Civil Engineering Reference
In-Depth Information
Fig. 2.7
Reference design of
bored tunnel
(distances/dimensions
in millimetres)
12,980
13,000
12,980
Cable
gallery
Escape
corridor
150
10 0
150
3,500
3,500
Cross connection - 500 m
of the Westerschelde moves and therefore a minimum value had to be
assumed for the design-life span-duration of 100 years. This deepest posi-
tion (the so-called 'lowest enclosed') is determined on the basis of meas-
urements during the period 1955 to 1990. Considering the fact that the
position of the bedding was 'managed' by maintenance dredging over the
past decades, it is highly unlikely that in the future larger fluctuations will
occur in the bedding position. It was therefore justified to take this period of
35 years as a point of reference. In order not to be confronted with unpleas-
ant surprises, a safety margin of 2 metres was maintained. Together with
a maximum slope of 4.5%, this led to a maximum depth of more than
50 metres (crest level of the tunnel) under the average water level located
at the Pas vanTerneuzen and 35 metres below Everingen.
In order to keep the depth of the starting and receiving shafts as shallow as
possible for the required bed covering, use could be made of surrounding
dykes that would be constructed. With this the length of the tunnel was also
determined at 6,474 metres.
Commencement of tendering procedure
In 1991 the Province of Zeeland requested companies to participate in the
designing and tendering for the crossing, including access roads. Of the six
parties who reacted, two did not comply with the conditions set, the remaining
four were invited to submit their design and tender. Two parties responded to
the invitation: Kombinatie Middelplaat Westerschelde (KMW) and the
Konsortium Westerschelde Oeververbinding (KWOV). They started with their
designs in the spring of 1992.The fundamental details and the reference limit-
ing conditions were used as a guide for this purpose; the reference design
'bored tunnel' had been completed so late, that this was not issued to the con-
tractors so as not to frustrate their design processes. At the end of 1992 it
appeared that they had both set their sights on a bored tunnel under the
Westerschelde across the entire width, in which in a general sense, the designs
by KMW and KWOV were comparable. Aside from that, KMW came with an
alternative plan which consisted of two bored tunnels under the fairways with
a dam in-between on Middelplaat.The three designs were evaluated and com-
pared on the basis of a so-called multi-criteria analysis. Eventually the main
design by KMW came up trumps because of the best price/quality ratio.
Original KMW design
The original design by KMW provided for a bored tunnel which consisted
of twin tubes with an outer diameter of 12.30 metres and a thickness of the
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