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6.7.6
Alternatively, TBMs could be launched from the Turnout T2 cavern at Ch 25+000m. This
would only be applicable to Option B, as the provision of a cavern is based on the
requirement for Heathrow spur turnouts. This would provide easier access for TBMs to
the site, by way of Harvil Road or Breakspear Road. However, the railhead connection
would be further away from the tunnel bores, creating a logistical challenge until
tunnelling reaches the railhead chamber at which point the TBMs can be serviced via the
railhead. This would also increase the length of the Northolt tunnel in comparison to the
previous option, which would impact on the programme.
6.7.7
For the 6.2km Northolt tunnel extension through the Colne Valley, it is proposed to
launch twin TBMs south from the gap structure at the Chiltern tunnel south portal. This
would provide a more convenient site for servicing the tunnel drives, with more space
available at the adjacent sites for plant and materials. Four TBMs, two driving north
constructing the Chiltern Tunnel, and two driving south constructing the tunnel through
the Colne Valley, would therefore need to be serviced via the gap structure. SSE Energy
Supply Ltd have confirmed that there would be capacity for a new 43MVA power supply
as required for servicing the tunnelling operations, subject to detailed design. However
the risk that other resources may require power supply in the region could have an
impact on future capacity, and as a result more expensive works may be required to
provide the power supply for the tunnel drives.
6.7.8
Tunnel logistics would be via public roads for both tunnels, until the tunnel under Colne
Valley reaches a point past Ch 24+200m (Shaft F) where it could be serviced by the
railhead. It is also noted that the tunnel extension of 6.2km could have a detrimental
impact on programme time.
6.7.9
As an alternative, a single TBM could be launched from the Chiltern tunnel south portal
and turned round within the turnout chamber at Shaft F to give some relief to the
worksite at the Chiltern tunnel south portal, before boring back to the gap structure. For
the return drive, logistics could be via the railhead connection. This would increase the
programme time for the 6.2km Northolt tunnel extension, which could impede its
viability.
6.7.10
As a further alternative, the tunnel under Colne Valley could be driven north from the
more space-constrained open cut site at Shaft F. However, co-ordination of the
tunnelling works with that of Northolt tunnel (driving south) could be a challenge, as
both tunnels would require use of the railhead connection to service tunnelling
operations subject to sufficient train paths being available. Removal of excavated
material from the two tunnels would therefore present a number of challenges as
described in Section 7.3 of the report.
6.7.11
For the purpose of this assessment, it is therefore proposed to drive the Northolt tunnel
south from Shaft F, which agrees with current methodology for the Proposed Scheme.
This would require TBMs to be launched in ‘semi-short’ mode from the 80m x 30m open
cut. The length of the Northolt tunnel would be slightly increased in comparison to the
Proposed Scheme (in which the TBMs are launched from West Ruislip portal at Ch
23+480m). Although the TBMs for the Northolt tunnel would be launched from the
chamber containing the railhead turnout at Ch 24+200m, most of the construction
support system logistics would be centred at the main compound between Harvil Road
and Breakspear Road. Temporary infrastructure and facility provision requirements
would include:-
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Northern Extension of Northolt Tunnel SIFT Report
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Precast segment storage areas;
General storage areas (pipes/rail sleepers etc.);
Grout batching plants;
Excavated material storage/handling/buffer areas;
Workshops and fabrication areas; and
Office space for staff/Project offices if appropriate.
6.7.12
It is proposed to construct the 6.2km Northolt Tunnel extension under the Colne Valley
from the location of the Proposed Scheme Chiltern Tunnel south portal, and service the
tunnelling operations via the gap structure until the TBMs reach a point past Ch
24+200m where the tunnelling would be serviced via the railhead.
6.8
Impact on Northolt Tunnel Construction - General
6.8.1
Combining the Northolt Tunnel with the Northolt tunnel Extension in the Colne Valley
would create several issues in terms of construction of the tunnel works. Current plans
for the Northolt tunnel construction site compounds would be affected by the possibility
of extension of the tunnel.
6.8.2
Moving the turnout from the West Ruislip portal (Ch 23+480m), further west towards
Breakspear Road (Ch 24+300m), would create an extension of Northolt tunnel works of
approximately 500m. The construction and logistics methodology for the Northolt
tunnel would need to be reviewed.
6.8.3
The possibility of two contracts sharing a site at Ch 24+300m (Northolt tunnel driving
south, Northolt tunnel Extension driving north) could complicate matters further.
6.8.4
Moving the railway into tunnel throughout the Colne Valley would require that the
vertical gradient is no less than 0.5% for drainage purposes, whereas the current
alignment is as low as 0.3% at the Northolt tunnel West Ruislip Portal. Therefore the
vertical alignment through this area must be modified to meet required HS2 standards,
impacting on the alignment further south.
6.8.5
If the decision is taken to include passive provision for Heathrow spurs, and to combine
the cavern at Ch 24+300m (see 6.9.6 Option B), then the horizontal alignment would
need to be adjusted to allow for a straight section of track to accommodate the turnout.
This could have significant effects on the alignment many hundreds of metres away
from the turnout, not least of which are the impacts on neighbouring residential
properties within the town of Ickenham. At present, this particular aspect of the tunnel
option has not been assessed.