Definition of overburden drilling. i. A technique developed in Sweden that involves the sinking, by percussive-rotary drilling, of a drill casing through the overburden to where it seats in the underlying rock. A rotary percussion drill hole is then continued to the desired depth in the rock. With overburden drilling boreholes, the casing (outer pipe) and drill pipes (inner pipe) are driven either in a ‘rotating – rotating’ or ‘rotating – hammering’ manner, depending on the system in use. The flushing medium is pressed through the drill pipes (inner pipe) and is forced back out again at the mouth of the borehole or at the discharge head, along with the loosened drill cuttings, through the annular gap between the drill pipes (inner pipe) and the casing (outer pipe).