Pavement Design as per UK, USA & Australian Standards and their Comparison

UK PAVEMENT DESIGN:

The Design Manual for Roads and Bridges (DMRB) in the UK provides a comprehensive set of guidelines used for the design, assessment, and maintenance of road infrastructures. Developed by National Highways (formerly Highways Agency & Highways England) on behalf of the devolved administrations of Scotland, Wales, and Northern Ireland, the DMRB includes detailed standards for pavement design. The DMRB Standards used for pavement design are:

• CD224 - Traffic Assessment
• CD 225 - Pavement Foundation Design
• CD 226 - New Pavement Construction

Example Pavement Design Process According to DMRB:

Step 1: Foundation Design

The subgrade surface modulus shall be used in the pavement foundation design. CBR is traditionally used as indirect test for sub grade strength. The following equation is used in the estimation of subgrade surface modulus:

E = 17.6 (CBR)0.64 MPa

Design approaches for foundation:

1. Restricted Design Approach?– It allows a limited number of designs to be applied for foundation classes 2 and 3 and is particularly intended for use on schemes of limited extent.

2. Performance Based Design Approach?– This method is used to enable the efficient use of materials. This method is used for wider sites. The foundation designs shall be subject to performance testing. The foundation classes are given below:

• Foundation Class 1 ≥ 50MPa – Capping Layer only – not used for traffic > 20msa – for non-trunk roads only.
• Foundation Class 2 ≥ 100MPa – Capping Layer plus/or granular subbase material
• Foundation Class 3 ≥ 200MPa – Hydraulically bound material (using cement, furnace slag, fly ash etc.) are used
• Foundation Class 4 ≥ 400MPa – Hydraulically bound material (using cement furnace slag, fly ash etc.) are used

Step 2: Traffic Assessment

Design Traffic = Commercial vehicle loading over the design period (expressed as the number of equivalent standard axle load (80KN or 8.16T)

For new scheme AADF from Traffic Appraisal Manual 12.1.1

For existing road scheme, a classified count shall be carried out over a 12, 16 or 24-hour period. AADF is flow in one direction. AADT is traffic on both directions.

Step 3: Commercial vehicles at opening (F):

Min 3.5 tonnes gross weight

PSV – Public Service Vehicles - Buses and Coaches

OGV1 – 2 & 3 Axle rigid

OGV2 – 4 or more axle articulated

Percentage of commercial vehicles within the AADF (in one direction) For new road schemes, the commercial vehicle flows by class / category shall be determined from traffic transport analysis using the principles described in the Department for Transport's WebTAG Unit

Step 4: Design Period (Y):

A design period of 40 years is used.

Step 5: Growth Factor (G):

A table is used for growth factors.

Step 6: Wear Factor (W):

Structural wear is proportional to 4th power of axle load. Thus a 50% increase in Axle load results in a five-fold increase in calculated structural wear. The wear factors to be used for the Maintenance (Wm) and New Design (Wn) cases are shown in a table.

Step 7: Percentage of Commercial Vehicles in Heaviest Loaded Lane (P):

As all lanes are designed as for the heaviest loaded lane. For new and existing carriageways with 2 or more lanes in one direction, the proportion of vehicles in the most heavily loaded lane shall be estimated using a table provided in DMRB.

Step 8: Design Traffic = 365*F*Y*G*W*P*10-6

Step 9: Pavement Thickness

Nomographs for determining the design thickness of flexible pavement are shown in DMRB CD226.

AASHTO PAVEMENT DESIGN:

The American Association of State Highway and Transportation Officials (AASHTO) pavement design method is a widely used approach for designing both flexible (asphalt) and rigid (concrete) pavements. This method has evolved over time, with significant updates reflected in various editions of the AASHTO Guide for Design of Pavement Structures. The method is grounded in empirical research and practical experience, primarily derived from the AASHO Road Test conducted in the late 1950s. Here’s an overview of how AASHTO pavement design works:

Example of Pavement Design Using AASHTO Guidelines:

Step 1: Develop Equivalent Single Axle Load, W18

Traffic is represented in the AASHTO method by the equivalent single axle load (ESAL), or the number of 18-kip (or 18 Tons) equivalent single axle loads that will pass over the pavement during its initial service lifetime.

Step 2: Develop soil resilient modulus, MR

The AASHTO correlation below gives reasonable agreement between the California Bearing Ratio (CBR) and the soil resilient] modulus. Unless site specific investigations determine different resilient modulus-CBR correlation factors, the AASHTO correlation should be used.

Mr = 1500(CBR)

where: Mr = Resilient Modulus (psi)

CBR = California Bearing Ratio

Step 3: Determine the overall standard deviation, So

The overall standard deviation is a dimensionless parameter that accounts for random variation in the traffic projections and normal variation in the pavement parameters. Simply put, it provides a means of accounting for areas of weaker than average pavement receiving higher than expected traffic. A value of 0.45 for So is commonly used for flexible pavement materials.

Step 4: Select the level of reliability, R

The level of reliability describes the degree of certainty that the pavement will last as long as the design service period. The level of reliability is represented in the AASHTO equation by the standard normal deviate, ZR, and in the design nomograph by R.

Step 5: Select design serviceability loss, PSI

The pavement serviceability is a general measure of the pavements ability to service the traffic which must pass over it. Serviceability ranges from 0 (impassable) to 5 (ideal) and represents a quantification of subjective impressions about the roadway quality. The design serviceability loss is obtained by simply subtracting the final value from the initial value, and so describes the amount of degradation of service which is acceptable during the design lifetime.

Step 6: Solve for the structural number, SN

The preceding steps 1-5 were independent. However, a value must be obtained for each one in order to complete step 6, solving for the structural number. The structural number can be solved for using in an equation, using a trial and-error procedure.

Step 7: Determine pavement and base thickness

Once determined from step 6, the structural number is used to determine the thickness of each pavement material layer using the appropriate material coefficients from a prescribed table.

Various combinations of pavement materials of various thickness are possible to meet or exceed a given structural number. Once the structural requirements are met the combination and thickness of the individual pavement material sections is based on such factors as aggregate availability, aggregate size, cost of various pavement materials, minimum recommended thickness, restrictions on overall thickness, number of lifts required.

AUSTRODES PAVEMENT DESIGN:

Austroads is the peak organisation of Australasian road transport and traffic agencies, including those in Australia. It plays a crucial role in researching, publishing guidelines, and setting standards for road design, construction, maintenance, and safety, including pavement design. Austroads’ work is highly influential in shaping pavement engineering practices across Australia and New Zealand, aiming to enhance the efficiency and safety of road networks while considering environmental sustainability.?

A typical example of pavement design according to the Austroads standard involves several key steps:

1. Determine the Pavement Design Traffic:

This involves calculating the number of Equivalent Standard Axle Loads (ESALs) that the pavement is expected to support over its design life. The calculation considers factors such as traffic growth, distribution of heavy vehicles, and axle configurations.

2. Subgrade Assessment:

The strength and properties of the subgrade material are evaluated using the California Bearing Ratio (CBR) test or other relevant tests. The subgrade performance influences the thickness and materials needed for the pavement layers.

3. Material Selection:

Based on the subgrade assessment and the expected traffic loads, suitable materials for the subbase, base, and surfacing layers are selected. Material selection also considers local availability, cost, and environmental factors.

4. Structural Design:

The structural design process uses the determined traffic loads and material properties to calculate the required thicknesses of the pavement layers. Austroads provides methods for designing both flexible and rigid pavements. For flexible pavements, the design might involve the use of empirical formulas or mechanistic-empirical design principles. For rigid pavements, considerations such as slab thickness, joint spacing, and reinforcement are made.

The specific details, such as layer thicknesses and material types, would depend on the outcomes of the design process steps, including traffic loads, subgrade strength, and environmental conditions.

COMARISON OF UK, USA and AUSTRALIAN PAVEMENT DESIGNS STANDARDS: ?

• Design Philosophy: The UK and Australia both use a mix of performance-based and empirical methods, with a strong emphasis on sustainability and the use of recycled materials. The US, while historically relying on empirical methods from the AASHO Road Test, is moving towards more mechanistic-empirical methods with the adoption of the MEPDG.? ?
• Methodology: The UK and Australia utilize the CBR method for subgrade evaluation, while the US has traditionally used the Structural Number concept and is now incorporating more advanced mechanistic-empirical models through MEPDG.? ?
• Sustainability: All three countries are increasingly focusing on sustainable pavement design, though the extent and methods of incorporating sustainability into design standards may vary.? ?
• Adaptation to Local Conditions: Each country’s methodology is adapted to its specific environmental conditions, traffic patterns, and material availability, reflecting the need for local customisation in pavement design.? ?
• Evolution of Design Practices: There is a global trend towards incorporating more mechanistic-empirical design methods that can more accurately predict pavement performance and consider a broader range of factors, including environmental impacts.

Overall, while there are similarities in the basic principles of pavement design among the UK, USA, and Australia, each country has developed its standards and methodologies to suit its specific needs, resources, and experiences.