LOTS OF FINE POINTS
MAKING ENDS MEET
v On Wednesday a general outline of the big drainage work in hand under Wellington was given in "The Post," but there was not space in that article • to more than mention details of the w.ork" described to a rerporter when he went down shafts and along drives, never less than forty feet below street Svel and in places a, hundred feet— though when you are down there it VJoks about the same either way. .except where there are particular reasons tunnels are driven from several points. The Tawa Flat and Mount. Victoria tunnels had several simultaneous working faces, from either end and either way from shafts; the Orongbrongo water tunnel had 1 ■ only two faces, one at each end; the Waiwhetti tunnel had only one, working upgrade, so that the water seepage would get away easily- (but the Waiwhetu tunnel never got through to the Wainui Valley and is a rather sad example to quote to illustrate any point, and had better be forgotten, as those who put their money into it, have been •trying to do ever since). Multiple working faces make for more rapid work, but they do pile on engiiieeririg a<id surveying difficulties, because the engineer, working with the surveyor, has to design so that each section of the tunnel will meet the.next section and the surveyor has to; steer them so that they ; do, not just right, but dead right. . ! SPECIAL fROBLEMS. . .;,The; drainage tunnel under Wellington is being driven from half a dozen shafts, with the .work going either ! way,:and,;6n top of. that, it is not a straight tunnel, but,has v turns " and, bends and angles,- generally following the street lines above, instead, of barging right ahead,Srom one end of the city ' to. [the other. -When,a.Mount Victoria or a Tawa Flat tunnel is being driven the surveyor; can get. well back and set'out his. line, through theodolites arid with plumb lines and that sort -of, thing, on mile-lqng base lines, so that the.sighting and the aiming of one tunnel drive to meet the other has something to come and go on, but when he has to work from a base line only a few feet long (the width of the shaft) and has to go so; many chains ."north and then turn east for so many more to, meet another - section' coming towards him round other cor-' ners, then he has reason for checking and headaches. . i. Ih>. new- countrj*: the surveyor can if need'be cut a line through a mile of bush 'to get a common checking point, -$iit people won't have houses cut Through for any surveyor's sighting; a'onsequeritly he must check up by aomplicated- triangulation, and then cl'a'eck 'that up by triangulation round tne'other way. It was all explained', by''the .engineers. The reporter did iiot understand it, but he did.get the i&ta'. thai the tunnel sections, a dozen or \taore of them, whether they .are fSrb7 'feet down or a hundred feet, tiav^i to' meet wall for wall and floor for.aobr. .'"'/:■/. . EXACT GRADING. Thefcjfloor for floor meeting is more importnni than the side for side, for the.'floiy through the main channel will : be .by gravity and as the highest practicable |i6int at the north end of the ciiy is not much above sea level the grade" haV> to be very fine) as little as one foot '<)ffall in 1000 feet of length— in some Tongths .even finer than this— and steps jmd drops just will not fit in. A.:gra'de;ofi one in ,1000 is imperceptible to- the ;eye,',; and there will be sections of-the .work), which will be a good deal more level ithan the/ average house floor, :b'*ut i<i is normal engineering practice and: well within fine surveying work. ,\\ , .If is not a fob which can be patched up afterwards}, for the lining of .'the tunnel section^l proceeds as the drive goes on aheadi in work just as exact as the design. and survey checking of the line and fal^ of the drive. : _ ,; There are ■ several quite. new ideas in these Wellington tunnels, and .the method of liningljis one of them. The finished- tunnel has a section of a flat-sided-ellipse (unknown to geometricians, but fairly diescriptive) six feet from top to. bottoni curves and three feet from side wall to side wall. The curved floor and Some.-roof are bb- , tamed in an ingenious , way, by cutting a three-foot otarneter spun concrete pipe in two" liengthwise, and.using one half for the floor and the other for the dome. As fair as is known this is the first time that 'ithe idea has been applied. It is rapid and the work can be laid exactly; practically no boxing is needed,l but there Vare some short sections where other* methods _may have, to be employed. ;, NEATER WORIC STILL. ! When the Cambridge : Terrace-Ade-laide Road stormwateir work—the stormwater and' sewerage; systems are quite distinct-rwas IbeingL planned an estimate was. made on me basis of standard working methods, but as the job went ahead the idea otf driving the six. and four-foot concrete gpipes underground by hydraulic jacks Ipoked more and .more attractive. It ) was tried, found' to deliver the goods in excellent "order, and was adopted as standard practice for the greater of the job. Two jacks, ■ giving a driving / force of about 200 tons, were -used, and 60 feet of pipe could be handled be- . tween shafts. As the pipe' moved ahead the core was cut out anJA shifted back by men working inside; tfoe road surface, except where the jacksf shafts were put down, was undisturbed, and excavating costs were just* about cut in half. The system will not, of .course, work in all classes of ground, but where the formation is suitable.it is economical beyond expectations* It will be employed in some sections of ..the sewerage work, with fourJoot diameter pipes. Tunnel driving to exact and fine grade is one sort of a nice piece of collaboration .between engineer, surveyor, and tunnel workers, and this jacking of pipes is another. To make this grade straight jacking and core cutting is perhaps just too fine, bot there is "a 1 way past that—to drive the pipe three or four inches below the designed grade and to finish the flooi: to exact grade by a concrete invert, lining. ' All the nice work is by no. means done above ground. There are other interesting points about this tunnelling under Wellington: foolproof hoists for spoil, a lighting system which allows a light to be 1 plugged in anywhere along a rubber flex without cutting it; automatic pump controls for handling the water seepage; different cutting tools for varying classes of formation; the air supply and ventilation system, and so on. The methods adopted are unviual and the equipment and facilities for working below and above ground level are fully up to date, and have been particularly remarked upon by the Government's inspecting officers, but the outstanding point is that after the engineers have drawn the plans for work under hills and hollows and round corners andbends, the survivors can lay out the line precisely, and the sctual construction can follow the line, dfcfd, right.
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DRIVING TUNNELS, Evening Post, Volume CXXV, Issue 59, 11 March 1938
DRIVING TUNNELS Evening Post, Volume CXXV, Issue 59, 11 March 1938
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