Designing For the Tunnel Legacy Beneath the Hunter Expressway

New road infrastructure has the capacity to greatly improve connectivity between communities, fostering sustainable development and unchartered growth.

Beneath many modern highways connecting rural towns today, sits remnants of infrastructure that supported past industry in the region. This presents challenges for the teams tasked with designing a solution that is both best for project and preserves the local environment and heritage. 

 

Such was the case with the Hunter Expressway in New South Wales, Australia. Two heritage listed rail tunnels, a testament to the region’s rich coal mining history, left behind a challenging legacy for geotechnical engineers and the Alliance commissioned to deliver the project. Construction impacts, blasting vibrations, and the ongoing expressway traffic loads had the potential to negatively impact the sensitive brick lined tunnels. The role of the Alliance was to ensure the tunnels suffered minimum disturbance.

 

Managing the Impacts of a Coal Mining History

Opened in 2014, the AU$1.7b Hunter Expressway features 40 km of new dual-carriageway connecting the M1 Pacific Motorway near Seahampton and the New England Highway west of Branxton. It has cut travel times between Newcastle and the Hunter by 30 minutes, reduced the number of heavy vehicles travelling through townships on the New England highway and is central to meeting the growing freight demand for the region.

 

The Hunter Expressway Alliance, consisting of WSP, Roads and Maritime Services, CPB and Arcadis, was tasked with design and construction of the Stage 2 eastern section. This consisted of some 13.3 km of road, with significant geotechnical challenges featuring poor quality earthworks material available on site, and historical coal mine workings underlying much of the alignment. Furthermore, project requirements meant that the Alliance had an overriding accountability to minimise clearing of native vegetation, protect important local waterways and manage impacts to the two heritage listed rail tunnels.

 

Delivering Best For Project While Preserving Our Heritage

WSP is designing new road infrastructure with the Tunnel Legacy beneath the Hunter Expressway.

The new expressway alignment passes near two existing heritage-listed brick-lined rail tunnels that formed part of the former Richmond Vale Rail Line – previously tasked with servicing coal mines at Minmi, Stockrington, Pelaw Main and Richmand Main, before their disuse in 1988 with the fall of the local mining industry.

 

The two tunnels, one 160 m in length the other 372 m in length, were constructed some 100 years ago. The expressway design meant that the deepest part of alignment was excavated to within 10 m above the crown of one tunnel, necessitating structural assessment to assess its capacity to withstand the construction impacts. This was complicated by a gap that was thought likely (and subsequently proved) to exist between the brick lining and the rock, which posed a tunnel stability risk when construction and excavation loads were considered.

 

“Large scale tunnel intervention or stability works by their very nature will disturb a tunnel, regardless of whether road construction causes direct impacts, and so we had to minimise the extent of the intervention,” says Andrew Noble, WSP Technical Executive for Hydropower & Tunnels.

 

“One of the goals of the Hunter Expressway Alliance was to preserve the integrity of the heritage listed tunnels and cause no damage, meaning that a balance needed to be struck between minimising the amounts of intrusive testing to preserve the heritage aspects, while gathering sufficient information to support structural assessment of the tunnels.

 

“Intrusive investigations were limited to drilling a vertical borehole 1.7 m away from the tunnel to assess the surrounding rock, along with using six core drills through the lining at various points to assess its properties. This was supplemented with non-intrusive surveys and investigations such as visual observations and topographical surveys. The results of this investigation enabled the formulation of a detailed geological and structural model. Subsequent vibration and structural analyses then demonstrated that a “do nothing” minimal intervention approach was viable. A rigorous construction phase monitoring regime also gave confidence to stakeholders that risks were managed.

 

“The approach proved to be the right one – four years after opening, despite significant construction and blasting loads, as well as the ongoing vibrations associated with the expressway traffic, the two tunnels remain intact. The ‘do nothing’ approach has preserved the tunnel’s heritage value, which was so intricately linked with the region’s identity, as well as ensuring unnecessary cost was not incurred on the project.”

 

This is the topic of a paper co-authored by Andrew Noble and Robert Kingsland entitled, ‘The Case for the ‘Do Nothing’ Solution for Two Heritage-Listed Brick Linked Tunnels Impacted by Motorway Construction Above’. The findings on the Hunter Expressway Alliance’s approach to these tunnels was presented by Mr Noble in April at the International Tunnelling Association’s World Tunnelling Congress 2018 in Dubai.

 

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