Void states perspective for critical hydraulic gradient of internally unstable non-cohesive soils

The critical hydraulic gradient ([Formula: see text]) for internally unstable soils is observed to be significantly less than the [Formula: see text] given by Terzaghi's classical equation. The difference is due to the uneven stress contributions by fine and coarse fractions of internally unstable soils, and thus an empirical stress reduction factor ([Formula: see text]) was considered for [Formula: see text] predictions. Internally unstable soils are coarse fraction dominant with a void ratio of the fine fraction higher than the void ratio of overall internally unstable soil. The differential stress states lead to washout of the fines from the local voids of the coarse fraction at a hydraulic gradient of less than the [Formula: see text] given by the classical equation. This study presents a theoretical approach for predicting [Formula: see text] based on the notion of the effective stress of the fine fraction within the voids of a coarse fraction being equal to zero for internally unstable soils. A comparison of predicted [Formula: see text] based on the proposed approach and experimental observations obtained from the literature and the current study has demonstrated the validity of the proposed approach. The applicability of the proposed approach is also illustrated for the estimation of a critical hydromechanical envelope.