Civil Engineering Reference
In-Depth Information
Introduction
The shield of a tunnel boring machine (TBM) consists of a steel cylinder
with a front part, a middle section and a tail section. The front part con-
sists of the pressure section with the cutting wheel, the excavation cham-
ber and the working chamber. The partition between the excavating and
working chamber, which do have an open connection to each other, is
formed by the submerged wall.
The front part of the TBM is sealed off from the tail section by a steel par-
tition, the pressure wall. The cylinder has an open structure at the tail sec-
tion (or tail seal) which is where the lining is constructed from segments.
The soil and water pressures of the environment are present on the
outside of the shield and pressure wall. Inside the TBM and the tunnel
are normal atmospheric conditions. Due to the great depth (maximum
of 60 m), the shield and the pressure wall of the Westerschelde Tunnel
TBMs had to be designed for a maximum pressure difference of 8.5 bars,
including a safety margin. In order to construct the Westerschelde Tunnel,
two boring machines were built which were named after two sisters who
had previously owned the land where the boring was started: Sara (the
eastern TBM) and Neeltje Suzanna (the western TBM).
Choice of type of boring shield
In principle two types of shield are suitable for boring into the soil of the
Netherlands: the slurry shield and the Earth Pressure Balanced shield (EPB).
In addition, it is also possible to use a mixed shield boring machine. A mixed
shield can be converted and can be used either as a slurry shield or as an
EPB shield.
For the boring of the Westerschelde Tunnel, after a thorough consideration
of all the advantages and disadvantages of the various methods, the choice
was made to use the slurry shield method. An EPB shield as such, is very
suitable for boring in clay, but at the same time it is less suitable for excav-
ating sand - judging by the technology available at the time the tunnel was
designed. In addition, only a few EPB machines were built with such a large
diameter (more than 11.30 metres). Furthermore, EPB shields had never been
used to bore at such great depths and the corresponding soil and water
pressures which were involved when constructing the Westerschelde
Tunnel.
The main reason for not using the mixed shield was the loss of time that
would be entailed in converting a slurry shield into an EPB shield.
Decisive factors to be considered in making the choice are as follows:
- the slurry shield method enables the high soil and water pressures to be
better controlled;
- the excavation chamber of a slurry shield machine is accessible to divers
who have to carry out inspections and repairs;
- the risk of wear and tear is less when using a slurry shield than when
using an EPB machine;
- the power to be installed is considerably higher in an EPB machine;
- by choosing one type of machine and not a convertible mixed shield
machine, it is possible to optimize the cutting wheel, the excavation
chamber and other parts of the shield to prevent clay sticking to the
machine. Compromises could be involved in the case of a convertible
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