Recently, in Japan, not a few dam sites are composed of soft and weak rocks or sand and gravel deposits. Since these dam foundations are low in strength, it is required to evaluate seepage failure potential in the designing stage.
There are two types of seepage failure tests (SFT), that is, laboratory SFT for undisturbed specimens and in-situ SFT. Although in-situ test is more expensive than laboratory one, heterogeneity and existence of very weak part of foundation can be taken into account with in-situ test. Therefore, in-situ test can give us more realistic results than laboratory one. Conventional in-situ SFT is the test by use of the water injection from a single borehole (single-hole injection type in-situ SFT). However, because the water flow from a borehole is not onedimensionalbut radial, large potential loss takes place around a borehole. This seepage is much different from the under seepage in dam foundation, and the evaluation of critical hydraulic gradient is very difficult.
In this paper, we have explained this problem about single-hole injection type in-situ SFT quantiatively by using a mathematical model. In addition, we have developed new in-situ SFT by use of the water injection from plural boreholes drilled on the line parallel to the open side plane (multi-hole injection type in-situ SFT) in order to establish approximate one-dimensional flow condition. We have shown the advantage of multi-hole injection type in-situ SFT by use of a mathematical model, and reported results of case study of this test.
