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uppermost - extended - steel segment. The opening is 1.5 metres wide and
2.9 metres high. Due to the over-height in respect of the structure gauge dif-
ferences could be taken up as a result of cross super-elevation, boring toler-
ance and circumferential rotation. The lowest steel segment is shorter than
a normal concrete segment and for practical and economical reasons it is
also made of steel; if it were made of concrete that would have meant that
a separate formwork casing would have to be made for only 52 elements.
Steel connecting beams below and above the door opening provide stiffen-
ing, the taking up of connection forces between the bored tunnel rings and
to restrict the deformations during the construction phase.
Fig. 13.4
Steel segment
Construction concept 'NATM'
The construction method chosen for the construction of the cross connec-
tions is also known as the 'New Austrian Tunnelling Method (NATM)', which
is always applied in stable rocky soil. In this method the soil - in the case of
the WesterscheldeTunnel, is not in stable soil but within the protection of the
circular-shaped frozen soil - excavation is phased both crosswise as well as
in the longitudinal direction. Simultaneously the perimeter of the excavation
is provided with a layer of reinforced shotcrete.This layer forms the outer lin-
ing of the actual cross connection and when the concrete has hardened, it
takes over the water-retaining and bearing function of the frozen soil. After
the entire excavation is complete, reinforced concrete is applied as the inner
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