Study on Mechanism of Piping and Its Preventive Methods

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Seepage failure is a problem in hydraulic structures, cofferdams at the location of high ground water level. Once the seepage failure occurs, it causes serious damage to the work itself and/or neibouring places from the economical point of view. There are still many problems unknown, in the seepage failure phenomena, which have to be made clear.In this study, the seepage failure problem of soil within a cofferdam was mainly considered. First the existant theries of seepage failure given by Harza and Terzaghi are investigated precisely, and Prismatic failure -A new method of calculating stability against boiling of sand within a cofferdam is presented. Typical three examples (isotropic and antisotropic one-layred soils, and two-layred soil) were analyzed using these theories, and the characteristics of these theories of seepage failure were then clarified precisely.Second eight examples of case histories on seepage failure were reported and analytical considerations of these problems we … More re given. The possible causes of seepage failures for these examples were discussed and classified as follows : (1) Desige was mistaken or wrong countermeasures were taken. (2) Unexpected situation occurred. (3) Creep length was too short. (4) Inhomogeneous subsoil (including multi-layred soil) was encountered. (5) Seepage flow concentrated three dimensionally into the soil surrounded by a cofferdam. (6) Permeability of soil was highly anisotropic. (7) Rock foundation was weathering in deep excavation. (8) Seepage flow concentrated into the end of a structure constructed within a cofferdam during dewatering. (9) Filter was clogging with the passage of time.Finally, 47 cases of experiments were made on seepage failure of soil within a cofferdam. Analysing the state of soil (homogeneity, anisotropy of soil), changes in formation of soil surface and distribution of equipotentials, the relationship between hydrautic head difference and discharge, hydraulic head differences at which soil starts being deformed and failure occurrs, and angle of repose of soil in water. The hydraulic head difference at which the soil starts being deformed is estimated from Prismatic failure (Friction). Relative density of soil makes effects on modes of failure, hydraulic head differences at being deformed and at failure. Less