SOLOMON'S KNOB DAM

Dominion, Volume 2, Issue 332, 20 October 1908, Page 7

SOLOMON'S KNOB DAM

TO COST NEARLY £50,000.

MARTIN, HURRELL, AND SNADDON'S TENDER ACCEPTED.

—: — / A FERRO-CONORETE STRUCTURE. -The tender of Messrs. Martin, Hurrell, and Snaudon, of Wellington, contractors, has been accepted by tlie City Council for the construction of tho now reservoir dam at Solomon's Knob, half a mile abovo the prelent basin at Wamui-o-niata. Tho price is vbout- £43,01)0. ' The need for a better stand-by water supply for Wellington City was realised livo years ago, and at any timo during that period the matter of erecting a dam at a favourable, point above tho present reservoir has been taiked and written about. Three years ago the matter was considered an urgent one, two years ago the City Council awoke to the fact 4 and the partial drought of last summer showed how real the need was of a supplementary water supply to meet the. demands of''this growing city. To relievo the immediate strain, it was considered advisable to erect a storage reservoir pt Karori, which work was undertaken by Messrs. Mitchell and King, and was completed about the end ■of February last. This dam has filled up during tho past winter, and is now ready for any demands that a thirsty summer- may. mako upon it. At Karori, however, the stream is as faithless as the one that worried Eugene Aram. It is not- above drying .up completely in summer, whioh is not surprising, as the catchment area is small; and the land is cleared.

As soon as the hew dam at Karori was finished- the City Engineer (Mr. W. H. Morton) set to work to get out plans for the new reservoir at Wainui. It was decided to deviato from the old-style solid dam in favour of a new American style of ferroconcrete dam, which experts stato presents far less risk of accident than is the case with the solid-wall dam. Tho new style of construction consists of the face wall, 3ft. in thickness, lessening to 2ft., sloping upstream at an angle of 45 degrees, having for its support a series of vertical ferroconcrete walls erected twelve feet apart for tho entire' length of tho dam, these walls, of course, being erected at right angles to the face, and extended down stream for a: distance that, when the pressure of water is calculated mathematically, the centre pressure is in the middle third of the wall. In some cases the downstream side of the 'dam is just a row of wall-ends or buttresses, either sloping slightly or vertical. In the case of the dam at Solomon's Knob, that side of the dam mil be walled half-way up, and the earth that will have to be removed in sinking the foundations, etc., will be thrown against this wall, and so act as an extra stiffe'ner. The total length of the dam will be 540 ft., 107 ft. of which will be a by-wash situated one-third the way across (in which section the back of the iam will be walled to tho top), which is another innovation in dam construction. The height of tho sloping face is 64ft., and the'perpendicular height from the ground to the crest of the dam will be 47ft., and to the crest of the by-wash 43ft. The supporting walls are to be only a couple of feet thick, so that though they occur at 12ft. intervals the structure is hollow'compared with the old style dam. There will be a good deal of work to do in gaining good foundations, "and it" is" known .that in some places they will have to go down to the rock over 20ft. below the' surface of the ground.

With only a 3ft. deck or face wall against the water, the coarse common order of concrete is barred, and a fine c'ose mixture is specified, which has to be tamped and prodded into position, so; as not to leave the Bmallest chance for a leak. The whole of the concrete work will be knitted together with a net-work of steel bars and girders.

The maximum depth of water, when the new dam is filled will be -50ft., and when full the surface of the water will be 40ft. abore the level of the sea, and the area covered will be 26 acre 3. The capacity of the dam will be about 140 million gallons.