MANGAHAO NEARS COMPLETION.

Shannon News, 26 August 1924, Page 3

MANGAHAO NEARS COMPLETION.

j PRESENT POSITION OF A GREAT ENTERPRISE.. POWER NEXT MONTH. Public interest in the great work, that has been developing in Uie iffijjs behind Shannon for the past tlnee ye;ir s is becoming intensified is this vast enterprise approaches me pumt of fulfilment. Sceptics have scoffed at the idea of a State enterprise woiking out to schedule, but nevertheless the Engineer-iiniGharge—Mr Dinnie, was able last week to state that, witu a fair run of weather a certain amount of power would be' available next month.

A “News” representative visited Mangahao on Wednesday and through tiie courtesy of Mr G. P. Anderson, Engineer-im-Charge at Arupeti, and .Mr T. A. Johnston, who is m charge at the Mangahao Dam, had every opportunity oi gaining a Tull knowledge of the progress of the work}, and learnt many interesting details pertinent to it. MANGAHAO’S SUDDEN FLOODS. As is generally understood, the Mangaliao liver is now fiowing through tiie by-pass, constructed m divert the water mom the bed of the stream across wbicn trie dam is being built. This by-paiss lias a capacity or 1200 a compound word vvnicii means tiie x nuinoer oi cubic ieet per second. Tne normal flow in tne river is 30 cusecs, or one-fortieth qi this total, me ' ny-pass then w.ouiu appeal; to be ample lor afi needs, uuu yet .such is the rise of tiie river m sudden fiood that m half-un-hour, as was. explained by Mr Johnston, tiie how. has risen from the normal 30 to the nuge total ol 18,000 cusecs, sweeping .through the by-pass in .a roaring rtqod, swelling over the coffer*dams and then pouring in a soiid mass over tiie work under construction, and sweeping away tiie framing used- in tiie concrete work and tne green concrete- work itself.

These sudden floods havev. determined the policy of construction which has been adopted i’n tiie Mangahao dam. To construct a by-pass capable oi taking noiod waters would have been possible, although immensely costly, but to confine the water -to tins ciianiiel during flood would have necessitated, tiie provision ol “cofferdams, ”■ as tiie temperary dams whicn protect the works under construction the called, of a strength almost as great as uie finished -Mangaliao dam will be.

The only alternative, then, was to submit to a degree to the lorces oi nature. The dam was commenced from both sides, leaving a gap of 40it. in the middle, through which floodwaters might escape. This was further necessitated by the fact that the door which will eventually close tho by-pass tunnel did not arrive for some months, after the. work was finished and it was necessary to divert the stream hack into its original bed to enabl.e this door to he fitted. POSITION LASr WEEK.

The work as it stood last week appeared Uhu>: ui the -394 f- which comprises the total length ol the dam, an out 4t)it in Uie middle have been finished, this inciuuing • 208 it of what is known as “core-wail” which is driven into the side of the hill, where behind a facing of rock" oju tile Eastern bank of tne river, an old riverbed filled wit’ll houldefs land. isanu Was encountered and which wouiu nave allowed percolation to such an extent Uiat Uiis core wail was necessary., The total amount oi concrete which will be used in the dam is 22,090 yards, .and of this 18,000 yarns have already been placed in position, leaving some 4000 yards to complete tne work. Of tills amount 7000 yards was used for the cor-e-waii, which lias a depth up 103 ft, Uiis. carrying it sxt below, tiie boulder-pack into the solid rock below. It was also found necessary to face- this boulder puck on Uie lace next Uie dam with a. retaining wall of concrete to obviate, any possibility of the action of me' water causing a. subsidence into the dam, fhis lacing-wall joining up with the core wall and the main portion oi the daaii. The cone-wall itself is 26ft wide at the base .rising to 4it at tiie top, and is in reality a, wedge of solid concrete driven into the side of the hill to strengthen the rotten formation. As has been stated, the main dam, with the' exception of tiie 40ft gap has now been completed, being built out from Uie Eastern side for a distance of some 60it and from the western side for 80it, and presenting two solid wails of concrete facing each other on each side of the old stream bed and rising to. a height of 10811, above the level, mere being a further 22ft beneath the normal water level, where the concrete base,.9oft wide is locked five feet into the soiid rock. As it rises the wall of the dam decreases in width with the lessening of tiie pressure whiefi it will have to hear, until at the top it is 26ft. FILLING IN. THE GAP. Work is now proceeding rapidly with ttie filling-in of Uie gap in the wail. Already the concrete lias been raised to a height of 25 feet above Uie bed of the stream and every three work days will raise it a further seven ieet. By Uie end of this week it is hoped to have the dam raised sufficiently high to allow of the water being turned in by a stopping of the by-pass, and raised to such a height as will send the flow through No. J tunnef to the Arapeti dam, from which No. 2 tunnel will convey a head of water sufficient to run -one turbine. GETTING THE WATER PRESSURE: As will be easily understood it is the pressure of water which creates tiie power, and with a system such as is in operation at Mangahao it is the weight o.f water above the tunnel mouth which, forcing the Water through the tunnel in a solid head will develop the full horse-power of

which Uie scheme is capable. Until tne uam is raised to ns iufi lieigin tins necessary weignt oi water win not be avaiiaole, but m tne meantime it is hoped that by the unddfe ol September, or earlier, sufficient power wiif nave been developed to supply tne needs qii ffie Hjorowlienua Power district, which will* be the first area to receive the benefit of Uie. scheme.

WORKING THE CLOCK ROUND. Provided tliat an exceptionally laige hood does not do some unu.vpected damage, it is hoped to complete tlie dam and have the 6 cheme in lull operation by the middle of February next. Work is proceeding night and day in eight hour shifts, and from the suspension bridge which spans the river 150 ft above the level ol tlie stream, and a similar dis. tance below the aam the scene of operations presents a spectacle of rather bewildering bustier which on examination resolves itself into a well-organised whole. Straight in front at about the level of the bridge towers the completed portion of the wall, rising straight and white from the stream-bed below. In the 40ft gap In the centre a score or more of men, looking like ants in the depth and in their bustling activity, toil in the spreading of the concrete, which is fed down to them in a continuous stream from two chutes. THE CONCRETE ELEVATOR. Rising 130 ft above the level of the dain on the eastward side is a huge wooden and steel tower from the base of which every 9Q secs, a steel bucket rises containing 18 cubic feet of concrete from a mixer at the base. From the top of the tower a steel cable stretches across the river to the further bank, and slung from this a system (Oif cliutes capable of being directed to any portion of the work, conveys the mixed concrete to the scene of operations. Situated at the ievei of the top of the dam, a further mixer of 28 cubic feet capacity, sends a steady stream oi concrete ’by gravitation,, down another chute to the men below, and set beside it an 80ft derrick 'every minute swings out from the bank and lowers, into the gap, a huge bucket containing boulders, each weighing, a cwt. or more, and which are known technically as “plums,” these being thrown into ihe concrete bed at intervals. Leading to these mixer, s and hoist a system of tramways convey an unending stream of trucks loaded with cement, gra.vel, and boulders, to tlie top of the dam, the whole so systematised that each part dovetails perfectly into the other, and presenting an undoubted tribute to the organisa* tion of the Engineer-in-Gharge.'

DAM-BUILDING IN SECTIONS.*

Tlie. system adopted in placing: tide concrete is to commence a seven i'eei high section the whole length of the gap in the dam from, side to side anu one-third of the total width. The boxing is first placed in position ana work commenced in the filling-of.this section, which is completed in one day, the down-stream section being first built. On. tire following day the next section on that level is done and on the third day the lift of 7ft is completed, this. necessitating a total of 500 cubic yards of concrete. The procedure adopted is the result of the • conditions obtaining: Owing to -the sudden floods which in an hour may send the water pouring over tire work which, has been done, it' is necessary to work ,sQ as to save), in as far as possible, the green, or unset concrete being carried away by, such a flood. Were the days work 01 180 yards or-so spread evenly over the surface, a flood would carry,, away the whole of it. Further tne greatest pressure the water ‘is exerted on the inner side qf the dam where the water strikes the dam fust. Where it flows over providing there is a smooth-face on the lower side it drops straight away from the top of tne wall with nothing to impede its flow, .with the, result that it doe s not du any great damage, Tlie position then I is' that should a flood strike tlie wall at a time when the lower section i only was completed, and provided a j did not carry it away bodily, little ( damage would be done. Were this, ( section raised on the top side, how- \ ever, the force of’Water pouring over it, and on to the still partly green concrete Of the other sections below would inevitably carry these away. The justification oi the policy adopted is shown in the fact that to date, although a number of floods have swept over the gap, only 30yds of concrete has been carried away. 1 THE SPILL WATS. - With the •oinpletion oi the daiii proper, the next operation will be tne erection qf tlie spiil-wa.ys or gates constructed of wood in a steel frame- J work, which will automatically regulate the height of the water in tne dam. Of these there will be two in the width ol the dam, each 461 t in length,, and standing 61t above the level of the concrete dam, and supported at .each end on piers. The principle of these gates is roughly us follows. Tlie gateg are swing ’ at about the central line, go that they are sunk some six feet into a channel cut in the concrete bed, the water being kept out of this by skilfully constructed trapdoors which fit up i against the gate. The gates, pivoted ; a,t the ends, are counterweighted so , as to take a pressure equivalent to that of the level of water which it is . desired to maintain within tile dam. , Once this is exceeded the pressure , begins to press the gate outwards, the action being accelerated by a syphon i arrangement which admits water, through a valve into a tunnel at tlie i back of the counter-weighted lower end of the gate. 1 Once the water has been reduced > to normal, again by spilling over the top the gate automatically Swings back, the water in the tunnel escaping through another set of valves operated by the closing of the gates. CLEARING THE GROUND. Evidences are numerous that ihe

huge work commenced' in August, is nearing completion. Already the demolition, of auxiliary"' works has commenced. The extension tramways which lead to the’ quarry and gravel pits from whence the stone and sand for the dam were procured, have already been removed, and the bridge which spanned the stream to carry the tramline from this spot, to the crusher is in course of removal Nothing is being wasted. Bolts and ironwork are being taken back into store and the bridge timbers are being sent out to be uised on other Public Works jobs. This apples to the whole of the material—a huge, 'total—which has gone to make up the equipment both mechanical and incidental at the camp. The greater part of it will be either rafted or taken by tram-line-up the mile and a half dam to the site of the “Tap Dam” which, on the completion of Mangahao, will be commenced to provide an additional 106 million cubic feet of water as a provision against an exceptional drought, and which is expected to take a further four or five years to complete.