australian society for concrete pavements papers/paper 30... · australian society for concrete...

Australian Society for Concrete Pavements

4th Concrete Pavements Conference

Learnings from Detailed Design of Tunnel Pavements

Chloe Leng BEng. MPhil.

Senior Pavement Engineer Transport Services

Aurecon Australia

ABSTRACT

The recent infrastructure boom in Sydney will provide the city with three new road tunnels by the

end of the decade. While the Austroads Guide to Pavement Technology and the various model

drawings by state road authorities provide thorough guidelines for the design, maintenance and

rehabilitation of highway pavements, guidelines for the design and construction of tunnel

pavements are very limited.

The space constraints and associated safety concerns of tunnel paving contribute to unique

construction challenges not usually encountered in road pavement design. Space proofing and

interface coordination with other disciplines, such as drainage and underground structures,

became significant drivers of design.

This presentation will discuss general learnings from the detailed design of tunnel pavements

and highlight common issues encountered during the design process. Although all three tunnel

projects have adopted continuously reinforced concrete pavement (CRCP), a comparison of key

tunnel features across different projects will provide an insight to the advantages and

disadvantages of their impacts to pavement design and construction.

ASCP 4th Concrete Pavements Conference 2 Learnings from Detailed Design of Tunnel Pavements Chloe Leng

1 Introduction

Most highway pavement components in Australia are governed by design, construction,

maintenance, inspection, and operations standards and guidelines of the various state road

authorities and the Austroads Guide to Pavement Technology. However to date, highway

pavement in tunnels in Australia do not have comparable guidelines and regulations. Still, road

tunnels are increasingly being built under cities to relieve surface congestion, especially with the

increase in land value and a heightened public environmental awareness. In Sydney alone,

three new road tunnels are currently under construction with completion forecasted towards the

end of the decade.

This paper discusses general learnings from the detailed design of tunnel pavements and

highlights common issues encountered during the design process. Although all three tunnel

projects have adopted continuously reinforced concrete pavement (CRCP), a comparison of key

tunnel features across different projects provides an insight to the advantages and disadvantages

of their impacts to pavement design and construction.

The space constraints and associated safety concerns of tunnel paving contribute to unique

construction challenges not usually encountered in road pavement design. Space proofing and

interface coordination with other disciplines, such as drainage and underground structures,

became significant drivers of design.

This paper explores the need to develop guidelines for the design and construction of concrete

pavement in tunnels to improve the efficiency and homogenisation of the design process and the

construction outcomes.

2 Common design features

The three tunnel projects currently underway in Sydney have many similarities in pavement

design features. This is partly attributed to the similar rock stratigraphy in the Sydney area, but

more importantly the designs were governed by standard requirements of the state road authority

– Roads and Maritime Services (RMS), and the Austroads Guide to Road Tunnels Part 2:

Planning, Design and Commissioning.

2.1 Pavement profile

As per the recommendations of the Austroads Guide to Road Tunnel and the project’s Scope of

Works Technical Criteria (SWTC), all three tunnel pavements consist of identical layer

configurations in the driver tunnel as shown in Figure 1.


Figure 1. Typical tunnel pavement profile

The tunnel pavement has two main advantages compared to the pavement in open roads.

Firstly, the rocky tunnel floor constitutes a stiff subgrade, especially compared to expansive clay

subgrades, leading to reduced traffic stresses. Secondly, the interior tunnel climate means

thermal gradient with less magnitude and less duration leading to reduced thermal stresses.

Table 1 summaries the main design features of each tunnel project.

Table 1. Summary of tunnel pavement design features in recent projects

Project

Name

Tunnel type Pavement type Pavement

design life

Asphalt

surfacing

Future lane

change

Project A Roadheader CRCP over NFC

subbase

40 years No No

Project B Roadheader CRCP over NFC

subbase

40 years No No

Project C Roadheader CRCP over NFC

subbase

40 years No Yes – from 2

to 3 lanes

2.2 Past road tunnel projects in Sydney

Tunnels Wearing Course Base Interlayer Subbase

M2 tunnel (1996) 30 AC10 230 PCP 25 AC10 125 NFC

Eastern Distributor (1997) 30 AC10 230 CRCP 25 AC10 150 NFC

Cross City Tunnel (2003) 45 AC14 220 CRCP 25 AC10 220 NFC

Lane Cove Tunnel (2004) 50 AC14 260 CRCP 25 AC10 220 NFC

Table 2 summarises the pavement profile in past road tunnel projects. Compared to past tunnel

pavements with asphalt wearing course, it is interesting to note that all three tunnel projects have

adopted concrete surfacings. This change can mostly be attributed to higher fire safety

precautions in tunnels following a number of tunnel fires in Europe. The European Concrete

Paving Association has been actively urging tunnel operators and regulatory authorities to take

measures to specify concrete pavements in all new tunnel construction. The pure mineral

composition of concrete provides an inert, non-combustible and non-toxic material, which in case

of a fire, enables safe evacuation and protects the tunnel equipment and its structure. Asphalt on

Continuously reinforced concrete (CRCP)

Asphalt interlayer with AR450 binder

No fines concrete subbase

Blinding concrete correction layer


the other hand burns at a temperature of around 500 degrees, causing wide-spread fire, emitting

toxic fumes and weakening the structural integrity of the tunnel. For these reasons, the Austrian

Decree of September 2001 requires concrete road pavement for new tunnels longer than one

kilometre.

Apart from fire safety, the light colour of concrete pavement absorbs less light, thereby reducing

energy cost of the tunnel while also increasing driver safety. While the low maintenance durable

concrete surface offers many advantages over asphalt, it also creates other challenges which are

explored in the next section.

2.3 Tunnel subsurface drainage

Figure 2 shows a typical cross-section of concrete pavement in the tunnel. Due to the limited

space in a roadheader mined tunnel, it is common for the longitudinal drainage line to be pushed

away from the edge of pavement to leave space for the conduit trench. As the subsurface

drainage pipe is often placed in the same trench as the drainage line, the no-fines concrete

(NFC) drainage layer is required to be graded independently from the crossfall of the road

towards the drainage line.

Transverse subsurface drainage pipes are typically placed every 30m throughout the tunnel.

They are connected to vertical collector pipes at the back of the barriers to drain runoffs from the

tunnel wall.

Figure 2. Typical cross-section of concrete pavement in tunnels

3 Common issues

3.1 CRCP terminal joints

Since CRCP are constructed with no transverse joints, provisions are made to either restrain or

accommodate end movements wherever the CRCP abuts other pavement types or structures to

protect both the CRCP and adjacent structure. The Concrete Reinforcing Steel Institute (CRSI)

notes that ‘the free end of a CRCP can be expected to move up to 2 in (50 mm) longitudinally

due to environmental changes in temperature and moisture depending on the frictional resistance

or the viscoelastic properties of the underlying layer’. CRCP terminal joints are often required in

tunnels, not only at the interface with surface pavement, but also with structural slabs installed

over ventilation tunnels or to support other elements across the carriageway.


3.1.1 Terminal end anchor joints

Roads and Maritime Services currently specifies a three terminal anchor system in the CRCP to

restrain end movements against other pavement types or structures as shown in Figure 3.

Although this movement restraining system may be suitable for an open weather road

environment, it is not the most practical nor optimal solution in a tunnel. It is very difficult and

costly to install the terminal anchors in a rock profile, especially under the tight space constraint

of a tunnel.

Figure 3. Pavement terminal anchor and RMS standard detail for CRCP terminal anchors

Anchor lug systems are installed at the end of CRCP sections to restrain most of the terminal

movement by transferring movement forces into the soil mass through passive and shear

resistance of the soil. The requirement for the three 1200 mm terminal anchors was derived

under the assumption that materials resisting the anchors are soil, not rock, as is the case in a

tunnel environment. Moreover, as the temperature in a tunnel is far less variant than an open

environment, the CRCP movements from thermal expansion and contraction is also expected to

be less significant.


Furthermore, as a drainage line is required continuously on the low side of the tunnel and

approach retaining structure, the anchors will require blockout to cater for the drainage pipe as

shown in Figure 4.

Figure 4. Modified Type 12 anchor at drainage pipe

3.1.2 Wide Flange Beam Terminal Joints

An alternative approach to address CRCP end movement is to accommodate the movement with

special expansion terminal joints. The most commonly used treatment in the U.S. is the wide

flange steel beam joint as shown in Figure 5. This joint is typically formed by partially casting a

wide flange beam into a reinforced concrete sleeper slab that supports the CRCP on each end of

the steel beam. The top flange is flush with the CRCP surface and compressible joint filler such

as polyethylene foam is inserted on one side of the beam web to allow for expansion movement.

A bond breaker is also provided on top of the sleeper slab to allow the pavement ends to move

freely. A detailed illustration of the wide flange beam joint is shown in Figure 6.


Figure 5. Wide flange beam terminal joint

A thicker or wider flange can also be provided for improved fatigue resistance where significant

heavy vehicle traffic is expected. Corrosion protection can be provided for the wide flange beam

using a corrosion inhibitor to improve the durability of the beam and reduce the maintenance

activity in a tunnel, which is always desirable. The wide flange beam joint is commonly used

together with doweled expansion joints constructed between the CRCP transition joint and the

adjacent structural slab or pavement to provide allowance for additional expansion. A subgrade

beam similar to that specified by RMS must be provided at the expansion joint.

[1]

Figure 6. Details of the wide flange beam joint

The beam joint both provides room for CRCP expansion and a means to maintain joint load

transfer efficiency. A field study by FHWA in 1998 has shown that wide flange beam joints are

increasing in popularity in the U.S. as a means to control CRCP end movements. Approximately

50% of all terminal joints inspected in the test were wide flange beam joints. This may be related

to the lower cost of beam joints compared to terminal anchor systems, but also the Federal

Aviation Administration (FAA) noted that experience with highway CRCP showed that attempts to

restrain end movement have not been too successful. The FAA noted that more favourable


results are achieved where end movement is accommodated rather than restrained. The beam

joint system has also been successfully implemented in Europe, on the Antwerp Ring Road in

Belgium for instance.

The FHWA field study also discovered fewer distresses such as spalling and faulting of the

transition joint on wide flange beam joints than on terminal anchors when the study examined the

performance of existing CRCP sections in several States. However, this may be attributed in part

to the younger ages of the wide-flange beam joints.

The wide flange beam joint could be a much more cost-effective option for CRCP terminal ends

in a tunnel setting with solid rock subgrade. Currently the rigidity of standard drawing details

discourages project orientated solutions that actually address site specific conditions.

3.2 Installation of traffic detection loops

As the SWTC for recent tunnel projects require traffic volume monitoring to an accuracy of 99%

in 15 minute intervals, inductive loop detection system becomes mandatory as it is the only

readily available technology to achieve this level of accuracy. This then creates a conundrum

with its installation in CRCP as the following clauses are specified:

Vehicle detection loops must be installed in the pavement without damage to the pavement

substructure and without affecting pavement life, while providing for a long life of the detection

loop itself;

and

Pavement provided as part of the Project Works must not be cut except as is necessary for

pavement joints and for the installation of traffic signal loop detectors in asphaltic concrete.

Figure 7. Typical in-pavement inductive loop detector layout

Some have suggested to cast-in the traffic detection loops to avoid cutting the pavement, but

experiences from contractors have been less than satisfactory with this practice. There are a

wide variety of possible causes to the high malfunction rate, such as damage to insulation during


shipment or poor installation techniques, but the two most obvious reasons for installation in

CRCP are presence of steel reinforcement and wire breakage during paving. Experience from

Tintenbar to Ewingsdale (T2E), for example, has found cast in preformed loops to perform poorly

due to interference from the reinforcing steel.

3.2.1 Impact of reinforcement bars

Inductive loop detectors (ILD) rely on a magnetic field to detect disturbances as the metal of the

vehicle passes over the loops, and the abundant steel reinforcement in CRCP reduces the

sensitivity of the detector. Although some literature suggests that modern ILDs are capable of

detecting vehicles with the presence of reinforcement bars, various manufacturers such as

Swarco, Marsh Products INC and Nortech Detection, all stated in their installation guidelines that

reinforcement steel at close proximity to the loops impacts the sensitivity of the loop detection

system. Study by the Federal Highway Administration Research and Technology (FHWA) also

suggested that closely spaced steel bars generate a greater reduction to the loop sensitivity as

the current can flow through the steel bars. This induced current could fully or partially cancel the

vehicle-induced current in the inductive loop.

While Marsh Products recommends to cut the reinforcement back to at least 600 mm from the

outer perimeter of the loop, Nortech recommends to minimise the effect of steel by providing a

minimum spacing of 150 mm between the cable and steel bars. Although none of these

recommendations are practical in CRCP, they do offer insight to possible reasons leading to

malfunction. Figure 2 shows a common method to secure loops directly over steel bars prior to

concrete pour. The direct contact of loop with steel is likely to cause considerate impact on

vehicle detection accuracy.

Figure 8. Common method of installing preformed traffic loops over reinforcement in CRCP

3.2.2 Placement and installation

Notwithstanding the interference of the steel, secure placement of the loops over the

reinforcement to withstand paving operations is another challenge. ILD manufacturers

recommend the loop to be installed no more than 50 mm below the wearing surface, as the

deeper the loop the less sensitive the system becomes. This is in line with Roads and Maritime

Services’ guideline on vehicle loop depth clearance. For a typical 240 mm CRCP, this means that

the loops will be sitting approximately 50 mm over the longitudinal bars. Secure placement of the

detection loops so close to the surface is an uncertain task without impacting the structure of the

concrete because unlike steel bars, the preformed loop is very light-weight and delicate even

when encased in PVC pipes, and the push of concrete during paving operation could generate


enough movement to stretch or break the wires and connections, causing the loop to

malfunction.

Due to poor experiences in the past, contractors are now weary of casting in preformed loops as

they are not accessible for repair. Moreover typical warranty for detector loop is 15 years and

they will likely require replacement during the life of the concrete pavement, which will inevitably

require cutting of the concrete pavement. Roads and Maritime Services Technical Direction

TD2012/09 Pavement depth for vehicle and bicycle loop detectors allows the cutting of concrete

pavement for loop installation, however, project specific requirements often overrules these

technical directions. Nevertheless, current tunnel projects have opted for the cut-in option as the

calculated design thickness is generally considerably less (between 50 mm and 60 mm) than the

minimum required thickness by the Austroads Guide to Pavement Technology Part 2.

Currently Roads and Maritime Services does not have any approved preformed inductive loop

products that are tested for use in concrete pavements. There may be manufacturers with loop

detecting technologies that can tune out the steel bars, but thorough research is needed to check

and certify such product.

3.2.3 Alternative technologies

Alternatively, the road authorities may want to consider other traffic monitoring devices that are

not invasive to the pavement, such as video image processing (VIP) through closed circuit

television (CCTV) surveillance. Vehicle count accuracy was reported for various detector

technologies in a test on I-4 in Florida. ILD was the most accurate with 0.2% error and the VIP

system had 2.1% error compared to the ground truth counts. It is undeniable that ILD is the most

accurate technology to date but the cameras offer more flexibility with lane changes, multilane

monitoring, non-invasive installation and accessibility for maintenance. The stable environment of

a tunnel without weather impact and variable lighting may further assist with visual detection

system’s greatest shortcoming. However, a detailed cost and benefit analysis must be completed

prior to implementation.

3.3 Wearing Course

The issue of the traffic detection loop is related to the adoption of concrete wearing surfaces in

the tunnel. While the SWTCs of all three tunnel projects allow the option of either dense graded

asphalt or concrete (CRCP) wearing course, all contractors have opted for the concrete option for

cost reasons.

The relative merits of concrete versus asphalt wearing surfaces in tunnel environments have

been long debated. Currently, major road bodies internationally including Austroads (Australia),

FHWA (US) and PIARC (Europe) all allow the use of both concrete and asphalt wearing courses

in road tunnels. However, in response to the recent tunnel fires in Europe, the European

Concrete Paving Association has been actively urging tunnel operators and regulatory authorities

to take measures to specify concrete pavements in all new tunnel construction. In particular, the

Austrian Decree of September 2001 requires concrete road pavement for new tunnels longer

than one kilometre.

The use of open graded asphalt wearing course however is not recommended in tunnel

pavement due to its high porosity as the voids may retain flammable or toxic spillages arising

from an incident.


The advantages and disadvantages of both concrete and asphalt wearing courses are listed in

Table 3.

Surface Advantages Disadvantages

Asphalt • Better contrast to tunnel walls and

line marking

• Enables the installation of traffic

loops

• Smoother ride

• Continuity with dark coloured

surfacing outside of tunnel

• Emission of harmful gas when

combusted

• Combustible at high temperatures

causing wide-spread fires

• Additional installation cost

Concrete • Mineral composition of concrete

makes it non-combustible and non-

toxic thereby providing better fire

safety

• Light coloured surfacing reflects

more light thereby reducing energy

usage for lighting

• Lower maintenance

• Lower construction cost - no

additional asphalt cost and slightly

reduces excavation

• Difficult to install traffic loops

• Requires greater precision during

paving and tining to achieve riding

quality

• Lower contrast with line marking

and tunnel walls

4 Conclusion

Concrete pavement design and construction in tunnels currently follows the same design

standards and construction specifications as pavement on open roads. At a glance, the tunnel

pavements may appear to be very similar to open road pavements as they are subjected to the

same traffic loading. However, the unique conditions under which the tunnel pavement is

constructed as well as the high strength rock subgrade distinguishes tunnel pavement from open

road pavement. In-depth examination of the current standards should be undertaken to evaluate

their suitability to tunnel projects. Alterations to specifications and design standards are

recommended to better address issues identified in this paper.


5 References

[1] EuPave, “Contribution of Concrete Pavement to the Safety of Tunnels in Case of Fire,”

2010.

[2] CRSI, “Continuously Reinforced Concrete Pavement Design & Construction Guidelines,”

2011.

[3] A. Tinni, “Check of CRCP Terminal Anchor Lug Requirements,” 2010.

[4] Caltrans, “Concrete Pavement Guide Part 2: New Construction,” in Chapter 200 –

Continuously Reinforced Concrete Pavement (CRCP), 2015.

[5] N. J. Delatte, Concrete Pavement Design, Construction, and Performance, CRC Press,

2014.

[6] FAA, Airport Pavement Design and Evaluation, 2009.

[7] Norwegian Public Roads Administration, Road Tunnels Standard, 2004.

[8] Marsh Products, “The Basics of Loop Vehicle Detection,” 10 Nov 2000. [Online]. Available:

http://www.marshproducts.com/pdf/Inductive%20Loop%20Write%20up.pdf. [Accessed Mar

2017].

[9] D. Gibson, M. K. Mills and D. Rekenthaler Jr., “Staying in The Loop: The Search for

Improved Reliability of Traffic Sensing Systems Through Smart Test Instruments,” Public

Roads, vol. 62, no. 2, 1998.

[10] Nortech Detection, “Vehicle Detector Loop Installation Guide,” [Online]. Available:

http://nortechdetection.com.au/wp-content/uploads/2016/04/Loopinstallation_an.pdf.

[Accessed Mar 2017].

[11] S. Vandebuerie, “Out of the Loop,” 2004. [Online]. Available:

http://www.traficon.com/mediastorage/FSMLDocument/1393/en/ART_TTI_Nov04_Final.pdf.

[Accessed Mar 2017].

[12] FHWA, “Traffic Detector Handbook: Third Edition,” Volume 1, 2006.

[13] N. Habesch, K. Jehanian and F. Awadallah, “Evaluation of Wide-Area Detection Systems,”

[Online]. Available: http://www.kmjinc.com/wp-content/uploads/Evaluation-of-Wide-Area-

Detection-Systems.pdf. [Accessed Mar 2017].

[14] Caltrans, “Standard Plans,” 2015.

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