Northrop

Northrop provided structural engineering services for the adaptive re-use of a former coal bin at Catherine Hill Bay, into a multi-level public viewing platform having uninterrupted views of the Pacific Ocean. 

Background

Mining operations ceased at Wallarah Colliery in 2002. Lake Coal engaged Northrop in 2013 to assist them with remediating the large coal storage bin on the site.

The coal storage bin is cut into the cliff face and constructed from a series of reinforced concrete beams and columns funnelling into a tunnel at the base that contains a conveyor to transport the coal to the ship loading facilities on the jetty.

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When Northrop became involved in the project the metal roof had been removed and the ocean end of the tunnel had been sealed. The site was subject to unauthorised entry and illegal rubbish dumping into the coal storage bin. There were serious concerns for public safety and the owners were at a loss as to how the rubbish and remaining coal could be removed from the bin safely. There were also concerns around the long-term stability of the reinforced concrete bin structure that was subject to deterioration from concrete cancer and how to make it safe.

Solution

The construction works involved fully filling the 30m deep bin with approximately 18,000m³ of material. To minimise cost and limit construction traffic through the neighbouring residential area, the existing concrete and bitumen pavements within the former mine site were crushed and used as fill material within the coal bin. The recycled concrete and bitumen were bound together with a cementitious grout using fly ash, a waste product from a nearby power station. The use of recycled waste materials for the bulk filling of the bin significantly reduced the environmental impact of the works compared with filling with standard concrete. It also achieved a project cost saving of over 50%. 

Once the redevelopment of the former mine site into a residential subdivision is complete, the new viewing platform will be an outstanding community facility and likely a very popular spot for dolphin and whale watching.     

This project demonstrates excellence in a number of areas

• In satisfying the clients brief to provide a solution that was safe to construct without the need to access the inside of the coal storage bin.
• In providing a solution that didn’t think about sustainability as an add on or afterthought but embodied it into the project from the outset. This is demonstrated by the reuse of material from the site, not removing material from site and disposing of it in landfill, using waste product as the majority (over 90%) of the fill volume and reducing cement content by over 50%.
• The solution produced a reduction of approximately 3,200T of C0₂ in a single remediation project.
• The solution provided protection to the environment in a highly sensitive area. Encapsulating the remnant coal and dumped rubbish in the bottom of the coal bin prevented leaching and keeping the structure of the existing coal bin allowed all construction materials to be contained within the site. There were no environmental issues throughout the project.
• The solution provided used established engineering principles but applied them to a new and unique problem to provide an outstanding outcome.
• The engineering solution reinstated the site for community and ecotourism use without leaving a legacy issue for future generations to address. The site can be repurposed in the future for a myriad of different purposes.
• The engineering solution respected the local community and minimised the impact of traffic, noise and air pollution on the local community during construction.

In a world where global emissions and climate change are at the forefront of everybody’s mind, engineers have an important role to play in leading the solutions to the challenges we face. On this historical industrial mine site we have been able to demonstrate how clever engineering solutions can be used to reduce our impact on the environment and improve safety for those working on construction sites.

Project team Anna Lansley, Dean Jarvis and Neil Petherbridge