GB2222841A - Packwall structures - Google Patents

Abstract

A noncombustible packwall structure is formed from a plurality of adjacent stacks of generally square, preformed elements 10 each having a hole 11 therethrough and being dimensioned to be of such volume and weight that a mine worker can stack one element 10 on another element 10. The stacks extend from a mine floor to near the mine roof. Packing (16 Fig.3) is provided between the stacks and the mine roof to form a seal therebetween. Preferably, at least one face of the structure is covered with a sealant (30 Fig.3) to prevent airflow through the packwall structure. <IMAGE>

Description

TITLE Packwall Structures DESCRIPTION: Field of the Invention This invention relates to packwall structures used during the underground mining of coal.

Description of the Prior Art Underground mining of coal is carried out by various methods such as by the room and pillar method and by advancing and retreating longwall mining method. Coal deposits occur predominantly as bedded flat deposits in sedimentary strata. These flat deposits or seams may be of fairly uniform thickness which may range from a fraction of an inch to more than 100 feet. Coal seams may extend mostly horizontally over distances of many miles but can be inclined also. If the coal occurs near the surface, it can be mined by removing its rock cover or overburden. When the coal seam occurs at a depth which makes surface mining uneconomical, it can be mined by underground mining methods.

The room and pillar method employs the driving of tunnels or entries which are laid out mostly in a rectangular grid pattern. This way rectangular pillars are created which support the rock or mine roof which is located on the top of the.coal seam. Once the entries are driven to the boundaries of the mining operation, the resulting square pillars can be recovered by retreat mining. Room and pillar mining requires many equipment movements and unproductive work. Also if the mine roof is structurally weak, it may be necessary to leave the pillars in place, thereby abandoning a substantial tonnage of the coal reserves.

To increase the efficiency of mining underground coal seams, the longwall system of mining is used. With this method the coal seam is mined by developing a long front, that is a long wall, and then removing most of the coal in slices or cuts by means of a coal shearing machine or a coal plow. A conveyor along the longwall face removes the broken coal to one of the access entries or head gate for the longwall. As the coal shearer or plow advances with the removal of the coal from the face, it is followed by powerful hydraulic roof supports behind which the mine roof is allowed to cave in to an area called gob. With the advance of the longwall, the coal seam releases a great deal of gas and coal dust which must be promptly diluted and removed by ventilation air which is brought into the mine from the outside.In addition, men and equipment have to be moved to the longwall. It is therefore inevitable that entries must be made for access to the longwall. The entry which is used for access by the mining crews, for bringing in fresh ventilation air to the face and contains the coal conveyor for removal of the mined coal, is called the head gate. The entry on the other end of the longwall is called the tail gate which is important for exhausting the ventilation air which is brought in through the head gate. Air from the head gate is forced to sweep by the longwall face, collect gas and coal dust, and exhaust through the tail gate.

Obviously it is vital that both the head gate and tail gate must be kept open for the prevention of unsafe mining conditions which are caused by insufficient ventilation air. In many longwall operations, the caving of the mine roof behind the hydraulic supports at the longwall face causes the rapid and severe deterioration of the head gate and tail gate entries, thereby reducing the cross sectioned areas of these entries. Such a reduction decreases the amount of ventilation available for the longwall face to an extent where mining becomes unsafe and where material and men cannot be moved to their work stations. To prevent such deterioration, solid timber cribbing or rock pack timber cribbing is used together with steel beams on posts or with steel arches that are bagged and backfilled.Other methods which attempt to maintain the initial cross section of longwall entries consists of two rows of timber cribbing between which are placed bags which are filled wj.th fresh cementitious concrete material. These methods do not tend to eliminate the problems caused by the reduction of the entry cross section because timber cribbing in the packwall needs to be compressed by 15% to 20% of its initial height before it becomes effective as a substantial support. In addition the strength of timber decreases significantly with the passage of time because of the effect of fungi and other organisms which cause timber to rot. Such weakening of timber in the humid environment of underground mines also may cause the closure of entries.

Supports made from bags filed with concrete material become effective only when they are fully cured, What happens frequently is that upon mining the coal, the roof rock moves down before the concrete is fully cured.

Loading of the.uncured concrete destroys its integrity.

Such loading causes movement of the concrete. Then separation occurs between the aggregate and the sand in the concrete mix which is being subjected to movement before the concrete becomes rigid.

As mentioned above, when the mine roof rock is not supported sufficiently, the cross section of the entries may become reduced. Such a reduction may be dangerous if insufficient volumes of air enter the longwall face area.

To alleviate this problem, severely damaged steel entry supports must be extracted. Roof rock must be removed to regain the cross section of the initially made entry.

New steel supports must then be installed at great expense. Closure of the entries during their repair causes significant production losses from the longwall.

Another problem with presently used packwall systems is the loss of air from the head gate entry and the problem of keeping the air which has passed by the longwall face from mixing with the fresh air which is directed into the head gate entry. When wood cribbing is used, air passes through the packwall and mixing occurs. Efforts have been made to seal wooden packwalls. However, presently used timber cribbing has rough irregular surfaces with large gaps. Consequently, the thorough sealing of the presently constructed packwalls is very difficult and only partially successful. To make up for these air losses, very large amounts of air must be moved into the longwall mining area by large ventilating fans which require a great deal of energy. Another important factor is the problem of using mine roof supports which are not fireproof.In case of a mine fire, timber cribbing will burn and thereby contribute to the dangers of an underground fire.

Summary of Invention In consideration of the above mentioned severe problems, we have developed a packwall structure which eliminates the difficulties which are being encountered with packwalls for longwall operations.

Our packwall system serves two primary functions. It provides a readily constructed support to take the load from the mine roof after removal of the coal and acts as an economically sealable wall to contain the airflow to the longwall face. Furthermore, our structures are essentially noncombustible. Consequently, the hazard of timber which may burn is eliminated.

In our packwall system, we provide a plurality of superposed, preformed, fully cured concrete elements and each element being square in configuration and of uniform thickness and having a circular central opening to provide maximum strength at corners. Each element is readily stackable by one person and preferably hzs corners chamfered to reduce corner chipping during handling. Preferably high strength concrete elements are preformed with internal bonding of metal fibers as is disclosed in Chlumecky United States Patent 4,565,469.

These elements are, however, different from those of Chlumecky in being generally square in external configuration. If necessary for severe conditions, we propose to place between designated joints of the stacked elements load transfer discs made of fibrous sheet such as panel board, cardboard, and wood or made of plastic.

The load transfer discs assure equal distribution of load throughout the packwall. We close the gap between the wall structure and the mine roof with wedged timber blocking, mortar or bagged concrete to make the wall structure immediately load bearing against the roof movements and to make it tight to prevent the loss of ventilation air through the wall. We also prefer to seal at least one face of the wall structure.

In the foregoing general description we have set out certain objects, purposes and advantages of this invention. Other objects, purposes and advantages of this invention will be apparent from the following description and the accompanying drawings.

DRAWINGS Figure 1 is an isometric view of one packwall element which is to be used in our packwall system; Figure 2 is an isometric view of a load transfer disc which may be used in our packwall system; Figure 3 is a side view, of a longwall layout in a mine with our packwall in place; Figure 4 is a top plan view showing a longwall layout in a mine with packwalls in place according to our invention; and Figure 5 is a diagrammatic view of a longwall layout in a mine using our packwall wherein arrows indicate air flow.

Description by Reference to Drawings Referring to the drawings we have illustrated in Figures 1 and 2 packwall elements to be used in our invention.

As shown in Figure 1, we provide a generally square, high strength, fibre reinforced, precast concrete element 10 having a circular central opening 11 and chamfered corners 12. Each of these elements are dimensioned to contain a volume and weight which can be stacked one by one by a mine worker. We prefer to make each element 10 generally about 22 inches along each side of the square and about 3 inches thick. We provide a central opening 11 which preferably is about 12 inches in diameter. We prefer to make the chamfer 12 about 1/2 inch in each direction.

To attain high support capacity of the stacked packwall elements, load transfer discs 14 may be placed at designated intervals between top and bottom surfaces of the stacked packwall elements. Load transfer discs 14 are preferably about 22" x 22" square shape and thin, being of not more than 3/8" thickness as shown in Figure 2. Load transfer discs can be made of fibrous sheets, plywood, cardboard, plastic or other material which helps with the load distribution from one stacked element 10 to an adjacent element 10. The elements 10 are preferably stacked as shown in Figure 3 one on top of another in closely touching side by side stacks 25 on floor 13 to form packwalls as shown in Figures 3, 4 and 5.

Preferably the stacks are covered and sealed on one side with mortar or other sealant 30. The resultant packwall is topped with wood blocking 16, cement bags (not shown) or other packing between the top of stacks 25 and mine roof 15. Unlike the prior art wood cribbing, our packwall is almost entirely noncombustible. Our packwall will perform the function of the underground support described in United States Patent 4,425,057 with the packwall element manufactured outside of the underground -mine and of cured strength. To improve the load distribution, load transfer discs 14 can be placed in horizontal joints of stacked elements.

The most important functions of the packwall structure are that it provides prompt support of the mine roof and that it acts as a ventilation control. Figures 4 and 5 shown how the ventilation air flow as indicated by arrows 35 and 36 is directed through headgate 18 towards and sweeping longwall face 19. Most of the air flows along the packwalls 22 and longwall 19. Eddy currents indicated by arrow 35 are created and partially ventilate caved area or gob 20. Air escapes through tailgate entry 21 thereby diluting and removing toxic gases and coal dust from longwall face 19. The packwall structure 22 generally prevents the closure of headgate and tailgate entries and generally prevents the damage to steel mine roof supports 23.

Figure 3 also shows how advancing hydraulic shield support 24 can be followed closely in increments of generally 22" with packwall cribbing stacks 25 thereby providing prompt support for mine roof 15 as coal 26 at longwall face 19 is mined with a shearer indicated by box 27 and broken coal 28 is removed by conveyor 29.

As shown in Figures 4 and 5, the packwall structures 22 can be two spaced apart walls 32 and 34 used in combination with packwall bags 31. We prefer to provide similar structures near the headgate entry 18 and the tailgate entry 21 as shown in Figures 4 and 5.

For mining conditions which are less severe than those anticipated for situations which require a packwall construction as shown by the plan view of Figures 4 and 5, a more economical packwall can be constructed.

Again, two parallel rows of packwall stacks 25 are provided. However, instead of packwall bags 31, a void of suitable width can be established between the rows. A yet more economical packwall construction can be installed by using in place of one row 34 a second row of cribbing stacks on generally 36" centers. Such stacks can be made up of elements of donut cribbing as described in United States Patent 4,425,057.

In the foregoing specification, we have set out certain preferred practices and embodiments of our invention however, it will be understood that this invention may be otherwise embodied within the scope of the following claims.

Claims (19)

CLAIMS:

1. A packwall structure comprising: a) a plurality of adjacent stacks of generally square, preformed elements, each element having a hole therethrough and being dimensioned to contain a volume and weight so that a mine worker can stack one element on another element, said stacks being of sufficient height to extend from a mine floor to near a mine roof; and b) packing placed between the stacks and the mine roof in a manner to create a seal therebetween.

2. A packwall structure according to claim 1 wherein the generally square elements are preformed from fibre reinforced concrete.

3. A packwall structure according to claim 1 or claim 2 wherein the generally square, preformed elements have chamfered corners.

4. A packwall structure according to any preceding claim wherein the generally square, preformed elements are of substantially uniform thickness.

5. A packwall structure according to any preceding claim wherein the strength of the preformed concrete elements is of generally uniform quality.

6. A packwall structure according to any preceding claim wherein the packing is placed between the top of the stacks and the mine roof in a manner to provide rigid and substantial support to the mine roof thereby essentially preventing downward movement of the mine roof.

7. A packwall structure according to any preceding claim wherein the packing is formed from wood or cementitious material.

8. A packwall structure according to any preceding claim further comprising at least one load transfer disc placed between any adjacent elements of the stacks.

9. A packwall structure according to claim 8 wherein the or each load transfer disc is formed from fibrous material, panel board, plywood, cardboard or plastics material.

10. A packwall structure according to any preceding claim wherein the stacks are arranged in a single elongate row.

11. A packwall structure according to any of claims 1 to 9 wherein the stacks are arranged in two parallel elongate rows.

12. A packwall structure according to claim 11 in which the two parallel elongate rows are spaced apart.

13. A packwall structure according to claim 12 in which the space between the two parallel elongate rows is filled with packwall bags.

14. A packwall structure according to claim 10 further comprising a second parallel structure formed from donut cribbing and sized to support the mine roof.

15. A packwall structure according to any of claims 10 to 14 wherein at least one elongate face of the structure is covered in a sealant to prevent airflow through the elongated structure.

16. A packwall structure according to claim 15 wherein the sealant is mortar.

17. A packwall structure according to any preceding claim and being so positioned in a mine as to direct air flow from a head gate to a longwall face.

18. A packwall structure according to any of claims 1 to 16 and being so positioned in a mine as to direct air flow from a long wall face to a tailgate.

19. A packwall structure substantially as described herein with reference to the drawings.