This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol2", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
(Contributed by T. H. Bishop, A.R.I.B.A.)
Of all the units that make a satisfactory house, the drainage should receive the most careful attention; a good system being absolutely necessary for the health of the inhabitants.
At the present time bad drainage should be considered a disgrace, although Acts of Parliament and Local Bye-Laws make a high standard of sanitation compulsory. Yet sanitary work in general leaves much to be desired.
It is the duty of householders to see that their drains, manholes, and other fittings are kept clean and periodically flushed. Unfortunately, however, the average householder imagines that drains are automatic, and need little or no attention.
A solution of 1 oz. of sulphate of iron in 2 gallons of water, if passed through the drain, will sufficiently cleanse it.
In constructing a satisfactory scheme of drainage the following points should be observed:-
4. Drain pipes should be as small as possible, and of glazed stoneware, laid to self-cleansing gradients.
The general rule as to gradients of drains is:-
1 in 40
1 „ 60
1 ,, 90
These gradients give velocities of from 2 1/2 to 5 feet per second. A considerable saving in excavation is made, and the best results obtained, by adopting the moderate gradients in the table on next page; steep gradients are undesirable, as they allow the liquid to run away, leaving the solids behind to putrefy and cause a nuisance.
The following table shows self-cleansing velocities (Beardmore):-
will not disturb clay and stones.
will not move along coarse sand.
will move along fine gravel the size of peas.
will move along round pebbles 1 inch diameter.
will move along angular stones 1 3/4 inch diameter.
From this it will be seen that the velocity should never be less than 3 feet per second.
It has been proved by experiment that the velocity of the flow of liquid in a pipe varies as the when
R is the "hydraulic mean depth" (which is equal to sectional area of ABCE }and s the gradient of the the wetted perimeter ABC pipe.
The hydraulic mean depth of pipes running full or half full is diam, always in feet. For intermediate 4 depths the hydraulic mean depth can be calculated in the following manner, which avoids the use of trigonometry (see Fig. 183).
The wetted perimeter should be first found by the following rule:-
3 Assuming a 9-inch pipe running a quarter full (see Fig.
We have now to find the chord of half the arc, namely, the length of AB.
The other variations of HMD can be found in a similar manner.
A formula which is easily remembered is Beardmore's variation of Eytelwein's. This gives flows which are slightly less than the actual, and it is widely used on account of its simplicity:-
When V = velocity in feet per minute.
HMD = Hydraulic mean depth in feet, - for pipes running full and half full. 55 = Constant. Example, find the velocity and discharge of a 7-inch drain running full, and laid to a gradient of 1 in 340. 1 in 340 = 15.5 feet per mile.
HMD = 7 inches 4x12
V = 55x2.13.
V = 117.15 feet per minute.
V = 117.15 1.95 foot per second.
The discharge of this pipe per second is diameter2 x .7854 x 1.95 x 6.25, = 3.24 gallons per second. The following table worked on the above formula gives the Rate of Inclination for velocity per second for pipes running full:-
Diameter of Sewer.
Rate of Inclination of Velocity per Second.
1 in 182
1 in 83
1 in 46
6 „ .
1 „ 278
1 „ 123
1 „ 70
7 „ .
1 „ 33°
1 „ 43
1 „ 81
9 „ .
1 ,, 416
1 „ 185
1 ,, 104
Plate V. shows the drain plan of a suburban house, and may be taken as a typical example, as in larger houses the several points are duplicated, whilst the principles remain the same.
The trenches having been excavated to the required depths and gradients, a bed of concrete, 6 to 1, varying in thickness with the diameter of pipes (see Fig. 184), is deposited in the trenches, lightly rammed, and graded to the required falls, with bricks lightly bedded in same where the socketing of pipes occur. These bricks are taken out when the drains are laid, so that the pipes bed firmly on the concrete. The concrete should be 12 inches wider than bore of pipes, and not less than the following depths:-
4-inch pipe . 3 inches deep.
7-inch pipe . 4 inches deep.
6 ,, . 4 „
9 „ .4 „
The pipes are then laid in position, a good fillet of neat cement or cement mortar 1 to 1 is laid on the bottom of the socket and on the top of the spigot of the adjoining pipe and gently driven home. The remaining portion of the joint is then made, the cement weathered off, and the surplus cement inside cleaned off by a dolly or scraper. On the completion of the line of pipes they should be tested, and fine concrete bedded round to a height equal to half the diameter of the pipe, sloped off as shown on Fig. 184. The earth is then returned to the trenches and lightly rammed, care being taken that the pipes are covered to a depth of at least 18 inches before the ramming commences. If this precaution is neglected the newly made joints are liable to be broken.