CO129-459 - Individuals - 1919 — Page 266

10

JAFFÉ ON HONG KONG WATERWORKS,

[Selected

section. The sheet-piles were of sawn Oregon pine 12 inches by 6 inches, and were V-grooved for a 2-inch-square hardwood tongue secured in place by nails. The piles for both coffer-dams ranged from 20 to 41 feet in length, 9 feet being left projecting above the stream-bed. All the piles were driven to refusal, and only three failed to reach the rock.

Before the coffer-dams were completed the stream-diversion cut was finished and two culverts for the passage of the water through the dam were built, each 12 feet in width by 10 feet in height, and lined throughout with ashlar masonry. The invert was 5 feet above stream-bed level, and, below each culvert, a 21-inch diameter cast- iron scour-pipe for draining the reservoir was laid at stream-bed level.

The coffer-dams were completed and the stream was diverted into the cut in March, 1914. The reservoir scour-pipes proved of suffi- cient size to take the normal flow of the stream, and the culverts only came into operation during flood times.

The excavation for the foundations of the dam in the stream-bed was, with the exception of a layer of small water-worn boulders overlying the rock, entirely in sand. Rock was encountered at about the estimated depths, and was nowhere, except in the key trench, excavated to a greater depth than 30 feet below stream-bed (27 feet below L.W.O.S.T.). The rock, which is a schist, was generally sound. The excavation of the stream-bed was made of sufficient width to permit of side slopes being formed to a batter of 1 to 1, at which the sand would stand, and no timbering was required. On the up-stream side of the foundation a tongue or key, ranging from 10 to 20 feet in width and about 16 feet in depth, was cut into the rock to ensure greater watertightness.

For draining foundations a 10-inch steam-driven ram-pump, assisted at intervals by a 6-inch centrifugal pump, was sufficient, but two additional centrifugals driven by oil-engines were pro- vided as stand-by. There were, unfortunately, many days when work had to be discontinued owing to the partial breakdown of the pumping-plant, but the excavation was not excessively delayed on account of this, and by January, 1915-i.e., 9 months after it was commenced part of the foundation in the stream-bed was ready for concreting.

The key into the rock, being deeper than the remainder of the excavation, would naturally have acted as a drain for the water percolating through the up-stream coffer-dam, but this was pre- vented by constructing, parallel with the tongue, a channel formed of two concrete walls about 6 feet in height, separated

!

Papers.]

LOW-LEVEL RESERVOIRS AND WORKS FOR PUMPING. 11 262

from one another by a distance of 2 feet (Fig. 4, Plate 1). The outer wall was built on a foundation of concrete in bags, between which all leakage water freely passed to the channel. An 8-inch cast-iron pipe was laid from this channel spanning the key trench and discharging into a sump within the foundations of the dam.

Immediately clear of the down-stream toe of the dam, another channel for the better collection of the sea-water leaking through the down-stream coffer-dam was similarly constructed. The sump already referred to was utilized during the depositing of concrete in the key trench and the forming of the inner and outer toes of the dam. On the completion of this part of the work, another sump was formed in the channel at the down-stream toe of the dam, the pipe from the up-stream channel being connected with it.

The blasting of the surface rock for the first few feet in depth of the key trench opened up the joints, and a wooden launder was fixed to collect the water percolating through them. During the first stages of the concreting, the water which collected in the launder was dealt with by hand-pumping, but later, when the concrete rose above the original surface of the rock, the fissures were stanched. The wooden launder which had been carried up with the concrete, was then removed and the space between the face of the concrete and the inner wall of the main drainage channel was arched over, grout-pipes 4 inches in diameter being concreted into the crown at intervals of 10 feet for the whole length of the supplementary channel, The pipes were carried up to a height of about 30 feet, and grout com- posed of 1 cement to 1 fine sand was poured simultaneously into alternate pipes until it rose in the intermediate ones. This did not occur in every case, and intermediate pipes in which it did not occur were filled with grout after no more could be poured into those on either side. The key trench was completed in the dry season, and was filled with concrete forthwith, as the leakage through the upstream cofferdam was then small, after which the inner toe of the dam was formed while the remainder of the excavation in the stream-bed was being completed.

The excavation was finished with pricking-bars and levers, and the surface was very carefully picked over to remove any small loose pieces. The few springs which appeared after the final picking- over were piped and either connected with a main pipe-drain which discharged into one of the culverts, or led into the flood-water channels. The quantity of water which had to be dealt with in this way was very small. The excavation in the stream-bed amounted to 16,429 cubic yards, 4,057 cubic yards being rock and the remainder sand. The excavation for the stream-diversion cut