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How To Effectively Calculate The Explosive Load For A Precision Presplit On 24-In. Spacing Along With The Stemming, Loading And Timing Of The Holes.

This is the second in a three-part article about Precision Presplitting. –Ed.

Precision Presplitting is a form of presplitting where the blast design is based on the rock properties and rock mechanics. The differences between normal presplitting and precision presplitting were discussed in the previous article (Konya & Konya, Precision Presplitting - The Basics, 2016).

Precision Presplitting uses very light loads to just break the rock by placing it under tension and minimize overbreak. This is typically done utilizing different strands of detonating cord and a 24-in. spacing between boreholes (Figure 1). At the bottom of the hole is a booster or chub of emulsion. This is not only to add energy at the bottom of the borehole, but to add weight to the detonating cord which makes loading the shot easier.

precision presplit

Figure 1 - Precision Presplit

In previous years the explosive load has been based on experience and using precision presplitting in different rock types. After an explosive load was estimated, field tests were done using different loads across the face. After the blast, the engineer, blaster or superintendent would determine which face had the best presplit at the lowest. This was generally based on the amount of half-casts (borehole marks) left on the final wall.

In 2016 the authors conducted a study to determine the explosive load based on the rock properties. This data came primarily from large construction projects, sandstone and aggregate quarries that previously had tested the rock properties. Through this the explosive load for given rock properties was developed as functions and tables that are used throughout this article.

The first study that was completed was calculating the explosive load for precision presplitting based on the rocks compressive strength (Konya & Konya, Precision Presplit Design, 2015). The compressive strength of the rock is the ability of the rock to withstand compression. The way a precision presplit functions is:

1. A rapid buildup of gas pressure within the borehole.

2. Compression of the rock surrounding the borehole.

3. Tension failure between holes to form a smooth crack.

Table 1 shows some of the rock types that where considered for this study:

Table 1 - Rock Types and Explosive Loads

Rock Type

Explosive Load (grains/ft.)

Explosive Load (g/m)

Siltstone

100

21

Sandstone

250

53

Shale

300

64

Limestone

550

117

Granite

700

149


Figure 2 shows the graph for determining the explosive load based on the unconfined compressive strength of the rock. The points marked are the rock types shown above with siltstone being the bottom point and sandstone being the top point.

comp strength to explosive load

Figure 2 - Compressive Strength

The other rock property that was analyzed was the Young’s Modulus (Konya & Konya, Precision Presplitting Optimization, 2016). The Young’s Modulus is the distance the rock deforms horizontally (strain) as it is compressed vertically (by stress). This is a common parameter that is given in geologic reports for construction sites and is normally given when the unconfined compressive strength is not given. Table 2 shows the data that was used for this study. The graph for this data is shown in Figure 2. Both of these studies have explosive loads estimated on a 24-in. spacing.

 
Table 2 - Young’s Modulus Study (Konya and Konya)

Rock Type

Young’s Modulus (GPa)

Explosive Load (grains/ft.)

Explosive Load (kg/m)

Siltstone

8.5

100

0.021

Sandstone

15

250

0.053

Shale

20

300

0.064

Limestone

30

550

0.117

Granite

40

700

0.149

young modulus to explosive load

Figure 3- Young’s Modulus for Explosive Load

This data was then compiled into tables which are very useful for determining the explosive load based on the rocks properties. The following tables are quick references to determine the explosive load for a precision presplit based on a 24-in. spacing. These are grains per ft. based on compressive strength (Table 3), grains per ft. based on Young’s Modulus (Table 4), and grains per ft. based on average rock type (Table 5).

Table 3 - Compressive Strength to Grains Per Ft.

Compressive Strength (MPa)

Grains of Detonating Cord per Ft. of Borehole

30

45

40

70

50

100

60

130

70

170

80

210

90

250

100

290

125

415

150

550

175

700


Table 4 - Young’s Modulus to Grains Per Ft.

Young’s Modulus (GPa)

Grains of Detonating Cord per Ft. of Borehole

5

60

10

150

15

250

20

340

25

430

30

530

35

620

40

715

45

800

50

900


Table 5 - Explosive Load for Different Rocks

Rock Type (multiple sources)

Grains of Detonating Cord per Ft. of Borehole

Siltstone

100

Marble

150

Dolomite and weak Limestones

250

Sandstone

250

Rock Salt

275

Shale

300

Strong Limestone

550

Quartzite

610

Gneiss

620

Granite

700

Diabase

1150

Basalt

1250

Now that an explosive load has been determined, the next step is to load the shot. When loading the shot, the bottom of the hole will have a ½ lb.+ booster or a chub of emulsion explosive to provide weight to easily load the hole. Inside of the booster or chub will be a cap (either electronic or nonelectric) and the detonating cord will be wrapped around and/or electrical taped to the chub (shown in Figure 4).

bottom load

Figure 4 - Bottom Load

In many situations the grain per ft. load of detonating cord load calculated does not match a single grain load of detonating cord available from the manufactured. In this case one must mix and match different grain load strand to get the load required. For example, to get a 250 grain per ft. load one may use two 100 grain coeds plus a 50 grain cord. This can be done by taping together the different strands and onto the leg wire of the cap to keep all cords going down the hole together. This taping is typically done in 5- to 10-ft. increments. The next challenge then becomes how to efficiently and quickly load this shot. Figure 5 shows the picture of a truck which has become a common way to load the precision presplits detonating cord charges. This is done with a pipe that goes across the truck bed and can hold the spools of detonating cord.

Loading the multiple strand of detonating cord

Figure 5 - Loading the Multiple Strands of Detonating Cord

At the top of the borehole on a precision presplit the stemming should be 10 to 12 times the diameter of the borehole. The stemming material should be drill cuttings, with the aim of them blowing out the top of the borehole. Drill cuttings are used in place of crushed stone to only momentarily confine the gas pressure, then blow out the hole and finish forming the crack to the top. Using crushed rock increases chances of overbreak in the collar of the hole, especially in weaker rocks.

The detonating cord must stop beneath the stemming material and very often a stemming plug is also used. The layout for the top of the hole is shown in Figure 6.

Top of borehole

 

As previously stated, all spacing in this pattern is set at 24 in. The next variable that one must consider is the timing of the pattern. This will depend on the caps chosen by the operation. If electronic caps are chosen, generally 4 to 8 milliseconds (ms) between holes is best with one hole firing after the other. The time period between holes can be extended or shortened as needed. A few other common timing techniques with the electronic caps:

  • Fully instantaneous (all caps fired on the same delay).
  • Bunches where multiple fire instantaneously (to meet vibration limits).
  • 8 ms between holes (to account for vibration).
  • 1 ms between holes (to help guide shear better).

When using electronic caps it is important to pay attention to precompression of the cap in the adjacent holes. In some cases electronic caps may precompress and not fire or fire late. In critical areas, caps and boosters that can help reduce this may be used.

If nonelectric caps are being used, then an 8-ms or 17-ms delay can be used between holes and similar results will be obtained. Care must be taken with using nonelectric caps and firing instantaneously because firing out of sequence can cause backbreak and negative effects which is common when attempting to fire caps instantaneously from cap scatter.

This article reviewed how to effectively calculate the explosive load for a precision presplit on a 24-in. spacing along with the stemming, loading and timing of the holes. In the next article we will discuss how to calculate explosive loads at varying spacing either less than or greater than 24 in. If you have any questions or concerns, please email This email address is being protected from spambots. You need JavaScript enabled to view it. for more information.

Dr. Calvin Konya is the president of Precision Blasting Services, and Anthony Konya is a project engineer for the company.