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Boxing clever

Delivering new infrastructure through urban areas is challenging enough but work on the Brisbane Airport Link meant the new road had to pass under a major rail link - without interrupting services. GE takes a look at the project.

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The Brisbane Airport Link has required a complicated jacking of two concrete boxes

From next month, the new Brisbane Airport Link in Queensland, Australia, will provide a dual carriageway connection from the city’s central business district to the airport. It is expected to provide fast travel times and reduce congestion.

But before air passengers could benefit from the link, joint venture contractors Theiss John Holland had to find a way of constructing the road without impacting on the rail link to the airport.

Part of the £2.9bn route had to pass below an embankment at Toombul. The embankment carries six railway lines used by up to 380 trains a day on the North Coast Railway and the Airtrain and work had to take place without interrupting services.

The solution was to jack two precast concrete boxes through the embankment, but challenging ground conditions and tight settlement tolerances on the rail lines meant that the solution was not simple.

The two boxes are 65m long, 12.5m high and have a combined width of 38m. The site teams worked around the clock to jack the boxes through the embankment in just 36 days. Ground conditions in the embankment were challenging and varied from firm to very soft alluvial clays with water bearing alluvial sands and residual soils up to 17m deep through to weathered siltstones and sandstones.

In addition to the variable ground, there was the added risk of historic timber structures, old sewers and concrete footings in the embankment fit itself.

The complexity of the Toombul jacked boxes is illustrated by the fact that the list of companies involved in the work reads like a who’s who of ground engineering. Design of the temporary works was carried out by Benaim and reviewed by Arup and RD Geotech, while permanent works designs were undertaken by a joint venture of Parsons Brinckerhoff and Arup. On the construction side, the main work was undertaken by Theiss John Holland, while the specialist aspects of the work were subcontracted with Tunnel Corp carrying out canopy tube installation, nailing and fracture grouting carried out by Keller, jet grouting by Menard, mining shield construction by Terratec, and the actual box jacking carried out by VSL.

Track maintenance and surveying were also key elements of the work and ensured there was no impact on rail users. These elements were carried out by Queensland Rail and Geometric, respectively.

Inside

Around 40 people worked to excavate ahead of jacking

The jacked boxes were constructed in a jacking pit adjacent to the embankment. At one end a 15m high vertical pile headwall was built to support the dig and minimise lateral movement in the embankment.

There were a number of other geotechnical challenges too. Soft clays and running sands were considered to be unstable at the jacking face so ground improvement was needed ahead of jacking and there was also concern that the bedrock may be subjected to high water pressures that could impact on the jacking.

Advance works

The headwall was formed using 900mm diameter contiguous bored piles toed into the bedrock at 1.4m centres. A jet grouted trapezoidal block was also created immediately behind the wall to improve the soils and minimise water ingress. The wall was also restrained using three levels of ground anchors. The strength of the soft clays and sands was further improved by soil fracture grouting ahead of box jacking. This was done by horizontally drilling glass-reinforced plastic soil nails between the gaps in the contiguous piled headwall over a distance of 65m. Grout sleeves were wrapped around the soil nails for multiple fracture grouting.

Toombul

Above: Ground improvement was carried out ahead of jacking

The alluvial sands were also permeation grouted using the sleeves to reduce the risk of high face losses during the jacking. In total 17,480 Geonails and 346M.t of cement grout was used in this operation. In addition, 220m of horizontal drains were installed to accelerate pore water pressure dissipation to allow the soft soils to gain strength following the fracture grouting.

Each box was jacked through the embankment using two sets of jacks - one used 26,000t of jack force and the other 22,000t. This was delivered with a series of 1,000t push jacks and 750t pull jacks using tendons

Other issues that had to be considered included planning the construction sequence to ensure that jacking the boxes 65m through the embankment could be instrumented and monitored to ensure safety and to allow the rail way to remain in operation.

Canopy tubes were installed, after construction of the headwall but ahead of jacking, above the top of the box alignment. These were interlocking, concrete-filled steel tubes at 900mm centres 75mm above the top of the jacked boxes. The structure was designed to provide local face support to the embankment by redistributing the load onto the box roof and minimising ground loss at the excavation face. The canopy structure was also tied to the top of the header wall to provide additional horizontal constraint.

The rail authorities set minimum settlement criteria of 25mm but the design team realised that it was likely to exceed this and established a daily routine of re- levelling the tracks to ensure settlement remained within the set limits.

The jacking forces were also substantial as they had to overcome the frictional resistance of the ground. So the structural components - jacking shield, permanent reinforced slab and the jacking box - had to be designed to resist these forces with sufficient redundancy to avoid compromising the integrity.

During the work, a rigorous programme of instrumentation and monitoring of the structural performance of the jacking operation was made. This consisted of measuring pore pressures within the ground, measuring vertical ground movements in horizontal inclinometers installed within the canopy structure and conducting insitu vane and horizontal static cone penetration testing of the improved ground around the soil nails.

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Two sets of jacks pushed each box through the embankment

Movement limits set by rail authority Queensland Rail were checked using 200 survey prisms located on various railway structures and three automatic survey stations continuously scanned the prisms with alerts about movement sent by text and email. Average settlement during the work was 115mm, and this was managed by ground tamping at key stages of construction.

With the ground improvement in place, the box jacking was carried out in a continuous 24 hours a day, seven-days-a-week operation, with excavation and jacking progressing in 200mm cycles. Around 40 people at a time worked to excavate the 27,000m3 of material from the compartmentalised face shield.

Each box was jacked through the embankment using two sets of jacks - one used 26,000t of jack force, and the other 22,000t. This was delivered by a series of 1,000t push jacks and 750t pull jacks using tendons. Progress averaged 1.1m a day but rates of up to 2.5m a day were achieved.

The jacking process was completed in June last year, clearing the way for construction of the carriageway to begin.

The project has already been recognised for its achievements with presentation of the Fleming Award in December last year.

But the Brisbane Airport Link is now in the running to become a double award-winner after it was shortlisted for a GE Award.

The project team hopes that the work at Toombul will be of a high enough calibre to score a win in the technical excellence category at the GE Awards ceremony on 4 May.

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