Earth Pressure Balance (EPB) Machines
These represent the majority of TBM’s used in China and also the majority of machines made by CREG.
As a closed face machine, support is provided to the excavated face, so that ground settlement is controlled. But this support is maintained by retaining material in the face, conditioning it so that it can perform the required function, and only allowing it to discharge in a controlled manner through the screw conveyor.
To achieve this successfully certain key components of the machine and the process have to be carefully designed and tailored to suit the anticipated conditions. Several examples are illustrated in project information
The first and probably the most important is the Cutterhead. It has to be able to excavate the ground whilst supporting it. Material must flow through it easily but still be able to be controlled in the screw conveyor.
The very wide range of grounds in China has enabled CREG to gain experience in designing cutterhead of varying configurations.
Each machine is designed to suit the anticipated ground.
Other features which are carefully considered during the design of the machine are the screw conveyor, ground conditioning system and the grouting system
The design of the screw conveyor is carefully considered, bearing in mind the type of ground, its permeability and the size of cobbles that must be handled. Both centre shaft and ribbon screw conveyors have been used.
Retraction for maintenance is important as is the design of the discharge gate.
A double discharge gate is normally fitted , although different designs have been incorporated where necessary.
Ground Conditioning Systems
These are another essential component of efficient tunnelling in Earth Pressure Balance mode. To ensure that the excavated material flows readily through the TBM and screw conveyor, but can be controlled without loss of water.
To achieve this both Foam systems and Bentonite systems are usually incorporated on to the back up behind the TBM.
In the foam systems, each injection point on the cutterhead is served by an independent supply , each with a separate injection pump, so that the process can be fully controlled.
Testing of the materials on site is important to determine the optimum type of injection, and facilities for both bentonite and polymer can be incorporated.
Single component grouting using Schwing piston pumps
Two component grouting using peristaltic pumps can be incorporated into the TBM design.
In this case auromatic flushing of the tailskin pipes is incorporated based on the TAC sytem of embedded tailskin pipes.