A compression ring is a circular or elliptical caisson wall system that does not require steel reinforcing to achieve the desired soil retention. A large diameter rotary drill rig is used to install interlocking piles. Bonding between caissons is achieved by drilling into neighboring caissons (interlocking). Guide walls and precision layout are required to maintain accurate interlocks and overall circular shape. Compression ring design uses the compressive strength of concrete while eliminating the requirement for tension reinforcing because the circular / elliptical shape resolves soil retention forces in the shoring wall to compression only.
The compression ring can be a cost-effective solution for applications that allow the circular / elliptical shape because it does not require steel reinforcing. Compression rings can be used below the water table and can be used to support hydrostatic pressure and seal an excavation to bedrock. Whereas a rectangular shoring system would need tension reinforcing (steel piles) and intermediate support (tiebacks or struts / walers), a compression ring provides an unobstructed opening for the construction work to take place inside the shaft.
Compression rings are typically used for tunneling and manhole applications. Tunnel boring machines can be hoisted and launched through the unreinforced caissons or in the rock below with no shoring design implications or use of fiberglass reinforcement. Manholes can be installed and connected without interference from intermediate shoring support elements.