(1) Soil investigation
The tradition of soil mechanics and geotechnical engineering relies on the subsoil investigation by which both spatial/geometrical information and those on material properties are obtained. More elaborate investigation costs more than what inexpensive construction affords. In case of a river levee, the problem is not only that the subsoil is variable at a spatial scale of 30m or so but also that the inside of a levee itself is highly heterogeneous. It is not rare that there is no information about the cross section and soil properties because manylevees have been constructed and expanded several times in the past hundreds of years (Photo 4). Because of the significant heterogeneity, desirably subsoil investigation should be carried out on a cross section of a levee at every 10 meters or so along the levee axis. Therefore, some efforts have been made in the recent times to use geophysical methods of investigation. However, the obtained resolution is not good enough from the geotechnical viewpoints.