Rock Excavation

Rock Characteristics

• Igneous Rock – granite, basalt• Sedimentary Rock – sandstone, shale,

limestone• Metamorphic Rock – originated as igneous

or sedimentary and then changed by heat, pressure, chemical reaction – slate, marble, schist

• Hardness – Moh’s scale 1- talc -> 10 diamond

Rock

• Difficulty in excavation due to– Rock type, extent of fractures, other planes of

weakness, amount of weathering, predominant grain size, brittleness or hardness

Investigation

• Drilling – core samples, tv dropped down shaft

• Test pits – only good for soft rock

• Seismic measurements

Rock Handling

• 4 phases

• Loosen – blast or rip

• Load – shovel or loader

• Haul – truck or wagon

• Compact - roller

Tunneling

• Old days drill and blast

• Now tunnel boring machines (TBM)– F 8-4– Usually 1 use machine– Rotating head with debris handling system

and

• Jumbo – large mobile frame so that you can drill a complete rock face at one time

Tunneling

• Hydraulic jumbo – self propelled machine with hydraulic arms with drills – drills blast holes in entire face F 8-5

• Mucking machine – form of shovel used to load fractured rock into haul units

Drilling Equip

• Percussion – jackhammer, drifters, wagon drill, track drill operates by hitting a bit which rotates between blows F 8-6

• Rotary drill – diamond tipped cutter head – well drilling machine F8-7

• Rotary-percussion T 8-2 • Down hole drills F 8-8

– Percussion device is mounted directly above drill bit at bottom of hole

– Less noise, less air required to clear hole, longer drill life higher efficiency

Drilling Equip

• Drill rate depends on rock hardness, drill type, drill energy, bit type Increased air pressure = increased drill production also increases costs

• Raise boring– Drill pilot hole down and then raise rotating

reaming head– 1,000,000 # upward thrust,400,000# torque

Drill Bits

• F8-10

• Tungsten Carbide steel

• Drill steel – connects drill bit to percussion machine

• Drill pipe connects rotary bit to machine

Drill Patterns

• Size of hole, depth, spacing & amount of explosive depends on degree of rock break desired, rock type, soundness, explosive used

• Usually test blast to determine exact pattern

• F 8-11 shows patterns

Drill Patterns

• Volume/hole = (S^2*H)/27

• S = hole spacing

• H = effective hole depth (* usually 90% of hole depth)

• Rock volume/ft of hole = Volume per ft of hole/ drill hole depth

Explosives

• Dynamite– Has been replaced by next three due to lower

cost and easier to handle than dynamite

• Ammonium nitrate

• Ammonium nitrate in fuel oil

• slurries

Detonators

• Electric blasting caps– Instantaneous or time delayed– Ignite primers – usually placed at bottom of hole

• Either primer or primacord

• Amount of explosive required to produce desired rock fracture = powder factor.– Pounds/bcy– Blast holes loaded with explosive except for last

several feet– Stemming is placed on top of explosive – inert

material