The Severn Tunnel/Chapter 2
CHAPTER II.
THE EARLY HISTORY OF THE UNDERTAKING.
Early history of the undertaking—1871.It has already been stated that the Great Western Railway Company had for many years been anxious to obtain a better access to South Wales. Their main line at first ran from London to Bristol. From this, in 1838, wishing to acquire the traffic from Gloucester to London, and probably fearing that any attempt to obtain powers from Parliament to bridge the Severn below that point would be defeated, they constructed the line passing by Kemble and Stroud to Gloucester. At Gloucester the South Wales Railway joined them.
The line through the Stroud Valley has very heavy gradients and many sharp curves.
Many schemes have been set on foot to avoid these curves and gradients, and the Bristol and South Wales Union Railway, with its ferry from New Passage to Portskewett, is one of them; but this line was only a single line, with one gradient of 1 in 60, and the fatal drawback of the steamboat-ferry.
Early history of the undertaking—1871.In November, 1871, Mr. Charles Richardson deposited plans in Parliament for the Severn Tunnel. He soon afterwards obtained the assistance of Sir John Hawkshaw, who agreed to act as consulting engineer, and the scheme being taken up by the Great Western Railway Company, was carried through Parliament, and an Act for the construction of the tunnel obtained in 1872.
The Great Western Railway Company lost no time in commencing the works, which they did early in 1873.
In order to test the strata, they sunk and lined with brickwork, on the Monmouthshire or west side of the Severn, a shaft 15 feet in diameter to a depth of about 200 feet; and from this they commenced to drive a heading eastwards under the river. The heading had a rise from the bottom of the shaft of 1 in 500, and was driven at the level necessary to drain the lowest point of the intended tunnel under the deep-water channel of the river.
When the works were commenced, the parish of Portskewett, in which they were situated, was a purely agricultural parish, with a population of men, women, and children, of 260. No buildings whatever existed near the site of the shaft, the nearest being a farmhouse, known as Sudbrook or Southbrook Farm, with two cottages, and at a slightly greater distance along the river-bank an inn, known as the ‘Black Rock Hotel,’ with three or four cottages occupied by the inspector of the
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| Charles Richardson |
| Engraved by W. H. Gibbs from a photograph by Maull & Fox |
Before commencing the works it was necessary to make some provision for the men who would be employed there. For this purpose land, to a small extent, was purchased by the Great Western Railway Company near the shaft, and upon it were built six cottages and a small office.
A single line of railway, or tramway, was laid from Portskewett Station to the shaft on land leased from the tenant of Southbrook Farm. Over this the winding engines, pumping engines, bricks, timber, and other materials were brought to the shaft, and six other cottages were built upon land belonging to the Great Western Railway Company, near the Bristol and South Wales Union line, between Portskewett Station and Portskewett Pier.
The progress made with so small a provision was necessarily slow, and in August, 1877, after four and a half years’ work, all that had been done consisted of the sinking of the one shaft, afterwards known as the ‘Old Shaft,’ and the driving of about 1,600 yards of 7-feet square heading under the river. A second shaft had been commenced, in which it was intended to fix the permanent pumps to drain the tunnel. This shaft in August was about half sunk, but not lined.
At this date the Directors decided that they would let the contract for the works, and advertisements Early history of the undertaking—1877. asking for tenders were published. Only three tenders were received, one of which was from myself. Sir John Hawkshaw, acting as consulting engineer, advised the Directors to accept the tender I then made; but, after considerable discussion, they being of the opinion that too great an amount had been estimated for contingencies, decided not to let the contract until they had further proved the ground, and, in fact, till they had driven a heading through the whole length of the tunnel.
They then entered into two small contracts, one with Mr. Oliver Norris, of New Passage, to sink a shaft on the Gloucestershire side of the river, and to drive headings east and west from that shaft; and another with Mr. Rowland Brotherhood, to sink two shafts, known as the ‘Marsh Shaft’ and the ‘Hill Shaft,’ and to drive headings both ways from these shafts. The Company continued to carry on the heading under the river themselves, and they also at a later period agreed with Mr. Norris to drive from the original or old pit 7-feet headings, westwards towards the Marsh Pit, and eastwards on the formation level of the tunnel under the river.
The Company also completed the pumping-shaft which they had commenced in 1877. This shaft they tubbed with iron, erected an engine-house over it, and in this fixed two Bull-engines, each with a 50-inch cylinder and 10-feet stroke, and each working a 26-inch plunger-pump.
The iron tubbing of the shaft did not reach quite
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| Plan of Sudbrook, 1873 |
This brickwork carried four wrought-iron girders, on which the pole-cases of the 26-inch plunger-pumps were fixed. There was a small iron door 2 feet square in the iron tubbing of the shaft opening outwards, through which access could be obtained to a cross-heading leading to the main heading from the bottom of the Old Pit, and in the brickwork between two of the wrought-iron girders was fixed a sluice by closing which water could be excluded from the pit, or admitted by opening it.
The girders when fixed proved too weak for the work they had to do, and an ordinary cast-iron pipe 15 inches in diameter was placed under one of the girders as a column. This pipe had, of course, a large flange at the top which afterwards proved a serious obstacle in the way of fixing other pumps. The valve, by which the quantity of water to be admitted to the pit was regulated, instead of being actuated by long rods brought up above the highest level of the water, was fitted with an ingenious (?) apparatus by which it was intended to shut or open it by turning on the pressure of water obtained from a spring behind the iron tubbing about 100 feet above the valve. When the shaft was full of water the action of this arrangement was most uncertain, and was the cause of much of the difficulty encountered in clearing the tunnel of water.
In the manner above described the work was Early history of the undertaking—1879.proceeded with till the 18th October, 1879, at which time a considerable length of heading had been driven under the land from the three additional shafts which had been sunk, and the heading under the river had been so far advanced that only about 130 yards intervened between the heading being driven from the Gloucestershire shaft, known as the ‘Sea-Wall Shaft,’ and the main heading from the Old Pit on the Monmouthshire side.
None of these headings, up to the 17th October, had given any large quantity of water.
There were fixed at the Hill Pit two 15-inch plunger-pumps; at the Marsh Pit and the Sea-Wall Pit, each two 15-inch plunger-pumps. In the Old Pit there was an 18-inch plunger-pump worked by a 41-inch Cornish beam-engine; and in the Iron Pit adjoining were the two 26-inch plunger-pumps, each worked by a 50-inch Bull-engine.
But 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. Fortunately no lives were lost, the men being warned as they were changing shifts in the long heading, and being able to escape by the Iron Pit with only a wetting.
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It was a melancholy result of nearly seven years’ work, and no doubt those in authority no longer undervalued the contingencies of such a work, which they had thought had been over-estimated by the contractors two years before.
The Directors then called in Sir John Hawkshaw, who up to this time had been consulting engineer, and asked him to take full charge of the works as chief engineer, and carry them on as he thought best himself.
Sir John Hawkshaw agreed to take charge only on the condition that they would allow him to let the works to some one in whom he could have confidence; and on their consenting to this condition, he did me the honour to send for me, and asked me if I were still willing to enter into a contract.
Even after this irruption of the water under the land (water which it was well known was perfectly fresh and sweet), no one seemed to be fully alive to the fact that the greatest dangers and difficulties were to be found there, but everyone still thought the danger lay in the construction of the tunnel Early history of the undrtaking—1879. under the deepest part of the river. Sir John had to explain to me that the Directors had determined, in the first place, to complete the tunnel under what was known as the ‘Shoots’ (that is, the low-water channel of the river), before proceeding further with the tunnel under the land, and he asked me to give him a price for executing that part of the work only.
After a little consideration, I declined to do this, but offered to carry out the works on the tender I had made in 1877, executing the work under the ‘Shoots’ first, if he thought that advisable. I gave to him then all the details of the tender I had made in 1877. The Directors accepted the tender, with certain modifications necessary in consequence of the lapse of time and the work that had been done, and the contract was entered into with me, and the works handed over to me on the 18th December, 1879.
Nothing could be more desolate than the appearance of the works at this time. There were, as I have stated, near the main shaft only six cottages and a small office, the necessary boiler-houses and engine-houses, a small carpenter’s shop, a fitter’s shop, a blacksmith’s shop, and two low buildings or sheds used as cottages also. The tramway which had been originally laid to Portskewett Station had been pulled up, and in lieu of it another tramway had been laid, following (on the surface of the ground) the centre line of the tunnel itself from the Old Shaft to the Marsh Pit, and joining the Great Western .png)
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The pumping-engines at Sea-Wall, Marsh, and Hill Pits were still at work, as the working of those pits had been in the hands of Mr. Norris and Mr. Brotherhood; but no work was being done below, orders having been given to these gentlemen to suspend their operations.
The pumps were able to keep the Hill Pit dry, but were not sufficiently powerful to drain either of the dipping headings from the Marsh or Sea-Wall Shafts while the main shaft and heading were full of water.
Immediately after the irruption of the Great Spring, and before the contract was let to me, the Great Western Railway Company, under Sir John Hawkshaw’s advice, had ordered two additional pumping-engines with large pumps to contend with the extra volume of water. The one was a Cornish beam-engine with a 75-inch cylinder, intended to work either a 38-inch bucket-pump or a 37-inch plunger-pump. The other was a 70-inch cylinder beam-engine, to work two 28-inch bucket-pumps.
It may be as well here, as this is intended to be a popular account, and not written for professional readers, to state the difference between bucket and plunger pumps.
Early hirstory of the undertaking—1879. The bucket-pump is simply the ordinary lift-pump used all over the country, with a bottom valve fixed as near as possible down to the level from which the water is to be pumped, and with large rods running down the centre of the pump attached to the bucket-valve which works up and down the length called the working barrel, the bucket-valve opening as the pump-rods descend, and allowing the water to pass to the upper side of the bucket; while the valves of the bucket close when the engine makes the upward stroke, and the valve at the bottom opening allows the water to enter, and to follow the bucket in its ascent.
The plunger-pump is a force-pump;the pump-barrel, or rising-main, has no rods inside it, but near the bottom is connected by a casting called the ‘H-piece,’ with a separate closed cast-iron cylinder known as the ‘pole case,’ There are two valves as in the bucket-pump, but they are both fixtures.
The top and bottom of the pole-case are closed, but the plunger at the lower end of the pump-rods works through a stuffing-box on the top of the pole-case, and when the rods are drawn upwards the bottom valve opens while the top one is held closed by the column of water above it, and the atmospheric pressure drives the water through the bottom valve into the pole-case. On the return-stroke of the engine the pump-rods descending close the bottom valve and force the water through the Early history of the undertaking—1879. ‘H-piece,’ and the top valve up the main column of the pump and out to the required level.
The plunger is much the better pump for continuous work, as any slight wear there may be upon the pole itself is easily made good by packing the stuffing-box, while the faces of the buckets in the bucket-pumps are continually wearing by friction against the sides of the working barrel. (The drawings of the large pumps and engines at Sudbrook, given at the end of this volume, will show the difference between the bucket and the plunger-pump.) Of course we should have preferred that all the pumps should have been plungers, but it is necessary in fixing a plunger-pump to get to the bottom of the pump to fix the pole-case as well as the bottom of the rising main securely, and after it is at work to pack the stuffing-boxes. It was impossible to put down plunger-pumps while the shafts were full of water, and it was for this reason that, in the first place, bucket-pumps were adopted and ordered.
The engines for working the pumps were of two sorts, beam-engines and Bull-engines—the latter being so called after the inventor. Both engines take steam only at one end of the cylinder.
The beam-engine takes steam only above the piston. The pressure of the steam when admitted forces down the piston, pulls down the inner end of the beam to the full extent of the stroke, raising the outer end of the beam which carries the pump-rods, and thus making the up-stroke of the pump. Early history of the undertaking—1879. When it has reached the top, the weight of the pump-rods (assisted, if necessary, by weights placed on the inner end of the beam) makes the down- stroke without using steam.
In the 75-inch engine, working a 35-inch bucket-pump, the weight of the wrought-iron beam was nearly 23 tons, and the weight of the pump-rods was nearly 12 tons. The engine could run eleven strokes per minute, raising 376 gallons of water 173 feet high at each stroke. Steam was admitted to the cylinder at 45 lbs. pressure. The vacuum was about 13½ lbs. Steam was cut off at less than one-third of the stroke. The engine worked best at about eight strokes per minute, giving 3,000 gallons raised in that time.
The Bull-engine takes steam only below the piston, the pump-rods being attached to and working in a direct line from the piston-rod. Steam being admitted below the piston, the pump-rods are raised by a direct lift. Their weight, which must be greater than the column of water, makes the return-stroke without steam. The Bull-engine is the most compact-looking, but does not work so economically as the beam. The heavy beam itself, being once set in motion by steam, acts as an accumulator, allowing steam to be used more expansively than is possible in the Bull-engine.
In the 50-inch Bulls, steam was admitted at 45 lbs. pressure, and cut off at one-third of the stroke. The pump-rods weighed about 5½ tons, and the Early history of the undertaking—1879. weight of the plunger and balance-weights was about 18 tons. The pumps worked best at eight strokes, raising 231 gallons per stroke each, but could be run up to thirteen strokes per minute.
The contract contained a general description of the work, stating that the tunnel would be 7,942 yards in length, or just over 4½ miles; that the total length of the railway included in the contract was 7 miles 5 furlongs; that the work under the deepest part of the river, known as the ‘Shoots,’ for a length of 660 yards, should be proceeded with before the other work was commenced, unless the engineer-in-chief should otherwise order. The contract then stated that certain drawings were signed as forming part of the contract; that the engineer had power to substitute other drawings and make any alteration that he thought necessary during the progress of the works; and it was provided that if any alterations were made, additions to or deductions from the contract sum should be made by the engineer to compensate for these alterations.
The work that had been done by the Company was then set forth in the contract, and the pumps and other plant already provided or ordered by them was also set forth in detail; and it was arranged that payment should be made for the use of this plant by the contractor.
The usual clauses were inserted as to the power of the engineer over the works, as to progress and other matters.
Early history of the undertaking
1879. The Company agreed to furnish all the land required permanently for the works; the contractor undertook to provide all that was required for temporary purposes.
The contractor was made responsible for the setting out of the works, for the lines and levels; and the Company, on their part, agreed to the usual terms for payment monthly as the works proceeded.
The open cuttings at both ends of the tunnel were to be surrounded by embankments several feet above the level of the highest recorded tide, so that the most extreme high tide should not, when it flooded the meadows, run down the open cuttings and drown the tunnel.
The tunnel was specified to be lined with brickwork, three bricks or 2 feet 3 inches thick, but only half the tunnel was to have an invert. The brickwork was to be of Cattybrook vitrified or Staffordshire brindle bricks, approved by the engineer.
The mortar was to be made of one part of Portland cement and two parts of perfectly clean, sharp sand.
The concrete, if used, was to be one part of Portland cement and five parts of sand and gravel.
There were to be a number of bridges over the open cuttings, which were all specified in detail.
The ballast for the permanent way was to be broken stone in the bottom, with gravel or hard slag for upper ballast. The rails and sleepers for the permanent way were to be furnished by the Early history of the undertaking—1879.Company, the road to be a cross-sleepered road with steel rails weighing 86 lbs. to the lineal yard of rail.
The original amount of the contract was very considerably increased, firstly, by the whole of the tunnel having an invert; secondly, by the brickwork lining being made 3 feet thick, instead of 2 feet 3 inches, over a considerable part of the length of the tunnel; and, thirdly, from the lowering of the whole of the gradients, which will be mentioned later.
The clause in the contract which stated that the 660 yards under the ‘Shoots’ were to be completed before the rest of the work was proceeded with, was inserted because the Directors and the engineer still thought the chief danger in constructing the tunnel was from an irruption of the river-water; and no one supposed that there lay hidden under the land, within a quarter of a mile of the western shore, a greater difficulty and danger than any to be met with under the bed of the river. The very precautions which were taken for security under the river were the cause of magnifying the other difficulties, and there never was a clearer case of ‘De Scylla in Charybdim.’