Soft rock <20 MPa | Highly weathered rock, weak shale, marl, mudstone, chalk, weak sandstone, and heavily fractured formations | Rotary drilling; rotary drilling with casing where the borehole is unstable; auger drilling in mixed soil–soft-rock profiles | Compact crawler or truck-mounted rotary rig with adjustable rotation speed, moderate torque, and casing capability | Drag bit, tungsten-carbide insert bit, tricone roller bit, or soil/rock auger | Approximately 75–250 mm, depending on anchor tendon, grout cover, and casing requirements | Air, water, foam, or drilling mud; use low-pressure flushing where the formation is erodible | Low ground disturbance, borehole stability, precise angle control, and reliable cuttings removal without excessive washout |
Medium-hard rock 20–100 MPa | Moderately strong sandstone, limestone, dolomite, marble, weathered granite, and interbedded rock | Down-the-hole (DTH) hammer drilling for long, straight holes; top-hammer drilling for shorter holes and restricted access; rotary-percussive drilling for variable ground | Crawler-mounted hydraulic rig with a high-pressure compressor, rotary head, feed beam, and optional casing advancement system | Button bit for DTH or top hammer; casing shoe and pilot bit for overburden or broken zones | Approximately 90–300 mm for common anchors, soil nails, and micropile pilot holes | Compressed air is common for DTH; water or foam may be added for dust control, borehole cleaning, or unstable formations | Compressor capacity, pullback force, feed stroke, rod handling, torque reserve, dust suppression, and performance in fractured rock |
Hard to very hard rock >100 MPa | Fresh granite, basalt, quartzite, high-strength gneiss, dense volcanic rock, and competent massive formations | High-energy DTH or top-hammer percussive drilling; rotary-percussive drilling with casing systems for fractured hard rock; wireline coring where geological verification is required | Heavy-duty crawler rig with high-impact-energy hammer, rigid feed beam, high-pressure compressor, robust rod carousel, and strong dust-control system | Large-diameter carbide button bit, high-strength DTH bit, casing-system pilot bit, or diamond core bit for investigation holes | Approximately 90–350 mm; larger diameters may require staged drilling or specialized casing systems | High-volume compressed air for DTH; water injection or mist for dust suppression and heat management where permitted | Impact energy, compressor pressure and volume, feed force, torque control, rod fatigue resistance, bit life, vibration, and operator safety |