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Repairing Sewers Without Digging

More and more ways are being devised to repair sewer pipes without digging up the road around them.

At present the Christchurch City Council spends about $2.1 million a year on sewer renewal work. The bulk of this is spent on renewing old, broken and cracked pipes, and fixing leaks.

Increasingly, sewer joints are found to be letting in ground water that overloads the system or they allow sewage to leak out into the ground. Various methods are now being used or investigated for use by the Council to tackle faults in the sewer pipes.

A method used widely in Australia and which could start here in July is injection grouting. This, put simply, is the injection of a polyurethane resin, mixed with water, into defective joints to seal them.

The mixture is very moisture sensitive and takes only 40 seconds to set to a firm gel. The sealant is pumped into a flexible, rubber "packer." This is put into the sewer pipe, along with a television camera, and winched to the site of the faulty joint. The joint is cleaned and then an operator in the control van in the street expands the packer with compressed air.

The joint area is sealed at each end of the packer allowing for it to pump out the resin to set around the joint It is a slow process with only 100m of pipes able to be covered in a day. But once Christchurch has such a system in place it could be working five days a week for the next three years on known leaks that need attention. This can be estimated because a close-circuit television survey and a sewer flow monitoring programme has already pinpointed leaks.

Over 400km or 40 per cent of the city's sewer pipes have been surveyed by close-circuit television. Life up to 20 years This repair system has the emphasis on the rehabilitation of the sewer rather than digging up the road and renovation. Grouting is low cost but limited to a lifespan of 10 to 20 years.

Another City Council low-cost repair system also involves a type of packer. However, this works by dragging a polyester resin and fibreglass liner on the outside the packer. As this reaches the spot in the pipe to be repaired the packer is inflated and the liner attached to the inner wall.

From the start of the liner preparation in the street to the "plastering" inside the pipe takes the operators about 40 minutes.

About 60 repairs have been made over two years using this method. The process is still being refined, as it is a messy job that has to be carried out quickly. It is estimated that the method saves about $1000 each repair over the digging method. The City Council's Waste Management Unit aims to make 100 such repairs a year.

Bursting method Another Council method for repairing sewer pipes mainly on hills is the bursting method. This entails sending a hydraulically propelled steel cone into a pipe and, as it is bigger than the pipe, smashes it. As it does so it drags a new, black polyethylene pipe behind it.

Yet another method will be seen in Goldsmith Street, Waltham, soon. This will be pipe reaming where the pipe is pulverised by a revolving steel cone with cutting attachments on it. The crushed pieces are dispersed and, in this first for Christchurch, the reamer will go under Brougham Street leaving the road undisturbed. The big advantage of the pipe reaming method is its ability to replace old pipes very close to the original grade.


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