Overburden drilling apparatus having a down-the-hole hammer separatable from an outer casing/drill bit unit

An overburden drilling equipment for drilling a hole, comprising:

• a cylindrical casing defining a longitudinal axis and having a diametrically reduced structure forming at least one keyway extending longitudinally therethrough;
• a drill bit disposed in the casing and including a cutting face located at a longitudinal front end thereof, and an anvil portion located at a longitudinal rear portion thereof;
• a generally cylindrical casing shoe attached to the casing and mounted to the drill bit for longitudinal movement therewith, the drill bit being rotatable relative to the shoe;
• a down-the-hole hammer disposed in the casing, the hammer including a cylinder, and a piston reciprocable in the cylinder due to hydraulic fluid being directed alternately to upper and lower ends of the piston to effect reciprocation thereof in the cylinder, each downward stroke inflicting an impact blow upon the anvil portion of the drill bit which extends upwardly within the lower portion of the cylinder, the cylinder being rotatable relative to the casing;
• a drill chuck mounted at a lower end of the cylinder and receiving an upper portion of the drill bit, the drill chuck including at least one key defining a largest radius of the drill chuck and sized to pass longitudinally through the at least one keyway, the hammer and the drill chuck being separatable from an assembly comprised of the casing, the casing shoe, and the drill bit by rotating the cylinder to align the at least one key with the at least one keyway and raising the hammer.