---
title: "Phase diagrams before and after compaction"
subject: "Soil Mechanics"
chapter: 2
key: "semester_4/soil_mechanics:phase-diagram-compaction-consolidation"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/phase-diagram-compaction-consolidation
site: BCT Engineering
---

# Phase diagrams before and after compaction

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

- Asked in: 2077 Chaitra, 2 marks
- 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-compaction-consolidation
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-2

## Question

Draw phase diagrams for a dry soil sample and a saturated soil sample before and after the compaction and consolidation processes, respectively.

## Short answer

Compaction reduces air voids in dry soil by mechanical energy, while consolidation expels water from saturated soil under load. In both processes solids volume stays constant, void ratio decreases, and dry unit weight increases.

## Answer

Compaction reduces air voids by mechanical energy (no water leaves); consolidation reduces voids by squeezing water out of saturated soil under a sustained load. Solids volume $V_s$ is constant in both.

#### Dry soil before and after compaction
```text
 Before            After (V reduced)
 +------+          +------+
 | Air  |          | Air  |  (less)
 |      |          +------+
 |      |          |Solids|  Vs same
 +------+          +------+
 |Solids|
 +------+
```
Air volume decreases, so the void ratio and total volume decrease; $W_s$ is unchanged.

#### Saturated soil before and after consolidation
```text
 Before            After (V reduced)
 +------+          +------+
 |Water |          |Water |  (less)
 |      |          +------+
 |      |          |Solids|  Vs same
 +------+          +------+
 |Solids|
 +------+
```
Water is expelled from the voids, so $V_w$ and $V$ decrease, $S$ remains 100% and $W_w$ decreases.

In both cases the void ratio falls and dry unit weight rises.

## Check yourself

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

1. During compaction of dry soil:
   A. Water is expelled
   B. Solids volume decreases
   C. Air volume decreases by mechanical energy; Ws is unchanged
   D. Weight of solids increases
   Answer: C. Compaction reduces air voids; solids stay the same. (Key point: Compaction mechanism)

2. During consolidation of saturated clay:
   A. Water is squeezed out under sustained load, Vw and V decrease, S stays 100%
   B. Air escapes
   C. Solids are compressed
   D. Saturation falls to 80%
   Answer: A. Pore water is expelled and the soil remains saturated. (Key point: Consolidation mechanism)

3. What stays constant in both processes?
   A. Void ratio
   B. Total volume
   C. Volume of solids Vs
   D. Water weight
   Answer: C. Solids volume does not change; voids shrink. (Key point: Constant quantity)

4. After both processes, void ratio and dry unit weight:
   A. Void ratio falls, dry unit weight rises
   B. Both fall
   C. Both rise
   D. Void ratio rises, dry unit weight falls
   Answer: A. Smaller voids with the same solids give higher dry unit weight. (Key point: Result)

## Related in Soil Mechanics

- [Compaction vs consolidation; methods](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Acompaction-methods-merits) — Soil Mechanics, chapter 5
- [Effect of compaction on soil properties](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Acompaction-effect-engineering-behavior) — Soil Mechanics, chapter 5, asked 3×
- [Soil structures by compaction; DDL](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Asoil-structure-compaction-ddl) — Soil Mechanics, chapter 4
- [Infinite slope FoS, dry cohesive soil](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Ainfinite-slope-dry-cohesive-derive) — Soil Mechanics, chapter 11
- [Saturated clay 20 cc numerical](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Asaturated-clay-void-ratio-gravity) — Soil Mechanics, chapter 2
- [Consolidation test, 90% time, k, Cc](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aconsolidation-test-50-90-percent-time) — Soil Mechanics, chapter 9

## Related in other subjects

- [Sludge volume before and after digestion](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asludge-volume-before-after-600kg) — Sanitary Engineering, chapter 8
- [Laplace: two switches, current before and after 8 ms](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_circuit_theory%3Alaplace-two-switch-s1-s2-8ms) — Electrical Circuit Theory, chapter 4
- [Test: mean weights of two processes](https://www.bctengineer.com/question.md?key=semester_5%2Fprobability_and_statistics%3Abulbs-factory-two-processes-weights) — Probability and Statistics, chapter 6