Trenchless activities to install underground piping are implemented in diverse regions and ground conditions. Ground can be classified as soft, medium or hard, based on the measured compressive strength. Soft ground has a compressive strength less than 10,000 psi. Medium ground is in the range of 10,000 to 30,000 psi. Ground that has a compressive strength above 30,000 psi is classified as hard ground.
Soft ground includes materials like loose soil, clay or soft limestone. Medium ground consists of hard shale and dolomite-type material while hard ground includes granite and other rock formations. Although a trenchless drilling operation will normally require moving through more than one type of ground hardness, a drill bit is selected that will be most effective for the most common ground type expected, even hard rock drilling.
Drill Bit Materials
Drill bits are exposed to extremely harsh conditions as they break up the ground ahead of the piping installation. They are subject to the pressure being applied to push the drill bit forward through the ground, as well as the rotational forces that keep the drill bit turning. In addition, there is the friction of the drill bit catching against the surface of the rock. As such, they are made from a variety of durable materials that will not break under the pressure of these impacts.
Steel tooth bits are the most common type of drill bit used in directional drilling applications. For a greater strength characteristic, sometimes tungsten carbide inserts are used for harder ground formations. The ultimate drill bits for very hard rock formations are polycrystalline diamond compact bits where synthetic diamonds are added to the carbide inserts giving a strength characteristic that is forty to fifty times higher than steel. Hybrids of these different drill bit materials are also available. Determining the type of drill bit to be used also requires a geological investigation, a compiled geological report and strict adherence to the information provided by the geo engineers.
Soft Ground Drill Bits
Drag bits or fixed cutter bits are made from a single solid piece of steel (sometimes with tungsten carbide inserts and polycrystalline diamonds imbedded) and have no rolling parts and associated bearings. The entire drill bit rotates with the drill string and cuts through the ground as the blades rotate.
Air or cutting fluid can be used with these bits to remove the broken ground from the cutting surface and out of the bore.
The advantages of drag bits are:
- The absence of bearings results in less possibility of damage and loss of effectiveness.
- The manufacture from a solid steel piece provides more resistance to breaking and leaving a part of the drill bit in the bore.
Drill Bits for Medium and Hard Ground
A common type of drill bit in use for directional drilling is the three-cone rolling cutter bit. Three rotating cones are placed at the top of the bit, with their points facing inwards towards the center. The cones roll on the rock surface while the entire bit is rotated by the drill string at the same time.
The angle of the rotating cones, as well as the shape and material of the inserts on their surface, give the rolling cutter bits their properties for specific ground types. Long and widely spaced teeth are suitable for soft ground, while tightly-spaced short teeth are more suitable for harder rock formations.
When the cone angle is high, the penetrating action of the bit on the surface is a scraping and twisting action while lower angles result in a crushing action. The harder the ground, the lower the cone angle should be in order to prevent excessive damage to the bit. The material of the inserts is selected on the basis of the hardness of the ground, with polycrystalline diamond being used for the hardest ground.
Rotating cutter bits are used with drilling fluid which is pumped through the bit so that a jet of fluid strikes the ground as the cone rolls. The purpose of the fluid is to remove the rock chips out of the way so that the next insert makes contact with the rock face again.
Measuring Drill Bit Effectiveness
The effectiveness of a drill bit selection can be seen in the speed of progress through the ground, which is defined as the rate of penetration. The selection of the best drill bit for a specific application may require some trial and error with a couple of different bits being used to test which one is most effective.
Another factor to take into account when judging the performance of a bit is the number of times the bit must be removed from the bore to be checked or replaced. Each time a drill is stopped and removed to work on the drill bit is costly in terms of time wasted, and so, drill bits with high-wearing characteristics have advantages.
Bearing wear is another consideration for the use of rolling cutter bits. A selection must be made between sealed or unsealed bearings, and journal or roller bearings. Each decision has an impact on the durability of the bit and its effectiveness for the application. (Learn more in "A Complete Guide to the Usage and History of Drill Bits and Tooling.")