FAARFIELD(FAA Rigid and Flexible Iterative Elastic Layer Design) Method for Flexible Pavement Design

The FAARFIELD uses the maximum vertical strain at the top of subgrade and maximum horizontal strain at the bottom of all asphalt layers as the predictor of pavement structure life . The design aims to minimize the deformation of the subgrade and pavement surface under aircraft loading. Key considerations include:

• Load Distribution: Aircraft load characteristics such as weight and wheel configuration influence stress distribution across the pavement layers.
• Material Properties: The strength of materials used for each pavement layer is crucial in determining the overall pavement thickness.
• Subgrade Conditions: The properties of the subgrade soil, often characterized by parameters like the California Bearing Ratio (CBR) and Modulus of Elasticity (E), affect the design to ensure the pavement can adequately support aircraft loads.

Materials and Thickness Recommendations

FAA’s guidelines provide recommendations for selecting materials and determining layer thickness based on aircraft traffic and subgrade characteristics:

• Full-Depth Asphalt Pavement: For aircraft weighing below 60,000 lbs, a full-depth asphalt pavement is typically recommended. The design accounts for parameters such as CBR, E-values, and the maximum allowable strain in the subgrade.
• Geosynthetics and Drainage: Incorporating geosynthetic materials improves separation and reinforcement, while efficient drainage layers prevent water accumulation that can weaken the pavement structure.
• Specified Pavement Layers: The FAA has established standards for different layers, including surface course (e.g., Asphalt P-401 & 403/ PCC P 501 & P404), base course (e.g., P-207,208,209,210,211,212 & 219), and subbase (e.g., P-154 & 213), each with specific material and compaction requirements to enhance structural performance.

Understanding E = K * CBR: Correlating Modulus of Elasticity with CBR

In pavement design, the relationship between the Modulus of Elasticity (E) and the California Bearing Ratio (CBR) is a common empirical correlation used to estimate subgrade stiffness. The formula:

E=K×CBRE = K \times \text{CBR}E=K×CBR

where KKK is a conversion factor (typically ranging from 10 to 30), provides a practical means for estimating the subgrade’s modulus based on its CBR value:

• High CBR Values: Indicate stronger subgrade materials, leading to higher E-values and potentially reducing the required pavement thickness.
• Low CBR Values: Suggest weaker subgrades that may necessitate thicker layers to distribute aircraft loads and prevent excessive strain on the pavement structure.
• Minimum CBR value 5%.

FAARFIELD Software:

FAARFIELD software revolutionizes airport pavement design by automating the iterative process of calculating the required thickness for each pavement layer. The software performs detailed analyses, considering factors such as aircraft type, departure frequency, and material properties. Its features include:

• Iterative Layer Adjustment: Users can modify the pavement structure by adding, removing, or changing layers to optimize the design based on calculated requirements.
• Cumulative Damage Factor (CDF) Analysis: FAARFIELD evaluates cumulative damage by considering the effects of various aircraft types within a traffic mix, ensuring the pavement meets structural requirements across its service life.
• Traffic Mix Customization: The software allows users to create specific traffic scenarios by selecting aircraft types, adjusting their gross weights, and defining departure frequencies, ensuring that the design aligns with actual airfield conditions.

Pavement Design Steps FAARFIELD

FAARFIELD enables engineers to undertake a systematic approach to flexible pavement design through a series of steps:

1. Program Initialization: Start by selecting the type of pavement (rigid or flexible) that will be designed.
2. Modify Pavement Structure: Customize the pavement layers by adding, deleting, or adjusting layer materials and properties.
3. Define Traffic Mix: Choose aircraft types from the software's library or use a stored mix to represent the expected air traffic. Adjust the number of departures and gross weight of each aircraft if necessary.
4. Run Thickness Design: Use the software to calculate the minimum required thickness for each layer, ensuring that the pavement can adequately support aircraft loads throughout its design life.
5. Compaction and Life Evaluation: Assess the subgrade's compaction requirements and pavement life under the expected traffic conditions.
6. Review Design Report: Generate a detailed report summarizing the design specifications, including layer thicknesses, material properties, and cumulative damage factors.

FAARFIELD’s iterative process helps refine the design by adjusting layer thickness based on the calculated stress-strain response of the pavement to aircraft loading. The software’s flexibility allows for different design scenarios to be explored quickly, ensuring that the final pavement configuration meets the FAA’s structural criteria.

Incorporating the Pass-to-Coverage Ratio (P/C) in Pavement Design

One of the unique factors in FAARFIELD’s methodology is the incorporation of the Pass-to-Coverage Ratio (P/C), which accounts for aircraft wander—the lateral movement of aircraft across a pavement surface. This phenomenon results in uneven load distribution as aircraft rarely follow the same path along a runway or taxiway. Key considerations include:

• Modelling Aircraft Wander: FAARFIELD uses a normal distribution to represent the variability in aircraft wheel paths, ensuring the design accounts for this lateral movement.
• P/C Ratios for Different Gear Types: Single Wheel: 5.18Dual Wheel: 3.48Dual Tandem: 1.84Boeing 747: 1.85DC 10-10: 1.82
• Impact on Cumulative Damage: The P/C ratio helps determine how many passes are required for a specific point on the pavement to experience one full load application. This information is used in cumulative damage calculations to optimize the design.

Cumulative Damage Factor (CDF) in FAARFIELD Analysis

The CDF is a critical measure used in FAARFIELD to assess the pavement's structural integrity under aircraft operations. It quantifies the ratio of the number of load repetitions applied to the pavement to the number of repetitions that would cause failure:

• Calculation of CDF: CDF=(Annual Dep. x Life in years)/ (p/c) x Coverage to Failure
• CDF Design Objective: When the cumulative damage factor reaches 1.0, the pavement has reached its design limit. FAARFIELD calculates the CDF for each strip of pavement to ensure the entire width meets the structural design criteria.

Conclusion

The integration of FAARFIELD software with the FAA’s pavement design guidelines provides a robust framework for airport pavement design, enhancing safety, reliability, and sustainability. The software’s capability to automate the design process, account for aircraft wander, and optimize layer thickness based on real-world conditions makes it an invaluable tool in the aviation industry.

FAARFIELD enables engineers to achieve efficient and durable airport pavements by considering every aspect of pavement behaviour and aircraft characteristics. This holistic approach ensures that airfield infrastructure can support a wide range of aircraft types, improving operational efficiency and maintaining high safety standards.

Reference:https://www.faa.gov/airports/engineering