Seepage and Stability – Onset of piping in soils Piping (subsurface erosion leading to heave and boils) is most likely to occur when:

Introduction / Context:
Piping is a critical hydraulic failure mode in earth structures and excavations below the water table. It involves progressive removal of soil particles by seepage flow, often observed as sand boils. Recognizing the governing condition helps in designing cutoffs, filters, and relief systems to maintain stability.

Given Data / Assumptions:

• Soil under upward or exit seepage near downstream toes or excavations.
• Total stress constant; pore-water pressure varies with hydraulic head.
• Terzaghi’s effective stress principle applies: σ′ = σ − u.

Concept / Approach:

Piping initiates when seepage forces counteract the submerged weight so that the effective stress vanishes. The critical hydraulic gradient i_c occurs when seepage force per unit volume equals buoyant unit weight: i_c ≈ (G_s − 1)/(1 + e). At i ≥ i_c, σ′ → 0 and soil particles can be lifted and carried away, leading to heave and internal erosion.

Step-by-Step Solution:

Verification / Alternative check:

Field observation of boiling at the base of cofferdams or weirs coincides with head differences that compute to i ≈ i_c for local soils, validating the criterion.

Why Other Options Are Wrong:

(a) High porosity alone is not sufficient without adequate gradient; (b) contrasts permeability but does not directly cause piping; (d) stratification may redirect flow but piping needs σ′ → 0.

Common Pitfalls:

Ignoring anisotropy in permeability and filter design, or calculating gradients based only on total head drop without considering flow path length and exit geometry.

Final Answer:

The effective stress (effective pressure) becomes zero