Internal stability refers to the ability for the coarse fraction of a broadly-graded soil to prevent the loss of its fine fraction due to seepage flow. The internal stability of a soil is influenced by geometric conditions (i.e. grain sizes and their distribution and pore sizes and their distribution)

hydraulic conditions (i.e. hydraulic gradient and flow direction)

and mechanical conditions (i.e. compaction efforts). From the geometric standpoint

soils are classified into two groups: well-graded and gap-graded

based on the information of 167 soils in a laboratory test dataset and understanding on the control variables for internal stability. Each group is further divided into three categories according to the fines content (< 0.063 mm). Finally

new internal stability criteria for each category of well-graded and gap-graded soils are proposed. From the hydraulic point of view

the influence of complex stress states on the initiation and development of seepage erosion is investigated. It is found that both the initiation hydraulic gradient and the failure hydraulic gradient increase with confining pressure. This is mainly due to the increase of friction between particles.