Pavement design is very important for making sure that road infrastructure is safe, structurally sound, and will last for a long time. Over the years, many design methods have been created around the world. One of the most well-known empirical methods is the AASHTO (1993) Guide for Design of Pavement Structures.

The AASHTO (1993) Guide [10] , which was based on the AASHO Road Test, is still a widely used empirical method around the world. To guess how well a pavement will work, we use Equivalent Single Axle Loads (ESALs), subgrade properties, and environmental variables. Using the Structural Number (SN) concept [8] [10] , it figures out the needed pavement layer thickness based on the properties of the material and the expected traffic over a certain analysis period. Some important design factors are

Particularly in nations where the initial road test conditions closely resemble local traffic and soil characteristics, the AASHTO method is a preferred standard worldwide due to its adaptability in incorporating both empirical data and localized adjustments [8] .

Nowadays, there are more advanced modeling tools available through mechanistic-empirical (M-E) techniques [12] - [14] . However, the AASHTO 1993 Guide is still often used when testing options are limited.

The Roads and Highways Department (RHD) published the Pavement Design Guide in 2005, which outlines the procedures for pavement design and construction in Bangladesh. This guide builds upon the AASHTO standards and considers local conditions, such as low CBR (California Bearing Ratio) soils, a monsoonal climate, high groundwater levels, and the capabilities of local construction practices.

The AASHTO method for predicting cumulative traffic loading over the pavement’s design life, using Equivalent Single Axle Loads (ESAL), remains a component of the RHD guideline. To address the practical limitations of the local context, certain aspects of the design process are streamlined. For instance:

The RHD guidelines specify Vehicle Equivalent Factors (VEF) that are tailored to the traffic composition in Bangladesh, which features a high proportion of overloaded trucks and buses. These guidelines recommend the use of improved subgrades, locally calibrated VEFs that account for overloading practices, stage construction, and modified layer coefficients for regional materials [4] .

The AASHTO equations for structural number (SN) and the Roads and Highways Department (RHD) values for California Bearing Ratio (CBR), Vehicle Equivalent Factors (VEF), and drainage coefficients are utilized in highway projects throughout Bangladesh. This is particularly true for projects funded by development partners, such as the Asian Development Bank (ADB), which adopt a hybrid approach [5] [9] .

To design pavement structures that are technically sound, economically viable, and resilient to the challenging traffic and environmental conditions in Bangladesh, a blended approach is employed. This approach ensures both technical accuracy and local relevance.

To gather the necessary input data, a thorough review of the Pavement Design Report was conducted. This review included examining axle load distributions, subgrade strength parameters, material properties, drainage conditions, traffic forecasts, and serviceability indices [4] . Below is a summary of the data sources and assumptions:

1) Traffic Data and ESAL Calculations

2) Subgrade and Soil Data

2) Design Thickness Calculation

Each pavement layer’s thickness was determined based on drainage values and layer coefficients.

(2)

Example (Section 1):

The cumulative Equivalent Single Axle Loads (ESALs) for a 20-year analysis period were determined using axle load distributions and traffic forecasts. To facilitate interpretation, the design ESALs were converted into Million Standard Axles (MSA). The calculations follow the AASHTO methodology and are based on Vehicle Equivalent Factors (VEF) and Annual Average Daily Traffic (AADT).

The increase in MSA over the 20-year design period is illustrated in Figure 1 for each section:

The AASHTO 1993 formula was used to calculate the necessary Structural Number (SN):

Therefore,

The adjustment reflects standard practice: the initial 215 mm result was theoretical, and the adopted 140 mm is supplemented by base and subbase layers to meet the required SN values.

Step 3 – Add base layers to satisfy (on subbase)

For base layer thickness,

In line with the stage construction strategy, a 50 mm overlay is planned after 10 years. This is not counted towards the initial SN calculations, but the choice of a 50 mm wearing course facilitates later overlay integration.

Thus, the adopted design thicknesses (50 mm wearing + 90 mm binder + 150 mm Base I + 200 mm Base II + 200 mm subbase, over a 250 mm improved subgrade) meet the required SN values while remaining constructible and consistent with specifications. The calculated SN values for asphalt, base, and subbase layers are summarized in Table 1 , which presents the properties of each layer according to AASHTO and RHD specifications.

* Table 1 presents the attributes for each layer according to AASHTO and RHD standards.

The final adopted layer thicknesses for each section are presented in Table 2 . These thicknesses align with the calculated Structural Number (SN) values and ensure constructability under local standards. The design incorporated the following components:

*A 250 mm layer of ISG is present in every section to support the pavement.

Over a 20-year period, traffic projections estimate ESAL values reaching as high as 81.34 million. The Structural Number (SN) values for the subgrade, subbase, and base are 4.73, 4.01, and 3.56, respectively. The designed thicknesses met or exceeded these specifications, with Section 1 having a thickness of 690 mm as an example.

The project’s pavement structure was designed based on geotechnical studies, regional climate conditions, and projected cumulative traffic loading over a 20-year period. The recommended materials—Dense Bituminous Surfacing (PG 64), Aggregate Base (Types I & Type II), Granular Subbase, and Improved Subgrade (ISG)—are locally suitable and economically viable. Additionally, these materials are technically sound and readily accessible in Bangladesh.

The estimated Structural Number (SN) for each section demonstrates that the layer thicknesses either fully comply with or slightly deviate from the requirements, ensuring long-term serviceability and providing an additional safety margin.

The design closely follows the RHD Pavement Design Guide (2005) [9] and aligns with its guidelines regarding:

The AASHTO (1993) method, which has been improved with RHD design values, is the best way to bring international best practices to a local level. The design has a 90% reliability level, which gives you a better guarantee of performance. This is especially important for the trade corridor across borders, though the RHD usually suggests lower levels of reliability for collectors in rural areas.

1) Climate and Flooding

The monsoon, which brings heavy rainfall from May to September, has a significant impact on Bangladesh’s climate. Waterlogging and flooding are common occurrences on roads, which hasten the formation of potholes, aggregate loss, and pavement stripping [6] . The design tackles climate resilience in this context by:

However, without the installation of adequate subsurface drainage systems during construction, there remains a risk of early pavement distress in areas susceptible to flooding or lacking proper drainage. To address these concerns, it is essential to reevaluate these risks through hydrological studies during the detailed design and construction phases.

2) Soil Conditions

The alignment goes through low-lying land with soft alluvial soils that aren’t forceful and can be easily compressed. To lower these risks, a 250-mm-thick layer of improved subgrade is put on top of the natural soil:

The phenomenon of differential settlement can lead to challenges, especially in areas undergoing widening or embankment work. Continuous geotechnical monitoring is essential during construction to adjust the subgrade treatment as needed. This monitoring is crucial for effectively addressing these issues.

3) Heavy Traffic Loads

The road is expected to handle a substantial volume of overloaded trucks, as it serves as a crucial trade route to Hili Land Port, especially during peak periods for industrial and agricultural transportation. This has been considered in the design with:

Following the allowed axle load limits is essential to keep the pavement from wearing out too quickly. If these limits are not followed, the pavement may fail before it should, even if the design is sound. This is because axle loads that are much higher than what was planned could cause the pavement to fail prematurely [3] [4] . Moreover, while the RHD 2005 VEFs provide a calibrated basis for design, actual truck overloading in Bangladesh likely exceeds these values, further reinforcing the urgency of enforcement [5] .

In addition, the pavement design incorporates resilience measures that mitigate some of the risks of overloading. The adoption of a 90% reliability factor ensures a conservative margin of safety, providing greater assurance that the pavement will perform under uncertain and variable traffic conditions. Similarly, the use of conservative material properties, such as high-quality aggregate bases (CCBR 50 - 80) and polymer-modified PG 64 asphalt, adds durability and resistance to rutting and fatigue. Together, these choices strengthen the pavement structure’s ability to withstand higher than expected loading, even if a strict environment is not always achieved.