Noise associated with an explosive blast may be undesirable from an environmental perspective, particularly where the blast occurs in the vicinity of a populated area. Such noise pollution has been the subject of environmental control regulations and there has long been need for a method of reducing such noise in an efficient and cost effective manner. The present invention seeks to address this need.

Accordingly, the present invention provides a method of attenuating noise associated with an explosive blast, which method comprises forming a waterscreen at a location and at a time relative to the blast such that noise associated with the blast is attenuated in a desired direction by the waterscreen, and wherein formation of the waterscreen is initiated before or at the same time as the blast. This method is simple to deploy, effective and economic.

The invention may be used to attenuate the noise due to such things as blasting operations, for instance in open-cut mines or quarries, demolition operations, for instance where buildings are demolished in the proximity of populated areas and"make safe"operations where unexploded munitions or terrorist devices are required to be neutralised or detonated.

The noise associated with an explosive blast may comprise noise from detonation of an explosive charge and, in addition, noise generated as a consequence of the blast. For instance, in rock blasting operations, a significant amount of noise may be due to fracture or movement of the rock after detonation of an explosive charge.

Formation of the waterscreen is initiated at a location and at a time relative to the blast such that noise is attenuated in a desired direction. The location of the waterscreen is typically its position relative to the blast source, i. e. the explosive charge which causes the blast. The time at which formation of the waterscreen is initiated is taken relative to the time at which the blast occurs, for instance the point at which an explosive charge is detonated.

The method of the invention is generally used to attenuate the noise of an above-ground (surface) blast, and this is certainly the case where the invention is used in commercial open-cut mining or quarrying blasting operations. Noise associated with underground blasts is usually not regarded as an environmental problem.

Formation of the waterscreen may be initiated before or at the same time as the blast. In either case, it is important that the waterscreen is suitably established and in place by the time the sonic wave from the blast reaches the location of the waterscreen. If the waterscreen is not adequately formed at this time, the efficacy of noise attenuation will be diminished. The intended formation of the waterscreen and the rate at which the waterscreen is generated will contribute to when formation of the waterscreen should be initiated relative to when the blast occurs. Usually formation of the waterscreen is initiated before the explosive blast.

The waterscreen may be formed in a number of ways. For example, the waterscreen is usually formed by detonation of an explosive charge adjacent to or in contact with a volume of water, such that at least a portion of the water is thrown up in the form of a screen. The explosive charge used to form the waterscreen will usually be a distinct explosive charge. A variety of configurations may be employed whereby a volume of water may be provided adjacent to or in contact with an explosive charge which results in waterscreen formation. Preferably the volume of water is contained in one or more plastic liners (or containers). Such water-filled plastic liners may be positioned adjacent to or over an explosive charge such that the liners are broken by the detonation of the explosive charge and the waterscreen formed adjacent to or above the detonation. In some embodiments, for example on uneven ground, it is preferable to use multiple short lengths of water-filled plastic liners so that the volume of water is substantially maintained and not drained to lower lying portions of a large liner. The liner (s) may be of any suitable material, such as polyethylene, of sufficient thickness not to burst until desired.

Generally the liner will not constrain the water in any way once the explosive charge has been detonated.

In a further embodiment the volume of water may be provided adjacent to the aforementioned separate explosive charge in a trench or channel formed for example with a backhoe or similar earthmoving equipment. The explosive charge may be positioned either within the water-filled trench or channel or in close proximity thereto.

The waterscreen may be provided by means other than the detonation of an explosive charge adjacent to a volume of water. For example, the waterscreen may be formed by spraying water onto a substantially vertical sheet. In this embodiment a sheet of plastic film may be erected spaced apart from the blast and water may be directed onto the plastic sheet to form a waterscreen. Alternatively, water may simply be sprayed into the air, for instance from a pressurised source, so as to provide the waterscreen.

The aforementioned (separate) explosive charge which is detonated to form the waterscreen is preferably a length of detonating cord of selected charge weight. Other explosive charges may be used although it has been found that detonating cord is particularly convenient due to its charge weight and ability to be readily laid in length-wise configurations. Typically detonating cord is supplied in effective charge weights of from 3 to 80 g/m. Higher charge weights may be obtained by running multiple detonating cords adjacent to the volume of water. Preferably the entirety of the separate explosive charge is covered by water so as to reduce noise resulting from the detonation of the (separate) explosive charge.

Preferably the detonating cord, or other explosive charge, has an effective charge weight in the range of from 5 to 60 g/m. Detonating cord typically comprises core charges of molecular high explosives such as PETN, RDX, HMX and the like. It should also be appreciated that the alternatives to detonating cord may be used to provide the aforementioned separate explosive charge which forms the waterscreen. For example, detonators, primers or small charges or cartridges of explosive may be placed in or adjacent to the volume of water. The explosive charge per unit volume of water may be in the range of 0.01 to 10 g/1. Preferably it is in the range 0.1 to 1.0 g/1. More, preferably the charge per unit volume of water is in the range of from 0.1 to 0.7 g/1.

The shape and height of the waterscreen may be determined by the effective charge weight and position of the detonating cord relative to the water-containing plastic liner (s). Advantageously, plural detonating cords or other explosive devices may be positioned in a predetermined array beneath the water-containing liner (s) such that when detonated the desired shape and height of waterscreen is obtained. Similar considerations apply to an explosive charge in a water-filled trench or channel.

The shape and configuration of the waterscreen may also be improved by the addition of a surface tension reducing agent, such as a surfactant, to the volume of water. It is well known that surfactants can be added to water to reduce surface tension and reduce the size of droplets.

Surfactants which are particularly suited to this application include dialkylsulphosuccinates (for example sodium diamyl-or dioctyl-sulphosuccinate), alkyl carboxylates (for example sodium laurate or sodium dodecanoate) or alcohols (for example octanol). In particular, commercial surfactants which are especially suited to very rapid surface formation are those based on alkyne-ethoxylates. Typical concentration ranges for surfactants may be by weight based on the weight of water but for economic reasons preferably in the range 0.001-0. 1% by weight. However it should be appreciated that other surfactants which act to reduce the surface tension of the water may also be used.

In order to further enhance the attenuating quality of the waterscreen, the addition of solid particles may be advantageous. The additional action of solid particles in reflecting or absorbing noise (kinetic) energy from the sound wave may enhance the noise reduction achieved by the screen. Such particles may be mixed into suspension in the water or may settle to the bottom of the water containers when used. Examples of suitable solids include talc, silica or, advantageously, fine rock dust from the quarrying or mining operation (for example the cuttings from drilled blastholes). The solid should preferably have low cost and environmental impact.

The solid may be used in conjunction with a surfactant as mentioned previously. Typically, particles in the range 1-500 microns are suitable, although larger particles of low density material, such as polystyrene or similar expanded polymeric materials, may also be effective in attenuating noise.

In certain applications, e. g. in mining and quarrying blasting operations, the waterscreen is usually formed and in place before the main blast is fired. The time delay between formation of the waterscreen being initiated and the main blast is chosen such that the waterscreen is sufficiently developed but has not substantially dispersed, fallen to the ground or otherwise deteriorated when the sonic wave from the blast reaches it. When using a separate explosive charge to form the waterscreen as described herein, this time delay would normally be in the range 1 to 10,000 milliseconds, dependent on the configuration and location of the waterscreen and the blast source. Preferably, the time delay would be in the range 10 to 2000 milliseconds.

To assist in accurate timing, the separate explosive charge which generates this waterscreen may be connected to the main blast charge and may, for example, be part of the ignition system for the main blast charge.

Effective noise reduction may be obtained by positioning the waterscreen in the range 1 to 100 metres from the blast. However, in some particular blasting configurations, for example over unstemmed blast holes, the waterscreen may initially be located on top of, or essentially right over, the blast or explosive charge. Preferably, in most blasting configurations, the waterscreen will be positioned in the range 2 to 50 metres from the blast. The location of the waterscreen relative to the blast will depend upon such factors as the size of the blast, the size and shape of the waterscreen, and the degree of noise attenuation required. Through controlled experiments it is possible to derive the optimum location for a given set of waterscreen and blast characteristics.

The positioning of the waterscreen in accordance with the present invention may be in a selected location, or locations, around the blasting charge. This may provide noise reduction in a direction or region of particular sensitivity, such as where nearby residences or places or objects of particular sensitivity to the blast noise are located. The positioning and dimensions of the waterscreen should be such that it shields a desired direction or directions from the sonic wave associated with the blast. If the positioning and dimensions of the waterscreen are inappropriate, sonic waves may pass uninhibited above and/or around the waterscreen.

Without wishing to be bound by theory, it is believed that the water attenuates the noise or sonic wave from the blast through a combination of physical mechanisms. These are believed to include reflections and scattering at air/water interfaces, the transfer of kinetic energy and momentum to the water droplets, heat transfer and possible evaporation of water.

The present invention will now be further described by way of example only with reference to the accompanying drawings, in which: Figure 1 is a series of cross-sectional representations of water-filled plastic liners laid over various arrangements of detonating cords; Figure 2 is a schematic diagram of a water-filled liner disposed over a length of detonating cord adjacent to a blast; and Figure 3 is a side view of a dynamic waterscreen shortly after the detonation of a detonating cord disposed under a water-filled liner. The waterscreen actually corresponds to that generated in Event 1.9 in Table 1 below.

Figure 4 is a side view of a dynamic waterscreen shortly after detonation of a detonating cord under a water-filled liner. The waterscreen actually corresponds to that generated in Event 1.4 in Table 1 below.

Figure 2 shows a bench (1), in which there are predrilled blast holes (2), which blast holes (2) are filled with the blasting charge (not shown). A length of detonating cord (4) extending in front of the blast face (3) has a water-filled plastics liner (5) disposed over it. Means for igniting the detonating cord and for firing the blasting charge are not shown, but these will be well understood by those skilled in the art.

The present invention will now be described with reference to the following non-limiting examples.

Example 1 Plastic liners having diameters of 270 mm and 350 mm were filled with an amount of water as specified in Table 1 below. Detonating cords providing the charge weights as detailed in Table 1 were positioned under the water-filled plastic liners in the cross-sectional arrangement indicated, corresponding to Figure 1. Waterscreens capable of attenuating blast noise were formed at the heights shown in Table 1.

The detonating cords were detonated and the waterscreen produced measured in order to determine maximum height and the time talcen to reach maximum height. The results are shown in Table 1 below.

Table1 Event Effective chargeCross-sectional Width ofplastic Approximate water Explosive charge per Max Time to max weight (g/m) arrangementof liner (m) volume perunit water volume (grams height (m) height (m) cords length charge explosives/litres (I/m) * water) 1. 1 0. 27 34 0. 15 1 100 1. 2 10 0 0. 27 34 0. 29 10 570 1. 3 10 0. 27 34 0. 29 4 270 1. 4 20 0. 27 34 0. 59 14 680 1. 5 20 0. 35 60 0. 33 6 415 T620**05870234430 1. 7 20-e 0. 27 34 0. 59 15 560 1. 8 40 -* 0. 27 34 1. 2 18 580 1. 9 40 0. 35 60 0. 67 17 830 2. 0 40 00 0. 35 60 0. 67 17 815 2. 1 30 0. 35 60 0. 5 9 750 2. 2 50 00 00 0 0. 27 34 1. 5 15** 840 2. 3 60 0.3560 1. 0 16 615 * cross sectional area calculated using theformulafor an ellipse,A=7t*ab, where a and b are the longest and shortestradii respectively<BR> ** Average height (m) along length of plastic liner.<BR> <P>These results show that the characteristics of the waterscreen may be varied by varying the nature and configuration of thewaterscreen-forming charge and<BR> the containment of the water.

Example 2 A quarry trial was performed in which waterscreens were formed by detonating lengths of 5 g/m detonating cord placed underneath flexible polyethylene bags filled with water, the ratio of charge weight to water volume being approximately the same as in Example 1. The waterscreens were formed Im in front of test charges, comprising uncovered lengths of detonating cord. The waterscreen charges were fired before the test charges by using short delay (17 or 25 ms) detonators to separate the two charges. This delay ensures that the waterscreen is in place before the sound wave from the test charge arrives. Similar test charges unshielded by waterscreens were also fired as controls. The control charges were fired 400 ms before the shielded charges. This period was sufficiently long to allow sound waves caused by the test charges to subside before detonation of the further (shielded) test charges but sufficiently short to ensure that test conditions such as wind speed and direction did not influence the test firings.

Sound levels were measured by an array of microphones placed at about 200m away. Readings from six separate trial runs showed that, on average, the noise levels from the shielded test charges were about 4.5dB lower than the unshielded control charges. These results show the efficacy of the present invention in attenuating noise.

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications which fall within its spirit and scope.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word"comprise", and variations such as"comprises"and"comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.