For smaller bores in soil, the slant-face spade bit is most commonly used as indicated in the general method description. For larger and deeper bores and in rock and hard soils, a different drilling and steering mechanism is used. In this system, the drilling mud is pumped to the drill head under pressure where it is used to rotate the drill bit via a mud motor. In addition, the section of drill pipe behind the drill bit is deviated by a few degrees which will cause the borehole to progressively deviate if the drill bit is advanced without rotation of the drill string. If the drill string is rotated while the drill bit is advanced, the borehole will advance in an approximately straight direction. Combining straight drilling with curved sections allows a preplanned borehole with curved sections to be followed. Other variations of drilling mechanism also are possible. These include drilling using air for cuttings removal and the addition of hammering methods to improve rock cutting ability.