Laboratory tests and analysis on piping in two-stratum dike foundation

The piping erosion phenomena are simulated in laboratory tests. The erosion is divided into 4 periods through the analysis of hydraulic head, flux and spring sand quantity: steady period before the superstratum breaking, period of the superstratum breaking, steady period after the superstratums breaking and period of the entire breaking. It is found that at the very beginning the superstratum bears the most of the head difference, which shifts to the underlying stratum after the superstratum breaking. Then the hydraulic gradient in the underlying stratum is enhanced greatly for the head difference shifting. And if the hydraulic gradient exceeds the critical value, the piping erosion will happen in the stratum. After a period of erosion, the upper hydraulic head is reduced and raised again for the next period of tests to simulate seepage erosion in breaking dike foundation. After some repeated tests, the capability of the dike foundation to resist the seepage erosion is deeply reduced and a concentrated leakage passage occurs in the underlying stratum. The foundation may burst for the development of the leakage passage if no effective measure is taken.