It is not only the artificial barrier that has to be watertight but also the country formation underlying the dam and surrounding the body of water. In certain instances—not in New Zealand—reservoirs have failed to fill because of undiscovered seepage paths underground. Foundation and abutment treatments, sometimes extending also to parts of the lake's rim, to tighten and strengthen these zones are an important feature of dam construction; such treatments (e.g., by pressure injections of fluid mixtures of cement and water and of other materials) may often extend in depth to hundreds of feet and can become quite elaborate and expensive. Foundation treatments at Karapiro and Roxburgh dams extend actually below sea level. Moreover, saturation of the foundation materials, whether of earth or rock, through ingress of lake water and the ensuing development of “pore pressures”, can endanger the stability of dams owing to the effects of partial flotation. Hence to tighten the formation in the vicinity of the water-face is not enough. It is also necessary to provide means for drainage from under and from within the body of the structure or embankment. Typical examples of these provisions are depicted in the diagrams.