The UK’s 109km long Channel Tunnel Rail Link (CTRL) is an integral part of the developing European High Speed Network and is being built by London & Continental Railways (LCR) with Union Railways Ltd, a subsidiary of LCR, acting as the client.
Rail Link Engineering (RLE), a joint venture of Bechtel, Ove Arup & Partners, Sir William Halcrow & Partners and Systra, is responsible as project managers for the design, planning, procurement, project management and commissioning of the entire project.
Some 25% of the route between London’s St Pancras Station and the Channel Tunnel is in tunnels, the longest being the 17.5km long, 7.15m i.d. twin tube London Tunnels between St Pancras and Dagenham. A second similar 3km long twin tube tunnel has been constructed to take the railway beneath the River Thames (T&TI, CTRL Supplement, Sept ’03).
Development of the design
As part of the design development and in particular the project’s safety concept, the CTRL project worked extensively with the local emergency services, specifically the London, Essex and Kent Fire and Civil Defence authorities and the Railway Inspectorate of the UK Health & Safety Executive.
RLE decided to review a range of significant recent fire tunnel incidents, and consider where lessons learned may be appropriate to the CTRL. An investigation into passenger train fires from 1971, up to the Channel Tunnel fire of 1996 was undertaken and the results included in a risk assessment developed to review the tunnel design.
It was recognised early on that procedures, especially those covering emergencies, should replicate – as far as was sensible – those used in the Channel Tunnel. This obviously had the greatest ramifications for the safety provisions in CTRL’s extensive London Tunnels section (Figure 2) and the required design that would enable the evacuation of passengers and crew from an incident tunnel to the other tunnel. It was therefore decided, at a very early stage in the design, that regular cross passages would be constructed to provide access between the twin bore tunnels. The challenge would be to find the optimum design passage spacings that would balance both cost and safety (Figure 1).
Cross passage spacing
The original analysis of cross passage spacing (in support of Union Railways Ltd. Outline Railway Safety Case), estimated that cross passage provision in the London Tunnels only marginally reduced the risk to passengers using the CTRL. However, reducing the risk to as low as reasonably practicable (ALARP) is the project’s primary safety criterion.
Using data taken from an early risk analysis of cross passage spacing schemes it was estimated that for spacings as far apart as up to 1200m, there was a negligible increase in risk to passengers in an emergency situation.
RLE had originally considered schemes based on a 1000m-1500m cross passage spacing, as this was consistent with the design of similar tunnels recently approved in the UK. The Heathrow Express, Jubilee Line and CrossRail tunnels all have, or have designed, ingress and egress points every 1000m.
The maximum spacing of the cross passages in the London Tunnels is, however, limited by electrical design requirements, as each will contain signalling and communication equipment.
Additionally, recent trials suggest that the maximum practical distance between a fire brigade bridgehead, where breathing apparatus is donned, and a tunnel incident should be 350m. Therefore, RLE decided to analyse design options based on a maximum cross passage spacing of 750m as it conforms to the requirements of the emergency services.
The London Tunnels will have a forced ventilation system that will provide smoke management during a fire and also fresh air for an approach to a train in one direction. Emergency services will be able to approach the incident, initially through the non-incident tunnel and then through the nearest cross passage, in clear air. Breathing apparatus will only have to be donned in the immediate area of the fire.
Risk analysis
A risk analysis was performed on the following three cross-passage schemes eventually considered:
a) 375m cross passage spacing – This scheme was the one assumed in the CTRL Outline Railway Safety Case and provides the same cross passage spacing as the Channel Tunnel. It was originally proposed for a tunnel length of 30km. This arrangement would lead to significant costs and an increased safety risk due to the extra construction efforts needed to build the estimated 39 cross passages required by the design.
b) Regular spacing of cross passages at 750m – With this scheme, cross passages would be spaced at regular 750m intervals throughout the tunnels. It was estimated that 17 cross passages would be required with this arrangement.
c) Provision of emergency response locations (ERL) – This option would provide ‘clusters’, cross passages at five ERLs, associated with the ventilation and access shafts. At each of the ERLs, extra cross passages would be constructed, resulting in a reduced cross-passage spacing in that area of the tunnel. To comply with Fire Brigade operational requirements, cross passages would be provided at a regular spacing between each ERL (it should be noted that Stratford International Station will also provide emergency de-training facilities). It was estimated that 24 tunnelled cross passages would be required with this arrangement.
The analysis drew on fire frequencies obtained from fault trees, which used data derived from incidents in older British Rail rolling stock, hence the fault tree tended to over estimate the frequency of fires.
The fire frequencies were then used in the event trees to estimate the consequences of fires in tunnels with probabilities assigned to the likelihood of:
From the event trees output, a risk table was developed for a series of incidents involving all passenger traffic types, and consequences estimated in terms of casualties and injuries. This data was then used to calculate the risk to passengers and staff.
The risk analysis estimated the safety benefit of the two potential design options, b) 750m and c) the ERL scheme, when compared to the original 375m cross-passage scheme a). It also assessed whether any significant safety benefit would have been gained using the enhanced 750m scheme (c), when compared to the regular 750m scheme (b).
The results
The original 375m cross passage scheme was not reasonably practicable, because the cost was grossly disproportionate to the safety benefit gained, even if the statistical value for each casualty avoided was taken to be US$7M. The difference in safety benefit between 375m and 750m cross passage spacing was also not considered significant. The increase in safety benefit provided by the ERL scheme was negligible, in comparison to the regular 750m spacing scheme (b).
For the above reasons RLE originally proposed that – in addition to the emergency smoke management and access facilities provided in the London Tunnels – a design that placed cross passages at 750m spacing was reasonably practicable and gave a considerable margin of safety for CTRL passengers.
The findings of the risk and cost benefit analysis were presented to HMRI and the local authority Fire Brigades, through the Safety Liaison Group (SLG). A series of workshops were held to explain, in detail, the methodology of the risk analysis and the robustness of its conclusions.
It was considered, however, that although not justified by the risk analysis, the ERL scheme (c) had the following advantages:
ERLs provided the optimum operational facilities for the emergency services. They enable entry to be made to the tunnel in close proximity to the train; also lifts and stairs direct to the FICP would be available for casualties.
ERLs provide sufficient cross passages for evacuation of variable length trains (Eurostar, domestic and open access)
The scheme also allows for the tunnelling of cross passages in the best conditions and reduced the need for hand tunnelling in ‘bad ground’, thus reducing the risk to construction workers.
The ERL design therefore received a letter of non-objection and was adopted as a practical scheme that satisfies both passenger evacuation and fire brigade operational requirements.
Lining design
In 1996, the major fire in the Channel Tunnel caused extensive damage to the precast concrete lining. The fact that the 500mm thick reinforced concrete lining was totally destroyed in the tunnel crown was a significant concern to the CTRL project. The Channel Tunnel was constructed in stable chalk marl whereas the London Tunnels’ alignment runs through Thanet Sands at up to 3 bar pressure. The loss of the lining under these conditions would result in an inundation of the tunnel with water and sand. RLE decided that it had to fire harden the 7.15m i.d tunnel lining.
CTRL tunnel linings represent the first use of steel and polypropylene fibres to provide a fire hardened corrosion resistant lining with a 120-year design life.
The use of a steel fibre ring was determined during pre-tender design reviews to counteract the highly saline conditions that existed for the Thames tunnel under the River Thames. The tunnel is subjected to pressures in excess of 4.5 bar and the groundwater around the tunnel is highly saline.
Experience from the Far East has shown that reinforced concrete linings are susceptible to corrosion caused by the wetting and drying effect of trains operating in the tunnels and require significant maintenance work after 15 years.
Whilst a plain concrete segment would have met the corrosion requirements, experience in the use of un-reinforced linings in large diameter tunnels built by Earth Pressure Balance Machines (like the CTRL London Tunnels) has resulted in significant damage during construction. The decision was made to use steel fibres to give the segments both ductility, and the ability to withstand the high shove forces from the TBMs.
Codes exist to cover the testing of fibre reinforced concrete as a material, but no codes exist at present to cover design of tunnel linings using steel fibres.
RLE undertook a testing programme to establish the design parameters for the segments to withstand a 5MW fire, handling forces, construction tolerances and long-term loading. The testing regime confirmed that steel and polypropylene fibres, coupled with granite or limestone aggregate would meet the structural performance and fire performance requirements.
The CTRL contractors have manufactured the steel and polypropylene fibre segments on site. The design was also adapted to take on board buildability adjustments proposed by the contractors, which included the use of spear bolts and a doughnut on the circle joints, additional rebates around joints and a change in taper on the Thames Tunnel.
All the segments manufactured at the CTRL’s three factories are interchangeable. This has allowed the Thames Tunnel segments to be used to negotiate the 500m radius curve on the London Tunnels C220.
Over 40km of fibre reinforced segments have been designed, manufactured and constructed with hardly any damage/spalling on any of the joints between the segments. The main damage has been cracking across some segments, parallel with the cross joint. As a remedial measure, these are being injected with an epoxy resin. The segments have proved both economic and easy to manufacture and construct. They also provide a fire hardened lining with no reinforcement that can corrode.
Conclusion
The CTRL long tunnels have been designed to remain consistent with the present Eurostar train emergency procedures. Many similarities exist in the tunnel safety provisions in the CTRL and Eurotunnel.
Recent Rail Tunnel fires have been reviewed and any consistencies noted. The design of the CTRL tunnels adopted relevant recommendations from previous emergency incidents. In order to conform to the principles of ALARP and to provide the most effective safety strategy for the CTRL, however, some safety provisions such as ventilation and cross passage spacing and tunnel lining design were adapted for the particular circumstances of the CTRL tunnels. Where designs have been developed in this way, a risk-based approach was used to determine the most effective safety scheme, in consultation with HMRI and the Fire Brigade.
Related Files
Fig 2 – Long section of the London Tunnels alignment
Fig 1 – Plan diagram of the CTRL London Tunnels’ ERL cross passage locations
