Analyses for failure patterns of blanket defects on initiation of backward erosion piping

Backward erosion piping (BEP), a critical mechanism of internal erosion in levees and dams, initiates at downstream blanket defects where hydraulic pressures exceed the resistance of cohesive soil layers. While prior studies focused on BEP with preset exits, this research investigates failure patterns under varying defect geometries and soil properties through laboratory experiments and a simplified analytical model. The model integrates defect dimensions (length, width, thickness), soil cohesion, and internal friction angle to predict three failure modes: shear, bending, and bending-shear. The model was verified by laboratory results and revealed the following: (1) the increase in soil cohesion and internal friction angle enhanced seepage resistance, but this effect diminished for defects with high length-to-thickness ratios (a/t) or width-to-thickness ratios (b/t); (2) Larger a/t or b/t ratios reduced the shear strength coefficient (η) exponentially, shifting failure dominance from shear to bending; and (3) the combinations of the ratio of the pressure head at the near-upstream edge and far-upstream edge to the head of the reference point (ξ₁ and ξ₂) had minimal influence on failure thresholds, but transient flow conditions (e.g., sudden head surges) could bypass gradual bending deformation, triggering abrupt shear failure.