---
title: "Short note: Porosity and permeability"
subject: "Engineering Geology II"
chapter: 3
key: "semester_4/engineering_geology_ii:porosity-permeability"
source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/porosity-permeability
site: BCT Engineering
---

# Short note: Porosity and permeability

Engineering Geology II · Chapter 3: Hydrogeology

- Asked in: 2068 Magh, 3 marks
- Video: [Porosity and Permeability](https://www.youtube.com/watch?v=8mfBomrw0rs) — GeoScience Videos
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/porosity-permeability
- Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-3

## Question

Write a short note on porosity and permeability.

## Short answer

Porosity is the percentage of void volume, indicating storage capacity; permeability is the ability to transmit fluid through interconnected pores, expressed as hydraulic conductivity. Clay has high porosity but very low permeability, while sand and gravel are both porous and permeable.

## Answer

#### Porosity
**Porosity ($n$)** is the ratio of the volume of voids to the total volume of rock or soil, usually in %.

$$
n = \frac{V_v}{V}\times 100
$$

- Primary porosity: pores between grains formed at deposition (sand, gravel).
- Secondary porosity: joints, fractures, solution cavities developed later.
- Depends on shape, packing and sorting; well-sorted rounded sand 30-40%, clay up to 50-60%, granite below 1%.
- Porosity measures storage capacity, not the ability to transmit water.

#### Permeability
**Permeability** is the ability of a rock or soil to transmit fluid through its interconnected pores. It is expressed as hydraulic conductivity $K$ (m/s) from Darcy's law, $Q = K A i$.

- Depends on pore size, connectivity, grain size, fractures and fluid viscosity.
- Gravel $10^{-2}$ m/s, sand $10^{-4}$ m/s, silt $10^{-6}$ m/s, clay $10^{-9}$ m/s.

#### Relation
A rock can be porous but impermeable (clay) because the pores are small and not connected. Good aquifers need both high porosity and high permeability.

#### Importance
Porosity and permeability control groundwater storage and yield, seepage under dams, reservoir leakage, drainage of slopes and foundation behaviour.

## Check yourself

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

1. What does porosity measure?
   A. Ability of the rock to transmit water
   B. Rate of flow per unit area
   C. Volume of solids divided by the voids
   D. Volume of voids divided by total volume, indicating storage capacity
   Answer: D. n = Vv/V x 100 gives storage, not transmission. (Key point: Porosity definition)

2. Which statement about permeability is correct?
   A. It is the ability to transmit fluid through interconnected pores, expressed as K in Darcy's law Q = KAi
   B. It is the percentage of voids in the rock
   C. It depends only on the total void volume
   D. It is highest in clay
   Answer: A. Permeability depends on pore size and connectivity and is quantified by hydraulic conductivity K. (Key point: Permeability)

3. Why can clay be highly porous yet practically impermeable?
   A. It has no voids
   B. Its pores are very small and not well connected
   C. Its voids are large and connected
   D. It has secondary porosity from joints
   Answer: B. Clay holds much water but the tiny pores hinder flow (K about 10^-9 m/s). (Key point: Porous but impermeable)

4. What does a good aquifer need?
   A. High porosity only
   B. High permeability only, with low porosity
   C. High porosity and high permeability
   D. Low porosity and low permeability
   Answer: C. Storage (porosity) and transmission (permeability) are both needed for good yield. (Key point: Good aquifer)

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- [Constant head test, k at porosity 39%](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aconstant-head-permeability-porosity) — Soil Mechanics, chapter 6
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