ABSTRACT
DRILL AND BLAST TUNNELLING Full face in good rock; benched or crown heading with subsequent breakout where bad rock needs support. Jumbo machines have 2-5 drills on arms; can swing to drill bolt holes. Drill holes converge round a central wedge which is blasted out first; delay fuses (1-100 ms) then break outer zone into the central hole. Smooth blasting leaves clean walls with perimeter holes spaced 1 m, lightly charged and fired simultaneously.
