NorthConnex and Hornsby Quarry Spoil Management
Construction of new road and rail infrastructure in Australia turns frequently to tunnelling as a preferred design solution because many of the associated environmental and community impacts are minimised or eliminated.
However, tunnelling in highly urbanised environments still presents numerous challenges, which include:
- spoil management,
- increased heavy vehicle movements on the surrounding road network,
- protection of sensitive heritage and ecological areas,
- noise and vibration impacts, and
- community and stakeholder relations.
Spoil management in urban areas isn’t easy
Disposing of spoil generated by tunnel excavation has its own considerable challenges:
- availability of spoil disposal locations in proximity to the construction sites
- limited spoil disposal locations available outside of standard construction hours
- spoil disposal locations often regulated by their own licences and planning approval conditions
- impact of spoil removal and disposal constraints on tunnelling production rates
- impacts on the local community (traffic, noise, safety) from increased truck movements on the road network
All NorthConnex tunnelling sites in Sydney, Australia have direct access onto the main arterial road network to minimise heavy vehicle movements on local roads. However, direct access and egress onto Pennant Hills Road means coordinating traffic movements onto one of New South Wale’s most heavily congested roads.
This, together with long haulage distances to the nearest disposal site, presented a significant challenge around gaining certainty of haulage cycles, travel times, and maximising the available hours to haul spoil to disposal sites.
These issues are not unique to NorthConnex, but they place pressure on the delivery of a tunnelling program. And they are likely to repeatedly occur with the increased prevalence of tunnel construction for new road and rail projects.
Hornsby Quarry presented an excellent opportunity
Looking to overcome these challenges, the NorthConnex project team recognised an opportunity to use Hornsby Quarry in Sydney’s north as a spoil disposal site. Hornsby Quarry is much closer than other spoil disposal sites in Sydney. It was also a space the local government was looking to fill-in and reclaim as a recreational park.
Beyond the obvious financial benefits of a short haulage route, the advantages of using this site included:
- dramatically reduced total kilometres travelled for spoil disposal,
- reduced risks associated with haulage delays,
- more certainty on haulage turnaround times, and
- overall more efficient removal of spoil from tunnelling site.
Identifying this opportunity is a good example of how a State Significant Infrastructure project can test its original scope in order to find a new approach to delivery — an approach that worked with the local government to find an innovative solution to a common project issue.
It’s important to forge strong partnerships
Having the same proponent, in this case Roads and Maritime Services, hold the planning approval for both projects (ie NorthConnex and the Hornsby Quarry) provided an unquantifiable benefit, opening up the opportunity to continually review site operations and propose potential efficiencies to maximise project outcomes.
This forging of a strong partnership between the State Government and the private consortia delivering a major infrastructure project contributed significantly to achieving optimum project outcomes.
Filling Hornsby Quarry had benefits, but also several challenges
The benefits to the community included:
- Minimising the distance travelled by trucks carrying spoil by about 3.7 million kilometres, reducing congestion on the road network, fuel consumption, and emissions by more than 16,000 tonnes.
- Reducing the cost of maintenance associated with the quarry for the community and Hornsby Shire Council.
- Paving the way for Hornsby Shire Council to create the Centennial parklands of the north.
However there were several challenges faced by the project team, including:
- Getting the spoil into the quarry void. The void is more than 100 metres deep, with steep and exposed sides, and has a total volume of about 3.3 million cubic metres. Prior to the start of construction the void was flooded with water about 11 metres deep.
- Up to 70 spoil truck movements an hour travelling through local roads to get to the site, which is surrounded on one side by residential development.
- Providing a brand new haulage road to access the site down a very steep gradient.
- Minimising noise impacts on the neighbouring TAFE.
- Retaining mountain bike track access around the construction site.
- Dense vegetation, including Berowra National Park surrounding the remainder of the site.
In order to overcome these challenges the team:
- Innovatively trialed and established a conveyor system that is normally used in the mining industry. This was used to safely carry the spoil to the bottom of the void.
- Carried out extensive consultation and achieved community and stakeholder buy-in for the work.
- Constructed a purpose-built temporary teaching facility to relocate the most affected TAFE classrooms during construction.
- Built bike crossings under the main haulage access road and the spoil conveyor to maintain mountain bike trail access throughout construction.
- Put strict environmental controls in place to minimise impacts to the local environmental.
There will always be challenges to deal with
The initiative to undertake the Hornsby Quarry project was motivated by the challenges of spoil management and disposal in sensitive urban areas and the drive to find sustainable approaches to infrastructure delivery.
Acknowledging that a vacant quarry in need of filling in close proximity to a major tunnel project is not always going to be the case, other measures should be considered early in the planning and environmental assessment process to further mitigate the risk associated with spoil management and disposal. These could include a permanent disposal site, or temporary storage site, available outside standard construction hours. Achieving a quality outcome in this area is likely not to be without its challenges.
In the case of Hornsby Quarry, overcoming the significant challenge to enable hundreds of haulage vehicles a day to travel through the town centre of Hornsby should not be underestimated. This was only achieved through high quality environmental impact assessments, a comprehensive community and stakeholder engagement strategy, and the collaboration between multiple organisations focused on delivering the best project, community, environmental, and sustainable outcomes.
Question standard approaches to infrastructure delivery
Future project teams, from either the government or private sector, should be encouraged to question the standard approach to infrastructure delivery and test the original project scope to find innovative solutions to common project challenges.
The risk of acquiring access to spoil disposal locations is often passed on to the contractor after project approval has been granted. This project risk can be mitigated by incorporating the procurement of those sites into contractual agreements and defining them in the environmental impact assessment process.
With spoil management forming a key element of successful and timely tunnel project delivery, early focus on this issue will ensure the best environment and community outcomes are achieved.
Bright future ahead for Hornsby Quarry
With the last load of NorthConnex spoil delivered to the Hornsby Quarry on 25 January 2019, the quarry has been partially filled with more than one million cubic metres of spoil and will allow Hornsby Shire Council to commence rehabilitating the site to create a park benefitting the local community.
Note: This article is based on a conference paper produced for the 16th Australasian Tunnelling Conference 2017.
tl;dr
We dug a tunnel so had lots of dirt to spare; Hornsby Quarry needed filling. Sustainable spoil management ensued, and Hornsby Shire will get a new park out of it.
