pressure balance shield is another important type of tunneling machine.
It is normally applied for varying soil compositions and is excellent
for smaller work sites. Within shield, the cutter excavates the site
through rotary motion. In the earth-pressure balance shield, the
excavated soil balances the excavating front. All excavation takes place
within a steel ring called the cutter bit. In the excavation chamber,
the excavated soil is mixed with a conditioning agent.
An excavated soil
is carried to a conveyor belt through a screw conveyor. From here soil
is taken to the slurryfying box, where it is mixed with water and is
taken out of the tunnel by the dredging pumps.
The excavated soil is mixed with additives and kneaded forcibly with
the kneading blades to improve its plastic fluidity and impermeability.
In the cutter chamber the pressure of the excavated soil is increased to
counterbalance the tunnel face earth pressure and the groundwater
pressure. Tunnel face can be sustained by muddy soil pressure to
minimize ground settlement.
The major applications of earth pressure balance shields are :
- For laying pipes for smaller areas.
- Used for laying cables to be used for various purposes.
- Excavation for drainage and municipal purposes.