THE HONGKONG GOVERNMENT GAZETTE, 18TH OCTOBER, 1890. 1041
10. Lime concrete used for encasing new drains shall be composed of four Materials, lime concrete. parts of good sound clean stone, broken to half inch cubes, two parts of red earth
and one part of lime thoroughly well mixed and well rammed into place.
11. Cement mortar used for the jointing of pipes or any other work shall Materials, cement mortar. be mixed in the proportions of three parts of clean sharp sand and one part of
good Portland Cement and used fresh.
12. No main house-drain shall be less than six inches in clear internal Size of drains. diameter.
13. Subject to the limitation mentioned in the preceding Bye-law, no main Size of drains, house-drain shall be larger than is necessary in the opinion of the Board to carry off the sewage of the dwelling, or the sewage with the rain-water, which, under con- ditions hereinafter specified in Bye-laws Nos. 38, 39, 40 and 41 shall be admitted to the house-drain.
14. Every house-drain shall have the maximum fall, throughout its length, Fall to drains. that the relative levels of the public sewer and of the most remote inlet, will admit of.
Provided always
(a.) That the maximum available fall does not exceed one in thirty (or 4 inches in 10 feet). If it does, then the part of the drain, more remote from the public sewer, may be laid with a fall of one in thirty; and the remainder, with such greater fall as may be necessary to connect with the public sewer.
(b.) That the excavation, necessary to obtain the maximum available fall, is not of such a nature as to endanger the stability of the adjoining or neighbouring property.
In these and similar cases the gradient may be reduced subject to the approval of the Board.
15. Whenever a reduction of fall, (below the standard grade 1 in 30), is Fall to drains. necessary or permissible, it shall, wherever practicable, be made in the portion of the drain, nearest to the outlet, rather than in the part nearest to the inlet and in
the main trunk of the drain, rather than in the branches.
NOTE.-The available fall in a house-drain, 100 feet long, is 2′ 6′′ or one in forty. It will be better to make the first thirty feet from the inlet, with a fall of 1 foot, or 1 in 30; and the remaining length of 70 feet with a fall of 1 in 46'66, than to give an uniform fall throughout.
1 IN 30
100'
B 1 IN 40
A
1 IN 46.6
It is clear that an obstruction at A would be more easily removed than one at B, in the pipe of uniform gradient, because it would have a greater head of water behind it; and in flowing from the inlet to it, the stream will have attained a certain momentum, tending to carry it on, through the flatter part of the pipe.
e.g. (2).
D-
-40'
B
Λ
,09
90'
A B is a drain 90 feet long, with an available fall of 3 feet, between A and B. At C, distant 60 feet from A, there is a branch CD 40 feet long; D being at the same level as B,
If the main-drain AB were given an uniform fall of 1 in 30, then
would be 1 foot below B and D. Then CD would have a fall of 1 in 40 only. The proper course to pursue would be to give CD a fall of one in thirty, or 1′ 4′′ in all. BC would then have a fall of 1′ 4′′ in 30, or 1 in 22‡, and CA a fall of 1′ 8′′ in 60 feet; or one in thirty- ΒΙΣ,
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