Interflow completes “impossible” project

The collapse of Lismore City Council’s sewer line required an innovative approach for rehabilitation within a narrow laneway in Lismore, NSW.

The 100-year-old wastewater asset required an urgent excavation, repair and relining to nine broken junctions buried 5 m below ground while nestled in a confined 5.5 m wide alleyway and popular Lismore tourist attraction, Larkin Lane.

As the laneway is flanked by two-storey buildings, the proximity of infrastructure meant that any additional load around the buildings would be imposed on the pipe through surrounding soil.

Interflow Project Manager John Adamo said this affected the excavation plan to access the collapsed junctions as the buildings’ weight would directly impact the walls of the trench.

“The pipeline also had high flow in the main and house service lines, making excavation especially difficult,” said Mr Adamo.

A geographical report of the area determined Interflow’s need to install a custom designed steel casing tube to stabilise the working area and provide a robust solution due to its ‘hoop strength’.

“Our steel casing design was strong enough to resist the ground pressure from the soil and withstand any weight imposed on the trench from the surrounding infrastructure,” said Mr Adamo.

As the confined area would not allow for a large excavator, Interflow used a 6 t excavator to lift each section of the 1,800 mm diameter casing into the ground and bolt them together.

The project also faced some challenging conditions including extremely muddy and unstable work areas, as well as coping with heavy pedestrian foot traffic.

“Deep excavation was needed in slippery back mud – it is a very sticky, wet and heavy clay and this type of ground collapses easily,” said Mr Adamo.

To ensure minimal disruption and mitigate the risk of injuries, safety management and community relations remained at the forefront of the project.

For more information visit the Interflow website.  

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