Abstract: In this study, experimental tests on seepage deformation of unsaturated gravel clayey soils are carried out, and the influence of soil saturation conditions and hydraulic gradient loading on the capability of wide-graded gravel clayey soil against seepage failure is studied. Results show that the saturation condition has a great influence on the capability against seepage failure. A pre-saturated soil sample can bear a larger hydraulic gradient before piping failure, while the failure occurs at a certain hydraulic gradient when the sample is not pre-saturated. All the unsaturated gravel soil samples fail when a relatively large hydraulic gradient is applied in a single stage. An increase in hydraulic gradient decreases the elapsed time at which fine particles begin to be brought out and the elapsed time at piping failure. Decreasing fines content also leads to a decline in the sample’s resistance to piping failure. For gravel soils with the same dry density and water content, the failure hydraulic gradient decreases with the decreasing fines content. The anti-piping capability of gravel soil is obviously improved by adding a filter protection, but the soil, even so protected, may still fail due to piping if its fines content is low.

Key words: gravel soil, piping, fines content, pre-saturated state, loading mode of hydraulic gradient