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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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constructed as a mined cavern either one large cavern or alternatively two narrower
parallel caverns (see Section 6.3).
6.10.7
Alternatively, the Heathrow spur Turnout T2 could be optimised and combined with the
railhead turnout at Ch 24+300m by extending the Heathrow spurs parallel with the
mainline for approximately 1km to the east. This would eliminate the need for two
chambers within 1km distance, which would significantly reduce the construction cost
and reduce programme time. The site at which the railhead turnout is located is large
enough to accommodate the extra chamber width (approximately 50m); however the
mainline would require horizontal realignment as high speed turnouts must be on a
straight section of tracks (Paragraph 5.5.1). This could affect the horizontal alignment
several hundred metres away in each direction from the chamber and in particular could
have significant impacts on the residential area at West Ruislip to the south of the
mainline (Ch 23+800m). As noted earlier, this variant has not been considered further.
6.11 Option C – Tunnelled Option without Passive Heathrow
Spur Provision
6.11.1
Option C considers the scheme if no passive provision for Heathrow spurs is made – that
is, that the scheme would not allow for construction of Heathrow spurs in the future.
6.11.2
Option C in general is identical to Option B: Locations of ventilation shafts, the railhead,
and the “gap” structure remain the same.
6.11.3
Without Heathrow spurs present, there is no need to provide Turnouts. Therefore, T2
would be eliminated completely, and there would be the possibility to reduce the
dimensions of the “gap” structure as the width would not need to account for four
tunnel bores. This could significantly reduce the cutting in the area of the “gap”, as the
width could be reduced by as much as 40m at the base.
6.11.4
If no passive allowance for Heathrow spurs is made, the alignment may be raised locally
to the T2 area, allowing easier connection to the railhead at Harvil Road, and the
possibility of optimising the tunnel vertical alignment towards Ruislip
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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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7 Comparison of Options
7.1
Construction Costs
7.1.1
The additional construction costs of Options B and C compared to the Proposed Scheme
have been estimated as follows:-
Option B additional cost: £215.0 million
Option C additional cost: £185.1 million
Note: Costs are inclusive of all works required between the Proposed Scheme West Ruislip portal and
the Chiltern Tunnel south portal.
Table 7.1: Colne Valley Tunnel Costs
7.1.2
In addition to the tunnel works, these estimates include all associated works required
for construction, including tunnel portals, highway diversions, retaining walls, temporary
railhead connection, and ventilation-intervention shafts. The costs include extended
time costs, indirect costs, and efficiency and value engineering savings.
Item
Option A
Proposed Scheme
(inc H’row spur
passive provision)
£m
Option B (Tunnel
inc H’row Spur
Passive Provison)
£m
Option A (exc
cost of H’row
spur passive
provision) £m
Option C (Tunnel
exc H’row spur
Passive
Provision) £m
Tunnels
£63.6
£523.3
£63.6
£487.9
Civil Engineering
£413.1
£103.7
£388.1
£88.8
Railway Systems
£111.1
£159.7
£111.1
£157.0
Sub Total:
Construction
£587.8
£786.7
£562.8
£733.7
Difference
£198.9
£170.9
Time related costs;
indirect costs; VE
and efficiency
savings
£16.1
£14.2
Total cost
difference with
Proposed Scheme
£215.0
£185.1