Timber Piling In Cass Bay Impractical

Press, Volume CVIII, Issue 31830, 7 November 1968, Page 9

Timber Piling In Cass Bay Impractical

A scheme for timber piling as protection from waves for a yacht harbour at Cass Bay was not new and did not offer an acceptable solution to the prob- z lem of providing a cheap form of breakwater in Lyttelton, the chief engineer (Mr A. J. Charman) reported to the Lyttelton Harbour Board yesterday.

Mr Charman was commenting on a scheme proposed by Mr A. W. Cordell as an alternative to the board's suggestion of moving moorings from the inner harbour to Purau and/or Governor’s Bay. The scheme had been investigated and reported on by former chief engineers on a number of occasions, Mr Charman said. The earliest report found was in 1928 by Mr P. W. Fryer and followed the driving of piles to test the practicability of such a scheme in either Magazine Bay or Cass Bay. He reported that a timber pile 68ft long and not pointed sank under its “own weight” to a point 37ft below water level, and that by resting the 2}-ton pile monkey on it this distance was increased to 52ft No firmer material was found by “driving” the pile beyond this and the pile was lifted out without effort.

Mr Fryer had commented: “These results are a confirmation of the fact that the nature of the bottom is quite unsuitable for a structure of this kind.”

On a reference by Mr Cordell to the wave break at Okahu Bay, Auckland, Mr Charman said that in 1962 when the possibilities of a piled breakwater were again being examined the then chief engineer of the Auckland Harbour Board had said the protection afforded was “largely psychological.” This opinion was well confirmed by technical papers and studies on the subject of closely-spaced piles used at breakwaters, Mr Charman continued. He quoted from a paper in the “Dock and Harbour Authority Journal” of September, 1961: “The use of a row of elosely-spaced piles appears to be illusionary as a relatively cheap breakwater.” An lift depth at mean low water springs quoted by Mr Cordell did not correspond with the most recent charted soundings of the area in which he assumed the yachts were proposed to be moored, Mr Charman said. The average existing depth over the area was from 4ft to 6ft at low water.

The pile costs given by Mr Cordell were about onequarter of what experience bad shown the actual driven cost would be for the type and length of pile required for the foundation and marine conditions' in the harbour.

Numerous investigations, research and reports on arti-ficially-protected yacht harbours in Lyttelton had been made over the last 40 years. A large number of people, many of them highly-qualified technically and others well versed in the aspect of harbour engineering had been involved.

“A low-cost solution has not been found, and there is no doubt in my mind that this is because a low-cost solution to the problems pertaining at Lyttelton does not exist," Mr Charman concluded.