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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
19
Uncontrolled when printed
6.3.12
The line speed for the Turnouts T1 and T2 is currently 230kph but given the tight
horizontal radius of curvature required it may be necessary to restrict the speed to
180kph which would impact upon journey time.
6.4
Development of Railhead Location - General
6.4.1
Close proximity of HS2 to the Network Rail Chiltern Main Line allows temporary railhead
sidings to be constructed near to Harvil Road. This would give rail access to be utilised in
construction of the mainline, enabling loading and unloading from trains delivering
material to the site, and excavated material to be removed. The use of railheads is
encouraged where practicable, as it provides a more environmentally friendly route for
servicing the tunnelling operations in comparison to use of public roads, subject to
sufficient train paths being available.
6.4.2
The connection to the Chiltern Main Line would be formed by linking to the northern
end of the railhead. The sidings require flat gradient for 440m long sidings to
accommodate 400m long trains, and therefore the point at which the railhead can start
to descend into tunnel is constrained.
6.4.3
The connection from the railhead to HS2 would be in both directions. This would be
feasible given that the chamber for the turnout connection is thought to be shallow
enough to be constructed as an open cut until tunnelling is complete, at which point it
would be covered and filled. If the chamber width is to be minimised by connecting the
railhead in one direction only (provided in a southerly direction), engineering or
construction trains accessing HS2 to the north of the turnout would be required to
reverse direction of travel for some 5km, before reaching the “gap” structure where a
crossover point is provided.
6.4.4
The location of the railhead would be governed mostly by the difference in level that is
needed between the railhead sidings and HS2 rails. Two options were considered for
locating the railhead, depending on where the turnouts were to be located.
6.4.5
Option 1 would locate the railhead sidings west of and under Harvil Road, at Ch26+100
(Figure 6.1). This would require a diversion of the road onto a new viaduct. The railhead
would immediately descend at 3.5% (assumed maximum gradient for engineering trains)
in a retained open cut adjacent to the Schering Plough Research Centre (MSD Animal
Health Pharmaceutical Products Manufacturers), and then descend into tunnel
underneath Breakspear Road and the River Pinn. Roughly 1 Diameter clearance can be
provided above the tunnel under Breakspear Road, however the gradient would be at a
maximum and it may be desirable to reduce this to create a shallower retained cut.
6.4.6
If the gradient is reduced, the clearance over the railhead would not be sufficient to
begin tunnelling before reaching Breakspear Road. In this case, it would be possible to
replace Breakspear Road deck with a slab (built incrementally, necessitating some
degree of traffic management). The railhead can sit in a box structure underneath the
road, with enough clearance from the rails to the soffit. This would therefore allow the
retained cut section to be shallower, as the reduction in gradient raises the vertical
alignment of the railhead.
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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
20
Uncontrolled when printed
Figure 6.1 – Railhead Location West of Harvil Road
6.4.7
The turnout would be positioned at Shaft F (chainage 24+300m), which would require
the construction of a chamber or box to accommodate it. The minimum clear spacing
between tunnels at the entrance would be 0.5xDiameter (4.8m); this would give a total
width at the wider end of the chamber of 35.6m (24m width at the narrower end). At
approximately 20m below ground level, the chamber would more likely be constructed
as open cut, and covered and filled on completion of the tunnels. This site would allow
the construction of Shaft F to be combined with the turnout chamber, condensing the
work sites to one location for both. However an additional chamber would be required
for connection to the Heathrow spur should passive provision be included – this would
be located just 1km to the west.
6.4.8
Locating the railhead sidings to the west of Harvil Road would encroach on land already
reserved for the site of Ickenham Auto-Transformer Feeder Station (ATFS) (see Rail
Systems Construction 5.6.1). This proposal would thus necessitate the relocation of the
ATFS site to an open area adjacent to Shaft E2. This new site would be combined with
that of Shaft E2, and would be likely to reduce the length of cables between the National
Grid substation and ATFS in comparison to its current proposed position. Access to this
site can be provided from Harvil Road directly.
6.4.9
Alternatively, the possibility of placing the railhead east of Harvil Road at Ch 25+200m
was considered. This would require the sidings to continue at grade over Breakspear
Road, over the River Pinn, and only then begin to incline at 3.5% maximum gradient into
a tunnel which would connect to the mainline at West Ruislip, Ch 23+800m. This would
involve construction of extensive temporary works such as bridges over the River Pinn
and Breakspear Road, and the turnout would be adjacent to a large residential area
close to West Ruislip station, which limits the availability of land to the south. It is
considered that the tight spatial constraints mean that the maximum gradient of 3.5%
cannot be achieved in the space available.