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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
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speed; as a first estimate, this could be less than 200km/h. Journey times would be
adversely affected.
Or
o
Development of new in-tunnel mitigation measures that reduce the gradient of the
pressure wave during propagation, such as air spaces engineered under walkways. A
substantial research and development exercise would be required in order to
validate the effectiveness and develop design details. This solution would enable the
open section excavation to be as stated elsewhere in this report (700m long included
100m long hoods at either end), and would also reduce the length of the perforated
hood at the northern end of Chiltern tunnel from the currently envisaged 200m. The
risks are (i) the research might find that the solution does not work as planned; (ii) it
is possible that the 8.8m internal diameter might need to be enlarged; (iii) it is not
certain whether this solution would still require a speed reduction through the
Heathrow spur cavern.
6.6.6 Construction
6.6.6.1
The key construction issue with the alternative proposal is the restrictive access to the
trace increasing the logistical constraints compared to the Hybrid Bill option. This will
increase risk to the overall rail systems installation. Hence complex logistics planning
and access are key to the delivery and would require risk mitigation to ensure delivery
6.6.6.2
The Proposed Scheme is for the tunnel fit out to be undertaken from the Chiltern tunnel
south portal. This alternative proposal will require the use of the gap structure which
will increase the safety and logistical complexity as well as add time to the construction
programme. This will become a key rail systems worksite resulting in a construction and
logistics constraint with the civil engineering works being required to be completed prior
to rail systems work completed. The gap will need to be designed to provide adequate
access to crane equipment and material to track level. Short rails (60ft / 18m) would
have to be lowered through the gap for track installation using the temporary rails, with
re-railing with and the S&C fit-out undertaken from the temporary railhead once the
track is completed. The use of the gap for tunnel fit out is further complicated by the
1.25% track gradient. Works train loading is normally undertaken on a 0.2% gradient (NR
Standard) and thus this will need to be carefully reviewed to provide sufficient safety
measures, which adds risks to the programme.
6.6.6.3
The revised access from the temporary railhead at Ruislip to the HS2 trace also adds
issues to the construction and logistics. The c. 800m long tunnelled railhead connection
would require longer to install. In addition a review is required to ensure sufficient
gauge to ensure delivery of the high speed S&C. The alternative proposal also only
allows a single connection to the HS2 trace. Thus track/temporary track would be
required to be installed at least 300m in either direction before train moves in either
direction could be accommodated which provides a further constraint to the
programme.
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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
30
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6.6.6.4
The implications of the above to the programme, compared to the Hybrid Bill, are as
follows (the following are approximate durations and further work is required to fully
understand the programme implications):-
1 month due to the logistics constraints reducing the number of overlapping
activities
1 month (worst case) to fit out of the crossover cavern and gap structure
1 month to install / remove temporary railhead connection
1 month for the logistical and additional installation complexity
1 month for the additional tunnel systems installations between 24 to 31km
(this is a slower operation than for open route)
6.6.6.5
Subsequently, the overall construction programme is likely to extend by a minimum of 5
to 6 months for the alternative proposal compared to the Hybrid Bill option.
6.7
Northolt Tunnel Construction and Logistics - General
6.7.1
Under Option A, the Proposed Scheme, the Northolt tunnel is to be constructed from a
tunnel portal site at West Ruislip. A temporary rail connection is to be made to the
Chiltern Main Line for construction logistics subject to sufficient train paths being
available. Under Options B and C, the HS2 rail alignment would be moved underground,
requiring the Northolt tunnel TBMs to be launched from Shaft F. This would significantly
influence the construction of the Northolt tunnel, as discussed below.
6.7.2
The railhead turnout chamber at Shaft F is suitable for the assembly and launching of
Northolt tunnel TBMs in “semi-short” mode. A work site supplying the machines with
tunnel lining segments and the handling of bulk excavated material would be located
adjacent to Harvil Road site as per Option A. The shaft is envisaged to be constructed as
a large D-wall shaft with ‘HAC’ vertical conveyor and segments installed by gantry. Fore
and back shafts in sprayed concrete lining (SCL) are envisaged to start tunnelling.
However, rail systems installation would need to be done from the temporary rail
connection.
6.7.3
It is assumed the major components of the Northolt tunnel TBMs would be delivered in
sections to the adjacent site by way of a temporary construction access to the north of
the turnout chamber. Significant craneage would be required to lower the assembled
TBMs into the open cut at Shaft F. The TBM support gantries would then be delivered
into the cut and assembled behind the TBM cutter head and segment erector.
6.7.4
The excavated material from Northolt tunnel construction works would discharge
directly onto rail wagons for onward movement via the railhead. A stockpile area for
excavated material has been proposed within the main compound area to take any
overspill from the railhead loading area if tunnel production exceeds capacity on the rail
network.
6.7.5
The railhead connection to Shaft F would need to be in place prior to start of tunnelling
works in order to facilitate tunnelling operations. This is likely to be a critical path
activity to ensure that the Northolt tunnel construction can be commenced.