Compaction of Subgrade in Construction

In any construction project involving roads, pavements, or building foundations, subgrade compaction is one of the most critical steps. Without proper compaction, the structure built above is prone to settlement, cracking, or even failure over time. This article explains what subgrade compaction is, why it matters, how it’s performed, and what industry standards govern the process.

What is Subgrade?

The subgrade refers to the native soil or improved soil prepared to support a pavement system or a building foundation. It serves as the base for other structural layers such as subbase, base course, and the surface layer in road construction or slab/foundation in building construction.

For optimal performance, the subgrade must be stable, uniform, and have sufficient bearing capacity. This is where compaction plays a vital role.

What is Compaction of Subgrade?

Subgrade compaction is the process of mechanically increasing the density of the soil that forms the subgrade. This is achieved by removing air voids through the application of external forces using rollers, compactors, or other construction equipment.

The goal is to enhance the load-bearing capacity, stability, and durability of the soil beneath roads and buildings. Proper compaction also reduces future settlement, minimizes water infiltration, and prevents deformation.

Importance of Subgrade Compaction

Compacting the subgrade has multiple technical and economic advantages:

1. Improved Load-Bearing Capacity

Properly compacted soil can handle greater loads without excessive deformation. This is crucial for roads that bear heavy vehicular traffic.

2. Reduces Settlement

Uncompacted or loosely compacted subgrades can lead to differential settlement, which can crack pavements and damage structures.

3. Prevents Water Seepage

Compacted soils are less permeable, reducing water ingress that can weaken the pavement structure.

4. Cost-Effectiveness

Well-compacted subgrades reduce maintenance costs over time by preventing early failure.

5. Durability

A compacted subgrade provides a stable platform, increasing the longevity of the structure above it.

Factors Affecting Subgrade Compaction

Several factors influence the effectiveness of compaction:

Types of Soil Used in Subgrade

Subgrade soils can be classified into the following categories:

• Granular soils: Sand, gravel—best for compaction.
• Cohesive soils: Clay, silt—require precise moisture control.
• Organic soils: Unsuitable for subgrade; must be removed or replaced.

Optimum Moisture Content (OMC)

Each soil type has an Optimum Moisture Content—the moisture level at which maximum dry density is achieved during compaction. Compaction done above or below the OMC results in sub-optimal density, weakening the subgrade.

The Standard Proctor Test or Modified Proctor Test determines this moisture content and the corresponding Maximum Dry Density (MDD) of the soil.

Equipment Used for Subgrade Compaction

Different types of machinery are used based on the soil type and site conditions:

1. Smooth Wheel Rollers

Used on granular soils; provides finish compaction.

2. Sheepsfoot Rollers

Ideal for cohesive soils like clay due to their kneading action.

3. Pneumatic-Tired Rollers

Distribute pressure evenly; used for both granular and cohesive soils.

4. Vibratory Rollers

Apply dynamic force; effective for compacting loose granular soils.

5. Plate Compactors / Rammer Compactors

Used in small areas or trenches.

Standard Compaction Procedures

Step 1: Site Preparation

• Clear vegetation, debris, and unsuitable soils.
• Grade and level the surface.

Step 2: Moisture Conditioning

• Adjust soil moisture to OMC.
• Use water tankers or drying methods.

Step 3: Layering

• Spread the soil in thin layers (usually 150 mm to 200 mm).

Step 4: Rolling

• Use appropriate compaction equipment.
• Roll in longitudinal and cross directions.

Step 5: Testing

• Perform field density tests to confirm compaction.

Field Compaction Tests

Field compaction is typically verified through:

1. Sand Cone Test (ASTM D1556)

Measures in-place density by replacing excavated soil with calibrated sand.

2. Nuclear Density Test (ASTM D6938)

Non-destructive test using a nuclear gauge to measure density and moisture.

3. Core Cutter Method

Used in cohesive soils to extract and weigh a known volume.

The compacted density should meet a specific percentage (usually 95%–98% of MDD) to pass quality control.

Challenges in Subgrade Compaction

1. Water-logged Soils
   Difficult to compact without stabilization.
2. Cold or Wet Weather
   Impacts machinery performance and moisture levels.
3. Inaccessible Areas
   Hand compaction or mini-equipment may be needed.
4. Variable Soil Conditions
   Require adjustments in technique and equipment.

Methods to Improve Subgrade Compaction

• Soil Stabilization: Using lime, cement, or bitumen to strengthen weak soils.
• Geotextiles: Prevent mixing of layers and improve load distribution.
• Blending Soils: Mix poor soils with better quality materials.
• Drainage: Proper site drainage prevents moisture-related compaction issues.

Subgrade Compaction Standards

Compaction requirements vary by project type and local standards, but some common benchmarks include:

• Highways: Minimum 95% of Modified Proctor MDD
• Airfields: 98% or more for runways
• Buildings: Based on geotechnical engineer’s recommendations

Relevant standards include:

• ASTM D698 (Standard Proctor Test)
• ASTM D1557 (Modified Proctor Test)
• AASHTO T99 / T180

Safety Precautions During Compaction

• Operate machinery only by trained personnel.
• Use PPE (Personal Protective Equipment).
• Monitor for underground utilities before starting.
• Implement traffic control measures if working on roads.

Conclusion

Subgrade compaction is a cornerstone of sound construction. Whether it’s a highway, runway, or residential slab, compacting the subgrade properly ensures that the structure remains stable, functional, and durable. Ignoring this step may lead to costly repairs and structural failures. Following standard procedures, using the right equipment, and verifying compaction with field tests can significantly enhance the success of any project.

FAQs on Subgrade Compaction

1. Why is subgrade compaction important in road construction?

It strengthens the foundation and prevents future settlement, rutting, and pavement cracking caused by traffic loads.

2. What is the optimum moisture content for compaction?

It varies by soil type and is determined by lab tests, but it’s the moisture level that allows the soil to reach its maximum dry density during compaction.

3. Can you compact subgrade in rainy weather?

It’s not advisable. Wet conditions can oversaturate the soil, making it hard to compact and potentially weakening the subgrade.

4. How thick should each layer (lift) be during compaction?

Typically, lifts are 150–200 mm thick to ensure uniform compaction throughout the layer.

5. What happens if subgrade compaction is not done properly?

You may face structural issues such as uneven settlement, cracks, surface deformation, and reduced lifespan of the pavement or foundation.

6. What are the most common tests used for checking compaction?

Sand Cone Test, Nuclear Density Test, and Core Cutter Method are commonly used to verify field compaction.

7. Can all types of soil be compacted equally?

No. Granular soils compact easily, while cohesive soils like clay require precise moisture control. Organic soils are generally unsuitable for compaction.

8. What is Proctor Compaction?

It’s a standardized lab test (Standard or Modified) used to determine the optimal moisture content and maximum dry density of a soil for effective compaction.

9. Are there standards or codes for subgrade compaction?

Yes, standards like ASTM D698, D1557, and AASHTO T99/T180 are widely used to guide compaction requirements.

10. What type of equipment is best for clayey subgrade?

Sheepsfoot rollers are most effective for compacting cohesive soils like clay due to their kneading action.