Proposal for Northern Extension of Northolt Tunnel sift report

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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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that based on a review of available plots of train performance, this indicates that in the 

Down direction to Birmingham the train may not be able to reach the tunnel 320km/h 

speed limit. However, the tunnel resistance used for the modelling is considered worst 

case and potentially the journey time for the alternative proposal may be closer to the 

Hybrid  Bill  option.    Further  analysis  would  be  required  to  fully  understand  the  tunnel 

resistance so that the journey time can be accurately modelled. 

 

(*)  Tunnel  resistance  increases  have  been  estimated  on  the  basis  of  drag  co-efficient 

increase due to the combined tunnel length increasing. Detailed analysis of this tunnel 

would be required to determine the exact effects. 

 

 

 

Time (hr:min:sec) 

Journey 

Hybrid Bill 

Alternative 

Proposal

(1)

 

Increase in Journey Time 

of the Alternative Proposal 

Old Oak Common to 

Birmingham Interchange 

00:28:44 

00:29:12 

00:00:28 

Birmingham Interchange 

to Old Oak Common 

00:28:59 

00:29:02 

00:00:03 

(1)

  Based on an assumed worst case tunnel resistance 

Table 6.2 – Journey Time Assessment Results 

  

6.6.2.2 

There  may  be  a  potential  requirement  for  a  speed  limit  due  to  pressure  waves 

generated when trains pass through the Heathrow spur cavern with likely impact to the 

signalling headway. 

6.6.2.3 

The  impact  on  the  operation  timetable  of  this  increase  in  journey  time  has  not  been 

assessed.  The timetable may be able to cope with the increase in journey time from an 

emergency  recovery  perspective,  but  it  is  supposed  to  have  this  margin  for  only  such 

purposes so any increase in journey time would impact the timetable robustness. 

6.6.2.4 

With the HS2 requirement for one train per vent shaft section, signalling headway was 

modelled for the alternative proposal to determine potential timetable constraints. The 

results concluded that there was no difference in the signalling headway compared to 

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 

 

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6.6.3  Traction Power and Overhead Contact System 

6.6.3.1 

There will be an appreciable increase in traction power requirements due to the 

additional 8km of tunnel running and the effective increase in both Northolt and 

Chiltern tunnel lengths which result in increased train resistance forces. The adequacy of 

the currently proposed traction power system design in being able to support this 

increase in loading  has yet to be assessed and presents an appreciable risk. 

6.6.3.2 

A particular issue would be that the current scheme has Ickenham ATFS (Auto 

Transformer Feeder Station) located adjacent to the open route portion.  This has 

numerous high voltage  cable connections between the ATFS and the overhead contact 

system for each of the  two tracks.  For the tunnelled alternative there would need to be 

cable shafts provided between the ATFS at surface and the tunnels or a surface cable 

route for c. 400m to shaft E2 and the shaft size increased to allow for the additional 

traction power cables. 

6.6.3.3 

There are factors related to the neutral sections in the overhead contact system in the 

area of Ickenham ATFS which would need further consideration.  A neutral section is the 

arrangement of the overhead contact system where the source of power changes from 

one feeder station to another.   Additional space is required at a neutral section which 

may be problematic to achieve and may require additional tunnel adits. 

6.6.3.4 

The associated relocation of Ickenham ATFS to the east side of the railway is dependent 

on sufficient land availability to accommodate the required feeder station and 

equipment layout which is not yet confirmed.  In addition, as it moves the ATFS closer to 

the National Grid substation it potentially introduces the risk of  hazardous voltage from 

electrical faults at the 275kV substation being transferred to the ATFS and railway, which 

is a risk that was not present for this site previously; this would be subject to further 

detailed evaluation. 

6.6.3.5 

The 700m long ‘gap structure’ would likely impact on the layout for the West Hyde ATS 

(Auto Transformer Station) location at 31.0km and may require additional lateral land-

take. 

6.6.4  Train Control and Telecoms 

6.6.4.1 

The high level review has not identified any significant technical issues with the 

alternative proposal with respect to train control or telecoms. 

6.6.5  Tunnel Aerodynamics 

6.6.5.1

 

The greater length of tunnels proposed under options B and C would require increased 

mitigation of micro-pressure waves (“sonic boom”) and in the case of Option B, due to 

pressure  waves  generated  when  trains  pass  through  the  Heathrow  spur  cavern. 

Mitigation could be achieved by either of two solutions:  

 

o

  Either: Increased length of perforated hoods at both ends of the 500m open section 

and  at  the  northern  end  of  Chiltern  tunnel.  Further  work  is  required  to  define  the 

length,  which  as  a  first  estimate  could  be  in  the  range  200-300m.  The  trough 

structure at the open section would then be 900-1100m in length. For Option B, train 

speeds through the cavern would have to be restricted (this applies to the main line 

in addition to the spur). Further work is required to define the maximum acceptable 

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