Northern Relief Sewer Line Confluence Chamber

Scope

• Installation of 820m of 1400mm diameter GRP pipeline
• Installation of two in situ concrete connections to structures at either end
• Installation of 6 Number Tees including risers
• Dewatering using a combination of spears and wells
• 790 metres of sheet piling

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Project Description

The Christchurch earthquakes severely damaged the Northern Relief sewer line, a main sewer trunk interconnecting with other parts of the wastewater network, that collects wastewater from the northern and western parts of the city and takes it to Bromley for treatment and discharge.

Seipp Construction were engaged to construct 820m of new 1400mm diameter GRP pipe in a residential area with challenging ground conditions and a high ground water table. The fiberglass pipe was laid in depths ranging from 2.5m to 5m by using a variety of shoring methodologies to enhance value for money delivery.

Shallower sections (up to 3.5 metres) were constructed using traditional trench shields and two of our 35tn excavators. Only the use of a sophisticated dewatering systems enabled us to use shields up to this depth without extracting fines, a major risk identified pre-construction.

As depth increased the shoring methodology changed to sheet piles and our adjustable waling prop frames. The clutched sheet piles together with the sophisticated dewatering system kept water and running sands outside of our trench.

Each collar of the pipe was individually wrapped with geotextile material to ensure no material would infiltrate the rubber collar in the event of another earthquake. Extensive ground testing was undertaken to ensure the ground was strong enough to support the pipe, soft rafts were constructed underneath the pipe in areas of insufficient strength.