by Bertil Löfquist, Royal Inst of Technology, Stockholm
This paper was first publsihed in GE’s June 1992 issue
Most cases of erosive leakage or excessive seepage in embankment dams have been explained in terms of ‘internal erosion’ or ‘piping’. However, strictly speaking, these terms describe only what can be suspected or seen at the dam, when the process has been going on for some time. They may tell only the end of the story. The real origin of the leak is usually difficult to discover or to explain. We lack adequate theories on leakage in embankment dams.
The classical piping theory and genuine piping, beginning at the downstream side of the dam, is in the proper sense, valid only under special circumstances. Nevertheless, ‘piping’ commonly stands for erosive leakage in general. The concept of hydraulic fracturing gives us one explanation of the development of long leak paths, starting at the upstream side of the dam. Another mechanism, with similar effect but physically quite different, is a consequence of wetting-induced collapse settlements. Such settlements may occur in some layers, not sufficiently compacted or watered, or as a general tendency in the soil. Hydrofracture and collapse settlements in combination must be a very effective mechanism producing leaks from the upstream side. Backwards progressive erosion may be the second phase of the process.
To deal adequately with the subject, a wider concept is needed. It may be termed ‘hydraulic penetration’nd is a question of how the reservoir water in general finds its way through more or less layered, cracked or inhomogeneous soil in a dam. This process is promoted by local irregularities, by hydraulic splitting forces and by wetting induced contraction. Hydraulic penetration may also be seen as the phase in the process, which generates or does not generate hydraulic fracture.