also pose problems with waterproofinggaskets. For this reason their use has beenconfined to: • rock tunnels where water ingress and

ground loads are not an issue• projects in Japan where high tolerance

moulds and bespoke steel connectorshave been employed to limit loss of planeand ensure a good fit

However, if a simple and cost-effectivemethod for early identification andcorrection of ring plane can be identifiedthen it is possible that these linings couldbecome an attractive alternative to normaltapered rings.

An additional benefit of hexagonalsegments is that it is possible to createopenings by simply removing a segment orpattern of segments in a way that allows

the load transfer through the remaininglining. While this would require additionalreinforcement in the segments around theopening, it could eliminate the requirementfor internal propping.

Allowance for internal structuresAs more and more tunnels are constructedwith internal structures, particularly whereintermediate decks are required, costeffective solutions for the construction ofthe internal structures will be required. Suchsolutions will need to:• connect to the lining in a cost effective

and buildable way• accommodate all the movements that

might occur to the lining over its designlife (including seismic)

LINING TECHNOLOGY

TUNNELLING JOURNAL 31

Segmental linings: a vision for the futureIn the Oct/Nov issue of Tunneling Journal North America[1] theauthors, Anthony Harding and Malcolm Chappell describedthe current state of the art in segmental linings. Followingthis, they explore some of the areas they think segmentallinings are likely to develop over the next decade.

Step 1: thrust from shaded rams and retract other rams to erect the segments outlined

Step 2: thrust from shaded rams onto segments just erected and retract other rams to erect remainder of ring

Erect

Erect

Thrust

ThrustThrust

ThrustErect

Erect

Gap

Offset(due to plane loss)

Figure 2: Gaps due to ring planeloss in hexagonal linings

Figure 1: Hexagonal lining erection sequenceWHILE TUNNEL LININGS have been getting bigger (see box), thethickness of linings has been going in the opposite direction in aneffort to save cost and excavated volumes. The authors have seencontractors consider a lining thickness of 240mm for a 6m Metrotunnel, but settle on slightly higher thicknesses having examinedrisks associated with fragility of such thin linings, particularly theimposed loads and deformations at cross passage openings. Ittherefore appears likely that we are approaching the practical lowerlimit for lining thickness in many parts of the world.

The largest rings still tend to use traditional rectangular segmentdesigns. However, at smaller diameters rectangular segments areincreasingly being replaced with trapezoidal and parallelogramshapes (often called the universal ring). These rings typically buildmore quickly, more easily, and more accurately than their straightsided counterparts. It is likely that they will continue to be used atever higher diameters until they are the geometry of choice exceptwhere particular project requirements dictate otherwise.

However, the geometry that warrants real development over thenext decade is the hexagonal lining. Hexagonal linings offersignificant productivity benefits over the traditional tapered ring, asthe TBM can thrust from half of the ring while the other half isbeing erected (see Figure 1). In their current form they suffer fromgaps that open due to imperfect ring plane (see Figure 2). Thesegaps tend to concentrate the hoop loads in just one half of the ringunless the gap is grouted with a high strength mortar. The gaps can

Are tunnel linings getting bigger?It seems that barely a few years passesbefore the latest world record TBM issurpassed by another tunnel of evenlarger diameter. This has been driven by acombination of tunnel function andconstrained urban environments. Wherecorridors are narrow (for example due tothe presence of tall buildings either sideof a street), there may not be enoughroom for a twin bore tunnel. This problembecomes even more apparent at theportals, where significant excavated spaceoutside the TBM perimeter is required. Sowhat do you do if you don’t have enoughwidth? One option is to stack theroadways within a single bore asillustrated in Figure 3, to obtain a loweroverall width.

This has been successfully implementedin a number of tunnels, including theSMART tunnel in Kuala Lumpur and theSocatop Tunnel in Paris. A three lanetunnel with full height traffic lanes wouldrequire a world record size TBM, as wasenvisaged for the Orlovski project in StPetersburg.

Such solutions also offer benefits ineliminating cross passages – ascommunication between roadways can beprovided with internal structures, andequipment and ventilation spacesprovided to either side of the roadway.Removing the requirement to hand minecross passages may be a significant

Twin bore

Single bore

Figure 3: Comparison of twin bore and single bore stacked configurations of a three lane tunnel

32 TUNNELLING JOURNAL

LINING TECHNOLOGY

• be easy to build without impeding TBMlogistics

The authors are hopeful that this willgenerate some innovative solutions thatimprove significantly on the traditionalmethod of drilling and fixing large numbersof anchors into the lining.

MaterialsThe predominant material in segmentallinings is concrete that uses OrdinaryPortland Cement (OPC) as a binder, and isreinforced either with steel or steel fibres.However, the author’s view is that there willbe an increasing trend towards lowercarbon, non-ferrous linings.

One way that carbon footprints will bereduced is by the replacement of OPC withgeopolymer, a product that uses slag andfly ash in place of cement, activated byadmixtures to produce comparable strengthand strength gain rates to conventionalOPC. Wimpenny and Chappell[2] havepresented research into the use of fibrereinforced geopolymer concrete for tunnelsegments that demonstrates that

geopolymer can achieve equivalent orbetter structural, durability and fireperformance than traditional OPC. Thistechnology is sufficiently mature to beadopted on a tunnel project. Critically, theirresearch showed that the CO2 required toproduce and deliver the segments would be70% less than the OPC equivalents.

The major barrier to the widespreaduptake of geopolymer has been that highvolumes of concrete are required to makethe cost of setting up a supply chain andmanufacturing facilities economicallyviable. However, a typical metro tunnelcontains at least 1,000m³ of precastconcrete per route kilometre in the tunnellining alone, and would easily providesufficient concrete volume to make for aviable the business case for geopolymer.Indeed, geopolymer binder could even becheaper, particularly if the geopolymer wasto be used for other concrete on theproject.

As well as material cost, geopolymer canalso save money in the manufacturingplant:

• geopolymer requires almost no curing,eliminating the requirement for steamcuring

• geopolymer is less vulnerable to earlythermal cracking

• improved fire performance may save onmicro-synthetic polypropylene fibres

These cost savings will be of interest tocontractors while the reduced carbonfootprint will be a concern to all in theindustry and a major selling point to clients.Therefore as soon as a trial project hassuccessfully demonstrated the effectivenessof this technology, it is likely that there willbe a proliferation of projects that usegeopolymer concrete in place of OPC.

FibresAs well as changing the concrete binder, theauthors also see macro-synthetic fibresbecoming a frequently employed alternativeto steel fibres. There is currently muchdebate in the industry as to whether steelfibres or macro-synthetic fibres are superiorfor the reinforcement of concrete segments.Both have limitations when compared to

LINING TECHNOLOGY

project benefit in some groundconditions. The single bore option doesreduce schedule flexibility somewhat butthis can usually be managed. Thereforewhere space is tight we can expect moremega-TBMs.

Will conventional tunnels get bigger?While the mega-projects grab theheadlines, the workhorses of thetunnelling industry are much smallertunnels: metro tunnels at around 6m,cable tunnels at 2-4m, and water tunnelsvarying from the very smallest tunnels upto diameters over 10m. One might thinkthat this blend of sizes might remain.However, even for normal tunnels theremay be upward pressure on diameter. Inmetro projects the solutions developedfor Barcelona Line 9 (see Figure 4), whichplace both tracks on separate deckswithin the same tunnel, offer a numberof benefits outlined in Table 1. The impactof these benefits will vary from project toproject, but it is likely that this kind ofscheme will be the best solution for someprojects.

There may be other circumstanceswhere we can add genuine value toprojects by making space for moreinfrastructure inside the tunnel. This willalso put upward pressure on the averagesize of segmental linings.

Figure 4: Double deck construction in a singletunnel from Barcelona Line 9

the other, but the authors believe that thelimitations can generally be designed forand there will be many circumstances whereboth materials could meet the requiredcriteria.

A further development that is relevant toboth fibre types, however, is increasedconcrete strengths and a desire to makesteel fibres work at ever higher diametersand more heavily loaded linings. At largerdiameters the higher hoop loads and higherTBM ram thrusts give rise to high burstingstresses on the longitudinal andcircumferential joints. While steel fibreshave been shown to increase the post-crackstrength of concrete joints in bursting, thestrength of the joints is still fundamentallytied to the strength of the concrete.

However, the authors are aware that atleast one of the latest steel fibre types offersincreased concrete splitting strengths at lowdosages. This, paired with high strengthfibres that perform well with high and veryhigh strength concretes, make thepossibility of solely fibre reinforced rings athigher diameters a realistic possibility.

Plain concrete???Another potential area of development is theuse of plain, unreinforced concrete. This mayat first appear to be a retrograde step backto the earliest segmental linings, many ofwhich were not reinforced at all[3]. However,the problem with plain concrete segmentswas that they proved very prone to damageduring handling, even at low aspect ratios(ratio of circumferential length to thickness).In recent years aspect ratios have beenincreasing in order to have fewer segmentsin the ring, with the consequent cost andschedule benefits. So it would appear thatmodern requirements make plain concreteeven more problematic. However there aretwo important recent developments thatmay put plain concrete back in the frame.

Firstly, increased concrete strengths andqualities mean that in many tunnels thereinforcement is not required for the tunnelin its constructed state. The hoop force inmany tunnels provides sufficient resistancefor the expected bending moments.

Secondly, handling systems that limitdamage have become widespread, primarily

to limit damage to steel fibre reinforcedsegments. The success of such systems isthat they ensure that the segments are notcracked by the handling activities. Given thatthe fibres only provide additional strengthafter the formation of the crack, it could beargued that the systems could also limitdamage to plain concrete segments even athigh aspect ratios.

The key differences between fibrereinforced and plain concrete are:• Fibre reinforced concrete provides

toughness against impacts that plainconcrete does not.

• In the event of a flexural failure the fibreswill hold the segment together andprovide residual resistance. Plain concretesegments could break into two or morepieces.

Where these obstacles can be overcome (andground loads allow) plain concrete liningsmay become more common.

ConnectingAs well as reducing or even eliminating steelfrom the main body of the segments, the

Table 1: Comparison of single metro tunnel to twin bore.

Element Conventional twin bore Single tunnel

Tunnel 2x 6m tunnel Single 12m tunnelFootprint 18m wide 12m wideCross passages Every 250m approx. Not requiredCrossovers Cut and cover box ~200m long Constructed within tunnelStations Cut and cover box ~150m long Driven by patronage, but could be as small as one 30m diameter shaftSidings Separate tunnel required Within tunnel.

TUNNELLING JOURNAL 33

LINING TECHNOLOGY

TUNNELLING JOURNAL 35

authors also expect that steel bolts willbecome less and less common. Push-fitplastic dowels have already replaced steelbolts on the circumferential joint in manyparts of the world, and there are projectswhere guiding rods have been used in placeof steel bolts on the longitudinal joint. Boltsare often included in designs simplybecause they have always been used,without regard for whether they are actuallyrequired[4]. However, resistance to theremoval of bolts will continue to be erodedas more projects demonstrate that bolts arenot required in normal circumstances.

Where tension across the joint must beresisted bolts could continue to be required.However, installing bolts can add time tothe erection process and providing longterm protection against corrosion is difficult.The authors have been working with onesupplier to develop a push-fit plasticalternative to bolts in order to providealternatives for some current projects, andhave some promising ideas. Other suppliersare also looking to develop systems.Therefore in ten years’ time it appears likelythat there will be no need to use ferrouselements of any kind to connect thesegments.

Grouting and sealingThe recent and welcome adoption of cast-ingaskets is likely to turn into widespreadusage in the next ten years. While it may bedifficult to see any big developments in theseals incorporated into the segments, thefirst line of defence is usually the grouting.Tail shield grouting is very widely adopted,but still needs very careful control to ensureeffective filling of the annulus. Poor fillingcan easily occur and requires significantsecondary grouting to mitigate – if detectedat all. Even where tail shield groutingoperation is good some check grouting isrequired to verify the continuedperformance. Check and secondarygrouting activities require additional crewshandling highly alkaline materials, and canoften slow progress of the TBM.

Therefore if an alternative can be foundthat provides an assured filling of the void(such as a low strength expandablematerial cast into the outside of thesegments, for example), then theseactivities and the associated risks ofmaterial handling could be eliminated. Avoid filling material that is expanded couldalso be useful in squeezing ground if thematerial could absorb inward movementswithout overloading the lining.

AutomationThe drive to improve both productivity andtunnel safety must be maintained if we areto push the industry forward. The goldstandard for safety is to remove operativesfrom the work area. Meanwhile the quality

of segmental linings and TBM performanceis subject to quality of workmanship.Automation of the tunnelling operation hasthe potential to remove operatives from thetunnel, while maintaining or improvingquality and productivity.

The items required to fully automate thetunnelling process are listed in Table 2. It isinteresting to note that the only technology

that has not been implemented successfullyis the offloading of TBM consumables, andeven this appears straightforward whencompared to systems employed in otherindustries.

Automation requires a lot of up-frontinvestment, but saves in the cost ofoperatives, so the cost-benefit is likely to bemost apparent in long tunnels in locations

where wages are high. However, the systemmust have a high degree of reliability.Downtime will increase if the operativesrequired to rectify the problem are notalready on site, so the incidence of failuresmust be minimised if an automated systemis to achieve performance that is as good asor better than current practice.

In addition to the tunnel, there are alsopotential benefits to automating other partsof the segment manufacture and supplyprocess. Automation at pit top could easilytie in with the automatic trains operating inthe tunnel. Meanwhile there are manyaspects of the manufacturing plant that canbe automated such as reinforcementwelding, automatic batching, segment

turning, marking, and craneage. In this areaautomation is already developing in apiecemeal way as automation of variousaspects becomes cost-effective, and thisprocess is likely to continue.

Quality controlThe automation of segment manufacture,transport and erection can also bring

benefits to quality control/quality assurance.In normal practice each segment isequipped with a unique identifier, oftensimply a number sprayed onto a segment.This number is usually manually enteredinto a database, and then tied to data suchas date cast, batch number, etc. Howeverwith an automated system this informationcould be automatically entered. A bar codecould be affixed to the segmentimmediately upon demoulding.Alternatively, a barcode or other binarysystem could be cast into the segment.

If all handling steps could record thespecific item that they were handling then adatabase could be created which linkedrecords of all inspection and test steps. Ifsuch steps were recorded electronically inwireless devices (such as tablets) then paperentry could be almost eliminated and failsafes used that prevent uncheckedsegments proceeding to the next stage.Much of this technology already exists andhas been applied to a few projects.However, tying the quality control systeminto an automated segment delivery anderection system could allow the full historyof the segment to be recorded, from mouldcleaning and preparation through to thetime, location, and orientation ofplacement. This could free up time for theactual quality control activities that requirehuman inspection, improving quality andfreeing up staff for more productiveactivities.

Intelligent segmentsAnother area that may see significantchanges over the coming years is intelligentsegments: segments that monitor theirperformance over their life span or someportion of it. A recently developed smalldevice known as Utterberry can locate itselfrelative to other devices very accurately andcommunicate wirelessly, and has already

Table 2: Components of tunnelling automation.

Excavation and advance Either from a TBM operator’s cabin located at the surface or completely automatedSegment erection Robotic controlled erector arm. All segment connectors are pre-installed push fit type.Segment identification Bar code or more durable cast in system to uniquely identify each segment. Grouting Automated two component type grout fed by pipes on self-extending system. TBM consumables supply Either by robotic processing of cars delivered by automated train or by self-extending supply lines Segment supply Automated trains with robotically controlled transfer station to place segments on segment feeder.Muck handling Self-extending conveyors.Survey for TBM guidance Access for survey could be targeted mainly at routine maintenance Routine maintenance Systems designed for lower levels of routine maintenance to limit personnel entry into tunnel

Automation requiresa lot of up-front investment, butsaves in the cost ofoperatives, so thecost-benefit is likelyto be most apparentin long tunnels in locations wherewages are high.

been deployed successfully in anunderground context[5].

Paired with the development of othertechnologies such distributed glass fibremonitoring it is entirely feasible that at leastsome segments in the tunnel may be‘intelligent’ – able to assess theirperformance against what they weredesigned for. Over the next decade it isreasonable to assume that the cost of suchsystems will come down significantly. Thepotential benefits of would be many, butone that the authors consider mostsignificant is for critical infrastructure wherecreating downtime for inspection andmaintenance is problematic, as theadditional cost of the system could be manytimes less than the cost of just a singleinspection.

Special circumstancesAs well as he more routine segmentdesigns, there will continue to be “one-off”designs that fulfil a particular need. Thesehave the potential to provide newtechnologies that push the industry indifferent directions. A few suchcircumstances are discussed below.

One-pass pressurised tunnelsThe traditional approach to pressurisedtunnels is to use the segmental lining toprovide a primary lining that resists externalground and water loads duringconstruction, and then to provide asecondary lining (of reinforced concrete,steel or other material) that provides theresistance to the tensile hoop forcesgenerated by the internal pressure. If thiscan be achieved in a one-pass system thenthe cost and schedule benefits are self-evident.

One solution is to thread post-tensioningcables through ducts specially cast into thesegments, and then use the cables tocompress the entire ring, as used in recent

projects in both Switzerland and Japan. Thissolution has much merit insofar as it keepsthe concrete and the gaskets between thesegments in compression under tensileservice loads, thereby enhancing durability(by minimising cracking) and minimisingleakage. However, the cable is hard toassure for the 100 year design life oftenrequired, and it can be difficult to install.However, if a robust and cost effectivesolution to these issues can be found thenit is likely that such solutions willproliferate.

A second solution is to design the liningto take the tension. This can be achieved bysystems that span the longitudinal joint,

such as bolts, oracross thecircumferential jointby dowels (Figure 5).A system has alsobeen developed inJapan where aprofiledcircumferential jointinterlocks with thesegments in theadjacent ring,allowing the tensionto be transferredbetween rings in thesame manner asdowels.

Integral protectionAnother area whereone-pass solutions

have been sought is in sewer tunnels,where protection from the aggressiveenvironment is required. Systems with anintegral internal plastic protection havebeen developed. Such solutions may usewelding or gaskets to ensure continuity ofthe protection (Figure 6). However, afrequent objection to the solution is thatthe plastic can mask the deterioration ofthe concrete behind, and significant levelsof degradation may be required before it isspotted. This is less of an issue for a groutannulus or secondary lining as they areeasier to repair than the primary lining thatprovides grout support. If a solution to thisobjection can be found then integrallyprotected one-pass solutions could becomethe preferred option.

Heat recoveryAnother area that is likely to receiveincreased attention is to enhance return onthe capital investment by utilising the tunnelfor a secondary use.

One such idea that has been proposed isto recover energy through a series of pipescast into the tunnel lining. These are inter-connected to form a continuous circuitthrough each ring of the lining which arethen in turn fed into a main to circulatewater to a surface heat exchanger. This canbe used as a very effective heat sink orsource of heat, providing a very economicsource of energy for heating and cooling.The energy that is able to be captured canbe quite staggering.

The authors are convinced that there

LINING TECHNOLOGY

36 TUNNELLING JOURNAL

1. Opening created by segments moving apart circumferentially

2. Segments moving apart circumferentially resisted by two dowels on each joint on both sides

3. Path of tensile forces across opening

Figure 5: Dowels transferring tension loads around a joint

Figure 6: Internal lining/gasket detail on Herrenknecht combi-segments

LINING TECHNOLOGY

TUNNELLING JOURNAL 37

could be other beneficial applications thatcould be applied if entrepreneurs\inventorswere given the challenge.

ConclusionsPrecast concrete segmental tunnel liningsmay appear to be a mature technology thathas changed little over the years. However,in their previous article[1] the authors showedhow the technology has changedsignificantly over the last 20 years. In thisarticle the authors have shown how there isplenty of scope for the technology tocontinue to develop over the next decadeand beyond. In some areas, such asgeopolymer concrete and non-ferrouslinings, the authors have been able topinpoint what the solutions might look like.In others the authors have merely been ableto highlight the problems and the benefitsof solving them. However, if the authorswere asked what the ideal lining might looklike in ten years’ time they would offer thefollowing:• Geopolymer concrete lining with macro-

synthetic fibre reinforcement.• Non-ferrous connections on longitudinal

and radial joints• Hexagonal segments to allow continuous

mining• Ability to resist internal tension loading

• Intelligent segments monitor tunnelperformance remotely, reducing therequirement for inspection andmaintenance

• Void filling assured by an expandingmaterial cast into the outside face of thesegment that actuates on exit from thetail shield

• Database that allows the QC and as-builtrecords of every segment on the job to beeasily called up.

This may appear a panacea, but the authorsare convinced that if we work together asan industry it can become a reality thatoffers cost, safety, and environmental

benefits to the industry and society as awhole. This will require commitment tocommunication and genuine problemsolving from clients, contractors, anddesigners alike. Surely we can manage that?

1. Harding and Chappell, 2014. State of the art precast linings. North American Tunneling Journal,Oct/Nov issue, TGS Media, Tunbridge Wells, UK

2. Wimpenny, D; Chappell, M; 2013. Fiber-reinforced geopolymer concrete—an innovative materialfor tunnel segments. In Rapid Excavation and Tunneling Conference. Washington DC, USA, 23-26Jun 2013. Englewood, Colorado, USA: Society for Mining, Metallurgy, and Exploration. 810-819.

3. Craig, R N; Muir-Wood, A M; 978. A Review of Tunnel Lining Practice in the United Kingdom; TRRLSupplementary Report 335, Transport and Road Research Laboratory, Crowthorne, UK

4. Harding and Chappell, 2014. Myth and Reality: Bolts in Modern Concrete Segmental TunnelLinings. North American Tunnelling Conference, Los Angeles, 22-25 June 2014, Englewood,Colorado, USA: Society for Mining, Metallurgy, and Exploration. pp66-75

5. UtterBerry sensor scoops award for industry. 2014. http://www-smartinfrastructure.eng.cam.ac.uk/20141022-UtterBerry-innovation-award

REFERENCES

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The authors would like to acknowledge thefollowing people for their contributions to thisarticle: Chris Smith of CRS consultants,Rolando Justa Camara, Director of Tunnelsand Marine Works Department of AccionaInfraestructuras SA.

ACKNOWLEDGEMENTS