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Context Sensitive Rock Slope Design Solutions

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3. Rock Excavation Methods

Drilling Equipment

Drilling horizontal holes is accomplished using a rig with a boom-mounted drill guide (generally a track rig) that has the ability to rotate the drill guide into a horizontal position and drill. Vertical and angled holes are bored using a downhole drilling rig or a track drilling rig as compared in Table 3.

Downhole Drilling Rig

Drilling with a downhole rig is best suited for vertical or near-vertical boreholes, deep drilling, and hard rock. Bit diameters range from 75 to 230 mm (3 to 9 in), allowing for precise borings at significant depths (although downhole drilling is engineered for a maximum depth of about 60 m or 200 ft, greater depths have routinely been achieved). Most of these drills are mounted on trucks or large tracks and therefore require wider, moderately graded benches to access the site.

Track Drilling Rig (Percussion Drill Head)

Track drills, also known as drifter drills, are the most commonly used drills in civil applications and can be used to advance vertical, angled or horizontal boreholes up to 12 m (40 ft) in depth. Bit size ranges between 40 and 150 mm (1.7 and 6.0 in.). The holes are advanced through percussion, either through a drill at the head or with tooling such as a downhole pneumatic hammer. They feature a boom that allows for borings along a slope face at a height determined by the boom length as seen in Figure 19. Track drilling rigs are generally smaller than downhole rigs and have better maneuverability, and consequently can access more difficult terrain.

Table 3. Comparison of vertical drilling rigs (modified from Konya and Walter 2003).

Most Common Application(s) Hard rock drilling of relatively deep holes. Can drill straighter holes and holes of different sizes with same rig. Rock drilling of relatively shallow holes (vertical, angled and horizontal).
Depth and Penetration Maintains a virtually constant penetration rate at all depths. Has higher average drilling speed for deep holes. Higher initial penetration rates, but drilling speed falls off with each steel added.
Air Requirements Can require less air because drill exhaust helps clean holes. Can use high-pressure air to increase drilling speeds. Requires air for both hole cleaning and drilling. High-pressure air can cause drift and shorten steel life.
Noise Impacts Comparatively low impact makes downhole drilling quieter, as exhaust noise is muffled in the hole. Requires drill exhaust muffler to reduce noise. Impact noise difficult to control.
Shanks and Coupling No shank pieces or coupling required. Uses standard API rod threads. Shank piece and coupling threads subject to higher wear rates and more frequent replacement.
Impact and Vibration Fewer moving parts. Almost all energy goes into rock instead of into mounting, meaning less wear on rig. Rig must withstand much of the drilling impact and vibration.
Photo. Common track drill used to advance vertical blastholes.
Figure 19. Photo. Common track drill used to advance vertical blastholes.
Portable Crane-Mounted or Hand-Held Drills

Drilling on slopes with limited access will require horizontal drilling (see above) and/or the use of portable crane-mounted or hand-held drills, which can drill both vertical and angled borings. When drilling blastholes deeper than 5 m (15 ft), the drill will be mounted to a rigid frame or platform, typically suspended from a crane, to ensure proper alignment. Maximum drilling depth for portable rigs is around 12 m (40 ft) and bit sizes range from 40 to 100 mm (1.7 to 4 in). For borings less than 5 m (10 ft) deep, a hand-held sinker drill or jackleg-mounted drill (a drill supported on a single leg) can be used. However, drilling with a hand-held drill is slow because its downward pressure is limited by the weight of the drill and the physical strength of the operator.

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