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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
37
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7.1.3
The Option C cost estimate is based on the assumption that there would no longer be a
requirement for a cavern at Ch 25+000m which provides passive provision for possible
future construction of the Heathrow spur.
7.1.4
These costs do not include estimates for possible new utility diversions associated with
the extra tunnel lengths. These costs do include reduction for diversions that are
required for Option A but not for Options B and C (see section 7.4).
7.2
Programme Impacts
7.2.1
Additional tunnel drive lengths associated with Options B and C are as follows:-
Northolt tunnel - (extended from Ch 23+480m to Ch 24+200m) +0.72km;
Chiltern tunnel - (extended from Ch 31+363m to Ch 31+100m) +0.26km; and
Northolt tunnel Colne Valley extension - (Ch 30+400m to Ch 24+200m)
+6.2km.
7.2.2
An average tunnel advance rate of 80m/week has been assumed to calculate the
additional duration of construction.
7.2.3
It is proposed to construct the Northolt tunnel extension through the Colne Valley with
two TBMs, each driving one bore of the additional 6.2km tunnel length. If a single TBM
were to be used, the TBM would need to be turned around before continuing to drive
the bore in the opposite direction. The time taken to complete both bores would
therefore be doubled in comparison to use of twin TBMs, plus further time allowed to
turn the TBM.
7.2.4
Therefore, the tunnel construction period for the Northolt tunnel extension associated
with Options B and C is 18 months assuming 2 No. TBMs. To this should be added the
programme time for trackbed and services. This construction period is assumed to run
in parallel with construction of the Northolt tunnel and the Chiltern tunnel.
7.2.5
The construction period for the viaduct Option A in the Colne Valley area is
approximately 66 months, which includes the period for diverting the National Grid
275kV overhead cables.
7.2.6
The additional construction periods associated with lengthening of the Northolt tunnel
(0.72 km) and the Chiltern tunnel (0.26 km) with drive rate of 80 m / week, under
Options B and C are estimated as follows:-
Northolt Tunnel +2 months
Chiltern Tunnel +1 months
These periods are additional to the programme time for these works under the
Proposed Scheme.
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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
38
Uncontrolled when printed
7.2.7
The overall rail system installation could thus be expected to extend by
approximately 5 to 6 months as described in paragraph 6.6.6.4, when compared
to the Proposed Scheme.
7.3
Excavated Material
7.3.1
The total excavated material that would be generated for the Northolt tunnel extension
under Options B and C amounts to approximately 4.3million m3. This includes material
generated from the construction of the ‘gap’ structure but excludes the excavated
material generated from the construction of additional ventilation-intervention shafts.
Material generated from the construction of the Proposed Scheme Chiltern tunnel
which equates to approx. 2million m3 is also included in this sum as nearly all this
material would need to be disposed off-site. Breakdown of the material generated as a
result of the Northolt tunnel extension is shown in table 7.2.
7.3.2
Majority of this material (approximately 4.2million m3) would be generated from the
construction compound by the M25. It is currently anticipated that this material would
be removed from site via public roads (M25), equating to approximately 990,000 two-
way lorry trips during the construction period.
7.3.3
For the additional 720m length of the Northolt tunnel extension up to Shaft F, under
Options B and C, the excavated material amounts to approximately 107,000m3 which
would be serviced via the railhead connection. The quantity of excavated material being
generated each week would require approximately 20 trains to move the material by
rail. This is in addition to the remainder of the 14km long Northolt tunnel, which would
generate an approximate volume of 2,000,000m3 excavated material from tunnelling
operations which is to be serviced via the railhead connection (as set out in Option A). A
stockpile area for excavated material has been proposed within the main compound
area adjacent to Harvil Road (as above) to take any overspill from the railhead loading
area if tunnel production exceeded capacity on the rail network.
7.3.3
Summary of the excavated material generated from the tunnel under Colne Valley is
presented in Table 7.2.