Confluence Chamber



  • Construction of new confluence junction chamber (4.5m x 2.5m x5.5m) at the intersection of two of Christchurch’s main trunk sewer lines
  • Installation of a 900mm diameter temporary by-pass
  • 90 linear metres of sheet piling
  • Complex dewatering and sewer bypass pumping
  • Commissioning of the new structure

Project Description

The Northern Relief Sewer Main suffered substantial damage from the Christchurch earthquakes. Sections of the pipeline had to be replaced or repaired, with other parts being completely relaid on new alignments. In order to connect the existing pipeline to the new line, a confluence chamber was designed to merge the flows.

The Retreat Road confluence chamber is a 5.5m deep concrete structure located on the edge of Christchurch’s red zone, directly adjacent to the Avon River. Particular challenges on this project were the varying ground conditions and high water table close to the river as well as the requirement to maintain the existing Northern Relief main trunk operational over the entire construction period.

To maintain the significant flows of up to 900l/sec at peak Seipp Construction opted for an innovative 900m diameter steel pipe bypass system, which was installed within the sheet piled excavation. This bypass was tied into the existing lines during a night shift with a comprehensive over pumping regime in place to maintain live sewer flows. A tight seal was achieved within a matter of hours with each fabricated piece fitting perfectly.

The 19t chamber base was pre cast off-site and craned into the excavation followed by the 4.5m high walls being poured in two lifts. Polystyrene block outs for both the pipe inlet and outlet were used to expedite construction. The main structure was heavily reinforced, which meant the concrete mix used had to be designed specifically for this project. This special mix design helped to guarantee the specified concrete cover besides achieving an impressive finish of the entire in-situ concrete structure.

The internal benching had to incorporate two removable weir gates and a mezzanine floor, along with the transition from a 1.2m diameter concrete pipe to a 1.4m diameter GRP pipe. Our civil structures team designed a formwork frame that could be suspended within the structure allowing the 2m high concrete benching to be poured in one, eliminating the need for time consuming benching operations within the confines of the structure.

To tie the structure to each of its inlet and outlet pipes, six specially fabricate gibaults were installed, achieving a live connection immediately after the temporary gravity bypass was removed. Pre measured and cut pipe sections were installed reinstating the flows. A temporary overland bypass pumping was required for both the installation and removal of the gravity sewer bypass.

With the innovative construction methodologies used on this project, the Seipp team completed this challenging and complex civil project two weeks ahead of an already ambitious programme.

Seipp Construction are dewatering experts. Our experience includes wellpoint systems, large and small diameter wells and surface dewatering. We have comprehensive controls in place, including 24-hour rem ote monitoring of all pumps, live web access to the data of turbidity metres installed at the point of discharge and pH sensors. This system is a New Zealand first and enables immediate action in the face of any issues. We also have an extensive range of sediment and decanting binds, including oil separators.

Client: SCIRT/Christchurch City Council
Start Date: September 2016
Completion Date: December 2016