Research Projects

Evaluating the Impact of Aircraft Lateral Turning Forces on Pavement Design and Performance

Assigned to Danny Xiao

This research would help airports understand, qualify and quantify lateral shear forces within the surface or subsurface layers of flexible pavements induced by landing gear lateral turning movements; develop models to take the excessive lateral shear forces into consideration in the design of runway asphalt concrete surface layers; formulate recommendations to include shear resistant asphalt concrete layer (or sublayer) in the pavement structure to minimize shear failures and avoid layer delamination in areas within the runway/taxiway that are more likely to experience excessive lateral shear forces; and recommend existing or revised test standards and threshold values to reduce the risk of such failures.

Background (Describe the current situation or problem in the industry, and how your idea would address it.)

The introduction of a new generation of very large aircraft, capable of traveling very long distances on intercontinental flights, has created new challenges to airport pavements worldwide. Aircraft such as the Boeing B777, or the Airbus A340 and A380, have gross weight well over 750,000 pounds at maximum takeoff weight. The vertical load applied on the landing gears is used to design the pavement structure. However, the design assumes only static loading. There is no consideration of lateral forces induced at the surface of the pavement. This problem gets more complicated when the vertical load is amplified due to surface roughness and the normal dynamic forces during taxing operations. There are two main situations when these forces are critical for the pavement. During taxing to takeoff, the aircraft turns or pivot to align with the runway center. At maximum takeoff weight, this turn generates very large localized shear forces at the surface where tire and pavement interact. The second situation is during fast exists after landing. In this case the centripetal acceleration during the fast exit creates large lateral forces at the surface or near surface of the pavement. Shear forces in both situations cause the pavement to deform and ultimately delaminate, which is a serious distress in runway pavements, capable of producing large cracks, potholes, foreign object debris (FOD), and at the worst case a partial or complete delamination of the surface layer. There have been several instances of catastrophic failure of the surface layer during pivot turning in airports around the world. Delamination near fast exists has also become a source of problems for airport administrators trying to maintain the runway in good structural condition and free of FOD. Currently, the FAA does not have specifications in its design procedure to include the shear failure of the surface of subsurface layers induced by pinned landing gear movements at runway ends, or fast exist turns on the middle of the runway build as flexible pavement. Understanding the problem and its consequences is the first step in developing a design procedure – or modifying an existing design procedure – in which lateral shear failure is mitigated.

Objective (What is the desired product or result that will help the airport industry?)

This research would help airports understand, qualify and quantify lateral shear forces within the surface or subsurface layers of flexible pavements induced by landing gear lateral turning movements; develop models to take the excessive lateral shear forces into consideration in the design of runway asphalt concrete surface layers; formulate recommendations to include shear resistant asphalt concrete layer (or sublayer) in the pavement structure to minimize shear failures and avoid layer delamination in areas within the runway/taxiway that are more likely to experience excessive lateral shear forces; and recommend existing or revised test standards and threshold values to reduce the risk of such failures.

Approach (Describe in general terms the steps you think are needed to achieve the objective.)

Proposed Tasks
Based on the objectives above and the overall goal of this research, the following tasks are proposed:
1)
Conduct detailed review of, but not limited to, existing research, literature, case studies, airport regulations
and best practices related to lateral shear damage on pavement structures. As a minimum, the review should
focus on:
a.
Understanding damages induced by excessive lateral forces.
b.
Examples of failures and corrective measures adopted by the industry.
c.
Pavement mechanics and the structural behavior as it relates to shear stresses.
d.
Pavement design procedures and how the
y can be modified to include shear stress resistance.
e.
Mapping the interaction load-
pavement response to understand and define loading threshold in
which the problem must be investigated during the design.
2)
Load characterization
a.
Types of landing gear and loading magnitudes.
b.
Gear speed and loading duration.
c.
Operational characteristics and limiting conditions.
3)
Pavement mechanics
a.
Investigation of best constitutive models and type of pavement simulations required to effectively
evaluate th
e high shear stress problem. (Note: not a model development, but rather a model search
from the ones commonly used in practice.)
4)
Aircraft
-Pavement interaction
a.
Investigate the interaction between various aircrafts and gear/loading configurations and t
he
pavement structure.
b.
Identify critical conditions and limiting cases in which the evaluation of high lateral shear stress is
required.
c.
Investigate pavement structure concepts to minimize lateral shear damage.
5)
Pavement design recommendation
a.
Evaluate pavement design methods, including FAA, to propose ways to incorporate a procedure to
evaluate high lateral shear stress damage and failure.
6)
Materials and construction practices
a.
Recommendations of asphalt concrete mixture requirements to produce shear resistant materials
(including lab tests of such materials).
b.
Construction good practices to avoid creating areas more susceptible to damage induced by shear
forces.
7)
Prepare guidebook to assist airport pavement engineers in dealing with the design, construction and
maintenance of pavement structures subjected to high levels of surface or subsurface lateral forces.

Cost Estimate and Backup (Provide a cost estimate and support for how you arrived at the estimate.)

Estimated Funding
The funding for investigating the problem of premature failure of pavement structures due to high levels of shear
stresses induced by lateral turning forces is estimated at $400,000
Estimated Research Duration
The estimated duration for the proposed tasks outlined above is 18 months, which includes an estimated 3 months for
the ACRP approval process.

Related Research - List related ACRP and other industry research; describe gaps (see link to Research Roadmaps above), and describe how your idea would address these gaps. This is a critical element of a synthesis topic submission.

Although the issue of lateral shear forces is a frequent problem, there aren't many efforts of investigating it reported
in the literature. This project would build on some scattered research related below:

  1. Airfield Asphalt Pavement Technology Program (AAPTP) Project 06-04: Identification Non-destructive
    Testing to Identify Presence and Extent of Delamination of HMA Airfield Pavements.
  2. Airfield Asphalt Pavement Technology Program (AAPTP) Project 07-04 Evaluation of Failure Mechanisms for HMA Airfield Pavements and Project 07-01: Improved Shear Strength to Accommodate Higher Tire Pressures
  3. Federal Aviation Administration: few articles and publications on delamination of HMA airfield pavements and instrumentation of fast-exit taxiways.
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Idea No. 19

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