---
title: "Phase diagram; void ratio, saturation"
subject: "Soil Mechanics"
chapter: 2
key: "semester_4/soil_mechanics:phase-diagram-void-ratio-saturation"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/phase-diagram-void-ratio-saturation
site: BCT Engineering
---

# Phase diagram; void ratio, saturation

Soil Mechanics · Chapter 2: Solids- Water –Air Relations and Index properties of soils

- Asked in: 2078 Baisakh, 2 marks
- Video: [“Soil Three Phase Diagram 💧 | Void Ratio & Degree of Saturation Made Easy | Civil Simplified”](https://www.youtube.com/watch?v=aOnp7F0f8qg) — AS - Civil Simplified
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/phase-diagram-void-ratio-saturation
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-2

## Question

Sketch the phase diagram for a soil and indicate the volumes and weights of the phases on it. Define void ratio and degree of saturation.

## Short answer

The phase diagram shows volumes and weights of air, water, and solids, with voids as air plus water. Void ratio e equals volume of voids divided by volume of solids. Degree of saturation S equals volume of water divided by volume of voids, expressed as a percentage.

## Answer

#### Phase diagram
```text
  Volume               Weight
 +---------+ ---    +-----------+
 |   Air   | Va     |     0     |
 |---------|  | Vv  |-----------|
 |  Water  | Vw     |    Ww     |
 |---------| ---    |-----------| W
 |  Solids | Vs     |    Ws     |
 +---------+        +-----------+
  V = Vs+Vw+Va        W = Ws+Ww
```
Here $V_v = V_w + V_a$ is the volume of voids; $W_w$ and $W_s$ are weights of water and solids.

#### Void ratio ($e$)
The ratio of the volume of voids to the volume of solids:
$$
e = \frac{V_v}{V_s}
$$
It is a ratio (no unit) and can be more than 1 (for example in soft clays).

#### Degree of saturation ($S$)
The ratio of the volume of water to the volume of voids, in percent:
$$
S = \frac{V_w}{V_v} \times 100
$$
$S = 0$ for dry soil and $S = 100\%$ for fully saturated soil.

## Check yourself

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

1. Void ratio is defined as:
   A. Vv/V
   B. Vv/Vs
   C. Vw/Vv
   D. Va/V
   Answer: B. e = Vv/Vs, a ratio with no unit. (Key point: Void ratio)

2. Which is true of void ratio?
   A. It cannot exceed 1
   B. It is always percent
   C. It is expressed in kN/m3
   D. It can exceed 1, e.g. in soft clays
   Answer: D. Soft clays can have e greater than 1. (Key point: Void ratio range)

3. Degree of saturation is:
   A. Vw/Vs x 100
   B. Vw/Vv x 100
   C. Va/Vv x 100
   D. Vw/V x 100
   Answer: B. S = Vw/Vv x 100; 0 for dry, 100% for saturated. (Key point: Degree of saturation)

4. Vv in the phase diagram equals:
   A. Vs + Vw
   B. Vs + Va
   C. Va only
   D. Vw + Va
   Answer: D. Voids are filled by water and air. (Key point: Voids)

## Related in Soil Mechanics

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