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Analysis of the Influence of the Vertical Coaxiality of the Pillars on the Stability of the Resistance Structures, from the Ocnele Mari Saline

Analysis of the Influence of the Vertical Coaxiality of the Pillars on the Stability of the... Revista Minelor – Mining Revue ISSN-L 1220-2053 / ISSN 2247-8590 vol. 28, issue 4 / 2022, pp. 42-50 ANALYSIS OF THE INFLUENCE OF THE VERTICAL COAXIALITY OF THE PILLARS ON THE STABILITY OF THE RESISTANCE STRUCTURES, FROM THE OCNELE MARI SALINE 1 * 2 3 Dacian-Paul MARIAN , Ilie ONICA , Ovidiu MARINA University of Petroșani, Petroșani, Romania, [email protected] University of Petroșani, Petroșani, Romania, [email protected] University of Petroșani, Petroșani, Romania, [email protected] DOI: 10.2478/minrv-2022-0029 Abstract: The Ocnele Mari rock salt deposit is exploited with small rooms and square pillars, on the horizons +226 m and 210 m. Following the exploitation of the lower horizon +210 m, the floor between the two horizons suffered instability phenomena, marked by fractures and cracks and local degradation of the pillars.Since the pillars at the two horizons have certain deviations from the coaxiality, the question was raised whether the instability phenomena were generated by the deviations from the coaxiality.This article presents the stability analysis of bearing structures, taking into account the actual geometry of pillars, 3D finite element analysis and analysis of safety factors at the ceiling level, calculated from 2D finite element models. The final conclusion is that the instability phenomena that occurred at the Ocnele Mari Saline were generated by the state of stresses and strains, produced by the spatial distribution of the underground voids and the geomechanical characteristics of the rock salt and insignificantly, by the deviation from the coaxiality of the pillars. Keywords: rock salt, rooms and pillars mining, pillar, ceiling, stability analysis, finite element, stress, strain, safety factor 1. Geology of the Ocnele Mari-Cocenești deposit The Ocnele Mari salt deposit can be found in the area of the subcarpathian hills of Oltenia, stretching from the East of the Olt river to the West of the Govora stream, passing through the territory of the Ocnele Mari locality in Vâlcea county. The Coceneşti perimeter is located in the eastern area of the Ocnele Mari deposit. Access to the deposit is on the road that goes to the town of Ocnele Mari, Vâlcea county. The morphology of the region has a hilly aspect, with heights between 250 - 450 m. The Ocnele Mari region, where the deposit is located, belongs to the Getic Depression, which is the most external unit of the Southern Carpathians. It was formed as a result of the laramic movements, as a consequence of the uplift of the crystalline-Mesozoic zone, in front of which a premontane depression was formed, with the role of an ”avant-fossé” that functioned in this way during the Paleogene and Neogene. From a stratigraphic point of view, the Ocnele Mari region includes Paleogene, Neogene and Quaternary geological formations. The horizon with rock salt deposits is presented in a lagoonal facies, with local distribution, being formed by salt masses, gypsum and salty marls. The rock salt deposit from Ocnele Mari is included in this horizon, and the salt massif appears in the area of axial uplift from Ocniţa - Ocnele Mari. The age of the rock salt from Ocnele Mari - Coceneşti is middle Badenian [1, 2]. The Ocnele-Mari salt deposit has the shape of an elongated lens in the E-W direction, measuring approx. 7.5 km, and to the N-S approx. 3.5 km, presenting an axial uplift in the Ocniţei area, with slopes to the N. The Corresponding author: Dacian-Paul Marian, Assoc.Prof. PhD. Eng., University of Petroșani, Petroșani, Romania, contact details (University st. no. 20, Petroșani, Romania [email protected]) 42 Revista Minelor – Mining Revue vol. 28, issue 4 / 2022 ISSN-L 1220-2053 / ISSN 2247-8590 pp. 42-50 thickness of the salt deposit is variable, reaching laminations in the northern and southern parts, the maximum thickness reaching 450 m in the central part of the lens. The rock salt in the deposit has a macrogranular structure with well-developed crystals, grayish white or blackish in color, depending on the contribution of terrigenous impurities. Banks of white salt alternate with those of darker salt, and in the lower part of the deposit, a bank of black salt with thicknesses from 5 to 30 m, impurized with anhydrite, having a higher hardness and compactness than the rest of the salt, frequently appears from the deposit [1, 2]. 2. Instability phenomena occurring at the Ocnele Mari saline In the period 1993-1996, the mining workings to open the Ocnele Mari salt mine were carried out. The information obtained with the opening workings completed the picture of the deposit, and the underground drillings carried out starting in 1994 allowed a more accurate outline of the rock salt – barren rocks limit and the start of exploitation of the deposit at the level of the +226 m horizon. The designed mining method, for the geo-mining conditions at this salt mine, is the method with small rooms and square pillars [3], arranged in a 30 x 30 m grid, with different sizes in the eastern and western wings (table 1). After mining the deposit at the +226 m level, mining continued under an 8 m thick rock salt ceiling, at the +210 m level. Table 1. Geometric parameters of the mining method with small rooms and square pillars, for Ocnele Mari - Cocenești Saline [4, 5] Horizon Starting year of Room Pillar Ceiling Level exploitation Width, Height, [m] Width, Height, [m] Thickness, Height, W/ E [m] W/E [m] [m] [m] +226m 1996 16/15 8 14/15 8 8 16 +210m 2001 16/15 8 14/15 8 8 16 After the deposit was extracted, a tourist base was set up in the exploited spaces of the western wing, in accordance with the feasibility study developed in 2009 [6]. Following the exploitation of the +210 m horizon, in the western wing, related to the tourist base, a series of cracks and microcracks appeared in the ceiling between the two horizons (table 2), associated with cracks and detachments of pieces of rock salt at the +210 m horizon, from the ceiling and from the walls of the pillars, in the area of rooms 24-25, horizon +226/210W. Also, a series of instability phenomena could be observed in the area of ventilation shaft 5, between pillars H22 and H23, in the form of cracks in the ceiling, with displacements both in the horizontal and vertical planes. Table 2. The situation of the cracks in the floor between horizontal +226m and +210m, in the area of rooms 24-25 [7] Fissure Room Between Fissure Fissure Room Between Fissure pillars length, m pillars length, m F1 H G25- 8.80 F4 H H24- 11.43 H25 G24 F2 24 H24- 6.42 F5 H H23- 8.45 H25 G23 F3 24 H24- 8.86 F6 G G23- 9.90 H25 F23 At the request of the salt mine, in 2014, SC MINESA SA [6] analyzed the causes that led to the appearance of these instability phenomena. In this study, based on the topographic measurements carried out on the resistance structures, it was concluded that, if the inter-rooms pillars do not overlap and have deviations from coaxiality, in the ”rooms-pillars-ceiling” system, the following instability phenomena may occur, among others [6]: - horizontal and vertical deformations of the pillars at the level of both horizons, manifested by the rounding of the corners of the pillars and the detachment of pieces of rock salt from their surface; - deformations of the floor at the border with the inter-rooms pillars, local shear fractures may occur in the respective areas; 43 Revista Minelor – Mining Revue vol. 28, issue 4 / 2022 ISSN-L 1220-2053 / ISSN 2247-8590 pp. 42-50 - at a short distance from the pillars, both at the floors and at the ceilings, cracks may appear, due to the increase in tensile stresses. Phenomena of local instability of the ceiling and pillars were also found in the directional exploitation room G, from the eastern wing, horizon +210E, more precisely in rooms G31, G32 and G33. After 8 years since the completion of the exploitation of the rooms, areas with different degrees of stability have been identified, the working being placed in an area with a potential risk of long-term instability, namely: the southern wall of chamber G is more stable and without visible signs of degradation; the northern wall is inhomogeneous, being more pronounced; the ceiling is stable but exposed to damage due to the state of stresses and strains developed in this area. In this section, with a length of 105 m, consolidation works have already been carried out with 2.5 m long cemented anchors and reinforced shotcrete [8], completed in 2020, and following the monitoring of the ceiling movements, the measured values of the movements were insignificant. In order to study the coaxiality of the pillars, the plan representations of the pillars from the horizons +226 m and +210 m were used. Following the overlapping of the outline of the pillars at the level of the two horizons (as in fig.1), for each separate pillar, were the following geometric characteristics were measured: the surface of the pillars from horizon +226 m and from horizon +210 m, in m ; the area of intersection between the pillars of the two horizons, in m and the center / axis of this area; the distance between the center / axis of the intersection surface and the axis of the designed pillar, in m. The measured values are summarized in the graphs in figures 2-4 which also contains the deviations, in %, from the geometric characteristics of the designed surfaces of the pillars. Figure 1. Correlation between the designed and realized axis of the pillars at horizons +226 m and +210 m Ax.pr. – the projected axis of the pillars; Ax.sp.- the axis of the overlapping surface; Dax – deviation of projected surface axis-overlap surface axis; - the surface of the base of the pillar at the horizon +226 m; - the surface of 226 210 the base of the pillar at the horizon +210 m; -the overlapped surface of the pillars from horizon +226 m and +210 m Horizon +226 - 10 ÷ 0 0 ÷ + 10 + 10 ÷ + 20 + 20 ÷ + 30 + 30 ÷ + 40 + 40 ÷ + 50 + 50 ÷ + 60 + 60 ÷ + 70 + 70 ÷ + 80 + 80 ÷ + 90 + 90 ÷ + 100 > + 100 Pillar surface deviation, (%) Figure 2. The percentage of pillars from the +226m horizon, depending on the bearing surface deviation in relation to the projected one, in % Percentage pillars, (%) Revista Minelor – Mining Revue vol. 28, issue 4 / 2022 ISSN-L 1220-2053 / ISSN 2247-8590 pp. 42-50 Horizon +210 40.00 35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00 - 12 ÷ - 10 - 10 ÷ 0 0 ÷ + 10 + 10 ÷ + 20 + 20 ÷ + 30 + 30 ÷ + 40 + 40 ÷ + 50 + 50 ÷ + 60 + 60 ÷ + 70 + 70 ÷ + 80 + 80 ÷ + 90 + 90 ÷ + 100 + 100 Pillars surface deviation, (%) Figure 3. The percentage of pillars from the +210 horizon, depending on the bearing surface deviation in relation to the projected one, in % 0 ÷ 0,20 0,20 ÷ 040 0,40 ÷ 0,60 0,60 ÷ 0,80 0,80 ÷ 1,00 1,00 ÷ 1,50 1,50 ÷ 2,00 2,00 ÷ 4,00 + 4,00 Pillar axis deviation, (m) Figure 4. Percentage of overlapping pillars at the two horizons, +226 m and +210 m, function of the deviation of the actual axis from the projected axis, in m As can be seen from fig.2, at the +226 m horizon, the percentage of pillars, out of the total number of existing pillars at the +226 m horizon, with a deviation of the real surface reduced by 10% compared to the projected one, is less than 13 %. It can also be seen that 87% of the pillars located at this horizon have an oversized surface (of which, more than 57% of the pillars have a bearing surface larger by up to 20%). In the case of the +210 m horizon (fig. 3), the percentage of pillars with a surface area 12% smaller than the designed one is approximately 25%, and the pillars with positive deviations from the load-bearing surface (larger than the designed one) are in a percentage of approx. 75% of the total pillars from this horizon. Regarding the deviation from coaxiality of the pillars located at the two horizons (fig. 4), it is as follows: 33% of the overlapping pillars have deviations below 0.4 m; 12% have deviations between 2 and 4 m, and most of the pillars, in proportion to 53%, have deviations between 0.4 m and 2.0 m. Although the values of deviations from the coaxiality of the pillars would seem quite important, we note that the deviation of the pillar axes is due to their over-dimensioning (sometimes, to the point of connecting two adjacent pillars together), which led to the increase of the safety coefficient and implicitly to the improvement stability of resistance structures. As can be seen from the previous analyses, most of the existing pillars at Ocnele Mari Saline have load- bearing surfaces that are much larger than their designed values, which means an important reserve of stability, and the deviation from coaxiality of the pillars at horizons +226 m and + 210 m are only the result of the Percentage pillars, (%) Percentage pillars, (%) Revista Minelor – Mining Revue vol. 28, issue 4 / 2022 ISSN-L 1220-2053 / ISSN 2247-8590 pp. 42-50 change in the geometry of the pillars. The negative values of the deviations from the load-bearing surfaces are insignificant, taking into account the fact that the pillars have been oversized since their design phase [4, 5]. Because we considered that the factors that contributed to the occurrence of the instability phenomena at the Ocnele Mari salt mine, at least the fractures that appeared in the ceiling between the horizons +226 m and +210 m, are much more complex, and the deviations from coaxiality are insignificant, in order to have a important influence on resistance structures, we tried to carry out a much more in-depth study on their stability. So a 3D finite element modeling was carried out, assuming the elasto-plastic behavior without hardening of the rock salt massif, calculated according to the Mohr-Coulomb criterion. The geometric model of the Ocnele Mari salt mine respected the geometry of the pillars and ceiling set by the project, without taking into account the deviations of the pillars and ceiling from reality (details are presented in [4, 5]). As a result of the calculations carried out, it was found that the resistance structures entered plasticization, exactly in the areas where instability phenomena were observed (fissures, cracks in the ceiling and pillars), respectively in the area of pillars 24-25, related to the tourist base (fig.5 ) and in the area of directional chambers G31, G32 and G33, from the 210E horizon (fig.6), which were reinforced with cemented anchors and reinforced shotcrete. Figure 5. Situation plan of horizon +226W, from the western wing of the Ocnele Mari Saline [5] Figure 6. Situation plan of horizon 210E, eastern wing of Ocnele Mari Saline [5] The results obtained from the 3D finite element modeling demonstrated very clearly that it is not the deviation from the vertical coaxiality of the pillars the factor responsible for the occurrence of the instability phenomena at the Ocnele Mari Saline (as was concluded in the study [7]), but it is the condition of stresses and strains generated by the spatial geometric configuration of underground voids and surface relief [9] and the geomechanical characteristics of rock and rock salt. 3. Finite element analysis of the influence of the axial deviation of the pillars on the stability of the resistance structures In order to quantify the influence that the deviation of the axis of the safety pillars has on the stability of the resistance structures, with the help of 2D finite element modeling, in elasto-plasticity without hardening, according to the Mohr-Coulomb criterion, several simulations were carried out under the conditions Ocnele 46 Revista Minelor – Mining Revue vol. 28, issue 4 / 2022 ISSN-L 1220-2053 / ISSN 2247-8590 pp. 42-50 Mari Saline, for horizons +260 m and +210 m. In the numerical models, the behavior of the resistance structures was analyzed, in the case of the deviation of the pillars at the +210 m horizon, with increments of 1 m, from 0 to 8 m, in relation to the pillars at the +226 m horizon. A detailed analysis of the change in the state of stresses and strains, as the value of the deviation of the pillar axis increases, would have been very complicated to achieve (fig. 7). Therefore, to simplify the analysis, starting from the calculated values of the main stresses, from the models with finite elements, the values of the safety factor [4] were determined. a) b) c) d) Figure 7. Maximum main stresses: a) coaxial pillars, b) pillars deviated by 4m; Minimum main stresses: c) coaxial pillars, d) pillars deviated by 4m The criterion for calculating the safety factor, for models with finite elements in 2D, starts from the intrinsic curve of the rock salt. In this sense, for a certain point, characterized by a certain stress state, the corresponding Mohr's circle is determined and related to the intrinsic curve of the rock salt. In this sense, the Mohr-Coulomb line is taken into account (defined by the relation: τ = C +σ ⋅ tgϕ ) and the following conditions are established [4, 10]: a) If σ < R , for R = (C⋅ ctgϕ − S )⋅ sinϕ , SF = R / R ; 2 t 1 c 1 b) If , σ ≥ R then SF=0. 2 t σ +σ σ −σ 1 2 1 2 where: S = represents the abscissa of the center of Mohr's circle; R = - radius of Mohr's 2 2 circle; R - the radius of Mohr's circle tangent to the Mohr-Coulomb right; SF - safety factor; R =1,200 kN/m - 1 t 2 o tensile strength of rock salt; C=4,000 kN/m - cohesion; ϕ =30 - the internal friction angle of rock salt; ,σ , σ - maximum and minimum main stress, respectively (see fig.7). In the calculations, for rock salt, the following were also taken into account: the modulus of elasticity . 6 2 3 E=1.5 10 kN/m , the Poisson's coefficient, ν =0.25 and the apparent specific gravity, γ =21.5 kN/m . As a function of the calculated value of the safety factor SF=R /R, three cases of stability are defined [4, 10]: a) SF = 1 - limit stability (Mohr's circle and the intrinsic curve of rock salt are tangent); 47 Revista Minelor – Mining Revue vol. 28, issue 4 / 2022 ISSN-L 1220-2053 / ISSN 2247-8590 pp. 42-50 b) SF < 1 - conditions for the appearance of failure phenomena (Mohr's circle and the intrinsic curve are secants); c) SF > 1 – diferent degree of stability depending on the value of the safety factor (the state of stress is far from the failure phenomenon). Since the state of stresses and strains at the level of the ceiling of the horizon +226m and at the level of the floor of the horizon +210 m is less affected by the deviation of the coaxiality of the pillars and rooms, the calculation of the safety factors was carried out through two horizontal sections, at the level of the floor +226 m (fig.8) and the ceiling +210 m (fig.9). This fact is evidenced by the distribution of principal stresses in figure 7, for the basic model, (a) and (c) and the model with the pillar axis deviated by 4 m, (b) and (d). Section floor horizon +226m 75.0 Dev. 0m 70.0 Dev. 1m Dev. 2m 65.0 Dev. 3m Dev. 4m 60.0 Dev. 5m 55.0 Dev. 6m Dev. 7m 50.0 Dev. 8m 45.0 40.0 35.0 30.0 25.0 20.0 15.0 10.0 5.0 0.0 0 15 30 45 60 75 90 105 120 135 Distance X , m Figure 8. The variation of the safety factor in the floor of the horizon +226m, depending on the deviation from the coaxiality of the pillars by 0, 1, 2,...,8m Section ceiling horizon +210m 150.0 Dev. 0m 140.0 Dev. 1m Dev. 2m 130.0 Dev. 3m Dev. 4m 120.0 Dev. 5m 110.0 Dev. 6m Dev. 7m 100.0 Dev. 8m 90.0 80.0 70.0 60.0 50.0 40.0 30.0 20.0 10.0 0.0 0 15 30 45 60 75 90 105 120 135 Distance X , m Figure 9. The variation of the safety factor in the horizon ceiling +210m, depending on the deviation from the coaxiality of the pillars by 0, 1, 2,...,8m SF SF Revista Minelor – Mining Revue vol. 28, issue 4 / 2022 ISSN-L 1220-2053 / ISSN 2247-8590 pp. 42-50 Analyzing the variation of the values of the safety factors at the level of the ceiling between the two horizons, it can be found that there is a significant variation between the maximum values of the safety factors, while the minimum values are kept within the limits of the value of SF=2-3, for values of the coaxiality deviation up to 2-3 m. There is an important decrease in the safety factor, up to SF=1.4 -1.5, for higher values of the axis deviation, towards 7-8 m. Considering the fact that the axis deviations, for most of the pillars, are below 2 m (see fig.4), the decrease in the safety factor is within acceptable limits. Moreover, in most cases, the deviation of the geometric axis occurred as a result of the increase of the load-bearing surface of the pillar, which makes this assumption have a certain degree of relativism. Following the analysis of the calculated values of the safety factors, it can be concluded that the deviation of the axis of the pillars, under the mining conditions of the Ocnele Mari Saline, had an insignificant importance on the stability of the resistance structures, especially due to the large thickness of 8 m of the ceiling between the horizons +226 m and +210 m, which allowed a redistribution of stresses in its structure, without exceeding the tensile and shear resistance limits of the rock salt. 4. Conclusions • The exploitation of the rock salt deposit from Ocnele Mari - Cocenești began in 1996, at the level of the horizon +226 m, by the mining method with small rooms and square pillars. • After the continuation of exploitation at the next level +210 m, phenomena of instability appeared in the ceiling between the two horizons, marked by fissures and cracks and the local degradation of certain exploitation pillars. The affected structures were in the tourist base at +226W, in the area of pillars 24-25, especially the ceiling between the horizons, and in the area of directional rooms G31, G32 and G33, in the horizon 210E. • In order to determine the causes that generated the occurrence of the instability phenomena of the resistance structures at the Ocnele Mari saline, the following were analyzed: the real geometry of the pillars and its deviations from the design; the results of the modeling with 3D finite elements, in elasto- plasticity, of the stability of the resistance structures from the Ocnele Mari Salt Mine; safety factors calculated from the results of 2D finite element modeling, in elasto-plasticity, for several theoretical situations of pillars deviation, with values of 1,2,..,8 m. • From the stability analysis with finite elements in 3D it resulted that the regions in the models affected by plasticization, respectively instability, are located in the area of pillars 24-25, horizontal 226W and in the directional rooms G31, G32 and G33, horizon 210E. Since the models were made according to the designed geometry of the resistance structures (rooms, pillars, ceiling), without deviation from coaxiality, it follows that the instability phenomena occurring in the resistance structures are influenced by the state of stresses and strains produced by the geometry and spatial distribution of underground voids, the variation of land surface relief and the geomechanical characteristics of rock salt. • Following the comparative analysis of the safety factors, calculated by 2D finite element modeling, in elasto-plasticity, for different deviations from the coaxiality of the pillars, in relation to the real situation in the field, it resulted that the deviations from the coaxiality of the pillars from Ocnele Mari Saline did not significantly influence the instability phenomena that appeared in this saline. References [1]. Hirian C., Georgescu M., 2012 The stability of old Romanian salt mines – conditions for their use in various domains (in Romanian), 2nd edition, Universitas Publishing. [2]. Marica D., 2011 Stability study of mining excavations from Ocnele Mari salt mine to increase the work safety degree (in Romanian), Ph.D thesis, University of Petroșani [3]. Covaci Ş., e.a., 1999 Mining exploitations (in Romanian), Corvin Publishing, Deva 49 Revista Minelor – Mining Revue vol. 28, issue 4 / 2022 ISSN-L 1220-2053 / ISSN 2247-8590 pp. 42-50 [4]. Marian D.P, Onica I., 2021 Analysis of the stability of the rooms and resistance structures at the Ocnele Mari – Cocenești salt mine, based on safety factor, Annals of the University of Petroşani, Mining Engineering, Vol.22 (XLIX), pag.99-112. [5]. Marian D.P, Onica I., 2021 Numerical modeling of the stability of the resistance structures from Ocnele Mari salt mine using the finite element method, Annals of the University of Petroşani, Mining Engineering, Vol.22 (XLIX), pag.113-126. [6]. *** , 2009 Opportunity study for arranging the underground tourist site from Ocnele Mari-Cocenești Salt Mine, Vâlcea County (in romanian), symbol 34-661-01/2009, S.C.MINESA-ICPM S.A. Cluj-Napoca. [7]. Pușcaș G. e.a., 2014 The ceiling support of chambers 24-25, horizon +210W, from Ocnele Mari Salt Mine, Stage I Study, (in romanian) Contract 693/21.01.2014, symbol 34-876, SC MINESA – ICPM SA Cluj-Napoca. [8]. Kovacs F., 2012 Minimal flux for rock salt grinding in underground Ocnele Mari Salt Mine, Râmnicu Vâlcea Mining Exploitation Branch, Volume II – Strenghtening the placement area for the grinding flux (in romanian). Technical project phase and task books, symbol 3/4/2012, S.C.DACITROM SRL Cluj-Napoca. [9]. Herget G., 1988 Stresses in rock, Balkema [10]. Jifang Lin, 1990 Modélisation des milieux anisotropes. Application du logiciel CESAR-LCPC à l’étude des ardoisiers d’Angers, DEA de Génie civil et minier, Laboratoire de Mécanique des Terrains, ENSM Nancy. This article is an open access article distributed under the Creative Commons BY SA 4.0 license. Authors retain all copyrights and agree to the terms of the above-mentioned CC BY SA 4.0 license. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Mining Revue de Gruyter

Analysis of the Influence of the Vertical Coaxiality of the Pillars on the Stability of the Resistance Structures, from the Ocnele Mari Saline

Marian, Dacian-Paul; Onica, Ilie; Marina, Ovidiu
Mining Revue , Volume 28 (4): 9 – Dec 1, 2022
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10.2478/minrv-2022-0029
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Abstract

Revista Minelor – Mining Revue ISSN-L 1220-2053 / ISSN 2247-8590 vol. 28, issue 4 / 2022, pp. 42-50 ANALYSIS OF THE INFLUENCE OF THE VERTICAL COAXIALITY OF THE PILLARS ON THE STABILITY OF THE RESISTANCE STRUCTURES, FROM THE OCNELE MARI SALINE 1 * 2 3 Dacian-Paul MARIAN , Ilie ONICA , Ovidiu MARINA University of Petroșani, Petroșani, Romania, [email protected] University of Petroșani, Petroșani, Romania, [email protected] University of Petroșani, Petroșani, Romania, [email protected] DOI: 10.2478/minrv-2022-0029 Abstract: The Ocnele Mari rock salt deposit is exploited with small rooms and square pillars, on the horizons +226 m and 210 m. Following the exploitation of the lower horizon +210 m, the floor between the two horizons suffered instability phenomena, marked by fractures and cracks and local degradation of the pillars.Since the pillars at the two horizons have certain deviations from the coaxiality, the question was raised whether the instability phenomena were generated by the deviations from the coaxiality.This article presents the stability analysis of bearing structures, taking into account the actual geometry of pillars, 3D finite element analysis and analysis of safety factors at the ceiling level, calculated from 2D finite element models. The final conclusion is that the instability phenomena that occurred at the Ocnele Mari Saline were generated by the state of stresses and strains, produced by the spatial distribution of the underground voids and the geomechanical characteristics of the rock salt and insignificantly, by the deviation from the coaxiality of the pillars. Keywords: rock salt, rooms and pillars mining, pillar, ceiling, stability analysis, finite element, stress, strain, safety factor 1. Geology of the Ocnele Mari-Cocenești deposit The Ocnele Mari salt deposit can be found in the area of the subcarpathian hills of Oltenia, stretching from the East of the Olt river to the West of the Govora stream, passing through the territory of the Ocnele Mari locality in Vâlcea county. The Coceneşti perimeter is located in the eastern area of the Ocnele Mari deposit. Access to the deposit is on the road that goes to the town of Ocnele Mari, Vâlcea county. The morphology of the region has a hilly aspect, with heights between 250 - 450 m. The Ocnele Mari region, where the deposit is located, belongs to the Getic Depression, which is the most external unit of the Southern Carpathians. It was formed as a result of the laramic movements, as a consequence of the uplift of the crystalline-Mesozoic zone, in front of which a premontane depression was formed, with the role of an ”avant-fossé” that functioned in this way during the Paleogene and Neogene. From a stratigraphic point of view, the Ocnele Mari region includes Paleogene, Neogene and Quaternary geological formations. The horizon with rock salt deposits is presented in a lagoonal facies, with local distribution, being formed by salt masses, gypsum and salty marls. The rock salt deposit from Ocnele Mari is included in this horizon, and the salt massif appears in the area of axial uplift from Ocniţa - Ocnele Mari. The age of the rock salt from Ocnele Mari - Coceneşti is middle Badenian [1, 2]. The Ocnele-Mari salt deposit has the shape of an elongated lens in the E-W direction, measuring approx. 7.5 km, and to the N-S approx. 3.5 km, presenting an axial uplift in the Ocniţei area, with slopes to the N. The Corresponding author: Dacian-Paul Marian, Assoc.Prof. PhD. Eng., University of Petroșani, Petroșani, Romania, contact details (University st. no. 20, Petroșani, Romania [email protected]) 42 Revista Minelor – Mining Revue vol. 28, issue 4 / 2022 ISSN-L 1220-2053 / ISSN 2247-8590 pp. 42-50 thickness of the salt deposit is variable, reaching laminations in the northern and southern parts, the maximum thickness reaching 450 m in the central part of the lens. The rock salt in the deposit has a macrogranular structure with well-developed crystals, grayish white or blackish in color, depending on the contribution of terrigenous impurities. Banks of white salt alternate with those of darker salt, and in the lower part of the deposit, a bank of black salt with thicknesses from 5 to 30 m, impurized with anhydrite, having a higher hardness and compactness than the rest of the salt, frequently appears from the deposit [1, 2]. 2. Instability phenomena occurring at the Ocnele Mari saline In the period 1993-1996, the mining workings to open the Ocnele Mari salt mine were carried out. The information obtained with the opening workings completed the picture of the deposit, and the underground drillings carried out starting in 1994 allowed a more accurate outline of the rock salt – barren rocks limit and the start of exploitation of the deposit at the level of the +226 m horizon. The designed mining method, for the geo-mining conditions at this salt mine, is the method with small rooms and square pillars [3], arranged in a 30 x 30 m grid, with different sizes in the eastern and western wings (table 1). After mining the deposit at the +226 m level, mining continued under an 8 m thick rock salt ceiling, at the +210 m level. Table 1. Geometric parameters of the mining method with small rooms and square pillars, for Ocnele Mari - Cocenești Saline [4, 5] Horizon Starting year of Room Pillar Ceiling Level exploitation Width, Height, [m] Width, Height, [m] Thickness, Height, W/ E [m] W/E [m] [m] [m] +226m 1996 16/15 8 14/15 8 8 16 +210m 2001 16/15 8 14/15 8 8 16 After the deposit was extracted, a tourist base was set up in the exploited spaces of the western wing, in accordance with the feasibility study developed in 2009 [6]. Following the exploitation of the +210 m horizon, in the western wing, related to the tourist base, a series of cracks and microcracks appeared in the ceiling between the two horizons (table 2), associated with cracks and detachments of pieces of rock salt at the +210 m horizon, from the ceiling and from the walls of the pillars, in the area of rooms 24-25, horizon +226/210W. Also, a series of instability phenomena could be observed in the area of ventilation shaft 5, between pillars H22 and H23, in the form of cracks in the ceiling, with displacements both in the horizontal and vertical planes. Table 2. The situation of the cracks in the floor between horizontal +226m and +210m, in the area of rooms 24-25 [7] Fissure Room Between Fissure Fissure Room Between Fissure pillars length, m pillars length, m F1 H G25- 8.80 F4 H H24- 11.43 H25 G24 F2 24 H24- 6.42 F5 H H23- 8.45 H25 G23 F3 24 H24- 8.86 F6 G G23- 9.90 H25 F23 At the request of the salt mine, in 2014, SC MINESA SA [6] analyzed the causes that led to the appearance of these instability phenomena. In this study, based on the topographic measurements carried out on the resistance structures, it was concluded that, if the inter-rooms pillars do not overlap and have deviations from coaxiality, in the ”rooms-pillars-ceiling” system, the following instability phenomena may occur, among others [6]: - horizontal and vertical deformations of the pillars at the level of both horizons, manifested by the rounding of the corners of the pillars and the detachment of pieces of rock salt from their surface; - deformations of the floor at the border with the inter-rooms pillars, local shear fractures may occur in the respective areas; 43 Revista Minelor – Mining Revue vol. 28, issue 4 / 2022 ISSN-L 1220-2053 / ISSN 2247-8590 pp. 42-50 - at a short distance from the pillars, both at the floors and at the ceilings, cracks may appear, due to the increase in tensile stresses. Phenomena of local instability of the ceiling and pillars were also found in the directional exploitation room G, from the eastern wing, horizon +210E, more precisely in rooms G31, G32 and G33. After 8 years since the completion of the exploitation of the rooms, areas with different degrees of stability have been identified, the working being placed in an area with a potential risk of long-term instability, namely: the southern wall of chamber G is more stable and without visible signs of degradation; the northern wall is inhomogeneous, being more pronounced; the ceiling is stable but exposed to damage due to the state of stresses and strains developed in this area. In this section, with a length of 105 m, consolidation works have already been carried out with 2.5 m long cemented anchors and reinforced shotcrete [8], completed in 2020, and following the monitoring of the ceiling movements, the measured values of the movements were insignificant. In order to study the coaxiality of the pillars, the plan representations of the pillars from the horizons +226 m and +210 m were used. Following the overlapping of the outline of the pillars at the level of the two horizons (as in fig.1), for each separate pillar, were the following geometric characteristics were measured: the surface of the pillars from horizon +226 m and from horizon +210 m, in m ; the area of intersection between the pillars of the two horizons, in m and the center / axis of this area; the distance between the center / axis of the intersection surface and the axis of the designed pillar, in m. The measured values are summarized in the graphs in figures 2-4 which also contains the deviations, in %, from the geometric characteristics of the designed surfaces of the pillars. Figure 1. Correlation between the designed and realized axis of the pillars at horizons +226 m and +210 m Ax.pr. – the projected axis of the pillars; Ax.sp.- the axis of the overlapping surface; Dax – deviation of projected surface axis-overlap surface axis; - the surface of the base of the pillar at the horizon +226 m; - the surface of 226 210 the base of the pillar at the horizon +210 m; -the overlapped surface of the pillars from horizon +226 m and +210 m Horizon +226 - 10 ÷ 0 0 ÷ + 10 + 10 ÷ + 20 + 20 ÷ + 30 + 30 ÷ + 40 + 40 ÷ + 50 + 50 ÷ + 60 + 60 ÷ + 70 + 70 ÷ + 80 + 80 ÷ + 90 + 90 ÷ + 100 > + 100 Pillar surface deviation, (%) Figure 2. The percentage of pillars from the +226m horizon, depending on the bearing surface deviation in relation to the projected one, in % Percentage pillars, (%) Revista Minelor – Mining Revue vol. 28, issue 4 / 2022 ISSN-L 1220-2053 / ISSN 2247-8590 pp. 42-50 Horizon +210 40.00 35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00 - 12 ÷ - 10 - 10 ÷ 0 0 ÷ + 10 + 10 ÷ + 20 + 20 ÷ + 30 + 30 ÷ + 40 + 40 ÷ + 50 + 50 ÷ + 60 + 60 ÷ + 70 + 70 ÷ + 80 + 80 ÷ + 90 + 90 ÷ + 100 + 100 Pillars surface deviation, (%) Figure 3. The percentage of pillars from the +210 horizon, depending on the bearing surface deviation in relation to the projected one, in % 0 ÷ 0,20 0,20 ÷ 040 0,40 ÷ 0,60 0,60 ÷ 0,80 0,80 ÷ 1,00 1,00 ÷ 1,50 1,50 ÷ 2,00 2,00 ÷ 4,00 + 4,00 Pillar axis deviation, (m) Figure 4. Percentage of overlapping pillars at the two horizons, +226 m and +210 m, function of the deviation of the actual axis from the projected axis, in m As can be seen from fig.2, at the +226 m horizon, the percentage of pillars, out of the total number of existing pillars at the +226 m horizon, with a deviation of the real surface reduced by 10% compared to the projected one, is less than 13 %. It can also be seen that 87% of the pillars located at this horizon have an oversized surface (of which, more than 57% of the pillars have a bearing surface larger by up to 20%). In the case of the +210 m horizon (fig. 3), the percentage of pillars with a surface area 12% smaller than the designed one is approximately 25%, and the pillars with positive deviations from the load-bearing surface (larger than the designed one) are in a percentage of approx. 75% of the total pillars from this horizon. Regarding the deviation from coaxiality of the pillars located at the two horizons (fig. 4), it is as follows: 33% of the overlapping pillars have deviations below 0.4 m; 12% have deviations between 2 and 4 m, and most of the pillars, in proportion to 53%, have deviations between 0.4 m and 2.0 m. Although the values of deviations from the coaxiality of the pillars would seem quite important, we note that the deviation of the pillar axes is due to their over-dimensioning (sometimes, to the point of connecting two adjacent pillars together), which led to the increase of the safety coefficient and implicitly to the improvement stability of resistance structures. As can be seen from the previous analyses, most of the existing pillars at Ocnele Mari Saline have load- bearing surfaces that are much larger than their designed values, which means an important reserve of stability, and the deviation from coaxiality of the pillars at horizons +226 m and + 210 m are only the result of the Percentage pillars, (%) Percentage pillars, (%) Revista Minelor – Mining Revue vol. 28, issue 4 / 2022 ISSN-L 1220-2053 / ISSN 2247-8590 pp. 42-50 change in the geometry of the pillars. The negative values of the deviations from the load-bearing surfaces are insignificant, taking into account the fact that the pillars have been oversized since their design phase [4, 5]. Because we considered that the factors that contributed to the occurrence of the instability phenomena at the Ocnele Mari salt mine, at least the fractures that appeared in the ceiling between the horizons +226 m and +210 m, are much more complex, and the deviations from coaxiality are insignificant, in order to have a important influence on resistance structures, we tried to carry out a much more in-depth study on their stability. So a 3D finite element modeling was carried out, assuming the elasto-plastic behavior without hardening of the rock salt massif, calculated according to the Mohr-Coulomb criterion. The geometric model of the Ocnele Mari salt mine respected the geometry of the pillars and ceiling set by the project, without taking into account the deviations of the pillars and ceiling from reality (details are presented in [4, 5]). As a result of the calculations carried out, it was found that the resistance structures entered plasticization, exactly in the areas where instability phenomena were observed (fissures, cracks in the ceiling and pillars), respectively in the area of pillars 24-25, related to the tourist base (fig.5 ) and in the area of directional chambers G31, G32 and G33, from the 210E horizon (fig.6), which were reinforced with cemented anchors and reinforced shotcrete. Figure 5. Situation plan of horizon +226W, from the western wing of the Ocnele Mari Saline [5] Figure 6. Situation plan of horizon 210E, eastern wing of Ocnele Mari Saline [5] The results obtained from the 3D finite element modeling demonstrated very clearly that it is not the deviation from the vertical coaxiality of the pillars the factor responsible for the occurrence of the instability phenomena at the Ocnele Mari Saline (as was concluded in the study [7]), but it is the condition of stresses and strains generated by the spatial geometric configuration of underground voids and surface relief [9] and the geomechanical characteristics of rock and rock salt. 3. Finite element analysis of the influence of the axial deviation of the pillars on the stability of the resistance structures In order to quantify the influence that the deviation of the axis of the safety pillars has on the stability of the resistance structures, with the help of 2D finite element modeling, in elasto-plasticity without hardening, according to the Mohr-Coulomb criterion, several simulations were carried out under the conditions Ocnele 46 Revista Minelor – Mining Revue vol. 28, issue 4 / 2022 ISSN-L 1220-2053 / ISSN 2247-8590 pp. 42-50 Mari Saline, for horizons +260 m and +210 m. In the numerical models, the behavior of the resistance structures was analyzed, in the case of the deviation of the pillars at the +210 m horizon, with increments of 1 m, from 0 to 8 m, in relation to the pillars at the +226 m horizon. A detailed analysis of the change in the state of stresses and strains, as the value of the deviation of the pillar axis increases, would have been very complicated to achieve (fig. 7). Therefore, to simplify the analysis, starting from the calculated values of the main stresses, from the models with finite elements, the values of the safety factor [4] were determined. a) b) c) d) Figure 7. Maximum main stresses: a) coaxial pillars, b) pillars deviated by 4m; Minimum main stresses: c) coaxial pillars, d) pillars deviated by 4m The criterion for calculating the safety factor, for models with finite elements in 2D, starts from the intrinsic curve of the rock salt. In this sense, for a certain point, characterized by a certain stress state, the corresponding Mohr's circle is determined and related to the intrinsic curve of the rock salt. In this sense, the Mohr-Coulomb line is taken into account (defined by the relation: τ = C +σ ⋅ tgϕ ) and the following conditions are established [4, 10]: a) If σ < R , for R = (C⋅ ctgϕ − S )⋅ sinϕ , SF = R / R ; 2 t 1 c 1 b) If , σ ≥ R then SF=0. 2 t σ +σ σ −σ 1 2 1 2 where: S = represents the abscissa of the center of Mohr's circle; R = - radius of Mohr's 2 2 circle; R - the radius of Mohr's circle tangent to the Mohr-Coulomb right; SF - safety factor; R =1,200 kN/m - 1 t 2 o tensile strength of rock salt; C=4,000 kN/m - cohesion; ϕ =30 - the internal friction angle of rock salt; ,σ , σ - maximum and minimum main stress, respectively (see fig.7). In the calculations, for rock salt, the following were also taken into account: the modulus of elasticity . 6 2 3 E=1.5 10 kN/m , the Poisson's coefficient, ν =0.25 and the apparent specific gravity, γ =21.5 kN/m . As a function of the calculated value of the safety factor SF=R /R, three cases of stability are defined [4, 10]: a) SF = 1 - limit stability (Mohr's circle and the intrinsic curve of rock salt are tangent); 47 Revista Minelor – Mining Revue vol. 28, issue 4 / 2022 ISSN-L 1220-2053 / ISSN 2247-8590 pp. 42-50 b) SF < 1 - conditions for the appearance of failure phenomena (Mohr's circle and the intrinsic curve are secants); c) SF > 1 – diferent degree of stability depending on the value of the safety factor (the state of stress is far from the failure phenomenon). Since the state of stresses and strains at the level of the ceiling of the horizon +226m and at the level of the floor of the horizon +210 m is less affected by the deviation of the coaxiality of the pillars and rooms, the calculation of the safety factors was carried out through two horizontal sections, at the level of the floor +226 m (fig.8) and the ceiling +210 m (fig.9). This fact is evidenced by the distribution of principal stresses in figure 7, for the basic model, (a) and (c) and the model with the pillar axis deviated by 4 m, (b) and (d). Section floor horizon +226m 75.0 Dev. 0m 70.0 Dev. 1m Dev. 2m 65.0 Dev. 3m Dev. 4m 60.0 Dev. 5m 55.0 Dev. 6m Dev. 7m 50.0 Dev. 8m 45.0 40.0 35.0 30.0 25.0 20.0 15.0 10.0 5.0 0.0 0 15 30 45 60 75 90 105 120 135 Distance X , m Figure 8. The variation of the safety factor in the floor of the horizon +226m, depending on the deviation from the coaxiality of the pillars by 0, 1, 2,...,8m Section ceiling horizon +210m 150.0 Dev. 0m 140.0 Dev. 1m Dev. 2m 130.0 Dev. 3m Dev. 4m 120.0 Dev. 5m 110.0 Dev. 6m Dev. 7m 100.0 Dev. 8m 90.0 80.0 70.0 60.0 50.0 40.0 30.0 20.0 10.0 0.0 0 15 30 45 60 75 90 105 120 135 Distance X , m Figure 9. The variation of the safety factor in the horizon ceiling +210m, depending on the deviation from the coaxiality of the pillars by 0, 1, 2,...,8m SF SF Revista Minelor – Mining Revue vol. 28, issue 4 / 2022 ISSN-L 1220-2053 / ISSN 2247-8590 pp. 42-50 Analyzing the variation of the values of the safety factors at the level of the ceiling between the two horizons, it can be found that there is a significant variation between the maximum values of the safety factors, while the minimum values are kept within the limits of the value of SF=2-3, for values of the coaxiality deviation up to 2-3 m. There is an important decrease in the safety factor, up to SF=1.4 -1.5, for higher values of the axis deviation, towards 7-8 m. Considering the fact that the axis deviations, for most of the pillars, are below 2 m (see fig.4), the decrease in the safety factor is within acceptable limits. Moreover, in most cases, the deviation of the geometric axis occurred as a result of the increase of the load-bearing surface of the pillar, which makes this assumption have a certain degree of relativism. Following the analysis of the calculated values of the safety factors, it can be concluded that the deviation of the axis of the pillars, under the mining conditions of the Ocnele Mari Saline, had an insignificant importance on the stability of the resistance structures, especially due to the large thickness of 8 m of the ceiling between the horizons +226 m and +210 m, which allowed a redistribution of stresses in its structure, without exceeding the tensile and shear resistance limits of the rock salt. 4. Conclusions • The exploitation of the rock salt deposit from Ocnele Mari - Cocenești began in 1996, at the level of the horizon +226 m, by the mining method with small rooms and square pillars. • After the continuation of exploitation at the next level +210 m, phenomena of instability appeared in the ceiling between the two horizons, marked by fissures and cracks and the local degradation of certain exploitation pillars. The affected structures were in the tourist base at +226W, in the area of pillars 24-25, especially the ceiling between the horizons, and in the area of directional rooms G31, G32 and G33, in the horizon 210E. • In order to determine the causes that generated the occurrence of the instability phenomena of the resistance structures at the Ocnele Mari saline, the following were analyzed: the real geometry of the pillars and its deviations from the design; the results of the modeling with 3D finite elements, in elasto- plasticity, of the stability of the resistance structures from the Ocnele Mari Salt Mine; safety factors calculated from the results of 2D finite element modeling, in elasto-plasticity, for several theoretical situations of pillars deviation, with values of 1,2,..,8 m. • From the stability analysis with finite elements in 3D it resulted that the regions in the models affected by plasticization, respectively instability, are located in the area of pillars 24-25, horizontal 226W and in the directional rooms G31, G32 and G33, horizon 210E. Since the models were made according to the designed geometry of the resistance structures (rooms, pillars, ceiling), without deviation from coaxiality, it follows that the instability phenomena occurring in the resistance structures are influenced by the state of stresses and strains produced by the geometry and spatial distribution of underground voids, the variation of land surface relief and the geomechanical characteristics of rock salt. • Following the comparative analysis of the safety factors, calculated by 2D finite element modeling, in elasto-plasticity, for different deviations from the coaxiality of the pillars, in relation to the real situation in the field, it resulted that the deviations from the coaxiality of the pillars from Ocnele Mari Saline did not significantly influence the instability phenomena that appeared in this saline. References [1]. Hirian C., Georgescu M., 2012 The stability of old Romanian salt mines – conditions for their use in various domains (in Romanian), 2nd edition, Universitas Publishing. [2]. Marica D., 2011 Stability study of mining excavations from Ocnele Mari salt mine to increase the work safety degree (in Romanian), Ph.D thesis, University of Petroșani [3]. Covaci Ş., e.a., 1999 Mining exploitations (in Romanian), Corvin Publishing, Deva 49 Revista Minelor – Mining Revue vol. 28, issue 4 / 2022 ISSN-L 1220-2053 / ISSN 2247-8590 pp. 42-50 [4]. Marian D.P, Onica I., 2021 Analysis of the stability of the rooms and resistance structures at the Ocnele Mari – Cocenești salt mine, based on safety factor, Annals of the University of Petroşani, Mining Engineering, Vol.22 (XLIX), pag.99-112. [5]. Marian D.P, Onica I., 2021 Numerical modeling of the stability of the resistance structures from Ocnele Mari salt mine using the finite element method, Annals of the University of Petroşani, Mining Engineering, Vol.22 (XLIX), pag.113-126. [6]. *** , 2009 Opportunity study for arranging the underground tourist site from Ocnele Mari-Cocenești Salt Mine, Vâlcea County (in romanian), symbol 34-661-01/2009, S.C.MINESA-ICPM S.A. Cluj-Napoca. [7]. Pușcaș G. e.a., 2014 The ceiling support of chambers 24-25, horizon +210W, from Ocnele Mari Salt Mine, Stage I Study, (in romanian) Contract 693/21.01.2014, symbol 34-876, SC MINESA – ICPM SA Cluj-Napoca. [8]. Kovacs F., 2012 Minimal flux for rock salt grinding in underground Ocnele Mari Salt Mine, Râmnicu Vâlcea Mining Exploitation Branch, Volume II – Strenghtening the placement area for the grinding flux (in romanian). Technical project phase and task books, symbol 3/4/2012, S.C.DACITROM SRL Cluj-Napoca. [9]. Herget G., 1988 Stresses in rock, Balkema [10]. Jifang Lin, 1990 Modélisation des milieux anisotropes. Application du logiciel CESAR-LCPC à l’étude des ardoisiers d’Angers, DEA de Génie civil et minier, Laboratoire de Mécanique des Terrains, ENSM Nancy. This article is an open access article distributed under the Creative Commons BY SA 4.0 license. Authors retain all copyrights and agree to the terms of the above-mentioned CC BY SA 4.0 license.

Journal

Mining Revue – de Gruyter

Published: Dec 1, 2022

Keywords: rock salt; rooms and pillars mining; pillar; ceiling; stability analysis; finite element; stress; strain; safety factor

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