---
title: "Soil stabilization factors; mechanical; cement-soil base"
subject: "Transportation Engineering II"
chapter: 3
key: "semester_7/transportation_engineering_ii:soil-stabilization-factors-mechanical-cement-soil"
source: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/soil-stabilization-factors-mechanical-cement-soil
site: BCT Engineering
---

# Soil stabilization factors; mechanical; cement-soil base

Transportation Engineering II · Chapter 3: Road Construction Technology

- Asked in: 2076 Asoj, 8 marks
- Video: [What is soil stabilization? || Methods of soil stabilization](https://www.youtube.com/watch?v=BSpDUzRQQZI) — Gurukul of Civil Engineers
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/soil-stabilization-factors-mechanical-cement-soil
- Whole chapter: https://www.bctengineer.com/md/semester_7/transportation_engineering_ii/chapter-3

## Question

What are the factors affecting soil stabilization? What do you mean by mechanical method of soil stabilization? Write down the procedure of constructing cement soil stabilized road base.

## Short answer

Factors include soil type, stabilizer type and amount, mixing efficiency, moisture and compaction, curing time, harmful materials, climate and drainage. Mechanical stabilization blends soils and compacts to improve gradation and density without chemical binder. Cement-soil base: prepare subgrade, spread and pulverize soil, spread cement, mix, add water, compact within 2 hours to 98% modified Proctor, cure 7 days.

## Answer

#### Factors affecting soil stabilisation
The success of stabilisation depends on:

1. **Type of soil** - its grain size, plasticity index, mineralogy and organic content decide the suitable method (cement for sands and gravel, lime for plastic clays).
2. **Type and amount of stabiliser** - cement 4-8 percent, lime 3-8 percent, bitumen 4-7 percent; too little gives no strength gain and too much is costly or causes cracking.
3. **Mixing efficiency** - uniform pulverisation and mixing of soil and stabiliser; clods must be broken (below 25 mm).
4. **Moisture content and compaction** - optimum moisture and high density are needed; delay between mixing and compaction (more than 2 hours for cement) reduces strength.
5. **Curing and time** - adequate moist curing for 7 days or more; temperature (cement and lime reactions are slow below 10 C).
6. **Presence of harmful materials** - sulphates, organic matter and salts retard cement and lime reactions.
7. **Climate and drainage** - freezing, wetting-drying and water access affect durability.
8. **Cost and availability** of the stabiliser and equipment.

#### Mechanical method of soil stabilisation
**Mechanical stabilisation** improves the soil by changing its gradation and density only, without a chemical binder.

- Two or more soils are blended (for example, a coarse gravel lacking fines with a clayey soil lacking coarse particles) to get a well-graded mix with the right proportions of gravel, sand and fines (plasticity index 4-10 percent).
- The mix is spread at optimum moisture content and compacted with rollers to high density (above 98 percent of modified Proctor).
- Used for gravel roads, sub-base and base of low-volume roads, and as the first step before chemical stabilisation. It is cheap but depends on a good proportion of materials.

#### Construction of cement soil stabilised road base
**Soil-cement (cement-stabilised) base** is a mix of soil, cement and water, compacted to give a strong, durable layer (IRC:SP:89, IS:4332).

**Materials**: granular or sandy soil (PI below 20, fines below 50 percent, maximum size 40 mm; clay and organic soil unsuitable); ordinary Portland cement 4-8 percent by dry weight (decided by laboratory trial: 7-day UCS 1.7-3 MPa); clean water.

**Equipment**: pulveriser (rotavator) or grader, cement spreader (or bags), water tanker, rollers (vibratory 8-10 t, pneumatic tyre), motor grader, curing sprayer.

**Construction procedure**
1. Prepare the subgrade, trim and compact; set line and levels.
2. Spread the soil in a uniform layer and pulverise to break clods.
3. Spread the cement in a uniform layer at the specified rate (by bags at fixed spacing or by mechanical spreader).
4. Mix dry cement and soil with rotavator or grader until colour is uniform; add water to the optimum moisture content and mix again.
5. Compact immediately (within 2 hours of adding water) to 98 percent of modified Proctor density, using a vibratory roller and then a pneumatic roller; shape with the grader.
6. Cure for 7 days with moist cover or a bituminous prime coat (bitumen emulsion) to retain moisture; keep traffic off.
7. Provide the bituminous surface or the next layer after curing.
8. Control: cement content (titration), moisture and density, UCS cubes, thickness and level.

## Check yourself

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

1. How does delay between mixing and compaction affect cement stabilisation?
   A. It improves strength by allowing hydration
   B. A delay of more than 2 hours reduces strength
   C. It has no effect
   D. It only matters below 10 C
   Answer: B. Cement starts hydrating when mixed with water, so delay beyond 2 hours before compaction cuts strength. (Key point: Delay before compaction)

2. Which materials retard cement and lime reactions?
   A. Gravel and sand
   B. Sulphates, organic matter and salts
   C. Silica
   D. Kaolinite alone in small amount
   Answer: B. Sulphates, organic matter and salts interfere with cement and lime reactions. (Key point: Harmful materials)

3. What does mechanical stabilisation do?
   A. Adds cement to bind soil grains
   B. Changes gradation and density by blending soils and compacting, without a chemical binder
   C. Heats the soil to dry it
   D. Injects bitumen into the soil
   Answer: B. Soils are blended to a well-graded mix and compacted to above 98 percent modified Proctor. (Key point: Mechanical stabilisation)

4. What typical cement content is used for soil-cement base?
   A. 0.5-1 percent
   B. 4-8 percent by dry weight, decided by lab trial
   C. 20-30 percent
   D. Same as concrete, 15 percent
   Answer: B. 4-8 percent cement by dry weight, fixed by laboratory trials to reach 7-day UCS of 1.7-3 MPa. (Key point: Cement content)

## Related in Transportation Engineering II

- [Soil stabilization methods and lime stabilized road](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Asoil-stabilization-methods-lime-road) — Transportation Engineering II, chapter 3
- [Soil stabilization methods; soil cement construction](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Asoil-stabilization-methods-soil-cement-road) — Transportation Engineering II, chapter 3
- [Mass haul diagram and soil cement base](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Amass-haul-soil-cement-base-construction) — Transportation Engineering II, chapter 3
- [Soil stabilization, equipment, mass haul characteristics](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Asoil-stabilization-definition-equipment-mass-haul) — Transportation Engineering II, chapter 3
- [Road construction equipment and DBSD procedure](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Aequipment-plants-activities-dbsd) — Transportation Engineering II, chapter 3

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