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Rock Mechanics Considerations for Roof Bolt-Based Breaker Line Design
Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 191 (2017) 551 – 559
Symposium of the International Society for Rock Mechanics
Rock Mechanics Considerations for Roof Bolt-Based Breaker Line Design Rajendra Singh*, Sahendra Ram, Arun Kumar Singh, Ashok Kumar, Rakesh Kumar, Amit Kr. Singh CSIR-Central Institute of Mining and Fuel Research, Barwa Road, Dhanbad, Jharkhand, 826001, India
Abstract Application of roof bolts to support in developed galleries during underground coal mining is a common practice. Mechanised depillaring (MD) applies these roof bolts to support the galleries at the goaf edge also. Goaf edge of a depillaring panel inherits a number of openings, which provides least resistance path for the caving roof strata to encroach the working areas. Influence of strata caving, inside the goaf, make it difficult to apply conventional roof bolt support norms in these openings at the goaf edge. Basic rock mechanics considerations find that the mining induced stress development improves the efficacy of roof bolt at the goaf edge. Application of, relatively, high density of bolts as roof bolts-based breaker line support (RBBLS) in the existing openings at the goaf edge to arrest the goaf encroachment is found to be working satisfactorily only if the surrounding natural supports are stable. The stability of a natural support, in and around the goaf edge, is found to be diluted due to side spalling/loosening caused by the mining induced stress. Different field experiences found that a high value of the stress causes side spalling/loosening in natural supports, standing in and around the goaf edge. Under the existing geo-mining conditions of some of the recent MD operations in Indian coalfields, the extent of spalling varied from 0.5 m to 2 m only. Considering an acceptable dimension of the rib/snook for these site conditions, the position of RBBLS is shifted out-bye from the goaf line as per the extent of spalling, which performed satisfactorily in the field. Presenting basic working principles of a RBBLS at the goaf edge; this paper discusses different field experiences for a suitable positioning of the RBBLS at the goaf edge. Published by Elsevier Ltd. © 2017 2017Published The Authors. © by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of EUROCK 2017. Peer-review under responsibility of the organizing committee of EUROCK 2017 Keywords: Roof bolts based breaker-line support (RBBLS); Mechanised depillaring (MD); Goaf edge; Numerical modelling; Rock load height and natural support
* Corresponding author. Tel.: +91-326-2296043; fax: +91-326-2296033. E-mail address: [email protected]
1877-7058 © 2017 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of EUROCK 2017
doi:10.1016/j.proeng.2017.05.217
552
Rajendra Singh et al. / Procedia Engineering 191 (2017) 551 – 559
1. Introduction Existing facts and figures about different developed coal seams (standing on pillars) by bord and pillar method find pillar extraction i.e. depillaring an important issue [1] for the Indian coal mining industry. Depillaring with caving is a challenging operation from strata control point of view. Frequent encounter of competent roof strata of Gondwana formations creates a large overhang inside the goaf. Caving of the roof strata after a large overhang generates high value of mining induced stress over natural supports in and around the extraction line [2]. However, value of the mining induced stress, generally, remains insignificant if regular caving of the roof strata takes place inside the goaf. As per different field observations, in addition to the competency, treatment of goaf is a significant parameter for the strata control issue of a depillaring panel. Filling of the void due to mining, generally, provides good strata condition in a depillaring but falls beyond the scope of this paper. However, depillaring with caving inherently leaves a number of remnants (like ribs and snooks) inside the goaf. These remnants should be competent enough to support the slicing operation [3] and, at the same time, should fail in controlled manner when it travels inside the goaf with the face advance. An appropriate design of these remnants, protect the slicing operation from goaf encroachment and, also, plays significant role in caving of the overlying strata inside the goaf [4, 5]. In spite of a suitable design of the rib/snook, goaf treatment like induced caving and judicious reduction of rib/snook are conducted as per the encountered geo-mining conditions of different sites. During caving of the hanging roof strata, there is a possibility that the caving extends into the working area (also known as goaf encroachment) through the existing openings along the extraction line. Each of these openings in a MD operation, along the extraction line, is supported by the roof bolts-based breaker line support (RBBLS) to restrict the goaf encroachment. RBBLS and surrounding rib/snook react against the extension of roof caving into the working area. Under these circumstances, the RBBLS along with the surrounding rib/snook act as pivot point between goaf and working area. The purpose of this pivot is to facilitate controlled caving of the hanging roof and is known as breaker line. Generally, a slower rate of pillar extraction deteriorates strata condition [6] during the depillaring operation. Accordingly, the Indian coal mining industry has recently deployed a number of MD operations [7] {applying a continuous miner (CM) for cutting and shuttle/ram car for haulage}, which provides improvements in production, productivity and safety too. Considering larger size of the developed pillars, a pillar is, first, split into two or more fenders (depending upon their size) by driving one or more split galleries. Slicing of the most in-bye fender is done after leaving an appropriate size of rib against the goaf (Fig. 1) and installation of breaker line support at the goaf edge. Thus, an appropriate design of the rib [8] and the RBBLS is vital to provide optimal resistance against the roof during the slicing operation.
Fig. 1. Splitting and slicing for a typical straight line extraction in a mechanised depillaring panel.
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2. RBBLS An appropriate design of the natural support as well as applied support along the extraction line is an essential requirement to provide an effective pivot. Conventionally, high density of the reactive supports (like props, chocks etc.) are erected in the existing openings along the extraction line as the breaker line support. This type of the breaker line support has some operational disadvantages [9] and is found to be unsuitable for a MD operation. To alleviate the problems of conventional breaker line support, MD in India adopted RBBLS and found to be effective in arresting the goaf encroachment [10]. In fact, the MD operations adopted high capacity, pre-tensioned, stiff and resin grouted roof bolts [11] to support the roof strata in the galleries of a MD panel. Presence of a compatible machine for the bolting, a higher density of the bolts were applied at the goaf edge (Fig. 2) of the MD panel as a breaker line support. Adopted design of the RBBLS at different MD panel is given by different consultants on the basis of their experiences gained at other sites. Observed performance of the RBBLS at different MD operations in the country have shown mixed results, probably, due to the uniqueness of rock-mass and complex geo-mining conditions of the Indian coalfields. Therefore field and laboratory investigations were carried out to visualise the working model of the RBBLS at different site conditions. On the basis of field studies, it is realized that the side spalling of the surrounding natural support is a major reason for the failure of a RBBLS. Such a failure can be tackled in two steps: (1) design of competent ribs/fenders/pillars surrounding the RBBLS and (2) improvement in competency of the installed RBBLS. The first step falls beyond the scope of this study, but the improvement in the RBBLS competency needs to be matched with the nature of the caving strata. For an efficient design of RBBLS, length and density of bolts in the RBBLS need to be adjusted as per the behaviour of overlying strata at varying site conditions. In presence of spalling, length and density of the bolts in RBBLS become a function of its position. RBBLS with, relatively, longer and denser bolts is also likely to experience failure, if it loses the shadow of surrounding natural supports due to the side spalling.
Fig. 2. Dimensional details of single pass pillar extraction at Pinoura mine and plan view of the adopted RBBLS.
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2.1. Working model of a RBBLS Inherent existence of different rooms (openings) along the extraction line in a depillaring panel provides an easy path for the caving strata inside the goaf to encroach the working area. Field studies found that the application of, relatively, high density of bolts as RBBLS in the existing openings at the goaf edge to arrest the goaf encroachment works satisfactorily only if the surrounding natural supports are stable [5]. Any instability in surrounding natural support is found to be influential over the performance of the RBBLS. In the beginning, three rows of roof bolts (each row with six-seven bolts) were used as RBBLS. But, on the basis of field observations only two rows of roof bolts are now, generally, used as RBBLS (Fig. 2). It is found that the high value of mining induced stress due to overhanging roof strata inside the goaf and high depth of cover creates spalling/loosening of sides of the natural supports. These spalling deteriorate the efficacy of a RBBLS. The performance of a RBBLS is found to be, mainly, influenced by two factors: 1) geology and 2) position. A RBBLS is observed to be ineffective if installed along a weak plane, running across the roof strata. Here, the effect of bolting to increase inter-strata cohesion gets diluted due to the discontinuity caused by the geological structure. The performance of a RBBLS is found to be influenced by its alignment with the edges of the surrounding natural supports. If the position of a RBBLS falls towards goaf side (not confined by natural supports from both sides) then its performance remained unsatisfactory. However, if the RBBLS is positioned inside the gallery, ahead (towards rib side) of the loosened part of the surrounding natural supports, it received confinement and worked acceptably. The alignment issue of the RBBLS also happens due to stress induced spalling of the surrounding natural supports. 2.2. Rock mechanics issues Application of roof bolts to support freshly exposed roof strata over a gallery is a well established approach in the coal mining industry. Here, mainly, width of the gallery and roof rock mass is characterised for the support design. On the basis of different field and laboratory studies, an empirical relationship [12] is developed to estimate rock load, which provides a basis to decide the density of bolts for the roof support. The scope of application of such an approach for the RBBLS is limited because the RBBLS is placed very close to the goaf. This closeness makes the RBBLS vulnerable to the strata dynamics phenomenon inside the goaf at different stages of the depillaring. It is observed that the natural supports, in and around the goaf edge, experience high value of mining induced stress. The high value of induced stress over these natural supports works in favour of the efficacy of the roof bolts but it also attempts to induce side spalling in the natural supports. As stated above, the side spalling of the natural support adversely affects the performance of the RBBLS. As the RBBLS is placed very close to the goaf, any design approach for the RBBLS has to consider the magnitude of strata dynamics, encountered by the goaf edge at different stages of the depillaring. The most dominant failure of a roof bolt is slippage at the cable-grout interface. Yazici and Kaiser (1992) [13] found that the bond strength is created as a result of friction between bolt and grout and proposed a bond strength model, which may be written as: W
V tan{ i o [ (
V V lim
E
) ] M}
(1)
where, IJ is shear or bond strength, ı is radial stress at the bolt-grout interface and ij is the frictional angle between the bolt and grout, ȕ is reduction co-efficient of dilation angle, ılim is limiting stress, ılim = ıcg and ıcg = compressive strength of grout. On the basis of this bond strength model, Kaiser et al. (1992) [14] found that the bond strength of a fully grouted bolt increases with the increase in stress and vice versa. A bolt in the roof strata of a gallery encounters increase in radial stress due to high value of mining induced stress over the nearby natural supports, which, ultimately, enhances its bond strength (Fig. 3). Mining induced stress over the natural support attempts to squeeze the rock mass, which, ultimately, improves confinement of the nearby roof bolt [15]. Therefore the effectiveness of the RBBLS, placed out-bye from the goaf edge, in the gallery is likely to be performing satisfactorily. A RBBLS, placed exactly at
Rajendra Singh et al. / Procedia Engineering 191 (2017) 551 – 559
the goaf edge, travels inside the goaf due to side spalling of the nearby natural supports (Fig. 4). Under the influence of spalling, the RBBLS could not receive the required hold from sides and performs poorly. Once the position of the RBBLS is in-bye side from the goaf edge, it finds distressed zone. Here, roof strata are under tension due to larger width of the excavation, which dilutes competency of the RBBLS. If a RBBLS is placed deep inside the gallery then there will be a loss of coal because the slicing of the pillar/fender can be done only out-bye side of the RBBLS. Therefore, it becomes important to find the amount of side spalling of the natural supports at the goaf edge and the ultimate size of the rib/snook to be left against the caving goaf to decide the suitable position of the RBBLS.
Fig. 3. Performance of a roof bolt inside the gallery under the influence of mining induced stress.
Fig. 4. A plan view of side spalling dependent dilution in the position of a RBBLS at the goaf edge during splitting & slicing method of pillar extraction.
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2.3. Rib/snook dimension Estimation of rib/snook dimension during a MD operation is a difficult task as it depends upon a number of geomining parameters. The formation of an irregular shaped rib/snook in MD of the existing rectangular/square pillars makes the estimation more difficult. A rib/snook is treated as a temporary applied support in depillaring [16]. It should be large enough to protect the slicing operation from the exposed goaf and gallery intersections but, at the same time, it should be small enough so that they do not inhibit the caving of roof strata inside the goaf. A recent field and laboratory studies [5] found that there is an increase in the size of a stable rib/snook with an increase in depth of cover. This finding contradicts the assumption that a rib/snook is like a temporary applied support. It is observed in the field that a natural support such as a pillar/fender experiences side spalling at deeper cover. The rib/snook is formed from these natural supports only and, therefore, a consideration of the side spalling increases the size of a stable rib/snook with depth of cover. An analysis of results of this study provided a relationship to estimate the stable size of the rib/snook (S), which is given as: ܵ ൌ ͲǤͷʹ ܪǤସ ܴǤଶଷ m2
(2)
where, H = depth of cover, m and R = CMRI-RMR [12]. An application of this relationship for the existing site conditions of the MD operations provides around 4 to 5 m width of the rib/snook in the considered MD panels. Therefore, in any case, the shifting of the RBBLS out-bye from the goaf edge cannot exceed these values inside the gallery. However, an optimal position of the RBLLS can be established after having an idea of the extent of the side spalling of the surrounding natural supports. 2.4. Roof characteristics The three important mining structures in a depillaring operation are: (a) Pillar/fender, (b) goaf and (c) applied support. The design of all these structures is done as per the nature of movement/caving of overlying strata inside the goaf. On the basis of different experiences of the depillaring operations [3], it is found that, generally, the area around intact pillars (standing ahead of the extraction line) does not experience much strata dynamics in a depillaring operation. In the beginning of the depillaring, a beam of overlying strata is formed over the goaf. The beam of the roof strata fails and forms a cantilever at the goaf edge after a sufficient increase in the span of the goaf. Here, the nature of failure of the roof strata is governed by its competency. The caving of strong and massive roof strata takes place after a large overhang inside the goaf and attempts to encroach the working area through the existing rooms along the extraction line. The caving of an extremely weak/laminated roof stratum brings good settlement of overlying strata inside the goaf. The observed extending nature of the competent roof strata is found to be absent during caving of the weak/laminated roof strata. The caving of strong roof induces side spalling in natural supports, which remained, generally, absent during caving of the weak/laminated roof strata. A field study of this fact during depillaring under different types of roof strata is found to be important for the position of the RBBLS. 3. Field studies Different MD panels of three mines: Anjan Hill, Pinoura and GDK 11 are studied to estimate the extent and nature of the side spalling. Depth of cover of these studied panels varied from 70 to 297 m and RMR of the roof strata varied from 42 to 53. Exact measurement of the extent of the side spalling is found to be a difficult task. The extent of spalling varied with positions of depillaring operation in a panel. Therefore, at each studied mine, three to four panels are studied for this estimation. The condition of natural supports, in and around the goaf edge, was visually monitored during the progress of the depillaring in each of these panels (Fig. 5).
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Fig. 5. Observed RBBLS performance during field study in the depillaring panel at GDK 11 mine.
Field studies found that the position of a RBBLS, installed in good alignment with the edges of the surrounding natural supports, goes off towards the goaf side (Fig. 6) due to the side spalling. The experienced side spalling is not found to be regular around a natural support and, therefore, the maximum observed extent of spalling is taken into the consideration. The observed depth of spalling in natural supports varied from 0.5 m to 2 m at the goaf edge of the three considered mines. MD panels of Pinoura mine were at shallow depth of cover (§100 m) and the overlying strata was easily caveable (RMR 42). Generally, the natural supports at the goaf edge of this mine did not experience any spalling. However, an overhang inside the goaf due to oversized left out ribs caused 0.5 m extent of side palling in natural supports at goaf edge. Different panels of the Anjan Hill mine, which had depth of cover similar to that of the Pinoura mine but the immediate roof was bit competent (RMR 52), experienced nearly 1m extent of the spalling at the goaf edge. MD panels of the GDK 11 Incline mine experienced side spalling in natural supports up to 2 m depth as these panels were located at higher depth (§200 m) with competent roof strata (RMR variation from 47 to 53). As per the extent of the observed spalling at these sites, the positions of the RBBLS were shifted by 1, 1 and 2 m towards out-bye side from goaf edge at Pinoura, Anajan Hill and GDK 11 mines respectively. This shifting in the positions at these three mines provided satisfactory performances of the RBBLS. However, as per the estimated stable size of a rib/snook at these sites, it is suggested to shift the position of the RBBLS by 2 m (Fig. 7) towards out-bye side from goaf edge. This shifting is in tune with the dimension of the rib/snook and will place the RBBLS under the confinement of natural supports from both sides.
Fig. 6. Observed problem of RBBLS due to side spalling in MD panel B2 at GDK 11 Incline mine.
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Fig. 7. A shifting in the RBBLS position by 2m to control of the goaf encroachment.
4. Position assessment A systematic investigation is reported on simulated model to estimate the length of bolt at different positions of the RBBLS in a gallery at the goaf edge [7]. Here, rock load height (RLH) was calculated at 0 m, 1 m and 2 m distance (out-bye from the goaf edge) from the extraction line. The obtained results of this simulation study found that the obtained lengths of the bolts at 0 m and 1m distance from the extraction line are not in tune with the field results. The obtained bolt lengths for these positions of RBBLS are found to be higher than the length of bolts being used at actual sites. Most of the successful MD operations in Indian coalfields adopted a roof bolt of only 2.4 m length for the RBBLS. These RBBLS worked suitably, when positioned at 2 m out-bye side from goaf edge. This observational fact is considered for field validation of the simulation results. The simulation provided a reasonable length (matching with field results) of the roof bolt at 2 m distance (out-bye from the goaf edge) from the extraction line. A competent roof stratum caves inside the goaf, generally, creates a large overhang, which brings dynamic loading over the mining structures, in and around the goaf edge. Therefore, it is important to consider the impact of this dynamic loading for the design of support density of the RBBLS. Accordingly, field and laboratory studies (on numerical models) are conducted to understand the extent of dynamic loading in a depillaring operation. On the basis of these studies, an attempt is made to quantify the extent of dynamic loading for the design of the RBBLS [17] in a MD operation. 4. Discussion and conclusion The application of RBBLS at the goaf edge is a beautiful example of a balanced amalgamation of science and technology. Scientific strength for the application of RBBLS is obtained from the stress redistribution in and around a goaf edge of the depillaring panel. A technological opportunity to accomplish the task of goaf edge support by RBBLS is created due to the availability of good underground machines for the installation of high capacity, pretensioned, resin grouted and stiff roof bolts. The confinement dependent performance of a plain reinforcement (cable/rock bolt) make it sensitive to small redial dilation. This sensitivity is, generally, overcome by a modified cable/bolt (Garford bulb) but it is not required at the goaf edge; especially in Indian coalfields. A RBBLS should remain effective ahead of the goaf edge only and it should become ineffective inside the goaf. If a modified cable/bolt is used in RBBLS, it will keep strengthening the overlying strata inside the goaf. Overlying strata of Indian coalfields, generally, break with large overhang, which will be worsened by the application of the modified cable/bolt. Observed side spalling/loosening of natural supports due to higher value of mining induced stress is found to be an important factor to decide the position of the RBBLS at the goaf edge. The observed extent of spalling in the studied depillaring panels is found to vary from 0.5 to 2 m. A MD operation at shallow depth and under easily caveable roof strata experienced, relatively, less amount of spalling in comparison to that under a competent roof stratum, located at deeper cover. However, there is need to generate more information about the extent of spalling from field for a wider range of geo-mining conditions.
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Acknowledgements Authors are obliged to Dr. P. K. Singh, Director, CSIR-CIMFR for granting permission to publish this paper. The support provided by the management of Anjan Hill, Pinoura and GDK 11 mines is sincerely acknowledged. References [1] M.P. Dixit, K. Mishra, A unique experience of on shortwall mining in Indian coal mining industry, in: Proceedings of 3rd Asian Mining Congress. MGMI Kolkata; 22-25 January 2010, pp. 25–37. [2] A.K. Singh, R. Singh, J. Maiti, P.K. Mandal, R. Kumar, Assessment of mining induced stress development over coal pillars during depillaring. Int. J. Rock. Mech. Min. Sci. 48(5) (2011) 805–818. [3] S. Ram, D. Kumar, P. Konicek, A.K. Singh, R. Kumar, A.K. Singh, R. Singh, Rock mechanics studies during continuous miner based coal pillar extraction in Indian coalfields, Transactions: a technical publication of the MGMI, 111 (2015) 89–114. [4] S.Z. Petho, C. Selai, D. Mashiyi, J.N. Van-der-Merwe, Managing the geotechnical and mining issues surrounding the extraction of small pillars at shallow depths at Xstrata Coal South Africa, Journal of the South African Institute of Mining and Metallurgy 112 (2012) 105–18. [5] R. Singh, A. Kumar, A.K. Singh, J. Coggan, S. Ram, Rib/snook design in mechanised depillaring of rectangular/square pillars, Int. J. Rock. Mech. Min. Sci. 84 (2016) 19–29. [6] R. Singh, T.N. Singh, B.B. Dhar, Coal pillar loading for shallow mining conditions, Int. J. Rock. Mech. Min. Sci. Geomechs. Abst. 33(8) (1986) 757–68. [7] S. Ram, A.K. Singh, D. Kumar, R. Singh, Design of Roof Bolt based Breaker Line Support in a Mechanised Depillaring Panel, in: Proceedings of 35th International Conference on Ground Control in Mining. Morgantown WV USA; 26-28 July 2016, pp. 155–61. [8] J.N.Van-der-Merwe, Fundamental analysis of the interaction between overburden behaviour and snook stability in coalmines, The Journal of the South African Institute of Mining and Metallurgy 105(1) (2005) 63–73. [9] R.D Singh, Principles and Practices of Modern Coal Mining, New Age International (P) Limited, Publishers, New Delhi, 1997. [10] P.K. Mandal, R. Singh, A.K. Singh, R. Kumar, A.Sinha, State of art vis-à-vis Indian scenario of application of continuous miner based mass production technology, Journal of Mines Metals and Fuels 54(12) (2006) 332–336. [11] J. Leeming, Joy introduces continuous miner: Technology into India, Coal International/Mining and Quarry World, (2003) 203–206. [12] V. Venkateswarlu, A.K. Ghose, N.M. Raju, Rock mass classification for design ofroof support—a statistical evaluation of parameters, Min. Sc. Tech. 8 (1989) 97–107. [13] S.Yazic, P. K. Kaiser, Bond Strength of grouted cable bolts, Int. J. Rock. Mech. Min. Sci. 29(3) (1992) 279–292. [14] P. K. Kaiser, S. Yazic, J. Nose, Effect of stress change on bond strength of fully grouted cables, Int. J. Rock. Mech. Min. Sci. 29(3) (1992) 293–306. [15] R. Singh, P.K. Mandal, A.K. Singh, T.N. Singh. Cable bolting based mechanised depillaring of a thick coal seam, Int. J. Rock. Mech. Min. Sci. 38(2) (2001) 245–57. [16] C. Mark, J.C. Zelanko, Sizing of final stumps for safer pillar extraction, in: Proceeding of 20th International Conferenceon Ground Controlin Mining. Morgantown, WV, USA; 7-9 August 2001, pp. 59–66. [17] S. Ram, A study of roof-pillar interaction for an efficient breaker line design during mechanised depillaring. Ph.D. Thesis (submitted) 2016.
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Symposium of the International Society for Rock Mechanics
Rock Mechanics Considerations for Roof Bolt-Based Breaker Line Design Rajendra Singh*, Sahendra Ram, Arun Kumar Singh, Ashok Kumar, Rakesh Kumar, Amit Kr. Singh CSIR-Central Institute of Mining and Fuel Research, Barwa Road, Dhanbad, Jharkhand, 826001, India
Abstract Application of roof bolts to support in developed galleries during underground coal mining is a common practice. Mechanised depillaring (MD) applies these roof bolts to support the galleries at the goaf edge also. Goaf edge of a depillaring panel inherits a number of openings, which provides least resistance path for the caving roof strata to encroach the working areas. Influence of strata caving, inside the goaf, make it difficult to apply conventional roof bolt support norms in these openings at the goaf edge. Basic rock mechanics considerations find that the mining induced stress development improves the efficacy of roof bolt at the goaf edge. Application of, relatively, high density of bolts as roof bolts-based breaker line support (RBBLS) in the existing openings at the goaf edge to arrest the goaf encroachment is found to be working satisfactorily only if the surrounding natural supports are stable. The stability of a natural support, in and around the goaf edge, is found to be diluted due to side spalling/loosening caused by the mining induced stress. Different field experiences found that a high value of the stress causes side spalling/loosening in natural supports, standing in and around the goaf edge. Under the existing geo-mining conditions of some of the recent MD operations in Indian coalfields, the extent of spalling varied from 0.5 m to 2 m only. Considering an acceptable dimension of the rib/snook for these site conditions, the position of RBBLS is shifted out-bye from the goaf line as per the extent of spalling, which performed satisfactorily in the field. Presenting basic working principles of a RBBLS at the goaf edge; this paper discusses different field experiences for a suitable positioning of the RBBLS at the goaf edge. Published by Elsevier Ltd. © 2017 2017Published The Authors. © by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of EUROCK 2017. Peer-review under responsibility of the organizing committee of EUROCK 2017 Keywords: Roof bolts based breaker-line support (RBBLS); Mechanised depillaring (MD); Goaf edge; Numerical modelling; Rock load height and natural support
* Corresponding author. Tel.: +91-326-2296043; fax: +91-326-2296033. E-mail address: [email protected]
1877-7058 © 2017 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of EUROCK 2017
doi:10.1016/j.proeng.2017.05.217
552
Rajendra Singh et al. / Procedia Engineering 191 (2017) 551 – 559
1. Introduction Existing facts and figures about different developed coal seams (standing on pillars) by bord and pillar method find pillar extraction i.e. depillaring an important issue [1] for the Indian coal mining industry. Depillaring with caving is a challenging operation from strata control point of view. Frequent encounter of competent roof strata of Gondwana formations creates a large overhang inside the goaf. Caving of the roof strata after a large overhang generates high value of mining induced stress over natural supports in and around the extraction line [2]. However, value of the mining induced stress, generally, remains insignificant if regular caving of the roof strata takes place inside the goaf. As per different field observations, in addition to the competency, treatment of goaf is a significant parameter for the strata control issue of a depillaring panel. Filling of the void due to mining, generally, provides good strata condition in a depillaring but falls beyond the scope of this paper. However, depillaring with caving inherently leaves a number of remnants (like ribs and snooks) inside the goaf. These remnants should be competent enough to support the slicing operation [3] and, at the same time, should fail in controlled manner when it travels inside the goaf with the face advance. An appropriate design of these remnants, protect the slicing operation from goaf encroachment and, also, plays significant role in caving of the overlying strata inside the goaf [4, 5]. In spite of a suitable design of the rib/snook, goaf treatment like induced caving and judicious reduction of rib/snook are conducted as per the encountered geo-mining conditions of different sites. During caving of the hanging roof strata, there is a possibility that the caving extends into the working area (also known as goaf encroachment) through the existing openings along the extraction line. Each of these openings in a MD operation, along the extraction line, is supported by the roof bolts-based breaker line support (RBBLS) to restrict the goaf encroachment. RBBLS and surrounding rib/snook react against the extension of roof caving into the working area. Under these circumstances, the RBBLS along with the surrounding rib/snook act as pivot point between goaf and working area. The purpose of this pivot is to facilitate controlled caving of the hanging roof and is known as breaker line. Generally, a slower rate of pillar extraction deteriorates strata condition [6] during the depillaring operation. Accordingly, the Indian coal mining industry has recently deployed a number of MD operations [7] {applying a continuous miner (CM) for cutting and shuttle/ram car for haulage}, which provides improvements in production, productivity and safety too. Considering larger size of the developed pillars, a pillar is, first, split into two or more fenders (depending upon their size) by driving one or more split galleries. Slicing of the most in-bye fender is done after leaving an appropriate size of rib against the goaf (Fig. 1) and installation of breaker line support at the goaf edge. Thus, an appropriate design of the rib [8] and the RBBLS is vital to provide optimal resistance against the roof during the slicing operation.
Fig. 1. Splitting and slicing for a typical straight line extraction in a mechanised depillaring panel.
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2. RBBLS An appropriate design of the natural support as well as applied support along the extraction line is an essential requirement to provide an effective pivot. Conventionally, high density of the reactive supports (like props, chocks etc.) are erected in the existing openings along the extraction line as the breaker line support. This type of the breaker line support has some operational disadvantages [9] and is found to be unsuitable for a MD operation. To alleviate the problems of conventional breaker line support, MD in India adopted RBBLS and found to be effective in arresting the goaf encroachment [10]. In fact, the MD operations adopted high capacity, pre-tensioned, stiff and resin grouted roof bolts [11] to support the roof strata in the galleries of a MD panel. Presence of a compatible machine for the bolting, a higher density of the bolts were applied at the goaf edge (Fig. 2) of the MD panel as a breaker line support. Adopted design of the RBBLS at different MD panel is given by different consultants on the basis of their experiences gained at other sites. Observed performance of the RBBLS at different MD operations in the country have shown mixed results, probably, due to the uniqueness of rock-mass and complex geo-mining conditions of the Indian coalfields. Therefore field and laboratory investigations were carried out to visualise the working model of the RBBLS at different site conditions. On the basis of field studies, it is realized that the side spalling of the surrounding natural support is a major reason for the failure of a RBBLS. Such a failure can be tackled in two steps: (1) design of competent ribs/fenders/pillars surrounding the RBBLS and (2) improvement in competency of the installed RBBLS. The first step falls beyond the scope of this study, but the improvement in the RBBLS competency needs to be matched with the nature of the caving strata. For an efficient design of RBBLS, length and density of bolts in the RBBLS need to be adjusted as per the behaviour of overlying strata at varying site conditions. In presence of spalling, length and density of the bolts in RBBLS become a function of its position. RBBLS with, relatively, longer and denser bolts is also likely to experience failure, if it loses the shadow of surrounding natural supports due to the side spalling.
Fig. 2. Dimensional details of single pass pillar extraction at Pinoura mine and plan view of the adopted RBBLS.
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2.1. Working model of a RBBLS Inherent existence of different rooms (openings) along the extraction line in a depillaring panel provides an easy path for the caving strata inside the goaf to encroach the working area. Field studies found that the application of, relatively, high density of bolts as RBBLS in the existing openings at the goaf edge to arrest the goaf encroachment works satisfactorily only if the surrounding natural supports are stable [5]. Any instability in surrounding natural support is found to be influential over the performance of the RBBLS. In the beginning, three rows of roof bolts (each row with six-seven bolts) were used as RBBLS. But, on the basis of field observations only two rows of roof bolts are now, generally, used as RBBLS (Fig. 2). It is found that the high value of mining induced stress due to overhanging roof strata inside the goaf and high depth of cover creates spalling/loosening of sides of the natural supports. These spalling deteriorate the efficacy of a RBBLS. The performance of a RBBLS is found to be, mainly, influenced by two factors: 1) geology and 2) position. A RBBLS is observed to be ineffective if installed along a weak plane, running across the roof strata. Here, the effect of bolting to increase inter-strata cohesion gets diluted due to the discontinuity caused by the geological structure. The performance of a RBBLS is found to be influenced by its alignment with the edges of the surrounding natural supports. If the position of a RBBLS falls towards goaf side (not confined by natural supports from both sides) then its performance remained unsatisfactory. However, if the RBBLS is positioned inside the gallery, ahead (towards rib side) of the loosened part of the surrounding natural supports, it received confinement and worked acceptably. The alignment issue of the RBBLS also happens due to stress induced spalling of the surrounding natural supports. 2.2. Rock mechanics issues Application of roof bolts to support freshly exposed roof strata over a gallery is a well established approach in the coal mining industry. Here, mainly, width of the gallery and roof rock mass is characterised for the support design. On the basis of different field and laboratory studies, an empirical relationship [12] is developed to estimate rock load, which provides a basis to decide the density of bolts for the roof support. The scope of application of such an approach for the RBBLS is limited because the RBBLS is placed very close to the goaf. This closeness makes the RBBLS vulnerable to the strata dynamics phenomenon inside the goaf at different stages of the depillaring. It is observed that the natural supports, in and around the goaf edge, experience high value of mining induced stress. The high value of induced stress over these natural supports works in favour of the efficacy of the roof bolts but it also attempts to induce side spalling in the natural supports. As stated above, the side spalling of the natural support adversely affects the performance of the RBBLS. As the RBBLS is placed very close to the goaf, any design approach for the RBBLS has to consider the magnitude of strata dynamics, encountered by the goaf edge at different stages of the depillaring. The most dominant failure of a roof bolt is slippage at the cable-grout interface. Yazici and Kaiser (1992) [13] found that the bond strength is created as a result of friction between bolt and grout and proposed a bond strength model, which may be written as: W
V tan{ i o [ (
V V lim
E
) ] M}
(1)
where, IJ is shear or bond strength, ı is radial stress at the bolt-grout interface and ij is the frictional angle between the bolt and grout, ȕ is reduction co-efficient of dilation angle, ılim is limiting stress, ılim = ıcg and ıcg = compressive strength of grout. On the basis of this bond strength model, Kaiser et al. (1992) [14] found that the bond strength of a fully grouted bolt increases with the increase in stress and vice versa. A bolt in the roof strata of a gallery encounters increase in radial stress due to high value of mining induced stress over the nearby natural supports, which, ultimately, enhances its bond strength (Fig. 3). Mining induced stress over the natural support attempts to squeeze the rock mass, which, ultimately, improves confinement of the nearby roof bolt [15]. Therefore the effectiveness of the RBBLS, placed out-bye from the goaf edge, in the gallery is likely to be performing satisfactorily. A RBBLS, placed exactly at
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the goaf edge, travels inside the goaf due to side spalling of the nearby natural supports (Fig. 4). Under the influence of spalling, the RBBLS could not receive the required hold from sides and performs poorly. Once the position of the RBBLS is in-bye side from the goaf edge, it finds distressed zone. Here, roof strata are under tension due to larger width of the excavation, which dilutes competency of the RBBLS. If a RBBLS is placed deep inside the gallery then there will be a loss of coal because the slicing of the pillar/fender can be done only out-bye side of the RBBLS. Therefore, it becomes important to find the amount of side spalling of the natural supports at the goaf edge and the ultimate size of the rib/snook to be left against the caving goaf to decide the suitable position of the RBBLS.
Fig. 3. Performance of a roof bolt inside the gallery under the influence of mining induced stress.
Fig. 4. A plan view of side spalling dependent dilution in the position of a RBBLS at the goaf edge during splitting & slicing method of pillar extraction.
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2.3. Rib/snook dimension Estimation of rib/snook dimension during a MD operation is a difficult task as it depends upon a number of geomining parameters. The formation of an irregular shaped rib/snook in MD of the existing rectangular/square pillars makes the estimation more difficult. A rib/snook is treated as a temporary applied support in depillaring [16]. It should be large enough to protect the slicing operation from the exposed goaf and gallery intersections but, at the same time, it should be small enough so that they do not inhibit the caving of roof strata inside the goaf. A recent field and laboratory studies [5] found that there is an increase in the size of a stable rib/snook with an increase in depth of cover. This finding contradicts the assumption that a rib/snook is like a temporary applied support. It is observed in the field that a natural support such as a pillar/fender experiences side spalling at deeper cover. The rib/snook is formed from these natural supports only and, therefore, a consideration of the side spalling increases the size of a stable rib/snook with depth of cover. An analysis of results of this study provided a relationship to estimate the stable size of the rib/snook (S), which is given as: ܵ ൌ ͲǤͷʹ ܪǤସ ܴǤଶଷ m2
(2)
where, H = depth of cover, m and R = CMRI-RMR [12]. An application of this relationship for the existing site conditions of the MD operations provides around 4 to 5 m width of the rib/snook in the considered MD panels. Therefore, in any case, the shifting of the RBBLS out-bye from the goaf edge cannot exceed these values inside the gallery. However, an optimal position of the RBLLS can be established after having an idea of the extent of the side spalling of the surrounding natural supports. 2.4. Roof characteristics The three important mining structures in a depillaring operation are: (a) Pillar/fender, (b) goaf and (c) applied support. The design of all these structures is done as per the nature of movement/caving of overlying strata inside the goaf. On the basis of different experiences of the depillaring operations [3], it is found that, generally, the area around intact pillars (standing ahead of the extraction line) does not experience much strata dynamics in a depillaring operation. In the beginning of the depillaring, a beam of overlying strata is formed over the goaf. The beam of the roof strata fails and forms a cantilever at the goaf edge after a sufficient increase in the span of the goaf. Here, the nature of failure of the roof strata is governed by its competency. The caving of strong and massive roof strata takes place after a large overhang inside the goaf and attempts to encroach the working area through the existing rooms along the extraction line. The caving of an extremely weak/laminated roof stratum brings good settlement of overlying strata inside the goaf. The observed extending nature of the competent roof strata is found to be absent during caving of the weak/laminated roof strata. The caving of strong roof induces side spalling in natural supports, which remained, generally, absent during caving of the weak/laminated roof strata. A field study of this fact during depillaring under different types of roof strata is found to be important for the position of the RBBLS. 3. Field studies Different MD panels of three mines: Anjan Hill, Pinoura and GDK 11 are studied to estimate the extent and nature of the side spalling. Depth of cover of these studied panels varied from 70 to 297 m and RMR of the roof strata varied from 42 to 53. Exact measurement of the extent of the side spalling is found to be a difficult task. The extent of spalling varied with positions of depillaring operation in a panel. Therefore, at each studied mine, three to four panels are studied for this estimation. The condition of natural supports, in and around the goaf edge, was visually monitored during the progress of the depillaring in each of these panels (Fig. 5).
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Fig. 5. Observed RBBLS performance during field study in the depillaring panel at GDK 11 mine.
Field studies found that the position of a RBBLS, installed in good alignment with the edges of the surrounding natural supports, goes off towards the goaf side (Fig. 6) due to the side spalling. The experienced side spalling is not found to be regular around a natural support and, therefore, the maximum observed extent of spalling is taken into the consideration. The observed depth of spalling in natural supports varied from 0.5 m to 2 m at the goaf edge of the three considered mines. MD panels of Pinoura mine were at shallow depth of cover (§100 m) and the overlying strata was easily caveable (RMR 42). Generally, the natural supports at the goaf edge of this mine did not experience any spalling. However, an overhang inside the goaf due to oversized left out ribs caused 0.5 m extent of side palling in natural supports at goaf edge. Different panels of the Anjan Hill mine, which had depth of cover similar to that of the Pinoura mine but the immediate roof was bit competent (RMR 52), experienced nearly 1m extent of the spalling at the goaf edge. MD panels of the GDK 11 Incline mine experienced side spalling in natural supports up to 2 m depth as these panels were located at higher depth (§200 m) with competent roof strata (RMR variation from 47 to 53). As per the extent of the observed spalling at these sites, the positions of the RBBLS were shifted by 1, 1 and 2 m towards out-bye side from goaf edge at Pinoura, Anajan Hill and GDK 11 mines respectively. This shifting in the positions at these three mines provided satisfactory performances of the RBBLS. However, as per the estimated stable size of a rib/snook at these sites, it is suggested to shift the position of the RBBLS by 2 m (Fig. 7) towards out-bye side from goaf edge. This shifting is in tune with the dimension of the rib/snook and will place the RBBLS under the confinement of natural supports from both sides.
Fig. 6. Observed problem of RBBLS due to side spalling in MD panel B2 at GDK 11 Incline mine.
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Fig. 7. A shifting in the RBBLS position by 2m to control of the goaf encroachment.
4. Position assessment A systematic investigation is reported on simulated model to estimate the length of bolt at different positions of the RBBLS in a gallery at the goaf edge [7]. Here, rock load height (RLH) was calculated at 0 m, 1 m and 2 m distance (out-bye from the goaf edge) from the extraction line. The obtained results of this simulation study found that the obtained lengths of the bolts at 0 m and 1m distance from the extraction line are not in tune with the field results. The obtained bolt lengths for these positions of RBBLS are found to be higher than the length of bolts being used at actual sites. Most of the successful MD operations in Indian coalfields adopted a roof bolt of only 2.4 m length for the RBBLS. These RBBLS worked suitably, when positioned at 2 m out-bye side from goaf edge. This observational fact is considered for field validation of the simulation results. The simulation provided a reasonable length (matching with field results) of the roof bolt at 2 m distance (out-bye from the goaf edge) from the extraction line. A competent roof stratum caves inside the goaf, generally, creates a large overhang, which brings dynamic loading over the mining structures, in and around the goaf edge. Therefore, it is important to consider the impact of this dynamic loading for the design of support density of the RBBLS. Accordingly, field and laboratory studies (on numerical models) are conducted to understand the extent of dynamic loading in a depillaring operation. On the basis of these studies, an attempt is made to quantify the extent of dynamic loading for the design of the RBBLS [17] in a MD operation. 4. Discussion and conclusion The application of RBBLS at the goaf edge is a beautiful example of a balanced amalgamation of science and technology. Scientific strength for the application of RBBLS is obtained from the stress redistribution in and around a goaf edge of the depillaring panel. A technological opportunity to accomplish the task of goaf edge support by RBBLS is created due to the availability of good underground machines for the installation of high capacity, pretensioned, resin grouted and stiff roof bolts. The confinement dependent performance of a plain reinforcement (cable/rock bolt) make it sensitive to small redial dilation. This sensitivity is, generally, overcome by a modified cable/bolt (Garford bulb) but it is not required at the goaf edge; especially in Indian coalfields. A RBBLS should remain effective ahead of the goaf edge only and it should become ineffective inside the goaf. If a modified cable/bolt is used in RBBLS, it will keep strengthening the overlying strata inside the goaf. Overlying strata of Indian coalfields, generally, break with large overhang, which will be worsened by the application of the modified cable/bolt. Observed side spalling/loosening of natural supports due to higher value of mining induced stress is found to be an important factor to decide the position of the RBBLS at the goaf edge. The observed extent of spalling in the studied depillaring panels is found to vary from 0.5 to 2 m. A MD operation at shallow depth and under easily caveable roof strata experienced, relatively, less amount of spalling in comparison to that under a competent roof stratum, located at deeper cover. However, there is need to generate more information about the extent of spalling from field for a wider range of geo-mining conditions.
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Acknowledgements Authors are obliged to Dr. P. K. Singh, Director, CSIR-CIMFR for granting permission to publish this paper. The support provided by the management of Anjan Hill, Pinoura and GDK 11 mines is sincerely acknowledged. References [1] M.P. Dixit, K. Mishra, A unique experience of on shortwall mining in Indian coal mining industry, in: Proceedings of 3rd Asian Mining Congress. MGMI Kolkata; 22-25 January 2010, pp. 25–37. [2] A.K. Singh, R. Singh, J. Maiti, P.K. Mandal, R. Kumar, Assessment of mining induced stress development over coal pillars during depillaring. Int. J. Rock. Mech. Min. Sci. 48(5) (2011) 805–818. [3] S. Ram, D. Kumar, P. Konicek, A.K. Singh, R. Kumar, A.K. Singh, R. Singh, Rock mechanics studies during continuous miner based coal pillar extraction in Indian coalfields, Transactions: a technical publication of the MGMI, 111 (2015) 89–114. [4] S.Z. Petho, C. Selai, D. Mashiyi, J.N. Van-der-Merwe, Managing the geotechnical and mining issues surrounding the extraction of small pillars at shallow depths at Xstrata Coal South Africa, Journal of the South African Institute of Mining and Metallurgy 112 (2012) 105–18. [5] R. Singh, A. Kumar, A.K. Singh, J. Coggan, S. Ram, Rib/snook design in mechanised depillaring of rectangular/square pillars, Int. J. Rock. Mech. Min. Sci. 84 (2016) 19–29. [6] R. Singh, T.N. Singh, B.B. Dhar, Coal pillar loading for shallow mining conditions, Int. J. Rock. Mech. Min. Sci. Geomechs. Abst. 33(8) (1986) 757–68. [7] S. Ram, A.K. Singh, D. Kumar, R. Singh, Design of Roof Bolt based Breaker Line Support in a Mechanised Depillaring Panel, in: Proceedings of 35th International Conference on Ground Control in Mining. Morgantown WV USA; 26-28 July 2016, pp. 155–61. [8] J.N.Van-der-Merwe, Fundamental analysis of the interaction between overburden behaviour and snook stability in coalmines, The Journal of the South African Institute of Mining and Metallurgy 105(1) (2005) 63–73. [9] R.D Singh, Principles and Practices of Modern Coal Mining, New Age International (P) Limited, Publishers, New Delhi, 1997. [10] P.K. Mandal, R. Singh, A.K. Singh, R. Kumar, A.Sinha, State of art vis-à-vis Indian scenario of application of continuous miner based mass production technology, Journal of Mines Metals and Fuels 54(12) (2006) 332–336. [11] J. Leeming, Joy introduces continuous miner: Technology into India, Coal International/Mining and Quarry World, (2003) 203–206. [12] V. Venkateswarlu, A.K. Ghose, N.M. Raju, Rock mass classification for design ofroof support—a statistical evaluation of parameters, Min. Sc. Tech. 8 (1989) 97–107. [13] S.Yazic, P. K. Kaiser, Bond Strength of grouted cable bolts, Int. J. Rock. Mech. Min. Sci. 29(3) (1992) 279–292. [14] P. K. Kaiser, S. Yazic, J. Nose, Effect of stress change on bond strength of fully grouted cables, Int. J. Rock. Mech. Min. Sci. 29(3) (1992) 293–306. [15] R. Singh, P.K. Mandal, A.K. Singh, T.N. Singh. Cable bolting based mechanised depillaring of a thick coal seam, Int. J. Rock. Mech. Min. Sci. 38(2) (2001) 245–57. [16] C. Mark, J.C. Zelanko, Sizing of final stumps for safer pillar extraction, in: Proceeding of 20th International Conferenceon Ground Controlin Mining. Morgantown, WV, USA; 7-9 August 2001, pp. 59–66. [17] S. Ram, A study of roof-pillar interaction for an efficient breaker line design during mechanised depillaring. Ph.D. Thesis (submitted) 2016.
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