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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
15
Uncontrolled when printed
6 Option Description
6.1
General
6.1.1
This report considers the following design options:
Option A – the Proposed Scheme as set out in the hybrid Bill and
Environmental Statement;
Option B – a subsurface alignment, including passive provision for the
Heathrow spurs; and
Option C - a subsurface alignment, without provision for Heathrow spurs.
6.1.2
The qualitative assessment against the SIFT criteria is contained in Section 7.
6.1.3
The options have been developed to address the following requirements:-
Limiting the length of any section of the tunnel to 20km by inclusion of an
Intervention Gap/Firefighting point structure;
Including intervention / ventilation shafts at nominally 3km spacing and
vertical re-alignment to minimise shaft and Intervention Gap/Firefighting
points;
Identifying the means of providing the turnouts for the Heathrow spur
(Option B);
Routing the Heathrow spur tunnels either over or below the mainline tunnels
as necessary (Option B); and
Temporary connection to the Chiltern Main Line railway for construction
logistics.
6.1.4
The options identified have also been assessed to establish how they relate to the
efficiency of construction planning, for example, from where to launch / retrieve the
tunnel boring machines and service their activities.
6.1.5
An initial alignment for the option of tunnelling under the Colne Valley has been
developed using minimum line gradients as shown on Drawing Nos C222-ATK-RT-DSK-
020-011303 and C222-ATK-RT-DSK-020-011304, which indicate that a suitable
configuration is feasible although it does require the linking of the Chiltern and Northolt
tunnels. The alignment has been considered against the TSI recommendations, the
influence of high speed turnouts, the limiting ground cover, the line speed variation
between the mainline and the spur, the optimisation of shaft depths, and the alignment
of the spur connection (where required). However, the implication of such requirements
will be discussed in the following sections.
6.2
Intervention Gap Development
6.2.1
Given that the option of tunnelling under the Colne Valley would join up the Northolt
tunnel and the Chiltern tunnel, the commencement of the alternative option tunnel
would thus be in the east, at Ch 10+060m. The termination would be at the north end of
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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
16
Uncontrolled when printed
the Chiltern tunnel at Ch 44+635m, a total length in excess of 34km. Thus to limit
individual elements of the tunnel to a length of 20km, the permissible envelope for the
open excavation shall be within the section from Ch 24+635m to Ch 30+063m.
Unfortunately, the section of the route through which the River Colne flows is from Ch
26+000m to Ch 29+400m which limits the possibilities for the open structure to two
sections of length 1365m in the east and 663m in the west, located either side of the
river, SSSI, canal and lakes. In addition, both of these positions appear to be coincident
with high parts of the topography which means that the depth of the gap excavation
would be in excess of 50m as shown at Section 30+400m on Drawing No C222-ATK-RT-
DSK-020-011306, and would represent a considerable design and construction
challenge.
6.2.2
It should be noted that the 20 km limitation is subject to a location of firefighting points
according to the 2014 edition of the TSI, and it is possible that detailed design of tunnel
safety systems could lead to a tunnel solution without a gap. In addition, it may be
possible to limit the depth of excavation by lifting the alignment, although this would
impact on the minimum required ground cover beneath the lakes, and thus increase the
risk of adverse ground conditions affecting the tunnel drives.
6.2.3
A better solution, and that which has been adopted for the proposed alternative tunnel
options, would be to use the Chiltern Tunnel Main Compound. This means that the
extended Northolt Tunnel under the Colne Valley will 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 most excavation limited to approximately 20m.
6.2.4
The provision of the Intervention Gap structure at the Proposed Scheme southern works
area would also support the option for removing excavated material from both the
Chiltern tunnel and the proposed alternative options, rather than creating an alternative
enlarged ventilation shaft. The location of the gap structure allows room for temporary
storage of excavated material within the Hybrid Bill limits.
6.2.5
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 Hybrid Bill limits, provide an option for
launching the Tunnel Boring Machines (TBM) in both directions, and provide a route for
servicing both the mined and bored tunnelling operations.
6.2.6
A consequence of Options B and C is that the Chiltern tunnel south portal would not be
immediately adjacent to the M25 (as opposed to Option A) and the tunnel would be
lower under the M25, reducing settlement risks.
6.2.7
Given the need for provision for the Heathrow spur (Option B) it is assumed that the
“gap” would include turnouts T1, but in order to provide appropriate separation
between the tunnel bores, the structure would need to be a tapered structure with a
maximum of width of 70m, as shown in Drawing No. C222-ATK-RT-DSK-020-011306.
However, it is anticipated that this could be reduced slightly with further development
of the vertical and horizontal alignments and assessment of engineering impact. It
should also be noted that for Option C (without provision for Heathrow Spur), the “gap”
structure width would be reduced as the additional space for the turn outs would not be
required. In the case where passive provision for the Heathrow spurs is not required,
there is scope to raise the vertical alignment allowing a shallower gap structure,
however this is beyond the scope of this report.