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Figure 6.2 – Turnout Chamber
6.4.10
The availability of open space at Shaft F (Ch 24+300m) and the location of the railhead
sidings to the west of Harvil Road indicate that this option would be the least intrusive
and more easily constructed.
6.5
Development of Intervention Shaft Locations - General
6.5.1
For tunnelled railway, shafts shall be located at nominally 3km intervals to comply with
the project requirements, although the particular influence of caverns would not be
included at this stage. For this option, it is assumed that the following shafts currently
included within the Proposed Scheme areas shall be fixed i.e.:-
Chalfont St Peter Shaft, S1, at Ch 34+050m which serves the Chiltern Tunnel;
and
South Ruislip Shaft at Ch 20+740m which serves the Northolt Tunnel.
Ref
Chainage
Separation
(m)
Location
CSP
34+050
-
Chalfont St Peter
GAP N
31+100
2950
700m “gap” structure
GAP S
30+400
-
E
27+500
2900
On perimeter of SSSI, adjacent to Moorhall
Road
F
24+200
3300
200m East of River Pinn
SR
20+740
3440
South Ruislip
Table 6.1: Initial Proposed Shaft Locations (Options B and C)
Breakspear Road
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6.5.2
Located on a bench of land in between SSSI protected gravel pits, the initially proposed
Shaft E would have been adjacent to a groundwater abstraction point. This is an
extremely sensitive location as contamination to the abstraction point would be likely to
occur during construction. The shaft would also fall within a SPZ1 site (Groundwater
Source Protection Zone 1), which indicates that the groundwater source within this area
is susceptible to contamination as it is within 50 metres of the water table.
Contamination of the source could lead to unpotable water and may take a long period
of time to remediate. Alternative shaft sites were therefore proposed.
6.5.3
To keep the spacing between shafts as close to 3000m as possible, two alternative shafts
would be needed: E1 and E2 (Figure 6.3).
Figure 6.3 – Shaft Locations
6.5.4
Shaft E1, located at Ch 29+400m, would be situated in a field which is classed as
‘woodland’ according to the Environmental Baseline drawing for Colne Valley CFA07 (ref
CFA7 map book, map CT-10-012), however it does not have SSSI protected status. The
location of Shaft E1 would be constrained by the Heathrow spurs to the south (Figure
6.4). Shifting the shaft further east would either impinge on the A412 road or Ancient
Woodland (Battlesford Wood), which also lies within a SSSI area. All possible locations
for Shaft E1 are situated within a Source Protection Zone 1 (SPZ1), in a location likely to
give concerns for the Environment Agency and licensed abstractors, Affinity.
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Figure 6.4 – Shaft E1
6.5.5
Shaft E2 would be bounded by the Hillingdon Outdoor Activity Centre (HOAC) to the
north (Ch 26+250m), and the East Heathrow spur (to the west). To limit the spacing
between shafts, Shaft E2 would be placed as far north as possible without impacting on
the HOAC. This location would also fall within a SPZ2 (Groundwater Source Protection
Zone 2) site, which is more desirable than nearby alternatives within SPZ1. The shaft
would also lie within a floodplain and may require flood defences. Under Options B and
C, Ickenham Feeder Station could also be located at this site, which would give extra
benefit to locating the shaft here.
6.5.6
The locations of the shafts, particularly when adjacent to or within the influence of
bodies of water would need to be carefully selected. Depending upon the outcome, the
connections to the tunnels, the depth of shafts, the requirement for 500m2 rescue
areas, and their internal arrangement would need to be re-assessed for compliance at
detailed design stage. In addition their architectural form, construction form, and
constructability would be considered in further detail.
6.5.7
However, with the introduction of caverns necessary to accommodate the turnouts
there would be a requirement to re-assess the overall fire life safety and ventilation
strategy for this revised system of tunnels given the presence of two lines converging,
i.e. there is a possibility of smoke being blown down both the incident bore and the bore
forming the place of relative safety without some form of mitigation being in place.