ENGINEERING FEAT

Rotorua Morning Post, Volume 1, Issue 52, 23 October 1931, Page 5

ENGINEERING FEAT

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WORK IN DIFFICULTIES CONSTRUCTION OF BOROUGH SEWERAGE PLANT IN KING ST. WATER AND BURIED TREES. One of the most difficult feats of drainage engineering undertaken in tbis couritry is now nedring conipietion in Rotorua, where after overcoming the disabilities of working in groundhoneycorabed -with ;undergrotmd streams and of.excavating among the remains of a buried totara forest,. the contractors have now . reached the stage where they can look forward to the eompletioh of the constructiori of the new borongh..sewerage plant in King Street, r. .. : \.r : Not .only the- plant itself, but the layihg of .the mains over the extensiye suburhan area - whieh it is designed to serve, have .presented pro- . hlems of nnusnal difificulty.- : In several cases while the men have . been -working on the ■ hiains, . water, has broken through from the subterranean streams, usnally by toedium of hollbw buried tree trunks which have become . transformed into natdral fountains, to disturb the - opdrations of the worknien. : The exeavating of ; the site f or -the sewerage tank, however,- has been one succession of difficulties^ whieh have only been overcome by pertinacity and resouree to ususual ■ ;expedients. Wben a hnge totara trunk, jincovered at a depth of 14 feet,. suddenly developed into a spouting fonntain gush- : ing hundreds of gallons of water into ; the cavity, a. diver . had to be .sent down to saw through the, trunk under water and to excavate 200 cubic yards : of spoil. Then it was fouiid -imposr sible to hold the watef ih check while ' the concreting of.the tank;was carried ; out, and onee again a diver had to be nsed to lay a concfete fldor qapable of withstanding an upward water pressure of 14001bs to the square foot. Working fdr six to Uine hours a day under 23 feet of water over an almost unbroken period of foUr weeks, one diver carried ont a great part of this most difficult work, nntil the combined action of the water and the concrete broke the skin of his wrists and made it necessary for him to be relieved. , Tribute to Contractors. A high .tribute to the resouree of the, contractors in meeting the unusual conditions, is paid by the borough drainage engineer, Mr. J. F. Lenihan, wbo yesterday oiitlined the progress of.the work to a, "Post" representative. " Trouble was encountered almdst at the beginning, ; Mr. Lenihan stated, when after 14 feet of exeavation had been carried ont oh one site, a shattered pile allowed water to pour.intp the cavity. Coincident with this, an alarming development occurred, when the ground at the rear of a neighhouring house commenced to Subside and the stability of an onthouse was threatened, Work was quickly stopped and after the services of a water diviner had been obtained to determirie the course bf the itnderground streams responsible for the trouble, it was decided to commence excavations on a new site. Parallel Streams Two parallel subterranean streams were found to be running quite near the surface, and the new site for the tank was placed between the two. Work proceeded uneventfully again to a depth of 14 feet, where the top of a large buried totara tree was uncovered. From tbis point, the water trouble recommenced and it was discovered tbat the hollow trunk of the tree was acting as a channel from the streams beneath. Apparently the roots, nfany feet underground, hacl rotted, leaving radiating channels, which carried the water from either side. Large Totara Tree. Finally it was decided to allow the • water to fill the cavity and thns regain its level. A diver was then employed and went down to commence the work of sawing through the log. Automatic "grabs" were installed and directing these under water, the diver proceeded with the work of exeavation. The hase of the tree was discovered to be 36 feet below ground level, and where it was sawed through it was 17 feet in circumference. Using a cross-cut saw under water, it toolc the diver the hest part of four days to cut through the log. The trhhk, when raised to the surface, was discOvered to be in a remarkable state of preservation, with the exception of the core, which was completely rotted away. The top of the : tree Was lying at approximately the same level as the lake-hed, and it had apparently been buried for many scores of years. Concreting Bottom. With the tree removed, the diver proceeded to carry the exeavation to the necessary depth of , 23,. feet.. All the spoil was raised from beneath the water level and as sdon as this was completed, a statt was made on the concreting. This had to be most carefully carried ont, as with the water removed from thfe exeavation, the. floor of the tank wonld be subjected to tremendous pressnre. Mathematically coihputed, this was asceftained to be 1400lhs to the square foot, or 300 tons over the whole area — equal to the weight of thirty ten-ton steam rollers. To meet this pressnre, concrete two to three feet in thickness Was laid, heavily reinf orced, _ and the reinforcements were tied into the heavy 36 fbet piles which were driven right f bund the site. _ In order to ensure the setting of the concrete at a depth of 20 feet under water, care hadi to be taken to maiiitain the ,water in the cavity at an even lhvhl aiid undisturbed, so that ahy Wash which Would loosen the setting would be avoided. Diver Incapacitated. Bad luck, howeverj seemed.to dog the joh and just when the laying of the bottom was half completed, the diver had to be relieved. The action of the concrete ihnd .the water, together with the chafing of the wristbands of his diving dress, hroke the pldn on the arms, and Made it impossible for the maii, to .continue. One of the contractor's employees was (then put on the work, and althoiigii ihe had very- little experience of concreting under ! such unusual conditions^ he made good progress and had almost completed the. laying of the floor when he, also, became incapacitated from the same cause as ,

his predecessor. Fortunately, h'owever, by. this time the flrst hian had rec'overed, and was- able to complete the remainder of the concreting in a little over a day. ' j • - Critical Stage. . . After the concrete had Set, came the critical stage when the water had to be pumped out and the bottom siibjected fco the full pressufe of the underground streams. The' water was taken out ih gr,adual stages by two high capacity pumps, and when the bottom, was finally uncovered, the concreting was found to be. everi better than had been aiiticipated. Leaks had been expected, but only fwo developed, due prohably to silt pockets in the concrete. Through these a steady stream Of water is flowing into the cavity, hnt this has been easily kept under control by the pumps, whiie the workmen have pfoceeded with the concreting of the side' walls. As soon as these *are conipleted, the. tank will he allowed to refill with Water and the diver will bd sent ; down again to 'fil! in th'e leaks. Well Forward. The work is how well forward and it, is . expected that the concreting of the side walls will shortly be 'completed. These walls will he .two feet thick and heavily reinf orced lo a height of 18 feek Althotlgh the major difficulties have now been successfully overcome, it is expected that further difficulties will he. encountered in laying the feeder main. eonnecting the tank with the • reticulatiiig system. This main will pass through further difficult couhtry, but it is expected that the work will be carried out with out any serious delays. In carrying out the exeavation, the workmen passed through about 14 feet of clay, but at that depth, - which is approximately the level of the lakebed, a hand of Ioose sand about a foot deep was encburitered. Below this again was mofe clay to a depth of approximately 30 feet, where a mixture of pumice and silt intervetled. At the present rate of progress, and provided no further serious setbacks are encountered, it is expfected that the Whole - of the work' will he completed and linked up with the retieulation within twb ihenths' time: