DRAINAGE WORK

Evening Post, Volume CXXVIII, Issue 36, 11 August 1939, Page 11

DRAINAGE WORK

TUNNEL UNDER CITY

EIGHTEEN MONTHS'JOB

Work on the main city sewerage | system has been proceeding uninter- j ruptedly for over eighteen months. It j is the biggest city work for years, since the driving oJ: the traffic tunnel' through Mount Victoria, but is the least noticed by the average citizen of any work in hand. The principal feature is the construction of a second sewer, either by a tunnel of about seven by three feet in i the rough drive, or by huge reinforced concrete pijaes, from a junction with the old sewer in Drummond Street (just off Adelaide Road) through to Dixon Street (where the existing sewer ends) and extended on to the most northern point in Tinakori Road. From Drummond Street the old system will continue as it is at present, for it has ample capacity to meet requirements for some years to come. From Drummond Street north to Dixon Street the old system will be approximately paralleled by the supplementary system, and from Dixon Street to Tinakori Road the new work will make it possible to handle a great part of city waste by gravity, whereas under the old system an intricate and costly pumping system is wholly relied upon, but this pumping system will be retained in commission alter modernisation, so that the work, to cost £200,000 in all, is an augmentation and modernising of the drainage system, and not a replacement system. TUNNELS AND PIPING. The tunnel has been completed between Drummond and Vivian Streets, and two headings are approaching each other, one from Vivian Street and the other from Dixon Street south, in some lengths in circular pipe which is driven underground, without preliminary tunnelling, by pressure from powerful jacks and excavation from the inside of the pipes. The jacking of pipe sections has advantages in economy and in minimum disturbance of the road surface where the ground formation and the working depths are suitable, but cannot be applied throughout. The tunnel has been completed bej tween Dixon and Church Streets, and a heading is being pushed out towards | Everton Terrace, where a working shaft has been sunk in the bed of the gully. Spoil taken out from the tunnel will be used on the spot for the fill i across the gully to replace the old wooden bridge with a two-way roadj way; at present there is only a narrow | fill. Some difficulties have been ex- | perienced in sinking the Everton Ter- ! race shaft owing to an unusual flow of water from a wet patch of country, but these are being overcome and it lis anticipated that driving ,in two directions from this shaft will comj mence next week. | Plans have been completed now for j the whole length of the sewer, and the j next main step will be the commencement of the terminal length leading j from Molesworth Street to Tinakori Road, but a different plan of working may be followed here for more rapid and economical- working. Open excavation, will probably be resorted to. but in some short sections the jacking system may be used or pipes may be laid in a tunnel drive. It is anticipated that the tunnelling and pipe placing will be completed in another fifteen months, but there will still be a great deal of work ahead in the modernising of the pumping system for the old sewer and in attention j Ito sections which have gone past the | i limit of useful life and give constant ! trouble. i The plan of augmentation and modernisation is complex, with simultaneous advance along several , lines. The tunnelling and pipe jacking., for instance, are being prosecuted from a number of working faces, Avith tunnel finishers following up the drives and truing the fall to grades as exact as one in 1200; the replacement of old pumping and other equipment is already in hand. The main pumping station at Clyde Quay has been brought to modern standards by a change-over from the forty-year-old steam plant to electrically-driven compressors which have been functioning very satisfactorily for six months, and although the greater system is still a long way from operation the additional capacity of the new compressors at Clyde Quay has brought a considerable improvement in the pumping of sewage from the low-lying mid-city section.