Wall and pillar drains in puddle clay embankment dams

Puddle clay core embankment dams were built prior to the use of rolled clay core dams and comprise a large proportion of UK dams, the dams relying for their watertightness on adding water to (puddling) the clay until it had an undrained shear strength of around 10kPA, and thus had increased ability to deform without cracking. Such dams sometimes had early forms of unfiltered drains, comprising coarse cobbles, in the downstream shoulder, either to assist construction, and/or to lower the phreatic surface and improve slope stability once the reservoir was in operation. These drains are separate from foundation drains dealing with springs encountered in the foundation excavation and drains to deal with surface runoff. This paper gives examples of such drains in dams constructed between 1863 and 1940. These drains were installed prior to the development of classic filter design rules and tools to analyse seepage. This is important as the drains have no filter, and are thus vulnerable to internal erosion which, if not detected, could lead to failure of the dam. These drains may also be the true root cause of historic incidents at these dams, due to embankment fill materials washing into the drain and creating sinkholes.