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Mine Pressure and Rock Displacement Temporal Variation Analysis for Jiu Valley Directional Drifts in the Context of “n” Stability Criterion/Analiza Ciśnień I Chwilowych Przemieszczeń Skał W Badaniach Kierunkowych Wyrobisk W Kopalniach W Dolinie Jiu Z Uwzględnieniem Kryterium Stabilności ‘N’

Mine Pressure and Rock Displacement Temporal Variation Analysis for Jiu Valley Directional Drifts... Arch. Min. Sci., Vol. 59 (2014), No 2, p. 455­466 Electronic version (in color) of this paper is available: http://mining.archives.pl DOI 10.2478/amsc-2014-0032 MIHAELA TODERA*, ROLAND IOSIF MORARU*1 MINE PRESSURE AND ROCK DISPLACEMENT TEMPORAL VARIATION ANALYSIS FOR JIU VALLEY DIRECTIONAL DRIFTS IN THE CONTEXT OF "n" STABILITY CRITERION ANALIZA CINIE I CHWILOWYCH PRZEMIESZCZE SKAL W BADANIACH KIERUNKOWYCH WYROBISK W KOPALNIACH W DOLINIE JIU Z UWZGLDNIENIEM KRYTERIUM STABILNOCI `n' Ensuring mine workings stability over their entire operation period largely depends on the chosen support system and on their interaction with the surrounding rock. Looking at how the main horizontal mine workings are supported in Jiu Valley coal basin, we found that they fall into the category fulfilling a reinforcement role. From the data provided by the documentation of the collieries within this basin, it was found that there were generalized different metal ­ type supporting systems that with respect to operating and working mode are with constant or malleable strength and increasing ­ strength (rigid supporting systems). Research conducted on the stability analysis of horizontal mine workings aimed at elucidate the intensiveness and characteristics of mine pressure, the deformation character and contour displacement of rocks, the interplay between geo-mechanical deformation conditions and deformation extension, as good as the influence of these parameters on the supporting system's behavior. This paper presents a methodology for determining the main laws of mine pressure regime distribution, the results of the burden load values and displacement of support, considering the "n" stability criterion as a complex parameter which can express the laws of variation, for the specific location and operating conditions of directional galleries within the floor of coal seam 3 in Jiu Valley. Keywords: stability ­ reliability, mine pressure, stability criterion, deformation, interaction, time, dependence Zapewnienie stabilnoci wyrobisk w trakcie calego okresu prowadzenia prac górniczych w duej mierze uwarunkowane jest przez dobór odpowiedniej obudowy i kontroli oddzialywania ssiadujcych warstw skalnych. Obserwujc w jaki sposób zabezpieczono i podparto poziome wyrobiska glówne w kopalniach zaglbia wglowego w dolinie Jiu, stwierdzilimy, e obudowy te pelni rol wzmocnie. Na podstawie danych dostarczonych przez kopalnie w rejonie zaglbia wglowego, wiadomo, e w wikszoci UNIVERSITY OF PETROANI, DEPARTMENT OF MINING, SURVEYING AND CIVIL ENGINEERING, 20 UNIVERSITY STR., 332006, PETROANI, ROMANIA CORRESPONDING AUTHOR: E-mail: roland_moraru@yahoo.com byly to rónego typu obudowy metalowe o stalej lub regulowanej wytrzymaloci, uwarunkowanej przez dobór metody wybierania, lub obudowy o zwikszonej wytrzymaloci (sztywne podpory). Przeprowadzono badania obejmujce analiz stabilnoci poziomych wyrobisk majce na celu okrelenie wielkoci i charakterystyki cinienia, charakteru odksztalcenia i przemieszcze warstw skalnych, a take zalenoci pomidzy geo-mechanicznymi warunkami odksztalcenia i rozszerzaniem w trakcie odksztalcenia i wplywu wyej wymienionych parametrów na zachowanie podpór. W pracy przedstawiono take metod okrelania zasad rozkladu cinie w obrbie kopalni, wyniki bada obcie wskutek nadkladu oraz przemieszcze podpór przy wykorzystaniu jako kryterium stabilnoci zloonego parametru `n', który okrela prawa rzdzce zmiennoci tych wielkoci w zalenoci od specyfiki miejsca i warunków prowadzenia wydobycia w kierunkowych chodnikach w obrbie pokladu 3 w kopalni wgla w dolinie Jiu. Slowa kluczowe: stabilno, niezawodno, cinienie, kryterium stabilnoci, odksztalcenia, oddzialywania, zaleno czasowa 1. Introduction In the several past decades, an extensive amount of work has been done about the analysis of stresses and displacements around a horizontal drifts considering various rock mass behaviors and failure criteria (Brown et al., 1983; Carranza-Torres, 2004; Hejmanowsky et al., 2009; Lee & Pietruszczak, 2008). As outlined by Kortas (2013), the physical phenomena occurring in the rock mass are described in various size and time scales. The time scale, applicable to rock deposit space or particular workings, is limited by second-long dynamic processes and century-long geological diastrophic processes. In their natural state, the rock massifs as object mining processes, are permanently under the influence of gravitational forces, of tectonic nature forces, under the action of water in its various forms and of time, causing the onset and development of complex stress state known as the primary or natural stress state, which as long as the rock mass is impelled and can not be deformed, is contained in a latent form. When the rock massive is disturbed by conducting underground works of any kind, the natural stress state in the surrounding rock is amended, amplified and redistributed, creating a new stress state called secondary stress state T. The new stresses that arise can affect at greater or lesser extent the functionality and therefore the stability of underground workings. Two aspects of the stability were emphasized, as follows: · natural stability of the excavation, which can form itself, by the very nature of the rock massif and factors specific to the situation in which mining work is done. It results from the proportion these excavations creates with their execution between the secondary stress state T and the reaction or limit state lim of the surrounding rocks, i.e. T < lim. Mining workings executed in such circumstances need not be supported on their entire operational time span; · required (imposed) stability of underground excavations, as a result of the disturbance in the balance created between rock and excavation, the working development being no more favored by natural, technical mining and conditions; this means that T >> lim and as a result, the stability of the rock massif ­ mine working can be provided only on based on a counteracting force reaction developed by various methods, such as, for example: consolidation, support ­ system mounting or a combination of the two, namely by creating an imposed stability (Todera, 2006). In addition to these resources, mining stability analysis is based on a series of criteria for assessing, in concrete mining and technical conditions, to accurate natural and/or imposed stability of mine workings assessment. The involvement of these criteria in theory design creates prerequisites the ultimate objective to find links between the configuration and size of the pressure regime and working conditions of supporting systems (Letu et al., 1981; Popescu et al, 1982). Furthermore, the resort to the stability criteria of rock massif ­ mine working and supporting system parameters in an imposed stability context of the interaction mechanism may be the basis for choice of alternative of a forecasting model for calculating pressure regime, establish logical links between classes and forms of manifestation of these regimes, state the fundamental principles of a procedure for calculating the development of support systems to address stability and reliability of mine workings throughout their entire life cycle. 2. Directional drifts stability analysis based on ,,n" criterion The ability to quantitatively reflect the implications of geo-mechanical characteristics and of certain technical and mining parameters, including the depth of mine working location, can be achieved through the use of various stability criteria. Given the stability criterion concept definition, this concept was applied in the analysis of experimental results regarding the pressure regime, obtained by in situ investigations (Todera, 1992). In light of these explanations, in a study on the status of surrounding rock deformation around directional drifts, considering rock massif ­ support system interaction for geo-mining conditions in Petrila Colliery perimeter, Todera (1992) analyzed the opportunity of employing the ,,n" stability criterion to elucidate the issues related to dependence of this criterion on the following parameters: compressive breaking strength rc; depth, n = f (H); deformation of mine working's contour, n = f (u); necessary bearing to support; contour deformation of the working; change in pressure and the displacement in time; dependence of pressure and displacement on the value of the stability criterion. Stability analysis can be substantiated by detailed knowledge of the geo-mechanical characteristics of the rocks and their structure changes occurring over time due to moisture and other agents of deterioration, the depth of mine working's location, the influence of cutting ­ supporting technology, the impact exerted by other workings and faces etc. The ,,n" stability criterion relationship can be expressed as: n rcM Cs kld k w rc 1 kct k1k3 a H k f k1k3 a H (1) where: Cs -- structural weakening coefficient of the rock, kld -- long-term strength coefficient of the rock, kw -- coefficient that assesses the influence of humidity on the compressive breaking strength of the rock, kct -- stress concentration factor, which for underground workings with straight walls is 2.5-3.0, k1 -- stress concentration factor induced by neighboring mine workings, k3 -- face-related influence coefficient, with values ranging from 1.0 to 3.5. The situation met in the mine workings located in the floor of seam 3 from Petrila Colliery can be described through the data synthesized in Tables 1 and 2. TABLE 1 Horizontal mine workings situation in the floor of seam 3 from Petrila Colliery Rock type Depth [m] Value of ,,n" index Characterization Sandy clay Compact clay Mine working is highly unstable, rock deformation occurs in the form of plastic flow all around the working. Displacements become unlimited, support must work under simultaneous deformation regime with the surrounding rock and have a lift pi = (300-600) kN/m2 to restore the original balance of the massif. The working is unstable, tending to greater instability. Non-elastic deformations occur throughout the mine working contour. 450 mm or even larger. Displacements are comprised in 100 < u 450 mm or even larger. Support must work under simultaneous deformation with rocks and have a lift of pi = (60-200) kN/m2 to restore the original balance of the massif. The working is of average stability, displacements being u = 50-100 mm. Support is working in given load regime. It is recommended, however, a constant resistance malleable support, pi = (10-60) kN/m2 TABLE 2 Arenaceous clay Argillaceous sandstone Siliceous sandstone Contour displacements of horizontal mine workings, seam 3 floor Displacement values Depth [m] outside the seam, u [mm] Roof Walls Displacement values considering seam's influence, u [mm] Roof Walls Rock type Characterization Sandy clay Compact clay Arenaceous clay Argillaceous sandstone Siliceous sandstone Rock contour is unstable to very unstable especially in the side walls of the working, fully passing to ,,very unstable" category in the influence area of seam 3 mining operation. Outside the influence area of seam 3 mining the workings are stable; otherwise, they move in the average stability domain. Analyzing the stability of mine workings according to "n" dependency criterion and the contour deformation "u" through relationship: n 1,35 u ln 1 0,1 a (2) and considering Figure 1 for the geo-mechanical and mining conditions in Jiu Valley, it appears that supporting systems of these workings are working under heavy loads when n < 0.35, while the contour deformation of simple profile mine workings is included in the 100 mm-410 mm range or 200 mm-600 mm for double profiles (Todera, 1999). Fig. 1. The ,,n" stability criterion dependency on rock displacement on the mine working's contour ,,u" 3. Stress ­ strain regime emergence in directional drifts located in seam 3 floor at Petrila Colliery During 3 years measurements were carried out on the deformation of the support in galleries from levels +50, ­50, ­100 and ­150, and the results (Todera, 1992) allowed mathematical statistics processing and setting: the dependence on specific minimum bearing capacity required to metal arched support according to the values of the stability coefficient; dependence of the support movement according to the coefficient of stability; the pressure dependence of the support with respect to time and the rock displacement versus time dependence for calculated stability coefficients. A) Dependence on specific minimum bearing capacity required to metal arched support according to the values of the stability coefficient For each of the stations tracking displacements it was determined the specific bearing capacity of the real support installed (Pr) by multiplying the specific bearing capacity of an arch to the number of arches, N, installed in 1meter of mine working (3 arches/ meter) i.e.: Pr = N ·Psc (3) The specific bearing values obtained were inlaid in the coordinate system: stability coefficient n on the x-axis and bearing capacity on the y-coordinate line (Todera, 1992; Todorescu, 1993). Based on experimental data were approximated for each Pr interval, the points which interlinked yielded a curve that determines the minimum bearing capacity required to ensure a reliability level of about 0.95. B) Experimental dependence of support displacement on stability coefficient values Such a dependence was obtained by statistical processing of data values recorded at mine levels +50, ­50, ­100 and ­150, resulting in the following regression equation: U 0, 0414 0, 043 0, 019 2 n n (4) C) Pressure dependence on the support versus time span The same results obtained from measurements (Todera, 1992; Todera, 1999) led to the establishment of pressure relationship that we consider to approximate the experimentally obtained curves of pressure values; in the resulting correlation, parameters a and b were determined according to stability coefficient ,,n". Such a correlation is a curve of the form: p 1 t b ln a b (5) where: a and b are dimensionless factors determined by the method of least squares, or by replacing in Eq. (5), the values of the pressure p, for t = 200 × 24 hours, respectively, t = 1000 × 24 hours and whose expressions are having the shapes given bellow Level ­50: a 0, 664 n 2 0,1785 n 0, 0839 b 0, 000824 n 2 4,948 n 0, 000839 Level ­100: a 0, 00426 n 2 0,517 n 0, 000443 b 0, 00768 n 2 4,952 n 0, 0017 Level ­150: a 0, 00161 n 2 0,521 n 0, 000806 b 0, 00644 n 2 4,942 n 0, 00304 (8) (7) (6) Through pressure calculations according to relation (5) there were obtained corresponding time tracking values; Table 3 synthesizes the results for the directional gallery located at ­150 level. TABLE 3 Pressure variation with time for the directional drift level ­150, Petrila Colliery Rock type Time [days] Sandy clay Compact clay Arenaceous clay Argillaceous sandstone Siliceous sandstone 0 65.59 73.53 76.01 78.38 81.57 83.82 85.39 8.,6 87.69 88.81 88.83 Solving equations (6), (7) and (8) according to ,,n" stability criterion values allowed to obtain the a and b coefficients values as given in Table 4. TABLE 4 Values of a and b coefficients according to ,,n" stability criterion ,,n" stabilitaty criterion 0.138 0.2098 0.390 0.664 0.920 Coefficient values obtained a b 0.072 0.682 0.1153 1.038 0.204 1.929 0.348 3.285 0.482 4.551 In the cases considered, Eq. (5) assessing the load exerted on metal supports of directional drifts located in the floor of seam 3 at Petrila Colliery, will have the shapes given bellow (see Figure 2): Level ­50: ln p t 0, 000824 n 2 4,948 n 0, 000839 0, 000824 n 2 4,948 n 0, 000839 0, 664 n 2 0,1785 n 0, 0839 (9) Level ­100: ln p t 0, 00768 n 2 4,952 n 0, 0017 0, 00768 n 2 4,952 n 0, 0017 0, 00426 n 2 0,517 n 0, 000443 (10) a) b) c) Fig. 2. Graphs of pressure variation in time in directional drifts located in seam 3 Petrila Colliery ­ Jiu Valley basin: a) Level ­50; b) Level ­100 c) Level ­150 Level ­150: ln p t 0, 00644 n 2 4,942 n 0, 00304 0, 00644 n 2 4,942 n 0, 00304 0, 00161 n 2 0,521 n 0, 000806 (11) Similarly, by processing the results of measurements based on the values support displacement according the ,,n" stability coefficient, we have reached a corresponding relationship, as it follows: ut 28,571 t 0,174 n 2 0, 436 n 1 0, 28 0,174 n 0, 436 n 0, 28 e3,94 n (12) on which were built the graphs relating the rock contour movements of the mine working, to the duration of observations and the stability coefficient ,,n" (see Figures 3 and 4). a) b) Fig. 3. Graph of rock displacement variation on working's contour versus ,,u" the "n" stability criterion Expressing the experimental results in the form of dependencies p = p(n) and u = u(n), a correlation relationship can be observed between the two parameters considered: p 0, 091 0,112 n 1 0, 068 n 2 u 0, 0016 0, 0195 n 1 0, 043 n 2 (13) From the above, it follows that the index ,,n" can be considered as a complex parameter through which we can express most of the main laws of pressure regime manifestation and assess the mining loads on support and it's displacement. Fig. 4. Diagrammatic representation of the rock contour displacement ,,u"dependence on observation time span "t" and the ,,n" criterion of stability Analyzing further the implications of such a stability criterion it was found that for values of n < 0.2 and n > 1 practically the displacements do not depends on the value of cross-sectional area of the mine working, or the angle of rock stratifications. For the range 0.2 < n < 1, the displacement increase is proportional to the increase in the cross ­ sectional area of the mine working. With increasing angle of rock stratifications, = 0°-90°, it proportionally decreases the rock and support displacement, approximately with 30% for cross-cut galleries and only (10-15) % for directional drifts. 4. Conclusions Objective primary information on the laws that can characterize in geomechanic terms the rock massif describe in a qualitative and quantitative way the deformation of the rock massif and elucidates the pressure regime manifestation in all its forms. They are needed for creating improvement opportunities of the existing techniques and development of new methods aimed to find means to ensure the mine workings stability according to the present level of scientific and technical development. To achieve such a goal complex research was carried out, combining laboratory and in situ field tests with thorough analytical research. The concept of stability is cumulative, aimed firstly at knowledge of the real geo-mechanical characteristics of the rock, a natural state of tension, the secondary stress-strain state, i.e. the pressure of the rock massif, and secondly trying an explanation of geotectonic processes and groundwater presence in the very context of production activities related to the construction of underground excavations. Currently, there is an intensive development of analytical research on the evaluation of the mine workings stability. However, they allow yet only approximate evaluations, guidance and therefore it requires in almost all cases, laboratory test for experimental verification (modeling) and in situ observations ­ measurements. Laboratory and in situ research techniques alone, allow to obtain only the cumulative effect of all determinants, without being able to separate the effects of each. This is why in our case study regarding Petrila Colliery from the Romanian Jiu Valley coal basin we applied this combined approach. Based on the same analysis, diversification can be achieved in several main groups of the manifestation character of the pressure regime from the Jiu Valley, with direct reference to Petrila Colliery, namely: a) for n 0,7, the load on the support for a period of (30-60) days increases to an average of 30 kPa, following a reduction thereof by about (20 to 25) %, followed by a further but slow increase (1.2 kPa/year), tending to a constant value in t (2.5-3) years. As a result, the load on the support, for t > (10-15) will be (40-50) kPa and basically depends on the malleability of the support system. This mode of manifestation of the pressure corresponds to the formation of a natural dome of equilibrium at the mine working's roof, as confirmed by both displacement measurements and direct observations during excavation and support setting stages; b) for 0,2 < n < 0,7 the burden increases more intensively on the support, although in this case both the pressure and displacement curves gradually flattens. The value of load on the support depends essentially on the malleability of the support system. In such circumstances, around the mine working the non-elastic deformation area (pseudoplastic deformation behavior) is formed, whose size can change quite broad limits; c) for n 0,2, rock around mine working will manifest as a viscous ­ plastic massif. The loads on the support are quickly increasing and rock movements, when backward push is not enough, are having a viscous flow character. The load on limited supporting malleability after (6-10) months of operation does not really depends on the characteristics of support, reaching; the load burden reaches (0.8 to 2.2) MPa at a depth of 800 m. Conclusively, in the context of stability concept are included three main categories of problems: (i) issues related to determining the deformation and fracture behavior of rocks in which mine workings are located, due to the complex actions occurring around them; (ii) problems involving the phenomenon of rock massive elements interaction with each other, and with the elements of the mine structures based on the geo-mechanical conditions and characteristics of the support; (iii) problems related to the influence of mining operations on the workings, particularly the undermining phenomenon, and respectively overmining. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Archives of Mining Sciences de Gruyter

Mine Pressure and Rock Displacement Temporal Variation Analysis for Jiu Valley Directional Drifts in the Context of “n” Stability Criterion/Analiza Ciśnień I Chwilowych Przemieszczeń Skał W Badaniach Kierunkowych Wyrobisk W Kopalniach W Dolinie Jiu Z Uwzględnieniem Kryterium Stabilności ‘N’

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Abstract

Arch. Min. Sci., Vol. 59 (2014), No 2, p. 455­466 Electronic version (in color) of this paper is available: http://mining.archives.pl DOI 10.2478/amsc-2014-0032 MIHAELA TODERA*, ROLAND IOSIF MORARU*1 MINE PRESSURE AND ROCK DISPLACEMENT TEMPORAL VARIATION ANALYSIS FOR JIU VALLEY DIRECTIONAL DRIFTS IN THE CONTEXT OF "n" STABILITY CRITERION ANALIZA CINIE I CHWILOWYCH PRZEMIESZCZE SKAL W BADANIACH KIERUNKOWYCH WYROBISK W KOPALNIACH W DOLINIE JIU Z UWZGLDNIENIEM KRYTERIUM STABILNOCI `n' Ensuring mine workings stability over their entire operation period largely depends on the chosen support system and on their interaction with the surrounding rock. Looking at how the main horizontal mine workings are supported in Jiu Valley coal basin, we found that they fall into the category fulfilling a reinforcement role. From the data provided by the documentation of the collieries within this basin, it was found that there were generalized different metal ­ type supporting systems that with respect to operating and working mode are with constant or malleable strength and increasing ­ strength (rigid supporting systems). Research conducted on the stability analysis of horizontal mine workings aimed at elucidate the intensiveness and characteristics of mine pressure, the deformation character and contour displacement of rocks, the interplay between geo-mechanical deformation conditions and deformation extension, as good as the influence of these parameters on the supporting system's behavior. This paper presents a methodology for determining the main laws of mine pressure regime distribution, the results of the burden load values and displacement of support, considering the "n" stability criterion as a complex parameter which can express the laws of variation, for the specific location and operating conditions of directional galleries within the floor of coal seam 3 in Jiu Valley. Keywords: stability ­ reliability, mine pressure, stability criterion, deformation, interaction, time, dependence Zapewnienie stabilnoci wyrobisk w trakcie calego okresu prowadzenia prac górniczych w duej mierze uwarunkowane jest przez dobór odpowiedniej obudowy i kontroli oddzialywania ssiadujcych warstw skalnych. Obserwujc w jaki sposób zabezpieczono i podparto poziome wyrobiska glówne w kopalniach zaglbia wglowego w dolinie Jiu, stwierdzilimy, e obudowy te pelni rol wzmocnie. Na podstawie danych dostarczonych przez kopalnie w rejonie zaglbia wglowego, wiadomo, e w wikszoci UNIVERSITY OF PETROANI, DEPARTMENT OF MINING, SURVEYING AND CIVIL ENGINEERING, 20 UNIVERSITY STR., 332006, PETROANI, ROMANIA CORRESPONDING AUTHOR: E-mail: roland_moraru@yahoo.com byly to rónego typu obudowy metalowe o stalej lub regulowanej wytrzymaloci, uwarunkowanej przez dobór metody wybierania, lub obudowy o zwikszonej wytrzymaloci (sztywne podpory). Przeprowadzono badania obejmujce analiz stabilnoci poziomych wyrobisk majce na celu okrelenie wielkoci i charakterystyki cinienia, charakteru odksztalcenia i przemieszcze warstw skalnych, a take zalenoci pomidzy geo-mechanicznymi warunkami odksztalcenia i rozszerzaniem w trakcie odksztalcenia i wplywu wyej wymienionych parametrów na zachowanie podpór. W pracy przedstawiono take metod okrelania zasad rozkladu cinie w obrbie kopalni, wyniki bada obcie wskutek nadkladu oraz przemieszcze podpór przy wykorzystaniu jako kryterium stabilnoci zloonego parametru `n', który okrela prawa rzdzce zmiennoci tych wielkoci w zalenoci od specyfiki miejsca i warunków prowadzenia wydobycia w kierunkowych chodnikach w obrbie pokladu 3 w kopalni wgla w dolinie Jiu. Slowa kluczowe: stabilno, niezawodno, cinienie, kryterium stabilnoci, odksztalcenia, oddzialywania, zaleno czasowa 1. Introduction In the several past decades, an extensive amount of work has been done about the analysis of stresses and displacements around a horizontal drifts considering various rock mass behaviors and failure criteria (Brown et al., 1983; Carranza-Torres, 2004; Hejmanowsky et al., 2009; Lee & Pietruszczak, 2008). As outlined by Kortas (2013), the physical phenomena occurring in the rock mass are described in various size and time scales. The time scale, applicable to rock deposit space or particular workings, is limited by second-long dynamic processes and century-long geological diastrophic processes. In their natural state, the rock massifs as object mining processes, are permanently under the influence of gravitational forces, of tectonic nature forces, under the action of water in its various forms and of time, causing the onset and development of complex stress state known as the primary or natural stress state, which as long as the rock mass is impelled and can not be deformed, is contained in a latent form. When the rock massive is disturbed by conducting underground works of any kind, the natural stress state in the surrounding rock is amended, amplified and redistributed, creating a new stress state called secondary stress state T. The new stresses that arise can affect at greater or lesser extent the functionality and therefore the stability of underground workings. Two aspects of the stability were emphasized, as follows: · natural stability of the excavation, which can form itself, by the very nature of the rock massif and factors specific to the situation in which mining work is done. It results from the proportion these excavations creates with their execution between the secondary stress state T and the reaction or limit state lim of the surrounding rocks, i.e. T < lim. Mining workings executed in such circumstances need not be supported on their entire operational time span; · required (imposed) stability of underground excavations, as a result of the disturbance in the balance created between rock and excavation, the working development being no more favored by natural, technical mining and conditions; this means that T >> lim and as a result, the stability of the rock massif ­ mine working can be provided only on based on a counteracting force reaction developed by various methods, such as, for example: consolidation, support ­ system mounting or a combination of the two, namely by creating an imposed stability (Todera, 2006). In addition to these resources, mining stability analysis is based on a series of criteria for assessing, in concrete mining and technical conditions, to accurate natural and/or imposed stability of mine workings assessment. The involvement of these criteria in theory design creates prerequisites the ultimate objective to find links between the configuration and size of the pressure regime and working conditions of supporting systems (Letu et al., 1981; Popescu et al, 1982). Furthermore, the resort to the stability criteria of rock massif ­ mine working and supporting system parameters in an imposed stability context of the interaction mechanism may be the basis for choice of alternative of a forecasting model for calculating pressure regime, establish logical links between classes and forms of manifestation of these regimes, state the fundamental principles of a procedure for calculating the development of support systems to address stability and reliability of mine workings throughout their entire life cycle. 2. Directional drifts stability analysis based on ,,n" criterion The ability to quantitatively reflect the implications of geo-mechanical characteristics and of certain technical and mining parameters, including the depth of mine working location, can be achieved through the use of various stability criteria. Given the stability criterion concept definition, this concept was applied in the analysis of experimental results regarding the pressure regime, obtained by in situ investigations (Todera, 1992). In light of these explanations, in a study on the status of surrounding rock deformation around directional drifts, considering rock massif ­ support system interaction for geo-mining conditions in Petrila Colliery perimeter, Todera (1992) analyzed the opportunity of employing the ,,n" stability criterion to elucidate the issues related to dependence of this criterion on the following parameters: compressive breaking strength rc; depth, n = f (H); deformation of mine working's contour, n = f (u); necessary bearing to support; contour deformation of the working; change in pressure and the displacement in time; dependence of pressure and displacement on the value of the stability criterion. Stability analysis can be substantiated by detailed knowledge of the geo-mechanical characteristics of the rocks and their structure changes occurring over time due to moisture and other agents of deterioration, the depth of mine working's location, the influence of cutting ­ supporting technology, the impact exerted by other workings and faces etc. The ,,n" stability criterion relationship can be expressed as: n rcM Cs kld k w rc 1 kct k1k3 a H k f k1k3 a H (1) where: Cs -- structural weakening coefficient of the rock, kld -- long-term strength coefficient of the rock, kw -- coefficient that assesses the influence of humidity on the compressive breaking strength of the rock, kct -- stress concentration factor, which for underground workings with straight walls is 2.5-3.0, k1 -- stress concentration factor induced by neighboring mine workings, k3 -- face-related influence coefficient, with values ranging from 1.0 to 3.5. The situation met in the mine workings located in the floor of seam 3 from Petrila Colliery can be described through the data synthesized in Tables 1 and 2. TABLE 1 Horizontal mine workings situation in the floor of seam 3 from Petrila Colliery Rock type Depth [m] Value of ,,n" index Characterization Sandy clay Compact clay Mine working is highly unstable, rock deformation occurs in the form of plastic flow all around the working. Displacements become unlimited, support must work under simultaneous deformation regime with the surrounding rock and have a lift pi = (300-600) kN/m2 to restore the original balance of the massif. The working is unstable, tending to greater instability. Non-elastic deformations occur throughout the mine working contour. 450 mm or even larger. Displacements are comprised in 100 < u 450 mm or even larger. Support must work under simultaneous deformation with rocks and have a lift of pi = (60-200) kN/m2 to restore the original balance of the massif. The working is of average stability, displacements being u = 50-100 mm. Support is working in given load regime. It is recommended, however, a constant resistance malleable support, pi = (10-60) kN/m2 TABLE 2 Arenaceous clay Argillaceous sandstone Siliceous sandstone Contour displacements of horizontal mine workings, seam 3 floor Displacement values Depth [m] outside the seam, u [mm] Roof Walls Displacement values considering seam's influence, u [mm] Roof Walls Rock type Characterization Sandy clay Compact clay Arenaceous clay Argillaceous sandstone Siliceous sandstone Rock contour is unstable to very unstable especially in the side walls of the working, fully passing to ,,very unstable" category in the influence area of seam 3 mining operation. Outside the influence area of seam 3 mining the workings are stable; otherwise, they move in the average stability domain. Analyzing the stability of mine workings according to "n" dependency criterion and the contour deformation "u" through relationship: n 1,35 u ln 1 0,1 a (2) and considering Figure 1 for the geo-mechanical and mining conditions in Jiu Valley, it appears that supporting systems of these workings are working under heavy loads when n < 0.35, while the contour deformation of simple profile mine workings is included in the 100 mm-410 mm range or 200 mm-600 mm for double profiles (Todera, 1999). Fig. 1. The ,,n" stability criterion dependency on rock displacement on the mine working's contour ,,u" 3. Stress ­ strain regime emergence in directional drifts located in seam 3 floor at Petrila Colliery During 3 years measurements were carried out on the deformation of the support in galleries from levels +50, ­50, ­100 and ­150, and the results (Todera, 1992) allowed mathematical statistics processing and setting: the dependence on specific minimum bearing capacity required to metal arched support according to the values of the stability coefficient; dependence of the support movement according to the coefficient of stability; the pressure dependence of the support with respect to time and the rock displacement versus time dependence for calculated stability coefficients. A) Dependence on specific minimum bearing capacity required to metal arched support according to the values of the stability coefficient For each of the stations tracking displacements it was determined the specific bearing capacity of the real support installed (Pr) by multiplying the specific bearing capacity of an arch to the number of arches, N, installed in 1meter of mine working (3 arches/ meter) i.e.: Pr = N ·Psc (3) The specific bearing values obtained were inlaid in the coordinate system: stability coefficient n on the x-axis and bearing capacity on the y-coordinate line (Todera, 1992; Todorescu, 1993). Based on experimental data were approximated for each Pr interval, the points which interlinked yielded a curve that determines the minimum bearing capacity required to ensure a reliability level of about 0.95. B) Experimental dependence of support displacement on stability coefficient values Such a dependence was obtained by statistical processing of data values recorded at mine levels +50, ­50, ­100 and ­150, resulting in the following regression equation: U 0, 0414 0, 043 0, 019 2 n n (4) C) Pressure dependence on the support versus time span The same results obtained from measurements (Todera, 1992; Todera, 1999) led to the establishment of pressure relationship that we consider to approximate the experimentally obtained curves of pressure values; in the resulting correlation, parameters a and b were determined according to stability coefficient ,,n". Such a correlation is a curve of the form: p 1 t b ln a b (5) where: a and b are dimensionless factors determined by the method of least squares, or by replacing in Eq. (5), the values of the pressure p, for t = 200 × 24 hours, respectively, t = 1000 × 24 hours and whose expressions are having the shapes given bellow Level ­50: a 0, 664 n 2 0,1785 n 0, 0839 b 0, 000824 n 2 4,948 n 0, 000839 Level ­100: a 0, 00426 n 2 0,517 n 0, 000443 b 0, 00768 n 2 4,952 n 0, 0017 Level ­150: a 0, 00161 n 2 0,521 n 0, 000806 b 0, 00644 n 2 4,942 n 0, 00304 (8) (7) (6) Through pressure calculations according to relation (5) there were obtained corresponding time tracking values; Table 3 synthesizes the results for the directional gallery located at ­150 level. TABLE 3 Pressure variation with time for the directional drift level ­150, Petrila Colliery Rock type Time [days] Sandy clay Compact clay Arenaceous clay Argillaceous sandstone Siliceous sandstone 0 65.59 73.53 76.01 78.38 81.57 83.82 85.39 8.,6 87.69 88.81 88.83 Solving equations (6), (7) and (8) according to ,,n" stability criterion values allowed to obtain the a and b coefficients values as given in Table 4. TABLE 4 Values of a and b coefficients according to ,,n" stability criterion ,,n" stabilitaty criterion 0.138 0.2098 0.390 0.664 0.920 Coefficient values obtained a b 0.072 0.682 0.1153 1.038 0.204 1.929 0.348 3.285 0.482 4.551 In the cases considered, Eq. (5) assessing the load exerted on metal supports of directional drifts located in the floor of seam 3 at Petrila Colliery, will have the shapes given bellow (see Figure 2): Level ­50: ln p t 0, 000824 n 2 4,948 n 0, 000839 0, 000824 n 2 4,948 n 0, 000839 0, 664 n 2 0,1785 n 0, 0839 (9) Level ­100: ln p t 0, 00768 n 2 4,952 n 0, 0017 0, 00768 n 2 4,952 n 0, 0017 0, 00426 n 2 0,517 n 0, 000443 (10) a) b) c) Fig. 2. Graphs of pressure variation in time in directional drifts located in seam 3 Petrila Colliery ­ Jiu Valley basin: a) Level ­50; b) Level ­100 c) Level ­150 Level ­150: ln p t 0, 00644 n 2 4,942 n 0, 00304 0, 00644 n 2 4,942 n 0, 00304 0, 00161 n 2 0,521 n 0, 000806 (11) Similarly, by processing the results of measurements based on the values support displacement according the ,,n" stability coefficient, we have reached a corresponding relationship, as it follows: ut 28,571 t 0,174 n 2 0, 436 n 1 0, 28 0,174 n 0, 436 n 0, 28 e3,94 n (12) on which were built the graphs relating the rock contour movements of the mine working, to the duration of observations and the stability coefficient ,,n" (see Figures 3 and 4). a) b) Fig. 3. Graph of rock displacement variation on working's contour versus ,,u" the "n" stability criterion Expressing the experimental results in the form of dependencies p = p(n) and u = u(n), a correlation relationship can be observed between the two parameters considered: p 0, 091 0,112 n 1 0, 068 n 2 u 0, 0016 0, 0195 n 1 0, 043 n 2 (13) From the above, it follows that the index ,,n" can be considered as a complex parameter through which we can express most of the main laws of pressure regime manifestation and assess the mining loads on support and it's displacement. Fig. 4. Diagrammatic representation of the rock contour displacement ,,u"dependence on observation time span "t" and the ,,n" criterion of stability Analyzing further the implications of such a stability criterion it was found that for values of n < 0.2 and n > 1 practically the displacements do not depends on the value of cross-sectional area of the mine working, or the angle of rock stratifications. For the range 0.2 < n < 1, the displacement increase is proportional to the increase in the cross ­ sectional area of the mine working. With increasing angle of rock stratifications, = 0°-90°, it proportionally decreases the rock and support displacement, approximately with 30% for cross-cut galleries and only (10-15) % for directional drifts. 4. Conclusions Objective primary information on the laws that can characterize in geomechanic terms the rock massif describe in a qualitative and quantitative way the deformation of the rock massif and elucidates the pressure regime manifestation in all its forms. They are needed for creating improvement opportunities of the existing techniques and development of new methods aimed to find means to ensure the mine workings stability according to the present level of scientific and technical development. To achieve such a goal complex research was carried out, combining laboratory and in situ field tests with thorough analytical research. The concept of stability is cumulative, aimed firstly at knowledge of the real geo-mechanical characteristics of the rock, a natural state of tension, the secondary stress-strain state, i.e. the pressure of the rock massif, and secondly trying an explanation of geotectonic processes and groundwater presence in the very context of production activities related to the construction of underground excavations. Currently, there is an intensive development of analytical research on the evaluation of the mine workings stability. However, they allow yet only approximate evaluations, guidance and therefore it requires in almost all cases, laboratory test for experimental verification (modeling) and in situ observations ­ measurements. Laboratory and in situ research techniques alone, allow to obtain only the cumulative effect of all determinants, without being able to separate the effects of each. This is why in our case study regarding Petrila Colliery from the Romanian Jiu Valley coal basin we applied this combined approach. Based on the same analysis, diversification can be achieved in several main groups of the manifestation character of the pressure regime from the Jiu Valley, with direct reference to Petrila Colliery, namely: a) for n 0,7, the load on the support for a period of (30-60) days increases to an average of 30 kPa, following a reduction thereof by about (20 to 25) %, followed by a further but slow increase (1.2 kPa/year), tending to a constant value in t (2.5-3) years. As a result, the load on the support, for t > (10-15) will be (40-50) kPa and basically depends on the malleability of the support system. This mode of manifestation of the pressure corresponds to the formation of a natural dome of equilibrium at the mine working's roof, as confirmed by both displacement measurements and direct observations during excavation and support setting stages; b) for 0,2 < n < 0,7 the burden increases more intensively on the support, although in this case both the pressure and displacement curves gradually flattens. The value of load on the support depends essentially on the malleability of the support system. In such circumstances, around the mine working the non-elastic deformation area (pseudoplastic deformation behavior) is formed, whose size can change quite broad limits; c) for n 0,2, rock around mine working will manifest as a viscous ­ plastic massif. The loads on the support are quickly increasing and rock movements, when backward push is not enough, are having a viscous flow character. The load on limited supporting malleability after (6-10) months of operation does not really depends on the characteristics of support, reaching; the load burden reaches (0.8 to 2.2) MPa at a depth of 800 m. Conclusively, in the context of stability concept are included three main categories of problems: (i) issues related to determining the deformation and fracture behavior of rocks in which mine workings are located, due to the complex actions occurring around them; (ii) problems involving the phenomenon of rock massive elements interaction with each other, and with the elements of the mine structures based on the geo-mechanical conditions and characteristics of the support; (iii) problems related to the influence of mining operations on the workings, particularly the undermining phenomenon, and respectively overmining.

Journal

Archives of Mining Sciencesde Gruyter

Published: Jun 1, 2014

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