ABSTRACT
EARLY PUDDLE CORE DAMS 10. The systematic use of puddle clay for l ining canals in porous ground is generally attributed to Brindley in the 1760s. Moreover puddle clay was also used in the side embankments of canals either as a sloping core, as on the Forth & Clyde canal in 1768 (ref. 4) or, probably more often, as a vertical central core (ref. 12). The idea of using puddle clay when an impervious core had to be incorporated in a dam would therefore have been obvious to the canal engineers. 11. Specifications for dams dating from the end of the 18th century and the first half of the 19th century all indicate that the clay (after being left to weather, if necessary) was brought to the embankment to be placed in layers 15 to 20 cm thick, watered and allowed to soak overnight, or for 24 hours, and then trodden, cross-cut and heeled in . In the second half of the 19th century more usually the clay was watered and worked up at the borrow pit (or 'clay field1) before being brought on to the dam. Finally, in the 20th century, the clay was prepared in pugmills and placed generally in 10 to 15 cm layers. The second, and especially the third of these procedures resulted in more uniform material but in all three methods the average strength must have been much the same: suitable for the unaltered process of treading and heeling. Tests on several puddle cores soon after placing show an undrained shear strength around 10 kPa (ref. 16). 12. Butterlev. Though very probably not the first, the earliest puddle core dam of which any details are known to the writer is at Butterley; 10 m high built 1793-95 to Jessop's design for the Cromford Canal (ref. 17). The bank fi l l consisted mostly of spoil from a deep approach cutting to the Cromford summit tunnel in shales and sandstones of the Coal Measures; the core, vertically-sided, was 2.7 m wide and extended in trench a few metres below ground level. 13. Rudyard. A improved form of puddle core, tapering symmetrically from 6 ft (1.8 m) at the top to a width of 12 ft (3.6 m) at ground level, was adopted by Rennie for the 10.5 m high Rudyard dam; see Fig. 2. Built 1798-1800 for a branch of the Trent & Mersey (ref. 18) the dam displays logical design in all its features and is maintained today in excellent condition after nearly two centuries of uninterrupted service. Rennie's specification includes a requirement that the bank fill should be placed in 60 cm layers with large stones excluded. It is probably made of weathered sandstone and shale of the Millstone Grit Series, obtained from borrow pits within the reservoir area. From this date symmetrically tapered cores became standard practice. 14. Glencorse. The first deep cut-off was constructed at Glencorse dam (1819-23, H = 23 m) for Edinburgh's water supply: design by James Jardine with Rennie and Telford as joint consultants. The puddle trench had to be taken to a depth of 38 m below crest level, in water-bearing coarse gravel, before reaching sound rock. To avoid any risk the core (and top of the puddle cut-off) was made 18 m wide at ground level at the deepest section, while the upstream and downstream slopes of the embankment were built at 3:1 (ref. 19). The trench, excavated in open cut, had a bottom width of at least 4.5 m.
