Civil Engineering Reference
In-Depth Information
To a major degree the longitudinal profile of the tunnel is dictated by the
condition of the soil at the location of the Westerschelde. More about the
geotechnical circumstances can be found in
Chapter 7.
Further, the vertical alignment of the tunnel was determined by the follow-
ing limiting conditions:
- the maximum permissible gradient of 4.5% with its associated maximum
and minimum radii of respectively 10,000 metres (crest vertical curve)
and 2,500 metres (sag vertical curve);
- the minimum gradient required of approximately 0.15% with regard to the
drainage of e.g. leak water and the necessary discharge capacity of the
drains for fuel leaks, in cases of disaster with a petrol tanker;
- the minimum soil covering required.
For the necessary safety measures against flotation and bursting of the tunnel,
a minimum covering of 10.5 metres was necessary. In connection with the
uncertain positioning of the river bed (current), an extra margin of 1.5 metres
was placed on top, whereby the minimum soil covering maintained amounts to
12 metres. The maximum depth positioning of the tunnel therefore lies at the
location of the Pas vanTerneuzen, at a depth of more than 60 metres below NAP.
Fig. 3.4
Representation of
alignment of the
tunnel
Pas van Terneuzen
Middelplaat
Everingen
1
26
2
25
3
24
4
23
10
11
12
13
14
15
22
16
5
9
17
21
18
19
20
8
6
7
The alignment as seen from the South
As seen from the south, the road declines from the dyke surrounding the
access ramp (at approximately 6.5 metres
NAP), with a maximum gradient
of 4.5%, to the deepest point under the Pas vanTerneuzen. Having the minimal
amount of required covering, the road surface is situated at approximately
57 metres
NAP, a height difference of more than 63 metres. Here the
tunnel, having its underside at approximately 60.5 metres
NAP, is situated
fully in the so-called Berg sands. The maximum water pressure under the Pas
van Terneuzen - the deepest point - amounts to about 6.5 bar. The alignment
then continues further with a minimal almost horizontal gradient. On the
southern side of Middelplaat, the road inclines with a gradient of approxi-
mately 3.5% to approximately 39 metres
NAP (top of road surface). Here the
tunnel lies in the middle of the clay stratum (Boom clay) which is present here.
Under Middelplaat the alignment declines again at a gradient of approximately
0.33%, more or less in accordance with the course of the top of the clay stra-
tum. In order to limit the depth positioning of the tunnel at the Everingen, the
choice was made not to follow the clay stratum as far as possible - which con-
tinues to get deeper towards the north - and then to go through the sandy soils
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