There is growing evidence that commercial use of electronic delay detonators for quarry blasting is close at hand. Claimed benefits include improvements in vibration, air-blast control, fragmentation, digging productivity and highwall control.

Faced with sharply rising regulatory and cost requirements, quarries, surface mines and construction operations around the world have been conducting an increasing number of trials of electronic detonators. Commercial use has been reported in Africa.

In North America, which has the largest total number and greatest variety of such operations, numerous trials have been conducted over the past two years. Sources within the quarry industry report that full-scale acceptance at some operations awaits only the availability of commercial quantities.

Typically, the quarry industry does not accept major innovations without field trials such as the series conducted in early 1998 at the Ogdensburg, N.Y. quarry of Hanson Aggregates, a division of Hanson Building Materials America. The details of this test series reveal a closely controlled focus on all aspects of electronic detonator performance compared to conventional detonators. Significantly, both the quarry operator and the down-the-hole explosives supplier have reported a willingness to accept a higher cost for electronic detonators in exchange for improvements in overall safety, regulatory compliance, performance and productivity.

Importance of accurate delays A number of companies worldwide are developing electronic detonators. A common objective is to provide a series of delay electronic detonators having precise, repeatable firing times without the scatter that is said to be an unavoidable characteristic of conventional pyrotechnic detonators.

"The potential effectiveness of available explosives energy in breaking and displacing the rock mass is directly proportional to the effective burden that energy must overcome," according to Daveyfire Inc., Powell, Ohio, sponsor of the Ogdensburg electronic detonator trials. "This relationship is crucial. Any variation in hole detonation timing that would result in a hole being fired prior to or after its nominal firing time will result in energy-to-burden relationships that can have adverse impacts on blast performance."

Adverse results include:

* poor fragmentation;

* large amounts of oversize;

* high vibration levels;

* high air-blast levels;

* fly rock incidents;

* more secondary blasting; and

* high excavation and crushing costs.

Test site sought In early 1997, Daveyfire technical representatives began seeking a suitable test site for their version of the electronic detonator, known as the Daveytronic. Hanson Aggregates volunteered its quarry at Ogdensburg, N.Y. Explosives for this operation are supplied on a down-the-hole basis by St. Lawrence Explosives of Adams Center, N.Y.

The Ogdensburg quarry was selected for a variety of reasons. Perhaps most important was the quarry's consistent hard rock geology which ensured uniform testing conditions. Other selection factors included state-of-the art drilling and blasting practices, including bulk loading; good bench preparation; demonstrated concern about vibration, air blast and potential fly rock; and management cooperation over a two-week trial period.

A series of four test blasts in adjacent sections of a 250-ft-long, 30-ft-high quarry wall was planned, with the first two blasts using conventional pyrotechnic detonators followed by two blasts using electronic detonators. Laser profiling of the four-blast area was conducted to ensure identical burden conditions. Drilling conditions were uniform and drilling was closely supervised.

Blast patterns for normal operations vary, but for the trial, each blast consisted of 18, 4-in.-diameter holes drilled 33 ft deep, including 3 ft of subdrill. A staggered pattern comprised three rows of six holes on a 10- Yen 10-ft spacing. Holes were top and bottom primed with bottom initiation and loaded with ANFO. Column heights were carefully measured and holes were plugged, then stemmed with 31/48-in. crushed stone.

Measurement and recording Several technologies were used to gather and analyze blast data:

* A one-hole blast-vibration waveform signature was obtained prior to blasting. This is an ongoing procedure instituted by Hanson and its vibration consultant and is used as a means of predicting vibration and developing compliant blast designs.

* Two LoCam high-speed motion picture cameras with 500-frames-per-second filming speeds were located at positions best suited for recording face velocity and hole firing times as well as post-detonation surface swell, stemming ejection and vertical rock throw.

* Three Larcor Inc. Mini-Seis digital ground-motion seismographs were set up at structures that have been principal points of concern for vibration levels.

* After-blast muckpiles were measured and then photographed and videotaped for a WipFrag optical fragmentation analysis. Digging was videotaped repeatedly to analyze the interior muckpile and to record excavator productivity.

The trial began after firing the signature shot, which indicated that delays of 25 milliseconds (ms) between holes and 96 ms between rows would result in acceptable vibration levels and frequencies. Then, in normal succession, the first two conventional-detonator holes and the first of the electronic-detonator holes were loaded and fired. A new one-hole signature shot for the fourth and final test hole indicated the need to change to 22 ms between holes and 86 ms between rows.

Conventional electric and non-electric delay detonators normally are supplied in a series of fixed nominal firing times in increments such as 25 ms, 50 ms and 75 ms. Sequential electric blasting machines or non-electric delay connectors provide additional delay periods, such as 12 ms, 15 ms or 42 ms, to standard nominal times. The electronic delay detonator, however, can be programmed for 1 ms increments of firing-time delay and does not use intermediate delay devices.

Less vibration, better fragmentation Maximum peak particle velocities (PPV) for the electronic detonator holes at the most critical of the three seismograph-monitored points of concern (LaRose residence) were 33% to 48% less than the maximum PPV of the conventional detonator holes (Table 1). The electronic detonator holes were closer to this point of concern than the conventional detonator holes. A shift to a higher frequency for the fourth blast-after the second signature shot-was indicated by a result of 42.6 Hz (radial) at the LaRose residence compared to readings at this point of 39.3 Hz, 39.3 Hz, and 30.1 Hz at 70-ft increments of increasing distance (Table 2).

Analysis of hole-detonation times revealed a maximum deviation of 44 ms for conventional detonators with all but four holes detonating at variations of 3 ms or more from nominal firing times. All of the electronic detonators detonated within 3 ms of their programmed firing times-the limit of measurement accuracy available with the type of high-speed cameras used.

WipFrag optical analysis indicated an improvement in fragmentation using electronic detonators (Table 3):

* 25% reduction in average maximum size;

* 38% reduction in average minimum size; and

* 19% reduction in overall average size.

These improvements were accompanied by a 22% increase in uniformity of size. Generally, improved fragmentation will reduce costs in downstream operations such as digging, crushing and equipment maintenance.

On the outside, the Daveytronic detonator looks similar to a conventional pyrotechnic detonator. It has the exact size and shape of standard electric and non-electric detonators to permit use in all commercially available cast primers and explosive products.

Inside, however, the difference is clear (Figure 1). The pyrotechnic delay element along with its inherent inaccuracies is replaced by a miniature micro-processor-controlled delay circuit. The fully programmable circuit provides timing accuracies to within 0.1 millisecond (ms) from 1 to 4,000 ms, according to the manufacturer. This high accuracy eliminates the timing scatter found in the actual firing times of pyrotechnic delays that are subject to batch variations in delay composition and manufacturing methods.

Inside the detonator, the explosive charge is totally isolated from the leg wires and any outside energy source by the integrated electronic delay circuit. This microchip and the two-capacitor system allow safe communication with the detonator. Only during the final charging sequence step of the blasting process does the firing capacitor receive electrical energy. The detonator must receive the digitally encoded firing command from the blasting machine to detonate, providing high immunity to radio frequency and stray electric currents. The Daveytronic system also has numerous self-testing and diagnostic procedures to improve safety.