ABSTRACT
Backward erosion piping is one of the causes of levee damage. Breach of levee without overtopping occurred in the Yabe River during the 2012 Northern Kyushu Flood. Possible cause of this breach is backward erosion piping in the sandy ground. It is considered that levee is more vulnerable to piping when its foundation consists of relatively low permeable surface layer over permeable layer with dead end in the protected side. Following the study by Koito et al. (2016), centrifuge model tests are conducted to examine the piping progression in such a layered levee foundation. In the tests, impact of the dead end location of the permeable layer on onset and progression of the piping is examined.
Typical model levee in the centrifuge tests is shown in Fig. 1. In the tests, only the slope on the protected side is modelled. The seepage length is 200 mm and the thickness of the foundation ground is 50 mm in the model scale. Seepage tests are conducted in a centrifugal acceleration field of 50G. Corresponding prototype seepage length is 10 m and the thickness of the foundation ground is 2.5 m. Silica No. 8 (less permeable layer) and No. 5 (permeable layer) are used for the model foundation ground and Kaolin clay is used for the model embankment. The ground water level on the protected side is maintained at the ground surface level. Rise of flood water level is modelled by supplying water to the reservoir on the flood side.
Figure 2 shows side views of the model after the test. Test results reveal that, when the permeable layer exists below the less permeable layer, rise in the flood water causes upward seepage flow in the protected side. This makes the required flood water level for piping lower compared to the uniform one-layer case. Marked contribution of this permeable layer to the piping vulnerability is seen when the dead end of the permeable layer is located near the toe of the slope on the protected side.
