Ammunition and explosives – Blasting – Patterned blasting
Reexamination Certificate
2000-05-10
2002-07-23
Nelson, Peter A. (Department: 3641)
Ammunition and explosives
Blasting
Patterned blasting
C102S313000, C102S320000
Reexamination Certificate
active
06422147
ABSTRACT:
This invention is concerned with a method and apparatus for initiating sequential detonation of a series of explosive charges.
It is well known that improved blasting of a rock face can be achieved by arranging explosive charges in rows of spaced boreholes, and by initiating sequential detonation of the charges in each row, and also sequential detonation from one row to another. The purpose of this is to create a “free face” after each explosion before a successive explosion takes place.
In recent years, so-called “electronic detonators” have been used to an ever-increasing extent, with a view to achieving greater accuracy of control in the time interval between successive detonations. An electrical control circuit is provided to control the initiation of a blasting sequence, and which is intended to trigger detonation of each successive explosive charge at a predetermined time interval after the preceding detonation.
A considerable amount of research work has been carried out into the subject of control of the time interval between successive explosions. First of all, theoretical studies are carried out to determine the most favourable time interval, dependent upon (a) the nature of the rock medium and (b) the spacing-apart of the explosive charges. Then, electronic control apparatus and related software have to be developed, with a view to achieving in practice detonation at successive intervals which correspond as accurately as possible to the theoretically desired time intervals.
The behaviour of an explosively driven vibration in any particular rock medium is complex, and particularly when a sequence of explosively-derived vibrations is applied through the same rock medium, and inter-acting with each other. There is much published literature on the subject, and which might lead one to assume (erroneously) that blasting technology is now an exact science.
It is of course true that use of modern technology can give more efficient fragmentation of rock than the cruder techniques used in the past e.g. by use of fuses, but despite modern technology being available (including design of sophisticated software to control the blasting operation), in practice results can be of variable quality.
A desired fragmentation of a rock medium normally involves production of a major proportion of fragmented rock material reduced in size below a predetermined size, and without generation of (a) substantial amounts of larger fragments and (b) generation of excessive amounts of unusable small fragments and dust.
Furthermore, to the uninitiated, it might be thought that it would be a positive advantage to generate harmonic vibrations in a solid rock medium i.e. so that successive explosively driven vibrations reinforce each other to apply harmonic vibration to the entire rock mass. However, in practice this gives undesirable ground vibrations.
In particular, despite the use of sophisticated blasting techniques i.e. using theoretical calculations plus sophisticated electronic control equipment to implement the theory, it happens from time to time that harmonic vibrations are set-up in a particular rock mass as a result of a controlled sequence of explosions.
The harmonic vibrations may result in undesirable fragmentation of the rock, and also can give rise to significant environmental problems, which may generate unacceptable noise levels being generated and also by potentially damaging ground vibrations. Quarry sites often are located near to buildings e.g. houses or factory buildings, and environmental requirements are that noise and vibration levels must be kept below set limits.
Vibration measurements are normally required, prior to carrying out regular blasting operations, with a view to meeting requirements of local authority or other agencies controlling quarry operations. However, this involves extra costs which many site operators choose not to bear, with consequent adverse effects on residents living or working nearby.
It Is known from U.S. Pat No. 4,725,991 (Shell) that damaging vibrations can be set-up in the ground, during a rock blasting programme, and which can have adverse effect on (a) the quality of the fragmentation, (b) the efficiency of usage of the explosives and (c) the foundations and structure of any nearby buildings. The Shell patent also acknowledges that this subject has been addressed by many learned papers and publications e.g. by the US Departments of Mines, and all are agreed that very complex waveforms (at differing frequencies) are set-up in a rock mass as a result of a series of detonated explosive charges.
Also, it is recognised that relatively low frequency vibrations can have an adverse effect on building walls and foundations (a) from the point of view of horizontal waveform propagation, (b) vertical waveform propagation, and (c) lateral (shaking) displacement of the walls.
There are also desirable time intervals from the point of view of required fragmentation of a rock mass, and efficient usage of explosives.
In the Shell patent reference, while there are many references to the desirability of achieving optimum blasting timing (i.e. for good fragmentation while simultaneously avoiding undesirable ground vibrations affecting buildings), the actual teaching of the Shell patent is:
(a) to carry out a test explosion in a rock mass at a new site;
(b) measure the vibration profiles at selected measuring sites spaced from the test explosion; and,
(c) use mathematical calculations to derive a desired singular best time interval between successive explosions of a series of charges spaced apart in boreholes in the rock mass, derived from best shot vibrational data
The Shell reference teaches an elegant mathematical model utilised to reach the calculation of desired time intervals, but what is an essential aspect of this teaching is that the calculated time interval applies to the entire blasting programme, and which is a constant time interval between successive explosions in the row.
In particular, the teaching of the Shell reference can only deal with one frequency at a time, whereas the invention does not need a “test hole”. Further, the Shell reference has the frequency depending upon charge weight.
Therefore, while the present invention is based on a shared recognition of the problem of simultaneously achieving (a) efficient fragmentation and (b) minimising undesirable building foundation-rocking vibrations, the solution offered by the Shell reference is fundamentally different from that provided by the invention.
The present invention thus seeks to alleviate this problem by providing improved and different means to control the timing of a detonation initiation system, with a view to overcoming, or at least mitigating the risk of harmonic vibrations being generated in a rock medium as a result of initiation of a sequential detonation of a series of explosive charges.
According to the invention there is provided a control system for controlling the initiation of detonation of a series of explosive charges spaced apart from each other in boreholes formed in a rock medium to be blasted, said system being operative to apply controlled time difference in the time interval between successive detonations of at least one phase of the series of charges, and which includes at least three successive detonations, so as to reduce the probability of consecutive stimulation and amplification of ground vibration by reason of the detonation of the charges in the rock medium.
Preferably, the system includes an electrically operated control device which is operative to initiate energisation of detonators associated one with each explosive charge in a respective borehole, and time interval control means for controlling the intervals between successive energisation of at least said one phase of the series of charges.
The means whereby the electrically operated control device initiates energisation of successive detonators can take any suitable form, including direct electrical connection lines, radio transmission or through use of “shock tubing” systems known per se.
Each detonat
Hatorex AG a Swiss limited liability company
Madson & Metcalf
Nelson Peter A.
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