There are many types of overburden drillings systems, from the DR System, Symetrex System, Atlas Copco and Odex Drilling System.
The Odex system is widely used and can be used with either sacrificial casing or casing which is retrievable and can be reused. It is a piling mechanism suitable for drilling through rough and sub-terrain formed objects such as boulder, shale and rock. It can be used to simultaneously drill through overburden/soil and rock, flush out unwanted material and bring a steel casing with the drill down the hole.
Once drilling starts, the Odex eccentric bit moves out creating a hole with a diameter greater than that of the steel casing. This allows the steel casing to traverse down behind the drill bit without having to first remove the drill bit to insert the casing.
If the pile is required to be planted into the bedrock, the drill bit has the capability to drill and remove the rock. Once desired depth has been reached, the eccentric bit moves back in allowing the drill to be removed back up through the casing.
Casing tubes that are to be permanently fitted are then usually sealed with a cement grout. The borehole is then filled with concrete/grout and reinforcement is placed.