Wonders of World Engineering

 © Wonders of World Engineering 2014-21 |  contents  |  site map  |  info@wondersofworldengineering.com

Mobile Site

For thirty years obstacles baffled and tragedy thwarted all attempts to complete a tunnel beneath the Hudson River, New York, but at length the engineers achieved a lasting victory over the forces of Nature



A BEND IN THE HUDSON RIVER TUNNEL. As finally planned, the tunnel descends from the New Jersey side at a gradient of about 1 in 50, before rising to the Manhattan side. The depth below the river bottom varies from 5 to over 60 feet. The north tunnel has an external diameter of 19 ft. 5£ in., and an internal diameter of 18 ft. 1£ in. The south tunnel, built to accommodate tramcars, has an external diameter of 16 ft. 7 in. and an internal diameter of 15 ft. 3 in.

THE growth of New York, with its ever-increasing volume of traffic, induced American engineers in the ’seventies to consider the problem of finding a speedier outlet from Manhattan across the Hudson River to Jersey City on the mainland. Something more than the dozen lines of public ferries that plied to and fro on the mile-wide river was required to serve the needs of the population. A practical solution was a tunnel under the river, which would serve to link up the railways that had their termini on the New Jersey side of the river with the railways on the other side of the river in New York.

When the Hudson Tunnel Railroad Company was formed to build the tunnel there was little underwater tunnelling to which the engineers could look for guidance. As early as 1818 Sir Marc Isambard Brunel had taken out a patent for driving a tunnel by means of a circular shield that was to be forced forward by hydraulic jacks. The shield that he used to accomplish his stupendous task of tunnelling under the Thames did not, however, use hydraulic jacks.

In 1868 Barlow and Greathead drove a second tunnel under the Thames in about eleven months by using the Greathead shield. This tunnel was quite small, with a diameter of slightly less than 6 ft. 9 in. For a length of 1,350 feet it was driven in the London clay, through which the shield forced its way with great ease. This small tunnel for pedestrians, however, was noteworthy, because it was the first in which the Greathead shield was used, the first to be lined with iron rings and the first in which the space between the iron lining and the material round it was filled with liquid cement to consolidate the whole mass in the earth.

The Severn Tunnel, which was still in the making, could teach the American engineers little, because it was being cut through rock, whereas the Hudson tunnel would have to be built through silt and sand, with just a short length through rock. The problems of tunnelling under the Thames, under the Severn and under the Hudson were, therefore, distinct.

Having surveyed the best route and tested the river bed, the engineer in charge, Dewitt Clinton Haskin, began in November 1874 to sink a shaft from which to start the tunnel. He planned to use compressed air for the first time in history to keep back the water while he tunnelled under the river. Brunel and Lord Dundonald had formulated plans for using compressed air to tunnel through waterlogged soil, but no one had ever tried it. With great courage Mr. Haskin decided to put it to the test.

On the New Jersey shore, which was here reinforced with piles and stones to make a solid embankment, the American engineer started work 83 feet from the river bank. A circular trench having been dug with an outside diameter of 38 feet and an inside diameter of 30 feet, he built inside the trench a circular shoe of massive timbers, 10 in. by 12 in. As seen in section, the timbers were built in the form of a triangle standing on its point, which was surmounted by an iron cutting edge. The base of the triangle, which was uppermost, formed a wooden platform 4 feet wide, and the shoe was 4 feet deep, from the top to the point sticking in the bottom of the trench.

Upon this wooden platform running round the trench, the bricklayers began to build a circular wall 4 feet wide. As the brickwork increased in weight, so the cutting edge of the shoe was driven downward, the sinking being hastened by the removal of the material from beneath it. The first section of 9 feet through which the shaft sank was made-up ground composed of ashes. Then came the silt into which the shaft sank at a steady pace, while the bricklayers added course after course to the top, to increase the weight and the downward thrust.

This ingenious method of sinking the shaft by its own weight, instead of first digging out the shaft and lining it with brick as the excavation increased in depth, had many advantages. It enabled the bricklayers to work on the ground outside the shaft at a high speed, while the men excavated the material from inside as the shaft sank down. This method also obviated the risk of the work being stopped by the sides caving in when they reached the water-bearing soil, for the bottom of the shaft had always a firm foundation, and a single hand-pump could cope with the inflow of water.

The bricks were set in a special cement that formed the whole shaft into a rigid structure. To enable the tunnelling to be started towards the river and also in the opposite direction, the engineer planned to build two elliptical portions of the shaft walls, 26 feet wide by 24 feet high, in ordinary mortar to facilitate their removal.

It was a sound idea, but the ground toward the river bank proved to be unstable. As the shaft sank, the pressure of this made-up ground began to drive the weakened brickwork inward. Baulks of timber were at once put in across the shaft to prevent the ellipse from being forced in before the time was ripe. Then, course by course, the bricks were added to the shaft and the sinking continued.

In thirty working days the cutting edge of the shaft reached 14 feet below the high-water mark of the river or 20 feet below the surface of the ground. Just when the work was getting into its stride, it was stopped by a legal action, and five years passed before it was possible to resume, in September 1879. As soon as the work had been resumed, it was pushed on with such energy that by November 3, 1879, the shaft was completed, with the shoe thrust through the silt and into the sand 60 feet below the surface of the land.

The penetration of the water-bearing sand by the cutting edge of the shoe allowed the water to flow into the shaft. The rate of flow was only about 200 gallons a minute and the water could easily be kept in check by pumps. Unfortunately it washed away a good deal of material from round the shaft, and this in turn allowed the silt to move down the outside of the shaft and squeeze up under the edge inside. As the silt moved down, its place was taken by the loose ashes above until the ashes had penetrated a further 20 feet or more down that side of the shaft facing the river.

THE GREAT SHIELD designed by Sir Ernest Moir, for tunnelling under riversInstead of the 9 feet of ashes at the top of the shaft, there was thus a depth of about 30 feet of ashes down the outside of the shaft, reaching to the spot where the brickwork was to be removed to allow the tunnel to be started. But for this movement, the shaft above this point would have been set in the silt, which would have gripped it so tightly that neither water nor air could have penetrated down the outside. Ashes will allow air and water to pass through them easily, and their movement has been detailed because it was in this way that the stage was set for the terrible Hudson Tunnel disaster of 1880.

THE GREAT SHIELD designed by Sir Ernest Moir, for tunnelling under rivers. The shield was made unusually strong by its division into sixteen cells. It was driven through the soil by hydraulic rams. Airtight, except at the face, it was entered through air-locks in the tail. The men worked in the shield under compressed air, which kept back the water.

The first thing the builders strove to do, when the shaft was in position, was to put in a concrete bottom to make it watertight. They built some traps for the water and dug a central well into which a series of pipes radiated from the sides. Then they covered the driest portions of the bottom with stone to a depth of half a yard, over which were placed layers of concrete until a watertight bottom was made. The continuation of the concrete floor over the central well baffled them for a time because the water kept washing away the concrete before it had a chance of setting. Then they hit on the idea of stopping the well with bags of concrete which were packed in tightly. The covering of sacking prevented the concrete from being washed away and enabled the engineer to floor the bottom completely.

A large air-lock 15 feet long and 6 feet in diameter had been made to the plan of the engineer, with doors 4 feet high and 3 feet wide at either end. In each door was a glass bullseye an inch thick, strong enough to stand any air pressure it would be called upon to bear. A dozen pipes were carried through the lock for various purposes, for electric light, telephone, water, air and so on. Once the air-lock was in place, it was imperative that there should be no aperture through which the compress ad air could gush into the shaft from the workings: hence the necessity of carrying all supply and service pipes through the lock.

After they had removed some of the bricks that were set in mortar, the bricklayers made a circular cavity in the side of the shaft just over 6 feet in diameter, big enough to take the end of the air-lock. They took out all the courses of bricks in this aperture except the outer course giving access to the silt outside the shaft.

Then the air-lock was lowered from the top and the end was manipulated into the cavity in the shaft wall, after which hydraulic jacks slowly forced the airlock through the last course of bricks in the side of the shaft until it was driven some 4 in. into the silt. The wall in the shaft all round the air-lock was built up solid again, and the day arrived for the American engineer to prove whether his theory was right or wrong, whether the silt would indeed be impervious to air, as his tests had indicated, and thus enable him to keep out the water by filling the workings with compressed air.

His men entered the lock and the air pressure was increased to 12 lb. to the square inch. Under this pressure the men found it was possible to deal with the silt at the other end of the lock. Before long they had cut away the silt outside the air-lock to a depth of 4 feet and to a width of 15 feet.

Excavation in Compressed Air

For several days this newly-excavated chamber was subjected to the compressed air. As the silt began to dry it developed minute holes through which the air escaped. The escaping air served to turn these tiny holes into cracks through which water began to percolate. The roof, however, was held up for four days by the air pressure, before bits began to drop. Having sensed danger, the tunnellers withdrew into the air-lock and soon afterwards the silt, which had grown much softer because of the amount of water that had filtered through the cracks, flowed into the cavity and completely filled it. The engineer was quick to profit by this experience. He instructed the men to dig a big hole over the top of the air-lock and laid in the bottom of it a waterproof tarpaulin to prevent the water from running down the side of the shaft and on the roof of the chamber which was to form the beginning of the work. This was merely a temporary measure to protect the roof until he could get iron plates in place to hold it up. To prevent the air-lock door from being jammed by a fall of roof, and to provide a temporary entrance to the lock until the main entrance was excavated, the builders bolted another section of cylinder some 8 feet long to the far end of the lock.

Then they began to excavate in earnest. The air-lock was set in the top of the tunnel. From the floor of the lock to the bottom of the tunnel there was a drop of 22 feet; so the men cut their way down in a series of steps to the bottom line of the tunnel. At first the work was slow, no more than twelve inches a day.

TWIN AIR-LOCK in the Hudson River tunnel

TWIN AIR-LOCK in the tunnel, showing shifting rails for entering lock chamber. The north tunnel, for railway traffic, was equipped in 1903 with a cable-hauling system. This was built in three independent sections, separated by the air-locks. The first section, 1,575 feet long, reached from the foot of the shaft to the first air-lock. The second section, 1,660 feet long, connected the first and second air-locks. The third section, of variable length, extended from the second air-lock to the working face. The object of the air-locks was to prevent the compressed air from gushing into the shaft from the workings. Air-locks were installed also in the south tunnel.

They had to learn the best method of handling the silt. They worked with the utmost care, feeling their way forward, putting the excavated silt into a truck, which was passed through the air-lock to be dealt with. Later on they found the best way of dealing with the silt was to mix it with water and pump it out through a pipe. This was a big advance on the truck-handling method.

They worked under an air pressure of 16 lb. to the square inch, which was found sufficient to prevent the iron plates from being distorted by the weight above. They learned that they could cut a groove two feet deep in the silt, into which it was possible to insert an iron plate that was bolted to the preceding plate. In this way they could fix a ring of plates in which to build their brickwork tunnel.

Men were continually on guard watching for leaks of air in the silt. Sometimes they could hear the compressed air whistling through a hole, at other times they were able to detect leaks by passing a lighted candle over the roofs and sides, when the escaping air deflected the flame. All apertures discovered in this way were sealed again with silt, which proved a most efficient stopping.

Even when the tunnel was bricked there was the same problem of preventing the compressed air from escaping. No shield was being used for the work, and therefore the engineer was compelled to keep everything airtight. The bricks, being porous, allowed the air to escape. At first, it was thought that a coat of red lead would remedy the matter, but the compressed air forced its way through just the same. Eventually it was found that three or four applications of liquid cement sufficed to make the brickwork fairly airtight.

Trapped in the Workings

At the beginning it was planned to build two tunnels, one to take the northbound traffic and the other the southbound traffic, both tunnels to be united under one arch after they had been driven a short distance. Work was at first concentrated on the north tunnel, and while it was in progress the behaviour of the silt was most carefully studied.

Thus Mr. J. F. Anderson, the superintendent of the works, discovered a new method of driving the south tunnel. His idea was gradually to scoop out a ring in the silt 5 feet deep in which to set two bands of plates, supporting these by short props set against the core of silt. Then he proposed to lay the brickwork under this core of silt and up the sides to the top, afterwards allowing the central core to settle down in the bottom section of the tunnel to stiffen it while the work was in progress.

The men carried out instructions and the iron plates of the tunnel were put into place. The moment came for testing whether it was possible for the men to lay the bricks on the bottom in safety without their being buried by the mass of silt above. Mr. Anderson picked up a trowel and some bricks, inserted himself under the core and started to lay the brickwork at the bottom of the tunnel. The core fortunately remained in place and justified his observations. The idea was his and he calmly took the risk of proving to the men as well as to himself that his new method was safe. This new way was a distinct advance over the old, and gave the south tunnel a much more accurate line than the beginning of the north tunnel.

Having burrowed to the edge of the river, the builders found that the piles from the falsework on the river bank were right in their path. It was a difficult task to cut them off and get the stone out of the way, but it was accomplished and a roof was put in to enclose both tunnels as well as the inclined way and steps up to the air-lock.

When this work had been completed, the builders started to drive the north tunnel under the river bed. Just after 4 o’clock on the morning of July 21, 1880, there were twenty-eight men in the workings. It was customary to split the men into two parties and allow one to rest while the other carried on the work. The men at the face had knocked off and made their way up to the entrance of the air-lock, when they heard and felt a big escape of air.

DECOMPRESSION CHAMBER designed by Sir Ernest MoirAt that moment eight of the men, including Mr. Anderson, the superintendent, were in the air-lock, while twenty stood outside the second cylinder, which was designed to protect the airlock door and afford a means of escape in an emergency. No one anticipated serious danger. Their main concern was to stop the air from escaping. As they stood there by the door suggesting what to do, the roof came down.

DECOMPRESSION CHAMBER designed by Sir Ernest Moir to guard the tunnellers against “bends” or compressed-air sickness, which was often fatal. Men who were suffering from “bends” were placed inside the decompression chamber. The air pressure was raised again to relieve them of pain and then gradually lowered to normal atmospheric pressure. The decompression chamber was most successful.

By the irony of fate, the door of the chamber that was designed to protect the door of the air-lock was itself wedged fast by the fall of the roof. The twenty men trapped in the workings struggled in vain to open it. They, could prise it no more than 8 inches.

Those outside the air-lock in the shaft at once sprang to the rescue. They looked through the glass bull’s-eye and saw the eight men within the air-lock. They sought some way of getting all the men out. They were completely baffled. There was no way. The only thing they could possibly do was to smash the glass bull’s-eye of the airlock to reduce the pressure sufficiently to allow them to open the door and rescue the eight men inside. As the air escaped, the water gushed through between the loose stones of the river bank and down through the ashes, flooding out the workings and drowning the twenty imprisoned men.

The victims were not at the time under the river bed as is generally supposed. They were within 20 feet of the shaft. If only they had realized their danger directly they detected the leak of air, there was apparently enough time for them all to have saved themselves by entering the air-lock.

Behind a Barrier

The tragedy startled America and shocked those who planned the tunnel. From the first their main concern was to protect the lives of those engaged in the workings. They had foreseen the danger of a fall of roof jamming the air-lock door and cutting off the escape of those in the workings and had done their best to prevent it.

Unhappily, their precautions were nullified because someone engaged on the work credited the silt with greater strength than it really had. For some reason, one or two iron plates were removed from the roof just over the addition to the air-lock, and the unsupported silt caved in.

Pumps were at once got to work in an attempt to pump out the shaft. The water was beyond control. A diver found the air-lock half full of silt, with the water flowing freely through the lock, and he struggled vainly to shut off the water.

Having realized that it was impossible to clear the workings through the shaft and air-lock, the builders decided to build a massive cofferdam enclosing a space 46 feet square reaching from the side of the shaft and embracing the air-lock and the beginnings of the two tunnels. This was designed to act as a barrier against the shifting earth so that men could work in safety behind it. The guide piles of the dam, 12 in. square, were most carefully driven to a depth of 40 feet so as not to damage the work below. The task was to drive the piles over the tops of the two tunnels without penetrating or damaging them, but this was successfully done.

Meanwhile, a caisson weighing 460 tons was built to the design of Mr. Anderson, with two vertical air-locks through which men could pass in and out. On top of the caisson an open box 12 feet high was built and loaded with 350 tons of earth and 300 tons of bricks and rails. This was erected in the space enclosed by the cofferdam, from whose sides the whole mass of 1,100 tons was suspended and gradually dropped into position by excavation under the edges. The men inside the caisson worked under compressed air, which kept out the water, and by October 1880 the caisson was sunk in place, 42 feet below the top of the shaft.

Then began the struggle to make a connexion between the caisson and the air-lock through the side of the shaft. A slit was cut in the side of the caisson which, being under compressed air, did not allow the water to enter. Through this slit was passed a plate to connect up with the top of the air-lock. Then other plates were added until a complete ring had been formed, with the edges projecting into the caisson. The caisson was sunk so accurately that it was only 2 feet away from the end of the air-lock, and when the connecting ring was joined to the air-lock and the joints were sealed with silt, it was a simple matter to drive the water out of the air-lock by compressed air.

By a great deal of careful excavation and building within the caisson, the bottoms of the tunnels, which had not yet been built here, were completed, and the caisson itself was built up with arches and walls until it was turned into a strong working chamber.

FRONT VIEW OF A  LATER TYPE OF SHIELD, used for working on the south tunnel in 1903. Twelve years elapsed between the third abandonment of the workings in 1891 and their resumption by the New York and New Jersey Railway Company. Both the north and south tunnels were completed by the Greathead shield system, first used by Barlow and Greathead for a tunnel under the Thames in 1868.

Connexion with the north tunnel was not so difficult, because a good length had been built, but the water was flowing in and out of the south tunnel with the tide, and to effect an entry was a most puzzling problem. Before they could start tunnelling at all it was essential to have something solid in front of them that would prevent the air from escaping. As the water ebbed and flowed in the tunnel, there were many channels which would allow the air to escape as fast as it was pumped in.

In their dilemma, the builders decided to fill the tunnel with silt again so that they would have something solid through which to tunnel. Tiny holes were drilled with the utmost care through the side of the caisson and plugged immediately to prevent the air from escaping. Then the engineers managed to insert a 2-in. pipe. Large quantities of silt were mixed with water to the consistency of thick cream; when one end of the pipe was inserted in the mixture, the valve was opened and the silt was driven into the tunnel by the compressed air in the caisson. In this way the bottom of the tunnel was covered with silt.

When they could get no more silt through this small pipe, they used a larger pipe. Then as the heading of silt prevented them from forcing any more through the larger pipe, they rolled the silt up in balls, dipped the balls in water for a moment to make the outsides slippery, and placed them in a pipe. When the pipe was full, four men drove the silt through with a ramrod, and they went on ramming home the stuff until a section of the tunnel just outside the caisson was filled solid with it.

The brilliant idea was effective, for the men were then able to cut away part of the caisson and build a bulkhead across the tunnel mouth. The builders suspected that if they attempted to clear the tunnel to its full size at once they might easily be overwhelmed. They were also aware that however hard they rammed home the silt, they could not make it so compact and airtight as silt which had never been disturbed. The difficulty spurred the inventive genius of Mr. Anderson, who once more rose to the occasion by inventing a system which is now known as the pilot system of tunnelling.

Pilot Tunnel

Having cut through the bulkhead, he inserted a series of plates to form a little tunnel 6 feet in diameter. When this was complete the silt was rammed solid at the.end of it to enable the next ring of plates to be added. So they drove forward foot by foot. By the use of the pilot tunnel as a base for operations, it was possible to clear the rest of the tunnel, and the builders found the new method gave better results than any they had tried.

With the pilot tunnel driving on in advance, they could see what sort of ground lay ahead, and the tunnel itself made a firm foundation against which to set the props to hold up the plates of the larger tunnel before the brickwork was built in. The props radiated from the pilot tunnel as do spokes from the hub of a wheel. On the New York side the tunnellers found themselves in a quicksand. It was a far worse problem than tunnelling in the silt, for the silt was solid, whereas the sand flowed almost as if it were water. Engineers of the highest standing asserted that the difficulties could never be overcome, but the builders were not to be turned aside by doubts or difficulties. They devised a bulkhead built up of airproof iron plates which would suffice to keep back the sand. Strips of metal were inserted and slipped back over the top ring of the tunnel to hold up the sand above while a new section of plate 15 in. wide was set up and bolted in place to the preceding section. Then the top strip of steel in the bulkhead was advanced and held in place, the next strip following it until the whole bulkhead was advanced fifteen inches.

It was difficult work that called for continual watchfulness on the part of all concerned. They tried many things to stop the air from escaping, but found the silt was best. One trick they learned was to let the escaping air carry with it powdered cement until a hole was plugged. Sometimes a barrel or two of cement were used up in this way before the hole was filled. If the blowouts were too big to be stopped by powdered cement or silt, the men met the difficulty by packing bags of cement in the holes and plastering silt over all to make the place airtight again.

MODIFIED FORM OF SHIELD used for the Hudson River tunnelNo praise can be too high for the men who first tried to make a tunnel by working in compressed air. They faced difficulty after difficulty and overcame them; death and disaster did not stop them. After having driven 2,500 feet of tunnel, however, they were forced to cease work in 1883, due to lack of money, the expenditure to that date being the moderate sum of £220,000.

MODIFIED FORM OF SHIELD, showing moving cantilever platform. The shield measured 19 ft. 6 in. inside the tail piece, and was advanced by sixteen 8-in. hydraulic jacks. The pressure developed in the jacks corresponded with the amount necessary to force the shield forward, and varied according to the nature of the soil. The shield used by S. Pearson & Sons had been designed to deal with silt only. To make it suitable for working in rock an apron was built extending 6 feet in front of the face of the cutting edge.

There was a dangerous blowout in the quicksands on the New York side in August 1882 which flooded out a section of the tunnel. The men at work felt the air going - it was compressed at 26½ lb. to the square inch - and managed to reach the air-lock before the water overwhelmed them. A diver found that some of the plates in the bulkhead were distorted, and it took forty-five days to repair the damage and get the work going again.

The initial disaster in which twenty men had lost their lives by drowning seems rather to have blinded the engineering world to the magnitude of the feat which Mr. Haskin and his colleagues accomplished. Although they put men to work in compressed air for the first time, their precautions were so skilfully devised that not one of their workmen lost his life through f compressed air sickness. This is not generally realized.

Every man before being engaged for the work was examined by a doctor, and only men with perfect hearts and lungs were accepted. After the medical examination a man was put into the air-lock to undergo a pressure test, and if he was affected in any way ho was rejected. After their spell at work the men were prevented from taking violent exercise; they were forbidden to take alcohol and were served up with strong coffee as they came out of the air-lock.

Sometimes they were working in pressures up to 36 lb. to the square inch. There were not many cases of “bends”, and this was probably due to the fact that the men had to pass through two air-locks on the way out, the first being near the working face where the pressure was highest, after which they had to walk through a length of tunnel about 100 yards long where there was a moderate pressure, before passing through the last air-lock into the outer atmosphere. This happy combination of two air-locks with a gentle walk between them prevented any fatal cases of compressed-air sickness.

Six years after the tunnels had been abandoned, Sir Benjamin Baker was consulted about completing them. He saw that the only possible way to carry through the task was to use a shield combined with compressed air, and asked a young engineer, Mr. E. W. Moir (later Sir Ernest Moir), who had helped him on the Forth Bridge, to design a shield for the work. This Mr. Moir did before he went out to supervise the work.

The British engineer, when he came to start work early in 1890, found the north tunnel blocked for several hundred feet where a strong brick bulkhead into which an air-lock was built kept back the silt. The little pilot tunnel, about 80 feet long, was wedged in silt across the main tunnel. Moir knew that the influx of all this silt from the river bed must have left a big hole, and he was faced with the difficulty of clearing the tunnel and setting up his shield, which weighed 80 tons, right under this hole in the bed of the Hudson.

It was his first experience of tunnelling, but he was young and full of ideas. The first thing Moir did was to build a balloon of sailcloth which was filled with clay, straw and hay being added to bind the whole mass together and to prevent the clay from being washed away. This was weighted with old iron and bricks and lowered into the hole from a floating derrick.

“Medical Air-Lock”

Having left the balloon to settle for a few days, Moir went down with his men into the tunnel and managed to open the door of the air-lock slightly. They were working under a pressure of 34 lb. to the square inch, and as soon as the door was opened the silt began to flow into the chamber. As the silt moved along the tunnel, the clay balloon was sucked down tightly into the hole until the silt ceased to flow.

The men cleared the silt carefully from the tunnel, and then worked forward until they were burrowing through the bottom of their clay balloon. Here, under a pressure of 40 lb. to the square inch, they built a brick chamber big enough to permit them to rivet the shield together. In these stifling and dangerous conditions they heated the rivets and drove them safely home, allowing the fumes to escape through a lengthy pipe that passed back through the workings for 2,000 feet until it reached the shaft. Although the shield solved some of the old troubles, it brought a new one in its train. Its weight was such that when they forced it into the silt it began slowly to sink at the face. This not only threatened to throw the tunnel out of alinement, but also made it difficult to place the key plate of the ring. The men had to work at great speed to fix a ring within three-quarters of an hour to get the key plate home. With a little practice they were able to do this. But to counteract the movement of the front edge of the shield, they had to drive it so that it was 9 in. higher than the tail, to allow for it sinking as they fixed the ring. Sir Ernest Moir said afterwards that he was afraid sometimes that he was building a shaft instead of a tunnel.

AFTER THE THIRD SUSPENSION of work on the Hudson Tunnel Sir Ernest Moir designed an improved shield for the Blackwall Tunnel

AFTER THE THIRD SUSPENSION of work on the Hudson Tunnel Sir Ernest Moir, fortified by his American experience, designed an improved shield and equipment for the Blackwall Tunnel, London. Four steel caissons made for the tunnel had circular openings cut in their sides to allow the shield to pass through. The photograph shows the sinking of one of the caissons. When the caisson had been sunk into place, the shield was lowered into it and forced through the openings by hydraulic rams.

Eventually they were tunnelling at the rate of 10 feet a day, but the loss of life from compressed-air sickness was so alarming that the New York newspapers grew critical. According to Sir Ernest the death rate from this cause among the tunnellers averaged 25 per cent annually. As was to be expected, the tunnellers became chary of the work and labour difficulties began to arise, for the tunnel acquired a bad reputation.

Alarmed at the mortality, Moir providentially recalled that while sinking one of the caissons for the Forth Bridge, the Belgian and Italian labourers who suffered from compressed-air sickness were at once relieved when they went back to work in the compressed air. In a flash he reasoned that it was coming out of the compressed air too quickly that caused the illness. He therefore designed the first decompression chamber for use on the work so that the men who were attacked by “bends” could be placed inside and the pressure raised again to relieve them of pain and gradually lowered until it was the normal atmospheric pressure.

His “medical air-lock”, as he termed it, was a brilliant inspiration. As soon as it was installed it brought the death rate down to about 1 per cent. The decompression chamber has proved invaluable to divers and other workers in compressed air ever since.

Moir carried through the tunnelling at such a rate that in about a year he added 2,000 feet to that already built. The difficulties were never-ending, and one of the greatest was to balance the pressure of the air so that it was sufficient to keep back the head of silt, but not powerful enough to force its way through the silt and cause a blowout. Yet blowouts were unavoidable, however much care was taken, and stone and clay had frequently to be dumped into the river bed to fill the holes.

Another difficulty was that a pressure high enough for the top of the tunnel was too low to prevent sand and water from rising from the bottom. Between the top and the bottom was a depth of 19 ft. 5 in.; in theory, therefore, the air pressure at the bottom ought to have been about 9 lb. higher than that at the top. As the air pressure was necessarily the same all over the tunnel, they were compelled to balance it as well as they could.

Twelve Years' Delay

The contractors were wresting a triumph out of failure when a financial crisis again stopped the work. For some months Lord Cowdray, head of the firm of contractors, S. Pearson & Sons, kept the air-compressors running at his own expense, hoping in vain that the funds would be forthcoming to enable him to finish. At last he bricked up the shield, made the tunnel as watertight as he could, and withdrew from the work.

Twelve years later the New York and New Jersey Railway Company sent their men down the shaft. They found the tunnel intact. So carefully had the British engineers sealed it off that they were able to uncover the shield and drive it through to the other side.

The Hudson Tunnel was a great undertaking, fraught with tragedy. The builders laid down the lines on which future underwater tunnels could be built through strata of the kind encountered. The original American engineers achieved a miracle by putting men to work under compressed air for the first time without losing the life of a single man through “bends”. Sir Ernest Moir discovered a cure for the disease which had baffled medical science.

When he had returned from the United States after having sealed off the incomplete Hudson Tunnel, Sir Ernest started to design the shield and equipment for building the Blackwall Tunnel under the Thames. His American experience was invaluable and the Blackwall Tunnel shield was a big advance on the Hudson Tunnel shield. The four steel caissons made for the Blackwall Tunnel had circular openings in their sides to allow the shield to pass through.

Lord Cowdray and Sir Ernest Moir worked together in complete harmony. Their firm, S. Pearson & Son, went out again to New York and drove four tunnels under the East River to link up the Pennsylvania Railway between Manhattan Island and Long Island.

[From part 14 and part 15, published 1 and 8 June 1937]

You can read more on “Conquest of the Severn”, “The First Thames Tunnel” and “London’s Underground Railways” on this website.

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

Tunnelling the Hudson