Bulroc has developed a range of "Concentric" Overburden Drilling Systems for the continuous casing of medium to large diameter holes - up to 850mm diameter. The equipment gives excellent performance for drilling through loose overburden materials down into solid bedrock. At the start of the drilling operation, the segments are in the closed position to allow the system to pass through the lead casing and seat against the casing shoe. When drilling commences, the segments swing out to drill the hole to the desired diameter. The machines can be withdrawn through the casing by rotating the System anti-clockwise against the solid base of the hole or up against the casing shoe.