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in the year 1947/48 for instance, 91.16% were up to the standard of B. Coli absent in 100 c.c, and in the case of samples taken at random from consumers taps the figure was 79.83%. Similar figures for Physical and Chemical Analyses show 99.19% satisfactory at the filters, and 100% at the taps. In all, during the year 1947/48, 59.9 tons of Chloride of Lime, 19.9 tons of Chlorine gas, 203.2 tons of Alumina, 41.3 tons of Lime and 68.1 tons of Soda Ash were used.
SERVICE RESERVOIRS.
The service reservoir is the last reserve against breakdown, and also a balancing tank which enables a steady filtration rate on one side to meet a diurnal variation in demand on the other. There is, therefore, a definite minimum size of service reservoir for any given filter beds, such that it will just be filled during the low night time draw-off, in other words roughly 12 hours output from the filters. But this leaves practically no reserve in case of either fire or break- down so that the lowest practical minimum is one day's supply. It is generally considered, however, that a two day's supply is desirable, though with a large number of interconnected systems such as are available in Hong Kong this requirement can be somewhat relaxed. The summary in Table No. 6 however, shows a very poor position which it is hoped to remedy within the next three or four years by the building of three or four extra reservoirs on the Island and two more in Kowloon, quite spart from the extra ones required for the proposed Tai Lam Chung Scheme.
The siting of new service reservoirs in Hong Kong is not easy. On the Island they usually have to be built on a hillside which involves heavy excavation with serious risk of slips and inevitably a query as to the stability of the downhill wall of the reservoir. The use of former filter beds has already been referred to, but this is really begging the question as these sites have been similarly formed in the past, with the additional complication therefore that original site plans having been lost it is difficult to decide just where was the original ground line and one would not knowingly found a gravity section retaining wall on fill even if it were 30 years old.
One solution used on a small tank at Peak Road was to design the walls as cantilever retaining walls with the long leg of the base inside the tank. This results in a very steep resultant pressure which is well inside the middle third and consequently a reasonably low pressure on the outside toe. Unfortunately the steel shortage has made this solution undesirable for the present and in a similar tank now being constructed the walls are plain concrete with a full gravity section.
These reservoirs constitute an interesting study in Engineering Economics. Given the capacity required there are a number of alter- native solutions possible. The variables comprise:-
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(i) the depth of water--the greater the depth, the smaller the area, and the cheaper the roof covering, but the walls although shorter are much thicker.
(ii) the relative cost of mass concrete walls against reinforced
concrete-assuming steel is available.
(iii) the relative costs of excavation and construction materials -on a hillside site the deeper the tank proper the less the excavation of overburden but the heavier the concrete walls as at (i) above.
In view of the construction programme referred to above, the author is endeavouring to find the lowest common denominator of the above variables, or at least to establish some rules which can be applied to any given case to determine the most economical solution.
With one exception - Albany - all the service reservoirs are covered. This is highly desirable as the water is filtered and chlorin- ated ready for consumption and should therefore be safe against contamination. The earlier roofs were supported by brick jack-arches carried on masonry piers, but the more recent ones are all of rein- forced concrete. Generally they are of a beam and slab construction, though for the Kowloon Chai Reservoir a flat-slab roof with dropped panels was used. It is very doubtful if this is economical for the comparatively light loads these roofs have to carry.
An interesting repair job was carried out last year in the Botanical Gardens Reservoir. The north-west corner of this reservoir was founded on made ground, and despite piling and later underpin- ning, the mass concrete walls settled with resultant leakage. This was so bad by 1945 that one half of the reservoir could not be used, and as the division wall was only two-thirds full height, the capacity was reduced to less than one-third. The whole of the defective corner was therefore cut off by means of a reinforced concrete wall supported on A-frames in such a way that there was a minimum ground pressure. The work necessitated cutting through the concrete roof and floor and erecting a new wall between the existing columns, and then reinstating the roof. The whole work was successfully completed and the reservoir can now be used again to the full with a loss of only 0.34 M.G. from the original capacity. Details are shown in Plate 4.
PUMPING STATIONS.
It will be readily appreciated that with the relatively low level of many of the reservoirs compared with some of the residential districts on the Island, a considerable amount of pumping is required. In fact eleven separate pumping stations are maintained on the Island but so far none has been required on the Mainland.
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