Design Optimization of Longwall Chain Pillars Prof. Kazem Oraee, PhD University of Stirling, UK Behdeen Oraee, MSc University College London, UK Amir H. Bangian, MSc Azad University, Iran 29th International Conference on Ground Control in Mining Morgantown, WV

Introduction Longwall is an underground mining method High rate of production capacity Mechanization ability Soft rock flat-lying bedded ore bodies A widespread mining method in European coal mines (beginnings of 19th century). In early 1960’s , developed in US coal mines by development of self advancing supports mechanisms.

Longwall Mining System The Shield Supports Shearer Armoured Face Conveyor Shield Supports Stage Loader The Longwall Mining System

The main reasons of this improvement: During 1990s and beyond, production rate of underground coal mines using longwall doubled around the world. The main reasons of this improvement: high level of mechanization providing higher safety optimum design of panels efficient ventilation systems

An appropriate design of panels consists of: An accurate layout of the entries Proper ventilation condition A satisfactory level of the safety in the entries and the coal face Adequate system for conveying the extracted coal along the face

Appropriate depth of seem in longwall method can be: From less than 100m to more than 2000m Obviously, to design a panel that provides stability and safety at the entries is more complex at higher depths compared to shallow depths. Obviously, to design a panel that provides stability and safety at the entries is more complex at higher depths compared to shallow depths.

Therefore : The number and the width of chain pillars in both sides of a deep panel should be increased to provide the required safety. The recovery rate of coal in a deep longwall mining project is decreased by increasing the number and the width of the chain pillars. Total cost of the project under such circumstances is raised by decreasing the recovery rate of coal extraction. Therefore, cost of ventilation increases.

Thus: Pillar design has to be carried out based on a highly accurate procedure including; Assessment of load distribution on pillars Stress analysis in order to provide high safety with minimum possible pillar dimension

Tabas Coal Mine – Iran One of the largest coal resources in Iran Has created the required conditions for using the longwall mining method: Large volume of coal reserve Appropriate geometry of coal seams

C1 Coal Seam The main coal seam in Tabas Average thickness 1.8m Inclined openings Longwall Panels’ Width: 200m to 220m Panels’ Length: about 1000m Retreating method

Sketch of Central Mine Development

Sketch of Tabas Mine Longwall Panels

Sketch of Tabas Mine Longwall Panels

Geomechanical parameters of Coal & Overburden Rocks of C1 Tabas Coal Seam

Coal Pillar Design Methods Should achieve two goals: High level of safety High level of coal recovery

Before development of personal computers, mining designers designed coal pillar by manual procedures. Therefore the empirical designing methods were gradually created to design coal pillars based on experimental results. Nowadays coal pillar design is carried out by using advanced personal computers with high speed calculations but the design procedure has not been changed. Currently empirical coal pillar design methods are acceptable procedures worldwide

Empirical formulae are the equations which have been developed based on the extracted experimental data of a given coal pillar. The attained results from empirical formulae have a good conformity with the original field data obtained from the experimental tests. Developments of personal computers and numerical methods have allowed the mining designers to apply numerical methods to coal pillar design. In this approach some main criteria are compounded together and then the obtained results are applied to coal pillar design.

Customary equations for assessment of coal pillars strength Some of the most applicable formulae: Bunschinger (1876); Bieniawski (1967); Bieniawski (1968); Holland (1973); New Formula (2007);

Numerical Modeling Numerical coal pillar design methods have recently been widespread in engineering modeling because of the development of personal computers and the progress in advanced numerical techniques. These methods are unlike empirical methods that simulate and analyze the stress during loading of coal pillars. FLAC3D software was used to analyze coal pillar strength and stability.

In order to determine the loading capacity of a pillar, a typical pillar has been modeled on the basis of the average coal characteristics of Tabas coal mine. In the model the height of the coal pillar is 3.2m, the length of the coal pillar is 40m and the width of the coal pillar varies from 20m to 60m. In this procedure, the loading rate on the coal pillar increases with the increase in width of the coal pillar.

A sample of the modeled coal pillars by FLAC3D & the Displacement Velocity Model

Comparison of the different coal pillar strength assessment methods

The trend of coal pillar strength with respect to increased UCS of intact coal

The increasing trends of the obtained data by the new formula & FLAC3D based on: - Changes of the width of the pillar - Intact coal sample UCS

Conclusion Chain pillar design has a significant effect on safety, economics and performance of longwall coal mining. The results show that both the new formula and FLAC3D are applicable methods. Although as the new formula has been developed based on the specifications of the coal in Tabas, the obtained conformity amongst the achieved results and the field data was predictable.

As the produced data of the new formula are lower than that of the produced data of FLAC3D, a higher safety factor than the results of FLAC3D will need to be applied. Loading capacity of coal pillars increase with the increase of compressive strength of the intact coal. The ratio of the obtained results by the new formula has a linear trend but the obtained results by FLAC3D for range of UCS of the intact coal changes from linear to a decreasing trend after 7.5MPa.

It was also proved that an increase in the UCS of the intact coal and an increase in the width of the coal pillar simultaneously creates a higher level of loading capacity for the coal pillar. The attained results were proved by both the new formula and FLAC3D software and it is finally concluded that, using simultaneously the new formula and FLAC3D software can provide reliable results for coal pillar design in the Tabas coal mine.

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