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3. Rock Excavation Methods

Drilling Methods

Blast holes are drilled at various orientations, from vertical through horizontal. To create vertical holes, which are used almost exclusively in production blasting, rock slope excavation uses two types of drilling: downhole and step drilling. Horizontal drilling is used for both production and controlled blasting because of limited drill rig access or geometry requirements. Angled drilling can be performed as determined by slope face angle requirements.

Downhole Drilling

Also known as vertical or production drilling, this technique is used in production blasting using a conventional rotary tri-cone blast hole rig or a rotary percussion rig if smaller blast holes are adequate.

Step Drilling

Step drilling is another type of vertical drilling that's also used in production blasting, most often to produce relatively flat and benched slopes (usually shallower than 1:1 H:V). It's similar to downhole drilling, but creates holes that gradually increase or decrease in depth to allow for a stepped slope "break" line shown earlier in Figure 14. (This method is technically not part of a controlled blasting operation because it relies on backbreak to form the slope face.) If done properly, step drilling can produce a slope face that shows minimal signs of blasting-just drill holes entering the slope face, which are noticeable only by someone looking directly at the face. On projects involving step drilling, drillers have been awarded a pay item for tightening the drill hole pattern and using lighter, distributed loading to avoid performing excessive blasting charges along the slope later.

Step drilling is limited to ideal geologic conditions, such as blocky volcanic rock, where breakage at the bottom of each blasthole is reasonably well controlled, but it can provide good results with minimal backbreak. It has proven popular with contractors who favor the vertical drilling setup.

In step drilling, the blast holes are loaded with more explosives (about 25% greater charge density) at the bottom of the hole, which helps to ensure proper fracturing along the base of the excavation. However, this heavier loading will also increase the amount of radial fracturing and backbreak along the final slope face and create the need for a widened catchment area. Step drilling should be used only when the driller and blast designer are experienced in the practice because of the potential for excessive backbreak and a ragged slope face. In most instances, the vertical drill holes are extended beyond the final cut line (a practice known as sub-drilling) to ensure proper fragmentation and achieve a more natural final cut face as illustrated in Figure 15.

 Illustration. Cross section of downhole and step drilling with sub-drilling techniques (modified from Cummings
              2002).
Figure 15. Illustration. Cross section of downhole and step drilling with sub-drilling techniques (modified from Cummings 2002).
Horizontal Drilling

Horizontal drilling is an effective technique for starting new excavations and for small excavations with poor access at the top of the slope. There are two basic techniques used for horizontal drilling. The first uses blastholes drilled perpendicular to the final rock face as shown in Figure 16, while the second uses holes drilled parallel to the rock face. When drilling perpendicular to the face, angled holes are typically required to mobilize and fragment the rock at the toe (these holes are called toe lifters) and the ditch (ditch lifters) to achieve the proper final slope configuration.

The second method sometimes uses a fan configuration, which can leave a distinctive pattern is shown in Figure 17.

Illustration. Horizontal drilling design concept (modified from Cummings 2002).
Figure 16. Illustration. Horizontal drilling design concept (modified from Cummings 2002).

In horizontal drilling, it can be difficult to maintain drill hole orientation, location, and depth. However, this is critical to avoid excessive fracturing in the final cut slope. In some cases, special drilling equipment (such as drills suspended from a crane) may be required to access the upper portions of the rock face. Special loading and timing allowances also may be required, and the drilling pattern must be adjusted to keep the drill steel from drifting downward.

The loading of explosives is also more complicated in horizontal drilling. It typically requires either packaged product or pneumatically loaded bulk product and stemming (filling, such as drilling cuttings, that's used to fill the blast hole above or between charges to "stem," or retain the explosive force within the hole).

Photo. Drill hole traces left by horizontal drilling parallel to the rock face (fan drilling).
Figure 17. Photo. Drill hole traces left by horizontal drilling parallel to the rock face (fan drilling).

In addition, horizontal drilling and blasting can produce badly fractured slope faces, as the production blastholes typically extend all the way to the final rock face. For this reason, a widened catchment area is recommended for slopes excavated this way.

Figure 18 depicts blast damage to a rock face that was excavated using horizontal blasting methods. Note the draped wire mesh in the upper portion of the photo, which was installed to control rockfall that was caused, in part, by blast damage.

Photo. Blast damage caused by horizontal drilling perpendicular to the rock face.
Figure 18. Photo. Blast damage caused by horizontal drilling perpendicular to the rock face.

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