Below the belt: part 1

This week on the blog we take you on a journey down the South Belt sewer, one of Christchurch’s many 19th century wastewater sewers. Located deep below the east-bound lane of Moorhouse Avenue and more than a kilometre in length, construction of this sewer began in 1881 and was completed in early 1882. Recently, as part of SCIRT's horizontal infrastructure rebuild program, their Downer delivery team and sub-contractors Donaldson Civil replaced a 30 metre long upstream section of this sewer where a blockage had occurred. In this part 1 of a 2 part sewer archaeology special – we look at how this sewer was built, how it got blocked, and how it got fixed. Enjoy! Before we got digging, we put a sewer inspection robot down into the sewer, the footage it recorded helped us to determine the location of the blockage, and thus where to dig. Image: Hamish Williams.

We dug down more than 2.5 metres to reach the brick crown arch, downstream of the blockage location. It was neat to see a thin smear of cement mortar had been applied to the top of the arch – where the bricklayer more than 130 years ago had cleaned off his trowel. Image: Hamish Williams.

Using a concrete saw, we cut through the crown arch...Image: Hamish Williams.

...and were most surprised to find a 30+ metre long sewer snake trapped inside! This snake (actually a high pressure sewer cleaning jet) had got stuck some time ago while trying to swim upstream. There was no flow in the sewer at all, only 60 mm of stinky sewage water. Images: Hamish Williams.

Of an oviform or ‘egg’ shape, the base of the sewer (that’s what the invert is called in pipelaying speak) was made of unreinforced concrete. The upper crown arch was formed of specially shaped taper bricks, 13 of which were required to span the arch. In the photo on the left you can see the resin impregnated fabric liner that was installed inside the sewer circa 2009, and at right one of engineer William Clark’s original 1878 oviform sewer design drawings. The sewerage system that he designed for the Christchurch Drainage Board became fully operational in early September 1882, and many parts of this system are still in use today. Images: (at left) Hamish Williams and at right, after Clark (1878) Drainage Scheme for Christchurch and the Suburbs.

A section of this liner was cut out and used as a mould to custom make two PVC plastic transition pieces, as we were replacing the damaged section of sewer with pipe of a circular shape. Image: Hamish Williams.

A special wire cutting saw was brought in to make a clean cut through the sewer, so we could firmly fix the downstream transition piece to it, before this join was encased in reinforced concrete. Future archaeologists should have no issues determining when this concrete was poured! Images: Kane Reihana (at left) and Hamish Williams (at right).

When we got to the blockage, our suspicions about the cause of the blockage were confirmed. Although the sewer itself had not suffered any form of structural collapse, liquefaction silt had entered the sewer through cracks in the brickwork and had constricted the liner, blocking the flow of sewage. Images: Kane Reihana (at left) and Hamish Williams (at right).

Ben McConochie fits the upstream transition piece in place with epoxy mortar before the concrete is poured. Image: Hamish Williams.

All done! Image: Hamish Williams.

Many thanks to SCIRT, Downer and Donaldson Civil for a job well done, and especially to Moorhouse Avenue businesses and motorists for their patience while SCIRT has been working on fixing this and other damaged horizontal infrastructure in the area.

Hamish Williams.