Wonders of World Engineering

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Below the streets of London is an amazingly complex system of tunnels which carry thousands of miles of electric cables, gas mains and the like, in addition to the underground rivers which drain the huge area


ELECTRIC CABLES are carried beneath the River Thames in a tunnel from Deptford Power Station

ELECTRIC CABLES are carried beneath the River Thames in a tunnel from Deptford Power Station, London, to the north side of the river. The large cables are so heavy that they are made in short lengths and spliced together in position. The jointer is at work splicing the cables in the tunnel. In the London area are over 300,000 miles of underground telephone wires and nearly 4.000,000 miles of underground local wires.

APART from the railway tunnels and tubes which run beneath the streets of London, and which deserve a chapter to themselves, there are innumerable tunnels containing electric cables, gas mains and so on. Perhaps the most important of these underground systems is that which drains the city. There is something almost uncanny about the strange and extensive underground rivers which run through the length and breadth of London, far below the level of the busy and familiar streets. Those rivers are the city’s sewers, all of which are under the control of the London County Council.

London’s sewerage is carried out on what is known as the “combined” system. One set of channels carries off the waste from houses, offices, factories and slaughter houses, as well as the normal flow of rain water from the surfaces of the streets. In other cities the two types of sewage are sometimes kept separate, but this plan has not been adopted in London. Early in the

nineteenth century the sewers consisted simply of the old London streams - the Fleet, the Tyburn, the Walbrook - and the other lesser rivulets, the names of some of which have survived in those of certain well-known streets.

After 1847, when the old Metropolitan Commission of Sewers was formed, it was made compulsory for houses to be drained into the sewers, and the whole of the city’s effluent was carried in one set of channels. The engineers were faced at once with the double problem of inadequate channels and dangerous pollution of London’s river where it flowed through the heart of the city. So, during and after the ’fifties, they began the building of a complete new system of channels, leading the flow eastwards to outfalls in the lower Thames. The old brooks and the new storm water sewers were allowed after that to carry away abnormal flows of rain water from the streets.

Thus later developments have been carried out on the “combined” system of sewerage which was tried and proved in the middle of the nineteenth century. On the whole this arrangement, old as it is, works well. The London streams could never carry off the entire amount of water which even a fairly sharp shower sends through the street gullies. The planning of London’s main drainage was originally carried out by Sir Joseph Bazalgette.

The total flow is much increased in rainy weather, and London’s engineers had to plan accordingly. Nearly all the water from the road surfaces and from the roofs of buildings finds its way into the sewers. If all rain water had to be carried off through one channel, half an inch of rain in one hour over London north of the Thames would need a canal 500 feet wide and 10 feet deep, flowing at a speed of 200 feet a minute. Yet during severe rainstorms or thunder showers the sewers have not infrequently to cope with a fall of more than one inch an hour over part of London. Even were it practicable to build such a giant channel, difficulties would still arise in outlying parts of London a long way from the Thames and poorly served by natural watercourses.

AN INVERTED SIPHON is built to carry a sewer under an obstruction

AN INVERTED SIPHON is built to carry a sewer under an obstruction such as an underground railway or an open watercourse. This diagram shows the usual arrangement of an inverted siphon. From an inspection chamber equipped with a manhole the sewage passes through a sliding gate or penstock into an inclined passage leading to a vertical upflow channel provided with a sump.

The “separate” system of sewerage might have been carried out had London’s sewers been developed later and more rapidly than they were. The system has many advantages. The boroughs of Greenwich, Woolwich, Lewisham and Wandsworth, south of the Thames, and Hackney on the north side, were equipped later than other parts of London. A partly “separate” system was tried by their engineers and found successful. The main backbone of the London system was carried out by the old Metropolitan Board of Works, which succeeded the Metropolitan Commission of Sewers in 1856 and gave place, in its turn, to the London County Council in 1889.

The main sewers of London run from west to east, and the storm water relief sewers - the old brooks - run into the Thames approximately at right angles. The sewers of South London converge at a point near Plumstead, whence an embankment carries two of them to the Crossness Outfall, a third being laid below ground level at the side of the embankment. The layout on the northern side of the Thames is somewhat different, the sewers of the farther districts converging at Old Ford on the head of the great Northern Outfall Sewer, which is met again at Abbey Mills by the sewers coming from the districts lying nearer the Thames.

There are five great sewer mains on the north side. Nearest the river are Low Level Sewers Nos. 1 and 2, both beginning at Hammersmith and ending at Abbey Mills. No. 1 runs along under the Chelsea Embankment, then under the Victoria Embankment; No. 2 passes under South Kensington and the Strand. Next come the two middle-level sewers. Of these, No. 1 originates at Wormwood Scrubs and passes under Bayswater and Piccadilly, and No. 2 runs from Kensal Green, under Paddington and the district north of Regent’s Park and Islington. Finally, there is the High Level Sewer, which comes from Hampstead and takes a rather more devious route than its fellows, passing under Stamford Hill and then Hackney. The last three mains all converge on the Northern Outfall Sewer at Old Ford.

On the south side of the Thames are two big sewers, Low Level Nos. 1 and 2, running between Wandsworth and Deptford. From Putney to Deptford runs Southern High Level No. 1, serving Wandsworth, Clapham and Brixton on the way. Southern High Level No. 2 runs under Brockley, Lewisham and Blackheath, parallel with the two Southern Outfall Sewers between Plumstead and Crossness on the Thames. Finally, there are various large branch sewers serving the crowded neighbourhoods of South London in the Lewisham and Dulwich areas. These nine great sewers, on either side of the river, are served throughout their courses by innumerable street sewers, one of which lies under every roadway in London. They receive also the flow from the sewerage systems of several local authorities lying outside the area administered by the London County Council.

In 1935, the latest year for which figures are available, there were 400 miles of main sewers in London - enough to form a continuous channel from London to Edinburgh. The aggregate length of ordinary street sewers amounts to more than 2,500 miles.

TPNEUMATIC SEWAGE EJECTOR invented by Isaac Shone he ordinary drains connecting houses with street sewers are a familiar sight to everybody. In the houses all are provided with traps. A trap consists of a bend in the pipe constantly filled with water to sufficient depth to cover the top of the bend, thus providing a complete seal between the outlet pipe and the air of the habitation. The standard trap for use in baths and sanitary fittings consists of a curved S-bend, completely free from angles and corners, which remains clean automatically through the flow of water passing through it. The history of this simple and effective device takes us back many years. In 1775 a Mr. Cummings sought to prevent the dangerous passage of foul air into houses. His invention took the form of a water closet with an S trap beneath it, the first of this kind in the world, and now a universal arrangement. Iron soil pipes lead the effluent from water closets to the house drains. Pipes of small diameter carry off washing water and bath waste. These pipes lead into open heads or gullies, thus providing a complete break between the house drain and the bath, basin or sink.

PNEUMATIC SEWAGE EJECTOR invented by Isaac Shone and designed to raise sewage from low-lying drains to the main sewers. Sewage coming from the inlet fills a chamber until it raises a float which opens a valve admitting air under pressure This forces the sewage up the discharge pipe to the main sewer.

The gully is trapped in its turn, and a large trap has to be passed by the entire effluent of the house before it reaches the street drain. The house drain consists of earthenware pipes 4 in. or more in diameter, according to the size of the house. They are jointed together and rendered watertight by cement, the joints being perfectly smooth on the inside. An inspection chamber with an airtight lid, through which the house drain passes on its way to the sewer, enables any local stoppage to be located.

The possibility of water siphoning out of the traps is prevented by adequate ventilation of the house drain through pipe shafts communicating with the outside air. With this battery of traps, each house is protected against the possibility of sewer gas entering the rooms. Want of experience in the ventilation of trapped drains was the cause of much illness in days gone by. Modern methods have completely eliminated such dangers from the well-regulated drainage system of London. The sewers are not pipes, but channels, sometimes on a large scale. The ancient sewers were rough brick tunnels. During the last century, careful designers evolved an even brick channel, egg-shaped in section, with the more pointed end of the “egg” downwards. Access to it from the street is provided by manholes placed at more or less regular intervals, and ventilating shafts communicate with the outer air in the crown of the road, where the possible discharge of sewer air becomes diffused before it can reach pedestrians or the inmates of houses flanking the street. To cope with the extra needs for ventilation, shafts are carried high above the street level in iron columns, resembling the ventilating extension of a house soil pipe, but rather wider. A problem arose because solid or greasy matters adhered to the sides and caused stoppages. This was solved by glazing the brick linings.

As Large as Tube Tunnels

The larger sewers of to-day may be of brick, concrete or iron. They vary in diameter from 4 ft. 6 in. to 11 ft. 4 in. The largest may, therefore, be compared with the tunnels of tube railways. Gauges, which record the maximum rise in the level of the water, are fitted at many inspection points. In showery weather such rises are sometimes alarmingly rapid, and adequate means have to be provided to enable workmen engaged in the sewers to escape from a sudden spate. The familiar grids, through which rain water passes from the street gutters into the sewers, are always equipped with traps, cutting off the possible escape of sewer air to the pavements. An accumulation of solids, such as dead leaves, empty cigarette cartons and all the jetsam of the public street, forms in these traps and has to be removed periodically.

The coming of motor transport on the London roads brought new and almost waterproof road surfaces, which greatly increased the flow of surface water into the sewers. The war of 1914-18 delayed improvement for a while, but in due course miles of new sewers and modern appliances for pumping and diverting the water had to be brought into use. These included the Hammersmith pumping station, which is capable of dealing with 1,000 tons of storm water sewage every minute.

A sewer may have to pass under an obstruction such as an open watercourse or an underground railway, the bottom of which is below the level of the sewer. The problem involved was overcome, after experiments, by what is known as an inverted siphon. In this the sewer enters an inspection chamber provided with the usual manhole.

LINING THE INVERT of a sewer with blue bricks

LINING THE INVERT of a sewer with blue bricks. This sewer runs from Hammersmith to Bow, and its cast-iron segments resemble those of a tube railway tunnel. In London alone there are 400 miles of main sewers and more than 2,500 miles of ordinary street sewers.

From this inspection chamber the sewage passes over a penstock into a steep downward dip, then under the obstruction at its usual inclination into a perpendicular upflow channel. This also gives on to an inspection chamber, at the bottom of which the upcoming sewage passes over a dip and resumes its interrupted course. A sump is provided at the bottom of the upflow, from which any deposit formed can easily be removed. Inspection chambers attached to the sewers are sometimes of considerable depth, and iron steps are let into the perpendicular walls to enable the sewermen to reach the platforms, or benches on either side of the channel, without the use of ladders.

Another interesting feature of sewer design is the form of junction used where a lesser sewer passes into one of the main sewers, the latter being situated at a lower level. Between the two is a drop pipe, the flow through which is controlled by a penstock situated in a spacious inspection chamber. Entry from the street above is provided by the usual manhole. Another manhole gives access from the penstock chamber to the benches on either side of the main sewer down below.

The ideal sewer has to have a perpetual slope and must keep itself clear by the force of gravity acting on its contents. In such a complicated area as London we cannot expect to find such an easy state of affairs as this. A considerable amount of pumping has to be done to keep the sewage perpetually on the move, whatever the time of day and whatever the state of the weather. A stagnant sewer becomes nothing more or less than an elongated cesspool, and as such is not to be tolerated for a minute. With a good flow and a fair gradient all is well; to be self-cleansing a 21-in. sewer running half full of water needs a gradient of 1 in 600. The narrower the sewer and the smaller the flow, the steeper must be the inclination. An 8-in. pipe running one-quarter full needs an inclination as steep as 1 in 85.

In days gone by, sewage pumping did not exist and many channels were stagnant. To cope with this the Victorian designers adopted steam pumping engines. Steam power is now largely superseded by gas, oil and electricity.

Five Parallel Tunnels

Electric and oil-driven centrifugal pumps are used at Abbey Mills and Deptford; these pump most of the sewage flowing in the Northern and Southern Outfalls. At one time certain Continental towns, particularly in Holland, used a pneumatic system of removal to the total exclusion, of the familiar water-carriage system. British engineers, however, did not favour this peculiar arrangement. In the latter part of the last century, the problem of raising sewage from low-lying drains to the main sewers was solved by an ingenious application of compressed air.

The pneumatic sewage ejector invented by Isaac Shone Was first applied in the West London District Schools, and its use extended to various public buildings, including the Houses of Parliament. Its principle is simple. When the sewage in the low-level chamber reaches a certain level, it lifts a float, which is in connexion with an air valve. Immediately this valve opens, the surface of the sewage is subjected to powerful air pressure, which forces it Upwards through pipes to the higher level, whence it runs off into the sewers.

The level having fallen again, a certain amount of water is left in a dish-shaped vessel below the float, on the same axis, the weight of which causes the float to fall again, closing the air valves and allowing for another charge of water in the low-level chamber. Care has to be taken with the installation of pneumatic appliances for dealing with sewage, because of the consequent escape of sewer air into the outer atmosphere. No air escape should have its outlet anywhere near windows, chimneys or ventilators.


SUBWAY FOR CABLES AND MAINS in Holborn. This subway, 12 feet wide and 7 ft. 6 in. high, contains gas mains in brackets on either side, water and hydraulic mains (left), and two runs of iron troughing containing electric cables. Telephone cables are carried in the racks seen on the right. Subways such as these enable inspections and repairs to be carried out without disturbing the surfaces of roads.

The long, straight embankment of the Northern Sewer Outfall is a familiar sight to those who know the Essex flats. Between Abbey Mills and the Thames at Barking Creek, it consists of five parallel tunnels 9 feet by 9 feet, built partly of cast iron and partly of brick. Between Abbey Mills and Old Ford there are four tunnels. At first, crude sewage passed straight into the river at the outfalls, whence the tide carried it upstream, and formed noisome mud-banks out of the drifting sludge. To prevent this from happening to-day, the engineers have built huge precipitation tanks in which the sludge or solid matter is allowed to settle. The fluid constituents pass into the Thames estuary and the solid is passed on to big hopper steamers, of 1,000 to 1,500 tons gross. These vessels carry it out to Black Deep in the North Sea, some twelve miles off the Essex coast, and it is released into the purifying salt water through large valves forward of the screws, which diffuse the sewage as it is discharged.

Until 1916 the fluids passing into the Thames were treated with 4 grains of lime and 1 grain of proto-sulphate of iron to a gallon. Wartime difficulty in obtaining the necessary chemicals put a stop to this method. The activated sludge process is now used for part of the flow. By this system, the liquid is treated with chlorinated copper sulphate in settlement tanks and then allowed to flow along lengthy channels into another set of settlement tanks where the inflow is at the centre and the outflow at the periphery. This causes retention of the solids and discharge of the liquid sewage. For the rest of the flow further purification is not deemed necessary after the effluent has left the settlement tanks.

On the south side of the Thames, the Southern Outfall between Plumstead and Crossness consists of three circular section brick channels, each 11 feet in diameter, one of them being an extension of the Southern High Level Sewer No. 2. The same means are adopted at Crossness as at Barking Creek.

The normal discharge of the outfalls amounts to 250,000,000 gallons in twenty-four hours, but their capacity is 1,500,000,000 gallons a day. To this must be added 1,136,000,000 gallons of storm water, which gives a total volume of 2,636,000,000 gallons in one day. The storm-water sewers can conduct a further 4,000,000,000 gallons into the Thames in the course of one wet day. That is what London has to show for a rainfall of 2½ inches in one day.

Such are the means by which London is kept healthy and clean. Bringing them to their present state of efficiency has been a long fight. April showers seem trivial enough, and we think little of emptying a bathful of soapy water. Neither could be dealt with were it not for the vast, hidden channels which stretch in the fashion of a giant gridiron below the surface of the city.

Although the water supply and sewerage systems are the most important of the subterranean services in London equally vast installations exist in the interests of gas, electricity supply, telephones and telegraph services, and all their accessories. The story of these systems is one of continual straightforward work compared with the struggle against inexperience and unknown difficulties which had to be carried on by the designers of the sewers.

9,000 Miles of Gas Mains

The main supply pipes of the Gas Light and Coke Company, the largest and oldest of the London gas authorities (see pages 59-64), vary in diameter from 4 in. to 48 in., and consist of cast iron or cast steel tubes. Formerly a certain amount of internal corrosion went on, but modern science has eliminated the presence of water from coal gas, and the bubbling pipes so familiar to our grandparents are no more. Two 48-in. mains connect London with the great “gas town” of Beckton, in Essex.

JUNCTION BETWEEN LESSER AND MAIN SEWERSLow-pressure gas is a poor substitute for electricity in street lighting. The gas engineers vindicated their industry by increasing the pressure for large street lamps from 15 lb. to 19 lb. to the square inch, and separate mains had to be provided for this purpose.

The Gas Light and Coke Company has over 5,000 miles of mains in the London area. The pressure in the mains is automatically regulated by pressure governors, some installed underground and some on the surface. Altogether the London area contains more than 9,000 miles of gas mains, spread over an area of more than 900 square miles.

JUNCTION BETWEEN LESSER AND MAIN SEWERS is effected through a penstock and a drop pipe connecting the two. The flow through the drop pipe is controlled by the penstock, which is situated in a spacious inspection chamber. Access to the inspection chamber is given by a manhole at ground level and by a series of steps in the walls. The main sewer is at a lower level than the branch sewer, and another series of steps give access to it from the inspection chamber.

In addition to the great channels and pipe lines comprising London’s gas, water and sewerage systems, there are thousands of miles of electric cables, telegraph and telephone wires. Modern practice sends the telephone wires underground in great cities. London is no exception to this rule, by far the greater proportion of the telephone wires being carried along under the streets. This makes for neatness, and also protects the services from the effects of stormy weather. The London area contains more than 300,000 miles of underground telephone trunk lines, and nearly 4,000,000 miles of underground local wires.

The electric supply cables, carried under the streets, consist of stranded copper wire surrounded by insulating material and armoured with an outer lead sheath. Outside this, in its turn, there may be an additional reinforcement of hemp and steel strands, which are proof against normal corrosion. Particularly corrosive soil, however, had to be guarded against. The electrician took defensive measures by enclosing the entire cable in a pipe or trough, or a series of cables in a system of compound pipes, each resembling an elongated honeycomb.

The London electric light cables sometimes find their way into strange places. The Tower Subway, London’s first tube, was built in 1870 and fell into disuse when the Tower Bridge was opened. As a tube it was rather a fiasco, but it now carries electric cables under the river, a purpose for which it is ideal.

One more underground public service which remains to be described is the Kingsway Subway. This is the only tunnel in London devoted exclusively to the passage of street tramcars. The Kingsway Subway was built in the first decade of the present century to connect the great tramway systems of North and South London. Under Kingsway, the building of the tramway tunnel and of the street went on simultaneously, the “cut and cover” system being used, as it was on the earliest sections of the underground railway system. Entry from Southampton Row, at the northern end of the subway, is effected by an inclined plane. At the southern end, below Waterloo Bridge, the cars pass to and from the Victoria Embankment through an ordinary tunnel mouth. Two intermediate stations are provided, at Holborn and Aldwych.

A Subterranean Artery

The subway was opened on February 24, 1906, between the inclined plane at Southampton Row and the station at Aldwych. The final section from Aldwych to the Embankment, with its numerous sharp curves, was a more difficult piece of work. Large buildings were above the route, with a network of sewers, mains and cables beneath them. The L.C.C. engineers had to divert these obstructions without interruption of the service. Not until 1908 did cars begin to run right through the subway, the opening date being April 10. As originally built, the subway could accommodate only single-decked cars, and as trailers were not allowed, this caused a great measure of overcrowding. The overcrowding became worse as more people realized the great usefulness of this link in London’s passenger transport system, and at the end of the nineteen-twenties the London County Council, which then worked the trams, decided to enlarge the tunnel.

It was closed to all traffic in 1930, and work on increasing its internal height began immediately. This was not easy. It was impossible to raise the roof under Kingsway. So the floor of that section had to be excavated from end to end. At the same time it was not practicable to sink the southern section below the level of the Embankment. So at that end the roof had to be raised and the old obstructions were again encountered. In spite of great difficulties, the whole rebuilding programme was carried out in less than a year, and on January 15, 1931, the first standard double-decked cars, universal on other London tramways, passed through the subway.

The Kingsway Subway has an approximate length of three-quarters of a mile. Its straight section, north of Aldwych, is 20 feet wide and 16 ft. 6 in. high from rail-level to roof. The electric conductor rails are carried in underground conduits between the running rails, as on many of the tramway routes formerly administered by the London County Council.

UNDER THE RIVER THAMES a tunnel with a diameter of 10 feet

UNDER THE RIVER THAMES a tunnel with a diameter of 10 feet was built to carry electric cables across the river from Battersea Power Station. The tunnel - one of two - is about 1,800 feet long. The second tunnel has the same length but a diameter of 8 feet.

[From part 18, published 29 June 1937]

You can read more on “The First Thames Tunnel”, “London’s Underground Railways” and “The Story of Gas Production” on this website.

You can read more on “Railways Under London” in Railway Wonders of the World.

Subterranean London