As part of the 21st-century modernisation of the Great Western Main Line , the tunnel was electrified in The tunnel has good clearances and was relatively easy to electrify, although due to its age the seepage of water from above in some areas provided an engineering challenge. Rather than the use of standard overhead cabling, the tunnel was instead electrified with a solid conductor rail mounted on the tunnel roof.
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The finished tunnel is horseshoe-shaped in cross-section, with a concave floor. Its height is 6. An enclosed drainage channel, in the form of an upturned semicircular tunnel mm high, is built onto the tunnel invert, 1.
On 5th September , a special train steamed through the tunnel. However, water seepage remained an issue. It was assumed that the continuous brickwork lining of the tunnel would withstand groundwater pressure and the drainage sluice valve on the side heading had been closed and all but one of the pumps removed.
On 20th December , the pressure rose so high up to kN per sq m that bricks began popping out of the lining. The sluice valve was opened gradually and the pressure slowly subsided.
Evidently, greater pumping capacity was required to combat the force of the Great Spring. In , a brick-lined shaft of 8. Their purpose was to pump out around 90 million litres of water per day, mainly entering from the Great Spring.
Three of the engines remained in service until Sudbrook also had a ventilating fan A smaller pumping station was constructed on the Gloucestershire side. The tunnel opened to goods traffic on 1st September and to passenger traffic between Bristol and Cardiff three months later. On October 18 they had penetrated to within yards of each other. But on that day the workmen in the heading from the Old Shaft on the Welsh side tapped a great underground spring.
Thousands of gallons gushed out over the workers, extinguishing lights and bowling over skips and barrows. There was nothing for the men to do but run, and they bolted for their lives down the long heading to the bottom of the shaft.
Had they hesitated they would have been drowned. They had no time to wait and close the great iron door in the heading which would have confined the water to its upper end. The heading filled up; the shaft filled up. The water rose through the shaft until it reached the tidal level of the Severn. The tunnel was planned for a total length of 4 miles yards. Despite the great danger, Diver Lambert descended the shaft and after many attempts succeeded in closing the iron gate which would have confined the flood waters to the upper end of the heading.
Under- river working on the Welsh side was completely at a standstill. The formidable Big Spring, as it came to be known, cont- inued to discharge water into the workings at a rate far faster than the existing pumps could pump it out. It was in the face of this catastrophe that one of the great epics of engineering history took place. Nothing could be done with the flooded workings until that iron gate was shut, and the flood could be confined to the uppermost end of the heading.
Nobody but a diver could hope to reach the water gate, and that diver needed to be a thoroughly experienced and superlatively courageous man. In those days diving appliances were far less suited to deep- water work than they are to- day, and such a mission as the flooded Severn Tunnel workings would involve a heavy strain on the toughest constitution. The contractors found their man in Diver Lambert, a man of quiet habits, few words and tremendous grit.
Lambert said he thought he could shut the water gate. Hoping and fearing, they lowered him down the flooded shaft, and he vanished into the dark depths until only the trail of silver air bubbles remained to show that he lived and breathed far below the surface. From the bottom of the Old Shaft, Lambert went cautiously along the pitchy dark length of the heading, with the tremendous pressure of the water surrounding him.
He had to pick his way over deserted tools and barrows, moving slowly forward, step by step. It was his air pipe that defeated him. It needed the greatest care and skill to avoid severing the frail tube against some sharp edge. One breakage or stoppage of the pipe and Lambert could never hope to see daylight again.
For three hundred yards the brave diver groped his way along the point he could not drag his unwieldy air pipe. The man was ready to succeed, but the apparatus failed. He had to make his way back to the shaft, there to be hoisted to the surface with his mission unaccomplished.
At that time an inventor called Fleuss was demonstrating his new diving dress and apparatus in London. The great feature of the Fleuss appliance was that it eliminated the necessity for trailing an air pipe, as a tank of oxygen was carried on the back of the diver in the same way as a pack.
The contractors got into touch with Fleuss and induced him to attempt the task at which Lambert, with his ordinary diving dress and air pipe, had failed. The self- contained nature of his apparatus enabled him to move in the heading without fear of cutting off his own air supply.
But after one experience of the dark heading, with the great pressure of water all round him and all manner of unseen objects impeding his progress, nothing would induce Fleuss to pursue the quest further and he gave up without having closed the gate. Now Lambert volunteered to make a second attempt, and suggested that he should borrow the diving dress and apparatus of Fleuss.
At first Fleuss demurred, but after some persuasion he allowed Lambert to put on the dress, with its helmet and reservoir. Lambert at first made some trial dives to accustom himself to the use of the Fleuss apparatus before risking himself in the flooded deeps of the workings. Finally he considered himself proficient in the use of the Fleuss apparatus. It reached within yards of the west heading from the Sea Wall Shaft. Work on the other headings in each direction had also progressed favourably, including that on a heading from the Old Shaft at a higher level than the original one.
On October 16, , a terrible setback occurred. A big fresh- water spring was suddenly pierced. Volumes of water poured in at the rate of 6, gallons a minute from this source alone. The pumps had previously dealt successfully with more than 2, gallons a minute, but they could not cope with this additional burden.
They were overpowered after having worked at full speed for nearly twenty- four hours. The work associated with the Sudbrook Shaft under the river was entirely flooded, cascades falling down the 40 ft depth of the shaft.
Fortunately none of the men at work suffered any injury other than a wetting, as they were able to escape through the cross- heading and the door into the Iron Shaft. The spring thus freed, thenceforth known as the Great Spring, has ever since demanded its daily quota of energy from the powerful pumping plant with which first the works, and finally the tunnel, had to be equipped.
This was indeed a blow for the railway company. Sir John Hawkshaw, the Consulting Engineer, was immediately asked to undertake the chief responsibility, Mr.
Charles Richardson acting as joint engineer. On the advice of Sir John, Mr. Thomas A. Walker a contractor with whom be had shared previous tunnel experience was entrusted with the task of completing the work. He took possession on December 18, The desolation of the scene which confronted him can be imagined. Sir John now decided to lower the tunnel under the Shoots fifteen feet. This entailed deepening all the shafts, and the building of a new drain from the new bottom under the Shoots to the pumping shaft, converting the existing bottom heading into a top heading.
Larger pumps and engines were ordered and four new shafts constructed - one at Sudbrook, another at the Sea Wall, and two at a point twenty- six chains west of the Sudbrook shafts beyond where the Great Spring had broken in. The object was to collect all the water from the spring and to keep the rest of the work dry. Owing to various delays and unforeseen incidents the works were not cleared of water until December 7, Work underground was now pushed forward with great energy.
A strong wall was built to imprison the Great Spring, thus isolating it completely from the works. By this time a small town, accommodating the workmen, had sprung up at Sudbrook; it had its mission room, school, hospital and post office, besides engine houses and the buildings of the tunnel works. The dotted lines indicate the cross- section of the tunnel. At the Sea Wall Shaft on the English side of the river, the brick arching of the tunnel had been begun in December, and bricklayers were at work when, quite unexpectedly, salt water burst in from the roof, putting them to flight.
Above, at low water, was a pool near the shore called the Salmon Pool, where the thickness of roof over the tunnel was only about three feet and where it had been originally intended to concrete over the river bottom. A number of the men, joining hands, were sent to walk through the pool to find the hole. One of them found it and suddenly disappeared, having to be rescued by his neighbours.
The river bed at the spot was overlaid with large quantities of clay, and pumping operations, which had meanwhile been stopped to prevent the enlargement of the leak by the inrush of water, were resumed without incident. The brick arching was extended rapidly past the spot with a thickness of 4 ft instead of the normal 2 ft 3- in.
The severe snowstorm of January, , caused great hardships, and held up the supply of coal and other material for three days. Coal was borrowed from farms and cottages in the immediate vicinity and timber was also pressed into service. Thus there was enough fuel to keep the pumps going, although the severe frost which followed prevented all surface operations for about a fortnight. They were rescued the following day.
On September 26, , the two Seven Foot Headings were joined up and extended as one clear passage- way or preliminary tunnel for the full distance of two miles and a quarter from Sudbrook to the Sea Wall Shafts.
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