The Dam Embankment
Once the River Hodder had been diverted through the culvert tunnel it was possible to begin work in earnest in the area of the main trench and dam embankment.
Once impervious bedrock had been reached across the full width of the dam, that part of the trench in the bedrock was scrubbed clean and grouted and was then filled with 11,876 cubic yards of concrete. The top of the concrete was shaped to form a concrete "shoe" in the base of the main trench. This was designed and shaped to form a foundation key for the "puddle clay" waterproof centre of the dam embankment. The "shoe" ensured that the clay could not slide away from the bedrock and allow the dam to leak.
Concrete "shoe" in the bottom of the main trench
The puddle clay wall rises from the top of the concrete "shoe" in the main trench to 5ft. above the top water level of the reservoir. Clay for the puddle trench was dug from fields within half a mile of the dam site. Pneumatic spades were required to dig the "blue" clay as it was extremely hard but it was of excellent quality for the purpose with virtually no stones in it.
Excavating puddle clay c.1928
Photograph used by kind permission of Lancashire Library Service - Clitheroe
69,842 cubic yards of clay were used, all of which had to be laid in layers by hand and "puddled" or trodden in by the navvies in thigh-length boots working constantly in water pumped into the trench to make the clay workable. The maximum width of the puddle clay wall is 24ft. and the minimum 8ft.
Puddle clay trench (centre left) and embankment works
As the puddle wall increased in height it was possible to commence the building of the embankment on each side of the puddle trench.
Ceremonial commencement of the dam embankment above ground level: the chairman of the Fylde Water Board, Councillor Robertson of Fleetwood, tips a wagon load of "fill" - 19/6/1925
The dam embankment is formed of 732,000 cubic yards of earth filling, the greater part having been excavated from within the reservoir. The large quantities of "fill" were excavated using Ruston "Steam Navvies", mainly from the reservoir bed within the area to be flooded by the dam. This material was then loaded into "Manchester Ship Canal" type (M.S.C.) side-tipping wagons and hauled on temporary narrow gauge railway lines to the dam embankment.
Ruston steam navvy excavating and loading "fill" from the reservoir side into "M.S.C" type wagons
Photograph from the late John Heap collection, used by kind permission of Mr. A. Walmsley
The length of the embankment at the top is 1,160 ft. The greatest width at the base is 678ft.; the width at the top is 20ft. The height above the bed of the River Hodder is 11oft. The downstream face has a slope of 2½ to 1, with two "berms" (or ledges) each 20 ft. wide and one berm 68ft. 9 ins. wide. The upstream face has a slope of 3 to 1 and is shingled and pitched with stone, the area covered being 24,000 sq. yds.
The first water is impounded by the culvert bulkhead and the forebay wall. The embankment rises behind the forebay wall. The base of the valve tower may be seen above the culvert entrance
The shingling for the upstream face of the dam was excavated from a gravel bed found beneath the site of Grange Hall when it was demolished in 1926. This gravel extended down in a layer approximately 8ft. deep into the reservoir bed.
Steam navvy excavating and loading gravel into "M.S.C" type wagons hauled by 0-4-0 tank engine (possibly "Ogden"?) for transport to the dam embankment
Photograph used by kind permission of Mrs. J. Lawson
The stone "pitching" used to face the upstream side of the dam and thereby prevent erosion of the embankment was quarried at Jumbles Quarry and transported by rail to the dam site.
Embankment, Valve-Tower & Board House 1931
Stone pitching work on the embankment being undertaken.
Laying topsoil and turf to complete the dam embankment. The stone facing of the upstream side of the dam can be seen - temporary railway line being removed in the background
Photograph used by kind permission of Mr. A. Walmsley
Completed dam embankment c. 1932
Photograph used by kind permission of Mrs. J. Lawson
In the 1990s the embankment was modified by the building of a wall along the top of the dam embankment. This wall was built to eliminate the risk of the dam being eroded by wind-driven waves breaking over the top of the earth embankment. It is called a "wave wall" and is now commonly installed on many earth embankment dams.
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