Raise boring and shaft drilling have been demonstrated to be a viable alternative for augmenting ventilation systems that are stretched to capacity. By adding intake and exhaust openings closer to active sections of the mine, improved ventilation can be achieved which can supplement or even replace existing ventilation systems.

For areas that have already been developed, raise boring offers a quick and safe option for opening ventilation shafts. Raise boring is the most popular method currently being used to excavate raises both vertically and inclined. A raise-drill machine, typically located on the upper of two levels, uses a pilot bit to drill to the lower level. Once penetration is achieved, the pilot bit and stabilizer-sub are removed, the raise head is attached, and the head is pulled back to the raise-drill machine while being rotated. The compressive load applied on the face by rolling cutters breaks the rock and "drills" the material away.

As raising continues, the rotation of the head causes the rolling cutters to react on the entire face of the excavation and cut a smooth bore back to the machine. The cuttings fall down the opening to the level below, where they are typically removed by a Load-Haul-Dump (LHD) unit.

Roatry shaft drilling is typically used to create economical openings into undeveloped areas quickly. The blind hole shaft can be used to degasify and or dewater the local area prior to the advancement of the development drifts. A benefit of shaft drilling is that excavation of the shaft does not interfere with the development and construction activities occurring underground. Rotary shaft drilling typically relies upon fluid circulation to carry the broken rock from the face to the surface, therefore, there is little danger of igniting any methane present.

Shaft drilling with rotary equipment is accomplished by applying weights directly above the cutter head to provide the thrust required to cut the rock. While drilling, the weights also act as a pendulum to maintain a vertical hole. Stabilizers are used to assist in maintaining the orientation and alignment of the hole. A drill machine, located at the surface, supplies the torque required to turn the drill string.

There are several advantages to raise boring and shaft drilling over conventional drill and blast methods. The diameter of the hole is the size desired, not the size required to work in. Holes can be drilled in any diameter from 2 to 15 feet, though the most common hole sizes are drilled at even-foot increments.

The geometry of the opening is round, and the hole walls are relatively smooth and unfractured when drilled in fairly competent grounds, unlike holes produced with conventional drill and blasting techniques. Though the sides of the hole can be lined, the hole is typically smooth enough that impedance to flow is small. These advantages tend to result in a strong hole with a low roughness factor and minimal surface area which can improve ventilation efficiency.

The strength of these holes allows for a departure from conventional shaft ventilation geometries which involve a single large shaft. Use of shaft drilling or raise boring techniques can produce multiple, smaller holes with an equivalent cross-sectional area that would allow better utilization of the equipment. For example, a single 17 feet diameter hole could be replaced with two 12 feet diameter holes located next to one another. The ventilating efficiency would be slightly reduced with the increased surface area but the holes would be produced at an improved cost savings.

Raise boring and shaft drilling exhibit a high degree of safety and typically have minimal labor costs. Safety results from the fact that men are never in the opening during the excavation phase, and by the absence of explosives. The environment is also improved by the absence of the gases by the explosives. Labor costs are typically low since many raise boring operations now run with only a single operator per shift.

Raise boring and shaft drilling are typically more predictable and holes can be completed quicker. The inherent nature of raise boring and shaft drilling also provides the ability to cope with varying adverse geologic conditions, since the operator works from the surface. Additionally, as the length of the hole increases, the cost per foot decreases since the initial investment is spread over more footage.

The rolling cutters for raise boring and shaft drilling are available in a wide variety of sizes and with an assortment of cutting structures applicable to meet most rock cutting requirements. These cutters are used with saddles to allow the cutters to be replaced when worn without performing any cutting or welding procedures. All that is required is to remove the saddle fastener, substitute a new cutter, and refasten it to the saddle.

Raise boring cutters come with either a kerfed tungsten carbide or a randomly placed tungsten carbide insert cutting structure. Shaft drilling cutters are available with both of the tungsten carbide cutting structures and with steel teeth.

Steel-tooth cutters are typically ap¬plied in soft to medium formations and have long, sharp teeth with a tungsten carbide hardfacing on one side of each tooth. The hardfacing promotes self-sharpening of the teeth so that cutting efficiency is maintained throughout the life of the cutter. The steel teeth gouge and scrape the rock to remove it from the face of the hole.

Kerfed cutters have rows of tungsten carbide inserts which provide good rock breakage in medium to hard formations. The tungsten carbide inserts have improved life over steel teeth. The adjacent kerf rows cause the rock in between them to fail in shear. The rock fragments produced by a kerf-type cutting structure are typically large. When good hole cleaning is available, these cutters can achieve high penetration rates.

Random cutters (Figure 6) have tungsten carbide inserts that are placed on the cutter shell in a patented, random de¬sign. This design provides for dense axial carbide coverage with each revo lution while decreasing the actual number of inserts on the rock face. This results in higher loads per insert, lower drilling torques, and higher penetration rates which provides more efficient rock cutting. The chips .produced by these cutters are typically quarter-sized and are easy to remove from the hole. These cutters are applied in medium-hard tc hard formations.

The ease of producing a hole neai developing mine areas can offer an alternate solution when faced with dwindling ventilation. The wide variety oi equipment available allows for applica tion of the raise boring and shaft drilling techniques in most rock conditions. The ability to substitute cutters to meet chang ing geologic conditions and still utilize existing capital equipment can prove tc be an economic, long-term solution for: mines that are developing their resources.