Upward seepage and zero effective stress: When seepage pressure equals the submerged unit weight-induced pressure, effective stress drops to zero and particles tend to move upward along flow. This phenomenon is commonly known as:

Introduction / Context:
Under upward seepage, seepage forces act opposite to the self-weight of soil grains. If the upward hydraulic gradient is large enough, the effective stress can reduce to zero. Understanding terminology used for this critical state is essential for evaluating construction dewatering, filter design, and retaining works.

Given Data / Assumptions:

• Upward hydraulic gradient through a saturated sandy or silty deposit.
• Seepage force equals buoyant (submerged) weight at the critical gradient.
• Static conditions otherwise (no external reinforcing).

Concept / Approach:

Critical hydraulic gradient i_c is approximated by (G_s − 1)/(1 + e). At i ≈ i_c, effective stress approaches zero and grains are suspended by flow. The phenomenon is referred to as the “quick condition,” “boiling,” or “quicksand.” All three terms describe the same physical state: a loss of intergranular contact forces and the visible “boiling” appearance at the surface due to water and sand ejections.

Step-by-Step Solution:

Verification / Alternative check:

Field observations near sheet piles or pump wells under high gradients demonstrate sand boils and quick behavior—consistent with the above definitions.

Why Other Options Are Wrong:

Each term (a), (b), and (c) is valid; therefore (d) “All the above” is correct. “Piping only” describes progressive removal but is not the standard term for zero-effective-stress state by itself.

Common Pitfalls:

Confusing quick condition with liquefaction under cyclic loading; ignoring filter/drain measures to prevent sand boils.

Final Answer:

All the above