RELIEF SEWER TO RICCARTON

Press, Volume LXXXVII, Issue 26441, 7 June 1951, Page 3

RELIEF SEWER TO RICCARTON

Drainage Board’s Big Job STEADY PROGRESS MADE Steady progress is being made with the first stage of the biggest drainlaying job undertaken by the Christchurch Drainage Board, the laying o a 43-inch relief sewer along St Asaph street almost to Colombo street. The work, which when completed to Deans avenue will give relief to over-loaded sewers to the north and west of Christchurch, it not only the biggest, but one of the most intricate jobs the board has hac to do. The work is being done by a private contracting firm, under the supervision of the board’s resident engineer, Mr N. L. Samson. Mr Samson said yesterday that the main problem was to keep the trench dry while the work was proceeding At present the main operations are at the corner of St. Asaph and Manchester streets, and the depth of the trench has reached its maximum, 17 feet. Water near the trench is being drawn off by vacuum pump at a rate of 12,000 to 15,000 gallons an hour for 24 hours a day, seven days a week. This pump, which was used when the centennial baths were being constructed, is capable of discharging 58,000 gallons an hour. Before such pumps were available, Mr Samson said, tongue and grooved timber was used, with a battery of pumps to remove seepage. The equipment being used consists of the pump, a header pipe which is put in position horizontally near the trench, and wellpoints at intervals of five feet along the header pipe. The wellpoints, 20 feet long, are thin tubes which are driven into the ground to their full length and draw water from the ground near the trench through extremely fine filters. The water is drawn up by the pump to the surface, a head of 23 feet. After the ground has been broken by ordinary post-hole diggers, the wellpoints are inserted. Water is jet-pumped into the ground, forcing the soil away, and the wellpoint’s 20 feet can be put in in less than a minute. Should the pump break down, there is a standby plant which can be put into action in 20 minutes. The greatest care was taken to check frequently the level of the pipes, said Mr Samson. The rise in the section done so far, about 43 chains, had been only one foot in 820 feet, which was equal to about one-eighth of an inch difference in the level of each of the eight-foot pipes. A Constant Problem One of the problems, and an everchanging one, was clearing existing services—M.E.D. cables, Post and Telegraph Department cable ducts, water mams, existing sewers, stormwater pipes, and gas pipes. This problem was particularly tricky at intersections. Some of the equipment put in in the early days of the city was not fully recorded, which did not make the job any easier. At the Madras street intersection, there was a clearance of only an inch and a half between the underside of an existing stormwater pipe and the top of the new relief sewer. A great deal of timbering was needed to support the existing services as they were met and uncovered. An average of four or five of the huge pipes, each of which weighs a ton and three-quarters, was laid each day, said Mr Samson. In the earlier stages of the job, the ground was usually very puggy, but now the work was proceeding in shingle, the best conditions possible. The 28 men on the job were supported by a huge modern trench digger, a grab, and other equipment. Every pipe and joint was given a most minute inspection. Rubber ring joints were used. . Below the ground, they did not deteriorate through changes in temperature and would last a lifetime. As soon as the day’s string of pipes was in position, on concrete blocks at the bottom of the trench, they wefe set in concrete to about onethird of their diameter.

Mr Samson said the public had taken a particularly keen interest in the work. He paid a tribute to the cooperation the workmen had received from merchants along St. Asaph street, and from the traffic department of the City Council.