PRECISION PRESPLITTING IS BEING USED IN THE BLASTING INDUSTRY TO INCREASE SAFETY AND PREVENT OVERBREAK IN WEAKER ROCKS.
By Anthony J. Konya and Calvin J. KonyaThis is the first in a two-part article about Precision Presplitting. –Ed.
|Figure 1 - Normal Presplit|
Driving through the mountains you may look up and see a wall nearly falling over with hanging boulders and loose rock. As you get closer, you may see small portions of half casts scattered along the wall. We all think the same thing: “Someone must have underbid the presplitting on this project.” However, is this really the result of a bad job of drilling and blasting or is there a problem with the current methods of explosive presplitting used in today’s blasting industry?
The normal presplitting used in quarries and construction projects involves loading a drillhole with a presplit powder, small-cartridge emulsion or dynamite. This is done with little consideration to the rock types and the geologic depositional environment with typical presplit powder loads able to break the strongest of granites and limestones.
Generally, when one chooses to presplit the weak or heavily jointed with typical presplit blasting, the blasts are overloaded and the results can be poor. It makes no sense then to use these high powder loads, which leave the wall in bad condition with scattered half casts, bad backbreak and hanging boulders. This becomes a safety issue when loading and guarding must be put up to keep employees away from the crumbling face.
|Table 1 - Normal Presplit Design|
|Bench Height||35 ft.|
|Drillhole Diameter||3 in.|
|Powder Load||0.32 lb./ft.|
|Stemming||36 in. (3 ft.)|
For example, a highly jointed limestone quarry with 35-ft. benches needs to be presplit to provide a smooth final wall near a structure such as a crusher. The drill rig is capable of drilling a 3-in. hole. The question then becomes, what blasthole spacing should be used and what would be the powder load?
A rule of thumb for normal presplitting would be to use a spacing that is 12 times the drillhole diameter, which in this case is 36 in. (3 ft.). Next the powder load needs to be determined, which unfortunately is normally the presplit powder that is available from the manufacturer.
In this example, the available powder is 0.32 lb. per ft. The stemming for a normal presplit is estimated to be 12 to 14 times the drillhole diameter. Using a 3-in. hole, this is a stemming of 36 in. (3 ft.). The main concern with this is that while a large amount of stemming may reduce blowout, a smooth crack does not form to the top of the presplit, causing overbreak at the top of the blast. Little attention is paid to timing and generally the presplit is fired instantaneously.
|Figure 2 - Original Presplit Test|
This presplit is then drilled out and loaded and the blaster hooks up the initiators. The blaster is waiting in eager anticipation that a smooth wall will result and that will eliminate all the safety concerns with the highwall.
The blast goes off and after the dust settles everyone watching is upset that the wall is still badly broken and the highwall still poses a risk to employees. In many cases the blaster would just give up believing that bad geology is to blame and that this type of rock cannot be presplit. However, it is not the geology but the problems with our typical presplit design that is to blame.
About 30 years ago we were facing these same problems when trying to presplit highly weathered and very soft rocks. Needing to create a design that would work in all situations, we tried what is now called “precision presplitting.” This new form of presplitting would use the rock characteristics to help determine the spacing and explosive load. Figure 2 shows the original test of precision presplitting in a sandstone quarry.
While testing this new method, it was realized that the continuous powder loads used in traditional presplitting was too powerful and this is what was causing the large destruction (overbreak) of the back walls. By switching the explosive from continuous lengths of presplit powders to detonating cord with small dynamite, charges spaced out on the detonating cord.
|Table 2 - Precision Presplit Table|
|Bench Height||35 ft.|
|Drillhole Diameter||3 in.|
|Spacing||24 in. (2 ft.)|
|Powder Load||0.08 lb./ft.|
Later it was found that multiple strands of detonating cord were easier and faster to load and the exact explosive requirements can be set and accurately loaded. Using this method, even weak siltstones can be presplit to form smooth, solid walls.
In the previous example the blaster decided to try this new technique of precision presplitting. He again had a 35-ft. highwall and a drill rig that could drill a 3-in. hole. With precision presplitting that typical spacing used was 24 in., and this proved to work in almost all situations. The explosive load for this blast was 550 grains of detonating cord per ft. (0.08 lb. per ft.) This time a stemming of only 10 times the drillhole was used; 30 in. and the entire presplit will be fired instantaneously.
The blast was tied in and set off, and after the dust settled a smooth wall was shining in the distance. They have done it and the blaster cannot be happier!
|Table 3 - Varying Explosive Loads|
|Rock Type||Elevation||Explosive Load|
|Limestone||0-20||550 grains per ft.|
|Shale||20-25||300 grains per ft.|
|Limestone||25-35||550 grains per ft.|
In this example, the presplit was in homogenous rock, but in many cases the presplit has to run through multiple rock layers: siltstone, shale, limestones and sandstones. In this situation one can vary the explosive load at the proper intervals to account for all the different rock types. This can be done by taping different lengths of detonating cord to increase the explosive load where needed and having the main strand of detonating cord be for the weakest zone. For example:
This has worked well on many large, sensitive projects, including one in Grundy, Va. (Figure 4). In this blast there were four different rock types that varied throughout the face and a straight highwall had to be maintained. This increased flexibility in precision presplitting allowed for more control over the final wall at a mining operation.
|Figure 3 - Precision Presplit|
Another feature of a precision presplit is that it’s bottom-primed with either a cast booster or a cap-sensitive chub of emulsion or dynamite. While this does add a little extra power to the bottom of the blast, the main benefit is the extra weight it gives on the detonating cord. This makes the blast much easier and quicker to load.
While precision presplitting uses much less explosives, it generally increases the total drilling because of a reduction of the spacing. Looking at design variables and the economic variables one can analyze the differences between the two methods of presplitting. Once an economic consideration is placed on the benefit of having a smoother wall, a mine can then make an evaluation of whether precision presplitting is worth the investment. In the table below a comparison of these two forms of presplitting is done.
|Parameter||Traditional Presplit||Precision Presplit|
|Bench Height (ft.)||35||35|
|Drillhole Diameter (in.)||3||3|
|Powder Load (lb. per ft.)||0.32||0.08|
|Lb. per sq. ft.||0.12||0.05|
|Drilling per sq. ft.||0.33||0.50|
For instance, if a quarry had a drilling cost of $3 per linear ft. and similar cost per lb. of explosives of $3 per lb. In this case the quarry would reduce the explosive cost of their presplit by $0.21 per sq. ft.. At the same time, their total drilling cost per sq. ft. would increase by $0.51. Therefore, the quarry would increase the cost of presplitting by $0.30 per sq. ft. Little if any scaling of the final wall would be required.
Next the quarry needs to determine the monetary gain by receiving a smooth wall. If this quarry spends $50,000 per year in scaling and berms to improve their safety near the face and has a total of 3,000 ft. of a highwall (which is 35-ft. high), this is a total savings of $0.48 per sq. ft. In the end, by switching to precision presplitting this mine would save $0.18 per sq. ft. for a total of $18,900.
|Figure 4 - Crundy, VA|
The current methods of presplitting used in the mining and construction industry today are extremely ineffective when weaker or geologically unique rock is being presplit. Unfortunately, a presplit is normally necessary in a weaker rock to increase safety and prevent overbreak of the face.
The new form of precision presplitting is designed specifically to form a smooth crack to the top of the highwall in all rock types. Precision presplitting has now been proven to work on many projects such as the Folsom Dam, Kentucky Locks, Panama Canal, etc., and is being used in the mining industry today to increase safety and prevent overbreak in weaker rocks.
In the next issue we will discuss how to calculate the explosive loads for a precision presplit based on the rock types and characteristics.
Dr. Calvin Konya is the president of Precision Blasting Services, and Anthony Konya is a project engineer for the company.