---
title: "Quick sand condition"
subject: "Soil Mechanics"
chapter: 6
key: "semester_4/soil_mechanics:quick-sand-condition"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/quick-sand-condition
site: BCT Engineering
---

# Quick sand condition

Soil Mechanics · Chapter 6: Principle of Effective Stress, Capillarity and Permeability

- Asked in: 2079 Asoj, 2 marks; 2078 Baisakh, 2 marks; 2078 Chaitra, 1 marks; 2075 Bhadra, 2 marks; 2078 Poush, 2 marks; 2076 Baisakh, 2 marks (asked 6 times)
- Video: [Quick Sand Condition | Soil Mechanics](https://www.youtube.com/watch?v=V1N6R8kUhLw) — APSEd
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/quick-sand-condition
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-6

## Question

What is the quick sand condition? Explain the quick sand condition (during upward seepage flow).

## Short answer

Quick sand condition occurs when upward seepage makes effective stress zero in cohesionless soil, so it loses shear strength and behaves like a liquid. The critical hydraulic gradient is ic = gamma'/gamma_w = (G-1)/(1+e), about 1 for typical sands.

## Answer

**Quick sand condition** is the state in which the effective stress of a cohesionless soil becomes zero because of upward seepage of water. The soil then has no shear strength and behaves like a liquid. It is not a type of sand; any cohesionless soil (sand, silt) can become "quick".

#### Explanation (upward flow)

Consider a soil layer of thickness $L$ with water flowing upward under a head difference $h$.

```text
   h  ↑  water level in standpipe
  ----+----------------
      |   water
  ----+----------------  top of soil
      |   sand  L
  ----+----------------  base of soil
        ↑ ↑ ↑ upward flow
```

At the base of the soil layer:

- Total stress $\sigma = \gamma_{sat} L$
- Pore pressure $u = \gamma_w (L + h)$ (excess head $h$ added)
- Effective stress $\sigma' = \gamma_{sat}L - \gamma_w(L+h) = \gamma' L - \gamma_w h$

As $h$ increases, $\sigma'$ falls. When $\sigma' = 0$:

$$
\gamma' L = \gamma_w h \quad \Rightarrow \quad i_c = \frac{h}{L} = \frac{\gamma'}{\gamma_w} = \frac{G-1}{1+e}
$$

At this critical hydraulic gradient the seepage force just balances the submerged weight of the soil. The sand particles lose contact, boil up and the soil loses bearing capacity. For most sands $i_c \approx 1$.

Quick condition often causes failure of excavations, cofferdams and the downstream toe of hydraulic structures (piping).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Quick condition is when:
   A. Pore pressure is zero
   B. Sand is fast moving
   C. Soil is very dense
   D. Effective stress of cohesionless soil becomes zero due to upward seepage
   Answer: D. Seepage force balances submerged weight. (Key point: Definition)

2. Critical hydraulic gradient:
   A. (1+e)/(G-1)
   B. (G-1)/(1+e)
   C. G/(1+e)
   D. (G+1)/(1+e)
   Answer: B. i_c = g'/gw = (G-1)/(1+e). (Key point: Critical gradient)

3. Effective stress under upward flow:
   A. gsat L
   B. gw h
   C. g'L + gw h
   D. g'L - gw h
   Answer: D. Falls as h rises. (Key point: Effective stress)

4. Typical i_c for sands:
   A. About 0.01
   B. About 1
   C. About 10
   D. About 5
   Answer: B. G about 2.65, e about 0.65 gives about 1. (Key point: Typical value)

## Related in Soil Mechanics

- [Critical gradient; quick condition in sand](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Acritical-hydraulic-gradient-quick) — Soil Mechanics, chapter 6
- [Upward seepage stress variation diagram](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aupward-seepage-stress-diagram) — Soil Mechanics, chapter 7
- [Infinite slope, cohesionless, steady seepage](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Ainfinite-slope-cohesionless-seepage) — Soil Mechanics, chapter 11

## Related in other subjects

- [Control of sub-surface water and seepage](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Asubsurface-drainage-measures) — Transportation Engineering I, chapter 4, asked 15×