CCFWA Earth Awards 2020

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Department of Planning,
Lands & Heritage

Urgent major remediation and stabilisation works to the Swan River escarpment behind the Hill60 Apartment Buildings in Rivervale were required. Temporary remedial works had been undertaken following a landslide in 2012 however the slope continued to be affected, and a 60m section of dual-purpose path adjacent to the apartments had been heavily undermined.

The recommendation was an engineering solution involving the weighing down of the toe of the slope by constructing a rock revetment. A toe extending into the Swan River would create the structural integrity to achieve slope stability.

The Swan River, or Derbal Yerrigan in Nyoongar language, is one of the most significant Aboriginal cultural sites in the Perth region. The project would require cultural monitors to be present during the excavation stages to look for uncovered artefacts and to ensure that the construction techniques would be conducted with minimal impact to the Swan River and its spiritual and environmental values.

The only available access to the site was through a recently refurbished park owned by the City of Belmont. The extremely steep trail was cleared of trees, to enable the access track to be cut. Initial earthworks to establish the track were carried out with a mini excavator. The lower side of the track was supported with steel sheet piling and was continually monitored during construction and upgraded when necessary. During periods of heavy winter rain, the track was deemed too slippery and works were halted.

Due to the extremely limited laydown area – just 120 square metres – MMM had to ensure materials hauled into the stockpile were relocated and placed immediately along the embankment. Scheduling of deliveries was imperative and played a crucial role in the success of this project.

Scheduling the works due to tidal movements and weather conditions was paramount. Some elements of the works required staging around low tide, however, due to the scope of the project and time constraints this was not always possible, and work carried on with varying tide levels.

Before any work commences on the Swan River a silt curtain is required to be installed and maintained during the entire duration of the works, to prevent blooming from construction demolition or materials entering the river. These works required a double silt curtain – one to contain the entire works and a second to contain the current work area. A professional dive team was required to maintain the silt curtain and reinstate it after tides and interference. Twenty anchors were installed to secure the curtain to the riverbed.

Accurate placement of filter rock is crucial for the revetment, as it provides a foundation for the overlaying armour rock. Due to the restraint and restrictions on-site, a solution was required to use the existing machinery to reach 8.4m. A 3m-wide blade “pusher arm” was fabricated to MMM’s design and attached to a 13t excavator. This proved highly successful, enabling the riverbed to be lightly excavated and shaped to meet the design levels.

To enable the placement of 4t armour rock to reach the end of the toe design, a steel railway line was fabricated to place the rock onto so that it rolled down and out to the depth of the toe. This solution was necessary because the maximum sized machine that could be mobilised down the access track was a 20t excavator, which did not have the reach to place the furthest rock.

Two 5t dump trucks were purchased and shipped from Queensland, to enable the maximum material to be carted from the laydown without compromising the stability of the slope. The dump trucks had rotating seats so that they did not need to turn around on the embankment, which was not possible in the initial placement of blue metal and filter rock.

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