Internal stability of granular filters

Internal stability of a granular material results from its ability to prevent loss of its own small particles due to disturbing forces such as seepage and vibration. Internal instability results from the inability of a soil to act as a filter to prevent loss of its own small particles.Within pores in the load-bearing fabric of a cohesionless, granular material there can exist loose particles, and whether or not these particles can be removed by seepage depends on (i) particle size distribution curve of the whole material, (ii) density of the compacted material, and (iii) severity of the disturbing forces.Results of seepage tests are presented for a variety of compacted, cohesionless materials, some of which experienced loss of small particles (unstable gradings) and others of which experienced no such loss (stable gradings). From a synthesis of these results a method is proposed for evaluating the potential for grading instability based on the shape of a material's grain size curve. However, the surest method of making such an evaluation is to perform seepage tests following the procedure described in the paper. Key words: internal stability, grading stability, suffosion, tests, filters.