Many of these devices suffer from the disadvantage that they do not yield to the descending action of the hanging wall with time.

According to a first aspect of the invention there is provided a pre-stressing device for a timber mine prop including: a generally tube shaped member having an open end which is configured to be driven co-axially in to an end of the timber prop and a closed supporting end that is adapted to bear against a surface to be supported; and a valve means for allowing a space defined between the tube shaped member and the said end of the timber prop to be pressurised by a fluid, thereby to urge the supporting end away from the said end of the timber prop and towards an engaging pre-stressed condition with the surface to be supported.

Typically, the tube shaped member may define a right circular cylindrical chamber between its longitudinally extending side wall and the closed and open ends.

The closed end may have a generally arcuate sectional form. A spacer means may be provided on the closed end to permit spacing of the end of the timber prop from the closed end when the tube shaped member is driven into the timber prop thereby to protect the valve means from being damaged by the timber prop as it advances in to the tube shaped member. The spacer means may be provided by one of more dimple formations formed in the closed end.

The pre-stressing device may include a piston member which is mounted displaceably in the tube shaped member. The piston member preferably comprises a circular plate

which is configured to extend between the side walls of the tube shaped member and to span the area of the end of the timber prop bounded by the tube shaped member.

A spacer member may be arranged on the piston member to permit spacing of the end of the timber prop from the closed end when the tube shaped member is driven into the timber prop thereby to protect the valve means from being damaged by the timber prop as it advances in to the tube shaped member. The spacer member may be provided by a projection which extends outwardly from the circular plate or by deforming the plate so as to form raised or shoulder portion.

A sealing means may be arranged on the piston member to provide or improve sealing engagement between the side wall of the tubular member and the piston member. A slot may be defined along a peripheral region of the piston member for receiving the sealing means.

The sealing means may include an O-ring or a sealing ring having a generally"V"-or "U"-shaped cross section.

A headboard arrangement may be mounted on the closed end. Conveniently, the prestressing device may be adapted to yield into the headboard on a load being applied to the prestressing device.

The valve means may include any suitable non-return valve and preferably a non-return valve as described in South African Patent Application No. 98/5755. An aperture may be defined in the side wall for receiving the valve means, which aperture is preferably located near to the support end of the tubular member.

According to a second embodiment of the invention there is provided a mine support element comprising timber prop and a prestressing device as described above mounted thereon.

According to a third aspect of the invention there is provided a method of prestressing

a timber mine prop including the steps of : mounting a prestressing device as described above on the timber prop to form a mine support element as described; positioning the mine support element between the hanging wall and the foot wall of a mine; introducing a pressurising fluid into the space defined between the tubular member and the end of the timber prop through the valve means, thereby to urge the supporting end away from the said end of the timber prop into a pre-stressed condition. the end face of the headboard under the action of the pressurising fluid.

The fluid may either be introduced into the chamber under pressure or may be an expandable fluid, such as an expandable polyurethane or epoxy foam.

The invention and the manner in which it is to be applied will now be described by way of various non-limiting examples with reference to the accompanying drawings.

In the drawings:- Figure 1 is a pictorial view of a mine support element in accordance with the invention, in use in a mine stope; Figures 2 to 10 are sectional side views of various embodiments of pre-stressing devices in accordance with the invention; Figure 11 is partially sectioned side view of a pre-stressing device in accordance with the invention; and Figure 12 is sectional view of a seal of the pre-stressing device.

Referring to Figure 2, in a first embodiment of the invention, a pre-stressing device 10 comprises a generally tube shaped member 12 which has a closed supporting end 14 and side walls 16 which define a cylindrical chamber 18. An inlet 19 is provided in the side wall 16 to the chamber 18 near to the closed end 14 for introducing a pressurising fluid into the chamber 18, the fluid flow through the inlet 18 being regulated by a non- return valve 20 mounted screw threadedly in the inlet 19. An open end 22 of the tube member 12 is driven into an end 23 of a timber prop 24 which is strengthened radially

by wire strands 26 which are secured around the circumference of the prop 24. In this embodiment, an expandable material such a polyurethane foam or epoxy (which is capable of expanding up to 200%) is pumped into the chamber 18 to cause the required increase in pressure in the chamber 18 to pre-stress the device 10.

Figure 3 shows a second embodiment of the invention which further comprises a piston member in the form of a metallic circular plate 28 and synthetic plastic or rubber cup seal 30 located between the valve inlet 19 and the open end 22 of the tube 12. The cup seal could also be suitably reinforced by for example, glass fibres. Also provided is a spacer 32 which maintains a preselected distance between the closed end 14 and the cup seal 30. In this embodiment the cup seal 30 and the plate 28 form an end wall of the chamber. The plate 28 and the cup seal 30 are in slideable engagement with the side walls 16 to allow the volume of the chamber 18 to expand and contract.

Figure 10 shows a further pre-stressing device of similar form to that shown in Figure 2 save that a spacer 40 is defined in the closed end 44 for spacing the plate 28 and seal 30 away from a non-return valve 42 (as described in South African Patent Application No. 97/2805) so as to prevent damage thereto as the prop 24 is advanced in to the tube 12 prior to pressurising. In particular, the spacer 40 is provided by a dimple or shoulder formation in the closed end 44.

Figures 4 and 5 show third and forth embodiments of the pre-stressing device which, further to the first embodiment described above comprise circular metallic plates 34.1 and 34.2 located between the inlet valve 19 and the open end 22 of the tube 12. An O-ring 36 is located in a groove 46 provided around the perimeter of the plate 34 to improve the seal between the plate 34 and the walls 16 of the chamber 18. The o-ring 36 may have a generally"U"-or"V"-shaped section and which is shown clearly in Figure 12 Figures 6 and 7 show yet further embodiments of the pre-stressing device wherein the closed ends 70 are of generally domed or arcuate form. Spacers 72 and 74 are connected, preferably welded, to the plate 34.2 and closed end 70 respectively. The

spacers 72 and 74 are of tubular form. In particular, Figure 7 shows the plate 34.2 in a first position (dotted lines) when the tube 12 is initially driven into the prop 24 and a pre-stressed or pressurised condition as will be described in detail below.

Figure 8 shows a pre-stressing device wherein the piston member is formed by a plate 80 having a central stepped or shoulder portion 82 which serves to act as a spacer in similar manner to that described above.

Figure 9 shows a pre-stressing device wherein the plate 34.2 does not have a spacer mounted thereon, the spacing away from the extremity of the closed end 44 to protect the valve 42 being provided in this instance by the spacer 40.

Figure 11 shows a pre-stressing device having a generally domed closed end 90 with circular strengthening ridge portion 92 and a flared rim portion 94.

During manufacturing, the headboard of the requisite size is cut from a timber post, wire strands are then secured around the post to help strengthen it radially, the opposite ends of the strands having been welded to one another. The open end of the cylinder 12 described above is then driven into an end of the post at a force of 8 to 10 tons.

Figure 1 depicts the pre-stressing device 10 in use. The pre-stressing device is positioned atop a mine prop 24 with the closed end 14 of the prestressing device bearing against or connected to a headboard 38 which in turn bears against a hanging wall or ceiling of the stope. Once in this position, the mine prop is ready for prestressing and a hydraulic hose 60 is connected to the non-return valve 20. Fluid is then pumped into the chamber 18 under pressure. The increase of pressure in the chamber 18 urges the tube 12 out of the prop 24 thereby driving the headboard 38 into engagement with and against the hanging wall. Once the required prestressing of the mine prop is achieved the pressure in the chamber 18 is maintained by the non-return valve 20.

In the embodiments shown in Figures 2 to 11, which have special sealing arrangements, a liquid such as water is pumped into the chamber 18 to cause the increase in pressure. This is conveniently done by connecting a piston-type hand hydraulic pump 62 to the valve 20 or 42 by means of quick connect coupling. Water is then pumped into the chamber 18 until the headboard 38 is pressed against the hanging wall. Typically, the water is pumped to a pressure of 2 500 psi. Once the required pressure is reached the quick coupling is removed. The pressure exerted by the headboard 38 on the hanging wall in this configuration is in the order of 22 tons.

As the embodiment shown in Figure 2 does not include a sealing arrangement the pressurising fluid which is pumped into the chamber 18 is typically an expandable material such as a polyurethane or epoxy foam.

After the mine prop and prestressing device have been installed and prestressed the hanging wall is supported with time, the hanging wall descends and exerts pressure on the mine prop. The configuration and assembly of the prestressing device within the prop 24 allows the tube 12 to be forced thereinto, compressing the prestressing device and allowing the prop to yield, to some degree.