Quick Sand (Boiling) Condition – Identify the Correct Statements In geotechnical engineering practice, the term “quicksand” (a boiling condition under upward seepage) refers to a phenomenon in cohesionless soils when the effective stress becomes zero due to seepage forces. Which of the following statements about quicksand is correct?

Difficulty: Easy

The phenomenon of quicksand generally occurs in cohesionless soils such as sands and silts
At the critical hydraulic gradient, a saturated sand becomes quick as effective stress tends to zero
The critical hydraulic gradient depends on both void ratio and the specific gravity of soil solids
Quicksand conditions are more likely in fine sand and silt than in very coarse material
All of the above

Correct Answer: All of the above

Explanation:

Introduction / Context:
“Quicksand” is not a special type of sand; it is a hydraulic condition that develops in cohesionless soils under upward seepage when the seepage force balances the submerged unit weight and the effective stress approaches zero. Understanding when this occurs is crucial for safe dewatering, excavation below the water table, and evaluating piping/boiling risks near cutoffs or sheet piles.

Given Data / Assumptions:

We are dealing with saturated cohesionless soils.
Upward seepage exists (e.g., under a hydraulic gradient beneath a structure).
Critical condition occurs when effective stress → 0 at some point.

Concept / Approach:

The critical hydraulic gradient i_c is approximated by i_c = (Gs − 1) / (1 + e), which depends on the specific gravity of solids Gs and void ratio e (or porosity n). When i ≥ i_c, seepage force equals the submerged weight; the soil skeleton loses intergranular contact, leading to a “boiling” appearance and loss of shear strength in sands/silts. Cohesive soils do not exhibit classic quick conditions because their undrained shear strength resists boiling.

Step-by-Step Solution:

Recognize that quick condition is an effective stress issue: σ′ = σ − u → 0 under upward seepage.Note that i_c depends on packing (e) and mineral density (Gs), not on grain size alone.Finer cohesionless soils (fine sand/silt) are more susceptible due to lower permeability and higher upward gradients under the same head loss.Thus each individual statement aligns with theory; collectively, 'All of the above' is correct.

Verification / Alternative check:

Design checks against piping at exits of dams/cutoffs use exit-gradient limits consistent with the same i_c concept. Field boiling tests near excavations confirm the dependence on Gs and e.

Why Other Options Are Wrong:

Each of (a)–(d) is correct; choosing any single one ignores the rest, so only (e) fully captures the truth.

Common Pitfalls:

Assuming quicksand is a soil type; overlooking that clays rarely “boil” like sands; forgetting that upward gradients from improper well-point spacing can trigger boiling.

Final Answer:

All of the above

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