Fire and Water – the London Crossrail Experience

Mike King – CH2MHill

Abstract Like many major urban centers, London is faced with an ever increasing population and the need to

transport people safely and efficiently across a sprawling city landscape. The Crossrail project will

provide an East-West link from the suburbs and across the city enabling an additional 200 million

journeys each year linking up major business centers and existing transportation interchanges, while

reducing congestion at street level and within the existing busy underground system.

This paper will outline the scheme, and in particular, examine the technical approaches taken by the

project to deal with waterproofing underground structures and the fire protection strategy adopted for

the main structural materials in the tunnels, with a summary of the methods used and tests undertaken,

including some lessons learnt and approaches that might be used again or refined further on future

projects.

Introduction The idea for an East-West rail link across London has its origins in the Regents Canal & Railway Company

of the late 19th Century. The concept of the link went through several other evolutionary developments

over the following 90 years including the post second world war Abercrombie Plan, leading up to the

more recent 1974 London Rail Study where “Crossrail” was finally born. Had the scheme been built at

any of these previous times we would probably be looking at a very different product to the one now

showing its face to the world today, which uses the latest materials and technology to meet the

demanding performance requirements imposed on the project. With a combination of a design life of

120 years and some exacting performance demands for the structures, for example the requirement to

withstand the extremes of a 12000C fire and resist 40m head of external water pressure, the project has

developed solutions based upon previous project experience and robust engineering to satisfy

requirements that could be argued to require conflicting solutions for the materials in terms of their

permeability at the very least.

This paper summarizes the key aspects of the project including an overview of the strata that the

tunnels were constructed in, and then presents the project requirements for the tunnel structures in

terms of fire and water protection, and the tests undertaken to demonstrate compliance and allow

acceptance of the completed structures by the eventual owners, railway regulators, operators and

maintainers.

Overview of the scheme

General scheme The Crossrail East-West link across London will provide a 10% increase in existing rail capacity in London

enabling an additional 200 million journeys each year linking up major business centers and existing

transportation interchanges beneath the congested and built-up city. The whole scheme extends from

Reading in the West, to Shenfield and Abbey Wood in the East, but it is the central section from Royal

Oak in West London to Plumstead and the Olympic Stadium in the East that contains the vast majority of

the tunneling works (see figure 1). The central section has now completed construction of the 42km of

major running tunnel structures in Precast Concrete Segmental Linings (PCL), constructed behind

pressure balance tunnel boring machines. Sprayed Concrete Linings (SCL) and cast insitu linings within 5

new stations, 6 crossover and bifurcation caverns, and numerous access passages and crosspassages,

adits and ventilation tunnels are due for completion later in 2016.

Figure 1 – Central tunnel section of Crossrail

The high capacity train service will operate up to 24 trains per hour and will bring an additional 1.5

million people within 45 minutes commuting distance of London’s key business districts.

The main tunneling contracts with fire test and waterproofing test results discussed in this report are

summarized in table 1.

Table 1 – Summary of main tunneling contracts with test results considered in this report

Contract No.

Principal Contractors Main Construction elements with test results considered in this paper

C300 BAM / Ferrovial / Kier PCL from Royal Oak to Farringdon SCL at Fisher Street Crossover caverns

C305 Dragados / Sisk PCL from Victoria dock portal to Farringdon, and from Pudding Mill Lane to Stepney Green SCL at Stepney Green bifurcation caverns and Limmo shaft

C310 Hochtief / Murphy PCL from Plumstead Portal to N. Woolwich Portal

C410 BAM / Ferrovial / Kier SCL at Bond Street Station and Tottenham Court Road Station

C435 BAM / Ferrovial / Kier SCL at Farringdon Station Cast insitu at Farringdon Station

C510 Balfour Beatty / Morgan Sindall / Vinci

SCL at Liverpool Street Station, and Whitechapel Station and crossover caverns

Geological setting The geographic spread of Crossrail across London has meant that most of the strata beneath the city

down into the underlying chalk have been encountered and traversed by the tunnels on the project.

Starting from the west end of the central section, the tunnels are in the over-consolidated London Clay

from Royal Oak to between Tottenham Court Road and Farringdon Stations. The London Clay is the

favored tunneling medium in London where historically most of the tunnels across the city have been

constructed up until the time of the introduction of the more advanced face pressure stabilizing

technologies now available on standard tunnel boring machines. Although containing some sand lenses

and water which locally increases the permeability, the London Clay is a relatively impermeable and

stable tunneling medium. Further east towards the center of the tunneled section the tunnels and

stations at times move down into the Lambeth Group, which is a variable layer of sediments containing

clays, sands gravels and silts, up to 20m in thickness. Sand channels are more common, and the strata

becomes increasingly permeable towards the lower levels. East of Stepney Green the tunnels

intermittently also encounter the Thanet Sand, part of the principal aquifer under London, which

reduces in thickness towards the west but is present along the route with thicknesses of up to 20m.

With coarser sand grading towards the top of the strata and a basal unit of angular to rounded flint and

gravel, the strata can be locally highly permeable, subject to tidal pressure fluctuations and very

unstable without immediate support or dewatering.

Below these strata lies the Chalk which, along with the Thanet Sand was encountered on the alignment

beneath the river Thames between North Woolwich and Plumstead Portals. The chalk along the tunnel

route varied from weathered to structured in nature and can be very weak and highly permeable.

General description of tunnel lining systems

Precast Concrete Segmental Linings (PCL) The twin tunnels running between the stations are formed from precast concrete segmental elements

300mm thick. There are 7 segments and a smaller key in each ring that form the tunnel lining, with a

finished theoretical internal diameter of 6.2m (see figure 2). The rings are tapered across their diameter

and rolled for alignment control but are nominally 1.6m long and were erected with steel bolts across

the longitudinal joints, and dowels across the circumferential joints. The steel bolts were removed from

the ring above axis level sometime after ring erection to eliminate the risk of bolts loosening and falling

during the operation of the railway.

The segments are reinforced with steel fibers for the majority of the route, but at locations where

floating track slab is required for noise and vibration control, a light cage is also introduced to resist local

punching shear forces.

Figure 2 – Crossrail precast concrete segmental running tunnel

Sprayed Concrete Linings (SCL) The sprayed concrete lined tunnels on the project are built up with multiple layers of sprayed concrete,

waterproof membrane and fire protection as shown in schematic form in figure 3, with some layers

sprayed in 2 passes depending upon tunnel size and lining thickness, for example P1 and P2 for the

primary lining as indicated in figure 3. The majority of the SCL tunnels were generally close to circular

with finished internal diameters ranging from under 5m up to about 10m. The crossover and bifurcation

caverns were closer to an elliptical shape with an excavated diameter across the major axis (horizontal)

of over 17m and across the minor axis (vertical) of about 14m, but were built up with a similar layer

arrangement to that shown in the figure 3.

The different layers were generally steel fiber reinforced with the exception of the regulating layer,

which had a smaller aggregate and no fibers, and the fire protection layer, which had no steel fibers but

included monofilament polypropylene fibers as discussed later in this paper. At some locations, for

example close to junctions with crosspassages and in the larger and flatter crossover caverns, local

reinforcement was introduced in thickening layers inside the primary lining, or in the secondary lining to

resist the higher bending stresses. On Crossrail, the primary lining forms part of the load carrying

permanent works.

Figure 3 – Schematic of SCL layer build up

Tunnel structures under fire

Project performance requirements The requirements for the tunnel structures included the need to follow the European Technical

Specification for Interoperability – Safety in Railway Tunnels3 that was current at the time of scheme

development and project design. This Specification mandates the use of the RABT-ZTV (EUREKA) fire

curve (see figure 4) for structural design, and includes a requirement for the safety of people in the

tunnels while evacuation is undertaken, but makes no specific provision for the longer term stand-up

time and ground support. The Crossrail project made the decision that the hazards associated with the

risk of loss of ground support in this urban environment during or after the design fire scenario were too

great to contemplate, and specified a requirement that the ground support would be maintained, albeit

with a reduced factor of safety, even after the design fire event. This did not preclude the acceptance of

damage to the linings requiring some repair at a later date, and the limits listed in table 2 summarize the

designer’s specified requirements.

The Technical Specification for Interoperability is based upon hydrocarbon type fires and is applicable

only to structures directly surrounding the train pathway. Other underground structures such as

crosspassages protected by a fire door and escalator tunnels did not fall within this definition and have

generally only been required to meet local Building Regulation fire loads based upon a cellulose type

fire. There were some local exceptions to this such as ventilation adits as these had the potential to be

exposed to fires similar to the main running and platform tunnels. The tests and results discussed in this

paper are for the EUREKA fire curve exposure only.

Table 2 – Summary of project performance requirements of test samples

Precast segmental linings

Sprayed and cast insitu concrete linings

Spalling limit 25mm Fire protection layer + 25mm

Temperature limits 4500C at level of reinforcement. Suppliers declared acceptable limit at level

of waterproof membrane.

Residual strength after fire 70% of design 28 day strength

Figure 4 - Eureka curve time-temperature profile

Protection materials adopted Previous tests and projects had demonstrated that polypropylene fibers within the concrete mix can

significantly enhance the performance of the material under fire conditions1,2 by limiting concrete

spalling. The use of these fibers is also recommended within Eurocode 24. Variations of approach to the

use of this material were taken depending upon the construction methods adopted, and these

approaches are summarized below:

Segmental linings – polypropylene fibers incorporated throughout the full segment thickness for

both steel fiber reinforced segments and segments with local traditional reinforcement.

Sprayed Concrete linings – polypropylene fibers incorporated into a final layer of sprayed

material. Minimum thickness specified by the designer to be 50mm.

Cast insitu linings - polypropylene fibers incorporated throughout the full lining thickness for

both steel fiber reinforced sections and areas with traditional reinforcement (for example close

to openings).

In other parts of the project works that were required to be designed for similar fire conditions (for

example piled walls, diaphragm walls and insitu concrete slabs, which are not covered in this paper),

other proprietary products including both sprayed and protection board products were also used.

Testing regimes The final configurations and material selections for the PCL and SCL structures were required to be

verified under large scale fire testing to the specified EUREKA curve, and all tests were to be undertaken

under compressive loads representative of the loads on the structures in the Works. The stresses

applied to the test piece allowed for the worst case design load stresses, plus the effects of stress

increase due to lining expansion in the ground under elevated temperature conditions.

0

200

400

600

800

1000

1200

1400

0 50 100 150 200

Te

mp

era

ture

(0C

)

Time (mins)

EUREKA Fire Test Temperature/Time Curve

For the precast segmental lining tests, curved members representative of the tunnel segments which

were 300mm thick were used rather than flat panels. The test set-up was as shown in figure 5, and the

stresses in the lining during the test were controlled by relaxation of the rams to compensate for the

test segment expansion during the fire test. Deformation of the segments was also monitored during

the test but this was found to be minimal and had no significant impact on the performance of the

concrete under fire test conditions.

Figure 5 – Precast segmental lining test arrangement

For the SCL tests, initially small scale tests were undertaken on unloaded cylinder samples to compare

the performance of different fiber dosages, concrete mixes and, if required, fibers from different

suppliers. These were followed by large scale sprayed flat panel tests on samples with a total thickness

of approximately 500mm, with the test undertaken under compressive loads as shown in the

arrangement in figure 6. Most tests had a vertical panel arrangement rather than horizontal to suit the

furnace equipment available, although horizontal test set-ups were also acceptable.

Figure 6 – Test panel arrangement for representative SCL linings.

During the tests instrumentation and monitoring were included to measure the following: loads

(concrete stress); temperature in the concrete at various depths using thermocouples; and spalling

depths immediately after the test and after a cooling period for the samples.

Test results and compliant materials for PCL Table 3 below outlines the mix designs for the successful test segment samples and table 4 contains

summary details for the spalling depths measured. It should be noted that the results of fire tests are

very specific to the mix design and particular fibers and only provide guidance for possible successful

combinations on future projects unless very similar mixes and material sources are used. The stress

regime within the test samples can also influence performance and this should also be taken into

account.

Table 3 – General PCL concrete mix details.

Material Contract C300 Contract C305 Contract C310

CEM1 - Portland Cement (kg/m3) 287 337 258

PFA/GGBS (kg/m3) 123 113 172

20mm Limestone Aggregate (kg/m3) 643 723 263

10mm Limestone Aggregate (kg/m3) 684 361 706

Sand (kg/m3) 509 765 778

Steel fiber dosage (kg/m3) 35 30 30

Polypropylene fiber dosage (kg/m3) 1.25 (Propex 32

micron)

2.0 (Propex 32

micron)

1.5 (Baumhueter

Eurofibre HPR)

w/c ratio <0.35 0.32 0.33

Table 4 – Spalling depths from successful fire tests

Measured spalling depths C300 C305 C310

Average spalling depth (mm) 16 8 3

Maximum spalling depth 23 13 14

Figure 7 below shows smoothed lines for the maximum temperatures measured with depth into the

concrete (referenced to the face at the start of the test) from all test results. The maximum values were

measured on one of the C300 tests. Temperatures were measured using 2 thermocouples in each layer

being measured, spaced at 500mm along the center-line of the segment. Average values from all of the

successful tests at the higher temperature points along the curves shown in figure 7 at each layer were

about 70-80% of the maximum values shown.

Figure 7 – PCL “Smoothed” maximum temperature profiles with depth for EUREKA fire test

Test results and compliant materials for SCL The SCL had a higher variability in mix designs than the PCL, with more variance in the type and number

of different aggregates used and the nature and type of the admixtures. Table 5 below summarizes

principle quantities only for the main mix designs, with the final fire protection layer being of a similar

mix but with polypropylene fibers in the mix and no steel fibers.

Table 5 – General SCL concrete mix details.

Material Contract C305 Contract C410 & C435

Contract C510

Cement (kg/m3) 399 455 400

Microsilica (kg/m3) 52 50 52

Large aggregate - Limestone (kg/m3) 435 629 505

Small aggregate - Limestone (kg/m3) 435 - 590

Sand (kg/m3) 870 1168 580

Steel fiber dosage (kg/m3) – not in fire protection layer

35 35 35

Polypropylene fiber dosage (kg/m3) Fire protection layer only.

1.0 (Propex 32

micron)

1.0 (Adfil Ignis)

2.0 (Propex 32

micron)

w/c ratio 0.4 0.36 0.43

It was also necessary for the SCL contracts to test two panel types – one with the secondary lining

consisting of steel fiber reinforced concrete, and another with a secondary lining consisting of plain

concrete (no steel fibers) but with a representative quantity of traditional reinforcing steel to simulate

conditions at tunnel junctions where higher bending moments are present and they require additional

flexural strength.

Results from successful tests of temperature against depth within the SCL section are summarized in

figure 8 where the maximum results from the tests are plotted for both the steel fiber reinforced

sections and traditionally reinforced sections, all with the polypropylene fiber fire protection layer

sprayed at a minimum of 50mm. All distances are referenced to the concrete face at the start of the

test.

Figure 8 – SCL “Smoothed” maximum temperatures with depth for EUREKA fire test

Additional tests were undertaken by various contracts during the project to demonstrate alternative

arrangements for the linings and fire protection to suit particular construction circumstances. The

following summarizes the tests that met the specification criteria and which were accepted as solutions

for the permanent works:

C410 - 40mm layer of polypropylene fire protection layer sprayed over steel fiber reinforced

secondary lining (note that 40mm fire protection over reinforced concrete failed the

specification criteria on 1 out of 3 tests and was not accepted)

C510 - Light stainless steel mesh and anchors within the fire protection layer at 50mm cover to

assist with profile control

C510 - secondary lining sprayed in 2 main layers (nominally 150mm each) with polypropylene

fibers and steel fibers in the final layer, i.e. without a separate fire protection layer but with local

protection against snagging on steel fibers at the surface being required in some areas.

Generally there was more spalling and some delamination of the fire protection layer witnessed during

the SCL test compared to the PCL tests, but the loss of the protection layer when sprayed as a final layer

was acceptable for the design as long as spalling of the remainder of the secondary lining was less than

25mm. The majority of tests saw no spalling of the secondary lining although some spalling of up to

20mm was detected in a few panels. Figures 9a and 9b show typical results of damage to the fire

protection layer of the test panels, the majority of which occurred during the cooling phase after the

main test rather than being visible immediately after removing the sample from the furnace.

Figure 9a – Test panel immediately after test Figure 9b – Same panel after cooling

Lessons learnt from spraying fire protection layers A number of lessons can be taken from the Crossrail experience during the construction of the works of

spraying relatively thin layers of fire protection. The main considerations for future projects would

include the following:

Profile control and space proofing for the final layer needs consideration as tolerances are far

larger than can be achieved with cast insitu linings. Sufficient space needs to be allowed for

tolerances affecting every sprayed layer to achieve the minimum thicknesses and allow

sufficient space for subsequent layers, recognizing that far greater construction thicknesses are

also possible. Even allowing for tolerances of +/-30mm on the fire protection layer have proved

to be challenging, particularly at junctions.

Surface preparation is critical to achieve good bond between the layers and a process using

continuous cleaning with high pressure systems and only spraying small areas is required.

Bond below axis has proved to be more difficult to achieve. This is thought to be at least partly

related to the surface preparation and the constant need to meticulously remove both dust and

wash down water from the spray area.

Tunnel structures under water pressure

Project performance requirements The project general watertightness performance requirements are summarized below.

For tunnels containing track, the area above axis to be free from all leakage seepage and damp

patches and comply with the requirements of BS 81025 Table 2; grade 3 (No water penetration

acceptable; Ventilation, dehumidification or air conditioning necessary, appropriate to the

intended use).

For tunnels containing track, leakage from the area below axis to be limited to damp patches

and minor weeping of joints. The average level of water ingress in any drainage section to be

less than 0.12 liters/day/m2, except that within any 10m length, ingress of 0.24 liters/day/m2

would be permitted from any separate square meter.

Tunnels and shaft areas accessible to the public to be free from all leakage seepage and damp

patches to provide a dry environment.

Tunnels and shafts not accessible to the public to be limited to damp patches and minor

weeping at joints. Water ingress in any individual location to be limited to less than

1litre/day/m2 and the average shall not exceed 0.1 liters/day/m2.

Materials adopted To achieve these standards the segmental lining design required the use of gaskets on every segment of

every ring. Composite EPDM/hydrophilic gaskets were envisaged to be used across the project at the

design stage, to make use of both the mechanical pressure between gaskets to form a seal, and an

expanding component (hydrophilic) when in contact with water. A concession was granted where the

tunnels passed through zones of relatively impermeable clay to delete the requirement for the

hydrophilic portion of the gasket and to rely on the purely mechanical EPDM element.

For tunnels with sprayed concrete secondary linings, sprayed waterproof membranes were used, and

where cast insitu secondary linings were to be used the contractor had the choice of either using a

sprayed membrane or a more typical arrangement of geotextile fleece and sheet membrane. Sheet

membranes could be used locally, for example where the invert only was to be cast (figure 10a) or as a

full seal on its own where a full tunnel shutter was to be used to cast the secondary lining (figure10b).

There were also some local issues and exceptions to be taken into account, for example at escalator

barrels, where sprayed rather than sheet membranes were used with cast insitu secondary linings to

prevent connections between aquifers at different strata levels. Sheet membranes are a well-

established and tried and tested method of waterproofing tunnels and this paper will therefore only

deal with sprayed membranes in the following sections.

Figures 10a) and 10b) – Sheet membrane installation

Testing regimes All of the waterproofing materials were subjected to a wide range of material performance and

durability testing, but this section deals only with aspects related to meeting the watertightness

performance requirements of the project.

The gaskets for the segmental linings were tested for water pressure resistance under laboratory

conditions and replicated maximum lipping between segments as well as maximum potential joint

“gaps” due to the various tolerances for manufacture, construction and ring distortion under load.

Taking these elements into account led to the gaskets being tested for a water pressure up to 8 bar,

which was 2 times the expected working pressure.

Extensive use of sprayed waterproofing membranes had not been deployed on the scale required by the

Crossrail project in the UK before. There was limited experience available from Europe and a road tunnel

project in the South of England, and small scale use on other underground metro projects in London, but

Crossrail was to use the material in much greater quantities and in areas with more difficult ground

conditions. In addition to a number of site trials, physical testing of the sprayed membrane included the

following:

Membrane bond to substrate (greater than 0.5MPa required)

Permeability (zero penetration of water at 10 bar after 28 days)

Crack bridging

Flammability (temporary condition requirement)

Test results and compliant materials for the segmental linings All of the segmental lining contracts opted for a gasket supplied by Datwyler, which had a profile that

fitted within the same groove size whether the pure EPDM gasket was used, or the composite

EPDM/hydrophilic type was installed. Test results showed that gaps of up to 7mm could be tolerated

after 48 hours of the hydrophilic material expansion at a test pressure of 8bar and an offset of 10mm,

which exceeded the project requirements. Similar results were achieved for the gasket with no

hydrophilic component, although the gasket section height was slightly greater and the forces required

to fully compress the gasket slightly higher than the lower profiled gasket with the added hydrophilic

strip.

Test results and compliant materials for the sprayed concrete linings At the time material selection for the sprayed membranes was occurring, there were essentially only 3

manufacturers supplying sprayed waterproofing materials that were considered to have the ability to

meet the specification requirements. It is recognized that this is an area where manufacturers are,

relatively speaking, rapidly changing materials and developing solutions to suit new demands and

address observations that may be made as more experience is gained. It is not the purpose of this paper

to make comparisons between the materials available at the time, or the new materials that are now

making their way onto the market. At the time that testing was being undertaken on sprayed membrane

materials, all available sprayed membrane materials examined met the performance requirements

specified by the project.

The bullet points below include some observations made during the Crossrail works that, should similar

conditions be present on a project considering sprayed waterproof membranes, may need to be

addressed with the individual suppliers. Not all observations apply to all sprayed membrane material

types and are provided only to highlight areas that may need to be considered on future projects:

Blistering behind the membrane was locally an issue at some sites even where the substrate the

membrane was sprayed onto looked dry. This was due to moisture in the substrate evaporating

while exposed and appearing dry prior to sealing with a membrane. Re-sealing using crack

injection techniques and local re-spraying of the membrane was required to rectify this issue.

Preparation of the substrate is critical to the success of the installation particularly if reasonably

high levels of bond are required. More objective methods of measuring acceptable surface

preparation (for example roughness, undulation, cleanliness) would benefit future projects.

Specifications should require demonstration of bond between the membrane and the lining

sprayed directly onto the membrane (usually the secondary lining) as well as the bond between

the membrane and the substrate. This is particularly important if any form of composite action

is being considered in the design where issues such as bond, shear and elastic modulus impact

upon the design.

Designs should consider that some bond may be present even if the design does not require it

for structural action – this changes the distribution of stress between the lining layers and has

the potential to make stresses in the secondary lining more onerous.

Some membranes demonstrated a change in behavior under longer term testing and under

saturated conditions including a reduction in bond and a reduction in elastic modulus. This

becomes particularly relevant for any design utilizing a model of composite action between the

lining layers, although this was not a requirement on Crossrail.

Conclusions The Crossrail project has successfully developed tunnel lining structural and serviceability solutions to

meet the conflicting demands required by fire resistant and water resistant structures. There are lessons

to be learnt concerning the use of thin sprayed concrete layers and sprayed waterproof membranes and

this paper has attempted to highlight the high level areas that should be examined in more detail for

projects where similar solutions are being considered.

References 1 P. Shuttleworth, “Fire Protection of Concrete Tunnel Linings” 3rd International Conference

of Tunnel Fires and Escape From Tunnels, Washington DC, 9-11 October 2001

2 T. Lennon and N. Clayton, “Fire tests on high grade concrete with polypropylene fibers” 5th

International Symposium on the Utilisation of High Strength/High Performance Concrete,

Savdejord, Norway, June 1999

3 Technical Specification for Interoperability – Safety in Railway Tunnels. No 2008/163/EG.

(Note – this document now superseded by Commission Regulation (EU) No 1303/2014)

4 BS EN 1992-1-2:2004. Eurocode 2: Design of Concrete Structures. Part 1-2: General Rules –

Structural Fire Design. British Standards / European Committee for Standardisation.

5 BS 8102:2009. Code of practice for protection of below ground structures against water

from the ground. BSI Standards Publication.