Drains and rainwater disposal

The basic sewage and drainage system that we know today was devised only about a hundred years ago, after medical science had proved that there was a link between disease and the open sewers and piles of filth that lined the streets in those days. Their discovery provided the necessity which brought about the invention of the salt glazed drain pipe. However, the egg-shaped, butt-jointed pipe, which was the forerunner of our present-day circular salt-glazed pipe was unknown in 1840. Since that time the Victorian engineers devised and installed a drainage system which today forms the backbone of our sewage disposal system. Even so, the glazed, socketed, circular pipe was not in universal use until about 1900.

Water closets had been patented as early as 1775, but without the mass production of suitable pipes the water-carried sewage system was impracticable. The first system was of the combined type in which one large pipe carried all types of discharge from soil and waste fittings as well as taking the surface water. This was replaced later with a two-pipe system in which parallel pipes were laid, one to take the surface water and the other to take the discharge from soil and waste fittings. It was then found that the foul drain often became insanitary because of the lack of flow, so a partially separate system was devised in urban areas where the surface water drain took the discharge from the main roofs and gullies and the soil and waste sewer took the discharge from the back additional roofs and the paved yards to provide the necessary water which would assist the flow in the drain.

Insistance on inspection chambers at every change of direction was not by any means generally adopted and the drain layer often laid pipes to curves by knocking off a piece of the spigot end of a pipe so that it could be forced at an angle into the socket of another pipe. The necessity for calculating the proper falls was understood but every drain layer interpreted this according to his own ideas, so that drains followed their own way underground. Early gullies were inefficient and interceptor traps were incorporated in the drains as a first line of defence against foul air from the sewer, and the traps of the gullies and fittings served as a second line of defence. It has been said that the best period of drainage for surburban housing was from about 1890 to 1914 when the first world war put an end to it.

The joints of the early glazed pipes were caulked with clay and, not surprisingly, this was unsatisfactory. Later, when cement was used there was some difficulty in keeping the pipe central in the socket while the cement was wet. The wet cement would ooze through the joint at the bottom of the pipe and set to form a ridge on the inside and thus cause an obstruction to the flow.

This problem was overcome with the introduction of the tarred hemp gasket, which is inserted all around the spigot end of the pipe and driven into the socket rebate using a special caulking tool. A mix of 1 : 3 cement and sand is used to finish off the joint and this is neatly bevelled to about 45 degrees. Care has to be taken to ensure that the under side of the pipe is well caulked and filled with cement as it can be difficult to get at.

The traditional tarred hemp and cement method of jointing salt-glazed pipes has now given way to more modern methods, which include various patents. These new joints allow a degree of movement to take place without the joint or the pipe cracking. The new joints are based on the old principles of sleeves or O-rings but the materials are polyester or pvc. Building regulations require joints to remain watertight under all conditions including differential movement between the pipe and the ground.

Fall is of great importance to the design of a drain and it should be sufficient only to carry the sewage and the greatest quantity of rainwater. Where the pipe is used for sewage alone a moderate fall is better than a steep one. The liquid should not run away leaving deposits to form and adhere to the drain. Fortunately, there is a rule-of-thumb method of determining the fall required and this will be suitable for use when laying domestic drains or extending them. Known as Maguire’s rule it consists of simply multiplying the diameter of the pipe in inches by ten. Therefore, a 100 mm pipe needs a fall of 1 in 40 and a 150 mm pipe needs a fall of 1 in 60.

Inspection chambers, or manholes as they are sometimes called, are needed for proper access to the drain for repair and cleaning. They are situated at each change of direction of the drain and at 91 m intervals along straight runs of drains. There must be room in the chamber for a workman to be able to stoop down or kneel to fit stoppers and operate cleaning rods or other equipment. This is not so important where the invert depth is not more than about 915 mm. For depths of 1.83 m the inspection chamber must be at least 1 m X 660 mm. Extra width will be needed where there are a number of branches entering the chamber.

The chambers are generally built in brickwork and must be watertight. This is important not only to prevent the sewage seeping out, in case of a stoppage that fills the chamber, but ground water is not wanted in the drain so it must not be allowed to enter. The brickwork is built in English bond using a special quality brick, or dpc quality brick and a suitably strong mortar mix. There are also precast concrete manholes obtainable. These are made in circular sections which can be placed one on top of the other to provide the necessary depth. In this type of installation it is important that the joints between the sections are watertight.

All bends and branches into the drain must be in the direction of the flow and the channels are bedded in cement mortar. Benching has to be provided at the sides of the channels and this has to rise up to the height of the top of the outgoing sewer. These benchings are given a slope of 1 in 6 up to the sides of the manhole and they must provide a safe foothold for workmen. The benching is finished to a hard smooth surface using a mix of one part cement to one part sand.

There are also plastic moulded inspection chambers which, if they carry an Agrement Certificate, can be used with plastic, pitch fibre or vitrified clay pipes. The different local authorities take their own differing views of plastic drains and pitchfibre drains so that it is essential that enquiries are made before specifying these two types. . There are two methods of providing domestic drainage, one is the single-stack system and the other is the two-pipe system. In the two-pipe system the soil and the waste water are piped separately and in the single-stack system they are conveyed together. Whatever system is used, the whole object of the plumbing system, if it is to operate satisfactorily, is to ensure that the traps in the various appliances remain sealed at all times. These seals can be broken by the pull induced by siphonage caused by the full flow of the drain lower down the pipe line or by the full flow of the waste through the trap itself.

This self-induced siphonage is used in the siphonic w.c. suites to assist in the clearing of the pan. The traps are deliberately made to run at full bore to cause siphonage, then the siphon is broken so that the final wash of water fills the traps again.

The two-pipe system of domestic drainage is not installed now, but there are millions of old houses which use it. In this system, the soil stack takes the discharge from the w.c. or w.c.s only; the sink, bath and other wastes are discharged separately into hopper heads and downpipes to open gullies. The break at the hopper head and the gully and the ventilated soil stack ensure that the sewer gases do not enter the house. Unfortunately, the hopper head and the gulley can become quite foul and offensive so that some of the effect of the disconnection from the drain is lost.

Soil pipes for a domestic system are usually 100 mm diameter, and in the single-stack system the old style hoppers are done away with and waste pipes from the bath, handbasin and sometimes sink discharge into the soil stack which, as with the two pipe system, acts as a vent pipe and is carried up at least 1 m above the top of any window that is within 3 m of the pipe.

In this system the soil pipe and the waste pipes are situated within the building and are therefore not as unsightly as the old systems which snaked all over the exterior of many buildings. To work correctly, the single stack system needs to be installed carefully in order to prevent the lack of ventilating pipes allowing the traps to become unsealed. The stack itself must be straight and the bend at the bottom, leading to the drain must be slow. The sanitary fittings should be grouped as close as is practicable to the stack pipe which is 100 mm diameter cast iron or plastic.

Waste pipes to the fittings need to be fitted with traps giving 75 mm seal. The maximum length for a sink waste is 2.3 m and the pipe should be 38 mm diameter, the self-cleansing gradient for this pipe being between 1 in 12 and 1 in 24; this will also reduce the possibility of self-siphonage in the trap. Baths and washbasins have waste pipes 32 mm diameter and these need a fall of between 1 in 12 and 1 in 48. The maximum length for a bath waste is 2.3 m and for a washbasin it is 1.7 m. All the waste pipes must be connected to the soil stack separately above the w.c. connection or at least 203 mm below the centre line of the w.c. connection.

In many houses it is more convenient to carry the sink waste to an adjacent gully rather than move the sink to within the limits of the permitted length of waste pipe. If this is done the gully must be of the back entry type so that the waste discharges below the level of the grating and above the level of the water in the trap.

The soil branch for the w.c. must not be more than 1.5 m long. If there is a w.c. at ground-floor level it must be at least 460 mm above the bottom of the bend at the foot of the soil stack. The correct fall for the soil branch is an angle of 104 degrees to the vertical soil stack.

A minimum seal must be kept in the trap at all times and to do this there are various patent traps as well as the ordinary bends. For a single-stack system a seal of 75 mm is required for traps up to 64 mm diameter. An efficient trap should be incorrodible and as self cleaning as possible. There should be access for cleaning and the trap should hold as little water as possible while giving the correct depth of seal. A trap is essentially a U-bend in a pipe and the different patterns have different names; when one leg of the bend leads to a horizontal outlet it is called a P-trap. If there is a double bend so that the outlet leads vertically downwards it is an S-trap. A running trap is one in which the inlet and outlet are both horizontal. Bottle traps have a bend shaped like a bottle with a division down the middle.

There are also patent self-resealing traps in which a vent pipe, extra chamber or an enlarged chamber are used to break the siphon caused when the traps run at full bore, and then hold back enough water to reseal the trap.

The w.c. pan can be connected to the soil pipe with the usual hemp and cement joint, but a better method is to use one of the plastic connectors which are simply a push fit over the outlet spigot of the w.c. trap and also a push fit into the collar of the soil pipe. Using this type of connection means that any slight movement can be accommodated without the joint cracking as a cement joint may do. This is an important consideration when the w.c. is situated on a wooden floor, because it will not only be subject to thermal movement of the timber, but there may also be some slight springing of the floor which though otherwise unnoticeable, could bring about damage to a rigid pipe joint.

Waste pipes may be in copper, in which case they can be jointed either with the easily made, though clumsy looking, compression joint, or they can be soldered. The waste pipes are more likely to be plastic and in this case the joints can be simple, push-fit fittings which are sealed by means of a plastic or rubber O-ring or they can be a kind of compression fitting in which a large plastic nut drives a wedge-shaped plastic ring into the fitting to form a seal.

These plastic waste pipes can be uncoupled for cleaning; they are also used in conjunction with bottle traps, the bottom of which will unscrew for cleaning purposes. Where the U-bend type of trap is used there is generally a joint on the outlet leg of the trap which enables the outlet to be swivelled to face in any direction. Plastic waste pipes can also be of the solvent welded type which is a similar method to that used for the plastic soil systems.

Vertical plastic pipes need supporting brackets at about 1220 mm centres and horizontal runs need support at about 762 mm centres. Because of the amount of thermal movement which takes place in plastic piping, manufacturers make special fittings to accommodate this expansion and contraction and their recommendations must be carefully carried out. They also provide the necessary fittings and instructions for jointing the plastic system to metal or salt-glazed drain pipes.

As plastic pipe and fittings are manufactured to a good quality finish there is no need for any further work such as painting and decorating them. However, colours are limited mainly to grey, brown or black, dependent on the manufacturer, but some white products are also available.

Rainwater systems

When installing a rainwater system it is important that the gutters and downpipes are adequate for the area of roof which has to be drained. The flow capacity for a straight gutter depends on its cross section, its shape and the length, also the fall. The fall will increase the capacity up to a point, but there is a limit because too much slope would make too big a gap between the edge of the roof and the gutter so that much of the water would be blown against the wall before it reached the trough. Although the length does not influence the flow capacity it does determine the area of roof being drained and therefore the quantity of water collected.

A level gutter 11 m long and made of 112 mm half-round sections could carry 68 litres per minute. If the same length of gutter were given a fall of 25.4 mm in 5846 mm it could carry 118 litres per minute.

Of course, there are other factors to take into consideration; if there is a right-angled bend near an outlet it will reduce the capacity by about 20 per cent. If the bend is 1.8 m to 3.6 m from the outlet then the reduction in carrying capacity can be reduced to 10 per cent.

The position of the outlet also has a bearing on the gutter capacity. If the outlet is at the centre of the gutter, the gutter capacity required is only half that which would be needed if the outlet were placed at the end of the run. The correct positioning of the gutter depends on the type of roofing material. If it is a slate roof then the gutter is placed centrally under the edge of the overhanging slates and not more than 50 mm beneath them. Water running off roof coverings such as clay pantiles behaves differently and spreads out as it leaves the edge. This means that the gutter must be close to the edge and with its centre slightly forward. If the lower edge of the roof is rounded then the gutter must be placed more to the rear to catch the water which will be deflected backwards. The best type of roof edging is one in which the top edge is rounded and the bottom edge is sharp.

It is best if it is possible to have the rainwater discharged into the local authority stormwater drainage system or combined sewer if it is used, but often the water has to be taken to a soakaway. This is a pit dug at some distance from the property and filled with clean rubble. The rainwater is drained into this pit and allowed to soak away into the surrounding earth, if possible the rainwater is directed to stream.

The soakaway has to be carefully constructed otherwise it will soon fill due to clogging or saturation of the surrounding area. The pit must not be dug within 3 m of the house and twice that distance if the ground is clay, or if the pit has to be in such a position that the dispersing water will affect the foundations of the building.

When the pit has been dug and filled with clean brick rubble, it should be covered with a strong polythene sheet or a concrete slab about 300 mm below ground level to prevent the garden soil being washed into the rubble and clogging the pit. The pipes leading to the pit should be 100 mm diameter and can be laid to the same fall as the domestic drainage system. In soft ground the soakaway pit can be lined with perforated precast concrete rings, which are dry jointed. The tank which is formed can either be left empty or it can be filled with rubble. In either case it still needs to be covered over the top.

The drainage of the subsoil is essential on some sites, particularly sloping sites or low lying sites. These drains are known as field or French drains and are simply unglazed clay pipes set to falls and covered with clean rubble or gravel before back filling the trench. They are usually laid herringbone fashion leading to a slightly larger drain which discharges at a lower level, preferably into a stream.

The trenches for the drains should be dug as narrow as possible and the bottom should be given a fall of about 1 in 100. The distance between the trenches will, of course, vary with the type of soil and can be from 3 m to 9 m. The pipes are laid dry with butt joints, the uneven surfaces providing sufficient space for the water to get through. About 380 mm of gravel or fine rubble is laid over the pipes before the earth is replaced, loosly at first and then well rammed. On no account should the land drain to be taken underneath a building.

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