Roof support in underground coal mine

Roof support in underground coal mines

Roof support in underground mines is used:

• To prevent major collapses of the mine roof;
• To protect miners from small rock falls that can occur from the immediate roof skin;
• To control deformations so that mine openings remain serviceable for both access and escape, as well as for ventilation of the mine workings
• Ensure continuity of ore production to avoid shortage in profit. 
• Finally, keeping expensive machines in the underground mines safe (e.g. machines used in long wall mining.

When do we need to support mines?

• If there are weak or unconsolidated parts of rocks are present in the roof and walls of mines. 
• If there some geologic structures (e.g. shear zone) that weaken rocks. 
• If there are voids and spaces in the rocks.
• If there are some clay beds that can shrink and expand due to its swelling property, and hence sliding of rocks in presence of water can be expected.
• If there are underground water that helps in chemical weathering leading to loose rocks and corrosion of machines.

Material employed for support in underground mining

1. Timber, usually sal and in some areas teak is used for props, bars chocks or cogs, and laggings. 
2. Iron and steel in form of bars, props, arches, corrugated sheets and roof bolts. 
3. Brick or building stone masonry walls, or archings. 
4. Reinforced concrete or precast concrete block as roadway lining. 
5. Road way ripping, dirt bands and shales as pack walls. 
6. Sand, earth, boiler ash, washery rejects, small tailings, slage from blast furnace for smelting iron and crushed stone as packing of goaf and filling or voids. 

Timber

Most commonly used support material because it is cheap and easily available, it can be easily cut to fit the desire length, timber gives early warning when it swells. The disadvantages of timber as support in miens is the wooden trunk cannot be reused, it swells due to moisture, water, or decays due to bacteria activity and has relatively short life. Wood must be coated to avoid corrosion by mine water, insects, bacteria, etc.

Timber in mine is subjected to two main diseases during use.

1-     dry rot;         2-    wet rot 

The diseased timber is soft and weak in strength. Moreover it wraps and is subjected to attack by fungus leading to decay. The unseasoned timber (also called green timber) is prone to dry or wet rot. 

Wood/Timber drying

Wood drying (also seasoning lumber or wood seasoning) refers to reducing the moisture content of wood prior to its use. 

Drying starts from the exterior of the wood and moves towards the centre, and drying at the outside is also necessary to expel moisture from the inner zones of the wood. Wood subsequently attains equilibrium with the surrounding air in moisture content. 

 Broadly, there are two methods by which timber can be dried: Natura drying or air drying, and Artificial drying. 

Air drying 

 Air drying is the drying of timber by exposing it to the air. The technique of air drying consists mainly of making a stack of sawn timber (with the layers of boards separated by stickers) on raised foundations, in a clean, cool, dry and shady place. Rate of drying largely depends on climatic conditions, and on the air movement (exposure to the wind). For successful air drying, a continuous and uniform flow of air throughout the pile of the timber needs to be arranged. It is less expensive to use this drying method and air drying often produces a higher quality, more easily workable wood than with kiln drying. Its drawback is drying is dependent on the climate condition, it takes several months to a number of years to air-dry the wood. 

Kiln drying

The process of kiln drying consists basically of introducing heat. This may be directly, using natural gas and/or electricity or indirectly, through steam-heated heat exchangers, although solar energy is also possible. In the process, deliberate control of temperature, relative humidity and air circulation is provided to give conditions at various stages (moisture contents or times) of drying the timber to achieve effective drying. For this purpose, the timber is stacked in chambers, called wood drying kilns, which are fitted with equipment for manipulation and control of the temperature and the relative humidity of the drying air and its circulation rate through the timber stack.

Kiln drying provides a means of overcoming the limitations imposed by erratic weather conditions. In kiln drying as in air drying, unsaturated air is used as the drying medium. Almost all commercial timbers of the world are dried in industrial kilns. A comparison of air drying, conventional kiln and solar drying is given below:

• Timber can be dried to any desired low moisture content by conventional or solar kiln drying, but in air drying, moisture contents of less than 18% are difficult to attain for most locations. 
• The drying times are considerably less in conventional kiln drying than in solar kiln drying, followed by air drying. 

1. This means that if capital outlay is involved, this capital is just sitting there for a longer time when air drying is used. On the other hand, installing an industrial kiln, to say nothing of maintenance and operation, is expensive. 
2. In addition, wood that is being air dried takes up space, which could also cost money. 

• In air drying, there is little control over the drying elements, so drying degrade cannot be controlled. 
• The temperatures employed in kiln drying typically kill all the fungi and insects in the wood if a maximum dry-bulb temperature of above 60 °C is used for the drying schedule. This is not guaranteed in air drying. 
• If air drying is done improperly (exposed to the sun), the rate of drying may be overly rapid in the dry summer months, causing cracking and splitting, and too slow during the cold winter months. 

 The significant advantages of conventional kiln drying include higher throughput and better control of the final moisture content. Conventional kiln and solar drying both enable wood to be dried to any moisture content regardless of weather conditions. For most large-scale drying operations solar and conventional kiln drying are more efficient than air drying.

 The main elements of kiln drying are described below:

1.  Construction materials: The kiln chambers are generally built of brick masonry, or hollow cement-concrete slabs. Sheet metal or prefabricated aluminium in a double-walled construction with sandwiched thermal insulation, such as glass wool or polyurethane foams, are materials that are also used in some modern kilns. Some of the elements used in kiln construction. However, brick masonry chambers, with lime and (mortar) plaster on the inside and painted with impermeable coatings, are used widely and have been found to be satisfactory for many applications.
2.  Heating: Heating is usually carried out by steam heat exchangers and pipes of various configurations (e.g. plain, or finned (transverse or longitudinal) tubes) or by large flue pipes through which hot gases from a wood burning furnace are passed. Only occasionally is electricity or gas employed for heating.
3.  Humidification: Humidification is commonly accomplished by introducing live steam into the kiln through a steam spray pipe. In order to limit and control the humidity of the air when large quantities of moisture are being rapidly evaporated from the timber, there is normally a provision for ventilation of the chamber in all types of kilns. 
4.  Air circulation: Air circulation is the means for carrying the heat to and the moisture away from all parts of a load.

 Timber treated in special kilns is normally not used in mine s and the common method is to dry it in the open air for long period. This causes the sap to evaporate to a great extent. In the colliery yard the timber is so stacked that it is kept clear off the ground, free from dampness and permits air circulation to individual members. Turnover of the timber at intervals helps in maintaining the conditions which are not conductive to attach by dry or wet rot.