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Blithfield Reservoir Construction

River Blithe scheme

The River Blithe Scheme was authorised by Act of Parliament in 1939, giving the company permission to carry out a significant amount of work including:

  • Impounding the water in the River Blithe
  • Constructing Blithfield Reservoir
  • Creating a number of road diversions, including Admaston Road and Watery Lane
  • Constructing the Seedy Mill Purification Works
  • Extending the existing service reservoirs at Barr Beacon and Gentleshaw
  • Laying a trunk main between Blithfield Reservoir and the Seedy Mill works

Cut-off trench

At the site of Blithfield Reservoir, the River Blithe passed through a wide, flat valley, the floor of which consisted of alluvial sand and gravel, which contained a lot of ground water.

To create Blithfield Reservoir, the water company had to construct a dam across the River Blithe. This involved digging a “cut off” trench along the entire length of the dam's centre line, in order to stop water leeching under the dam. The trench was so deep that it met the hard marl underground. Prior to the excavation for the trench, interlocking steel sheet piling was driven down into the ground until it met the underlying hard marl. The excavation for the trench was then carried out in sections completely enclosed by the interlocking steel sheet piling. As well as providing support for the walls of the trench, this sheet piling was used to exclude some of the ground water held by the alluvial gravel in the valley floor. As the excavation proceeded, as much as 9.1 million litres per day of ground water had to be pumped from the various sections of the trench in which work was proceeding. The trench was filled with mass concrete in 1.4 metre lifts. The bulk of the sand and gravel required for the concreting operations was obtained from the reservoir basin below top water level, the material being washed, crushed and screened at the site.

A section of the trench near to the river was left unopened until it was possible to divert the River Blithe over a completed section of the concrete filling. The river was finally diverted through the discharge tunnel to allow the embankment to be completed across the valley.

Puddled clay core wall

Having prevented the water from seeping through the ground underneath the dam, the designers then turned their attention to the surface. Clearly, should the dam wall be made of the wrong materials, then the water would soak through the dam itself. Additionally, if the wall of the dam was just perched on top of the concrete cut-off, the water may be able to squeeze out between the two and so the dam would leak. Neither of these problems was acceptable so solutions had to be found.

The materials for the dam were carefully chosen. It was decided that a central core wall should be made using puddled clay. In fact, much of the clay was dug out from the local area. A deposit of suitable clay was found less than a kilometre up the valley from the embankment. This clay was brought to the dam and treated using "pug mills" to turn it into puddled clay. The puddled clay, which had a dense, putty-like texture, was placed in layers over the area of the core wall. It was then "heeled" into a compact mass that formed a water-tight barrier along the centre of the dam.

Having solved two of the three problems, all that remained was to find a way of preventing the water from passing between the cut-off trench and the puddled clay core wall. To solve this problem, a special joint was designed which allowed the puddled clay core wall to interlock with the concrete cut-off forming a seal that the water could not penetrate.

Completing the construction

The dam at Blithfield, otherwise known as the "Embankment", was a tremendous achievement in engineering terms. Following the construction of the concrete cut-off, the building of the dam continued with the creation of the puddled clay core wall. The marl that had been excavated to make the cut-off trench was used as "selective embankment material", helping to support the puddled clay wall on both sides. Alluvial gravel was dredged by drag-line excavators from lagoons in the floor of the valley within the reservoir area. A fleet of tipping trucks brought the gravel to the work area where it was then piled up against the marl. The movements of these trucks helped to compact the gravel, aided by bulldozers and vibratory rollers.

In order to prevent erosion of the face of the dam, the waterside of the dam was covered in thick concrete slabs. These slabs were actually made in casts "in situ", the joints between them being filled with fine gravel to allow easy drainage.

At the top of the protective facing provided by the concrete slabs, a specially shaped concrete coving was made. Its purpose was to act as a wave break to help prevent water from flying up and over the dam.

Soil and grass seed were spread on the downstream slope of the dam to create a picturesque field-like area, which is now used for grazing animals.

The labour force

Work commenced on the reservoir in autumn 1947. In order to attract labour, a camp capable of housing 130 men was constructed on the site, which became occupied largely by Irish labour. Roughly one-third of the total labour force was drawn from a unit of the Polish Re-Settlement Corps, which was billeted in a camp within a few kilometres of the reservoir. Coaches were run daily from the Potteries and from the Lichfield, Rugeley, and Cannock Chase areas, to convey men to and from the site. The maximum labour force was 495 men.

Work on the Admaston Road diversion commenced in June 1950. The maximum number of men employed on this was 190. By June 1952, the work was substantially finished, apart from the parapet walls, the construction of which was delayed until 1953 in order to give the embankment a chance to settle down and stabilise.

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