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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.
ASCP 4th Concrete Pavements Conference 3 Learnings from Detailed Design of Tunnel Pavements Chloe Leng
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
ASCP 4th Concrete Pavements Conference 4 Learnings from Detailed Design of Tunnel Pavements Chloe Leng
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.
ASCP 4th Concrete Pavements Conference 5 Learnings from Detailed Design of Tunnel Pavements Chloe Leng
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.
ASCP 4th Concrete Pavements Conference 6 Learnings from Detailed Design of Tunnel Pavements Chloe Leng
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.
ASCP 4th Concrete Pavements Conference 7 Learnings from Detailed Design of Tunnel Pavements Chloe Leng
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
ASCP 4th Concrete Pavements Conference 8 Learnings from Detailed Design of Tunnel Pavements Chloe Leng
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
ASCP 4th Concrete Pavements Conference 9 Learnings from Detailed Design of Tunnel Pavements Chloe Leng
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
ASCP 4th Concrete Pavements Conference 10 Learnings from Detailed Design of Tunnel Pavements Chloe Leng
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.
ASCP 4th Concrete Pavements Conference 11 Learnings from Detailed Design of Tunnel Pavements Chloe Leng
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.
ASCP 4th Concrete Pavements Conference 12 Learnings from Detailed Design of Tunnel Pavements Chloe Leng
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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