The chief Contractor for the building of the Severn Tunnel, Thomas Walker, gave the following details of the spring*...

At about 2 miles farther north than Caerwent, the hills of Wentwood are met with, with 'Grey Hill' standing in the foreground. The first spurs of the hills fronting the valley are composed of mountain limestone, the higher parts about Shirenewton of the old red sandstone.
On the east side of the Severn, and for some little distance on the western side, the new red sandstone formation is found in nearly horizontal beds. The first disturbance of this takes place behind Portskewett village, where the mountain limestone has been upheaved and the new red formation denuded. A mile further up the same limestone has been upheaved between Caldicot and Caerwent, and from there to the base of the hills the strata have been much broken, and the consequence has been that all the water from the hills, both from the mountain limestone and the old red sandstone, has found subterranean channels through this broken ground...

...on the 18th October, 1879, in the heading then being driven westwards from the Old Pit, a large body of water was tapped, which, although efforts were made to dam it out by timber placed across the heading, poured into the workings in such a volume, that in twenty-four hours the whole of the workings which were in connection with the Old Pit were full up to the level of the tide-water in the river.

After great difficulty this "Great Spring" was imprisoned behind a wall, and new shafts sunk...

On the 2nd June the winding-pit reached a bed of very hard con­glomerate rock at a depth of 96 feet. This rock, which was known to exist, and had been found in the Sudbrook Pit at nearly the same level, had been nowhere met with in a greater thickness than 9 feet, and in many places it was known to be only 3 feet thick. In this unfortunate shaft the thickness proved to be 26 feet. The rock was very jointy, and full of fissures, yielding immense quantities of water, the water spouting through the fissures under a head of about 100 feet; and it was not till the 18th July that the shaft reached the bottom side of the bed, and entered the fire-clay shale, which proved to be perfectly dry...we drove a cross-heading 4 feet 3 inches below the level of the invert of the tunnel in the direction of the pumping-pit. From this heading we sunk the bottom part of the pumping-pit in shale, which was perfectly dry till we reached within a few inches of the bottom, where a small spring was met with, which evidently was under a pressure of at least 100 feet...

With continual pumping, the tunnel was dried, and they decided to tackle the Great Spring, which was trapped behind a sealed door...

At the end of May an attempt was made to open this door, but it was at once found that a quantity of rock and shale had fallen down behind it, and that it was impossible to open it. On the 30th May holes were bored through the door with augers, and a piece 12 inches across was broken out. Through this a considerable quantity of soft material was forced by the pressure of the water behind, and now and then large rocks were brought down which stopped the hole. For more than two months the men continued to work, taking away material which was forced through the door by the pressure of the water behind, having continually to break up with long bars and 'jumpers' the lumps of rock which blocked the hole. At last it became evident that it would be an endless matter to attempt to work in this manner, so the bottom heading was pushed forward until it passed well beyond the point where this door was built.
A hole was then broken up from the bottom into the top heading, and all the water from behind the door allowed to pass that way.
Men then got up into the upper heading, and found that a length of 50 or 60 feet of the roof had fallen in, and that there was an enormous cavity above, but that little water was coming from that direction...

Walker gives this drawing of the cavity.

The total quantity of material forced through this door by the pressure of the water behind was 2,000 yards, showing the enormous extent of the cavity above, and the damage that had been done by not properly timbering the heading at first...

on the 10th October, 1883..the same foreman drove up to my house to report to me that the Great Spring had broken into the tunnel in larger volume than we had ever yet met with, and that it was rapidly gaining upon the pumps.
I went down with him at once to the works, and on descending the shaft found a river 16 feet or 17 feet wide of bright clear water, flowing along the invert of the tunnel, and leaping down the old shaft into the lower workings. As it had a drop there of more than 40 feet, the roar of the water, when you were in the tunnel, was deafening. My first action on reaching the bottom of the shaft was to taste the water. To my great relief I found that it was fresh, and we, therefore, had no reason to suppose that we should not be able to contend with this difficulty, as we had with so many before...I ascertained the follow­ing facts with regard to the inburst of the water.
The night-gang, working in the bottom heading westwards, had gone to work shortly after six o'clock, taking up skips with them, and had begun to shovel up the loose rock dislodged by the blasting, when the ganger said :' There is more water here than usual—the "Grip"' (a small ditch at the side of the heading) ' must be blocked. Push back a skip or two to clear it out.'
The men had hardly done so, when, to use the words of the ganger, ' the water broke in from the bottom of the face of the heading, rolling up all at once like a great horse.' It swept the men and the iron skips like so many chips out through the door and into the finished tunnel; and it was only when the water spread itself over the whole width of the tunnel that they were able to gather themselves up, and save themselves from being precipitated down the old shaft into the lower works. They were swept through the door without the power to check their passage, but they at once endeavoured to work their way back again up the heading, holding one another and clinging to the timbers at the side, to shut the door, if it were in any way possible. All their efforts failed, for the water was running down the heading in a stream 10 feet wide and 3 feet 6 inches deep, and with such rapidity and force that no man could stand against it...

On the morning of the 11th the water had risen against the pumps to the height of 52 feet. On the 12th the pumps, still working steadily, held the water at 132 feet from the surface.
A council of war was held, and it being the opinion of all that the inburst of the water might be from a subterranean reservoir, which would shortly exhaust itself, and that we should only have the same quantity of water ultimately to pump that we had before the inburst occurred, it was decided to continue the pumping for two or three weeks longer. After holding it a depth of 132 feet from the surface for two days, the pumps began to gain slowly. By the 22nd they had gained 9 feet 9 inches, and by the 26th, 13 feet.
The cubical contents of the tunnel and other works filled by the water, while the pumps were continually pumping at the rate of 11,000 gallons per minute, was accurately measured; and we found that the water must have run in at least at the rate of 27,000 gallons per minute, or 16,000 gallons more than we had pumping-power to lift...

Again the tunnel was dried out, and the area investigated...

Another cross-heading was driven from the tunnel from behind the head-wall to connect with the side-heading ; and the side-heading was pushed forward, and on the 19th December [1884] reached a large open joint in the strata, which had formed the channel for the subterranean water. A drawing is given showing a plan of this fissure, and the manner in which it was tapped by the side-heading.
By diverting the water of the spring into the side-heading the tunnel itself was left almost perfectly dry, except where, in taking out rhe invert, we crossed other fissures in the rock in which the water stood a foot or two higher than in the one tapped by the side-heading...

On opening out the full-sized tunnel, the fissure through which the Great Spring had passed was found to follow a most erratic course. In one place it passed directly across the tunnel from side to side, nearly at right angles to the centre line of the work. At another place it passed from side to side in an oblique direction, running for some small distance directly under one of the side walls. At another point where the tunnel had been perfectly dry, while the mining was done, the lifting of almost the last stone out of the invert set free an immense body of water which no pumps underground could cope with. At another point the water boiled up from a hole 18 feet in depth under the invert with such force that stones, the size of a man's fist, dropped into the water would descend about 10 feet, and then begin to flutter like a leaf in the wind, and be thrown out again by the water. Into this hole a cast-iron pipe was lowered, attached to a bend at the top to lead the water into the side-heading...

The minimum quantity of water pumped when dealing with the Big Spring was 23 million gallons daily; the maximum quantity 30 millions. For more than a year the average quantity pumped daily was 24 million gallons.
To give some idea of this immense quantity of water: It is sufficient to supply a town about the size of Liverpool or Manchester, and is about one-sixth of the quantity daily consumed in London. In one year it would form a lake about 1,000 acres in extent and 10 yards deep. The total pumping power provided — 66 million gallons per day, about half the supply of London would form in one year a lake nearly 3.000 acres in extent and 10 yards deep.
All the water pumped from the Severn Tunnel during the time it was under construction would form a lake about 3 miles square and 10 yards deep.