Room-And-pillar Panel Design Method

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Introduction

The main intention in using theroom-and-pillar mining method with-out pillar extraction is to avoid imme-diate and future surface subsidence.

However, surface subsidence doesoccur over such mines, with some ofthese unexpected subsidence eventsoccurring long after mining opera-tions causing significant surface dis-turbances. In some cases, cascadingpillar failure had occurred over largemine areas, causing substantial inter-ruption to mining operations (Morsyand Peng, 2001) and abrupt surfacesubsidence events. Investigations ofsuch subsidence events often foundthe root causes to be: (1) insufficient-ly sized pillars in the production pan-els and (2) failure of mine floor, roofor pillar parting that are clay-rich andcan be greatly weakened by mine wa-ter (Luo, 2011). To eliminate the pos-sibility of surface subsidence whileachieving reasonably high recovery

of the coal reserve, the room-and-pil-lar panel system should be properlydesigned.

In this paper, a systematic room-and-pillar panel design method ap-plying the pressure arch conceptincorporated with sufficiently sizedbarrier pillars is proposed. The pil-lars in the production panel are de-signed to only carry the overburdenload under the pressure arch so thata high recovery ratio can be achievedin the panel. The larger barrier pillarseparating the adjacent productionpanels is designed to withstand thecondition of extreme load, when thepillars in the production panels failcompletely. Through a design opti-mization process while consideringcommon production practices, the

Room-and-pillar panel designmethod to avoid surface subsidence

by Y. Luo

Y. Luo is associate professor,Department of Mining Engineering,at West Virginia University,Morgantown, WV. Paper numberTP-14-015. Original manuscriptsubmitted February 2014. Revisedmanuscript accepted for publicationJanuary 2015. Discussion of thispeer-reviewed and approved paperis invited and must be submitted toSME Publications by Oct. 31, 2015.

Abstract ■ In designing and operating underground room-and-pillar coal mines, it is alwaysdifficult to achieve a good balance between preventing surface subsidence and achievinghigh recovery ratio. In this paper, a design methodology applying the pressure arch theoryis proposed, and the principles and mathematical models involved are detailed. A computerprogram was developed to carry out the design optimization process and was applied to theexamination of a documented case in which a massive cascading pillar failure had occurredover a large area in a shallow coal mine. The results indicate that the proposed design methodcan be applied to prevent subsidence while achieving reasonably high recovery ratio for room-and-pillar mining operations.

Mining Engineering, 2015, Vol. 67, No. 7, pp. 105-110.Official publication of the Society for Mining, Metallurgy & Exploration Inc.

Resumen ■ Cuando se diseña y operan minas de carbón subterráneas con el métodode cámaras y pilares, es siempre difícil obtener un buen equilibrio entre la prevención desubsidencia de la superficie y la obtención de un alto coeficiente de recuperación. En estetrabajo se propone una metodología de diseño aplicando la teoría de arco de presión y sedetallan los principios y modelos matemáticos involucrados. Se desarrolló un programade computadora a fin de realizar el proceso de optimización del diseño y fue aplicado ala examinación de un caso documentado en el que había sucedido una falla masiva deun pilar en cascada sobre un área grande en una mina de carbón poco profunda. Losresultados indican que el método de diseño propuesto puede ser aplicado para prevenirla subsidencia y a la vez obtener un alto coeficiente de recuperación para operaciones

mineras de cámara y pilares.



106 JULY 2015 Mınıng engıneerıng www.miningengineeringmagazine.com

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overall recovery of coal reserves from such a room-and-pillar panel system can be maintained at a high level. Withthe proposed methodology, the possibility of pillars in theproduction panels failing is minimized and the effectsof any cascading pillar failure in the production panelscan be contained by the barrier pillars. Most importantly,through this design approach, the possibility of immedi-ate and future surface subsidence can be greatly reduced.

Design methodology The layout of modern room-and-pillar mines has be-

come increasingly like that of longwall mines, in whichrectangular production panels are separated by barrierpillars. Using the traditional room-and-pillar panel de-sign method, the recovery ratio in the production panelsis limited by a number of factors, the main one being theoverburden depth since each panel pillar is required tocarry the entire tributary overburden load with an accept-able safety factor. Among the possible shapes of panelpillars, square pillars offer the highest load-carrying ca-pacity per unit area of pillar and can thus achieve the bestreserve recovery for a given condition.

Different from the traditional design method, thisproposed design method considers the production paneland its adjacent barrier pillars as a system. The width of

the production panel should be properly sized so that apressure arch over the production panel can still exist inthe competent overburden strata. Under such conditions,the panel pillars only need to carry the overburden loadup to the pressure arch, so these pillars can be designedsmaller than those designed with the traditional method.To make the pillar designs more rational both for pro-duction practices and for better utilization of the pillarstrength, a unique panel pillar design system as shown inFig. 1 is proposed. The majority of the panel pillars aresquare in shape with size of W . Two lines of rectangularpillars are left in the center of the panel and each of thecentral pillars has width of W and length of L

c = 2W +

W r , where W

r is the room width. The larger central pillars

can not only provide better protection to the belt entrylocated between them but also carry the higher overbur-den load that is expected directly under the top of thepressure arch. Wider barrier pillars are left between theproduction panels to isolate any possible mining interac-tions from adjacent panels.

Pressure arch theory. The concept of pressure arch ap-plied in the design of the panel pillars had been proposedfor ground control in bedded strata as early as in the 1930s.The theory suggests that a pressure arch is developed asthe result of redistribution of weight and a distressed zoneis formed under the pressure arch (Institution of MiningEngineers, 1936). The beds under the pressure arch de-flect slightly and no longer carry the weight of the super-incumbent mass of strata. A pressure arch is thought to bepresent in the roof above every mining excavation, withthe load of the superincumbent strata transferred to thetwo abutments of the pressure arch (Institution of MiningEngineers, 1949). Based on this concept, the pillars in theproduction panel only need to carry the overburden loadunder the pressure arch. The pressure arch formed overa room-and-pillar panel is depicted in Fig. 2 and can bemathematically defined as an ellipse function:

(1)

In this approach, the semi-minor and major axes ofthe ellipse, a and b, are related by the angle of abutment,α, in the following equation. According to research bythe U.S. National Institute for Occupational Safety andHealth (NIOSH), an abutment angle of 21° is appropri-ate for U.S. coal mines (Mark and Chase, 1997). In theequation, a is one half of the width of the pressure archor the width of the production panel (W p). The height ofthe pressure arch (b) can be determined by the follow-ing simplified equation:

Proposed room-and-pillar panel system design.

Figure 1

Pressure arch and pillar loading.

Figure 2




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(2)

Therefore, the maximum allowable panel width forthe pressure arch to exist in the competent strata (W max)

can be determined by the thickness of the competentstrata (hc). The term h

c is the overburden depth subtract-

ing the thickness of unconsolidated materials near theground surface (Fig. 2). The height of the pressure arch at x distance away from the panel center is then determinedby:

(3)

Panel pillar design. Using Eq. (1), the area of thetransverse cross-section under the pressure arch can beanalytically determined. The total overburden load to becarried by the two lines of production pillars (shown asthe shaded area in Fig. 1) can be determined by the fol-lowing equation, considering the average density of theoverburden strata to be γ:

(4)

When a proper panel width, W p, is chosen, N entriescan be arranged in each half of the panel width exclud-ing the central entry (belt entry) and N − 1 lines of nor-mal production pillars plus the central pillar will be leftin each half of the panel width. The load capacity of eachof the square production pillars can be determined byapplying the Bieniawski pillar strength formula (Bien-iawski, 1981). It should be noted that other pillar strengthformulae can also be used in the design process accordingto the user’s preference.

(5)

In the above equation, m is the height of the pillaror the mining height in the coal seam. By converting arectangular central pillar of width W and length L

c into

an equivalent square pillar, the load capacity of a centralpillar can be determined using the following formula:

(6)

The in situ strength of the coal (σ i) in the two equa-

tions above can be determined from laboratory tests with

correction for the specimen size effect. However, basedon the recommendation of Mark and Chase (1997), 900psi (6.2 MPa) can be used for σ

i in most U.S. coal mines.

Furthermore, if clay-rich rock strata that can be weak-ened considerably by mine water are present in the im-mediate floor and roof or as pillar parting, σ

i = 600 psi (4.1

MPa) can be used. The total pillar load capacity of thetwo lines of production pillars and the two central pillars(shaded area in Fig. 1) is then determined by substitutingthe above two equations into the following equation:

(7)

To ensure the production panel does not fail, an ad-

equate overall safety factor (SF) for the production andcentral pillars should be specified in the design process,resulting in the following design equation:

(8)

The user should pay close attention to the units used inEq. (8) and the subsequent Eqs. (10) and (11).

Since the room width (W r ) is normally specified in acoal mining production, the only unknown variable to bedetermined from Eq. (8) is the width of the square pro-duction pillars (W ). Once W is determined, the size of thecentral pillar is also defined according to the proposedarrangement shown in Fig. 1). The recovery ratio in theproduction panel can be determined by:

(9)

It should be noted that the pillar design in the produc-tion panel using this pressure arch concept is an iterativeprocess. It should start with a rational panel width (W p)that permits the fitting of 2N + 1 entries, 2(N − 1) linesof square production pillars and two lines of rectangularcentral pillars in the panel, as shown in Fig. 1. The panelwidth should be kept smaller than the maximum allow-able panel width (W max) so that the top of the pressurearch still remains within the competent strata in the over-burden. It should also be noted that the recovery ratio inthe production panel is not necessarily the best when thepanel is designed up to the maximum allowable width.In order to achieve a good recovery ratio under the giv-en conditions, the panel width should be varied within arange practical for room-and-pillar mining operations.

As the pillars in the production panel are sized, thesafety factor of the individual pillars should also bechecked. For the central pillars, the safety factor is deter-mined using:

(10)

For a production pillar in the ith line from the centralpillar, its safety factor can be determined using:

(11)

The safety factors for the pillars in the productionpanel vary with distance from the panel center. Due tothe proposed special pillar design, the longer rectangularcentral pillars should have a safety factor higher than theoverall safety factor specified in the design of the produc-tion pillars. Figure 3 shows the resulting safety factors ofthe central and production pillars when the overall safetyfactor specified in the design is 1.3. Generally, the first lineof the production pillars beside the central pillars has thelowest safety factor. If any of the resulting safety factors




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is significantly less than 1.0, a higher overall safety factorshould be specified and the production panel should beredesigned.

Design of barrier pillar system. The barrier pillar

system separating two adjacent room-and-pillar panelsshould be much wider than the pillars inside the pro-duction panels. It should be designed to carry the entireoverburden load in the extreme condition, should allthe pillars in the adjacent production panels have failedcompletely as proposed by Zipf (2008), with safety factorof 1.0. The extreme load to be carried by the barrier pil-lar is determined as the entire overburden depth withina distance of the total width of the barrier pillar systemand the load above the pressure arch. The strength of thebarrier pillars is determined by applying the Bieniawskiformula. It should be noted that the barrier pillar systemdesigned in such way is extremely strong. A strong barrierpillar system will provide two benefits for mine safety andoperations: (1) it serves as the foundation of the pressurearches to ensure their stability, and (2) it contains any fail-ure of the production pillars within the panel.

There are commonly three types of barrier pillar sys-tems used in room-and-pillar mines to separate adjacentpanels (Fig. 4): (1) a continuous wide barrier pillar with-out crosscuts, (2) one line of wide barrier pillars separatedby crosscuts, and (3) two lines of barrier pillars separatedby a center entry and crosscuts. For the latter two types,the spacing of the crosscuts should be specified in the de-sign first.

Design optimization process. When the productionand barrier pillars of a room-and-pillar panel are de-signed according to the prescribed procedure with suffi-cient safety factors, it basically eliminates the cause forsurface subsidence. Therefore, if a room-and-pillar min-ing operation is conducted according to panel design us-ing the proposed method, the possibility of immediateand future surface subsidence should be very small. Whenthis is assured, the objective is to maximize the overallrecovery ratio of the coal reserve in the designed panel.

The design optimization goes through an iterativeprocess with user input. It starts with a width of the pro-duction panel that ensures the existence of pressure archin the competent strata. The size and the number of linesof pillars to fit in the production panel are determined.The width of the production panel is re-determined, andthe recovery ratio in the panel is calculated. Based on the

panel width, the size of the chosen type of barrier pillarsystem should be determined. The overall recovery ratiofor the room-and-pillar panel, defined as the ratio of totalarea of rooms in the panel and in the barrier pillar systemto the total area of a panel, is calculated. A spreadsheetprogram was developed in MS Excel, using its optimi-zation feature to carry out the design process. Figure 5shows the inputs and outputs from a sample execution ofthe design program. The inputs are basic geometric infor-mation, geomechanical properties and the safety factorfor the production pillars. After the iterative optimizationprocess, design parameters such as sizes of production andbarrier pillars, number of entries and width of the produc-

Variation of pillar safety factor.

Figure 3

Types of barrier pillar system.Figure 4

Inputs and outputs from an example of room-and-pillar panel

design.

Figure 5




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tion panel are determined, and the resulting recovery ra-tios in the production panel and overall are shown. Theexample has an overall recovery ratio of 60.65 percent.For comparison, the recovery ratio using the traditionalpillar design method without applying the pressure arch

concept for the same conditions was calculated, and therecovery ratio of the new design method was found to beabout 5 percent higher than that of the traditional designmethod.

Case demonstrationMorsy and Peng (2001) documented a cascading

pillar failure event in a room-and-pillar mine. The coalseam was about 6 ft (1.8 m) long. The overburden depthranged from 300 to 550 ft (91 to 168 m). The immediatemine roof was thick shale while the immediate floor hadthin claystone or fireclay underlain by a thick layer ofsandstone or sandy shale. The mine layout is shown inFig. 6. The pillars in the 10-entry mains were 30 by 50 ft(9 by 15 m). The mine rooms were all 20 ft (6 m) wide.The mining height was 8.5 ft (2.59 m) including extract-ing 2.5 ft (0.8 m) of the immediate roof. There were 11panels mined from right to left as shown in the figure.There was no barrier pillar left between the productionpanels and the mains. The dimensions of the pillars androoms in the first eight production panels were the sameas those in the mains. Each of the panels had eight linesof pillars, so the panel width was 420 ft (128 m). How-ever, the pillars in the last three production panels (thatis, Panels 9, 10 and 11) were apparently smaller thanthose in the other panels – about 15 by 44 ft (4.6 by 13.4m) – and the panel width was about 300 ft (91 m). Therecovery ratio in the first eight production panels was57.1 percent and that in the last three panels was about70.5 percent. Super-section mining technique was prac-ticed in mining the panels. No pillar retreating operationwas practiced in the mine. Various sizes of barrier pillarsbetween the panels were used. The width of the solid

barrier pillar between Panels 4 and 5 was 60 ft (18.3 m)while that between Panels 5 and 6 was 30 ft (9 m). Therewas no barrier pillar from Panels 6 to 8. From Panels 8to 11, the barrier pillars were 30 by 50 ft (9 by 15 m). Be-cause of the slightly dipping seam toward the northwest,

water was always found on the floor at the inby end ofProduction Panels 3 through 11.A massive cascading pillar failure event occurred in

the mine. Rib sloughing and roof-to-floor convergencewas suddenly observed around the belt transfer point (Tin Fig. 6). Thereafter, rib sloughing and entry convergencecontinued and propagated outby. Within 12 hours afterinitial observation, heavy rib sloughing and entry conver-gence were visible at the mouths of Panels 9 through 11.Within 24 hours, the event had propagated to and stoppedat a location marked as S in Fig. 6.

Because of the relatively large panel width (420 ft, or128 m) in a relatively shallow area (300 to 550 ft, or 91to 168 m), a pressure arch would not exist over each ofthe panels. In the area where the cascading pillar failureinitiated, the smaller pillars in the production panels wereunable to carry the tributary load imposed on them evenwithout considering the water’s weakening effect on theclay-stone and fireclay in the immediate mine floor. Thepillar safety factor in those three production panels wasback-calculated to be about 0.85, even using 300 ft (91m) overburden. When the pillars in the production panelbegan to fail completely, the barrier pillars separating thepanels only had a safety factor of about 0.19, definitelyinsufficient to contain the propagation of the cascadingpillar failure within the individual panel.

The proposed design method was then used to deter-mine two sets of room-and-pillar panel designs for over-burden depths of 300 and 550 ft (91 and 168 m), respec-tively. In each of the designs, conservative coal strengthof 600 psi (4.1 MPa) for soft floor (Chandrashekar et al.,1987) was used, with a line of rectangular barrier pillars,50 ft (15.2 m) long, to separate the adjacent panels. The

A room-and-pillar mine with cascading pillar failure (Morsy and Peng, 2001).

Figure 6




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design results are shown in Fig. 7.For the minimum overburden depth shown on the

left-hand side of Fig. 7, each panel contains four lines ofpillars and five entries. Each of the square pillars in theproduction panel is 26.3 ft (8 m) wide so the width of theproduction panel is 205 ft (62 m). The recovery ratio inthe panel is nearly 65.4 percent. The barrier pillar is 31.5ft (9.6 m) wide. With this design, the overall recovery ratiofor the panel (excluding pillars in bleeder system and thebarrier pillar beside the mains) is about 60.5 percent.

For the maximum overburden shown on the right-hand side of Fig. 7, the production panel width is 310 ft(95 m), fitting six lines of production pillars and seven en-tries. The square pillars in the production panel are 28.3 ft(8.6 m) wide. The recovery ratio in the panel is about 64percent. The barrier pillars are 93.3 ft (28.4 m) wide. Theoverall recovery ratio is nearly 56 percent.

In both design cases, the widths of the production pan-els are smaller than those in the documented case. Thesmaller panels make it hard to employ the super-sectiontechnique in mining these panels. However, because ofthese smaller panel widths, the pillars do not carry thefull overburden load, increasing their safety factors. Thelarger barrier pillars are more capable of containing pillarfailures, if any, in the production panel so that the cas-cading pillar failure can be prevented. The recovery ratiosin the production panels (64 to 65.4 percent) are higherthan those of the panels with larger production pillars

(57.1 percent) in the original case. In the last three panels,though the determined recovery ratio from the mine mapis higher than those resulting from the design program,the cascading pillar failure event also originated in thesepanels. Due to these benefits, the strata movement causedby pillar failure in the production panel would not propa-gate beyond the pressure arch and consequently wouldnot cause surface subsidence.

ConclusionsA new panel design methodology for underground

room-and-pillar coal mines is proposed. The concept ofpressure arch theory was applied in the systematic designprocess, considering panel width and sizes of the paneland barrier pillars. By specifying a sufficient safety factorfor the pillars, the pillars in the production panel are un-likely to fail. Even if some pillars fail unexpectedly in thepanel, the strata movement will be contained under thepressure arch to eliminate the possibility of immediateand future surface subsidence. The barrier pillar systemseparating the production panels is designed to withstandthe condition of extreme load for maintaining the pres-sure arch. With an optimization process, higher overall re-covery ratio than that resulting from conventional designcan be achieved. ■

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Proceedings of the 1st Annual Conference on Ground Control in Mining , S.S.

Peng, ed., pp. 13-22.

Chandrashekar, K., Nath, R., and Tandon, S., 1987, “Design of coal pi llars under weak

floor conditions,” Proceedings of the 28th Symposium on Rock Mechanics ,

Tucson, AZ, pp. 1073-1081.

Institution of Mining Engineers, 1936, “Third progress report of investigation into

causes of falls, and accidents due to falls in bord and pillar whole workings —

Roof fracture control in bords,” Trans Inst Min Eng , Vol. 90, No. 4, pp. 241-242.

Institution of Mining Engineers, 1949, “Seventh progress report of investigation into

causes of falls and accidents due to falls — Improvement of working conditions

by controlled transference of roof load,” Trans Inst Min Eng , Vol. 108, No. 11,

pp. 489-504.

Luo, Y., 2011, “Investigation of subsidence events over inactive room and pillarmines,” Transactions of the Society for Mining, Metallurgy and Exploration, Vol.

330, pp. 479-489.

Mark, C., and Chase, F.E., 1997, “Analysis of retreat mining pi llar stability (ARMPS),”

Proceedings of New Technology for Ground Control in Retreat Mining, NIOSH,

Pittsburgh, PA, pp. 17-34.

Morsy, K. and Peng, S.S., 2001, “Mine panel failure – A case study,” Transactions of

the Society for Mining, Metallurgy and Exploration, Vol. 301, pp. 11-19.

Zipf, R.K., and Mark, C., 2008, “Ground Control Design for Highwall Mining,” NIOSH, 5 pp.

Panel designs using the new method for the minimum and maximum overburden depths.

Figure 7

Room-And-pillar Panel Design Method

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