WHEN CONTRACTOR IC ICTAS-ASTALDIcame to fix the rebar for the secondary liningof the Riva Tunnel in Istanbul, it ran intoproblems. The 22m-wide tunnel requiredreinforcing bars up to 12m long to be fixed;this was proving difficult to do, introducingsafety risks and causing damage to the sheetwaterproofing membrane which sits betweenprimary and secondary lining.

The solution was an unusual one for Turkey:using fibre-reinforced concrete to create thepermanent secondary lining. Following carefulevaluation and tests at Istanbul TechnicalUniversity, designer EMAY International wasable to demonstrate that by using high-performance steel fibres a fibre-reinforcedconcrete lining would be just as good as onereinforced using steel reinforcing bars.

The Riva Tunnel is one of threeunderground projects to use the new Dramix5D series fibres which can be used to create amaterial which has a far greater residualstrength after cracking than other fibre-

reinforced concretes, behaving more liketraditionally reinforced concrete. In each ofthe three cases, designers and constructorsfaced challenges around constructability ordurability – or both – of the final lining whichhave been solved with the help of this newbreed of fibre.

A 40-year storySteel fibres have reinforced concrete since theearly 1970s. It was the 1990s that saw thestart of underground applications, in bothshotcrete and precast tunnel lining segments.

Back then, fibre-reinforced concrete wasalready being used for some permanentlinings. Costain Taylor-Woodrow used it as afinal lining in a ventilation tunnel on London’sJubilee Line and Taylor Woodrow employed iton the refurbishment of the Victorian BrunelThames Tunnel.

However a lack of regulation and standardshampered the spread of fibre-reinforcedconcrete for final tunnel linings. With the

publication of international design guidelinethe fib Model Code for Concrete Structures2010, this obstacle has been overcome anddesigners are gaining confidence in workingwith fibres.

This year there has been a flurry ofadditional guidance for fibre-reinforcedsegments: the American Concrete Institute’s(ACI) Report of Design and Construction ofFiber-Reinforced Precast Concrete TunnelSegments (ACI 544.7R-16); the InternationalTunnelling and Underground SpaceAssociation (ITA-AITES)’s Twenty Years of FRCTunnel Segment Practice: Lessons Learned andProposed Design Principles; BSI PAS8810:2016Tunnel Design - Design of concrete segmentaltunnel lining – Code of Practice; and ITAtechGuidance for Precast Fibre ReinforcedConcrete Segments – Vol. 1 Design Aspects.

Fibre reinforcement in the secondary liningsof Sprayed Concrete Lining (SCL) designs,such as those used for Crossrail’s station andplatform tunnels, is also becoming more

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FIBRE REVOLUTION

Bekaert Maccaferri’s UndergroundSolutions’ global business developmentmanager, Benoit de Rivaz, talks usthrough the evolution and uses of a newgeneration of steel fibres

commonplace. Designers now have theoption of fibre-reinforced concrete for cast-in-situ final linings too.

Steel fibres come in different sizes, shapesand qualities with each having its own effecton the concrete behaviour and quality. Thedosage of fibres needed to meet the desiredstructural performance will vary, dependingon the characteristics of the fibre itself.

The behaviour of fibre reinforced concrete ismore than a simple superposition of thecharacteristics of the concrete matrix and thefibres. To analyse the behaviour of thiscomposite material, the way that the loadstransfer between concrete matrix and fibrealso have to be taken into consideration.

For efficient load transfer, three conditionsmust be satisfied. There must be sufficientexchange surface, governed by the number offibres, their length and diameter; the nature ofthe fibre-matrix interface which allows for aproper load transfer; and the mechanicalproperties of the fibre such as Young’smodulus, tensile strength and anchoragestrength which must allow the forces to beabsorbed without breaking or overlyelongating the fibre.

Bekaert Maccaferri spent five yearsdeveloping the 5D fibre, trying manycombinations of hook design and wirestrengths in order to optimise the performanceand cost. The goal was to create a fibre thatcould be used in structural applications such asfoundations slabs, rafts, suspended structures– and the final linings of tunnels.

Dramix 3D – the name for the BekaertMaccaferri’s original steel fibre – and theDramix 4D, which uses higher-strength wireboth work in the same way. The kinked endsprovide ductility to the concrete by slowlydeforming as the wire is pulled out of theconcrete. This is the mechanism that generatesconcrete ductility and post-crack strength.

The Dramix 5D fibre, with four kinks ateither end rather than two or threerespectively, actually works in a totallydifferent way to its siblings the 3D and 4D: a

revolution rather than evolution. Rather thanrelying on the pull-out mechanism of thefibres to provide ductility, the 5D does not pullout but remains anchored, lengthening itself -

as rebar does - to provide ductility. The tensile strength of a steel fibre has to

increase with the strength of its anchorage,otherwise it would snap causing the concrete

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Figure 1: EN14651 beam tests

Lee Tunnel, London, UK

The Lee Tunnel in London was the firstelement of a wider programme calledTideway, which aims to preventenvironmentally harmful overflows ofmixed sewage and storm water runninginto the River Thames. The 7km, 7.2m IDtunnel is in operation today, acting as aholding vessel when rainfall events overloadLondon’s Victorian drainage system,directing the water to sewage treatmentplants.Constructed mostly through chalk, with

some sections of high hydrostatic pressuredue to fault zones, the Lee Tunnel’s primarylining is steel-fibre reinforced segments. Oneof the driving factors behind the design ofthe secondary lining is the surge pressures itmust withstand as it fills with waste water.At these times the internal pressure willexceed the hydrostatic pressure so that thelining will be in tension. Thames Water’s specification was

challenging: no damp patches or runningwater and a design life of 120 years. Toensure a long life, UnPS had to create asituation where the tensile strains did notlead to large cracks. Using 40kg of the high-performance fibres with the C40/50 concreteensures a high number of very small cracks,rather than fewer larger cracks.Contractor MVB cast the 250mm-thick

secondary lining using a full profile shutter,which has been done few times. Manymonths of concrete mix testing wererequired to find a solution which wouldprovide concrete that could be pumped,with the right early and long-termcharacteristics.In addition to durability issues MVB also

saved around 17,000 tonnes of steel,together with the commensurate reductionin Carbon emissions. The casting processwas also simplified and shortened byremoving the steel fixing operation.

FACT FILE:

Client: Thames Water

Main contractor: Morgan Vinci Bachy(MVB) JV

Designer: UnPS

Application: Secondary tunnel lining

Fibre: Dramix 5D 65/60

Dosage: 40kg/m3

Concrete: C40/50

Timeframe: secondary lining con-structed 2014/2015

Figure 2: Pull out forces

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Jansen Project, Saskatchewan, CanadaThe Jansen Project sees the possible developmentof an underground potash mine in east-centralSaskatchewan in Canada, 140km east ofSaskatoon. Though the collapse of prices in thepotash market means that a question mark hangsover whether the mine will go ahead as planned,contractor DMC Mining is already well-advancedwith the construction of two shafts which willprovide access to the mine.The production and service shafts both have

internal diameters of 6.5m, wall thicknesses thatvary between 800mm and 1.1m and are 1,030mdeep to reach down to the level of the potashdeposits. A ground freezing exercise involving 89freeze holes and monitoring wells was requiredbefore excavation of the shafts using ShaftBoring Roadheaders could begin in order to

prevent water from underground acquifersflowing into the shaft.Following behind the roadheaders, DMC

Mining is slipforming the shaft walls. By usingfibres to reinforce the concrete, rather thantraditional rebar, the whole operation becomesfaster and safer – as it removes the steel-fixingstep and the need to have additional peopleworking inside the congested area of the shaft.In August, BHP Billiton reported that the shafts

were about 600m deep with 300 or 400m to go."We'll finish the shafts around 2018-2019 and atthat point we'll face a decision as to whether weactually start to construct an operation aroundthose shafts and start to enter the market," BHPBilliton CEO Andrew Mackenzie told reporters atthe time.

FACT FILE:

Client: BHP Billiton

Main contractor: DMC Mining

Designer: Arup

Application: Shaft walls

Fibre: Dramix 5D 65/60

Dosage: 30 kg/m3 to 50kg/m3

Concrete: 60 Mpa

Timeframe: 2013/2018

to become more brittle. So the 4D, forexample, with its stronger anchorage is madeof stronger wire than the 3D. The 5D uses wirewith a tensile strength of 2.2N/mm2 andductility of more than 6 percent.

Tests carried out on concrete reinforced with5D fibres, as set out in EN14651 Test methodfor metallic fibre concrete, have been used toevaluate its behaviour in tension and showthat it exhibits far greater residual flexuralstrength values. Tests carried out at BRE in theUK have shown that mixes reinforced with 5Dactually exhibit deflection hardeningbehaviour.

Early adoptersVery soon after the launch of Dramix 5D fibresin 2013, UnPS became one of the firstdesigners to take advantage of the fibres’superior properties on London’s multi-million-pound Lee Tunnel. The decision to use thefibres was driven by the need to reduce crackwidths – although it yielded other benefits too.

Turkey’s Riva Tunnel, mentioned previously,came next. Initially EMAY investigated the useof synthetic fibres but rejected this solutiondue to synthetic fibres’ low E-modulus andsusceptibility to temperature changes.

The latest project to make use of 5D’scharacteristics is the Jansen project, thedevelopment of a new potash mine in east-central Saskatchewan, Canada. ContractorDMC Mining is slip-forming the walls of two1000m-deep shafts, aiming for productionrates of 3m a day.

The future will definitely see more use of 5Dfibres in the permanent linings of tunnels andshafts. Several projects are already beingdesigned using the fibre and others areconsidering its use as an alternative totraditional reinforcement for the final lining.Other underground applications have includedtrack slab inside rail tunnels and free-spanningslabs in car parks.

Riva Tunnel, Istanbul, Turkey

The Riva Tunnel in Istanbul will carry theNorthern Marmara Motorway, which willrun around the North of the city, bypassingthe congested city centre and crossing theBosphorous on the newly opened YavuzSultan Selim Bridge. JV contractor IC Ictas-Astaldi is

constructing the 115km central section ofthe motorway between Odayeri andPaşaköy including the main bridge, viaductsand two sets of tunnels as part of a Build-Operate-Transfer (BOT) deal which will seethe consortium run the motorway for 10years. This is a prestigious project,representing the largest ever investment ininfrastructure since the Turkish Republic wasfounded.IC Ictas-Astaldi constructed the primary

lining of the Riva tunnels, which runsthrough flysch, a combination of claystoneand sandstone, using the New AustrianTunnelling Method (NATM). The two four-lane tunnel, up to 22m in diameter, are626m and 564m long.

It was the dimensions of the tunnels –and hence the need to fix very long rebars –that drove designer EMAY International tolook for alternatives to the original design.EMAY decided on Dramix 5D fibres due totheir superior performance when comparedto 4D or 3D, retaining residual strength evenafter cracking. An automated dosing system ensured

that the right proportion of fibres wasadded to every batch of concrete. Thisprovided assurance to contractor IC Ictas-Astaldi that the quality of the lining wouldbe as-designed; a very importantconsideration given that this is a BOTproject.As well as achieving the main aim of

removing the safety risks posed by theunwieldy lengths of rebar, using fibre-reinforced concrete led to some significanttime savings: the lining was cast three timesfaster than a conventionally-reinforcedlining would have been, with 350m3

pumped into place every day.

FACT FILE:

Client: Ministry of Transportation,Turkey

Main contractor: IC Ictas-Astaldi JV

Designer: EMAY International

Application: Secondary lining

Fibre: Dramix 5D 65/60

Dosage: 20kg/m3

Concrete: C35/45

Timeframe: 2015/2016

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