Vyrnwy Aqueduct
First Vyrnwy Aqueduct
This aqueduct is largest structure on the Montgomery Canal. It crosses the River Vyrnwy, south west of Llanymynech.
William Hazledine built the aqueduct with John Simpson.
Difficulties during construction of the masonry aqueducts at Vyrnwy and Berriew led to parts of the structures collapsing. The engineer, John Dadford, resigned in 1796, and was replaced by his father Thomas Dadford senior (1730-1809). William Jessop was also called in to advise on remedial works.
1797 After 26km of the canal had been built, funding ran out and construction halted.
During the 1820s, the Vyrnwy Aqueduct underwent extensive repairs with cast iron strengthening bars.[1]
Second Vyrnwy Aqueduct
A second Vyrnwy Aqueduct was built, starting in 1881, to carry water to Liverpool from Lake Vyrnwy.
For the first five years of the project, the responsible engineers were Thomas Hawksley and George Frederick Deacon. On the resignation of Mr. Hawksley, Mr. Deacon successfully completed the work as Engineer-in-Chief.
Originally the aqueduct used a single pipeline, constructed generally of 42 in. (1.07m) internal diameter cast iron tubes, but where the route crossed the River Mersey the pipeline was of 32 in. (813mm) internal diameter riveted steel to facilitate maintenance; similarly the River Weaver was crossed by steel pipes.
Its route follows the watershed of the rivers Dee and Severn, maintaining high ground through Hirnant, Oswestry, Malpas and Cotebrook. It then crosses the basins of the rivers Weaver and Mersey to finish at Prescot service reservoirs, east of Liverpool, for onward distribution. The construction entailed tunnels, balancing reservoirs, valve houses, and river and railway crossings. The Norton Water Tower, near Runcorn, was built to hold a balancing reservoir at height above the Aqueduct.
1892 The Aqueduct was said to be the longest in the world when it was opened, conveying the waters of the largest artificial lake in Europe to Liverpool.
In 1892 The Engineer devoted an entire supplement to the project The Engineer 1892/07/15Supp. Engineering also provided a considerable amount of detail.
From Engineering 1892/06/17 and Engineering 1892/06/24:-
To pass under the River Weaver three lines of 32 in. bore riveted steel pipes, each 107 ft. long were laid in a dredged trench, and afterwards covered with a 12-inch layer of concrete. A cofferdam was built on either side of the river to enable the cast-iron syphon pipes to be connected with the tubes under the river. The bottom of the pipes was 21 ft. below top water level, and 8 ft. below the topmost part of the bed of the river. The cofferdam had to be made 24 ft. below top water level, and it extended 3 ft. or 4 ft. into the bed of the river.
The aqueduct was carried over the Cheshire Lines Railway at Delamere in 42 in. diameter steel pipes, 5/8 in. thick. The pipes rested at the ends on brick abutments. The clear span was 50 ft.
The aqueduct passes under the Manchester Ship Canal near to Old Randles sluice, and close to the point where the aqueduct crosses the River Mersey, the two being only 100 ft. apart. For the pipes under the canal a culvert 12 ft. in diameter was constructed of brickwork. It is 305 ft. long, the canal being 120 ft. wide at bottom, with embankments having a slope of 2 to 1. As the canal was not yet made when operations were commenced, a deep trench was sunk across the site of the canal and lined with sheet piling driven to the level of the rock which sloped so that one end of the brick culvert was embedded in the rock with concrete surrounding it and the other on the clay overlying the rock. A layer of concrete was placed below the invert of the culvert and the brickwork thereupon constructed. The culvert is completely surrounded with concrete. The top of the concrete is 5 ft. below the bed of the canal. At either end of the culvert, shafts 14 ft. 6 in. in diameter and approximately 60 ft deep were constructed. Through the culvert was laid steel piping 36 in. in diameter resting on cast-iron saddles.
The most difficult part of the work - technically, contractually, and in terms of reconciling differing views on the location - was the driving of the aqueduct tunnel under the Mersey, near Fiddler’s Ferry. The place of crossing was originally chosen with a view to the pipes being laid in the loose material of the river bed. Had a tunnel of any kind been intended, that site would certainly not have been selected.
The original intention of laying pipes in the bed of the River Mersey was vetoed by the relevant authorities who claimed that the Mersey might be deepened in the future. It was therefore necessary to drive a tunnel under the river. Poor ground conditions meant that the tunnellers had to work under compressed air. Whether there was ever any real intention to deepen the Mersey is unclear, but the changes incurred considerable delays, costs, and increased risks to the health and safety of the workers. Work proceeded slowly. By the time the second contractor gave up, two shafts had been sunk and only 182 ft of tunnel had been finished. Sir Benjamin Baker was called in as an independent consultant. In his report in October 1891 he stated that he was fimilar with many tunnelling projects worldwide, 'but in no instance was the soil to be tunnelled through so varying in character as that found at the Mersey Crossing. Past experience therefore afforded little help. He had no doubt in his mind that the delays and disappointment were not due to any inherent impracticability in the undertaking, but were consequent upon the several contractors underrating the difficulties, and making insufficient provision in the way of plant to overcome them. ..... Sir Benjamin pointed out that the contractors were attempting something which had never been done before, ....'. The Corporation decided that the work should be continued by Mr. Deacon and accepted his proposal to do it without the intervention of a contractor. More than a month was lost before the work could be resumed. It was then, so far as the driving of the tunnel was concerned, speedily finished in less than 4 1/2 months, despite major difficulties arising. The services of H. H. Cochrane, who had much experience of the work, were retained for the management of the men and materials, while A. W. Brightmore continued to act as resident engineer.
The challenges and difficulties of driving the Mersey aqueduct tunnel were also described by George F. Deacon in Engineering 1892/08/12 and The Engineer 1892/08/19
Increasing the capacity of the Vyrnwy Aqueduct
It was intended from the outset that there would be three pipelines, but only one was laid initially, although short lengths of three were laid in places, such as the Weaver crossing.
- Line 1 is made mainly of cast iron and was constructed between 1881 and 1892
- Line 2 is made mainly of cast iron and was constructed between 1902 and 1905
- Line 3 is made of steel and was constructed between 1926 and 1938.
1926 The aqueduct carrying water from Lake Vyrnwy to Liverpool is divided into a number of sections, of which the upper section, between Vyrnwy and Oswestry, carried unfiltered water. That section (13 miles, 21 km in length) was a double line of 42 in. diameter pipes, with capacity less than that of the sections below Oswestry. At the beginning of 1926 the Water Committee of Liverpool Corporation decided to start work to increase capacity of part of the aqueduct between the lake and Oswestry by initially adding an extra pipe of 4.5 miles (7km) length in parallel with the existing pipes, and divided into two sections, one on each side of the balancing reservoir.[2]
See Also
Sources of Information
- Engineering timelines [2]
- [3] 'The Vyrnwy Works for the Water-Supply of Liverpool.' by George Frederick Deacon, M. Inst. C.E. I.C.E. Minutes of Proceedings, Paper No. 2975