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This main was regarded as experimental as little information was available upon similar projects of such magnitude, and with the Shing Mun Scheme progressing, more water was available from the Mainland with the result that in 1935 a second pipe was laid. This time the main consisted of 18" internal diameter butt welded steel pipes, again " thick. In an endeavour to improve on the protection against corrosion this main was lined internally with a centrifugally spun lining of cement mortar " thick and outside with a composite wrapping of bitumastic, hessian cloth, brass tape, hessian cloth and finally sand. The laying procedure was generally similar to that for the original main, the new main being laid parallel to and 7'6′′ to the East of it and making use of the same anchor blocks.
However, this main too was not satisfactory and by 1937 the Waterworks Engineer-then Mr. W. Woodward-was reporting that "recent examinations of these pipelines reveal that the 12′′ main laid in 1929/30 has now nearly reached the limit of its useful life. Corro- sion has so affected the metal that in the event of a fracture of the pipeline it is doubtful if reinstatement could be effected. Examina- tion of the 18" main shows that corrosion of the metal is also taking place......
Investigations were thereupon commenced to find a more resistant protection to the metal of the pipe.
This time it was decided to try cement lining inside the pipe and a 21" sheathing of vibrated concrete on the outside. In addition the pipes were laid on a prepared bed as shown in Plate No. 3. The flanges of the lengths to be joined on the harbour bed were the only metal left unprotected by concrete, and these were covered by cast iron moulds which were packed with grease under pressure retained by rubber seal rings. Two 21" internal diameter pipes were laid, and finally coupled up in 1939, and to date are showing no signs of failure.
The disused 18" main was then recovered from the harbour bed. It was discovered that the sea water had penetrated between the overlapping of the brass covering and in places the steel was already corroded to a depth of over ". These spots were all cleaned out and filled in by welding, and the pipes re-used on a number of new mains on land. The 12" main was considered to be not worth recovering.
FILTRATION
With the exception of the Shouson Hill area all water on the Island and in the Kowloon Urban Area is now filtered, and it is hoped to start work on filters for Shouson Hill this winter.
The original filters were all of the slow sand type, designed to filter approximately 400 gallons per sq. yd. per 24 hours. This neces- sitates large areas of level ground, always a difficult proposition in Hong Kong. Eastern Filters for instance occupy an area of over 3.0
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