Investigation of gap-graded soils' seepage internal stability with the concept of void filling ratio.

Gap-graded soils from mountain areas are often used as subgrade filling materials, but problems associated with the gap-graded soils such as large permeability, poor uniformity, and poor seepage stability have to be solved. This article proposes a new terminology "void filling ratio" to study the seepage internal stability of gap-graded soils as subgrade filling materials. Laboratory seepage tests were performed to investigate the effects of compaction degrees of coarse grains, void filling ratios, and clay contents on the internal stability. Laboratory model tests were also performed to verify the findings from the laboratory seepage tests. It was found that the internal stability increased with increase of the void filling ratios, confirmed by both laboratory seepage tests and slope model tests. The increases of both void filling ratio and the clay content were able to change the type of internal instability from piping to the transitional type of internal instability. In laboratory model tests, surface areas lost more fine particles than the deeper area did in the models, but when the void filling ratio was increased, the amount of lost fine particles was significantly reduced. Finally, it was confirmed that void filling ratio was able to effectively describe the internal stability of gap-graded soils subject to different levels of hydraulic gradient.