Rotting joints threaten sewer system

Press, 2 September 1977, Page 1

Rotting joints threaten sewer system

By

PHILIP WORTHINGTON

Perishing rubber joints in sewer laterals and mains will give Christchurch a bigger headache in the next 10 years than any amount of stormwater flooding, the Christchurch Drainage Board's works committee learnt yesterday.

The gravity of the problem was highlighted in Nev Brighton a few weeks ago and will affect most areas of Christchurch, the board’s engineer (Mr P. J. McWilliam) said.

His report to the committee said that the laying of a big stormwater pipe in Pratt Street and Keyes Road, New Brighton, last month meant that some sewer laterals had to be dug out for rerouting. On dismantling the laterals serious deterioration was found in all of the rubber jointing rings. The jointing rings were first introduced in the 1950 s as a method of sealing joints between lengths of pipe and are now very common. The composition of the rings was changed afte’- the first perishing was discovered in 1961, but it now appears that all the rings used from their introduction until the early 1960 s are suspect.

Some of the rings removed at New Brighton were as much as 75 per cent eroded away by microbiological action.

“In the laterals so far exposed, there was no evidence of leakage or in filtration at the joints, but the deterioration of some joints was such that the time when this will occur

appears to be no more than a year or two away,” said Mr McWilliam.

The joints so far exposed — only a few in New Brighton — were laid in 1952 at the time the main sewer line (also jointed with the rubber rings) was laid. The house connections were not made to these laterals until 1955 and 1956, the perishing having occurred since then. “The majority of houses in central New Brighton were connected to the sewer between 1955 and 1957. In this area the sewer and the laterals to the boundaries of the properties were all laid with rubber-jointed concrete pipe,” Mr McWilliam said.

He estimated that in New Brighton alone there would be eight miles of sewer and about 1000 laterals with suspect jointing rings. Piping on the laterals inside propertv boundaries was sealed with a sealing compound not affected by microbiological attack.

Mr McWilliam said that perishing rings could not only allow sewage to seep out, hut allow sand and silt to seep in, blocking laterals and sewers.

The rings had been used in most areas of Christchurch before the new microbe-resistant ones

were developed and, although perishing was suspected in all of them, the rate and severity of perishing could vary from place to place. Mr J. Ryan, a committee member, suggested that the board study what was available overseas in the way of an internal grouting machine.

“It is obvious that this will be a bigger problem than any amount of stormwater flooding and it will be a mammoth and expensive job to dig up every sewer and lateral which was put down with the old rings,” he said. An internal grouting machine might be available which could reseal the joints from inside, making it unnecessary to dig up the pipes, he said. Mr McWilliam agreed that if such a machine could be found it would assist the board, particularly in those parts of Christchurch where "difficult ground” increased the cost of excavation and relaying. Mr McWilliam did not elaborate on how much piping had been laid in Christchurch with the old rings other than to say that “many kilometres” of both sewer main and laterals had been laid with them.