Seepage deformation is a phenomenon in which fine particles within the soil skeleton in granular materials are redistributed under the action of seepage

leading to the change in the internal fabric

hydraulic properties

and mechanical properties of soils. Seepage deformation has become one of the leading causes for the failure of sand-gravel foundation and embankment dams. The newly self-developed rigid wall permeameter was adopted to conduct the seepage test under the action of constant water head on gap-graded sand-gravel soils with different gradation and fines content. The spatial distribution of local hydraulic gradient along the specimen and the variation of vertical displacement were monitored during the seepage tests. The spatial distribution of soil particle size distribution was analyzed after the seepage test. The test results reveal that there appeared to be three kinds of packing state of fine particles

i.e.

under-filled

filled

and over-filled state

and these packing state determined the different contact modes between the coarse particles and the fine particles then affecting the permeability. At the end of the penetration test

the spatial distribution of fine particle loss along the sample height can be divided into three areas

namely the top loss area

the middle uniform loss area and the bottom loss area. The rapid decrease in local hydraulic gradient

accompanied by an abrupt increase in vertical displacement

implied the onset of seepage deformation. The local hydraulic gradient at the start of seepage deformation was more substantial than the global hydraulic gradient

which proves the necessity of carrying out the seepage test on large scale specimens. Specimens with fine particles in the over-filled state are still susceptible to seepage deformation

leading to significant settlement deformation

which is worthy of further investigation.