---
title: "Permeability method for clay"
subject: "Soil Mechanics"
chapter: 6
key: "semester_4/soil_mechanics:clay-permeability-lab-method"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/clay-permeability-lab-method
site: BCT Engineering
---

# Permeability method for clay

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

- Asked in: 2078 Poush, 1+1 marks
- Video: [Falling Head - Variable Head Permeability Method](https://www.youtube.com/watch?v=SS7o6sKw2Xc) — Elementary Engineering
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/clay-permeability-lab-method
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-6

## Question

At a site, the initial investigation showed that the soil is cohesive (clay). If you have to determine the coefficient of permeability of the soil, which method is most appropriate in the laboratory and why? Also write the expression to determine the coefficient of permeability.

## Short answer

The falling (variable) head test is most appropriate because clay has very low permeability, making constant-head flow too small to measure accurately. The coefficient is k = (2.303 a L)/(A t) log10(h1/h2), where a is standpipe area, L sample length, A sample area, and t is time for head to fall from h1 to h2.

## Answer

#### Method

The **falling (variable) head permeability test** is the most appropriate for clay.

**Why:** clay has very low permeability ($k \approx 10^{-7}$ to $10^{-10}$ m/s). In a constant head test the flow would be too small to collect and measure accurately in a reasonable time. In the falling head test, a standpipe of small area $a$ is used, so even a tiny quantity of flow gives a measurable fall in head.

#### Expression

For a sample of length $L$ and area $A$, with head falling from $h_1$ to $h_2$ in time $t$:

$$
k = \frac{2.303\,a\,L}{A\,t}\log_{10}\frac{h_1}{h_2}
$$

where $a$ is the standpipe area. (A consolidation test can also be used for clays, using $k = c_v m_v \gamma_w$.)

## Check yourself

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

1. The most appropriate laboratory test for clay is:
   A. Constant head test
   B. Falling head test
   C. Pumping out test
   D. Hazen's formula
   Answer: B. Falling head is suited to low-permeability soils like clay. (Key point: Choice of test)

2. Why not the constant head test for clay?
   A. Clay is too permeable
   B. Flow is too small to collect and measure accurately in reasonable time
   C. It needs a large pump
   D. Clay cannot be saturated
   Answer: B. Clay k is about 1e-7 to 1e-10 m/s so the flow is tiny; a small standpipe makes the head fall measurable. (Key point: Why)

3. The falling head permeability expression is:
   A. k = (Q L)/(A h t)
   B. k = 2.303 (a L)/(A t) log10(h1/h2)
   C. k = 2.303 (A L)/(a t) log10(h1/h2)
   D. k = (a L)/(A t) (h1 - h2)
   Answer: B. k = 2.303 a L/(A t) log10(h1/h2), a being the standpipe area. (Key point: Expression)

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