GB2324107A - Crib system and block therefor - Google Patents

Abstract

A crib system for use in supporting a mine roof comprises columns (15) of layered blocks (12), one block to a layer. The blocks are formed of fibre-reinforced concrete and include substantially flat parallel upper and lower surfaces, with a hole (13) therethrough. A rod (18) may be inserted through the aligned holes, for reinforcement. Layers of compressible board (17) may be placed between adjacent blocks to improve the compression and failure characteristics of the column.

Description

CRIB SYSTEM AND BLOCK THEREFOR This invention relates to irnprovements in crib systems for supporting roofs in mines and the like and to blocks for use in such systems.

Existing crib systems used in mines typically involve a construction consisting of a stack of rectangular blocks, or "crib", laid in parallel spaced apart pairs, alternate layers being disposed at 90 degrees to each other. Only those overlapping areas of the blocks which are in frictional contact with each other are effective in providing support whereas the remaining areas which are not in contact but which together occupy greater space than the contacting areas make no contribution to the supporting function. By virtue of the spacing of the blocks, such a crib construction occupies a considerable amount of room within the mine. It will be appreciated therefore that the existing system is inefficient both in terms of material costs and the valuable space taken up.

Laying the aforementioned rectangular blocks to provide a crib construction also demands much care in order to achieve a truly vertical stack having optimum stability.

Thus the existing system is also disadvantageous in that it relies very much upon the skill of the labourer in its construction.

A yet further drawback of the existing system resides in the failure planes which are inherent in the construction geometry of the crib, such failure planes being created where the edges of adjacent blocks contact. The existence of these planes of weakness means that collapse of the system can occur at any time, without prior warning, thus presenting considerable danger to personnel and equipment in the vicinity.

In retreat mine roadways, it is common to use wooden props as a secondary form of roof support in addition to the aforementioned crib construction. Wooden props are themselves prone to breaking horizontally under load, thus reducing their efficiency both drastically and suddenly. Although of some assistance, use of such wooden props does not represent a satisfactory solution to the problems associated with the crib construction itself.

There is therefore a need for a more efficient crib system which occupies a minimum of space within the mine, which reduces material costs and which relies less upon the skill of the labourer in its construction. There is also a need for such a system which does not fail in such an uncontrolled manner as the existing system is prone to do.

Accordingly, the invention resides in a crib system for supporting a roof in a mine comprising a column of blocks, the blocks being arranged in layers wherein each said layer is formed of a single block.

By means of the present invention, the problems of the prior art may be eliminated or at least substantially reduced. In particular, the configuration of a crib column according to the invention allows a construction that requires only one block per layer, thereby offering substantial savings in terms of material costs and space occupied.

Moreover, the crib columns are quicker and easier to construct compared with the aforementioned prior art system, the blocks simply being superposed one on top of another, rather than a pair of blocks in each layer having to be spaced evenly apart and with alternate layers being laid at right angles to each other.

The upper and lower surfaces in use of the block are, of course, generally flat and mutually parallel. In order to increase the support provided by each block and hence the overall strength of the column, the area of contact or overlap between upper and lower surfaces of blocks in adjacent rows should preferably be as large as possible, balanced with the need to occupy the minimum of space within the mine. This is most conveniently achieved when the blocks for use in the crib system of the invention are multi-directional, for example, having a generally circular or regular polygonal configuration, when viewed in plan. In this way, the weight of the column is substantially evenly distributed around its central axis thereby also improving the stability of the system.

The blocks are preferably superposed so that there is substantially complete overlap between blocks in adjacent layers. Such an arrangement makes the most efficient utilisation of each block's surface area and further minimises the "footpad" of the column. The reduction in surface area taken up by the completed crib column as compared with the prior art system permits ample clearance, for example to allow rail track access, without being prejudicial to the strength of the system.

By the above means, the deformation characteristics of the system are also vastly enhanced by comparison with the prior art crib construction. Tests show that the columns constructed in accordance with the invention demonstrate signs throughout their loading which indicate when failure is imminent, in which case the desirability of providing additional support can be considered. Moreover, deformation occurs in a controlled manner so that upon releasing the load, the columns remain intact. By means of the present invention therefore, sudden failure of the system may be avoided.

The column of blocks, or crib, is most conveniently constructed on a chock, preferably a wooden chock, laid on the mine floor. The column is built up to span the gap between floor and ceiling and is usually completed by placing a further chock between the uppermost block and the mine ceiling.

In order to provide additional stability and strength, the crib system preferably further includes a reinforcing member. Advantageously the reinforcing member comprises an elongate rod or bar which extends vertically through at least some and preferably all the layers of blocks of the system. Such an elongate rod or bar is ideally made of metal, steel being particularly suitable.

To accommodate the reinforcing bar or rod, the or each block is preferably provided with a hole extending between the upper and lower surfaces in use. Moreover, in order to avoid creating weaknesses near the edge portions of the block, the hole is preferably centrally positioned on the or each block.

The presence of a reinforcing rod extending through the layers of blocks provides benefits in terms of increased strength and stability of the system. In particular, the rod ensures that the column of blocks does not slide apart under load. A further advantage associated with use of a reinforcing rod is the ease with which the blocks may be aligned during the construction of the column. For example, insertion of the rod into the hole of the block forming the base layer enables successive layers to be correctly aligned relative to each other.

From another aspect, the invention resides in a crib block having substantially flat and mutually parallel upper and lower surfaces in use, wherein a hole is provided between said upper and lower surfaces which is adapted to align with a hole in an adjacent block when stacked in a vertical array and to receive an elongate reinforcing member extending through the aligned holes.

As previously described, the crib block is preferably multi-directional in order to provide high strength and good stability when stacked to form a column. In plan view, the blocks are more preferably circular or of regular polygonal shape, such as square, hexagonal, octagonal, etc.. Cylindrical blocks are especially preferred being the simplest shape to align with other blocks of the same shape.

The blocks according to the invention are advantageously made of concrete. Such concrete blocks may be made by either wetcasting or drycasting methods. Drycasting on a concrete block making machine is a particularly preferred method of manufacturing the blocks of the present invention.

In a typical drycasting method, a plurality of pallets made from metal or wood are fed by a conveying means into a block making machine which comprises a mould, a concrete filling means, one or more compacting devices, optional vibration devices, a stripping device, and an outgoing conveying means. In operation, a pallet, which forms the profile of one side of the block, is conveyed to be located under the mould in the machine. The mould comprises one or more side walls each defining a side of the desired block. A compacting and stripping device is preferably provided to form the final side of the block.

Once the pallet is located under the mould, the mould is lowered on to the pallet and held in frictional contact therewith, thus defining a cavity bounded by the upper surface of the pallet and the surfaces of the side walls of the mould. The cavity is then filled with concrete of the desired composition optionally using vibration or other levelling means to achieve a desired profile of concrete.

A compacting head, which preferably is also used for stripping, is lowered onto the upper surface of the concrete in the mould cavity and the concrete is compacted by the force exerted by the head, such force being in the form of direct pressure and/or vibration.

After compaction, the compacting forces are stopped and the mould is then raised from the pallet with both the pallet and the compacting head, now acting as a stripping device, being held in stationary alignment. Once the mould has been raised clear of the uncured or "green" concrete block, the compactinglstripping head is then raised clear of the green concrete. The resulting green concrete block is then conveyed away from the block making machine still on the pallet on which it was formed and carried to an area where it is allowed to cure to a hardened state.

When the block has cured sufficiently, it is then conveyed on the pallet to a depalleting station. The depalleted block is then transported to an area where it is packaged for delivery and the pallet is conveyed back towards the block making machine for re-use.

Concrete blocks are generally formed from a mixture of aggregate, sand or other fine material and cement. However, in order to provide improved properties under load, the blocks according to the invention preferably include a fibre reinforcement in the concrete mix.

The fibre reinforcement is preferably chosen from steel fibre, synthetic fibres, such as polypropylene, polyacrylamide, nylon etc., or natural fibres, such as wood and cellulose. Admixtures of such fibres may also be used to impart particularly beneficial properties to the block unit.

The properties of the crib system according to the invention may be further enhanced by the inclusion of at least one compressible board interleaved between adjacent blocks in the column. Such a board is preferably a timber board, and ideally in the form of oriented strand board, or "OSB", for example of the wood-cement type.

The inclusion of one or more compressible boards within the column permits greater deformation of the crib under load than is generally the case without their use.

Moreover, once the ultimate compressive strength has been reached, deformation still occurs in a controlled manner. In other words, there is no sudden failure and, upon releasing the load, the column remains intact. The presence of such boards therefore allows higher ultimate loading of the system without any significant detriment to failure characteristics.

A further benefit associated with the compressible boards is that they can be used to "fill" gaps which are inevitably present when the span between the mine floor and ceiling does not equate to a multiple of the block height.

The compressible boards are preferably interleaved between the layers of blocks at regular intervals along the length of the column, for example, at every sixth course.

Furthermore, when the crib incorporates a reinforcing rod, the or each compressible board is provided with a hole through which the rod may project.

The diameter of the compressible board is ideally chosen so that it projects beyond the boundaries of the upper and lower surfaces of the blocks between which the board is to be located. More preferably, and in order not to sacrifice the space savings offered by the system of the invention, the board is of no greater diameter than the chock upon which the column is normally laid.

The crib system according to the invention is particularly suitable as a primary roadway support, especially for providing support at the rib edge of the mine, between the unworked side of the mine and the rail track. However, the system is equally suitable as a secondary means of support, positioned adjacent to the coal face itself.

Whether it be for primary or secondary support purposes, a particularly preferred crib system is comprised of four basic components, namely centre-holed concrete blocks, a steel reinforcing rod, one or more oriented strand boards and one or two timber chocks. The actual crib construction can be varied to suit local conditions, for example, the number of blocks and/or boards used being dictated by the height of the mine ceiling.

From another aspect, the invention resides in a method of constructing a mine crib comprising laying a series of blocks to form a column extending between the mine floor and ceiling, wherein the blocks are laid in a vertical array such that each block overlaps with only one other block in an adjacent layer.

The method of the invention preferably further comprises reinforcing the column by means of incorporating a reinforcing element which extends through the vertical array of blocks, more preferably through aligned apertures provided in each of the blocks between their upper and lower surfaces in use.

In a further preferred embodiment, the method includes interleaving at least one compressible board between adjacent blocks in the vertical array.

The invention will now be illustrated, by way of example, by reference to the following drawings, in which: Fig. 1 is a side elevation of a prior art crib construction; Fig. 2 is a plan view of the construction of Fig. 1; Fig. 3 is a plan view of a crib block in accordance with the invention; Fig. 4 is a section through line A-A of the Fig. 3; Fig. 5 is a perspective view of the block of Fig. 3; Fig. 6 is a cross-sectional view of a crib system in accordance with the invention; Fig. 7 is a side elevation of a reinforced crib system in accordance with the invention; Fig. 8 is a side elevation of a ridge edge support system; Fig. 9 is a side elevation of a secondary support system; and Figs. 10 and 11 are charts illustrating deformation of cribs constructed in accordance with the invention.

Referring first to Figs. 1 and 2 which illustrate a typical prior art crib construction, rectangular blocks 1 are laid in pairs in spaced apart relationship with further layers of blocks 1 being disposed at 900 to the underlying pair. As will be seen from the plan view of Fig. 2 only those overlapping areas 2 of the blocks 1 are effective in providing support, whereas areas 3, 4, 5 are not in contact with blocks in an adjacent row and therefore do not contribute to the supporting function.

Figs. 3 and 5 show views of a crib block 6 in accordance with one aspect of the invention, and Fig. 4 is a section along line A-A of Fig. 3. The block 6 is generally cylindrical in shape having an upper face 7, a substantially parallel lower face 8, a side edge 9 and a centrally positioned hole 10 extending between the upper and lower faces 7, 8. The hole 10 affords the facility to place a reinforcement bar therethrough for horizontal loading thus ensuring that the column will not slide apart under load.

A basic crib construction is illustrated in Fig. 6. Blocks 12 provided with centrally positioned holes 13 are stacked vertically, each layer consisting of only a single block 12. The blocks 12 are superposed in overlapping relationship such that their central holes 13 are in alignment, The resulting column 15 of blocks 12 spans the gap between a mine floor and a mine ceiling, the lowermost block 12a in the stack resting upon wooden chock 16 and a further chock 16' being interposed between the uppermost block 12b and the mine ceiling.

A further crib construction in accordance with a preferred embodiment of the invention is provided at Fig. 7. The components of the construction which are the same as or substantially similar to the components of Fig. 6 have been given corresponding reference numerals. Essentially, the construction differs from that of Fig. 6 in that OSBs 17 are interleaved between selected courses of blocks 12. As with the blocks 12, each OSB 17 is provided with a centrally located hole which aligns with the aligned holes 13 of the blocks 12. Moreover, a steel reinforcing rod 18 is provided which spans the column 15 of blocks 12 and is located in the elongate aperture created by the aligned holes 13. The rod 18 ensures that the blocks 12 are maintained in vertical alignment and sideways movement of the blocks 12 which might otherwise occur under horizontal loading is effectively prevented.

Fig. 8 illustrates a mine crib system in the form of a rib edge support system. Roof bolts 19 are driven into the walls 20, 21 and ceiling 22 of the mine away from the coal face 23 being worked. A reinforced crib construction as shown in Fig. 7 spans the gap between the mine floor 24 and ceiling 22 adjacent the unworked side wall 20 of the mine. The width of the crib is such that it can conveniently fit between the side wall 20 and rail tracks 24 on which run wagons for conveying coal away from the coal face 23. Such an arrangement maximises the working space next to the coal face 23.

A crib construction in accordance with the invention may be used for other mining applications, for example, acting as a secondary support system as illustrated in Fig. 9.

Here, the crib shown generally at 25 is positioned closer to the coal face 23 being worked to provided added safety to personnel in the vicinity. Since the crib 25 is relatively compact, compared to prior art cribs, its presence near the coal face does not pose any great problems to personnel working at the coal face 23.

In order to demonstrate the properties of crib systems of the invention, reference is now made to Figs. 10 and 11. In particular, Fig. 10 shows a deformation chart or a failure analysis of a crib constructed in accordance with Fig. 6, being simply a vertical column 1.2m high of aligned cylindrical blocks without steel reinforcement. A load was applied to the crib and measurements of load against deflection were taken at incremental steps until the ultimate compressive strength of the crib was reached. At this point, the control mode was changed to deformation control in order to determine the mode of failure as the load bearing capability of the column diminished.

The same measurements were taken on a crib system constructed in accordance with Fig. 7, but without the steel reinforcement, the column in this case being 1.8m high and interleaved with oriented strand board every 0.6m. The results are shown in the chart of Fig. 11.

Figs. 10 and 11 demonstrate that the crib systems according to the invention are not only able to withstand high loads but also exhibit classic controlled failure. Thus, once ultimate compressive strength has been achieved, deformation occurs in a gradual, controlled manner, with no sudden failure. Moreover, in each of the tests, the columns remained intact upon releasing the load.

A comparison of the two charts in Figs. 10 and 11 highlights the benefit gained by the use of oriented strand board ("OSB") interleaving the layers of blocks. In particular, use of OSB allows higher ultimate loading of the crib. With regard to the failure mode, it is believed that the steeper, more sudden, slope of the crib with OSB interleaving results from the effect of elasticity in the timber.

While particular embodiments have been described, it should be appreciated that various modifications may be made without departing from the scope of the invention.

For example, the crib blocks may be provided with more than one hole passing through the upper and lower faces to allow for additional reinforcing members. The blocks may be formed in various sizes, both in terms of diameter and height, to suit the particular location where the system is to be constructed. Moreover, although the invention has been described in terms of a support system for roofs of mines, it should be appreciated that the system may equally be used for other applications, such as for shoring up unsafe buildings.

Claims (24)

1. A crib system for supporting a mine roof comprising a column of blocks, the blocks being arranged in layers wherein each layer is formed of a single block

2. A crib system according to claim 1, wherein the upper and lower surfaces in use of said blocks are of a generally circular or regular polygonal configuration.

3. A crib system according to claim 1 or 2, wherein each block is provided with a hole extending between its upper and lower surfaces in use for accommodating a reinforcing member.

4. A crib system according to claim 3, further comprising a reinforcing member in the form of an elongate rod which extends substantially vertically through aligned holes of blocks in the column..

5. A crib system according to claim 4, wherein said reinforcing member comprises an elongate steel rod.

6. A crib system according to claim 4 or 5, wherein the elongate rod extends through the full height of the column.

7. A crib system according to any preceding claim, further comprising at least one compressible board interleaved between a pair of adjacent blocks in the column.

8. A crib system according to claim 7, wherein a plurality of compressible boards are interleaved between adjacent blocks at regular intervals throughout the column.

9. A crib system according to claim 7 or 8, wherein the or each compressible board is provided with an aperture therethrough to accommodate a reinforcing member.

10. A crib system according to any of claims 7 to 9, wherein said compressible board is an oriented strand board.

11. A crib system according to claim 10, wherein said compressible board is a timbercement board.

12. A crib system according to any preceding claim, wherein the blocks are made of concrete.

13. A crib system according to claim 12, wherein said concrete is fibre-reinforced.

14. A crib system according to claim 13, wherein said concrete is reinforced with one or more of steel, polypropylene, polyacrylamide, nylon, wood or cellulose fibres.

15. A crib block having substantially flat and mutually parallel upper and lower surfaces in use, wherein a hole is provided between the upper and lower surfaces which is adapted to align with a hole in an adjacent block when stacked in a vertical array and receive an elongate reinforcing member extending through the aligned holes.

16. A crib block according to claim 15, wherein said hole is centrally positioned.

17. A crib block according to claim 16 or 17 formed of fibre-reinforced concrete.

18. A crib block according to any of claims 15 to 17, wherein said upper and lower surfaces are of a generally circular or regular polygonal configuration.

19. A method of constructing a mine crib comprising laying a series of blocks to form a column extending between the mine floor and ceiling, wherein the blocks are laid in overlapping relationship in a vertical array such that each block overlaps with only one block in an adjacent layer.

20. A method according to claim 19, father comprising reinforcing the column by inserting an elongate rod through aligned apertures provided in each of the blocks in the array.

21. A method according to claim 18 or 19, further comprising interleaving at least one compressible board between one or more pairs of adjacent blocks in the array.

22. A mine incorporating a crib system according to any one of claims 1 to 14 or as constructed according to any of claims 19 to 21.

22. A crib system substantially as hereinbefore described with reference to Fig. 6 or Fig. 7 ofthe accompanying drawings.

23. A mine rib edge support system substantially as hereinbefore described with reference to Fig. 8 of the accompanying drawings.

24. A secondary mine support system substantially as hereinbefore described with reference to Fig. 9 of the accompanying drawings.