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Collapse of the S&D 60 years ago THE COLLAPSE AND RECONSTRUCTION OF BATH ROAD VIADUCT SHEPTON MALLET
The original main objective of Acts of Parliament in 1856/7 to link the Somerset Central and Dorset Central Railways, providing a route from Burnham-on-Sea to Poole, was to join the Bristol and English Channels. The provision of a north-south route was initially only a secondary consideration. It was expected that the Bristol & Exeter Railway would lay standard gauge over its line from Highbridge to Bristol, thus giving the new line a link to Bristol and the north.
But by the time amalgamation to form the Somerset & Dorset Railway took place in 1862, it was clear that the B&E were not going to play, and other routes to Bristol and the north would have to be considered. The arrival of the Midland Railway at Bath in 1869 spurred the S&D to propose a link from Evercreech to Bath, which was duly authorised on 21st August 1871 and opened on 20th July 1874.
The major difficulty of the route was geographical. North-west of Wells, the Mendip Hills rise like a great rampart above the moors to an average of 800 feet. To the east around Shepton Mallet, however, the hills rise more gradually from the plain in a series of ridges and valleys, but it was still no easy task to drive the Bath Extension over the summit level of 811 feet at Masbury, with long stretches at the ruling gradient of 1 in 50. The southern slopes required major engineering features, four viaducts and one tunnel in a distance of 4 miles.
The opening of the Bath Extension brought a rapid Increase in traffic over the S&D, completely overwhelming the Company's resources and resulting in the line's joint lease by the London & South Western and the Midland Railways in 1875. Improvements were put in hand, including plans for doubling most of the northern half of the main line, completed by 1890.
One of the major features doubled on that section was Bath Road viaduct, about one mile north of Shepton Mallet station, just over 100 yards long. It has a maximum height of 75 feet, but its position makes it appear even more imposing, particularly when viewed from the west side. It was constructed in masonry with six brick arches, each of 50 feet span. The arches of the original viaduct had six rings of brickwork, but an extra ring was added to match the seven on the new viaduct, which was added on the up side. The viaduct is straight, on a gradient of 1 in 70 falling from north to south.
Trouble was experienced at an early stage, from water leaking through the longitudinal joint between old and new viaducts. Efforts were made to seal the joint with mortar from above.
A number of years later it was noticed the joint had opened slightly, but the movement did not develop and repairs to the arches were carried out by inserting new brickwork on the line of the joint. Between the wars, much defective masonry in the piers was cut away and replaced by blue brickwork, mainly in the newer portion. In 1945 it was noticed that the longitudinal joint had opened slightly in the two centre spans and the Divisional Engineer started the erection of tubular scaffolding for inspection in November, Inspection was made early in January 1946, which showed that remedial action was required and, while a scheme was being prepared, further movement was noted. On the 30th January, the widened part of the bridge was closed to traffic and single line working instituted over the older bridge.
Max Shore was a member of the station staff at Shepton Mallet at the time. He walked over the viaduct on 1st February to put oil in the signal lamps at Winsor Hill. Returning to Shepton in pouring rain, he climbed out onto the scaffolding at Bath Road Viaduct and sheltered under the arch, talking to a mason who was carrying out repairs. A train passed over and Mr Shore remarked to the mason on the amount of dust and small debris falling from the structure, to which the mason replied: "Do you think I would be working here if there were any danger?"
At 10.55pm on Friday 1st February 1946, the centre pier collapsed, carrying with it the arch on each side, but leaving the remainder of the newer section and the whole of the original viaduct standing. The track remained suspended across the gap. Fortunately, the road beneath was clear of traffic and no damage was caused to the nearby cottages. The overhead telephone and electric lines were brought down and a water main broken. Early Saturday trains were diverted via Highbridge and Bristol and a shuttle bus service run between Binegar and Shepton Mallet.
The Divisional Engineer inspected the old viaduct and found it unaffected by the fall. The down line was slewed towards the outside parapet by nearly two feet and opened to traffic with a speed restriction of 15mph later on the Saturday.
A contract for the whole of the reconstruction work was quickly drawn up and let to William Cowlin Ltd., Bristol, under the supervision of V. H. Short, Divisional Engineer SR and Resident Engineer G. F. George. Work began on the clearance of rubble on 10th February, taken by lorry to a nearby quarry. The next task was the removal of the suspended track. This was carried out on 22nd February by the use of explosives to cut all four rails simultaneously by Sappers of 591 Airborne Squadron RES from Burford. One pound high explosive charges were fixed to each rail and detonated electronically from a distance. The only damage was one broken window in the nearest house and within two hours the fallen track had been cut up and cleared away.
Rail traffic was maintained by single line working between Shepton Mallet and Masbury. The temporary works for traffic operation were completed on 24th February. Ballast on the open side of the down line was retained by timber baulks, but the absence of a parapet was particularly hair-raising and caused the practice of banking up freights to be discontinued. One Bath driver refused to run over the viaduct.
Reconstruction was carried out using brick faced concrete piers and concrete arches, the decision being related to the speed of construction and availability of materials. Materials for the foundations and lower parts of the piers were brought to site by road, the remainder brought by rail and unloaded in sidings created at each end of the viaduct. Pier foundations were taken to solid rock, 4-6 feet below ground level. Pier construction as far as the arch springing took only 3½ weeks. One particular feature of the work was the use of tubular steel scaffolding both for the construction of the piers and supporting the centring for the arches. Arch rings were 3 feet thick, cast in nine equal segments using galvanized corrugated iron shuttering and completed in two weeks. Centring was struck 3 weeks after completion of the arches with no measurable settlement.
On Thursday 1st August, the line was opened to normal traffic with a speed restriction of 15mph. The up line was opened with a slew of about 10 inches towards the down to enable the parapet wall to be completed between trains. The speed restriction remained until both lines were restored to their final positions on September 16th.
William Cowlin & Sons were paid £26,000 for their work which, with the alterations to track layout and signalling and compensation claims, brought the total cost of the reconstruction to around £30,000. Cowlins were awarded a further contract to examine the original portion of the viaduct. There was also serious concern regarding Ham Wood and Charlton, the other viaducts that had been doubled by Firbank at the same time as Bath Road. It was decided to impregnate cement grout under pressure into the piers and again Cowlins were awarded the contract to complete the trials and necessary repairs.
Frosts during the severe winter of 1946-7 revealed further weakness in the original piers of Bath Road Viaduct. The down line was closed from 8th February until 3rd March 1947, with single line operation instituted. Defective stonework was secured, cores of the piers grouted and the facing rebuilt as necessary. The frosts also damaged stonework on Nettlebridge, Ham Wood, Charlton and Prestleigh Viaducts. Yet again Cowlins carried out the repairs, bringing their total income from work to the S&D viaducts during 1946-7 to nearly £100,000.
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