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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
33
Uncontrolled when printed
6.9
Option A (Proposed Scheme) – Hybrid Bill
6.9.1
The baseline case is the Proposed Scheme as presented in the Environmental Statement
and shown on Hybrid Bill drawings.
6.9.2
The viaduct crosses the Colne Valley, including the River Colne, the Grand Union Canal
and number of lakes, between Ch 26+000m to Ch 29+400m and its height is generally
between 11m and 15m above the ground/standing water level.
6.9.3
In summary the viaduct:-
Crosses a stream at Ch 26+200m and Flood area Ch. 26+150m to 26+250m;
Crosses above Dews Farm locally listed building at 26+350m;
Passes over the Hillingdon Outdoor Activities Centre Ch 26+400m to Ch.
26+500m;
Passes under the Overhead line at Ch. 26+600m approx.;
Crosses access track at Ch. 26+950m approx.;
Crosses Grand Union Canal and towpath/walkway Ch. 26+970m;
Crosses Moorhall Road at Ch. 27+500m;
Crosses access track to Moorhall Road at Ch. 27+900m;
Crosses the Colne River Ch 27+950m approx. to Ch. 28+180m approx.;
Crosses an access path to a building at Ch. 28+975m;
Passes under the fight path of Denham airport, and
Crosses the North Orbital Road (A412) on a high skew angle at Ch 29+200m.
6.9.4
The constraints to the proposed Colne Valley viaduct are:-
The Viaduct location in an environmentally sensitive area which is recovering
from past gravel
quarrying activities;
The Viaduct bridges over the mid Colne Valley (SSSI) landscape and a number
of other features, from Durden Court to Dew’s dell;
Construction and maintenance access would not be straightforward due to
the topography of the site;
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Accepted
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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
34
Uncontrolled when printed
The overhead power lines at Ch 26+600m would provide constraints for any
craneage and construction plant required to work beneath until they are
diverted. It is crucial that this line is diverted early to enable the trestleway
jetty to be fully constructed;
The location of the pier south of the A412 which is constrained by the
presence of a high pressure gas main; and
The viaduct is partly constructed in a groundwater Source Protection Zone
(SPZ) 1 and piling works would need to take appropriate measures to avoid
groundwater contamination.
6.10 Option B – Tunnelled Option with Passive Heathrow Spur
Provision
6.10.1
As stated in Section 6.2, the positions available at which the 700m long excavation for a
“gap” structure and portal hoods can be located are limited, and as such it is proposed
that the existing southern works area be used. This means that the southern limb of the
tunnel would slightly exceed the 20km length, and detailed appraisal would be required
to validate this assumption. The position of the gap can be located in the flatter ground
of the proposed southern works area, outside the SSSI, with the depth of excavation
limited to approximately 20m.
6.10.2
This proposal has the additional benefit of being in a position which would allow access
to public roads for construction and operational purposes, allow routes to track level to
be developed without impinging on the currently designated hybrid Bill limits, provide
an option for launching the TBMs in both directions, and provide a route for servicing
both the mined and bored tunnelling operations.
6.10.3
For Options B and C, the Chiltern tunnel south portal would be further away from the
M25 (as opposed to Option A). Hence there would be a lower risk of excess settlement
of the motorway due to “learning curve” difficulties at the start of the Chiltern tunnel
TBM drive.
6.10.4
The Heathrow spur Turnout T1 would be accommodated within a tapered “gap”
structure, the dimensions of which would be defined by the track spacing. As indicated
in Section 6.2 the maximum width of the excavation could be some 70m
accommodating the two mainline tunnels and the two spur tunnels and the access to
the rescue area at existing ground level. However, it is expected that the separation
could possibly be reduced slightly and the overall structural headwall width reduced
accordingly.
6.10.5
The depth of excavation would depend in part upon whether the spur passes over or
under the mainline, being locally shallower by some 5m if the spur passes below as
shown in Drawing No C222-ATK-RT-DSK-020-011306. It may be possible to optimise the
mainline alignment further, however such re-alignment may impact the low point, and
thus the position of the tunnel sump and the distance that accumulated water would
need to be pumped to the nearest shaft or gap.
6.10.6
The Heathrow spur Turnout T2 would be located at Ch 25+000m, significantly deeper
than the railhead turnout at Ch 24+300m. It is therefore likely that the turnout would be