This invention relates to an improved slope of an open cut mine and a method of forming a reinforced rock slope in an open cut mine. The invention has been developed for an application in an open cut mine and it will be convenient to hereinafter describe the invention with reference to this particular application. It should be appreciated that the invention may have other applications in an open excavation environment, such as a quarry, and the invention is not to be limited to use only in a mine.

Mining companies use various methods of extracting ore from the ground, and an open cut mine is one method that requires the removal of a substantial volume of material. The excavation of the material develops a pit with a floor and a slope extending up from the floor. The slope is formed by a series of batters, with berms spacing the batters and providing rockfall catchment or a ramp for mining traffic accessing the pit floor. It is generally desirable to maximise the angle of the slope as it has the potential to minimise the extraction of waste material and maximise the extraction of ore.

A batter is generally excavated to an angle that, having regard to the characteristics of the rock, is naturally stable. This is generally no greater than 70° and normally more in the range of 50° to 70°. Where the berm at the top of the batter is acting as a ramp, or accommodating other critical mine infrastructure such as vent stacks or pumping stations, it is desirable to decrease the angle to account for the additional load and risk.

It ought to be appreciated that any angle other than 90° will typically result in inefficiencies. Where the batter is formed in the ore being mined, a batter of less than 90° will result in ore being left behind to maintain the berm above. Where the batter is to be formed in waste material, a batter of less than 90° may result in waste material being unnecessarily removed, particularly when it is outside the area of the ore deposit. Despite these inefficiencies open cut mines still tend to have batter faces at angles less than 70° to ensure natural stability. A safety issue with open cut mines relates to rocks falling from the face of the batter onto traffic or personal on the berm below. This is exacerbated where the batter is of less than 90° as the rocks bounce off the batter face and project out from the toe of the batter onto the berm, or past the berm and into lower working areas of the mine. It has been found that with a batter angle of about 60° the rocks fall generally between 2m and 9m from the toe of the batter but may well exceed this. This is a wide spread particularly given a berm may be generally around 10m.

The above discussed is included in the specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any of these matters form part of the prior art base or the common general knowledge in the field relevant to the present invention as it existed in Australia before the priority date of this invention. Throughout this specification the terms "open pit" and "open cut" may be used interchangeably.

According to one aspect of this invention there is provided an improved slope of an open cut mine including a plurality batters with a berm extending from a toe of each batter, each batter having an excavated batter face which is substantially vertical and extends from the toe to a crest of the batter, reinforcing means installed prior to the excavation of the face and spaced from the proposed crest, the reinforcing means including a plurality of reinforcing members each located in a substantially vertical borehole formed at said spacing from the proposed crest, each borehole extends to a depth of at least the distance between the crest and the toe, each reinforcing member extends substantially the depth of the each borehole

It is preferred that the depth of each borehole exceeds the distance between the crest and the toe by up to 15%. It is further preferred that each borehole is spaced from an adjacent borehole by between 1000mm and 5000mm. It is still further preferred that each reinforcing member is a bar or cable of a diameter of between 20mm and 100mm. It is preferred that the bar or cable is formed from steel, or alternatively each reinforcing member is formed from fibreglass. It is preferred that the reinforcing member is grouted in position in the borehole. It is preferred that the length of the reinforcing member exceeds the depth of the borehole, and in particular the reinforcing member extends out from the borehole to interact with a safety fence extending between the plurality of boreholes. Alternatively the length of the reinforcing member is less than the length of the borehole so that the reinforcing member is countersunk in the borehole. It is preferred that the substantially vertical face is within the range of 75° to 90°. It is further preferred that each borehole is spaced from the crest by no less than 800mm.

According to another aspect of this invention there is provided a method of forming a reinforced rock slope in an open cut mine including drilling a plurality of vertical boreholes to a borehole depth, locating a reinforcing member in each borehole which is substantially the length of the borehole depth, grouting the reinforcing bar in the hole, locating a crest of a batter which is spaced from the boreholes, excavating a face to the batter which is substantially parallel with the boreholes, the height of the batter face from the crest to a toe is no less than the depth of the borehole adjacent the face, wherein the excavation of the face is performed after the reinforcing member has been grouted in place in the borehole.

It is preferred to drill each borehole so that it is no greater than 300 mm in diameter. It is further preferred to space the crest of the batter from each borehole so that the reinforcing member is behind the crest of the batter by no less than 800mm.lt is still further preferred to space each borehole from its adjacent borehole at no less than 1000mm centres. It is still further preferred to provide a reinforcing member in the form of a reinforcing bar is no less than 40mm in diameter. It is still further preferred to drill the borehole depth to exceed the height of the face of the batter by at least 15%. It is still further preferred to provide a length of the reinforcing member that is less than the length of the borehole so that the reinforcing member is countersunk in the borehole. It is still further preferred that the method of excavating the face includes drilling and blasting to produce the substantially vertical face within a range of 75° to 90°. It will be convenient to hereinafter describe the invention in greater detail by reference to the accompanying drawings showing an example embodiment of the invention. The particularity of the drawings and the related detailed description is not to be understood as superseding the generality of the broad definition of the invention as provided by the claims.

Figure 1 is a side cross-sectional view of a schematic illustration of an improved slope according to the invention.

Figure 2 illustrates a schematic view of an unimproved slope prior to excavation of the batters.

Figure 3 illustrates an improved slope with the batter excavated.

Referring firstly to Figure 1 that illustrates a natural surface level 1 having been excavated to produce an improved slope 2 of an open cut pit. The improved slope 2 includes a plurality of horizontal berms 3, each spaced by a vertical batter 4. Each batter has a face 5 which extends from a toe 6 of the batter to a crest 7. The slope extends from the natural surface level 1 to the floor 8 of the pit.

Referring now to Figure 2 which illustrates a portion of the open cut mine prior to excavation. A crest 7 of the proposed batter 4 is selected, and a series of boreholes 9 spaced from that crest 7 are drilled to a borehole depth. This spacing is preferably no less than 800 mm, however this spacing x is merely preferred. Spacings x of other distances are clearly possible, however it is generally desirable that the spacing x be selected to be sufficient so as to reduce the likelihood that the borehole 9 will be exposed when the batter is excavated. The face 5 of the batter is exposed by a technique of drill and blasting which can cause the crest 7 to crumble. Accordingly it is preferable that the borehole 9 be spaced from the crest by 1500mm to avoid the hole being exposed when blasting. It is preferred that each borehole 9 have a diameter of no greater than 300 mm, however this dimension is merely preferred. The function of the borehole 9 is to accommodate a reinforcing member 10 and therefore the diameter of the borehole 9 is dictated to some extent by the characteristics of the reinforcing member 10. Furthermore, it is preferred that the reinforcing member 10 be grouted in position in the borehole 9, and therefore the diameter of the borehole will need to be selected to satisfy minimum cover requirements, particularly where the reinforcing member 10 is a steel bar. Accordingly, it is to be appreciated that the diameter of the borehole 9 may vary, however it is generally preferred that the diameter be less than 300 mm.

Each borehole 9 is preferably drilled behind the crest at a spacing y to space it from an adjacent borehole preferably by no less than 2000 mm, where the berm is not supporting critical mine infrastructure. Where the berm is acting as a ramp, or supporting critical mine infrastructure such as pumping stations or ventilation shafts, the spacing of the boreholes may need to be less than 2000mm. It is more likely that each borehole be spaced y from an adjacent borehole by between 2,000 and 5,000 mm. This spacing y will specifically be dependent on the characteristics of the rock structure being reinforced by the reinforcing member 10 and the desired level of reliability in the slope stability. If for example the rock structure comprises many closely spaced planes of weakness such as a fault, joints or fracture, it may be appropriate to have the spacings y of each adjacent borehole relatively close. In contrast, if the rock structure comprises very few planes of weakness it may be appropriate to have the spacings y relatively further apart. In either case, the spacings y can be set having regard to information revealed in a geological survey and variable between adjacent boreholes 9.

The reinforcing member 10 located in each borehole 9 may take any appropriate form. The preferred form of reinforcing member is a bar or cable formed from steel, however as an alternative it may be formed as a fibreglass rod. Where the reinforcing member 10 is a bar of steel, it is preferred that the diameter of the bar be between 20 mm and 100 mm. The diameter of the bar will be selected according to the characteristics of the rock, depth of borehole and/or the desired level of reliability in slope stability. The bar is placed in situ and grouted in position.

In one preferred embodiment it is preferred that the length of each reinforcing member 10 does not exceed the depth of the borehole 9. It is generally desirable that the reinforcing member 10 and grout are countersunk so as to reduce the likelihood that the top of the reinforcing member 10 is exposed. The top of the reinforcing member 10 might be exposed if the crest 7 crumbles, say as a result of blasting the batter face 5. Accordingly countersinking to a depth of 1000mm or greater is desirable. It may however be appropriate in certain applications that the reinforcing member 10 does exceed the depth of the borehole 9 as this enables the reinforcing member 10 to project from the borehole 9. The projecting reinforcing member 10 may be used for any suitable purpose, however it is preferred that it interacts with a safety fence 1 1 extending between the plurality of adjacent boreholes 9. This alternate embodiment is illustrated in figures 2 and 3.

The material 12 beyond the crest 7 that is to be excavated can be excavated by any suitable means. The preferred means illustrated in Figure 2 involves drilling a plurality of additional boreholes 13 at the crest 7 of the batter 4, and beyond the crest 7. Charges can then be placed in the boreholes 13 and the material be moved using drill and blast techniques understood and known by those people operating in this industry.

Referring now to Figure 3 which illustrates the improved slope with the exposed batter face 5 extending at 90° to the berm 3 below. The batter face 5 according to this invention is substantially vertical, and it is generally preferred that the substantially vertical face 5 be within the range of 75° - 90°.

The boreholes 9 including the reinforcing member 10 according to the invention extends to a depth at least equal to a distance between the crest 7 and toe 6 of the batter face 5. It can be seen from Figure 3 that it is preferred that the borehole depth extend beyond the toe, and it is particularly preferred that it extend beyond the toe by up to 15%. It has been found that by extending the reinforced borehole 9 by this distance reduces the likelihood of shear failure of the batter at the toe of the batter.

It ought to be appreciated from the foregoing that an improved slope 2 as hereinbefore described reduces the inefficiencies associated with a slope having a batter angle of less than 70°. Furthermore, it will reduce the distance from the toe within which rock fall occurs, providing a safer working environment. Still furthermore, reinforcing the batter 4 before excavating the batter face 5 is considered to be a relatively safe operation.

Various alterations, modifications and/or additions may be introduced into the invention without departing from the spirit or ambit as defined in the preceding specification.