Notes: Drilling, blasting and rock excavation (Pages 254-256)

Page 254

7.3 Drilling, blasting and rock excavation

7.3.1 Rock properties related to drilling

  • Hardness (as used here) is related to the rock’s compressive strength q_u, not Mohs’ scale. See Table 7.4 for compressive strengths of main rock types and implied drillability.
  • Abrasiveness measures how rapidly drill bits wear; correlates with (i) the minerals’ hardness, and (ii) grain size. Rocks with high silica content and fine texture are generally most abrasive.
  • Drill bit life (105 mm) varies significantly: about 60\,\text{m} of hole in fine-grained metamorphic quartzite vs up to about 600\,\text{m} in fresh quartz dolerite.
  • Tables referenced: Table 7.4 (compressive strengths), Table 7.6 (rock type percentages in three Scottish Midland Valley gravel pits), and Table 7.7 (abrasiveness of rock types).
  • 7.7 Abrasiveness (descriptive terms):
    • Abrasive: obsidian, rhyolite, aplite, granite, felsite, pegmatite; porphyry; flints, cherts, quartzite, siliceous sandstone, ganister, quartz or granite-cobbled conglomerate; metamorphic quartzite, hornfels, quartz-rich gneisses.
    • Intermediate: basalt, dolerite, gabbro, andesite, diorite, syenite, pyroclastic rocks; sedimentary sandstones, siliceous limestones, friable sandstones and grits.
    • (Notes: friable sandstones and grits may still be quite abrasive.)

7.3.2 Blasting techniques

  • Three main stages: drilling, blasting, and secondary blasting (or drop-balling).
  • Factors determining technique (a–d):
    (a) Type of rock (soft sediments require low-density/low-strength explosives; hard crystalline rocks require high-density/high-strength explosives).
    (b) Degree of fragmentation required (quarry work for loading/crushing machinery).
    (c) Shot-hole condition (dry vs wet). For water-filled holes, water-resistant explosives (e.g., gelignite, TNT-based slurry) are needed. If a hole is partially water-filled, use water-resistant explosives until the charge column is above water line.
    (d) Hole size (range 75\,\text{mm} to 300\,\text{mm}; in the UK 105\,\text{mm} is most common; holes \geq 300\,\text{mm} are unsuitable for rock blasting when rock strength exceeds 170\ \text{MN m}^{-2}).
  • Historically widespread but increasingly superseded technique: heading blasting (small tunnels driven, side tunnels, chambers packed with explosives; originally used black powder).
  • Drill hole types (a–c):
    (a) Small-diameter (\le 75\,\text{mm}) short holes for stepped benches. Vertical holes up to 5\,\text{m} deep; holes parallel to the free face with horizontal offset less than depth. Blasting ratio: 8\text{--}11\ \mathrm{tonnes\ kg^{-1}}.
    (b) Small-diameter long holes used to minimize ground vibration. Holes about 2.5\,\text{m} apart and 15\,\text{m} deep, parallel to the face. Blasting ratio: 11\text{--}18\ \mathrm{tonnes\ kg^{-1}}.
    (c) Medium-diameter (76--127\,\text{mm}) holes for blasting faces 6--30\,\text{m} deep; most quarry holes around 18\,\text{m} deep and spaced ~4\,\text{m} apart. Holes in line parallel to face, angled 70^\circ\text{ to }80^\circ toward the face. Each hole yields up to 660\ \text{tonnes}; blasting ratio \approx 10\ \mathrm{tonnes\ kg^{-1}}.
  • Explosive charges and firing: gelignite at the hole bottom with a detonator; top-up with a low-velocity explosive (e.g., ammonium nitrate/mineral oil mixture, ANFO) up to ~3\,\text{m} from mouth; remainder stemmed with sand or fine material. Gelignite detonates to initiate ANFO.
  • Detonators: delays from 1\text{ to }15\ \text{ms} to enable sequential firing at intervals of 1\ \text{ms} or more. If >15 holes, use mechanical or electronic delay setters.
  • Secondary blasting methods: break up larger blocks (pop-shotting, blasting-plaster shooting with charges attached to blocks). Drop-balling used to handle oversize blocks.
  • New developments: improved detonation cord (Nonel); larger drill holes (152\,\text{mm}). Lower section loaded separately from upper with inert stemming; fired separately, achieving a similar blast ratio to the 105 mm method with fewer holes.

7.3.3 Case history: the new Strome Road (A890), Wester Ross, Scotland

  • Rock types at site: Lewisian feldspathic and hornblende-gneisses (engineering granite group) with intercalated chlorite-schists and hornblende-rich rocks (schist group); younger Moine quartzite and quartz-rich gneisses (quartzite group).
  • Structure is complex; Moine rocks thrust westwards over Lewisian along the Moine Thrust; beneath thrust there is extensive fracturing.
  • Southern part of site contains Lewisian rocks both above and below the thrust; discontinuities were logged systematically by one author. There are five distinct joint sets.
  • Results shown in orientation diagram (Fig. 7.11): diameters represent the strike (in degrees magnetic) of the joint sets.

Page 256 (continued notes)

  • The five joint sets are characterized by their strikes; the diagram organizes discontinuities for stability assessment in rock slopes and cuttings.