FAST GOING

Evening Post, Volume CX, Issue 138, 9 December 1930, Page 10

FAST GOING

THE TRAFFIC TUNNEL

FINISH IN TWO MONTHS

ON THE LAST LAP

The driving and concreting of the traffic tunnel through the Mount Victoria ridge probably constitutes an easy New Zealand record, for though the first pick was driven last December the final concrete placing of the tunnel proper is to bo made to-night or tomorrow morning. The work is well ahead of contract time, and even of therosiest of congratulatory anticipations such as are expressed at first pick ceremonies. It has been a big job tackled in a big way, by methods not previously employed in New Zealand, but thoroughly proved for a work such as this. The contract is being carried out by Messrs. Hansford and Mills. Mr. A. P. Downer, who has been in immediate charge of the work, has had wide experience of tunnelling with the Public Works Department, one of his big jobs being the heavy work of starting the Tawa Flat railway deviation tunnels, but the system, which he recommended for the Mount Victoria job was brand new to everyone here, himself included. CENTEAL BLOCK SYSTEM. It departs from ordinary tunnelling practice in that two preliminary drives wore put through, along cither side of the projected tunnel walls for tlits full length, 2045 feet. These drives v,>jrc tackled from either end and also from a point reached by a vertical shaft put down from near the old Ellieo street quarry. Thus there were multiple working faces. Tie first drive was put through on 31st May, an occasion duly recognised by otherwise careful citizens and councillors walking up through the mud and slush of'the tunnel floor , and crawling uncomfortably through the ragged hole in the rock. The second parallel drive was completed a few months later, but was not an occasion for official rejoicings. Between the two drives was left a great block of solid rock, and along each drive was placed a.light tramway with electric haulers to handje rakes of trucks. Working upwards and inwards from either drive, the men broke out the arch of the tunnel, shooting the spoil by its own weight to the rakes of trucks on either side, leaving the block standing high to serve as a working platform for the next operations. As this arch cutting was pushed ahead the tunnel wall conercters followed behind, placing the heavy footing. Here the central rock block left by the preliminary drivers and arch cutters came in again, for a third tramway was laid along it, from either end, and over it was convoyed the concrete, mixed in machines placed high above each portal that the feed to the trucks would involve no handling. The elevation of ' the central tramway again made possible a gravity feed of the mix to the footings and lower tunnel walls. PLACING THE ARCH CONCRETE. As the lower walling went ahead thero followed again another crew whose work was to place the concrete of the arch, two feet in thickness. The arch, heavily timbered as the rock is taken out, is naturally pretty rough, but is remarkably even as tunnel arches go, thanks to the very even nature of the rock met. The timbering is left in and the finished arch sweep is given by huge steel "profiles," in place of the boxing used on the ordinary run of concrete jobs. The wet concrete has to be lifted to position in the arch, and this is done by means of a cement gun. Actually it is nothing like any gun on earth, but is rather a squirt; one with real authority, for it feeds out concrete through a sixinch nozzle at a pressure of about 80 pounds to the square inch, sufficient to pack the mixture very tightly. A quick-setting cement is used, and after twenty-four hours one set of profiles (four sots are used in all) is lowered, brought ahead of that last filled, and the cement gun gets busy again. The smooth steel surface of tlie mould and the heavy pressure put on by the gun pack the concrete down and when removed the profile leaves a surface every bit as smooth as a good hand-plastered job, except for narrow joint lines every ten feet. To-day the profiles are about twenty fcut apart, two more shifts and a few more hundred yards of gunning, and the arch is completed. The slush of the fii'st drives has disappeared, and walls, arch, and floor are dead dry. VENTILATION ARRANGEMENTS. That will not mean that the tunnel is completed. The raised footpath, about six feet wide and six feet above the floor level, has still to be put in, and in the top of the arch a flat ceiling is .being constructed to serve as an upper air duet, the space below the footpath serving as the lower duct. The first vertical air shaft, 125 feet deep, leading to the upper air duct, is completed, but the second, 230 feet, is still in hand. The council has yet to determine what provision is necessary for forced ventilation. There will still remain the central block, a great mass of solid rock, say twelve feet in section and over 2000 feet long. It will be the last to go, but Mr. Downer says that when it starts it will go quickly. It will be tackled first from the Hataitai end. Charges will loosen it up into "mullock" —a most expressive term, that —and an electrically driven shovel will scoop up the broken rock and deliver it to trucks running on the lines laid in the original side drives. In about six wocks' time, Mr. Downer considers, the block will have disappeared, all going well, and the tunnel, as far as the contractors are concerned, will be about completed. There will then remain the- approach work on the city side through the convent grounds, a general cleaning up of (the roadway on the Hataitai side, some day the tram, lines, and, of course, the meeting of the quite considerable interest bill on expenditure, for tunnel, approaches, compensation, tramways, etc., of something like £200,000.