THE SEWERAGE WORKS.

New Zealand Herald, Volume LXII, Issue 19177, 17 November 1925, Page 12

THE SEWERAGE WORKS.

OPERAT SONS AT ORAKEi. TAMAKI DISTRICT SERVICE. NEW TUNNEL INSPECTED. A visit of inspection to the main ontfal! works at Orakei waa made yesterday morning by the chairman and members of the Auckland and Suburban Drainage Board, principally to see the progress being made with construction of the new branch sewer under the full aft Orakei, designed to serve the seaward portion of the Taraaki Road District. The party made the trip t#' Orakei Wharf by the Harbour Board's launch Ferro, and waa there met by Mr. E. T. Walker, engineer in charge of the Orakei outfall plant. Several members walked through the tunnel recently driven under the Orakei hill and found that for most of its length of 2894 ft. the country was of firm papa rock requiring no timbering to hold the roof up. Recently a large portion of the Tamaki district waa brought into the drainage area and the Land on the seaward side which can be dealt with by gravitation is to be served by the sewer running under the hill. The contract for the work was let last March to Messrs. W. Stevenson and Sons, the price being £3589. This included the construction of the egg-shaped sewer within the tunnel, yet to be carried out.

The tunnel where it passes under the crown of the hill is at a depth of 143 ft. below the surface As the average depth is 80:1., it was not considered practicable to adopt the usual method and sink shafts from the surface so that the driving could be_ attacked from several points. Two gangs were employed, one working from either end, and three weeks ago, after seven months of driving, the hill was pierced. The work of concreting the sewer is to be commenced almost immediately,. and the contractors have made large numbers of blocks required for the undertaking. The invert blocks which'form the base of the egg will be set on a 4m. foundation of concrete and at the level determined for the proper fall of the sewer. Ready for Use Next March, Moulds will then be set up to form the flanging sides of the sewer to a height of 2ft. 4:n., and a concrete mixture will be poured in. This wilt carry the formation to the point where the spring of the top arch commences. The arch will be made up of shaped blocks, prepared in quantities outside the tunnel, and the joints grouted with cement mortar. The completed sewer will have an inside height of 3ft. 6in., and will measure 2ft. 4in. at it* greatest width, 14in. from the top. The work of concreting will be started from the middle, gangs working in both directions toward the ends. It is expected the sewer will be in readiness for use about nest March. In addition to the gravitation drainage from the Tamaki district, the sewer will carry storm water and sewage from the low-lying areas on the beach frontages of St. Heifers and Kohimarama, but this will be pumped by the Road Board into its own sewers, and thus delivered to the new sewer. The new branch sewer .ha* been designed to serve a population of approximately 42.000, although the present population of the Tamaki area in the drainage district Is only about 2100. The new sewer will have a capacity of slightly over 7000 gallons a minute.

The members of the board were shown other improvements and alterations being carried out at the outfall plant. A new chimney has jnst been erected, a new furnace is in course of construction, and a rearrangement of the screening plant is being carried oat. Several members have just taken their seat?, on the board and were consequently interested in the extensive system under their control Margin for Expansion. The engineer explained that the hage main sewer which carries drainage and sewage from as far west as Oakley Greek and runs from the city across Hobson Bay, under Hobson Point and across Okahu Bay to the outfall, is also built on the egg-shaped principle, and has an inside height of Bft. 6in. This sewer has been designed to serve a population of 230,000 and as the board's area has an estimated population of 140,000 it will be seen that there is ample margin for future extension of the system of branch sewers when the city's growth makes this necessary. Between the serening plant, housed in a substantial yellow brick building, and the outfall off Orakei Wharf are placed the storage tanks, which have a holding capacity of 7,750.000 gallons. The construction of the tanks was made necessary by the fact that the board is not permitted to discharge into the harbour at times when the tide is setting cityward. The Act of Parliament governing the operations stipulates that no discharge into the harbour shall take place earlier than two and a-half hours before high water, or later than four hours afterhigh water, but with the use of the storage tanks the board endeavours as far as possible to restrict the discharge to night tides, this again showing the farj sighted margin allowed in the original design of the system for the growth of the city. The outfall, which is 25ft. .beiow the surface some distance off shore, was discharging yesterday arid the launch was taken over" it. Beyond a of the sea water there was nothing to indicate that the citv sewage was passing seaward at this point. The engineer said that tests showed that the outflow was carried off bv the tide in the direction of the Motuihi channel, but before reaching that passage was swept by other currents into the main Rangitoto channel and so into the open sea. Provision for Future.

A point-about the storage tanks that appealed to new rncmbers_ was that the top casing; has been 'so laid that it will form part of the waterfront highway the development and population of the lands to the eastward justify the construction of the project ed esplanade. Iho concrete roof of the tanks is bui.t at road level and is wide enough to take a broad carriageway with a wide footway for pedestrians" on the water side, while on the shore side of the carriageway the concrete has been laid to a sufficient strength and thickness to carry electric tramcais. Further provision for the future was made when the main sewer was being thrown across Hobson Bay Ihe sewer is supported on concrete piles, allowing the tide to flow beneath, but it was recognised that it might be deemed expedient some day to reclaim the shallow hay, and a sum of £6500 was placed on fixed deposit to cover the cost of closing the gaps between the piles and thus making a strong retaining, wall. During the visit the members of the ItS.ird were entertained at morning tea by Mrs. Walker at her residence in Oka.hu Bay, Before leaving Mr. G. Baildon, chairman of the board, thanked Mrs. ; Walker for her hospitality.- i