For Section 2 of the UK’s US$4.7bn Channel Tunnel Rail Link (CTRL), the difficulties faced by engineers for the project’s 7.15m i.d. bored tunnels included requirements for exceptionally high durability and low maintenance, combined with a high resistance to the effects of fire. Additionally, some of the precast concrete segmental linings were required to accommodate future ground movements and loadings induced by adjacent construction works.
CTRL’s Section 2 includes over 20km of twin-tube running tunnel (T&TI, Sept 2003, Supplement). Much of the alignment lies in water-bearing sand with high hydrostatic pressures. Designed by Rail Link Engineering (RLE), a consortium of Arup, Bechtel, Halcrow and Systra, over 40km of fibre reinforced tunnel segments were manufactured for the project. This represented the first use of steel and polypropylene fibres to provide a fire hardened, corrosion resistant tunnel lining with a 120-year design life.
At the time of the 1996 Channel Tunnel fire, the CTRL tunnels were still in the design stages. This provided the RLE team with the opportunity to include more resistant specifications for the tunnel lining designs. These new design requirements included the capacity to withstand highly saline ground conditions, high temperature fires, increased tunnel construction tolerances and long term loadings, and minimal maintenance.
Steel fibre reinforced concrete technology is said to improve tunnel lining durability whilst maintaining structural integrity. At the time however, there were no codes covering the design of tunnel linings using steel fibres. Eddie Woods, RLE (Arup) project engineer, said: “To overcome the lack of standard test methods and codes for the tunnel linings using steel fibres, the linings were essentially designed as plane concrete segments, taking into account enhanced tensile stresses in the consideration of the joints.”
RLE undertook a testing programme to establish the fire design parameters for the new tunnel linings. They investigated limestone, granite and lightweight aggregates, coupled with two commercially available polypropylene fibres – monofilament and fibrillated. Steel fibres were also assessed. It was discovered that the segments containing polypropylene fibres maintained their integrity in severe fire conditions at an acceptable cost. These were thus specified for the manufacture of the concrete segments.
According to Peter Shuttleworth, RLE (Arup) senior engineer, “the test programme was designed to simulate fire effects on an in-situ lining with samples exposed to fire loading on one side only, and comparative data obtained on the propensity for spalling between the full range of mixes considered.”
All the samples tested indicated that the mixes containing polypropylene fibres would provide adequate fire resistance. It was established that the risk of concrete spalling was minimised through the use of monofilament polypropylene fibres, which will melt in the event of a large fire and allow steam generated from the concrete to escape.
Steel fibres coupled with limestone or granite aggregate enable the linings to meet structural performance requirements both before and after a fire. Additionally, the steel fibres give the concrete lining segments ductility and help withstand high shove forces from TBMs.
The final design of the tunnel linings incorporated all the new design specifications, while remaining flexible enough to include adjustments proposed by contractors. The tunnel lining segments manufactured for the project have since proved their robustness – there is very little damage or spalling on joints between segments. They were straightforward and economical to manufacture and construct, and provide a fire hardened lining with non-corrosive reinforcements.
Planning ahead at Moorhouse
The Moorhouse development has involved the demolition of an existing 1960s office block and construction of a new high-rise 29,000m² office and retail block with two basements and a car park. The basement includes a tube ticket hall entrance and an 8m diameter, 40m deep draught relief shaft for future Crossrail station and tunnels (T&TI Sept 2003, p22). The design for the Moorhouse Shaft incorporates an integrated and novel solution for construction within a new, deep piled foundation, where both elements are subject to significant future ground movements. The development building itself is supported on 45m bored, base-grouted piles bearing in the Thanet Sand, installed with low friction sleeves to minimise the draw-down effects of the planned Crossrail construction.
The segmental shaft lining design needed to incorporate a number of requirements, including:
Combined methods of analysis were applied to the challenge, including Arup’s in-house Oasys finite element software program SAFE, which provided the soil model for the London and Lambeth Clay strata. A Mohr-Coulomb soil model for the coarse grained strata and a linear elastic model was used to describe the behaviour of the draught relief segments. Alan Winter, Arup, lead designer, said: “Taking the results from the analysis, the design of the precast concrete segmental lining was developed to cope with the exceptional and varied performance requirements imposed by the loading situations.”
A familiar and well-proven lining pattern (developed by Charcon) was used as the basis for the final design solution and incorporated innovative sliding bearings in the circle joints.
The design led to financial savings for the client by mixing standard components, fibre reinforcement and a facility to accommodate future ground movements to optimise the lining design to cater for varied performance requirements.
Cast iron standards
London Underground Ltd has a long association with cast iron segment construction. Atkins, one of Metronet Rail’s five shareholders, is involved in the inspection and assessment of the Deep Tube Tunnel assets for London Underground. To assist with the assessment of the cast iron tunnel segments, various non-destructive techniques are being utilised including the Broadband Electro Magnetic (BEM) technique. This has been used in the pipeline industry for some time, but is relatively new to tunnel inspection.
The technique enables the segment thickness to be assessed and indicates cracking and areas of corrosion. A major advantage of the technique is that readings of the cast iron can be obtained through the secondary lining of infill concrete and tiled finishes. Baseline safe load analytical analysis of the cast iron linings are being carried out for the first time. The analyses are based on existing London Underground Standards, which limit the lining capacity to the elastic range. This may be potentially conservative. Knowledge on the segment types and behaviour of cast iron is being improved as the assessment work progresses. Collaboration with universities and research bodies has been established with the aim of improving the industry’s knowledge and approach to dealing with cast iron, a structural material that has a long history in the underground environment.
Originally, cast iron was used in the London Underground during the Victorian era. In fact, modern refurbishment projects are uncovering cast iron that has been in use for over 150 years and is still operational. Grey iron was the material traditionally used to manufacture tunnel segments prior to the introduction of ductile iron. The addition of spheroidal graphite (or ductile) iron (SGI) to the family of irons was introduced commercially some 50 years ago and has replaced the use of conventional cast iron, and steel in many areas. Ductile iron was first used for tunnel segments around 30 years ago.
The main difference between ductile and cast iron is that in cast iron the graphite is in the form of flakes, around 1mm in length, which can act as ‘stress raisers’ and as a result cast irons are comparatively weak with practically no ductility. Whereas in ductile iron the graphite forms in spheroids, which removes the potential crack effect and in fact the graphite nodules act as ‘crack arresters’. The change in graphite form is achieved by treating the molten iron with an addition of 1.50% magnesium ferro-silicon alloy, containing 5% magnesium. This results in a tougher material with an elongation (ductility) percentage ranging from 2% to 15% in the ‘as cast’ condition – that is, without any subsequent heat treatment.
Ductile Iron can be produced in a variety of grades within the specification: BSEN 1563:1997. This offers designers a great amount of flexibility, by modifying the chemical composition and cooling rates of the molten iron. The grade specified for tunnel lining segments is EN-GJS-600-3, which has a tensile strength of 600N/mm² with an elongation of 3%; in comparison cast iron would have a tensile strength of 200N/mm² with zero elongation. This substantial increase in properties has allowed for a significant reduction in the skin thickness and therefore weight of tunnel lining segments (approximately 30%).
Vast areas of the London Underground network are lined with ductile iron. Over recent years however, due to developments in pre-cast and sprayed concrete, its main area of utilisation has been in hand mined tunnels due to its reduced weight compared with pre-cast segments, enabling easier handling.
Typical applications include: cross passages, opening sets (hybrid ductile iron/pre-cast segments where cross passages break out into a main tunnel), lift shafts, escalator shafts, ventilation shafts, and sump shafts. Ductile iron tunnel segments are particularly useful where:
The inherent strength allows for thinner segments for a given load bearing capability. This in turn reduces the waste and spoil generated – typically by approx 15%.
Ductile iron’s greater strength and impact resistance over concrete segments eliminates breakage risks in transit and the risks associated with second comer damage or disruption. In addition to this, transport costs and associated environmental impacts are also lower per segment for ductile iron. It is environmentally friendly, being 100% recyclable, can be coated to client specification, and a typical specialist coating is available which provides low smoke emissions.
In short, low smoke, low toxic fumes, improved service life, reducing maintenance and long term operational costs, mean that ductile iron segments have much superior properties to grey iron segments, which themselves have already been in service for well over 100 years.
