The Story in a Tunnel
How a Tunnel Is Dug Under Water
Fig. 1. On the left is a cross section showing, in diagram, the back view of a shield. The heavy black circle is the “tail” or “skin.” The small circles within the tail are the hydraulic rams which at a pressure of 5,000 pounds to the square inch force the shield forward. The square compartments within the shield are the openings through which the men pass to dig away the ground. In the middle of the shield is shown the swinging “erector” which picks up the iron lining plates and puts them in position.
The view on the right is a longitudinal section of the tunnel showing the shield and the bulkhead wall across the tunnel with the air locks built into it. The front of the shield ahead of the doors is made with a sharp edge called the “cutting edge” and this makes it easier for the shield to advance in case all the ground in front has not been removed. This view shows how the tail overlaps the last portion of the iron lining.
Some distance behind the shield comes the concrete bulkhead wall with the air locks contained in it. There are two shown in the view. The upper one is the emergency air lock, always kept ready so that in case of an accident the men have a means of escape even though the lower part of the tunnel is filled with rushing water or mud. The lower air lock is for the passage of men and materials during ordinary working. This view also shows that all the tunnel ahead of the bulkhead wall is under compressed air while the finished tunnel behind the bulkhead wall is under the ordinary or normal air pressure. When the tunnel is finished the air locks and bulkhead walls are removed.
How Tunnels Are Built
These notes describe very generally the way in which tunnels are built through mud and gravel under parts of the sea or large rivers in such a way that the men who build them are protected and as safe as the carpenter who is building a house.
The way these tunnels are built is called the “shield” way because the machine used is called a shield. It is given this name because it shields the tunnel builders from the water and the mud which are ready at every moment to overwhelm them and kill them.
The shield was invented in 1818 by a great Engineer, Marc Isambard Brunel, who was a Frenchman living in England. The idea of the shield came to him as he saw how the sea worm which attacks the wooden piles of docks along the shore bores the holes it makes in the wood. The head of this worm is very hard and can bite its way through the hardest woods. As it goes through the wood its body makes a hard shelly coating which lines the holes which its head has made and prevents the hole from getting filled up. This is the general idea of a tunnel built by a shield.
The first shield was used by Mr. Brunel to make a tunnel across the Thames River at London, England. This is still the biggest tunnel ever built by a shield, although not the longest, and is still used by railroad trains. This tunnel was begun in 1825 and was finished in 1843, and provides a history of almost unexampled and not-to-be-excelled courage in attacking difficulties and skill in defeating them.
Since the days of Brunel many great improvements have been made in the shield and in the way of working it but the same idea is still there.
After the days of Brunel’s shield another great help was given to tunnel builders by the invention of the use of compressed air to hold back the water which saturates the ground in which the tunnel is being built.
WHO INVENTED THE COMPRESSED AIR METHOD
The first real invention of compressed air for this purpose was made by Admiral Sir Thomas Cochrane who, in 1830, took out a patent for the use of compressed air to expel the water from the ground in shafts and tunnels and, by this means, to convert the ground from a condition of quicksand to one of firmness. This patent covers all the essential features of compressed air working.
As suggested above, the thing which compressed air does in a tunnel is to push the water out from all the spaces which it fills in the ground, so that the men who are digging away the ground for the tunnel are working in firm dry ground instead of a mixture of earth and water which will run into and fill the hole they dig as soon as it is dug.
Whenever a tunnel is being built below a body of water through ground which is porous, or in other words through any ground except solid rock or dense clay, the water fills every crevice and space in the ground and is exerting a pressure of about half a pound per square inch above the ordinary pressure of the air, (which is 15 pounds to the square inch) for every foot of depth below the surface of the water; so that supposing the tunnel is 40 feet below the water the water has a pressure of nearly 20 pounds per square inch on every square inch of the surface of the tunnel. This pressure causes the water to flow violently into any hole or opening that is made in the ground, and, unless the water is prevented from moving by some means or other, the opening made would be very quickly filled with water and also with ground as the rush of water will carry the sand, gravel or mud with it.
By Cochrane’s invention the whole tunnel is filled with air under a pressure equal to the pressure of the water. This compressed air therefore balances the pressure of the water and holds it back from moving, and if the pressure of the air is made slightly greater than that of the water the water is driven back from the tunnels for a short distance so that when the tunnel is being dug the ground instead of being wet is quite dry.
This explains the principles of the shield and compressed air way of making a tunnel.
The following describes very shortly how these principles are put to actual use.
Most tunnels which are built by shield and compressed air under rivers or arms of the sea are lined with cast iron plates to protect the railway or roadway which is in the tunnel.
The tunnel is a circular tube, or shell, and the plates have flanges on all sides which are bolted together. This shell is put into place, plate by plate, by means of the shield which not only protects the workmen and the work under construction, but which helps to build the iron shell. In fact it corresponds to the sea worm which bores through the wood and lines the hole with a shell. In the case of the tunnel the shell is made of iron. The shield itself consists of a steel tube or cylinder slightly bigger in diameter than the tube or tunnel it is intended to build. The front edge of this shield is made up of a ring of sharp edged castings which form what is called the “cutting edge.” Just behind the cutting edge is a bulkhead or wall of steel, in which are openings which may be opened or closed at will. Behind this bulkhead are placed a number of hydraulic jacks or presses arranged around the shield and within it, so that by thrusting against the last erected ring of iron lining the whole shield is pushed forward. The rear end of the shield is a continuation of the cylinder which forms the front end, and this part, called the “tail,” always overlaps the last few feet of the built up iron shell.
HOW THE SHIELD CUTS THROUGH THE GROUND
The diagram, Fig. 1, shows more clearly what is meant. From an inspection of Figure 1 it is clear that, when the openings in the shield bulkhead are closed, the tunnel is protected from an inrush of either water or earth; the openings in the bulkhead may be so regulated that control is maintained over the material passed through. After a ring of iron lining has been erected within the tail of the shield, the shield doors are opened and men go through them and dig out enough earth for the shield to go ahead. The rams are then thrust out thus pushing the shield ahead. Another ring of iron is built up within the tail for which purpose an hydraulic swinging arm, called the “erector,” is mounted on the shield face. This erector picks up the plates and puts them into position, one by one, while the men bolt them together. Excavation is then carried on again and the whole round of work repeated, gaining every time the jacks are rammed or thrust out a length equal to the length of one ring of iron lining. In carrying out this work in ground charged with water the shield is assisted by introducing compressed air as described before. To use the compressed air thick bulkhead walls of masonry are built across the tunnel behind the shield and into the space between the shield and the bulkhead wall air is pumped, compressed to the same pressure as that of the water in the ground, or in other words the pressure of the air in pounds per square inch is about half the number of feet the tunnel is below the water surface. This dries the ground and simplifies enormously the difficulty of working in it. The diagram, (Fig. 1) shows a bulkhead wall across the tunnel. In order to pass from the ordinary air outside the bulkhead into the compressed air inside it, all the men and the materials have to pass through the “air locks” which are built into the wall. They are called air locks because they are like the locks on a canal which raise the water from a lower to a higher level or lower it from a higher to a lower level as the case may be. The difference is that an air lock enables one to pass from air at a low pressure to one of a higher, or vice versa. An air lock is made like a large boiler with a door at each end. If we wish to enter the compressed air we enter the lock from the outside. The door at the end has been tightly closed to prevent the compressed air from rushing out. We close the door behind us and are now tightly shut in the boiler-like lock. We now open a valve and compressed air begins to flow quickly into the air lock and the air gets hotter and hotter, due to the compression of the air. Very likely an intense pain begins to make itself felt in the ears but by swallowing hard and blowing the nose it may be relieved. It is caused by the air pressure being greater on the outside of the ear drum than on the inside. If the delicate ear passages are choked, because of a cold or some such reason, it is unsafe to go further or the ear drum may burst. When the pressure in the air lock has reached that in the working chamber, the door leading to the shield may be opened and we can pass to the working space and note the work going on. There is no especial bodily sensation to be felt except a slight exhilaration and it is curious to find that one cannot whistle. On leaving the compressed air we enter the air lock by the door we left; a valve is turned and the air begins to escape and the pressure in the air lock begins to go down. As it does so the air becomes colder and colder and the whole lock is filled with a wet fog due to the chilling by expansion of the air. The air has to be allowed to escape very slowly, as bubbles of air and gas otherwise form in the blood vessels and tissues of the body giving rise to the very painful complaint known to tunnel builders as “the bends,” and in very serious cases to paralysis and even death. The higher the air pressure the more slowly must one come out into the ordinary air.
MAKING THE JOINTS WATER TIGHT
This shows the erector building up the iron lining in one of the Pennsylvania tunnels at New York. It shows clearly how the iron plates are bolted together to make the rings of iron lining.
The last, or closing, plate of each iron ring is called the “key,” and is much shorter than the others. This photograph shows the shield erector on one of the Pennsylvania tunnels picking up and putting into place a key plate. This picture gives an idea of the mud and dirt and wet in which the men who work in tunnels have to do their work.
Wherever possible, every space and crevice outside the iron lining is filled with cement forced, in a liquid state, through the iron lining by compressed air. This photograph shows the operation of “grouting,” as it is called. The man at the left is in control of the grouting. He has the hose, through which the grout is forced, screwed to a pipe which passes through a hole made for the purpose in the iron lining plates and called a “grout hole.” The two men in the middle of the picture are attending to the “grouting machine” by which the work is done. Water and cement are fed into the small boiler-like tank, the tank closed and compressed air admitted thus blowing the liquid cement through the hose and behind the iron lining. When no more grout can be forced behind the iron lining all the space has been filled. The man on the right is the engineers’ inspector taking note of how much grouting is done, and seeing that the work is properly carried out.
This shows the process by which the iron lining is made perfectly water-tight, so that, when the compressed air is taken off, no water at all can get into the tunnel. Two operations are shown here. One is called “grommetting the bolts,” the other is called “caulking the joints.” The two men on the left, hanging on to the wrench, are tightening up the bolts as tight as they can after having put on, underneath the washers at the head and nut of each bolt, a ring of spun yarn dipped in red lead and oil or tar or some such water-proof material. A few of these “grommets” may be seen at the feet of the third man from the left. The other four men are caulking the joints between the iron plates by driving into the joints a mixture of sal ammoniac and iron borings. This sets as hard as iron and if properly done makes a perfectly water-tight joint.
When the shield has been pushed across the entire length of the water way which has to be tunnelled, and the whole of the iron tube or shell is in place, a thick lining of concrete is placed inside the iron shell to protect it and make the tunnel stronger. As an added safeguard wherever the tunnel is in rock, gravel, strong clay or other ground which is not so soft that it does not close tightly in on the outside of the tube, liquid cement is forced by compressed air through holes made in the iron plates for this purpose. This liquid cement enters every pore or crevice in the surrounding ground and when it has set hard it still further protects the iron with a coating of cement. Pieces have been cut out of the iron lining of a tunnel built under the river Thames at London, England, in 1869, which showed that the iron at all places was as good as the day it was first put in forty years before, and iron put in the lining of the Hudson River Tunnel about 1878 when removed after thirty years was in perfect condition.
SHIELD AT END OF JOURNEY
Sometimes, however, shields are not driven to meet one another, but end their journey at some shaft or in some other tunnel previously built, after having gone through thousands of feet of all kinds of ground, from the hardest rock, which had to be blasted out foot by foot before the shield could advance, through hard pan, gravel, boulders, piles, rip-rap, made ground and mud so soft that it flows like melted butter. Naturally, after an experience like this a shield does not look as spick and span as when it started in life. This photograph shows one of the shields of the Hudson and Manhattan Railroad in New York just reaching the end of its journey, battered and bent but still in the ring.
This shows a piece of curved tunnel near Morton Street, on the Hudson and Manhattan Railroad, and is given because of the clear showing it gives of the iron lining. The track and floor are only the temporary roads for use during construction.
Sometimes it is necessary to make borings of the ground below the tunnels. In some of these bore holes vast quantities of water are found at a much higher pressure than the tunnel compressed air. This picture shows a spouting bore hole in one of the Pennsylvania tunnels during construction.
The last thing to do before laying the track is to put the concrete inside the iron lining. This picture shows this work going on and the wooden forms or ribs for holding up the concrete while it is setting.
THE LAND END OF A GREAT TUNNEL UNDER THE HUDSON
This view is given to show how complicated an underground structure may have to be made to take care of the requirements of traffic. This view shows the three great reinforced concrete caissons sunk through the earth at Jersey City in order to contain the switches and crossings required to form the New Jersey connections of the uptown and downtown tunnels of the Hudson and Manhattan Railroad.
These caissons were sunk under air pressure by excavating below them just as though they were tunnels turned up on end. In sinking these caissons the material passed through was water-logged made ground, and the hulls of two sunken canal boats were encountered and had to be cut into pieces small enough to be taken out through the locks.
The usual passenger rushing at high speed in the trains between Jersey City and Newark and New York has little idea of the very complicated structure necessary to allow of his doing so.
The information in this article was supplied by Jacobs & Davies, Inc., Consulting Engineers, 30 Church Street, New York, the Engineers for the Pennsylvania Railroad, Hudson River Tunnels, the Hudson and Manhattan Railroad, and many other tunnels in various parts of the world.
The illustrations were kindly supplied by the Pennsylvania Railroad and the Hudson and Manhattan Railroad.
DANGERS OF TUNNEL BUILDING
This account of tunnelling by shield and compressed air is very short and gives no more than a bare statement of the principles and chief methods of such work. Nothing has been said of the engineering difficulties involved in the design of such work, nor of the delicate surveying work necessary if one should hope to start two shields a mile or two apart and have them meet as shown in Fig. 13 like two great glass tumblers placed rim to rim after having travelled through thousands of feet of every kind of ground. Nothing has been said of the men who work on this most arduous form of subterranean navigation, how they cheerfully face the dark and the water ever threatening above them and the unseen but not less deadly ally, and yet foe, the compressed air, with its dreaded result, the bends, or the men on the surface who keep the air compressors running without pause or stop day in and day out until the work is done so that their comrades below may work in safety. Nothing has been said of the curious accidents that are liable to occur as when the air pressure in the tunnel gets too high, overbalances the water pressure and blows a hole through the river-bed and forms a geyser in the river above. It gives no account of the special difficulties which arise when special conditions are found; for example, when the lower part of the tunnel is in rock and the upper part is in soft material. In fact it is nothing more than a bare outline but it hoped that some, who may not be clear in their minds as to how tunnels are built, may learn some of the first principles of this most romantic kind of work from this bald narrative.