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New road tunnel design delivers whole of life benefits

Cars in a tunnel

Aurecon Tunnels Expertise Leader Dr Harry Asche, EIC Activities Senior Engineer Gerry Bertakis and John Holland Design and Engineering Manager Ed Taylor, describe the key issues that arise in road tunnel construction and present a new design approach that provides owners with significant advantages during the operational phase.

Three-lane road tunnels driven by a tunnel boring machine (TBM) are becoming more common, but present special difficulties with respect to optimising the cross section. Fitting the road envelope within a circular tunnel makes for an inefficient use of space in an expensive tunnel. Constructing the road pavement can involve a large and time-consuming backfilling operation.

The position of the roadway in the TBM cross section for three lanes also affects the design of the cross passages and creates special difficulties for tunnel design and construction.

Cross passage design considerations

Cross passages are provided to allow people to escape in an emergency from one tunnel to the other (non-incident tunnel). The cross passages are located on the fastlane side of the road, and the passageway is set at the same level as the roadway (or walkway, if a walkway is provided). To allow the break in the road barrier, it is necessary to offset the leading edge of the downstream barrier away from the road; so that errant vehicles are deflected and prevented from hitting the end of the barrier head-on.

Cross passages are also used to allow services to cross from one tunnel to the other, thus enabling a series of circuit rings for HV and water main services to provide redundancy and reliability. In most tunnels, the cross passages are places where switchboards and deluge valves are located, and larger passages, additional passages or stub tunnels are often used for substations and low-point sumps.

From a construction perspective, the excavation and lining of cross passages in TBM driven tunnels is often delayed until, at least, the completion of the first drive and sometimes well into the second drive, as the tunnel excavation has passed one or a number of passage locations. Impacting drivers for the commencement of cross passage construction are conflicts with tunnel services (vent bags, pump and drainage pipes, cables, etc.), maintaining access for muck removal systems and tunnel supply (segments, etc.) systems as well as the logistics associated with the passage construction itself.

Activities such as preliminary ground probing and subsequent treatment, installation of often heavy temporary support in the passage breakout area - and then, of course, the excavation, support and mucking equipment - all require sufficient working room. This requirement, coupled with the fact that cross passages are located at a level related to the final tunnel roadway level, means that as the tunnel size increases, the passage openings can be increasingly higher from the tunnel invert; and, consequently, inaccessible unless either extensive temporary staging is used or there is some consideration given into the design of the permanent road deck that facilitates access to the cross passage opening.

The other major construction driver associated with cross passages is their physical size. Despite the fact that a larger opening facilitates the use of larger, often more efficient excavating equipment, there are also many constructability downsides with larger openings. The number of tunnel segments that will need to be removed can substantially increase the weight and complexity of the temporary opening support. Similarly, the same applies with the reinforcing, waterproofing and lining of the permanent collar as the design is required to accommodate an ever increasing load. The larger equipment clearly also calls for physically larger and higher capacity temporary staging, which can often have a negative flow-on effect with operation and productivity of mainline tunnel construction.

There are opportunities that exist in the larger road tunnels to reconsider what needs to be accommodated within cross passages, based on the fact that there is potentially other usable space within the tunnel envelope. This review can have a beneficial flow-on effect of physically reducing the size of cross passages and so enable potentially more innovative approaches to be adopted, with respect to their excavation and support. Such innovation can not only reduce the actual cost of cross passage construction, but also have a significant beneficial impact on the overall tunnel construction programme, by eliminating some impacts of their construction from the critical path. A solution with clear space under the road deck has the ability for construction of small sized cross passages from the upper deck to occur at the same time as tunnel services fit-out activities are occurring, unhindered below deck.

Invert options

In three-lane tunnels, the room above the vertical clearance is sizeable under most circumstances, and the space beneath the roadway is very large.

How can the tunnel invert space be better utilised and optimised from a construction perspective?

Examples of typical road base and invert treatments are described below.

• Backfill

On a major tollway built in Melbourne, the road pavement was placed onto a backfill constructed from crushed tunnel spoil and there was no smoke duct required.

An issue with this method is that it is difficult to place the backfill until the completion of the TBM drive. By the very nature of the curved tunnel shape, backfill methods can be complex and difficult to ensure road sub base compaction quality requirements. All below pavement services (HV cables, conduits, drainage, etc) may need to be excavated and laid conventionally in trenches excavated in the newly laid backfill. Despite the best design solution, hard spots are created under the pavement around drainage pits and soft spots in the trenches.

Another significant issue is the “burying” a significant portion of the tunnel permanent lining. This lining will not be visible for inspection over its design life and there are leakage management and durability issues to be addressed in the design.

There are potential construction cost benefits if excavated material can be re-used, otherwise there is extra cost for disposal of the excavated material plus procurement of backfill material.

• Box culvert, plus backfill

An option used in a recent project in Brisbane entailed precast culverts and engineered backfill placed either side of the culverts. This option has the advantage that some cabling can be undertaken early.

The system has the ability for the culvert installation and subsequent backfill to be completed relatively close to the TBM without impacting on TBM production. These spaces require ventilation to avoid early equipment and services corrosion. The drainage control of any sub pavement backfill must be thoroughly designed with the ability to be maintained to ensure there is no detriment to the adjacent void space. Access is required from the culvert space to each cross passage for cables and other tunnel services.

• Box culvert with roadway side planks

Another recent project in Brisbane used a precast solution comprising a road deck “table” unit with side wings that was installed within the backup gantry of the TBM.

The installation of this solution is fully integrated with TBM production. Any delays in placing the culvert will affect overall project productivity.

As with the culvert and backfill solution, this space needs to be ventilated to mitigate maintenance and potential durability issues. The culvert legs are heavy and despite the additional “air” space over a culvert and backfill solution, there is in reality little gained value for installation of additional services.

• Planking

A project in Madrid has road pavement sitting on precast planks supported by cast in situ corbels and a precast smoke duct also sitting on cast in situ corbels. The open, under-deck space provided sufficient room for emergency vehicle access.

Traditional design approach results in a high, ongoing maintenance cost

Historically the most common solution has been the box culvert with backfill either side. While some of the services can be located in the culvert, this solution requires significant equipment in cross passages or similar. A typical approach would include deluge valves in every cross passage, switchboards in alternate cross passages, which often involves cross passage tunnels alternating in width along the length of the tunnels, and additional passages for substations at approximately 1km spacing.

The cross passage openings are therefore large enough for egress, as well as allowing cables and pipes to transition in/out of the passages. The result is a tunnel which has frequent large cross passages, usually requiring a double segment opening to provide the width for egress and pipe/cables. Cross passage opening widths can increase with enlarged cross passage spacing due to the additional cables and pipes servicing a longer length of tunnel. A special cross passage would also be provided at the tunnel low point to allow a sump and pump installation. While the culvert and backfill solution can usually be provided without delay to the TBM driving the critical path is often through the cross passage construction.

Operational maintenance access is via the roadway for all of these installations, requiring closure of one tube at regular intervals, usually at night, which results in a high, ongoing maintenance cost.

New concept optimises space, construction, and maintenance

A solution is needed which allows integration across design, construction and operations with genuine whole of life benefits.

The TBM size required for a three-lane road tunnel is at least 14 m, with segment widths of at least 2 m. The three-lane traffic envelope is typically twice as wide as it is high, which is not ideal for fitting into a circular cross section, and which forces the roadway to be located high in the profile.

How can these disadvantages be turned into advantages?

A new concept, featuring an integrated road deck, cross passage and under-deck services, has been developed to functionally optimise the available space for a three lane road tunnel in a circular cross section (refer to Figure 1). The resulting outcome greatly simplifies cross passage construction and reduces tunnel operational and maintenance costs.

Concept of a three-lane tunnel with service tunnel/roadway

Figure 1: Concept of a three lane tunnel with service tunnel/roadway

• Deck

The deck consists of a precast unit with cast-in situ stitch pours providing continuity. The completed roadway solution provides a fully air/water/fire-sealed separation between the upper and the lower tunnel spaces.

- M&E access zone (service tunnel)

Beneath the deck, the following services are provided:

  • The drainage line, drain flush and access points
  • Fire mains, connections to deluge or sprinklers and the valves for these
  • Switchboards inside fireproof containers
  • Sub-stations within blockwork, fire-separated enclosures
  • A clear access roadway for the full length of the tunnel

The cross passages are now sized for egress, only. Typically, the egress width required is of the order of 1 200 mm. Consequently, cross passage construction is now greatly simplified, and an option exists to use precast boxes installed through a single segment opening. Figure 2 shows the option.

Figure 2: Cross passage using pre-cast box section
Figure 2: Cross passage using pre-cast box section

The single segment opening and jacked box solution significantly reduces the construction risks associated with poor ground scenarios or locations where groundwater inflow rates and periods need to be minimised.

The box would be jacked across with the reaction forces taken off the road deck or the segmental lining. The excavation can be carried out using a small remote controlled excavator or drill-and-blast or rock-splitting techniques, depending on the nature of the geology.

The box, as it is jacked between the two tunnels, is held on line and grade by steel guide rails installed as part of a pre-excavation probing and grouting regime.

A permanent structural collar is then constructed at the two openings, which also provide the completed groundwater seal. This solution eliminates the often difficult and risky exercise of providing a watertight seal of sheet membrane against the back of the tunnel segments.

Such a solution approximately halves the construction costs of more conventionally sized cross passages as well as halving the construction time, primarily because of the reduction in size and removing the need to install temporary ground support, blinding, waterproof membrane and a cast in situ secondary lining.

• Mechanical and electrical installation under-deck

By removing mechanical and electrical equipment from the cross passages and placing them under the deck, there are overall savings in civil works associated with the road deck solution and savings, in terms of time and cost in M&E installation and commissioning works.

From a programme perspective, by removing all equipment from the cross passages, the cross passage construction activity is no longer in the critical path, as the mechanical and electrical team do not need to wait for the excavation, lining and civil fit out of the cross passages or the installation of the conduits between them.

• Operations and maintenance benefits of the service tunnel concept

The under-deck solution significantly improves the availability of the tunnel, as the majority of maintenance activities can occur under the road deck without the need for costly and inconvenient tunnel closures.

With this solution, the O&M team can schedule their works on weekdays and without closing the tunnel, as they can drive in under the deck and perform the maintenance activities independently of the road users.

An integrated approach to tunnel delivery

A loss of workforce control by heavy recruitment at the peak of construction can have negative effects on industrial relations, productivity and cost. This history and the local context led to the thinking that the internal structure strategy for such large road tunnels should not just focus on the needs of the civil works team but take an integrated approach to tunnel delivery.

With this in mind, the various internal structure options developed in the past were overlaid with the impact each solution had from both an M&E and O&M perspective. It became very evident that an approach similar to that developed for a major project in Madrid, but adapted to Australian requirements, produced a solution that is cheaper and significantly faster, overall, minimising construction interfaces by providing delineated tunnel work areas and provides the most efficient solution for M&E installation and civil construction activities.

This can be achieved by adopting the road deck, cross passage and service tunnel solution, utilising precast components to the maximum extent possible and adopting installation gantries that enable a one-pass solution to construction.

A downside for the civil works is that this solution comes with additional costs and the need to use the road deck space to service the TBM; but, overall, the solution is potentially cheaper, faster, minimises labour, provides better quality control, carries less risk, is safer, and offers certainty of delivery.

A recommended approach to the installation of the interior structures -- including installation of major mechanical and electrical components -- would be to mechanise and modularise the installation as far as possible, which gives benefits such as minimal-sized work crews, improved safety and quality outcomes, and provides greater programme certainty.

In terms of economy, this tunnel design solution is considered to be cost-effective, considering all of the activities that occur inside the tunnel during the construction phase and the costs associated with the whole of life activities during the operational and maintenance period.

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