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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
5.3
Environmental Constraints
5.3.1
The proposed viaduct would cross through the Colne Valley, including the Colne Valley
Regional Park, over the Grand Union Canal, the Mid Colne Valley SSSI, the River Colne
and a number of lakes whereas the proposed tunnel alternatives would pass under all
these areas.
5.3.2
The landscape in the Colne Valley is defined by the floodplain of the river and is largely
agricultural, but the overwhelming influences are the lakes left from mineral workings.
Further development has since occurred, and as a result there are some new structures
and buildings in the area, particularly in the area around the possible tunnel Shaft F.
5.3.3
There is one Ancient Woodland within 50m of the HS2 centreline. There is one BAP
Habitat within 50m of the centreline. The site is located within an SSSI and a further
three SSSIs are within 2km of the centreline. The Colne Valley viaduct is partially within
Source Protection Zones (SPZ) 1 and 2 and the proposed tunnel options would similarly
pass through the same extent of the SPZ.
5.3.4
For further environmental details refer to Section 9.
5.4
Land and Property Constraints
5.4.1
Options B and C would require substantially different limits of land to the hybrid Bill
limits. At this stage there are no known constraints that would prevent land acquisition
and use under each of the options. Overall, a tunnel under the Colne Valley is likely to
require less land-take, both temporary and permanent, than a viaduct solution. In
particular, the Hillingdon Outdoor Activities Centre (HOAC) would not be affected.
5.5
Engineering Construction
5.5.1
In the broadest sense, the alternative proposal would entail the linkage of the two
currently separate 8.8m internal diameter tunnels (the Chiltern and Northolt tunnels) to
create a single tunnel, the overall length of which would be some 34km (excluding the
requirements for the Heathrow spur). It should be noted that apart from the Channel
Tunnel which is about 50km in length, such a length of tunnel would be unprecedented
in the UK mainland for a transport facility and would be about the 7th longest rail tunnel
in the world by the time it is constructed.
5.5.2
In order to accommodate the turnouts for the Heathrow spur, the form of tunnel
construction would necessarily include for large span (>20m) cavern structures and a
deep 700m long “gap” structure. Assuming that the ground around the caverns can be
dewatered, they would reasonably be expected to be mined with a sprayed concrete
temporary and permanent lining and with a waterproof membrane. The remaining
length of the spur tunnels from the turnout would also be expected to be mined, in part
at least, depending upon its overall alignment, length and the overall construction
logistics for the whole tunnel and be waterproofed.
5.5.3
The inclusion of a 700m long trough structure, located in the area currently identified as
the southern works area serving the Chiltern tunnel would aid tunnel construction. It
would allow a point from which pairs of tunnel boring machines could be launched to
both east and west, segments could be supplied, spoil managed and the system
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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
14
Uncontrolled when printed
serviced. Such a gap would also reduce the length of continuous tunnels to less than
20km as recommended by the TSI regulations, and thus keep similar fire and safety
requirements as for the Proposed Scheme.
5.6
Rail Systems Construction
5.6.1
An extension of the Northolt Tunnel would create a tunnel of 34.6km in length. A tunnel
of this length would create a number of rail systems issues that would need further
investigation for resolution should either Option B or C be adopted.
5.6.2
The areas of the rail system that would be impacted by the extension of the tunnel
would be:
Tunnel Ventilation and Smoke Control.
Traction Power
requirements
Journey Time
Aerodynamics
Rail safety
Vent Shaft Spacing
Maintenance
5.6.3
These areas and the issues are discussed in more detail in Section 6.6 of this report.