Designing for the Tunnel legacy beneath the Hunter Expressway
15 June 2018
WSP:
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
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.
Link to paper, https://www.wsp.com/-/media/News/ANZ/Document/HEA-Heritage-tunnel-paper-for-WTC-2018-Dubai_NOBLE-KINGSLAND.PDF
--ENDS--
Source: WSP - www.wsp-pb.com
Contact: N/A
External Links: The Case for the ‘Do Nothing’ Solution for Two Heritage-Listed Brick Linked Tunnels Impacted by Motorway Construction Above’.
Recent news by: WSP | PB