`Code
1 -
Accepted
Uncontrolled when printed
INTERNAL INFORMATION
Northern Extension of Northolt Tunnel SIFT Report
Northern Extension of Northolt Tunnel SIFT
Report C222-ATK-DS-REP-020-000034
Revision – P06
24
Uncontrolled when printed
Figure 6.5 – Shaft E2
6.5.8
Shaft F, required for Options B and C, would be located near to the current proposed
tunnel portal for the Northolt tunnel. It is proposed the shaft structure be combined
with the turnout chamber required for the temporary railhead connection. This shaft
would be the principal point from which the Northolt tunnel would be constructed.
`Code
1 -
Accepted
Uncontrolled when printed
INTERNAL INFORMATION
Northern Extension of Northolt Tunnel SIFT Report
Northern Extension of Northolt Tunnel SIFT
Report C222-ATK-DS-REP-020-000034
Revision – P06
25
Uncontrolled when printed
6.6
Impact on Rail Systems
6.6.1 Tunnel Ventilation and Smoke Control
6.6.1.1
The rolling stock TSIs require a capability to operate for 15 minutes at 80 km/h, which
implies a range of 20km. Consistent with this, rescue stations at a maximum spacing of
20 km would normally be considered. The rescue station would have the capability to
provide a tenable environment for passengers in the event of a fire. This could be a
subterranean facility, with local tunnel ventilation exhaust and supply, or an open
facility. The proposal for an open rescue location is considered reasonable considering
the shallow depth and lower complexity of a natural ventilation system.
6.6.1.2
The remainder of the tunnel ventilation system would be consistent with other HS2
tunnels and adopt longitudinal tunnel ventilation shafts at 2 to 3km spacing. The
signalling system would allow one train to enter each tunnel bore between the
ventilation shafts. The proposed shaft spacing of up to 3.2km is considered reasonable
from a tunnel ventilation perspective, albeit the shaft spacing is irregular which may
make airflow balances more challenging to achieve and may have an impact on
signalling and ability to meet the train technical headway. Further detailed study would
be required.
6.6.1.3
Tunnel heating is a key consideration for longer tunnels at these speeds. The
aerodynamic resistance of long tunnels generates heat which is mitigated for by the cool
air drawn in at the portal. Based on the prior analysis of 13.4km long Chilterns tunnel, it
is considered likely that tunnels longer than 13km would cause temperatures to rise in
excess of HS2’s 35°C summertime criterion. Warmer tunnels may be acceptable
depending on the degree, but could begin to have implications for maintenance workers
and for the sizing of the rolling stock air conditioning. They could also affect the ability to
control temperatures during train congestion and if too warm may affect tenability
during any in-tunnel evacuation. The life of tunnel based equipment is also negatively
affected by warm conditions. It is therefore likely that cooling would be required.
Cooling could be by either relief air shafts integrated into the ventilation shafts, or by
mechanical means, most likely from cooling pipes in tunnels. The use of relief air shafts
may be possible, but the sizes required might cause micro pressure wave issues as the
trains pass. Further detailed study would be required to understand whether a
reasonable design could be developed to balance the needs of achieving cooling but
minimising pressure waves. The shaft designs would need to change and potentially be
larger to accommodate the optimised relief air paths. Without this analysis it is
recommended to account for the need for cooling pipes near the portals, as adopted on
the Channel Tunnel. Pipe loops, concentrated near the portals, would pass up and down
the tunnels served by cooling plants served by air cooled chillers potentially located at
the end most ventilation shafts.
6.6.1.4
The open firefighting point potentially allows for cool air to enter the second
downstream tunnel, but careful design would be required to ensure that hot air from
one portal can dissipate and cool air can be drawn into the second tunnel. If this cannot
be achieved there may need to be localised extraction of the hot air at the rescue
station. Regardless of the provision of ventilation, passive measures such as dividing
walls extending some distance from each portal are likely to be required to prevent
transfer of hot or smokey air from one bore to the other. Considering the overall length
`Code
1 -
Accepted
Uncontrolled when printed
INTERNAL INFORMATION
Northern Extension of Northolt Tunnel SIFT Report
Northern Extension of Northolt Tunnel SIFT
Report C222-ATK-DS-REP-020-000034
Revision – P06
26
Uncontrolled when printed
of the tunnel, the fire authorities may request a special purpose vehicle at this location
for more rapid tunnel access. Further consultation would be required on this matter.
6.6.1.5
The proposed caverns and turn-outs present significant challenges for the tunnel
ventilation system. The spur tunnels are likely to require jet fans within them to both
control smoke in the event of a fire in these tunnels, and also to manage the air leakage
to and from these tunnels in the event of a fire in another part of the tunnel complex.
There may also need to be jet fans at the turn-out cavern location. If at high level these
would be a unique configuration and potentially challenging to maintain. The large cross
sectional area of the turnout may make it impracticable to control smoke at this
location. It may need to be accepted that smoke control can only be achieved in the
connected tunnel bores and not the turnout. If this cannot be accepted options might
include a ventilation tunnel constructed to connect to shaft E2 or F to provide location
specific smoke extraction at this point. Sufficient time would be required to analyse the
ventilation in detail and develop a working configuration. The design of the evacuation
walkway would also need careful design with track crossings suitable for persons of
reduced mobility a potential requirement.
6.6.1.6
The longer tunnels may affect air quality in the rolling stock. It may be necessary to shut
off the outside (fresh) air to the rolling stock when in tunnels to prevent pressure waves
affecting the pressure comfort of passengers. This loss of ventilation would cause carbon
dioxide (CO2) levels to rise. Whilst the TSIs require a long-term safety exposure limit of
5,000 ppm, there are uncertainties in relation to general air quality at levels above
2,000ppm. Operational practice in aircraft usually results in 1,500ppm of CO2. Levels of
around 2,000 ppm might be acceptable in rolling stock based on anecdotal evidence
from other long tunnels. Levels between 2,000 and 5,000ppm present a risk in terms of
general air quality. If the CO2 level could be controlled to 500 to 600ppm when leaving
the stations (outside air is about 400 ppm), the in-car CO2 content may rise to around
2,000ppm at the end of the proposed longer tunnel for the case of 50 people per car.
For a crowded car this would increase further and for slower train operations this could
increase again. To achieve even 2,000ppm at the end of the tunnel potentially a
supplementary rolling stock ventilation system would be required at the stations to
provide a high capacity purge of the carbon dioxide down to a lower starting condition
before the journey into the tunnels. It is known that some countries are considering
actively controlled pressure ventilation for rolling stock that may allow some ventilation
in tunnel when pressure waves were not near the train. Other countries are understood
to have developed a specialised air supply system, possibly from a pressurised reservoir.
6.6.1.7
Further work would be required to develop mitigation for HS2, but at this time it is
recommended to assume that some form of special measure would be required for the
rolling stock. Such a special measure might only be achievable on the captive rolling
stock, potentially affecting the ability for other rolling stock, including classic compatible
rolling stock to operate in the longer tunnel without risks associated with pressure
discomfort or poor air quality.
6.6.2 Operations
6.6.2.1
An assessment has been undertaken to determine the implications of journey time
between the Hybrid Bill and the alternative proposal. The results are shown below and
identify that there is a material difference, due entirely to tunnel resistance increases (*)
due to a significant increase in air mass movement for the combined tunnel. Note also