When blast mining, there are a number of procedures that can be used to create the necessary access through the rock or other substance. Holes are drilled into the surface in a strategic pattern and explosives are inserted to achieve the desired blast pattern. A fan cut is one type of drilling and blasting pattern used in tunnel mining that allows the mining efforts to reach the next free face, or clear area, for drilling. As drilling continues deeper into the tunnel, the fan cut can be used again to reach the next free face. The fan cut is named for the shape of the drill holes that are used, which form an arc or fan pattern in the surface from a series of v-shaped cuts.
Fan cuts are used in situations where the opening at the base of the tunnel for the next drilling action is wider than the entry, or tunnel advance. There must be sufficient width for the drilling equipment to reach the rock face and to be positioned in a way to create angled cuts. In this situation, a series of inclined holes can be drilled into the open face. The holes are positioned at angles that will cause the most breakage in the rock face so that the next layer of free face can be reached.
The v-shaped cuts can be made one upon another or next to each other, depending on the size of the tunnel and the type of equipment available. The fan cut helps widen the available rock face by creating a trench that permits deeper and deeper access to the rock face. The fan cut may be used in combination with several other cuts or techniques depending on the structure of the tunnel, the material being drilled, and the explosives being used.
The angle of the fan cut should not be very acute. If acute angles are necessary, however, there will need to be a stronger concentration of explosives used in the drill holes to achieve the desired effect. Ideally, the fan cuts should be more than 60 degrees. The cuts can be made in a double vee or as much as a quadruple vee if the space is available. Blasts will also need to be spaced at least 50 milliseconds apart to allow sufficient time for swelling and displacement. This will give the blast the impact needed to displace the most rock.
