Drill bits are designed to be used in specific soil and rock conditions.
Some drill bits are used to facilitate steering and to excavate the soil or rock at the face of the bore. Common types of drill bits used in the HDD industry are: 1) traditional slanted-face bits, 2) slanted-face rock bits and 3) hard rock or mud motor bits. Application guidelines are presented in the table on the next page.
If steering is unresponsive, in soft soils, it may be necessary to reduce fluid flow rate, while pushing without rotating. However, reducing flow rate may cause loss of circulation, which can lead to stuck drill pipe, hydrolock and other serious problems, so flow rates should be reduced with caution. If steering remains unresponsive, it may be necessary to trip out of the hole and change to a wider or longer bent bit. For shallow bores, a backhoe may be used to excavate down to the bit and use upward or downward pressure to redirect the bit. If not done carefully, this action can result in excessive stresses on the drill string and over-correction of steering. In stiff soils, it may be helpful to reduce advance rates while maintaining the flow rate to facilitate cutting of the intended path. However, steering usually is not a problem in hard soils.