1

ISRM International Symposium 2010 and 6th Asian Rock Mechanics Symposium - Advances in Rock Engineering 23-27 October, 2010, New Delhi, India

OPTIMUM PILLAR DESIGN OF ROOM AND PILLAR MINING FOR LAYER C1 IN PARVADEH COAL MINE

ARASH EBRAHIMABADI Department of Mining Engineering, Qaemshahr Branch, Islamic Azad University, Qaemshahr, Iran

MANSOUR HEDAYATZADEH Graduated Student of Mining Engineering, Tehran Southern Branch, Islamic Azad University, Tehran, Iran

Abstract : One of the major issues in room and pillar mining is to design of optimum pillar dimensions. The aim

of this research is to design the optimum pillar dimensions for layer C1 in Parvadeh coal mine, resulting in

significant improvement on pillar recovery and productivity for the mine. There are several methods for pillar

design including numerical and empirical methods. With this regard, FLAC software was utilized for numerical

analysis and Bieniawski's method was chosen from among the empirical methods due to its generality and

reliability. Using Bieniawski's method, the pillar length, width and extraction ratio were found to be 35 ft, 25 ft

and 63%, respectively. After verifying these results using FLAC software, the optimum pillar length, width and

extraction ratio were modified and found to be 25 ft, 25 ft and 68.2%, respectively. Consequently, these new

pillar dimensions can successfully be considered for future mining operation instead of the previous design.

Keywords: Pillar design, Room and pillar mining, Parvadeh coal mine, Bieniawski's method, FLAC software

1 INTRODUCTION

Room and pillar mining method is one of the underground mining methods that is commonly used for the

extraction of coal seams. In this method, coal extracted from rectangular shaped rooms with leaving part of the

coal that are used for supporting the hanging wall of the deposit. Generally, in the room and pillar mining

method, pillars are left for many purposes such as stability, control, safety and support. Excessive leaving of

coal, as a pillar, may overshadow the mine planning from economical point of view, on the other hands, low

leaving of it can lead to supporting problems, hence, design of precise pillar dimensions is significant in two

aspects; firstly, it keeps the stop safe and stable and to maximizing the extraction ratio, its dimensions must be

minimum. Therefore, applying the room and pillar method is reasonable, if the pillar dimensions would be

optimum. Nowadays, numerical methods are widely used in mining engineering while some researchers believe

that numerical methods, solely, can not be suitable for engineering assessment, although the combination of

empirical and numerical methods can be led to more precise analysis. The main purpose of this study is to

design the optimum pillar dimensions. Basically; there are three major parameters to design the pillars including

safety factor, pillar strength and in situ stresses. As a part of study, Tributary Area Theory was utilized to

determine the stresses acting on the pillars and in order to evaluate the pillars strength, bieniawesky's method

was employed due to its simplicity and popularity. After broad modeling and simulation, optimum pillar design

was performed using numerical method. In this respect, FLAC 2d software was utilized.

2 DESCRIPTION OF STUDY AREA

Tabas coal mine, the largest and unique fully mechanized coal mine in Iran, located in central part of Iran near

the city of Tabas in Yazd province and situated 75 km far from southern Tabas. The mine area is a part of

Tabas-Kerman coal field. The coal field is divided into 3 parts in which Parvadeh region with the extent of 1200

Km² and 1.1 billion tones of estimated coal reserve is the biggest and main part to continue excavation and

fulfillment for future years. The Coal seam has eastern-western expansion with reducing trend in thickness

toward east. Its thickness ranges from 0.5 to 2.2 m but in the majority of conditions it has a consistent 1.8 m

thickness. Room and pillar mining method is considered as the main excavation method in Parvadeh mine.

There are two continuous miners and four LHD machines excavating and handling the coal in the mine to meet

the determined production capacity of 200,000 tones of coal per year. Figure1 shows the overall plan view of

Parvadeh central mine [1].

Arash Ebrahimabadi, et al

2

Figure 1. Overall view of Parvadeh central mine (Room and pillar mining) [1]

3 PILLAR DESIGN FOR C1 LAYER IN PARVADEH COAL MINE USING BIENIAWKY'S

METHOD

In order to design the pillars in Parvadeh coal mine, three main steps were totally considered:

1) Application of empirical pillar design formulas.

2) Selection of suitable safety factor for designing of pillars based on back-analysis of available information.

3) Numerical modeling analysis to verify the previous design and test its robustness.

Over the years, many pillar design methods have been proposed for the assessment of coal strength, pillar

height, and pillar width. Among these methods, Bieniawski's pillar design formula has been successfully used in

the United States and other countries. One of the reasons for the wide acceptance and use of this method is that,

in addition to pillar width and height, the effect of pillar length is taken into consideration. Moreover, pillar

strengths estimated with the formula have been compared with over 100 case histories of actual pillar condition

with high correlation. Hence, in current study, The Bieniawski's method together with the Tributary Area Theory,

which is one of the most popular methods for assessment of pillar dimension, was adopted. In order to

determine the pillar stresses, the calculations were carried out based on rectangular shaped pillar due to its

generality and simplicity. This theory, to computation of the average vertical load acting on the pillars, is

defined as follows:

pL

pLoL

pW

pWoWHLp (1)

Where is rock average unit weight, H is depth to the mining level, oL and

oW are length and width of the

room, pL and

pW are length and width of the pillar, respectively.

Pillar design by bieniawsky's method has several steps including [2 and 3]:

1. Determination of rock uniaxial compressive strength

In the Tabas area, the uniaxial compressive strength of coal was found to be 6.62 MPa. Uniaxial compressive

test was accomplished on the samples gathered from the site in accordance to ISRM suggested method.

2. Determination of K (constant) based on c (uniaxial compressive strength)

DcK (2)

Where D is specimen dimension and c is the rock (coal) uniaxial compressive strength.

Considering inchmmD 12.254 and psiMPac 11.96062.6 , the constant of K is calculated as 1397.95.

Optimum Pillar Design of Room and Pillar Mining for Layer C1 in Parvadeh Coal Mine

3

3. Using one of the pillar strength estimation formulas (Agapitos formula)

Based on Agapitos formula for height of seam coal (H) more than 36 inchs (0.9 meter), the following equation is

considered.

361

K (3)

For C1 layer (1.83 meter), the σ1 is equal to 232.99 psi.

4. Determination of B (maximum entry width)

With regard to computed RMR in the sit and its related chart, mftB 55.16

5. Computation of pillar strength p

Bieniawski's pillar design formula is as follows:

)36.064.0(1h

wp (4)

Where σp is pillar strength, σ1 is in situ coal strength, h is pillar height and W is pillar width.

6. Computation of pillar loading (LP) on the basis of the Tributary Area Theory

pL

BpL

pW

BpWH1.1LP (5)

Where LP is average vertical load acting on the pillar, H is depth to the mining level, Wp is pillar width, Lp is

pillar length, B = entry width and is the coal average unit weight 1.1 .

With selection of reasonable safety factor ( 5.1sF ), the LP is calculated:

psiF

Ss

p

p 91.434

7. Assesment of pillar dimension using pS and H

With respect to drilled borehole number of 97, 104, 107, 108, 109, the average of depth to mining was

considered 50 meters. Having psiS p 91.434 , ftH 04.164 and ftB 5.16

and with taking into account into 10 equation, length and width pillar are finally computed as follows:

mftL 5.1035 mftW 5.725

8. Control of extraction ratio

To find out as if the mining operation is economic, extraction ratio based on the following equation must be

checked:

BL

L

BW

We

p

p

p

p1 (6)

With exerting the calculated values into the above equation, a 63% extraction ratio was computed. This

extraction ratio seems to be reasonable and acceptable. In the next section, it will be optimized by using

numerical method.

4 PILLAR DESIGN USING NUMERICAL METHOD

4.1 Geomechanical Parameters

The most important characteristics of the rock mass encountered in this area are: high alteration, low strength

and jointed rocks. Field investigations were carried out in-order to determine the structural characteristics of the

rocks. The collected data were then analyzed using the Dips software. Table 1 shows the dip and dip direction of

the joint sets encountered in the working face.

Arash Ebrahimabadi, et al

4

Table 1. Joints dip and dip direction.

Joint sets Dip (Deg.) Dip direction (Deg.)

Working area 1 65-70 325-345

Laboratory investigations were also carried out in accordance to ISRM suggested methods, in-order to

determine the physical and mechanical properties of rocks. Table 2 shows these properties. The Roclab software

was then utilized to determine the rock mass properties. The software is based on the generalized Hoek and

Brown failure criterion. Table 3 shows these properties.

Table 2. Geomechanical properties of encountered rocks

Rock Type Density (Kg / m3) Elastic Modulus

(GPa)

Uniaxial Compressive

Strength (MPa)

Poison’s Ratio

Coal 1320 1.35 6.62 0.29

Upper Sandstone 2410 2.00 90.94 0.32

Upper Siltstone 2300 1.10 28.84 0.31

Upper Mudstone 2210 1.00 16.25 0.30

Lower Sandstone 2500 1.84 64.26 0.31

Lower Siltstone 2350 0.85 44.44 0.31

Lower Mudstone 2260 0.80 17.26 0.29

Table 3. The rock mass properties

Rock Type Cohesion (MPa) Friction Angle (Deg.) Modulus of Deformation (GPa)

Coal 1.90 22.5 0.186

Upper Sandstone 1.67 38.0 0.304

Upper Siltstone 0.73 30.0 0.152

Upper Mudstone 0.70 29.0 0.138

Lower Sandstone 1.50 35.0 0.251

Lower Siltstone 0.60 28.0 0.116

Lower Mudstone 0.60 27.0 0.110

4.2 Stability Analysis

The aim of stability analysis in rock mechanics science is indicating how stresses effect underground spaces and

the determination of strain and stress distribution around the space. Usually there are four methods for stability

analysis of underground structures. These are mainly closed form solutions, empirical methods, physical and

numerical modeling. In recent years numerical modeling for strata control in rock mechanics has developed

significantly. In this study FLAC software was used [4].

The coal mine tunnel was simulated by the FLAC and the simulation process was checked by the resulting

unbalanced force. The model converged to an equilibrium state when the unbalanced force in the tunnel reached

an acceptable value. The model geometry was built up in accordance to joints directions given in table 1. After

modeling and analyzing the pillars, stability of the pillars having various dimensions were investigated. In this

respect, the smallest dimensions of pillars were chosen to be considered as optimum pillar sizes for future

operations. It should be noted that these dimensions should provide the appropriate safety factor. Considering

span (room) width of 5 m, after broad modeling and analysis, the pillar dimensions were found to be 7.5×7.5 m

(25×25 ft).

Figures 2 and 3 show the displacement in the X and Y directions using modified pillar dimensions. The results

gained show that there are small amount of displacements and movement of rock mass blocks in the openings

and hence, the tunnel is stable. Therefore there is no need to carry out further analysis and design in order to

stabilize the above area of the pillars.

Optimum Pillar Design of Room and Pillar Mining for Layer C1 in Parvadeh Coal Mine

5

Figure 2. X-Displacement with proposed pillar dimensions

Figure 3. Y-Displacement with proposed pillar dimensions

4 CONCLUSIONS

The aim of this research was to design the optimum pillar dimensions for layer C1 in Parvadeh coal mine.

Primarily, Bieniawski's method was used to design the pillar dimensions. Using the mentioned method, the

pillar length, width and extraction ratio were found to be 10.5 m (35 ft), 7.5 m (25 ft) and 63%, respectively. In

the next step, these dimensions were then verified using numerical method. With this regard, FLAC software

was utilized resulting in suggesting the modified pillar dimensions as 7.5×7.5 m (25×25 ft). The results

achieved from numerical analysis showed that with the proposed pillar dimensions very small amount of

displacements and block movements occur in the working area and the entries and rooms are safe and stable.

5 REFERENCES

[1] Tabas Geology Report, 2000, Technical Office of Tabas Mine.

[2] Bieniawski, Z. T., 1981, “Improved Design of Coal Pillars for US Mining Conditions”, Proc. 1st Annual

Conference on Ground Control in Mining, West Virginia University, Morgantown.

[3] Hustrulid, W. A., 1976, “A Review of Coal Pillar Strength Formulas”, Rock Mechanics, 8, pp. 115-145.

[4] Cundall, P. A., 1993, FLAC User’s Manual, Version 3.22.

“Arash Ebrahimabadi received his B.Sc. degree in Mining Engineering from the Islamic Azad University,

Tehran Southern Branch in 2000, Iran. He received his M.Sc. degree in Mining Engineering (Rock Mechanics)

from the Islamic Azad University, Science and research Branch in 2003, Iran. He obtained a Ph.D. degree in

Mining Engineering (Mechanical Excavation) at the Islamic Azad University, Science and research Branch in

2010, Iran. From 2005 he has been Faculty Member of Mining Engineering Department at the Islamic Azad

University, Qaemshahr Branch, Iran, where he specialises in interaction of roadheaders and rock mass,

engineering rock mechanics, tunnel geology and excavation”.

“Mansour hedayatzadeh was born on july, 1982, in babol, Iran. He received his B.Sc. degree from the mining

Engineering Department of the Islamic Azad University, Qaemshahr Branch, Iran, in 2003, He received his

M.Sc. degree from the mining Engineering (Rock Mechanics) Department of the Islamic Azad University,

Tehran Southern Branch in 2007. His research interests are the interaction of tunnel boring machines and rock

mass, engineering rock mechanics, tunnel geology and excavation”.