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英文原文
A new coal pillars design method in order to enhance safety of the retreat mining in room and pillars mines
E.Ghasemi*,K.Shahriar
Department of Mining of and Metallurgical Engineering,Amirkabir University of Technology,Tehran,Iran
Abstract:Most of the proposed methods of coal pillar design determine pillar dimensions using pillar estimation only through the tributary area theory.Designing pillar based on these methods is not appropriate in room and pillar mines with pillar recovery because retreat mining and gob creation generate abutment loads.Neglecting abutment loads in design stage may lead to pillar failure and destructive effects during retreat mining.Thus proper pillar design has a remarkable effect on mining effect on mining safety.In this paper,a step-bu-step method is presented to design pillars with square shape in room and pillar mines with regard to existing pillars in the active mining zone(AMZ) and estimating abutment loads according to experiment-method. This method has been applied to determine optimum pillar dimensions in the main panel of Tabas Cental Mine(TCM),located in the mid-eastern part of Iran.Obtained results show the abutment loads account for 27%of the total loads applied on pillars in AMZ in this panel.Pillar width,based on this method, is also obtained 11.6m.
Key words: Pillar design;Room and pillar;Retreat mining; Active mining zone;Abutment loads
1. Introduction
In underground coal mining ,room and pillar is the method of working preferable for flat,tabular deposits in thin seams,where rooms of entries are driven in the solid coal to form pillars in the development panels(Hustrulid,1982;Hartman,1978).Pillars o f coal are left behind to support the roof and prevent its collapse,thereby allowing miners to extract coal between them and to travel safely. In some cases, the pillars are removed partly or fully in a later operation, known as retreat mining(also known as secondary mining or pillar recovery operation). Coal mine pillar design has been the subject of sustained and intensive research in the major coal producing countries in the word. Pillar design and stability are two of the most complicated and extensive problems in mining related to rock mechanics and ground control subjects. Although these problems have been investigated for a long time, to date only a limited understanding of the subject has been gained. The subject of pillar design in the US goes back nearly a centry. Prior to this the dimensions of pillar were largely determined rules of thumb such research as there was tended to be isolated and sporadic. But nowadays, various pillar design formulas are developed, based upon laboratory testing, full-scale pillar testing, and back-analysis of failed and successful case histories. In 1980, field studies conducted by the US Bureau of Mines has developed the classic pillar design methodology. It consisted of three steps(Mark,2006):
1.Estimating the pillar load;
2.Estimating the pillar strength ,and
3.Calculating the pillar safety factor.
Th average pillar, in regular layouts of pillars can be estimated by tributary-area theory, each individual pillar is assumed to carry the weight of the overburden immediately above it. In the other words, a pillar uniformly supports the weight of rock overlying the pillar and one-half the width of rooms and entries on each side of the pillar(Peng,1978).Pillar strength can be defined as the maximum resistance of a pillar to axial compression(Brady and Brown,1993). Empirical evidence suggests that pillar strength is related to both its volume and its shape(Salamon and Munro,1967;Brady and Brown,1993).Numerous formulas have been developed that can be used to estimate the strength of pillars in coal mines, which Table 1 shows the most applicable of them. Each of these formulas estimates the to pillar strength in terms of two various; width to height ration and in situ coal strength.
Bieniawski(1981) represented very good classic approach to pillar design. He at fist described the issues involved in pillar design, and advantages and shortcomings of the available methods and then represented a logical, step-by-step approach to determine the coal pillars dimensions in room and pillar mines.
Table 1 Most applicable of empirical strength formula for coal pillars.
Pillar strength formulas(MPa)
Reference
Pillar cross-section
Remarks
Salamon and Munro (1967)
Square
Bieniawski (1968)
Square
—
Madden(1991)
Square
Mark and Chase (1997)
Square
—
Nowadays in most of the room and pillar mines in order to increase recovery and productivity,remanent pillars in panels are recovered by retreat mining. Since, the above mentioned methods are not appropriate for pillar design because these methods neglect the abutment loads due to retreat mining and creation of a mined out gob. Abutment loads affect on the pillar in the adjacent of pillar line and a load more than the one estimated bu tributary area theory applied on pillar(Mark and Chase,1997;Peng,1978).Studies conducted by van der Merwe(19990) confirm the increase of load on the pillars in the adjacent of the pillar line. He calculated the actual load applied on the pillar during pillar recovery using a two dimensional boundary element model and estimated the pillar safety factor for this condition.
Pillar design without the abutment loads to failure of pillar during retreat mining. Pillar failures continue to be one the greatest single hazards faced by underground coal miner, Pillar failure responsible for unsatisfactory conditions includes(Mark et al.,2003):
1.Pillar squeeze,
2.Massive pillar collapse, and
3.Coal pillar bumps.
The occurrence of pillar failure in underground mines entails detrimental effects on miners in the form of injury,disability or fatality as well as mining company due to downtimes, interruptions in the mining operations, equipment breakdowns,etc.For example in 1992,air blasts due to pillar failure at a southern West Virginia mine led to destroying of 103 ventilation stopping(Mark et al.,1997). On August 6th,2007,violent coal bump occurred in Crandall Canyon Mine in Utah, and caused the entrapping of six others(Heasley,2009a).So, proper pillar design is the key to prevent of pillar failure and the Analysis of Retreat Mining Pillar Stability(ARMPS) programs are used successfully for designing safe retreat mining(Tulu et al.,2010).LaModel is a PC-based program for calculating the stresses and displacement in coal mines or other thin seam or vein type deposits(Heasley and Barton,1999;Heasley,2009b).It is primarily designed to be utilized by mining engineers for investigating and optimizing pillar dimensions and layouts in relation to overburden, abutment and multiple seam stresses. The program was developed based on displacement-discontinuity variation of the boundary element method. Mark and Chase(1997) developed the ARMPS program based on empirical equations. ARMPS considering the active mining zone(AMZ) calculates stability factor(ARMPS SF) based on estimates of the loads applied to, and the load-bearing capacities of, pillars during retreat mining. More than150 cases of retreat mining were collected in US to verify the program(ARMPS help,2008).Analyses of all these cases show that pillar squeeze is the most frequent type of failure and occurs in about two thirds of cases. 14 cases of pillar sudden collapses were observed, which in every case occurred when the ARMPS SF was less than 1.5 and where the pillar width to height ration is less than 3. All but 3 of the 17bumps occurred when the depth of cover exceeded 400m.Mark and Chase(1997) understood that almost no considerable massive pillar collapses occurs when the pillar width to height ration more than 4 is selected. They also observed when the depth of cover is less than 200m; the minimum required stability factor to prevent massive pillar collapses is 1.5.
One of the keys to miners safty and efficient recovery of the reserves is to design sufficiently sized pillars that will prevent pillar squeezes, excessive pillar spalling severe floor heave, roof falls,and pillar bumps.Regarding the above mentioned comments, a new method to design coal pillars with square shape in room and pillar mines is presented in the following sections. The proposed method is suitable in determining optimum pillar dimensions in room and pillar mines where remanent,pillars are supposed to be extracted after preliminary mining completion. This method, in addition to considering abutment loads,lowers pillar failure risk. The goal of this method is to help ensure that the pillars developed for future extraction are of adequate size for all anticipated loading conditions.
2. Methodology
Similar to the ARMPS program, the proposed method in this paper considers the pillars in the active mining zone(AMZ) because these pillars are exposed to maximum load throughout mining process therefore the pillar dimensions obtained by this method is more satisfactory. Before describing the design method, a describing on the AMZ is necessary. As shown in Fig.1, AMZ includes all of the pillars on the extraction front(or pillar line), and extends out by the pillar line a distance of 5 times the square root of the depth of cover. This width of AMZ was selected because measurements of abutment load falls within its boundaries(Mark and Chase,1997).
The proposed method is based in five principles(Ghasemi et al.,2010a):
1. Calculating the maximum load applied on the pillars in AMZ(including development load, abutment loads),
2. Calculating the overall load-bearing capacity of pillars in AMZ,
3. Selecting an appropriate safety factor,
4. Calculating the pillar width, and
5. Correcting the pillar width to find the optimum pillar width.
The method is made up of twelve steps which are described below. Fig,2 also illustrates different steps of this method in a flow-chart plot. The symbols used here are provided in Table 2.
2.1. Step 1:Gathering essential data
Essential data to determine the optimum pillar dimensions in this method are as following:
Fig.1. Schematic show of the AMZ (Mark and Chase,1997)
1. Depth of cover:average overburden thickness over the pillar system.
2. Pillar height(Mining height):note that the value of pillar height is not necessarily equal to the seam thickness.
3. Entry width:entry width is usually determined base on roof rock quality, production rate and operational width of equipments. In this method, crosscuts are assumed to have the same width as the entries.
4. In situ coal strength.
5. Mean unit weight of the overburden.
6. Abutment angle:the abutment angle determines how much load is carried by gob. Measurement of longwall abutment loads indicated that an abutment angle 21° is appropriate for normal caving conditions. For example, if no caving has occurred abutment angle is 90°namely zero load transfer to the gob(Mark and Chase,1997).
7. Panel width: panel width is usually determined base on geotechnical conditions, stress state in the region, economic criteria, and environmental conditions. Panel width affects on stress distribution loading conditions and caving mechanism. An increase in panel width results in an increase of the abutment loads applied on the pillars adjacent to the gob area. The tension zone height developed in the roof of the gob area also increase as the panel width increase and may lead to a large failure in overburden(Bieniawski,1987). Based on width to depth ration(P/H), panel are divided into categories:
·Sub-critical panels(P/H<2tanβ)，and
·Super-critical panels(P/H≥2tanβ).
8. Coal Mine Roof Rating(CMRR):this index is used to evaluate roof rock quality. In 1994 the CMRR was developed to fill the gap between geologic characterization and engineering design(Mark and Molinda,2005). This classification system considers geotechnical factors such as roof rock strength, bedding and other discontinuities, moisture sensitivity of the roof rock, groundwater, etc. CMRR varies between zero and 100. Based on this index, roof rocks in coal mines are put in three categories(Chase et al.,2002):
·Weak(CMRR<45),
·Intermediate (45
65).
2.2. Step 2: Calculating AMZ dimensions
AMZ length and width are determined from Eqs.(1) and (2) respectively:
(1)
(2)
2.3. Step 3: Calculating development load
Development load are resulted from the overburden weight over active mining zone. Based on tributary area theory, development loads are obtained from the following equation:
(3)
2.4. Step4: Calculating the maximum front abutment load
Retreat mining starts with the extraction of the panel pillars. When enough of pillars have been extracted, the overburden strata above the extracted pillars start to cave. As a result of this roof caving, the active gob is carried by the gob, but a considerable amount of the original overburden load over the gob is transferred to the pillars in AMZ and barrier pillars as a front abutment load(see Fig.1). Front abutment load is calculated based on abutment angle concept(Mark, 1992;Tulu et al.,2010) and its distribution is different in sub-critical and super-critical panels(see Fig.3). Depending on whether the panel is sub-critical or super-critical , the maximum front abutment load is given bu Eqs.(4) and (5) respectively(Ghasemi et al.,2010a):
(4)
(5)
2.5. Step 5: Calculating side abutment load
The gob area beside the mining panel is the source of side abutment load. Two gob areas may exist beside each mining panel. The side abutment load is shared between the barrier pillar and the AMZ. This load the same as front abutment is calculated by abutment angle concept. Gob area width and barrier width are required to calculate side abutment load applied on AMZ. Depending load is given by Eqs.(6) and (7) respectively(ARMPS help,2008):
(6)
(7)
In both of them, regarding Eqs.(8),R is:
(8)
Factor R is transfer rate that shows the percentage of total side abutment load that is applied to AMZ.
2.6. Step 6: Calculating the maximum load on AMZ
The maximum load applied on the pillars in AMZ is calculated by summation of development load, maximum front abutment load, and side abutment load according to the following equation:
(9)
Fig.2. Flowchart for proposed coal pillars design method
2.7. Step 7: Determining number of entries
The number of existing entries is usually determined based on panel width,rock mechanics,operation equipments, and production rate. At least four entries are needed; one for accommodating the conveyor, one for fresh air, and wo others in two sides of panel to take the aie out (Stefanko, 1983). Economically and operationally, this number of entries is not adequate in continuous (mechanics) mining method and at least five entries should be planned which this number increases up to seven entries in mines with high production rate(Hartman,1987).
2.8. Step 8：Calculating the load-bearing capacity of AMZ
The load-bearing capacity of the pillars in AMZ is calculated by summing the load-bearing capacities of all of the pillars within its boundaries. The load-bearing capacity of each pillar is determined bu multiplying their strength by their load-bearing area(Mark and Chase,1997). In this method, pillar strength is estimated using the Bieniawski's strength formula. The number of existing pillars in AMZ is calculated according to the following equation:
(10)
Hence, the overall load-bearing capacity of pillars in AMZ is given by the following equation:
(11)
2.9. Step 9: Selecting an appropriate safety factor
The selection of an appropriate safety factor can be based on a subjective assessment of pillar performance or statistcal analysis of failed and stable cases(Salamon and Munro,1967;Mark,1992). According to the studies by Chase et al.(2002), Table 3 provides suggested safety factors for stability of the pillars in AMZ. These values are obtained from 250 analyses of panel design in US and as it is seen from table, safety factor depends on Coal Mine Roof Rating(CMRR) as well as depth.
2.10. Step 10:Calculating pillar width
In this step, putting the safety factor in Eqs.(12) and solving it, pillar width is obtained:
(12)
2.11. Step 11:Correcting pillar width to decrease the pillar failure risk
As it is pointed out before, one if the ways to decrease pillar failure risk ,especially large pillar collapse, is to choose a pillar width to height ration large than 4. In this step, if the ration of the obtained width from the previous step to pillar height is smaller than 4,pillar width is increased so a pillar width to height ration larger than 4 is reached. Of course, in order to control and avoid excessive increase of pillar width, the recovery rate is taken into consider. According to experiments and considering economic purposes in preliminary mining stage, the most suitable recovery rate varies from 40% to 60%. It should be notice 0.5m is added to the pillar width each time in this step.
Table 2 Used symbol in proposed coal pillars design method
Symbol
Description(unit)
AMZ
Active mining zone
H
Depth of cover (m)
Renewal table 2
p
Panel width (m)
h
Pillar height (m)
B
Entry width (m)
Mean unit weight of the overburden (KN/m3)
Abutment angle(°)
AMZ length (m)
AMZ width (m)
Development load (KN)
Maximum front abutment load (KN)
Side abutment load（KN）
Side gob width (m)
Barrier pillar width (m)
Transfer rate (%)
Maximum load applied on AMZ (KN)
Pillar strength (MPa)
Number of entries
Number of pillars in AMZ
Overall load-bearing capacity of AMZ (KN)
Safety factor
Pillar width (m)
Width difference
Optimum pillar width (m)
Fig.3. Abutment angle concept in sub-critical and super-critical panels (Mark,1992)
Table 3 Suggested safety factor for stability of the pillars in AMZ
Depth of cover (m)
Weak and intermediate roof()
Strong roof()
2.12. Step 12: Determining the optimum pillar width
In this step, the width obtained from previous step is corrected so that the optimum pillar width is determined based on the number od pillars in each row and the panel width. In order to at first △ should be calculated using Eqs.(13). If △ is less than or equal to the sum of pillar width and entry width, the optimum pillar width is obtained from Eqs.(14). Otherwise, the number of entries is added depending on △ value and calculating are repeated from step 8:
(13)
(14)
In the following section optimum pillar dimensions in the main panel of the Tabas Central Coal Mine ,located in mid-eastern part of Iran, is determined in order to validate the proposed method and results are interpreted. This mine is the first mechanized one in Iran designed as a room and pillar mine. The pillars are left behind in this mine are supposed to be extracted as retreat mining in future after the preliminary mining finishes. Therefore a proper pillar design can has a remarkable influence on higher safety and efficiency of the reserve recovery in this mine.
3. Tabas Central Coal Mine
Tabas Central Coal Mine(TCM) is the case studies here, located in Tabas coal region approximately 85km south of Tabas town in Yazd province in mid-eastern part of Iran (Fig.4). The mine is working seam C1 by room and pillar method using continuous miner and LHD. The C1 seam gradient is 1 in 5(11°) and seam thickness is about 2m. The immediate roof above the seam typically is weak(CMRR=37) and consist of 0.1~0.2m thick mudstone, siltstone/sandstone interfaces and sandstone channels in some areas within 3m which have potential to be water-bearing. The immediate floor is about 1~1.3m of weak seatearth/mudstone underline by stronger mudstones, siltstones/sandstones. The minable reserve accounts for 6 million tones of coking coal(Central Mine Design Report,2005). The in situ strengt