Internal stability of granular filters under static and cyclic loading
Results from a series of hydraulic tests conducted on four compacted granular filters in a one-dimensional rigid cell under static and cyclic loading conditions are presented. It is revealed that cyclic loading promotes premature washout failures in internally unstable and marginal specimens. The constant agitation of granular media combined with internal pore pressure affects the geometrical arrangement of stable primary fabric (coarse fraction). The results prove that washout becomes excessive at higher loading frequencies, such that marginally stable specimens also become increasingly unstable (higher effluent turbidity). The analysis also reveals that the existing criteria developed for static filtration cannot accurately capture the true potential for internal instability of some specimens tested in this study. Consequently, a revised geometrical approach is proposed that incorporates cyclic conditions and demarcates a more distinct boundary between internally stable and unstable soils. A broad experimental database adopted from published studies for validation showed a close agreement with the proposed approach.