CO129-459 - Individuals - 1919 — Page 265

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JAFFE ON HONG KONG WATERWORKS,

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(3) The provision of two further sets of pumping-machinery, each capable of raising 3 million gallons per day of 24 hours.

(4) The laying of two 18-inch diameter supply mains from the draw-off tower of the Taitam Tuk dam to the pumping-station and of two additional 18-inch diameter pumping-mains (making, with the existing main, which is of the same size, three in all) from the pumping- station to the gauge-basin.

TAITAM TUK DAM (Figs. 2-6, Plate 1).

The dam is constructed across the Taitam stream where it discharges on to the tidal flat at the head of Taitam Bay, and is therefore at sea-level (Fig. 2). Its length, excluding the tongues which extend into the hillside at each end, is 1,195 feet, formed in plan of two straight lengths of 728 feet and 310 feet respectively, inclined to one another at an angle of 144° 23′ and joined by a length of 157 feet curved to a radius of 253 feet (Fig. 3). The height from stream-bed to overflow crest is 117 feet and to the roadway 127 feet. As the foundations of the dam over the key trench in the stream-bed extend to a depth of 44 feet, the total height of the dam from the lowest part of its foundation to road-level is 171 feet. The stream-bed at the site of the dam is at a level 3 feet above L.W.O.S.T. The width of the dam at the lowest part of the foundation is 115 feet. The profile is a 1-in-15 batter for the up-stream or reservoir face, except the top 35 feet, which is vertical, and a curve of 400 feet radius for the down-stream face. The roadway has a clear width between parapets of 16 feet 6 inches, the masonry immediately below road-level being corbelled out on both faces of the dam to obtain this width. An overflow crest curved to a radius of 10 feet is formed for a length of 240 feet of the dam where it crosses the stream-bed, and the base of the dam over this section has been given an outward curve of 20 feet radius in order to better deflect the overflow into a water-cushion. The roadway over the overflow crest is carried on twelve arches of 20 feet span.

The valve-tower, which forms part of the dam, projects from the up-stream face and is divided into two dry valve-wells, each measuring 12 feet by 8 feet. The valve-wells connect at the base of the dam with two culverts 12 feet by 10 feet which were originally provided for carrying off storm-water whilst the foun- dation-work was in progress. A valve-house surmounts the tower.

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LOW-LEVEL RESERVOIRS AND WORKS FOR PUMPING.

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FOUNDATIONS OF THE DAM (Fig. 4, Plate 1).

Trial prickings made by driving 3-inch steel pipes into the stream- bed at numerous places indicated that rock would be met at depths varying from 15 to 23 feet below L.W.O.S.T., but these prickings did not establish the depths to which the excavation would have to be carried, because they gave little indication of the soundness of the rock. Trial-pits excavated on the centre line of the dam on either side of the stream-bed for its entire length, at intervals of 100 feet, indicated that soft rock would be met at depths ranging from 24 to 40 feet below the surface.

The Taitam stream above the site of the dam was diverted and the sea was excluded from the site by coffer-dams. A fold in the hillside about midway between the southern end of the dam and the stream-bed formed a natural line for a diversion cut, and it was also chosen as the site for the valve-tower. The culverts through the dam, where it crossed the fold, provided for the escape of the waters of the stream and served subsequently for housing the supply mains.

The up-stream coffer-dam, 245 feet in length, for diverting the stream into the cut consisted of an earth embankment 15 feet in height above the stream-bed, 10 feet top width, and 2-to-1 inner and outer slopes. The water slope was pitched with dry-rubble supported by a pell-mell rubble toe. A single line of close-driven grooved and tongued sheet-piles was driven down to rock on the centre line of the coffer-dam for 203 feet of its length. For the remaining length, where the depth to rock was not great, a trench was excavated and filled with clay puddle.

The down-stream coffer-dam, 174 feet in length, for excluding the sea from the excavation, consisted of a pell-mell rubble embankment 6 feet in height above stream-bed level, 18 feet top width, and 2-to-1 inner and outer slopes. The top and outer slopes were pitched with coursed rubble, part of which was set in cement mortar. A single line of grooved and tongued sheet-piling close- driven down to rock for the entire length of the dam, and a layer of selected filling material rammed against the projecting portions of the piles was all the provision made to secure watertightness. The guide-piles were pitched 10 feet apart and the lower waling was placed just above stream-bed level, the upper waling being 7 feet higher. Each sheet-pile was bolted to both walings after driving. The guide-piles were of squared Oregon pine 12 inches by 12 inches in cross section, and the walings 12 inches by 6 inches in cross

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