The Severn Railway Tunnel.
Chapter 8 - The Side Heading.
In his six-monthly report to the directors of the Great Western, Sir John Hawkshaw described that at the end of 1883, just over two miles of tunnel was completed with a further top break-up finished for another mile. In addition to the two steam-navvies working on the Gloucestershire cutting, was an additional steam-navvy on the Monmouthshire side. The original length of the tunnel was to have been 7,942 yards but a decision was taken in early 1884 to reduce the tunnel to 7,666 yards by increasing the length of the cutting on the Welsh side. This additional material excavated would make up the sidings at a little station near Rogiet, afterwards known as Severn Tunnel Junction.
Generally, the first nine months of 1884 were similar to the same period of 1883 in that the construction of the tunnel continued without any major interruptions. During the middle of February, pump breakdowns at 5 miles 4 chains and Marsh Shafts meant that this area was partly filled with water, but after repairs, the shafts became dry again by the middle of the following month.
Hawkshaw's report for July of that year showed that nearly three miles of tunnel was completed and in August the completed tunnel ran from under the Shoots to the cutting on the Gloucestershire side of the river, while in September, the tunnel was completed from 5 miles 4 chains to the cutting on the Monmouthshire side. The only area of tunnel, therefore, that needed to be finished was from 5 miles 4 chains, past the Great Spring and up to the Shoots. In addition to the work still in operation at both cuttings, a new shaft was sunk just inside the tunnel on the Welsh side which would pump out the rainwater that fell into the cutting. On the English side, the rainwater from the large cutting was channelled into a duct that ran down to the Sea Wall Shaft where the rainwater was pumped out with other water that accumulated inside the tunnel.
In order to deal with the water from the Great Spring, Hawkshaw decided to divert the water from the main heading by driving another heading parallel to the tunnel but 40 feet north of it. Running from the Old Shaft at Sudbrook to the point where it would intercept the Great Spring, the side heading would be driven at a gradient of 1 in 500 against the tunnel's rise of 1 in 90 so that the side heading would end 3 feet below the invert of the tunnel. The water would then be collected into a 12 inch diameter cast-iron pipe and taken to the pumps on the surface. This side heading was commenced in July but since the Great Spring originated from water which flowed through subterranean channels in the broken limestone and sandstone, most of the water was thought to emanated from fissures in the bed of the River Neddern. To reduce the amount of water from this source, the bed of the river was sealed with 7,000 tons of concrete over a distance of four miles.
By the end of September 1884, three of the large engines and pumps had been installed and it was decided to start work immediately on the Great Spring. The sluices in the bottom headwall were gradually opened and at the same time a hole was made in the brickwork at the top of the headwall. After three days pumping the heading was dry and it was possible to inspect the area where the spring had broken in. Since this area had been properly timbered up in October 1883, it was found to be in good condition and so work now started on the side heading. In addition, blasting resumed on the heading between Sudbrook and 5 miles 4 chains by working from both ends of this heading, until on the 17th of October the headings were joined just as Sir Daniel Gooch paid one of his surprise visits to the works. It was now possible to travel from the open cutting on the Gloucestershire side to the open cutting on the Monmouthshire side - although the small hole that joined the headings between Sudbrook and 5 miles 4 chains required a little "pushing and shoving", as Gooch, the first person through the hole with Lord Besborough would testify.
On the 19th of December, the side heading met a large open joint in the strata - the source of the Great Spring - and by diverting the spring into the side heading, the tunnel was left perfectly dry. Other fissures were discovered in this area as the headings were opened out into the full-sized tunnel; in one instance, virtually the last stone that was removed from the invert of the tunnel released a large body of water, while at another point the water boiled up from a hole 18 feet deep. At no time though were the works in danger with the massive pumping power at Walker's disposal.
The last of the brickwork was completed in the tunnel on the 18th of April 1885 and the pipe carrying the water from the side heading was fitted with a sluice valve and pressure gauge so that the quantity of water within the pipe could be regulated. The intention was that the valve would eventually be closed and the water would find its way around the tunnel and down to the river without the need for an expensive permanent pumphouse above the tunnel.
In July 1885, Sir John Hawkshaw reported that the tunnel was nearing completion in that the railway lines had been laid in most of the tunnel, the stone face at the entrance of the tunnel on the Welsh side was finished and the entrance on the English side was well in hand.
On the 11th of August, the valve in the 12 inch pipe from the side heading was closed and in the following days the pressure was recorded as follows:
Sir Daniel Gooch accompanied by his wife and a party of friends inspected the tunnel on the 5th of September in a passenger train hauled by a Great Western engine. It will be noted from the table above that on this date the water pressure had increased to 45½ pounds per square inch, indicating that the water had risen in the ground outside the tunnel to a height of 105 feet and at this pressure, a small quantity of water found its way through the joints in the six layers of brickwork creating a respectable shower in the tunnel.
Feeling the strain of almost six years continual work on the Severn Tunnel project, Walker left for Buenos Aires on the 7th of September to complete some estimates for dock works and he likely to be away for some three months. However, the pressure of the water on the tunnel increased so much that the bricks in the lining started to break, while in one area, the water found its way between the layers of bricks, forcing each layer apart. The most dramatic effect of this pressure of water was in an area of tunnel invert 40 square yards in size where the brickwork slowly rose under the pressure and then collapsed in a heap as the water found a means of escape. After this area had been cleared of the debris, the invert was rebuilt with new bricks but within a small time, the brickwork suffered the same failure.
In desperation at the end of October, Sir John Hawkshaw sent Walker a telegram insisting that he return home immediately.
Copyright © by John Daniel 2013