Symmetric Overburden Casing System
Symmetric is a system comprised of an ingeniously simple concentric method of drilling through overburden with casing.  Symmetric can drill straight holes at any angle (including horizontal) and to depths beyond 100 meters. Working together as one efficient drill bit, the components are made up of:
A pilot bit with large internal flushing holes and external flushing grooves.
Symmetrical ring bit (reamer) with internal bayonet coupling
Casing shoe for driving of casing.
The pilot bit is attached to any common DTH hammer shank or top hammer rod thread.
The casing shoe is welded to the casing. The pilot bit and ring bit are locked together by the bayonet coupling. They drill a hole large enough to allow the casing to be pulled into the hole. The pilot bit and ring bit rotate with the drill string while the casing shoe and casing do not rotate. When required depth is reached, the pilot bit is unlocked by a slight reverse rotation of the drill string. The drill string and pilot bit will be retrieved through the casing, the ring bits and casing shoe will be left in the hole for permanent casing system. Also the ring bits and casing can be retrievable through the special design for ring bits and casing shoe. We can manufacture both permanent Symmetric and Retrievable Symmetric system which can meet your comprehensive requirement.
There are two types Concentric casing system, one is Permanent type, the other is Retrievable type.

Operation Procedure kindly check below: 

Permanent

1. When drilling starts, the pilot bit drive the ring bit down to the hole, followed the casing shoe and casing tube.

2. At the bedrock, start Reverse circulation of tools and pull the pilot bit from the drill hole leaving the ring bit in the hole.

3. Pour the concrete or do the next constructions work.

Retrievable

1.After the drilling starts, the pilot bit drive the ring bit down to the

hole, followed the casing shoe and casing tube.

2. At the bedrock, start Reverse circulation of tools and pull the pilot

bit back from the drill hole leaving the ring bit in the drill hole.

3. Pull the casing tube out with the ring bit and at the same time pour the concrete.