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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
5
Uncontrolled when printed
1 Executive Summary
1.1.1
The options for extending the Northolt Tunnel reviewed in this report are similar to the
options proposed on behalf of London Borough of Hillingdon (LBH) and described in
their report titled ‘HS2 Tunnel Extension: Reducing the Environmental, Social and
Economic Burden in Hillingdon’ published in December 2014. An engineering review
has been undertaken of the LBH options and are reported in a separate report.
1.1.2
This document presents an engineering review of an alternative proposal by the Colne
Valley Community Forum to that outlined in the Hybrid Bill, which would replace the
Colne Valley viaduct and its approaches between Ruislip and the M25 Motorway with a
tunnel. This was requested in order to achieve a route that would cause minimal impact
to the surface, recognising that the area included much rural amenity including the
presence of a Site of Special Scientific Interest (SSSI), the Grand Union Canal and the
associated mooring basin, the River Pinn and the River Colne itself. The construction and
operational railway noise and the visual impact would expected to be reduced.
1.1.3
The proposal has been translated into an alignment option compliant with general high
speed rail, fire and safety requirements and the HS2 Project standards. The review has
shown that in order to provide adequate depth to the tunnel construction across the
Colne Valley this length of tunnel would become co-incident with the adjacent Northolt
and Chiltern tunnels. This would give an overall tunnelled length of some 34km
extending from the southern portal of the Northolt tunnel to the northern portal of the
Chiltern Tunnel at Mantle’s Wood. The alternative option is illustrated on drawings that
show the alignments and impact of the proposed works to the ground surface.
1.1.4
This report describes the process for the development of the alternative option and the
associated SIFT exercise for the replacement of the Colne Valley Viaduct with a tunnel.
The fundamental change comprises extending the Northolt tunnel beneath the Colne
Valley, such that it connects with the Chiltern Tunnel at some point, not necessarily at
the current south portal position just (inside) the M25. In doing so, the tunnel would
pass beneath the lakes in the Colne Valley, the River Colne and the Grand Union canal,
and depending upon the relative vertical positions, would impact upon them in varying
degrees (discussed in this report).
1.1.5
It has been assumed that the 8800mm ID tunnels would be constructed using tunnel
boring machines (as described in HS2 Information Paper D7: Tunnel Construction and
Methodology). Inclusion of passive provision for the Heathrow spurs means allowance
has to be made for the inclusion of turnouts. The turnouts would be included within
caverns each capable of containing two divergent tracks, and the excavations required
would therefore be of a significant size.
1.1.6
The following options have been assessed and compared against the Proposed Scheme
as set out in the Hybrid Bill (an alignment requiring a viaduct over the Colne Valley):-
Option B – a subsurface alignment to include a 700m long “Intervention Gap”
(including space required for the portal hoods) which provides compliance
with the European Technical Specification of Interoperability (TSI) 2014,
including passive provision for the Heathrow spurs.
Option C – as above, but without provision for Heathrow spurs.
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1 -
Accepted
Uncontrolled when printed
INTERNAL INFORMATION
Northern Extension of Northolt Tunnel SIFT Report
Northern Extension of Northolt Tunnel SIFT
Report C222-ATK-DS-REP-020-000034
Revision – P06
6
Uncontrolled when printed
1.1.7
The extension of the Northolt Tunnel under the Colne Valley would create around 4.3
million m
3
of earthwork in total. This material wold principally comprise Chiltern tunnel
excavated material (no longer required for mitigation earthworks in this area) and the
excavation material from the Intervention Gap as well as the new tunnel under the
Colne Valley. Approximately 4.2 million m
3
of this would require disposal by road via the
M25 from the construction compound adjacent to the Chiltern Tunnel south portal with
the potential of additional environmental impacts depending on disposal locations.
1.1.8
Although the Environmental assessment concludes that Options B and C would avoid
the majority of the above ground impacts identified in Option A during construction and
operation, it is concluded that the Proposed Scheme as set out in the hybrid Bill is
preferred, on the grounds of cost, programme implications, construction safety and
traffic effects as a result of removal of tunnel excavated material.
2 Introduction
2.1.1
The London Borough of Hillingdon, HOAC and the Colne Valley Community Forum
amongst other petitioners have asked HS2 Ltd. to consider an alternative proposal for a
section of the London - West Midlands route at Colne Valley which would comprise a
tunnel to pass beneath the River Colne and the Colne Valley lakes between Ruislip and
the M25. This would be considered preferable by the community because it would
address concerns for above ground disturbance from construction and would remove
visual and noise impacts during construction and operation. In addition, it is recognised
that the area includes much rural amenity including the presence of a Site of Special
Scientific Interest (SSSI), the Grand Union Canal and its associated mooring basin, as well
as the River Colne itself.
2.1.2
The hybrid Bill proposal for crossing the Colne Valley in Community Forum Area (CFA) 7
comprises a viaduct, 3.4km in length, located between chainages 26+000m and
29+400m (the Proposed Scheme). This proposed structure is positioned inside the M25
Orbital Motorway near the towns of Denham and South Harefield and passes through
the southern sections of the Colne Valley Site of Special Scientific Interest (SSSI), as
shown in Figure 5.1. The SSSI includes a number of disused gravel pits, which now form
lakes, the Grand Union Canal, and the River Colne. The Viaduct connects the line of HS2
close to where it emerges from the Northolt tunnel, west of West Ruislip station, to the
Chiltern tunnel portal south of the M25. The interface between the C222 Contract and
the C221 Contract is at chainage 25+800m at the eastern end of the viaduct approach
embankment, Structure 025-L1.
2.1.3
In the development of the railway alignment for the Proposed Scheme, passive provision
has been made for a spur line serving Heathrow Airport, the alignment of which is
shown in Figure 2.1.
2.1.4
The purpose of this report is to evaluate replacing the Colne Valley viaduct with a bored
tunnel. The outcome of this review is that the only way to provide this additional bored
tunnel is by making it co-incident with the adjacent Chiltern tunnel and Northolt Tunnel,
due to limiting gradients on the railway alignment. Given that this would then create an
effective 34km long tunnel the requirements of the 2014 European Technical
Specification for Interoperability (TSI) have been considered in the development of the
options. Drawings showing the assumed vertical alignment of an extended Northolt
Tunnel under the Colne Valley and which form the basis of this assessment are included
in Appendix A.