The disclosure of South African provisional patent application from which this International Patent Application claims priority forms an integral part of the disclosure herein as if specifically reproduced herebelow.

Field of the Invention

The invention relates to mine roof supports.

Background of the Invention

Background on dynamics of rock interaction: In mining setting, mine roof supports, either wooden or composite supports, are damaged from the impacts of rocks during a blast. This affects the functioning thereof as well as putting miners at risk of harm.

The background on the dyhamics of rock interaction are shown in Figure 7 and set out below.

1. The rock has both impact and cutting effects.

2. How these effects come into play depends on the angle that the rock impacts an object such as a mine support.

3. A direct hit on a centre line of a mine support is a shock impact.

4. An oblique hit to the side of a mine support can dominantly have a curtting effect because the rock surfaces can be very sharp. A need has thus been identified to shield or protect mine roof supports from slJcl ¾amageT Summary of the invention

According to a first aspect of the invention, there is provided a blast shield for protecting a mine roof support from the impacts of rocks during a blast, said blast shield including a plastic shell covering at least a portion of the mine roof support, which blast shield is, in use, secured to a portion of the outer surface of the mine roof support.

The plastic shell may be in the form of a sleeve. The plastic shell may be releasably secured to the outer surface of the mine roof support so that it can be reused.

The blast shield may be used on either wooden or composite supports. The blast shield may inhibit damage to the support.

-.. The blast shield may inhibit a mine roof support from being knocked out of position and thus requiring additional costs to re-install. The blast shield may include a shock absorption layer between the mine roof support and the plastic shell.

The plastic shell may be made of HDPE or LDPE or PP with flame retardant. The shock absorption layer may be made of foam, rock fibre, polyurethane, or EVA, or similar density material.

The plastic shell material may be plasticized so that it yields when the rock impacts and spreads its impact over larger areas. The material may be from 2 to 5 mm in thickness.

The shock absorption material inside the shell may be from 10 to 30 mm thick.

The shock absorption material may have suitable compressibility so that it absorbs the kinetic energy of rocks of 2 kg travelling at 15 m/sec while deforming to full compression. The shell and the shock absorption material may be joined together so that they move together around the support when the incoming rock hits the support at an oblique angle to minimise energy absorption.

The blast shield may be in the form of a sleeve including the shock absorption layer and the plastic shell.

The sleeve may be shaped and dimensioned to be clipped around the support for ease of application underground. However, the sleeve may be inserted around the support during or post manufacture thereof.

The blast shield may be made of non-flammable material which is plasticised to deform during impact with rocks.

The shock absorbing material may be sponge materials to give a cushioning effect. The isobutane content that is used as blowing agent of the sponge material may be limited so as to maintain a non-flammable rating. One embodiment is made from 3 mm HDPE alone (without foam or sponge like material) that is heat formed to into circular clips that go around 2/3 of the mine support radius. The outer protective plastic shell may be corrugated circumferentially to give the required cushion effect for direct hits.

The outer protective plastic shell may be extruded as a plate and folded around and secured to a mine support.

The plate may be corrugated or dimpled to improve shock absorption.

The plate may be folded about and secured to a mine support by means of cable ties.

The shield may only be needed at the blast front and not every mine prop installed underground would necessarily need a shield. Description of the Drawings

The invention will now be described with reference to the accompanying Figures. In the Figures, Figure 1 shows a blast shield after a test set-up with rock impacting at 13 m/sec;

Figure 2 shows direct impact tests on the blast shield of the invention; Figure 3 shows off-centre impact tests showing the force at which a prop can be blasted out of place;

Figure 4 shows a point of impact on a blast shield; Figure 5 shows the rock penetration throughout HDPE and EVA;

Figure 6 shows the minimal impact damage (from rock at 13m/sec) on mine support through EVA and HDPE shield of the invention; Figure 7 shows the dynamics of rock interaction with a mine support; and Description of Embodiments of the Invention Experiments proving concept

• The following tests were conducted (see Figure 1 to 6) with 3 mm thick HDPE plastic that was curved around 20mm thick EVA wrapped around the prop using cable ties (tying method just for experiment purposes).

• The tests indicate that the shield can:

- prevent damage to the prop.

- prevent the prop from being knocked out (thus minimising additional costs to re-install).

• The design (provisionally) is made from 3 mm HDPE that is heat formed to into circular clips that go around 2/3 of length of the support radius, as can be seen in Figure 8 :

Further tests were conducted with direct hits on the prop which was placed on bricks.

Appreciable lateral movement of the prop was observed indicating that appreciable forces occur that can dislodge props.

Figure 5 shows the effect of the external rock impact on the mine support onto which the blast shield of the invention has been attached, shown in Figures 1 to 4, on the inner surface of the absorbing layer at the point of impact on the outside by a rock. The minor damage is indicated by the arrow in the Figure 5.

Figure 6 shows the corresponding position to that of the arrow in Figure 5 on the mine support from which it can be seen that there is minimum damage to the mine support at the area to which to the arrow points in Figure 6 thus confirming that the blast shield has served the intended purpose.

A clip on type blast shield is shown in Figure 8, said blast shield is made from fire retardant resin and is molded into a sheet from which a clip on form is made which then permits the shield to be clipped onto a mine support underground with ease.