Civil Engineering Reference
In-Depth Information
The ideal material for EPBMs is a fine grained ('cohesive') soil with stiff
to soft consistency (I
C
= 0.5-0.75, consistency index is described in section
2.3.3), which extrudes through the openings of the cutterhead towards the
screw conveyor. If the excavated material does not conform to this ideal,
it must then be 'conditioned', i.e. artificially altered. This is important as
an optimally conditioned material helps maintain pressure in the plenum
and hence transmit the pressure to the face to maintain stability. It also
helps to control the material in the screw which also allows improved control
of the face pressure and settlement control (BTS/ICE 2005). EPB technology
has made significant progress over the last couple of decades. This is
particularly so in the area of ground conditioning which has enabled EPBMs
to be used in coarser grained materials.
Soil can be conditioned using:
•
water;
•
bentonite, clay or polymer suspension;
•
foam (surfactants mixed with water and compressed air);
•
foam with polymer (the polymer helps stabilize the foam).
These conditioning agents are injected via ports mounted across the face
of the cutterhead. There are also often facilities for the conditioning agent
to be injected into the plenum and even into the screw conveyor casing.
However, the most effective injection point is through the rotating cutter-
head as this ensures that the conditioner is mixed directly with the excavated
material. Trial tests to determine the correct conditioning regime are
essential. In granular soils the conditioning agent must create a stable plastic
consistency in the excavated material that will not degrade until the material
has been discharged from the screw conveyor or possibly until it reaches
the surface stockpile as it is easier to transport the material in this state
(BTS/ICE 2005). The conditioning for soft cohesive soils will generally
prevent the material forming into lumps and will therefore assist its flow
through the plenum and the screw conveyor.
Slump tests, as used for concrete testing, are used to determine the
appropriate conditioning of the excavated soil. An optimum slump for
EPBMs is in the range 5 to 15 cm. (BTS/ICE 2005)
EXAMPLES OF EPB MACHINES
Figure 5.31 shows an example of a 9.16 m diameter EPBM used for
constructing the Heathrow Airside Road Tunnel, UK.
Figure 5.32 shows an example of an EPB machine used as part of the
Channel Tunnel Rail Link (CTRL) project in the UK, which provides a
high speed rail line between London and the Channel Tunnel. Two similar
machines each measuring 8.16 m in diameter were used to construct two
parallel tunnels 4.7 km long through Thanet Sand. The internal diameter
of each tunnel on completion was 7.15 m. The cutterheads on these
