<![CDATA[Internal instability in granular filter materials can lead to suffusion, a critical mechanism contributing to embankment dam failure, yet its assessment is often secondary to conventional filter design. Variations in filter band limits among design standards may affect the likelihood of instability. This study evaluates the internal stability of well-graded sand used as granular filters and designed according to three standards: The United States Bureau of Reclamation standard; the Natural Resources Conservation Service standard; and the Indonesian National Standard. Three grain geometry analysis methods were employed to assess internal instability within the filter band limits specified by each standard: the Kenney–Lau, Kézdi, and Sherard methods. A well-graded, internally unstable filter was successfully developed within the filter band limits of the Natural Resources Conservation Service standard and the Indonesian National Standard. In contrast, the filter band limits of the United States Bureau of Reclamation standard did not allow the development of an unstable filter, indicating stronger control over internal instability. These results confirm the applicability of the Kenney–Lau method as a practical and conservative framework for assessing internal stability in well-graded soils. These findings also support the adoption of more restrictive filter band limits, since higher maximum-to-minimum grain size ratios increase the likelihood of instability.]]>
