Designing a durable road, driveway, or industrial pavement starts with one critical question: how thick should the pavement be?
The most widely used and practical approach for flexible pavement design is the California Bearing Ratio (CBR) method. It links soil strength, traffic loading, and material layers into a clear, buildable pavement thickness.
This guide breaks the process down step by step, expands on the theory, and adds practical engineering insights so the method can be applied confidently on real projects.
What Is the California Bearing Ratio (CBR)?
The California Bearing Ratio (CBR) is a measure of subgrade soil strength. It compares the resistance of soil to penetration against a standard crushed stone material, expressed as a percentage.
Key interpretation:
Low CBR = weak soil → thicker pavement required
High CBR = strong soil → thinner pavement sufficient
Typical CBR Ranges
| Subgrade Quality | CBR Value (%) |
|---|---|
| Poor Subgrade | 2–4 |
| Average Subgrade | 4–7 |
| Good Subgrade | 7–10+ |
CBR testing is usually conducted in a laboratory on remolded or soaked soil samples to simulate worst-case moisture conditions.
Pavement Structure in Flexible Pavement Design
1. Asphalt Surface Course
The asphalt surface course is the topmost layer of a flexible pavement and is the only layer directly exposed to traffic and environmental conditions. Its primary role is to provide a smooth, skid-resistant riding surface while protecting the underlying layers from water infiltration. This layer also helps distribute wheel loads to the base course beneath it. The thickness of the asphalt layer varies depending on traffic intensity and performance requirements, with heavier traffic demanding a thicker asphalt surface.
2. Base Course
The base course sits directly below the asphalt layer and functions as the main structural component of a flexible pavement. It is typically constructed using crushed aggregates or high-quality granular material capable of withstanding repeated traffic loading. This layer spreads applied loads over a wider area, reducing stress on the subgrade. A well-constructed base course significantly improves pavement durability and minimizes deformation such as rutting.
3. Sub-Base
The sub-base is positioned between the base course and the natural subgrade soil. Its purpose is to provide additional load distribution, enhance drainage, and create a stable working platform during construction. In areas with weak or moisture-sensitive soils, the sub-base plays a critical role in preventing water-related failures by allowing moisture to drain away from the pavement structure. The thickness of the sub-base is often increased when subgrade conditions are poor.
4. Compacted Subgrade
The subgrade is the foundation of the entire pavement system and consists of the in-situ soil that has been compacted to a specified density. Its strength is represented by the CBR value, which directly influences the overall pavement thickness. A weak subgrade requires thicker pavement layers to compensate for lower load-bearing capacity, while a strong, well-compacted subgrade allows for a thinner pavement structure.
Why the CBR Method Is Widely Used
The CBR method remains popular because it is:
Simple and field-proven
Suitable for low to medium traffic roads
Cost-effective for rural roads, access roads, and industrial yards
Supported by design charts and standards worldwide
It is especially effective where advanced mechanistic-empirical design tools are unnecessary.
Steps to Determine Pavement Thickness Using the CBR Method
Step 1: Determining the CBR of the Subgrade Soil
The first step in pavement design using the CBR method is to determine the bearing capacity of the subgrade soil. This is achieved through laboratory or field CBR testing, often under soaked conditions to simulate the weakest expected soil state. The lowest representative CBR value is typically adopted for design to ensure safety and long-term performance.
Step 2: Evaluating Traffic Loading Conditions
Once the subgrade strength is known, traffic loading must be assessed. This involves estimating the type, frequency, and weight of vehicles expected to use the pavement over its design life. Light traffic such as passenger vehicles requires less structural thickness, while heavy commercial or industrial traffic significantly increases pavement thickness requirements. Accurate traffic estimation is essential to prevent premature pavement failure.
Step 3: Selecting Pavement Thickness from CBR Design Charts
Using the determined CBR value and traffic category, standard CBR design charts or empirical formulas are consulted to obtain the required total pavement thickness. These charts are developed from long-term performance observations and provide a reliable relationship between soil strength, traffic load, and pavement depth. The thickness obtained represents the combined depth of the asphalt, base, and sub-base layers.
Step 4: Distributing Thickness Among Pavement Layers
After establishing the total pavement thickness, it is divided among the individual layers in practical proportions. This allocation depends on material availability, construction practices, and drainage considerations. Ensuring adequate thickness in each layer is essential to achieve proper load transfer and long-term pavement stability.
Example Pavement Thickness Calculation
Given:
Subgrade CBR = 5%
Traffic = medium-duty vehicles
From standard CBR design charts:
Recommended total pavement thickness ≈ 550 mm
Typical Layer Breakdown
Asphalt surface: 100 mm
Base course: 150 mm
Sub-base: 300 mm
Optimum pavement thickness = 550 mm
This configuration ensures sufficient load distribution while maintaining constructability and cost efficiency.
Factors That Influence Final Pavement Thickness
1. Drainage Conditions
Drainage plays a critical role in pavement performance, as the presence of water can drastically reduce soil strength. Poor drainage conditions often necessitate thicker pavement layers to compensate for weakened subgrade support. Incorporating free-draining materials, increasing sub-base thickness, or installing drainage systems can significantly enhance pavement lifespan.
2. Moisture Sensitivity of Subgrade Soil
Certain soils, particularly clays, are highly sensitive to changes in moisture content. When saturated, these soils experience a reduction in strength, leading to increased deformation and cracking. In such cases, pavement thickness is increased, or soil stabilization techniques such as lime or cement treatment are used to improve subgrade performance.
3. Quality of Construction and Compaction
Even a well-designed pavement can fail if construction quality is poor. Inadequate compaction reduces the effective strength of pavement layers and leads to early deterioration. Strict control of compaction levels and moisture content during construction ensures that the pavement performs as designed and achieves its intended service life.
4. Future Traffic Growth
Traffic volume and vehicle weight often increase over time, especially in developing areas. If future traffic growth is not considered during design, the pavement may become structurally inadequate sooner than expected. Designing with a margin for traffic growth helps reduce maintenance costs and extends pavement service life.
Advantages and Limitations of the CBR Method
Advantages
Easy to understand and apply
Minimal testing requirements
Proven performance for decades
Limitations
Less accurate for very heavy traffic highways
Does not explicitly model stress-strain behavior
Requires conservative assumptions for weak soils
For highways and airports, mechanistic-empirical methods are often preferred.
Build Strong, Durable Roads with Proven Expertise
Accurate pavement thickness design is the foundation of long-lasting road construction. By applying the CBR method correctly, supported by proper material selection, drainage planning, and quality construction practices, pavement structures can perform reliably for years with reduced maintenance costs. However, calculations and design alone are not enough—the real success of any road project depends on execution, experience, and technical judgment on site. Bewa Bina delivers road construction solutions backed by proven expertise and a strong track record of successful projects across various site conditions and traffic demands. From pavement design and earthworks to asphalt works and compaction, every project is handled with precision, compliance, and performance in mind.
If you are planning a road, driveway, industrial yard, or infrastructure project and want results that last, connect with a team that has already delivered. WhatsApp Bewa Bina today to discuss your project requirements and get professional guidance from a contractor with proven results on the ground.
