUSED BY UNDERGROUND EXTRACTION PŘÍKLAD LINEÁRNÍCH NESPOJITÝCH DEFORMACÍ ZPŮSOBENÝCH DOLOVÁNÍM Abstract The example of occurring surface discontinuous deformations in the shape of linear cracks caused by underground extraction of one of Polish coal mines has been presented in this paper. Mining-geological conditions in considered area have been described in connection to location of these deformations. There have been pointed main causes of their occurence as well. Introduction Underground mining influence on the environment includes wide variety of factors. The most significant ones among them are geomechanical transformations of rock mass and land surface. One can mention here: continuous deformations which accompany with every underground mining, irrespective of its mining-geological conditions as well as discontinuous deformations (surface fractures) which occur in case, when some specific conditions are fulfilled. Continuous deformations are observed in the shape of so-called subsidence troughs for many years. There are many mathematical methods that enable working out prognoses of the values of deformation indices that describe the subsidence trough [2, 4, 5, 6, 9]. Discontinuous deformations can be divided into two groups : surface fractures (cavities, sinks) and linear fractures (cracks, riffles). They are the subjects of research from the seventies of 20th century. Polish researches in this field were led in two centres : University of Science and Technology and Silesian Technical University. As a result of research, some methods were worked out for estimation the probability measures of deformation occurrence. These methods assume arising around extracted deposit part of caving zone connected with destressed and cracks zone. For example, in Chudek-Olaszowski method there is an assumption, that cavity will occur on the land surface in case when it penetrates into overburden built up with loose rocks. In Janusz-Jarosz method [3] it is pointed that it is sufficient that top of caving zone reaches the overburden layers. The exchange of knowledge between research institutions in this field made the description of cavity arising more accurate [3]. Two types of sinks were distinguished : Sinks that form suddenly, Sinks that form slowly The necessary condition for forming cavity of first type there is, that top of caving zone penetrates into loose overburden. It occurs in case when the cavity has sufficient width so called boundary width, because the height of caving zone is connected with its width. The necessary condition for forming cavity of second type there is penetration of fracturing zone into overburden. For many years the dominant type of discontinuous deformation in Upper Silesian Basin were deformations of surface type. The land surface sinks were caused by self-filling of caverns connected with an effect of shallow mining. The linear deformations occurred rarely. In recent years the 1 Dr. Hab., Ing., Prof., Silesian Technical University, Gliwice, Poland 2 Dr. Ing., Silesian Technical University, Gliwice, Poland 193

2 proportions of their occurrence are going toward inversion. Land surface cracks and riffles caused by underground mining led in few coal seams to common border are occurring more and more frequently and one should suppose that their number will be growing in the future. One can mention here that this type of discontinuous deformations is to be very dangerous one to buildings and objects of underground infrastructures. Another important thing to keep in mind is the fact that there are no methods of forecasting the probability of deformation occurrence. In this paper the analysis of reasons of discontinuous linear deformation occurrence in the area of one of Polish coal mine has been presented. The characteristic of recorded deformations In the considered area linear fractures in the shape of riffles run approximately parallel to street Z, crossing street K. These riffles have the throw 0.4 to 0.8m and have length about 400m. The characteristic phenomenon here there is different direction of their throw, observed on the K street, so they form in cross section some kind of wedge (pic.1, 2). Analysing further development of deformation one can observe that on the section of 250m length fractures run in parallel to railway and street K, and then they change direction towards NW, and they remains in parallel to each other. Discontinuous deformations had been revealing in different periods in the past, as mine documentation says. On the access road to shaft No 5 and near train station, cracks occurred in These deformations formed as the effect of overlayed edges of extraction that was led in coal seams 354, 357/1, 360, 361, 364, 401/1 at the depth from 370m to 450m with caving in the years (fig.1). street Z acces road to shaft 5 street S street K Fig.1 The location of deformations in correlation to extraction edges Other deformations in the area of railway station arose in 1998 in the shape of riffles. They passed through street S. Their length was about 100m and they were about 0.4m high. The reason of the deformation was the overlapping of edge exploitation conducted in beds: 354, 357/1, 360, 361, 194

3 364, 401/1, 405/3, 406/3. The exploitation was conducted with a fall of roof at the height of 0.9m - 2.5m. Another discontinuous deformations occurred in the south - western part of the railway station. In street S., about 130m from the crossroads with street Z. transverse fault at the beginning of In L. Street, behind a railway flyover in a distance of about 45 m. from the crossroad with street L was observed a fracture of surface but vertical dislocation did not occur. Picture 1. Faults in K. Street 195

4 Picture 2. Faults in K. Street Analysis of mining - geological conditions Rock mass situated in a given area was built of overburden layers and Carboniferous system. The overburden was developed in the form of Tertiary and Quaternary. The Quaternary form of volume about 18.5m was built of soil and sand clay layers. Under those layers occur Tertiary forms developed in the form of plasters with clay interbeddings, sand and dusted clays, clay shales and sandstones. The volume of Tertiary layers is about 147m. There occur Carboniferous forms below 165m, namely orzeskie layers developed in the form of clump and sand layers and sandstone mixed with coal beds from 352/1 to 364. The dominant part in those layers are shales. Ore layers occur below. They are formed of similar types of rocks but it should be mentioned that in the lithological profile the amount of arenaceous shales and sandstone increases. In order to analyse the influence of conducted mining exploitation on occurring of discontinuous deformations, there was prepared a map of thickness of extracted deposit in years The data connected with conducted mining exploitation were worked out on the basis of computer base of Mine. The map of thickness of extracted deposit was prepared by means of SurfCAD programme[8] and has been shown in fig.2. In the case of a total calculation of a thickness of a extracted deposit, the size of an elementary field was divided into exploitation plots as 10x10m. Analysing the map of thickness of extracted deposit we can say that a distribution of thickness of a extracted deposit was irregular and was about m. in the area of deformation. The mining exploitation was mainly conducted on the east and southern- west from street S. The characteristic thing is the fact that exploited plots were separated from each other by an non-extracted body of coal in a given beds that are connected with pillars remained for The East cross-cut no. 3 at the level of 250m, 350m and 450m as well as the 6th cross-cut. As the result of leaving the protective pillars for the mining excavation the coal beds were not exploited in a strip running to the north-east from street K. that was about 100m wide. The other strip of coal beds was not exploited, too. It took place in the southern part and the strip running to the west of street S. was about 150m wide. The surface, which was not exploited resembled the shape of V letter. The face of exploited beds has 196

5 stopped in a parallel line to street S. in the west direction. To the east of the street the beds were only exploited in the southern part. Analysing given materials we can generally say that a system of exploitation edges, conducted in a full range, laid a course of observed deformations. Thickness [m] powyżej Fig.2 The map of thickness of extracted deposit in years Summary On the basis of analysis of presented materials connected with the course of discontinuous deformations and geological-mining conditions, which lead to their occurrence, we can say that: Linear discontinuous deformations are the important aspect of an negative influence of mining exploitation on our environment. It is connected with the fact that occurrence of the deformation is becoming more frequent and it is a reason that vertical planes of mining exploitation overlaps in a few beds. Taking into consideration the fact that the extent of loosening the rock mass is becoming greater as a result of conducted mining exploitation we should suppose that the problem of the deformation will be observed more often. Linear deformations are a danger to building constructions, the more so because there are no methods that make a possibility to predict the occurrence. Even the probability of the deformation occurrence is difficult to predict. In the light of presented examples of the deformation occurrence we should remember about effective ways of decreasing the danger of faults. It means that the underground workings should not be conducted in a way that allow to overlap the edges of exploitation in a few beds. It requires significant broadening of pillars of every kind what leads to a loss of deposits. The geological-mining situation presented in this paper seems to be to some extent rather extreme because the exploitation edges overlapping only in a dozen or so beds during last seventy years of intensive mining 197

6 exploitation. The decrease of strength properties of a rock mass leads to the fact that deformations occur in an uncontrolled way. Both the scale and importance of the problem presented in this paper show the necessity for rational mining exploitation in the aspect of faults limitations. The other important thing presented in this paper is necessity to work out the method that can help to predict the course and scale of deformations. References [179] Chudek, M., Olaszowski, W. (1976): Określenie rodzaju i wielkości deformacji nieciągłych. Ochrona Terenów Górniczych nr 38. [180] Chudek, M. (2002): Geomechanika z podstawami ochrony środowiska górniczego i powierzchni terenu. Wydawnictwo Politechniki Śląskiej. Gliwice. [181] Chudek, M., Janusz, W., Zych, J. (1988): Studium dotyczące stanu rozpoznania tworzenia się i prognozowania deformacji nieciągłych pod wpływem podziemnej eksploatacji złóż. Zeszyty Naukowe Politechniki Śląskiej. Seria Górnictwo z Gliwice. [182] Knothe, S. (1984): Prognozowanie wpływów eksploatacji górniczej. Wydawnictwo Śląsk. Katowice. [183] Kochmański, T. (1956): Obliczanie ruchów punktów górotworu pod wpływem eksploatacji górniczej. PAN. Warszawa. [184] Praca zbiorowa pod red. J. Kwiatka (1997): Ochrona obiektów budowlanych na terenach górniczych. Wydawnictwo Głównego Instytutu Górnictwa. Katowice. [185] Strzałkowski, P. (2002): Wpływ płytkiej eksploatacji górniczej na zagrożenie powierzchni terenu deformacjami nieciągłymi. Z.N. Pol. Śl. S. Górnictwo z Gliwice. [186] Ścigała, R. (2003): Oprogramowanie dla celów tworzenia graficznego obrazu deformacji powierzchni terenu. Konferencja Naukowa: Górnictwo zrównoważonego rozwoju Zeszyty Naukowe Pol.Śl. s.górnictwo, z.258. Gliwice. [187] Sedlak, V. (2004): Deformation vector theory development in the specific conditions of mining subsidence. Transactions of the VSB. Civil Engineering Series, 2/2004 (13 Regionalni konference: Rozvoj seismologie, inzenyrske geofyziky a geotechniky) 198