Arguably, bit selection to obtain optimum drilling efficiency and economy for given ground condition is the most critical issue for HDD in rock. Pilot holes can be drilled using rotary, roller cone or percussive techniques dependant on the mechanical properties of the material to be drilled.
Depending on the type of ground conditions encountered, including overburden, type of rock including compressive strength, number of rock layers and length of bore, there is a choice of tooling to help overcome nearly all types of ground conditions encountered.
Drilling with Roller Cones – Tricones
Tricones have a range of designs and specifications making them suitable for a wide variety of rock formations. Mill Tooth tricones are designed for drilling rock up to approximately 6,000 Psi compressive strength, whereas TCI tricones have designs available that are suitable from 5,000 Psi to over 50,000 Psi. Rock types include soft sandstone and limestone through to Basalt.
The roller cones on a tricone bit rotate together and crush the rock under pressure provided by the drill rod from the surface. Inserts situated on the surface of the roller cones penetrate the rock under pressure and crush it.
Operating parameters vary depending on the ground conditions but typical values for Tricones are:
– RPM: 40 to 60
– Maximum Weight on Bit (WOB): 5-1/2” – 21,000 lbs, 6-1/2” – 24,000 lbs
Tricones have a number of advantages and include:
• Versatility – Option for nearly every type of rock
• Low in initial investment
• Excellent impact resistance which makes them suitable when the rock is fractured.
• Diameters from 2-15/16” to 26” typically
Rotary Drilling – PDC Bits
Rocks of low to medium hardness (up to 25,000 Psi compressive strength) are usually suitable for rotary drilling; these will include sand, shale, mudstone, sandstone and limestone.
A modern example of rotary cutting tools are PDC bits. They use a synthetic diamond cutter to shear the surface of the rock.
Operating parameters vary depending on the ground conditions but typical values for PDCs might be:
RPM: 60 to 80
Maximum WOB: 3,000 lb per inch diameter. E.g. for a 5-1/2” bit, maximum WOB is 16,500 lbs
Fluid Flow Rate: 20 GPM up to 5”, 30 GPM up to 5-7/8”, 40 GPM up to 7-7/8”
PDC bits have several advantages which include:
• They offer a higher rate of penetration, longer life and increased steerability compared to conventional methods
• No moving parts
• Less stress on the drill rig
• Lower torque required
• Less vibration on the drill rods
• Producing clean holes with a consistent size
• Diameters from 1-7/8” to 18” typically
Down the hole (DTH) Hammers are excellent when ground conditions are very hard, especially above 30,000 psi compressive strength. Rock types include hard limestone, granite and basalt.
DTH drilling uses a pneumatic hammer directly behind a carbide impregnated drill bit. The bit is held against the rock surface under load and the impact from the hammer shatters the rock.
DTH Hammers and bits have several advantages and include:
• Faster penetration rates in very hard rock
• Directional hammer bits are usually supplied as either Convex or Slant face bits depending on the ground conditions
• Lower wear rates in abrasive ground conditions, especially above 30,000 psi
• Suitable for solid rock
• Diameters from 4” to 7-1/2” typically
We have several resources available online to help their customers choose the best bit – https://www.infinitytoolmfg.com/resources/
Infinity offers advice to support you with your rock boring challenges. To discuss operating procedures when using rock tools or which products are best suited to your specific ground conditions, please get in touch with Infinity’s customer service team.