Quick (liquefied) condition in cohesionless soils: When a clean sand or other cohesionless soil reaches the quick (boiling) condition during upward seepage, it effectively loses which engineering attributes?

Introduction / Context:
The quick or boiling condition in cohesionless soils occurs when upward seepage reduces the effective stress to near zero. This is a critical stability issue beneath structures, cofferdams, and cutoffs where upward gradients can induce piping or heave. Recognizing the implications is vital for safe construction dewatering and filter design.

Given Data / Assumptions:

• Soil is cohesionless (c ≈ 0), such as clean sand or silt.
• Upward seepage increases pore-water pressure.
• Critical gradient ic ≈ (Gs − 1) / (1 + e) for typical cases.

Concept / Approach:
Effective stress principle states: sigma′ = sigma − u. As upward seepage increases u, effective stress falls. At the quick condition, sigma′ → 0, so shear strength τ = sigma′ tan φ → 0. Bearing capacity in sands depends on effective stress and friction angle; when sigma′ vanishes, capacity collapses and soil behaves like a fluidized suspension, unable to support loads.

Step-by-Step Solution:

Verification / Alternative check:
Field evidence shows heave and sand boils when gradients approach ic. Countermeasures include relief wells, filters, and increasing overburden to restore sigma′.

Why Other Options Are Wrong:

• A/B: Each alone understates the full loss; both properties are compromised.
• D: Contradicts effective stress mechanics.
• E: Unit weight does not alone capture the strength loss mechanism.

Common Pitfalls:
Confusing the quick condition with liquefaction under cyclic loading; both reduce sigma′ but mechanisms differ (steady seepage vs cyclic pore-pressure buildup).

Final Answer:
Both shear strength and bearing capacity