THE AUCKLAND WATERWORKS.

Manawatu Standard, Volume XLI, Issue 9476, 27 March 1911, Page 6

THE AUCKLAND WATERWORKS.

THE BUILDING OF THE DAM.

A HUGE UNDERTAKING

The building of tho dam in connection with the Auckland waterworks was one of the largest works carried out in the Dominion, and some particulars of the undertaking will bo of interest to our readers. For somo thirty years the city of Auckland has been provided with water from springs taking thoir rise in a volcanic watershod situated about three miles from tho city. Tho supply necessitated pumping. Of lato years tho population has increased considerably—there now being not far 6hort of 100,000 inhabitants in the water area, and it was founnd necessary to seek another sourco from which to obtain additional water.

In 1903 Mr Hugh Munro Wilson, of Auckland, was ontrnstcd with the work of surveying a source and with the developing and carrying out of a gravitation scheme. Ho choso a sight in the Waitakcroi ranges, which lie about 17 miles due west of the city. These ranges are densely wooded and are practically virgin country. They have an elevation in places of 1100 feet, and have several good streams which aro fed by an almost continuous rainfall for six months in tho year. After careful investigation and repeated gauging of the flow in various creeks, it was decided to tap tho Waitakrei stream at a point immediately at tho head of tho celebrated Waitakoroi Falls, which have a sheer drop of 300 feet into an enormous basin eroded out by the stream in the course of countless ages, in a cleft in tho conglomerate formation that constitutes the basis of these, ranges. To gain access to tho site, it was necessary to cut a road along the sido of ono of the spurs for half a milo to get to tho main range, through which a timnol, 7ft by 7ft, was driven half a milo in length. From the west end of this tunnel another mile of track, 10ft wide, was cut along the almost perpendicular side of the mountain ; this terminates at tho head of tho Falte, and at the site of the impounding dam. This work included four miles of ordinary bush road construction, beginning at Swanson railway station, ono and a half miles of mountain side cutting, ono tunnel half a mile long, and another tunnel 200 ft long; a total of, say, six miles. Along this a 2ft 6in gauge tramway

was laid from the railway station to the head of the Falls, The'work was done partly by tho City Council and partly by the contractor for tho concrete impounding dam. and the road was cmployed for hauling all materials, Horse

traction was used, the gradients rendering steam impracticable. It is not proposed to give details of all the various works, as they do not contain any unusual features, and we

shall confino ourselves to the construe-' tion of tho great concrete dam, which is the largest by far at tho present time in New Zealand. Briefly stated, howover, tho various works embraced in the Fcheme, including those already mentioned, were as follows:—(1) Four miles of bush road formation from Swanson railway station. (2) Ono and threequarter miles of graded track through tho Foot Hills, to take the 27in diameter locking bar pipe main from the break pressure reservoir in the open country. (3) The formation of half a mile of mountain side cutting, 10ft wide, on the

east side of the mountain. (4) The tunnel. 7ft by 7ft. through the main range, and half a milo long. (5) The formation of one mile of mountain side cutting, 10ft wide, along the west side of the mountain, including a tunnel 203 ft long, 7ft by 7ft. (6) Tho clearing of 65 acres of bush in tho impounding area. (7) The construction of a break pressure rcv-'vyoir on the Foot Hills at the east s ; dc of the range. (8) The construction o ? Ihe concrete impounding clam. Each c r t'.-.oso items, as given above, constituted separate contracts. The break pressure reservoir is of concrete with a principal roof, and has a capacity of 1,000.000 gallons. Its top water'level is at an elevation of 470,30 ft. The main tunnel offered no particular engineering difficulties. It went through stone rock; it was practically dry throughout, and no timbering was required. The approximate cost was 30s per lineal foot. The clearing of tho impounding area was romarkablo only for the enormous quantity of bush that had to bo removed. This ranged from kauri trees, 9ft in diameter, down to tho almost impenetrable undergrowth that covered every squaro foot of tho 66 acres. All this was grubbed 6in below the surface, and the whole area on completion was absolutely void of vegetation. Tho cost of this work, including the cutting-up of 90,000 6uper feet of tho timber for construction purposes, was a little over £4OOO. The site chosen for the concrete impounding dam _is admirable. It is so situated that it impounds every gallon of water discharged by no less than six different creeks or streams draining an area of about 3000 acres. In June, 1905, a 6tart was made to clear the bush from tho site in order to define the exact position of the dam. It was then found that it_ would have to be \vhere two converging spurs were cut through by tho stream, and to utilise these spurs for abutments a curved dam was decided upon, as a straight dam would have cut both spurs obliquely, A series of six trial shafts were then put down, some of which—those in tho spurs—bottomed on to a fine, hard, conglomerate reck, while three near tho stream bottomed on to a sandstone formation. This led to the conclusion that ths conglomerate overlayed the sandstone, as those that bottomed on tho latter were much deeper than tho others. All trial' shafts exposed an aroa of not less than 25 square feet.

In designing the work, Mr Munro Wilson decided to adhere to a gravity section notwithstanding the additional strength gained by tho curvature form as a guard against earthquake shock. Tho curve is to a- 382 ft. radius, and tho total length at top water Jovel is 533 ft. The lowest foundation is at level 602.41 ft, in the bed of tho creek, and it rises gradually, and practically uniformly, to 642.00 ft, at tho oast end, and 642.50 ft. at the west end. Tho top water level was originally fixed at 666.50 ft., but was afterwards raised to 668.50 ft, The foot of the parapet is 671.50 ft., and the latter is torraced at tho ends to 673.50 ft., thus giving a total height of 71.09 ft. from lowest, foundation level to the top of the parapet. The up-stream face batters gin. to tho foot, and tho downstream face Bin to tho foot up to the 649.50 ft. level, from which point the up-stream face graduates to a vertical at the 668.50 ft level, and tho do\vn-6tream side curves at a tangent and gives a crest top width of 3.50 ft. The parapets are 3ft high and 2ft thick, and an overflow weir, 100 ft long, has been left in the centre. I

The water is drawn oft' by a vertical 20in cast iron pipo column standing in a somt-circular chamber on tho up-stream Ride; within this chamber are assembled all the valves, which draw the water at three different levels. Tho bottom drawoff is at 618.87 ft, or 49.63 ft below the top water level. The intermediate valve is at 634.42 ft and the. top valve at 653.25 ft. In all cases (ho water passes through a \m mesh ntraincr. These strainers rest . on a bell-month and are free. They can be drawn up by a winch for cleaning purposes, tho arrangement of gear being such that they 'will descend into their places accurately. Tho approximate content of tho storage reservoir is 290,000,000 gallons, and the submerged area is approximately 50 acres. It was intended originally to build the work by day labour, but it was afterwards decided to lot it by contract, and a tender at a littlo under £30,000 was accepted, and tho contractor started work in August, 1906. The construction has, wc understand, proved perfectly satisfactory. The dam is stated to bo absolutely watertight, there- not king a sign of moisture on tho down-stream face, nor over the inside of tho valve chamber, the wall of which, though only an average thickness of 3ft 9in, being absolutely dry with 40ft of water against it at the present time. ~ The approximate quantities of materials used in the dam are w under:—ls,-

500 cubic yards total, 14,105 cubic yards mixed concrete, 1397 cubic yards stono plums, 5758 cubic yards sand, 30,300 bags cement 9169 cubic yards of l£in metal, 4992 cubic yards of fin metal. Tho whole of tho sand used wa6 brought from tho Kaipara ocean beaches, and although tho whole of it had to be screened to take out course foreign matter, it was otherwise clean and of good quality. Tho cement used was exclusively Wilson's Two Star Portland, of local manufacture, and it _ proved itself admirably _ adapted for this class of work, it having comparatively slow initial setting properties which suited the conditions of work better than a quick-setting cement would have done.

Tho total outlay on tho scheme, including engineering, cost of land, etc., was £150,000. The whole of the works were carried out under tho supervision of Mr Arnold Wilkins, inspector in charge, wlioso experience in water-works construction, particularly relating to largo concrete dams, was of a very extended nature, ho having built five such dams while in the New South Wales Public Works Department.

The above information is condensed from a diseription of tho works which was published in tho "Engineer" (an English publication) of December 30 last.