Patent ID: 7185717

Claim:
An apparatus, comprising: a casing shoe adapted for being substantially rigidly connected to a leading end of a casing in a substantially coaxial arrangement, with the leading end of the casing received within a trailing end of the casing shoe; a ring bit mechanically retained by the casing shoe in a substantially coaxial arrangement with the trailing end of the ring bit received within a leading end of the casing shoe, and rotatable relative to the casing shoe, the ring bit being adapted at its leading end for drilling a peripheral portion of a hole in a ground formation with a hole diameter sufficiently large to accommodate the casing shoe; and a center bit adapted at a leading end thereof for drilling a central portion of the hole in a ground formation, and adapted at a trailing end thereof for being rotated and percussively driven to drill the hole, wherein: the center bit and ring bit are adapted for engaging one another so that rotating and percussively driving the center bit also rotates and percussively drives the ring bit; the center bit and ring bit are adapted for engaging one another so that withdrawing the center bit from the hole also withdraws from the hole the ring bit and the casing shoe; the center bit and ring bit are adapted for enabling disengagement of the center bit from the ring bit and withdrawal of the center bit from the ring bit and the casing shoe; and an outer circumferential surface of the ring bit and an inner circumferential surface of the casing shoe are provided with mating reverse-angle shoulders for transferring to a casing via the casing shoe a percussive impact applied to the ring bit, and for urging the casing shoe radially inward when a percussive impact is applied to the ring bit.