Ammunition and explosives – Blasting – Borehole loading
Reexamination Certificate
1999-07-20
2002-09-24
Nelson, Peter A. (Department: 3641)
Ammunition and explosives
Blasting
Borehole loading
C102S312000, C102S315000, C102S323000, C102S324000
Reexamination Certificate
active
06453818
ABSTRACT:
The current invention relates to a method or blasting. In particular the current invention relates to a method of controlled blasting using dispersed explosive. More particularly the current invention relates to a method of controlled blasting comprising dispersing an explosive and then detonating the dispersed explosive.
Although the current invention will be described with reference to perimeter blasting, or presplitting, it is to be noted that the scope of the present invention is not limited to the described embodiment but rather the scope of the invention is more extensive so as to include the use of the current invention for blasting applications other than perimeter blasting or presplitting.
The objective of commercial blasting operations is to break rock and/or shift material in a useful way and explosive charges are positioned and detonated to maximise the desired effect. When an explosive charge explodes a powerful force is exerted in all directions but most movement will occur along the line of least resistance or least confinement.
In most mining applications it is desirable to locate explosives in such a way that the explosive force will break to a free face without too much confinement. In above ground mines the principal free face is called a “highwall” and may be dozens of meters in height.
Generally the most economical method of blasting involves detonating explosives located in a large mass of rock (called a “blast block”) such that the rock broken by the explosives is thrown into a conveniently located pile for easy loading into trucks. Blast blocks are kept as large as practicable to maintain economy of scale and minimise time lost in vacating and then re-entering the blast area.
Blasting results are influenced to a large extent by the shape and size of the blast block and the distribution and type of explosives within this volume. The location of blastholes, their depth, diameter, angle and quantity of explosive in each hole are parameters critical to the success or failure of a blast and can greatly influence the overall cost of mining.
Fragmentation and productivity are also generally improved by using staggered blasthole patterns rather than square or rectangular patterns. The sequence in which the blastholes are detonated is also important as the result of any multiple-hole blast depends on interactions between adjacent blastholes. The results of a well designed multi-hole blast are far superior to firing the same number of blastholes individually or at random.
In mining operations poorly designed blasts frequently cause overbreak and damage beyond the intended blast block volume. Overbreak and damage are frequently minimised using one or more of a number of techniques including:
(i) reducing the amount of burden rock which is pushed forward by each explosive charge and promoting progressive relief or burden during the blast;
(ii) reducing concentration of explosives energy within each blast hole; and
(iii) presplitting.
Presplitting is a method of creating more stable free faces or highwalls which are steeper and smoother than those which can be achieved by firing normal production blasts. Presplitting is generally carried out ahead of the production blast and can be utilised to improve control of blasts, by producing a constant burden for front-row blastholes. Presplitting involves drilling a row of closely-spaced parallel blastholes along the line of the proposed “new” free face or highwall and then very lightly charging these blastholes and detonating them simultaneously or in large groups. The objective of presplitting is to attain a pressure sufficient only to form a crack between the presplit blastholes while not causing excessive damage to the surrounding rock. Firing of the presplitting charges produces an intra-row crack along the proposed “new” free face or highwall and the subsequent production blast can break along this fracture.
A crank formed by firing of presplitting charges acts as a pressure-release vent for the explosive gases generated in front of the crack. The presence of a crack also causes partial reflection of blast-generated stress waves, and thus reduces the intensity of the strain wave experienced behind the presplit. As a result of this, the disruption and shatter of the subsequently exposed free face are much reduced.
In practice, the diameter of blastholes for explosive presplitting is usually the same as that of production blastholes. In the current practice of presplitting using large diameter blastholes, the blastholes for presplitting are often deck loaded. Deck loading or charging is the practice of separating or isolating short columns of explosives called “decks” within a single blasthole. The decks may be isolated from one another using drill cuttings, gravel, air or other inert material. Each explosives deck generally contains at least one primer and may consist of either a bulk explosives or packaged explosives. The decks are also usually “coupled”, that is the charge extends across the entire diameter of the borehole and is confined by the borehole walls.
One of the problems of this type of loading is that it may lead to uneven distribution of explosive and concomitantly uneven distribution of detonation energy. In general, excessive energy is released in the region of the deck charge and the highwall tends to be unevenly split. An unevenly thrown face burden requires more labour to collect compared to an evenly thrown burden. Furthermore, an uneven highwall face tends to be unstable and thus constitutes a safety hazard.
The use of decks of explosives in blastholes also has the disadvantage of being relatively time consuming to load and requires the use of a separate primer for each charge of explosive in the borehole. The longer the loading time for each blasthole, the fewer the blastholes which can be loaded per day and the higher the labour costs.
Currently, in underground mining operations, perimeter control blasting is often achieved by loading the perimeter blastholes with low density explosives. Low density explosives, particularly those with a density below 0.6 g/cm
3
, are expensive and often difficult to manufacture. Low density explosives also suffer from the drawback of being unsuitable for use in wet blastholes.
In order to overcome the aforementioned difficulties, attempts have been made to reduce explosive power by partially loading blastholes with explosives. However, partial filling involves difficult and time consuming loading procedures and often leads to explosives wastage. Presplitting and perimeter control may also be achieved by using decoupled charges to reduce the overall density of the explosives in the blasthole. For example, packaged explosives are used in small diameter blastholes for presplitting. However, these explosives are costly and decoupling may lead to misfires due to the well known phenomenon called “channel effect”. Furthermore, the handling problems associated with decoupled charges normally restrict their use to small diameter applications.
It has now been found that blasting, particularly where a reduced concentration of explosive energy is required such as in perimeter control, presplitting or soft rock conditions, can be controlled by a method which involves dispersing the explosives composition within a blasthole prior to initiating the blast. The dispersion of the explosive composition within a blasthole provides an in situ low density explosives composition. The in situ provision of low density explosives overcomes many of the problems associated in the past with the cost of loading low density explosives and the effects of uneven distribution of explosives detonation energy. The provision of in situ low density explosives also overcomes the need to provide decoupled explosives charges.
Accordingly, the present invention provides a method of loading an explosive within a blasthole wherein said explosive is supported by a retaining means, disrupting or removing said retaining means and dispersing said explosive within the blasthole to form a dispersed explosive ha
Nelson Peter A.
Orica Explosives Technology Pty Ltd.
Pillsbury & Winthrop LLP
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