---
title: "Phase diagram for different saturation"
subject: "Soil Mechanics"
chapter: 2
key: "semester_4/soil_mechanics:phase-diagram-saturated-dry"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/phase-diagram-saturated-dry
site: BCT Engineering
---

# Phase diagram for different saturation

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

- Asked in: 2075 Bhadra, 3 marks; 2076 Baisakh, 2 marks (asked 2 times)
- Video: [3-Phase Diagrams Part 1](https://www.youtube.com/watch?v=QrIaPVYS7og) — Introduction to Geotechnical Engineering
- 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-saturated-dry
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-2

## Question

Draw the phase diagram of soil for saturated, partially saturated and dry conditions.

## Short answer

A phase diagram shows soil as solids, water, and air blocks with volumes Vs, Vw, Va and weights Ws, Ww (air weight zero). In partially saturated soil all three phases exist; in saturated soil Va=0 and Vv=Vw; in dry soil Vw=0, Ww=0 and Vv=Va.

## Answer

A **phase diagram** shows soil as separate blocks of solids, water and air, with volumes on the left and weights on the right. Here $V_s, V_w, V_a$ are volumes of solids, water and air, $V_v = V_w + V_a$ is the void volume, and $W_s, W_w$ are the weights (air weight is taken as zero).

#### Partially saturated
```text
  Volume              Weight
 +--------+  ---   +----------+
 |  Air   | Va     |    0     |
 |--------|  | Vv  |----------|
 | Water  | Vw     |   Ww     |
 |--------|  ---   |----------| W
 | Solids | Vs     |   Ws     |
 +--------+        +----------+
   V=Vs+Vw+Va         W=Ws+Ww
```

#### Fully saturated ($S = 100\%$, $V_a = 0$)
```text
 +--------+  ---   +----------+
 | Water  | Vw=Vv  |   Ww     |
 |--------|  ---   |----------|
 | Solids | Vs     |   Ws     |
 +--------+        +----------+
```

#### Dry ($S = 0$, $V_w = 0$, $W_w = 0$)
```text
 +--------+  ---   +----------+
 |  Air   | Va=Vv  |    0     |
 |--------|  ---   |----------|
 | Solids | Vs     | Ws = W   |
 +--------+        +----------+
```

## Check yourself

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

1. In a fully saturated soil phase diagram:
   A. Va = Vv and Vw = 0
   B. Ws = 0
   C. Va = 0 and Vw = Vv
   D. S = 0
   Answer: C. Saturated soil has voids completely filled with water, so S = 100% and Va = 0. (Key point: Saturated diagram)

2. For dry soil:
   A. Vw = 0, Ww = 0, S = 0, Va = Vv
   B. Va = 0
   C. Ws = 0
   D. S = 100%
   Answer: A. Dry soil has no water; voids are filled with air. (Key point: Dry diagram)

3. In phase diagrams the weight of air is taken as:
   A. Equal to water
   B. Equal to solids
   C. Zero
   D. Negative
   Answer: C. Air weight is negligible and taken as zero. (Key point: Air weight)

4. Volume of voids Vv equals:
   A. Vw + Va
   B. Vs + Vw
   C. Va - Vw
   D. Vs + Va
   Answer: A. Voids contain water and air. (Key point: Void volume)

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