---
title: "Desirable properties of subgrade soil"
subject: "Transportation Engineering I"
chapter: 6
key: "semester_6/transportation_engineering_i:subgrade-properties"
source: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/subgrade-properties
site: BCT Engineering
---

# Desirable properties of subgrade soil

Transportation Engineering I · Chapter 6: Highway Materials

- Asked in: 2081 Ashwin, 3 marks; 2081 Chaitra, 4 marks; 2079 Chaitra, 2 marks; 2079 Ashwin, 3 marks; 2078 Poush, 4 marks; 2077 Chaitra, 4 marks; 2076 Bhadra; 2075 Baisakh; 2073 Magh; 2070 Bhadra; 2069 Bhadra (asked 11 times)
- Video: [Subgrade Materials and Properties - Highway Material - Transportation Engineering 1](https://www.youtube.com/watch?v=eYD_DKVbXmY) — Ekeeda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/subgrade-properties
- Whole chapter: https://www.bctengineer.com/md/semester_6/transportation_engineering_i/chapter-6

## Question

What are the desirable properties of sub-grade soil?

## Short answer

Desirable subgrade soil has high CBR (≥5%, higher preferred), low plasticity/swelling, good drainage, ease of compaction (≥97% MDD), low compressibility, durability when wet, uniformity, and absence of organic matter. It ensures pavement support and minimal deformation.

## Answer

The **subgrade** is the top 500 mm of natural or compacted soil that supports the pavement. Its properties decide the pavement thickness, because all the load finally reaches it.

#### Desirable properties
1. **Adequate strength and stability** - a high CBR (design CBR measured at the field density and moisture after 4 days of soaking; NRS 2070/IRC:37 use the CBR to find the pavement thickness; the minimum subgrade CBR is about 5%, and 8-10% or more is good).
2. **Low volume change** - low swelling and shrinkage, which means low plasticity (low liquid limit and plasticity index).
3. **Good drainage** - permeable soil that does not retain water; a high water table is avoided.
4. **Resistance to frost action** - non-frost-susceptible soil in cold areas.
5. **Ease of compaction** - well-graded soil that reaches a high dry density (maximum dry density and optimum moisture content) with ordinary rollers, giving good compaction (at least 97% of MDD, modified Proctor, by IRC/NRS).
6. **Low compressibility** - low settlement and consolidation, so the pavement does not deform.
7. **Durability** - strength retained in wet conditions; the soil does not soften under rain or traffic.
8. **Uniformity** - uniform soil along the length, so that the pavement has no differential settlement.
9. **Free of organic matter and deleterious material**.
10. **Good modulus** - a high resilient modulus, which gives less deflection under repeated loads.

## Check yourself

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

1. Which test is used to express subgrade strength for pavement thickness design?
   A. CBR
   B. Flakiness index
   C. Ductility
   D. Softening point
   Answer: A. The soaked CBR of the subgrade is used to decide the pavement thickness. (Key point: Strength measure)

2. Why should the subgrade have low plasticity?
   A. To reduce swelling and shrinkage with moisture change
   B. To increase its compressibility
   C. To make it hold more water
   D. To make compaction impossible
   Answer: A. Low liquid limit and plasticity index mean low volume change, so the pavement is not distorted by moisture. (Key point: Low volume change)

3. Why is uniformity of the subgrade along the road length desirable?
   A. To increase cost
   B. To avoid differential settlement in the pavement
   C. To avoid the need for compaction
   D. To raise the water table
   Answer: B. Non-uniform soil would settle unevenly and cause pavement deformation. (Key point: Uniformity)

## Related in Transportation Engineering I

- [Desirable properties of road aggregates](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Aaggregate-desirable-properties) — Transportation Engineering I, chapter 6, asked 5×
- [CBR test on subgrade soil](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Acbr-test) — Transportation Engineering I, chapter 6
- [Desirable properties of bituminous mixes](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Abituminous-mix-desirable-properties) — Transportation Engineering I, chapter 6, asked 3×
- [Desirable properties of bitumen](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Abitumen-desirable-properties) — Transportation Engineering I, chapter 6
- [Causes of moisture variation in subgrade](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Amoisture-causes) — Transportation Engineering I, chapter 4, asked 13×

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- [Boring type for road sub-grade](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Aboring-type-subgrade) — Foundation Engineering, chapter 2
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