An important railhead, Mount Isa also has an airport with facilities for jets and is a key road hub for the extensive system of highways linking Brisbane, Townsville, Alice Springs and Darwin. Mount Isa began as a mining town, following the discovery in 1923 of substantial deposits of lead. It is now one of the world’s top ten producers of copper, silver, lead and zinc. The city has grown to be a modern, well-appointed centre of 25,000 people.

The problem

Mount Isa City Council has responsibility for all infrastructure in the town and following a localised collapse on the West St sewer main they decided to investigate further. The suspected damaged sewer ran between and under buildings with limited access. It was decided therefore to expose the pipe just downstream of the length in question where it entered Council property. It had been reported that the walls of this 450 mm diameter concrete pipe were very thin in the vicinity of the ‘dry weather high’ sewage flow level compared to the crown of the pipe. A section of the pipe wall broke away during the excavation being carried out to expose the pipe.

A section was cut out of the pipe from the centre of the crown of the pipe down to the centre of the pipe on the side. A close up of this ‘cut out’ is shown in Figure 1.

Concerned with the condition of this pipe and its location and importance as a critical sewer, the Council called tenders for the rehabilitation of 384 m of the main with the following scope of works specified:

• Prove that the existing 450 mm diameter sewer main and manholes have been thoroughly cleaned, and that the CIPP sewer relining technique can be successfully undertaken; and,

• Reline the existing 450 mm diameter sewer main between manholes 19 and 24, using the CIPP technique.

The CIPP technique involves a flexible fibrous tube of any size and length which is impregnated with a suitable resin system. The resin impregnated tube is inverted into the existing conduit while still in a ‘soft’ state by the use of a hydrostatic head or pressure, and is then cured to a hardened state while held in intimate contact with the conduit. CIPP technology has been an integral part of the Trenchless Technology industry for over 25 years and is today the most widely used trenchless method for restoring full structural integrity to deteriorating pipelines.

A CIPP lining will effectively span cracks or gaps in the existing pipe and given the existing condition of this sewer, Council’s specification of a CIPP system for rehabilitation was certainly appropriate.

Lining installation

Kembla Watertech was awarded the tender based on their Enviroliner CIPP system. The 384 m length was made up of seven separate manhole to manhole sections with difficult access to many of the manholes. In fact, one manhole was located inside a motor vehicle workshop.

A long continuous length with changes of direction is the ideal situation for an experienced crew to show off the advantages of a CIPP system and so it was decided to line the entire length of 384 m in one installation. This of course also provided cost savings and less customer inconvenience.

The cleaning of this pipe proved to be the most critical aspect of the project. This concrete pipe did not exhibit the usual sewer gas attack to the obvert, instead it had extreme corrosion along the spring line. With this knowledge, a very careful cleaning methodology was instigated which involved lower pressure ‘passive ‘ water jetting with continual monitoring by a CCTV camera.

Despite repeated cleaning cycles it was found that the pipe would not remain clean for long enough to enable lining installation to take place. It was therefore decided to begin the lining installation but with the jetter and CCTV positioned inside the pipe and just ahead of the inverting lining so that the pipe was cleaned as the lining progressed. The lining was impregnated with resin off-site and transported in a refrigerated container to Mount Isa, as even in winter the average temperature in the region is in the mid twenties.

Installation of the 384 m length took place in one 24 hour period and the CCTV survey taken after installation shows the lining to be in excellent shape. The smooth internal finish and joint free nature of the lining will minimise any future maintenance problems and an actual increase in flow capacity is anticipated.

Council are very pleased with the final result. “The failing sewer mains in West St has provided Council with an opportunity to explore and trial alternative asset rehabilitation methods and because of the simplicity and non-invasiveness of the trenchless CIPP method, it will be considered for other programmed sewer renewal works,” said Engineering Services Director Emilio Cianetti.