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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
39
Uncontrolled when printed
Description
From
(Ch.)
To
(Ch.)
Approx. Spoil
Generated (m
3
)
Approx. 2-way
Lorry Trips
Proposed Scheme
location of Northolt
Tunnel north portal to
Shaft F
23+480
24+200
107,000
(Removed from
Shaft F)
-
(Removed via
railhead at
Ruislip)
Shaft F to Gap Structure
24+200
30+400
920,000
(Removed from the
compound by M25)
216,000
Gap Structure
30+400
31+100
1,260,000
(Removed from the
compound by M25)
300,000
Gap Structure to south
portal of Chiltern Tunnel
31+100
31+363
40,000
(Removed from the
compound by M25)
9,200
Proposed Scheme
Chiltern Tunnel
31+363
44+635
2,000,000
(Removed from the
compound by M25)
465,000
Note: Bulking of spoil is not included in calculated spoil volumes.
Table 7.2: Excavated Materials
7.4
Utility Diversions
7.4.1
There is a relatively dense network of major utility infrastructure in proximity to the
works for the Proposed Scheme (Option A). This includes high pressure gas mains, large
diameter water mains, large diameter sewers, and high and low voltage electricity lines.
In comparison, Options B and C would not require diversion of many of these utilities as
the tunnelled HS2 mainline would pass under utilities for the majority of the alignment.
7.4.2
At Ch 26+600m, a National Grid Electricity 275kV overhead line meets the proposed
viaduct (Option A). Studies undertaken by National Grid conclude there is no option to
increase the height of the overhead lines enough to achieve required HS2 clearances
and the line is redesigned in the Proposed Scheme. Options B and C would not require
this diversion, and therefore would provide a significant cost saving.
7.4.3
There are two high pressure gas mains running north to south between Harvil Road and
Breakspear Road. The larger of the two (48’’ steel gas transmission pipe) runs between
Harefield and Southall, and the slightly smaller (18’’ steel gas transmission pipe) runs
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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
40
Uncontrolled when printed
north to south between Fulmer and Haste Hill. Implementing Option A requires a
diversion which would require extensive planning to temporarily divert the routes.
These diversions would not be required for Options B and C.
7.4.4
The Proposed Scheme requires diversion of a 600mm ductile iron water main which also
lies to the west of Breakspear Road.
7.4.5
A Scottish and Southern Energy high voltage overhead line alongside the M25, adjacent
to the Proposed Scheme Chiltern Tunnel south portal, would also require diversion if
Option A is to be adopted. This diversion would not be required for Options B and C.
7.4.6
There are 18 other smaller scale utility diversions or protective measures required for
construction of the Proposed Scheme, such as BT overhead cables, water mains,
overhead high voltage power lines, and overhead low voltage wires.
7.4.7
Relative costs of utility diversion and protective works are included in the option costs
presented in Table 7.1
7.5
Environmental Impacts
7.5.1
The proposed tunnel routes A & B pass close to two of the largest source protection
zone (SPZ) 1 (TH177 and TH027) in the area; Route A passes within 50m of TH177 and
within 350m of TH027; and Route B passes within 350m of TH177 and within 100m of
TH027. Both tunnel options are within the SPZ 1 of these two sources. The ES HB
scheme passes at similar distances to these abstractions (less than 50m and 330m). Both
PWS abstractions are thought to be fed by large fissures in the Chalk, which supply most
of the water for these supplies.
7.5.2
Construction of the tunnel (along either route) has the potential to contaminate
groundwater through the production of turbidity (particulate) or the release of other
contaminants and affect surface water features and both SPZ1s (TH177 and TH027). As
stated within the ES, with the implementation of measures required by the draft CoCP,
contaminants will be controlled at source. However, these measures cannot eliminate
turbidity and it is likely that Affinity Water would have to temporarily close their sources
to avoid impact on Public Water Supply (PWS). This impact is also identified in the
Proposed Scheme due to piling for the Colne Valley viaduct and is mitigated in the ES by
the production of a Management Strategy. The effect magnitude and duration on
groundwater sources is considered similar under the Proposed Scheme and the two
tunnel options (A & B).
7.5.3
Vent shafts and cross passages would be required for the tunnelling options, which
would require groundwater dewatering. In the Colne Valley groundwater is thought to
support local surface water features, such as the River Colne and lake SSSIs. Therefore,
groundwater dewatering may have a temporary impact on water levels in the local
surface water bodies. Where possible, water could be recharged back to ground in the
vicinity of the abstraction in attempt to minimise any impact.
7.5.4
This groundwater dewatering activity could also have a short term temporary impact on
the PWS and private abstractions, where these are located close to the route.
7.5.6
Figure 6.3 indicates that a high proportion of the mainline falls within this area, and with
the proposed alignment highly constrained, there is little scope to reduce impacts
further.