July 2003

Refined methods and digital initiation technology help improve operations without disturbing the neighbors.

By Nick Lewis and Paulo Pereira

Production at Vulcan Materials Co.’s McCook Quarry reached 10 million tons in 2001, a daily shot rock requirement of about 45,000 tons.

With an annual production that reached 10 million tons in 2001 — a daily shot rock requirement of about 45,000 tons — drilling and blasting can significantly affect overall productivity and neighbor relations at Vulcan Materials Company’s urban McCook Quarry. Consequently, the company seeks to continuously improve its operations through testing and implementing new methods and emerging technologies.
The McCook Quarry, about 17 miles southwest of downtown Chicago, has been in operation for about 100 years. In 1998, Vulcan began mining reserves closer to some of the industrial and private structures that closely surround the quarry. In that area, about 50 percent of blasts are 500 to 700 ft. from structures; 25 percent are 400 to 500 ft. from structures; and 25 percent are greater than 700 ft. from structures.
Vulcan Material’s policy on maximum allowable vibration from blasting is considerably stricter than the state’s requirements. According to the company’s policy, peak particle velocity can not exceed 0.5 in./sec. and peak overpressure can not be greater than 130 dB. Independent consultants monitor 10 continuous seismographs surrounding the quarry to ensure blasts conform to these limits.
A detailed seismic study in 1999 by Vibra-Tech helped correlate blast vibration and frequency response with different zones in the quarry. It identified “hot spots” with higher than expected ground vibration frequencies at comparable peak particle velocities. Vibra-Tech recommended delay times and use of a conservative scaled distance of 75 throughout the quarry to avoid vibration problems.
To meet these parameters and the operation’s production requirements, Vulcan commonly made four shots per day, six days per week. Drilling proceeded 24 hours per day, seven days a week. In spite of this aggressive schedule, quarry operations sometimes could not keep up with the plant’s demand.

First steps
With a goal to increase the tonnage yielded from each shot without exceeding strict, self-imposed vibration and overpressure limits, Vulcan’s McCook Quarry implemented a new drilling and blasting program in 2001. Initially, the company changed from packaged to bulk-loaded explosives and began using global positioning system (GPS) receivers to improve the accuracy of measured distances between blasting faces and seismograph locations.
Vulcan previously used packaged explosives to meet a low pounds-per-delay requirement dictated by using a scaled distance of 75 and the close proximity to structures. In addition, blast holes typically needed to be dewatered prior to loading. Improved bulk loading measuring technology enables more precise explosives loading and use of emulsion blends that eliminate the need to dewater holes. Bulk loading a 70 percent emulsion/30 percent ANFO sensitized blend also allows Vulcan to expand the drilling pattern and reduce the number shots per year. The burden-to-hole diameter ratio was increased from 2.22 to 2.88.
To evaluate whether a scaled distance of 75 was appropriate for blasting in all areas of the quarry, Vulcan used GPS receivers to obtain distance measurements from each blast to the seismographs. The receivers provide an accuracy of +/-10 ft. without surveying. This, in conjunction with accurate measurement of explosives loading weight, provides a more accurate determination of scaled distance.
Using seismograph-recorded vibration levels and accurate scaled distance calculations, Vulcan determined that it could decrease the scaled distance for blasting all around the quarry from 75 to 50. The initial steps that enabled this decrease without increased vibration also produced significant cost savings. Bulk loading of holes reduced labor costs and eliminated dewatering and on-site storage of packaged explosives. Overall, with the initial changes, drilling and blasting costs decreased 34 percent.

Further refinements

i-kon electronic detonators allow in-hole programming of delay times in 1-millisecond increments from 0 to 15,000 milliseconds.

Blast designs developed with Orica’s SHOTPlus-i software can be downloaded directly to the i-kon Logger.

The initial recommendation to use a conservative scaled distance of 75 was based in part on uncertainties in using a non-electric blast initiation system with inherent inaccuracies in delay times due to detonator cap scatter. Cap scatter can alter the order in which detonators fire, affecting the timing of rock movement, which can affect vibration and fragmentation.
In early 2002, therefore, Vulcan began testing electronic detonators in a critical area of the quarry to see if it could further refine blasting operations. Electronic detonators, though more expensive, provide more precise and consistent delay times.
Vulcan chose to use Orica’s i-kon Digital Energy Control System. The system has three components — i-kon Detonator delays are defined with the i-kon Logger and programmed and initiated through use of the i-kon Blasters. Delays are programmable in-hole from 0 to 15,000 milliseconds in 1-millisecond increments. Orica’s SHOTPlus-i software can be used to develop blast designs that can be directly downloaded to the Logger. Two-way communication between control equipment and the detonators allows testing of the system integrity before a blast is armed and fired.
Vulcan’s tests with electronic detonators began using the same blasting parameters and the existing hole deck structure as with the non-electric initiation system. However, the company modified the timing configurations. By carefully controlling hole burden and spacing and the actual weight of explosives loaded, scaled distance was gradually decreased from 50 to 36.
Vulcan also decided to shoot a single, 100-ft. bench using the i-kon system instead of two, 50-ft. benches. In addition to increasing drilling and blasting productivity, this bench configuration shortened haul distances from the face to the primary crusher.
Use of i-kon detonators increased drilling and blasting costs by 16.7 percent compared to using non-electric initiation on 50-ft. benches with a scaled distance of 50. However, the following operating improvements, more than offset these higher costs:

• Haul costs decreased 30 percent by being able to shoot a single, 100-ft. bench;
• Drill utilization increased with a single bench;
• Tonnage per blast hole increased 44 percent, from 748 tons to 1,075 tons; and
• Primary crusher productivity increased about 13 percent, from 2,300 tph to 2,600 tph.

Improved fragmentation and crusher throughput resulted from eliminating one stemming zone (one bench instead of two) and from increasing the size of shots and the weight of explosives per delay.
Although use of electronic detonators did increase the average peak particle velocity from 0.26 in./sec. to 0.32 in./sec. — still well below Vulcan’s allowable maximum — this occurred using a much smaller scaled distance and larger shots (Table 1). When vibration control is critical, cap scatter with non-electric initiation systems is too unpredictable to be used in the same geometry.
Vulcan has mined more than 2 million tons from a critical area of the quarry, achieving successful vibration control with i-kon electronic detonators. Based on use of these detonators, the company is building a ground vibration prediction system for every location in the quarry and is experimenting with other blast design configurations to further enhance quarry productivity.

Reference
Lewis, Nick and Paulo Pereira, “Operating Improvements at Vulcan Materials McCook Quarry Using Electronic Detonators,” Proceedings of the International Society of Explosives Engineers 29th Annual Conference on Explosives and Blasting Technique, 2003.

Nick Lewis is drilling and blasting superintendent for Vulcan Materials Co., Midwest Division. Paulo Pereira is an AMS (Advanced Mining Solutions) projects manager for Orica USA.