Installation of the Underground Drains

The basic principles involved in the design of underground drainage systems have remained virtually unchanged for over half a century. Underground drains must be laid in straight lines, to a constant self-cleansing fall. They must be watertight and must remain watertight even if there should be slight soil settlement. Every point of the drainage system must be accessible for rodding in the event of a blockage occurring.

Branch connections must join the main drain obliquely in the direction of flow. The drainage system must be adequately ventilated. The development of new materials has however resulted in revolutionary changes taking place in the way in which these principles are observed.

Up to about 1950, only two materials were used for underground drainage work-either salt glazed stoneware pipes or heavy iron pipes suitably protected against corrosion. Glazed stoneware pipes were commonly used for domestic drains. They were 4in in internal diameter and 2ft in length, and were usually laid upon a 6in thick ‘raft’ of concrete to give them some stability in the event of ground settlement.

The multitudinous joints-each a potential point of leakage-were made by caulking a tarred rope grommet into the space between spigot and drain socket and completing the joint with either neat portland cement or a mixture of two parts cement to one of sand. The purpose of the rope grommet was to prevent the jointing material entering the drain to impede free flow and establishing a cause of blockage.

To be absolutely certain that no jointing material had leaked through it was usual to draw a sack through the completed drain by a length of rope. The inspector of the local authority would, before passing the drain for use, check on this point by observation with the aid of a mirror and an electric torch and, perhaps, by rolling a ball through the drain. After the drain was laid it was usual to haunch it over with concrete to increase its strength. Where a stoneware drain passed under a building it was covered entirely by a layer of concrete 6in thick.

Iron drainpipes were considerably more expensive and were used in good class industrial and commercial work. They were obtainable in 9ft lengths, thus reducing the number of joints. These were made by caulking lead wool into the space between spigot and socket. Iron drains were not required to be laid on a concrete base.

It has been established empirically that drains are self-cleansing when water flows through them at 3ft per second when one third full. A universally adopted rule of thumb allowed for a fall of 1 in 40 for the usual 4in drain and a fall of 1 in 60 for the 6in drains that might be used where a number of houses were drained in combination.

Even with the many-jointed stoneware drains this gave a flow in excess of 3ft per second if the drain were laid to a steady fall and it should perhaps be stressed that too steep a fall is almost as undesirable as a too shallow one. If the fall of the drain is too steep there is a tendency for liquid to flow on in advance of solid matter that may be left behind to form a blockage. However, for most purposes, the 1 in 40 rule worked well enough.

Access to every part of the drain was ensured by the provision of inspection chambers or ‘manholes’ at every change of direction of the main drain and at every point where a branch drain connected to it. Inspection chambers were constructed of brickwork on a 6in concrete base and it was usual to render the internal surfaces of the inspection chamber walls with a sand and cement mixture. The drain flowed through the inspection chamber in a half-channel built into the base and concrete was haunched up on either side of the half-channel and trowelled to a smooth sand and cement surface. Purpose made ‘three quarter bends’ could be built into the haunching to permit branches to connect in the direction of flow.

Access to the inspection chamber was obtained by raising a cast iron manhole cover set into a frame of the same material. On the rare occasions that it was necessary to construct an inspection chamber within a building the cover would be ‘double sealed’ and set into the frame in a bed of grease to prevent gases from the drain escaping.

The inspection chambers of iron drains did not need to be made watertight as special access sections were manufactured with bolted down iron covers. The bolts could be unscrewed and the cover removed when access was required to the drain.

In the final inspection chamber before the connection of the drain to the sewer it was common practice to provide an intercepting or disconnecting trap. The purpose of this trap was to prevent gases, and perhaps rats, from the sewer from entering the house drains.

The value of the intercepting trap was questioned as early as 1912 by the Departmental Committee on Intercepting Traps and House Drains. A well constructed and maintained sewer should contain neither rats nor offensive gases. If the intercepting trap were omitted the sewer would be ventilated very thoroughly by means of the soil and vent pipe of each individual drainage system. There would be no need for separate sewer ventilators. Furthermore, the intercepting trap was and-where it is installed-still is, the commonest site of drain blockage. The trap inevitably impedes the flow of water and tends to permit solid matter to accumulate in its base. To make intercepting traps rather less liable to blockage they were constructed with a sharp weir inlet and an easy outlet 2in lower than the inlet. For this design to have the desired effect of making the trap more or less self-cleansing it was, of course, essential that it should be set dead level.

Intercepting traps are provided with a rodding arm to permit the clearance of that section of the drain that lies between the trap and the sewer. The rodding arm has a socket inlet closed with a stoneware stopper. This stopper is a common cause of a particularly unpleasant form of partial drain stoppage that can sometimes remain undetected for months.

Any increase of pressure within the sewer—arising from, for instance, a surge of storm water—is liable to push the stopper out of its socket. It will fall into the inlet to the intercepting trap immediately beneath it and will promptly cause a blockage. The blockage will not however be discovered in the usual way because sewage will rise in the inspection chamber until it reaches the level of the, now open, rodding arm and will be able to flow down this arm to the sewer. The liquid in the bottom of the inspection chamber will, in the meantime, become steadily more and more foul until the defect makes itself obvious by the unpleasant smell that greets visitors near the front gate of the property. Complaints of ‘drain smells’ in front gardens are nearly always attributable to this cause.

Where this trouble has arisen it is usually better not to replace the stopper in the socket of the rodding arm. Cut a disc of slate or glass to size and cement it lightly into this socket. On the rare occasions that it may be necessary to rod through to the sewer the glass or slate can be broken with a crow bar and can then be replaced after the blockage has been cleared.

If a drain has an intercepting trap it will usually also be provided with a low level drain ventilator connected to the same inspection chamber. This consists of a length of drainpipe connected to the inspection chamber and protruding a few inches above ground level. Into the open end is inserted a metal box with a grille at the front. A hinged mica flap is suspended inside the ventilator against this grille. In theory air passing over the top of the open soil and vent pipe would aspirate air out of the drain. The reduction in pressure would result in the outside air pushing open the hinged flap and flowing into and through the drain from low level. In the event of a back pressure within the drain the mica flap would press tightly against the grille and drain air would not escape.

Unfortunately low level ventilators are particularly susceptible to accidental damage and to vandalism. Before long the grille becomes damaged and the mica flap either damaged or jammed in position. A glance into the front gardens of any suburban street developed during the 1920s or 1930s will confirm this. It will, in fact, be found that many householders have removed and sealed off the low level ventilator or ‘fresh air inlet’ as serving no useful purpose and being a recurring source of unpleasant smells. If the intercepting trap is omitted there is, of course, no need whatsoever for a fresh air inlet.

Modern underground drains are likely to be of p.v.c. or pitch fibre with push-on ring seal joints. Both the drains and the means by which they are joined are sufficiently flexible to absorb slight ground settlement without damage. A concrete base is therefore unnecessary. Proper preparation of the bed on which the drains are to be laid remains important however. Where the subsoil consists of heavy clay or chalk it may be necessary to prepare an imported bed of gravel to form a base. Infilling of the drain trenches must also be undertaken with some care to avoid the risk of accidental damage. Drain gradients may be considerably less than was considered necessary with many-jointed stoneware drains. Falls of 1 in 60 or 1 in 70 are in common use.

Inspection chambers are no less necessary than they were in the past and they may still be built of brickwork in the traditional manner. They should not however be rendered internally with sand and cement to make them watertight. Experience has shown that internal rendering is liable to crack and flake off the walls, causing drain blockages.

If an inspection chamber is to be rendered it should be to the external walls, before the soil is filled in round the chamber.

Complete prefabricated inspection chambers made of fibre-glass reinforced plastic are now available and can speed up installation work. Alternatively inspection chambers can be constructed on the site from precast concrete sections. Marley Extrusions have now produced a sealed access p.v.c. drainage system resembling in many respects the sealed iron systems referred to earlier in this post.

Blocked drains

Blockages may occur in any part of the drainage system, either above or below ground. Above-ground blockages are most likely to occur in the traps of baths, basins or sinks. The waste stopper is removed and the fitting fails to empty. The immediate course of action should be to try plunging with a sink plunger or ‘force cup’. In the great majority of cases this will produce a speedy remedy.

A force cup is a hemisphere of rubber or plastic usually mounted on the end of a wooden handle. Hold a damp cloth firmly over the overflow outlet of the fitting. Place the force cup over the waste outlet and plunge down forcibly three or four times. Since water cannot be compressed the force of this action is transmitted to the obstruction to move it. The purpose of the damp cloth held over the overflow outlet is to prevent the dissipation of this force.

If plunging fails to clear the blockage gain access to the trap. The traditional U bend trap has an access eye at or near its base from which the stopper can be unscrewed. The entire base of a bottle trap can be unscrewed and removed. Before attempting this place an empty bucket under the trap. Probing with a piece of wire after having gained access to the trap will usually dislodge the cause of the blockage.

A poor flow from a sink, bath or wash basin when the stopper has been removed suggests the presence of a partial blockage. This could be due to hair or other debris clinging to the waste grid. In this case the remedy is obvious. A build-up of grease on the interior surfaces of the waste pipe is another possibility. One of the proprietary chemical drain cleaners can be used to clear this. These chemicals usually have a caustic soda base and should be handled with care.

There are two ways in which a blocked underground drain may come to the attention of the householder. A gully may flood or drainage be seen escaping from under the cover of an inspection chamber; or a w.c, when flushed, may fill almost to the brim with water which will then very slowly subside.

If a flooded gully is the first indication first raise the grid to make sure that the trouble is not due simply to leaves or other obstruction on the grid itself.

Having cleared this point raise the drain inspection covers to establish the position of the blockage. If, for instance, the chamber nearest to the house is flooded but the one near the boundary of the property is clear, then the obstruction must be in the length of drain between these two inspection chambers.

A set of drain rods, or sweeps rods, are necessary to effect a clearance. Screw two or three drain rods together, plunge one end into the flooded inspection chamber and feel for the half-channel at its base. Push the end of the rod along this half-channel and into the underground drain in the direction of the blockage. Screw on more lengths of drain rod as necessary and continue to thrust into the drain until the obstruction is encountered and cleared.

A variety of tools are available that can be screwed onto the end of drain rods to clear difficult obstructions. Twisting the rods will make it easier to push them into the drain and to withdraw them afterwards. Be sure to twist clockwise only. Twisting in the other direction will result in the rods becoming unscrewed and lost in the drain.

If all inspection chambers are flooded and the drain has an intercepting trap, the probability is that it is in this trap that the obstruction is situated. It can be dealt with by plunging. Screw two or three drain rods together and screw a 4in drain plunger onto the end. This is a 4in diameter rubber disc with a screwed socket for connection to drain rods.

Lower the plunger into the inspection chamber containing the intercepting trap. Feel for the half-channel at its base and move along this channel until the drop into the trap is encountered. Plunge down sharply two or three times. The chances are that there will be a gurgle as the obstruction is cleared and water level in the inspection chambers will fall quickly as water flows through freely into the sewer.

In an emergency an old fashioned household mop on a long handle-or even a bundle of rags securely tied to a broom stick—can be pressed into service as a drain plunger.

After clearing a drain the sides and benching of the inspection chambers should be washed down with hot soda water, and taps should be left running for half an hour or so to flush out the drain and half channels.

Some legal considerations

House owners should be aware of the fact that their responsibility for their house drains does not end at the boundary of the property. It extends to the public sewer usually situated in the highway.

Blockages and other defects in the length of drain between the property boundary and the sewer are fortunately rare. Remedying them can be very expensive especially when it is necessary to excavate to expose the defective drain. Builders should therefore exercise extreme care in laying this length of drain, in making the actual connection to the sewer and in back-filling.

Other, somewhat complex, considerations arise where a number of houses are drained in combination. This obviously saves initial costs.

There is only one deep excavation to the sewer, only one sewer connection and only one reinstatement of the road surface. The difficulty arises when a section of ‘combined drain’ becomes blocked or otherwise defective and responsibility for remedying the defect has to be determined.

The legal position is that combined drains of this kind, constructed before the coming into effect of the Public Health Act 1936 on 1 October 1937, are ‘public sewers’- but they are public sewers with a difference. The sewerage authority is responsible for any repairs, maintenance or ‘cleansing’ that may be necessary but can recover the cost of maintenance and repair from the owners of the properties concerned. ‘Cleansing’ is generally taken to include the clearance of blockages and, except in the case of a recurring blockage due to a defect in the ‘public sewer’ this will usually be undertaken free of charge. Drains serving more than one property that were constructed subsequent to 1 October 1937 are ‘private sewers’ and are wholly the responsibility of the owners of the properties concerned.

Details of the connection to the private sewer and the responsibility that this entails should be set out in the deeds of each individual house. It cannot be stressed too strongly that owners of properties connected to such a sewer should establish, with the other owners concerned, responsibility for repair, maintenance and clearance before trouble actually occurs.

It is not unusual for as many as ten or twelve properties on a new housing estate to be connected to a single private sewer. A blockage or other defect near to the public sewer may have no visible effect whatsoever on the drainage of properties at the head of this private sewer though their sewage may be flooding the gardens of houses at a lower level. It is far too late to attempt to explain the situation once this has occurred.

When in doubt consult the Environmental Health Officer of the local District or Borough Council. His advice may lack legal authority but is likely to be based on common sense and experience of a score of similar situations.

Rural drainage

Everything so far written in this post has assumed the existence of a public sewer to which the house drains will be connected. This is by no means necessarily the case. There are many rural areas where no public sewerage system exists and many isolated houses to which a public sewer can never economically be taken. Such houses, if they are to have a water carriage system of drainage, must be connected either to a cesspool or a septic tank.

Details of the construction of a cesspool or septic tank system are hardly appropriate to a beginner’s guide to plumbing but it is important that all householders and builders who may be concerned with such installations should be aware of the differences between a cesspool and a septic tank and should be familiar with the principles involved.

A cesspool is simply a watertight underground chamber intended for the reception and storage of sewage until such time as it can be pumped out and disposed of. Cesspools may be constructed of brickwork rendered watertight with sand and cement, of precast concrete rings set into a concrete base or of fibreglass reinforced plastic.

Capacity may be as little as 500 gallons or may be 4000 gallons or more. The Building Regulations prescribe 4000 gallons as the minimum capacity of a modern cesspool but, of course, the great majority of existing cesspools were constructed long before the Building Regulations came into effect.

Cesspool owners are constantly surprised at the speed with which their cesspools fill and require emptying. A little thought will establish that there is nothing very surprising about this.

It has been estimated that water consumption for all domestic purposes-drinking, cooking and preparation of food, baths, laundry, w.c. flushing and so on-amounts to between 20 and 25 gal per person per day. Every drop of this water passes into the drain, and thence into the cesspool, in one form or another. Such a family could therefore fill a 500 gal cesspool in less than a week and a 1000 gal cesspool in under a fortnight!

The fact that cesspools do not often require to be emptied quite as frequently as this is usually attributable to the fact that few cesspools are wholly watertight. This too however can be a mixed blessing. A cesspool that permits its contents to leak out will permit subsoil water to leak in. A leaky cesspool, in an area where subsoil water level is high, may, during prolonged wet weather, fill up again almost before the Council’s cesspool emptier has returned to its depot.

The prospective purchaser of a property with cesspool drainage should check the capacity of the cesspool and the availability and cost of the local cesspool emptying service. The local authorities of districts in which unsewered properties are situated normally operate a cesspool emptying service. If they do not do so the Council’s Environmental Health Department will certainly be able to suggest a local private contractor. These are matters that deserve very careful consideration. Cesspool emptying is, at the best, a smelly and unpleasant operation. It can also prove to be a very expensive one for the householder.

A septic tank system is quite unlike a cesspool in that it offers a permanent solution for the disposal of sewage-not merely for its storage. It is, in fact, a small private sewage treatment plant. Small septic tank systems may serve individual houses while larger ones can be used to cope with the sewage of isolated groups of houses. To understand the operation of a septic tank installation it is essential to know something about the chemical and bacteriological processes of decomposition.

In the public mind bacteria or ‘germs’ are generally thought of as being malign organisms responsible for the spread of disease. In fact the bacteria responsible for disease are the exceptions. The overwhelming majority of the myriads of bacteria with which the world teems are wholly benevolent to other forms of life. Neither vegetable nor animal life could continue without their activities. These activities consist of the breaking down, or decomposition, of dead organic matter into its basic chemical constituents. The action of bacteria breaks down organic matter, first into offensively smelling ammoniacal compounds and then into harmless nitrites and nitrates. The nitrates provide nourishment for plants, some of which become the food of animals—including ourselves. Animals, and of course dead and dying plants, produce dead organic matter which is again broken down into its chemical constituents by bacterial action. This cyclical action is described as the ‘nitrogen cycle’. AH life on this planet depends upon the continuation of this cycle in which bacteria play a vital role. The purpose of a septic tank installation is to ensure that the bacterial process of decomposition takes place rapidly and under controlled conditions.

The septic tank itself is an underground chamber designed to retain sewage for at least twenty four hours. Sewage enters and leaves by means of dip pipes extending well below the level of the liquid in the tank when full. The surface of the liquid therefore remains undisturbed. A baffle, or system of baffles, may also be provided within the tank to prevent the rapid flow of water from inlet to outlet when, for instance, a bath is discharged. Within the septic tank sewage is liquefied by the action of anaerobic bacteria-bacteria which cannot live in the presence of free oxygen. A scum forms on the surface of the liquid in the tank, retaining the unpleasant smells, and a sludge forms at the base. The sludge must be pumped out from time to time. Twice yearly desludging has been recommended but this is probably a counsel of perfection. It is not unusual for septic tanks to operate satisfactorily for several years without desludging.

Dosing siphon

Anaerobic action within the septic tank is only the first part of the bacteriological process of purification. The liquid effluent from the tank must next be submitted to thorough and systematic aeration to encourage aerobic bacteria to complete the process. This is usually done by permitting the effluent to percolate through a ‘filter’ bed of clinker or granite chippings. One cubic yard of filtering or aerating material is required for every forty gallons of estimated daily flow.

Arrangements must be made to ensure that the effluent is distributed evenly over the filter bed. Larger installations will be circular and the effluent will be distributed by means of revolving arms activated by the weight of the liquid leaving the septic tank. Simpler arrangements are permissible with smaller installations. A tipper device may be used to spill the effluent onto perforated corrugated asbestos sheets, first on one side of the filter and then on the other. The Gibson-Ingol annular septic tank and filter unit has a septic tank, circular in plan, in which the inlet to the tank is at its centre and the effluent spills over onto a circular filter bed provided round the outside wall of the tank. The effluent from the filter can usually be discharged directly into any convenient ditch or stream. With large installations however a further small settlement tank, with baffles, may be provided to trap the small particles of black ‘humus’ present in the final effluent.

Where there is a light and absorbent subsoil, no sources of water in the vicinity and a sufficient area of land, it may sometimes be possible to dispense with the filter section of the septic tank unit and to dispose of the effluent by subsoil irrigation. Land drains for this purpose should be laid flat, or almost flat, on a 1ft deep bed of clinker or brick-bats. There should be a further 1ft of this material on each side of the pipe line and above it. To reduce the risk of the irrigation system becoming clogged with grit washing down from the soil above it is a good idea to cover the bed of clinker with a polythene sheet before back-filling the trench.

Traditionally, land drain pipes are of earthenware and are laid butt jointed. However perforated pitch fibre or p.v.c. land drainage pipes are obtainable in long lengths and offer a much more satisfactory modern alternative.

It is usual for the effluent from the septic tank to flow directly into the pipes of the irrigation system. This tends to result in the soil in the immediate vicinity of the tank becoming very heavily charged with sewage while little or no effluent reaches the further end of the pipe line. The provision of a final dosing chamber with an automatic siphon will prevent this. The level of effluent will rise in the dosing chamber until it reaches the inverted U bend of the siphon. Siphonic action will then take place, emptying the dosing chamber and distributing the effluent throughout the land drainage system. This ensures even distribution of the effluent, prevents the soil in the immediate vicinity of the septic tank from being overloaded and soured and ensures a period of time, after each flush, in which the soil bacteria can act upon the discharged effluent. A properly designed septic tank installation requires little or no maintenance beyond periodic removal of sludge from the septic tank.

Excessive use of disinfectants should be avoided within the home since these will destroy the bacteria upon which septic action depends as well as the germs of disease. For the same reason the salt wash from a mains water softener should not be permitted to flow into a septic tank. Brine has antiseptic qualities.

For a rather different reason the excessive use of household detergents should be avoided in a house drained to a septic tank. These tend to emulsify the fats present in sewage. Instead of a scum forming on the top of the septic tank and a sludge at the bottom, the tank will be filled with a liquid of soup-like consistency that will be washed through to clog the filter or land drainage system.

Rain water, whether from roofs or yard surfaces, should of course be rigidly excluded from any cesspool or septic tank drainage system.

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