APPLICATION OF STEM PLUG

Role of Stemming

Traditionally, the accepted procedure for directing the explosive energy into the surrounding rock mass is to load the blast hole with explosives and the remainder of the hole is filled with drill cuttings or imported aggregate. Drill cuttings are the most convenient stemming material, but are generally inadequate to fully contain explosive gasses if used with the optimum charge height for maximum blast efficiency. The stemming length is usually increased in an attempt to compensate for the loss of explosive energy. This result in mediocre blast result, usually with oversize material at the top of the shot.

Figure 1: Traditionally Stemmed Blasthole

• To little stemming will allow the explosive gasses to vent, creating fly rock and air blast problems as well as reducing the effectiveness of the blast.
• Too much stemming will result in poorly fragmented rock near the top. This is especially apparent with hard cap rock formations.
• It is generally accepted that the shock from initial detonation of explosives in a blasthole is responsible for the cracking, spalling and weakening of the rock around a blast hole. The following rapid expansion of gasses provides the heave and resultant fragmentation. Thus, confining the gasses in the hole for as long as possible is important in maximizing the blast efficiency. This has been substantiated by studies indicating an inverse relationship between stemming ejection velocity and face velocity.

Product Description

The Stem Plug is a cone-shaped device constructed of high impact polystyrene. This material has a 15,000 psi compressive strength and is highly resilient. Stem Plug are available in 12 standard diameters ranging from 76 mm to 311 mm.

Table 1:Diameter of Stem Plug Based on Standardized Borehole Diamaters

Each plug is designed to occupy approximately 90% of the actual borehole diameter for the following reasons: (1) allowing space for stem plug to freely pass down wires and (2) compensating for drill bit wear. For maximum effectiveness always use the correct nominal size plug for stated borehole diameter range. For example, use 6-inch stem plug for a 6-inch diameter borehole.

Figure 2: Stem Plug

Method of Application

The first step in utilizing the Stem Plug is to create a buffer between the explosive column and the plug. This buffer be 1 1/2 times the diameter of the borehole and consist of a component stemming material. The purpose of the buffer is to protect the plug from superheated gas while still allowing the plug to provide the desired energy confinement.

Figure 3: Installation Step 1


Table 2:  A Good Rule of Thumb of Stemming Material (Standardized-Crushed Stone)

Next, the Stem Plug is lowered onto the buffer with the appropriate insertion tool. Tamp the stem plug on the buffer to ensure that it is properly seated.

Figure 4: Installation Step 2

Prior to disengaging the insertion tool from the Stem Plug, add at least one borehole diameter of stemming material to the borehole. This will secure the plug in place, allowing the insertion tool to be removed from the borehole while leaving the stem plug properly positioned within the stemming column.

Figure 5: Installation Step 3&4

Continue stemming the charged borehole to the collar or designated height. The stemming column is now equipped with the best available technology in blast energy confinement.

Figure 6: Installation Step 5

Upon detonation, explosive energy drives the stem plug upwards into the stemming column, the typical path of least resistance, and engages the stemming material in the borehole wall. Essentially a self-driving wedge, the stem plug will consistently replicate the "clogged gun barrel" effect to confine blast energy.

Figure 7: Stem Plug Mechanism

Insertion Tools

There is a pole which is available in a variety of 3-foot sections that may be joined together to accommodate most applications. Note the reverse thread on the tip. This allows the plug to be disengaged from the tip without loosening the pole sections.

 Figure 8: Standard Loading Pole Sections

Figure 9:Another Alternative Loading Pole for Use With 76 to 200-mm-stem plug

Blasting Benefits

The stem plug has one primary function is to seal the stemming column upon detonation. When applied correctly to a properly functioning shot, the benefits of this plug can be tremendous.

1) Higher Powder Columns
With improved stemming efficiency, the powder  column can be safely raised to utilize more of the borehole. The improved energy distribution reduces oversize generated by excessive stemming column lengths.

2) Improved Decking Efficiency
Studied show that maximum utilization of energy is obtained with a combination of the longest air deck and the shortest stemming deck that will provide adequate confinement of borehole gasses.

3) Air Blast and Dust Reduction
Lower stemming ejection velocities and reduced venting translate to reduce air blast. Reductions between 8 to 25 decibels may be achieved, depending on the rock type.

4) Flyrock Control
In all applications, it has been found that eliminating the venting of explosive force through the borehole collar greatly reduces or even eliminates fly rock. Thus, in addition to improving blasting safety in general, it is possible to extend the safe use of explosives into areas in which it would otherwise be marginal.