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INTERNAL INFORMATION
Northern Extension of Northolt Tunnel SIFT Report
Northern Extension of Northolt Tunnel SIFT
Report C222-ATK-DS-REP-020-000034
Revision – P06
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Uncontrolled when printed
6.2.8
The design of the “gap” structure would require the inclusion of an access road from rail
level to the surface rescue area. Given the depth of excavation, and the need to provide
access from both portal structures, it would be anticipated that such a route would
emerge from it centrally and approximately normal to the mainlines.
6.2.9
The form of the excavation would require either a retained cutting, an open cut with
engineered side slopes through the chalk and groundwater control, or a combination of
the two. For a retained cutting, the width of the excavation would preclude
conventional propping and it is expected that ground anchorages would need to be
utilised. It should be noted that the groundwater levels are currently assumed to be at a
depth of about 5m below existing ground level, so the influence of water during
construction and operation could be a significant constraint depending upon ground and
flow characteristics. Permanent de-watering may be required.
6.2.10
Consideration should also be given to the probable negative impact of a major
excavation on the principal chalk aquifer which is designated as Source Protection Zones
(SPZ) 1 and 2 by the Environment Agency.
6.3 Provision for Heathrow Spurs - General
6.3.1
In respect to making an allowance for the Heathrow spurs there are three locations
where the tunnel would need to include turnouts in order to access Heathrow Airport as
shown on the plan in Figure 2.1, namely:-
Turnout 1 (a and b) - on the mainline immediately east of the existing
Chiltern tunnel,
Turnout 2 (a and b) - on the mainline immediately west of the Northolt
tunnel, and
Turnout 3 - where the foregoing lines meet in turnout and crossover to
continue towards Heathrow Airport. Note that Turnout T3 is not
designed or costed for this report.
6.3.2
Section 6.2 identifies the connection details which are related to the requirements for a
gap structure that would be located at Turnout T1.
6.3.3
The principal requirement for the turnouts is that they are included on straight sections
of track. As such they dictate the horizontal and vertical alignment of the spur lines
(from Birmingham at Turnout T1 and to London at Turnout T2) and whether they pass
below or over the HS2 mainline. In both cases, the required vertical separation between
the main and spur tunnels is assumed to be equivalent to one tunnel diameter. In the
approach to the Turnout 3 the four lines are required to merge to form a twin track
railway, by the inclusion of both turnouts and crossover.
6.3.4
It has to be recognised that high speed trains require matching high speed turnouts, and
for these high speed turnouts the rate of divergent separation of the tracks is required
to be at a ratio of 1:49, and the vertical alignment through the turnout is required to be
flat. Generally, to operate the very long and heavy switch blade there would need to be
multi-point motors and these would probably need to be accommodated within niches
in the tunnel / cavern, and heaters would be required.
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INTERNAL INFORMATION
Northern Extension of Northolt Tunnel SIFT Report
Northern Extension of Northolt Tunnel SIFT
Report C222-ATK-DS-REP-020-000034
Revision – P06
18
Uncontrolled when printed
6.3.5
When these requirements are combined with the need for the alignment of the spur to
fall / rise and arc beneath / above the mainline, a large span space may be required to
accommodate the overall configuration. This space would be provided by a cavern
excavated within the chalk, the dimensions of which may be significant with design and
construction influenced significantly by the presence of groundwater.
6.3.6
For a fully tunnelled option (Option B) Turnout T2 would be located in a cavern at
significant depth to suit the mainline rail alignment. It is expected that this cavern
would be mechanically excavated and lined with a sprayed concrete lining. Two options
are generally possible for the size and shape of this cavern:-
To have one very large cavern, which would accommodate:
o
Entry of the two 8.8m ID bored tunnels;
o
Turnouts which diverge to both directions; and
o
Exit points for four 8.8m ID bored tunnels with a specified minimum
separation.
To have two smaller parallel caverns which, noting the 21m separation of the
mainline tracks, should be achievable with a chalk rib of some 10m width between
them. The rib condition and size would need to be assessed in detail to ensure that
conditions of stability can be created, noting that the cavern would not be
excavated by the tunnel boring machine but by “normal” excavators in a staged
sequence.
6.3.7
The principal construction issue would probably be related to groundwater which is
currently identified to be at a level of about +40mOD consistent with the surface level of
the lakes. Construction of this cavern would be significantly at risk of any poor ground
conditions at this location.
6.3.8
A further key concern with the cavern construction is that although relatively deep (40-
50m to rail level) the cavern works would be in close proximity / underneath the
pharmaceutical research facility with particular concerns over vibration impacts.
6.3.9
In order to minimise dimensions for the single cavern option, it would be required to
optimise alignments by bringing the mainline tracks closer together. Even with this
adjustment, such an option is unlikely to be feasible as for a minimum separation of
0.5xDiameter between tunnels, the cavern width would be in excess of 50m at the wider
end.
6.3.10
For the twin parallel cavern option, T2 would require to include cavern structures of a
height of 14m, a span of greater than 20m and a length of some 300m to accommodate
the turnout. After this point the tunnels are separated by at least one tunnel diameter
vertically as the alignment climbs over the mainlines as illustrated by the sections
included as Drawing No. C222-ATK-RT-DSK-020-011305.
6.3.11
In the case of Turnout T1 the Option B issues are discussed in Section 6.2.