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TUNNEL BORING MACHINE (TBM)
A Tunnel Boring Machine (TBM) is a machine used to excavate tunnels with a circular cross section through a variety of soil and rock strata. TBMs have the advantages of limiting the disturbance to the surrounding ground and producing a smooth tunnel wall. TBMs can be categorised into two main groups, open face TBMs and closed face TBMs. Open face TBMs are the machines without a system for pressure regulation at the tunnel face. Open face TBMs are usually used in stable ground conditions where stability of the ground or groundwater pressure is not an issue. Three main types of open face TBMs are: Gripper TBMs, Single Shield TBMs and Double Shield TBMs. Closed face TBMs are the machines with a system for pressure regulation at tunnel face. This is to control the ground and groundwater at the tunnel face and provide stability. Closed face TBMs are usually used in unstable ground with high groundwater pressure. These types of TBMs are common in construction of tunnels in soft ground in urban areas. Two main types of closed face TBMs are: Earth Pressure Balance (EPB) TBMs and Slurry TBMs.
Conventional Tunnelling is the construction of underground spaces of any shape with a cyclic construction process of:
- excavation by drill and blast methods or basic mechanical excavators
- ground support
Conventional Tunnelling is carried out in a cyclic execution process of repeated steps of excavation followed by the application of relevant ground support. The term Conventional Tunnelling could be applied to any tunnel that is not excavated by a Tunnel Boring Machine (TBM).
As an alternative to conventional tunnelling, tunnels can be bored by special purpose tunnelling machines which are in principle more automated and complex than the mechanised tools used in conventional tunnelling. The best example of mechanised tunnelling is the use of Tunnel Boring Machine (TBM) for excavation and support of a tunnel. Mechanised tunnelling is not limited to TBM but TBM is the most common form of mechanised tunnelling.
CONSTRUCTION OF SHAFTS
There are different methodologies for construction of shafts depending on the ground and groundwater condition. In tunnelling projects shafts are often used for launching the tunnel excavation and are usually on the critical path of the project. Shafts can be constructed to provide ventilation, escape route or other types of services for metros, railways, highways and other mass transit tunnels. Two of the most important elements in design and construction of shafts are the method of excavation and the ground support technique.
Gripper TBMs are used in stable rock conditions with low water ingress. The Gripper machine locks itself laterally against the tunnel walls using gripper shoes to establish the required face pressure. When the machine is locked, the tunnel is advanced by using hydraulic jacks which push the cutterhead forward.
SINGLE SHIELD TBM
Single Shield TBMs are usually used in unstable ground conditions. Installation of segmental tunnel lining is an essential stage in this type of TBM. The pushing forces are maintained axially against the installed lining segments. During the installation of segments the tunnel cannot progress in Single Shield TBMs.
DOUBLE SHIELD TBM
A Double Shield TBM combines features of the gripper and single shield machines. In fact the machine has a front shield with a cutterhead as well as a gripper section with gripper shoes. Both parts of the machine are connected by a section called the telescopic shield. When the ground condition is good the grippers can be used and installation of segments and excavation can be done at the same time resulting in higher progress rate. When the ground is not stable and grippers cannot be used, it can be used like a single shield machine. In single mode condition the pushing force will be maintained against installed segments again. In single mode condition the excavation should be stopped during the installation of segments and this results in slower progress rate.
EARTH PRESSURE BALANCE (EPB) TBM
EPB machines are TBMs used for the excavation of soils where face support and counter-effect of groundwater pressure is obtained by the excavated material itself. The special feature of Earth Pressure Balance machines is that they use the excavated soil directly as support medium. In cohesive soils with high clay and silt contents and low permeability this method is the preferred option. Excavated material is removed from the excavation chamber by a screw conveyor which allows a gradual reduction of pressure.
Slurry TBM provide face support by pressurising slurry in excavation chamber to balance soil and water pressure on working face. The excavated muck is transported by a slurry circulation system. These machines are characterised by stabilising working face in sandy to gravely ground with precise settlement control.
VARIABLE DENSITY TBM
Variable Density TBM combines the advantages of EPB TBM and slurry TBM. This machine can switch between four tunnelling modes in the tunnel without significant mechanical modification.
This means that geological and hydrogeological changes along the alignment can be managed with extreme flexibility.
The change between the modes can be done gradually under permanent and total control of the tunnel face pressure without needing chamber interventions. This machine can also be operated using a high density in the excavation chamber that would be too dense for a classic slurry operation but too fluid for a traditional EPB operation.