---
title: "Define relative compaction"
subject: "Soil Mechanics"
chapter: 5
key: "semester_4/soil_mechanics:relative-compaction-define"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/relative-compaction-define
site: BCT Engineering
---

# Define relative compaction

Soil Mechanics · Chapter 5: Soil Compaction

- Asked in: 2079 Asoj, 1 marks; 2078 Chaitra, 1 marks (asked 2 times)
- Video: [11# Relative compaction of Soil | Soil Mechanics | GATE | ESE | Shubham Sir](https://www.youtube.com/watch?v=as-HsdapMd0) — Back Benchers
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/relative-compaction-define
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-5

## Question

What is relative compaction?

## Short answer

Relative compaction is the ratio of field dry unit weight to laboratory maximum dry unit weight from Proctor test, expressed as a percentage (RC = γ_d,field/γ_d,max,lab ×100). It is used for field compaction control, typically requiring 95% for embankments and 98-100% for pavement layers.

## Answer

**Relative compaction (RC)** is the ratio of the dry unit weight (density) achieved in the field to the maximum dry unit weight obtained in the laboratory compaction test (Proctor test), expressed as a percentage:
$$
RC = \frac{\gamma_{d,field}}{\gamma_{d,max,lab}} \times 100
$$
It is used as the **field control of compaction**. Specifications normally require RC of 95% (standard Proctor) for embankments and 98 - 100% for pavement layers or the top of fills. Not to be confused with relative density $D_r$, which is for cohesionless soils.

## Check yourself

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

1. Relative compaction is:
   A. e_max - e / e_max - e_min
   B. Lab dry unit weight / field wet unit weight
   C. Water content / OMC
   D. Field dry unit weight / lab maximum dry unit weight x 100
   Answer: D. RC compares field dry density with lab MDD. (Key point: Definition)

2. Relative compaction is used for:
   A. Classifying soil
   B. Field control of compaction
   C. Strength of rock
   D. Measuring permeability
   Answer: B. It checks whether field fill meets specifications. (Key point: Use)

3. Typical RC specification for embankments is:
   A. 75%
   B. 95%
   C. 50%
   D. 120%
   Answer: B. Embankments normally need 95% of standard Proctor MDD. (Key point: Typical value)

## Related in Soil Mechanics

- [Compaction test: curve, MDD, OMC, RC](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Acompaction-test-curve-mdd-omc-rc) — Soil Mechanics, chapter 5, asked 2×
- [Compaction test, RC and saturation](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Acompaction-test-curve-saturation-degree) — Soil Mechanics, chapter 5, asked 2×
- [Compaction vs consolidation; methods](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Acompaction-methods-merits) — Soil Mechanics, chapter 5
- [Cylindrical specimen, 95% compaction](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Acylindrical-specimen-relative-compaction) — Soil Mechanics, chapter 5