Published in 20th World Road Congress
(Seminar on airfield pavements), Montreal, Canada, 3-9 September,1995.
 

AN IMPROVED FLEXIBLE AIRPORT PAVEMENT DESIGN METHOD

WARDLE Leigh
(MINCAD Systems Pty. Ltd., P.O. Box 2114, Richmond South, Vic. 3121)
AUSTRALIA

RODWAY Bruce
(Federal Airports Corporation, Locked Bag 28, Botany, N.S.W. 2019)
AUSTRALIA
 

Key-words: elasticity, flexible pavement, mathematical model, pavement design, rutting
 

Introduction: Most current procedures for flexible airport pavement design and analysis, including the U.S. Federal Aviation Administration (FAA) design method and the International Civil Aviation Organisation (ICAO) system for aircraft load classification, are based on the empirical CBR method. This method assumes a failure mode that consists of surface rutting caused by over-stressing the subgrade. Design life is considered to have expired when the pavement surface has rutted to an extent that renders it unsuitable for aircraft. Pavement failure due to fatigue cracking of the bituminous surfacing layer or cracking of other bound layers is not addressed by the CBR method and must be separately considered by the designer. The CBR design method involves increasing pavement life by increasing pavement thickness to further protect the subgrade.

The method involved using elastic pavement models to calculate a selected indicator of the rate at which rutting develops, typically the maximum subgrade vertical strain or deflection that is caused by the passage of an aircraft. The number of repetitions of the strain or deflection to cause unacceptable rutting must be established by calibration against full-scale trafficking tests. Extension of the method to cater for aircraft and pavements for which no test data is available is attempted using elastic theory.

At the time these procedures were developed it was necessary to make a number of major simplifications:

These simplifications are no longer required given developments in the layered elastic method and the computational power of modern desk-top computers.

The FAA have recently developed sophisticated design software based on layered elastic theory (FAA, 1994). The system retains the PCR concept.

APSDS (Airport Pavement Structural Design System) is a software package that has been developed to overcome some of the limitations of the existing systems caused by the simplifications listed above.

Methodology: APSDS is based on a concept described by Monismith et al. (1987). The important new and unique feature is that subgrade strains, or alternative indicators of the rate at which surface rutting develops, are computed for all points across the pavement to capture all damage contributions from all the aircraft wheels in all their wandering positions. This contrasts with previous methods that computed single maximum values of the damage indicator. In the case of fatigue failure of bound layers, APSDS would typically compute the tensile strains at the undersides of the relevant layers.

The strains are converted to damage using a performance model of the form:

(1)

where N is the predicted life (repetitions of e )
k is a material constant
b is the damage exponent of the material
e is the load-induced strain (unitless strain)

More complex performance relationships could be accommodated by the program if required.

The Damage Factor for the i-th loading is defined as the number of repetitions (ni) of a given damage indicator divided by the ‘allowable’ repetitions (Ni) of the damage indicator that would cause failure. The Cumulative Damage Factor (CDF) is given by summing the damage factors over all the loadings in the traffic spectrum using Miner’s hypothesis.

Cumulative Damage Factor = (2)

The pavement is presumed to have reached its design life when the cumulative damage reaches 1.0.

The method incorporates the design repetitions of each aircraft at their various operating weights, the wander of the aircraft, and the material performance properties used in the design model.

Figure 1 shows the general concepts for a single gear. The basic procedure is:

1. The strain distribution across the transverse section is determined.

2. The number of aircraft passes is determined for a series of equally spaced intervals that span the wander distribution.

3. The damage contributions due to each of the wander distribution intervals are summed at a series of points across the section.


APSDS has a menu-driven interface that runs under Microsoft Windows and is based on the layered elastic program CIRCLY (Wardle, 1977) that has been used on numerous pavement design projects over the last 20 years. CIRCLY is at the core of the Austroads Pavement Design Guide and is routinely used for highway design in Australia.

Figure 2 is a typical cumulative damage ‘profile’ across the pavement generated by APSDS.

In the case of a rutting failure mode, parameters k and b have been established from back-analysis of the full-scale aircraft trafficking tests conducted at the Waterways Experiment Station, Vicksburg, Mississippi. The most recent test series was carried out in the late 1960s and early 1970s (Ahlvin et al., 1971). Details of the back-analysis procedure are given by Wardle and Rodway (1995). The APSDS model provides a better fit to the Corps of Engineers full-scale test data than previously reported. For the fatigue mode, k and b can potentially be obtained from laboratory testing.

Figure 1 Schematic of effect of aircraft wander on cumulative damage
 

Figure 2 Sample cumulative damage plot

The extensive performance history of bound materials under highway conditions is expected to be adequate to calibrate laboratory-derived performance models for use in the airfield situation.

Aircraft Wander: The airfield situation differs from highways in that wheel loads are much more evenly distributed across the pavement width. This is because traffic flow is far less channelised and because of the large variety of aircraft wheel configurations. Field observations of aircraft movements have shown that successive passes of aircraft along a pavement are statistically normally distributed about the pavement centreline. The degree of "wander" can be reasonably characterised by a standard deviation and is found to be significantly different for runways, taxiways and aircraft docking bays. This spreading of aircraft wheel loads across the pavement width has a major effect on the amount of pavement damage caused.

As mentioned earlier, conventional pavement design and analysis systems relate pavement life expressed in aircraft passes to the maximum value of a chosen damage indicator, typically subgrade vertical strain or deflection calculated from elastic models. Because only a single maximum is computed it was necessary to introduce the pass-to-coverage (PCR) concept to account, in an approximate way, for the effect of aircraft wander. A point on the pavement is said to receive a ‘coverage’ when any part of a tyre’s contact area passes over it. The PCR is defined as the number of passes of a wandering aircraft that is statistically required for the most frequently covered point to receive one coverage. The PCR depends upon wheel configuration, tyre width and the degree of aircraft wander. The PCR concept solely addresses the statistics of load distribution at the pavement surface and, therefore, incorrectly implies that the reduction in pavement damage due to aircraft wander is the same for all pavement thicknesses.

APSDS does not use the coverage concept. Instead the strain distribution (not just a single maximum strain) at all points across the pavement for a given depth is used to capture the damage contributions of all the aircraft wheels in all their wandering positions.

The user can specify the standard deviation of wander that is appropriate to the particular pavement. The standard deviation for a taxiway is typically taken as 773 mm and for a runway as 1546 mm (Ho Sang, 1975). These correspond to wander widths of 1778 mm (70 inches) and 3556 mm (140 inches) where wander is defined as the pavement width within which the centrelines of aircraft are contained 75% of the time. For a parking bay, a standard deviation of the order of 100 mm may be appropriate.

A simple example (Figure 3) illustrates how the coverage concept becomes increasingly inappropriate with increasing depth to subgrade. This shows the effect of aircraft wander upon damage for two depths, as calculated using APSDS. In the 500 mm case (Figure 3a), taxiway wander reduces damage by 80% of that caused in the channelised, no wander case. This contrasts with the 1500 mm case (Figure 3b) where taxiway wander reduces the channelised damage by only 30%.

Pavement Damage Due To New Large Aircraft: The recent arrival of the Boeing 777 (which has six-wheeled, three-axled landing gears) and the necessity to evaluate gear configurations of very large future generation aircraft has focussed attention on the need to extend models to accommodate larger numbers of wheels in a gear and the interaction of all gears. There is no test data that addresses this issue. The full-scale tests used to calibrate pavement models were essentially limited to single gear assemblies having no more than two axles. The FAA has proposed a 7-year research program, including extensive full-scale accelerated tests, to quantify the effects of more than four wheels on a strut and interaction effects between closely spaced struts (FAA, 1993).

(a) Depth = 500 mm

(b) Depth = 1500 mm

Figure 3 Effect of depth and wander on pavement damage

Past and present methods of pavement analysis use single computable ‘damage indicators’, typically vertical components of deflection, strain or stress at subgrade level. This simplified approach ignores the shape of the strain distribution. Multiwheel landing gears proposed for some of the future generation aircraft will be likely to produce more laterally extended, near-uniform loadings than those produced by the loading assemblies used in past full-scale tests. They will cause large maximum deflections, strains and stresses at subgrade level. Consequently design methods that use a single parameter such as the maximum strain as an indicator of rutting will predict very high damage. However, the high degree of uniformity may mean that significantly less rutting will actually occur.

APSDS has been used to study multiple gear interaction effects for a Boeing 747 using a range of alternative damage models (Wardle and Rodway, 1995). Results from the study show that the successful calibration of simplified design models against the full-scale test data does not create a capability to confidently extrapolate beyond the limits of the test data. The study showed that simple damage models give unrealistic predictions for the damage caused by all sixteen wheels when compared to that computed for a single isolated 4-wheel gear. Three different performance models, each of which gave a similar ‘goodness of fit’ to the full-scale test data, gave greatly different predictions of the damage caused by the interactions of the sixteen main wheels. The differences between the alternative predictions increase with increasing depth to subgrade.

Conclusions: APSDS has unique features that will enhance and optimise the analysis and design of airport pavements:

The system’s transparency, speed and flexibility enables design specialists to readily change all problem inputs including aircraft wander, aircraft numbers and mass, layer thicknesses and material properties and also the performance models. This allows rapid assessment of the sensitivity to each component input and for all design assumptions.

APSDS models the lateral distribution of traffic in more detail than alternative methods that only address the load distribution at the pavement surface. The APSDS model provides a better fit to the Corps of Engineers full-scale test data than previously reported. This suggests that APSDS realistically models the effects of wander at subgrade level and so includes the influence of pavement thickness and properties on the amount of damage reduction that results from aircraft wander.

An APSDS study of the interaction of multiple gears has demonstrated the major difficulty in predicting the impact of future generation large aircraft on the thickness requirements for flexible pavements. Although alternative performance models can give a similar ‘goodness of fit’ to the full-scale single gear test data, widely different damage predictions are obtained for conditions beyond the limits of the test data. Further full-scale testing, as planned by the US Federal Aviation Administration, is required to quantify damage caused by large multiwheeled gears and by interaction of aircraft gears.

Acknowledgements: The original system concept was developed by Ian Rickards of Pioneer Road Services Pty. Ltd, Australia. Pioneer financed the initial development and have supported the promotion of APSDS to the international airport design community. The authors wish to acknowledge Ian Rickards for his enthusiastic support and active participation in our research. Permission of the Federal Airports Corporation to publish this paper is gratefully acknowledged.

References

Ahlvin, R. G. et al. (1971). Multiple-wheel heavy gear load pavement tests. Technical Report S-71-17, Vol. I, US Army Corps of Engineers, Waterways Experiment Station, Vicksburg.

Federal Aviation Administration (1993). Airport pavements. Solutions for tomorrow’s aircraft. US Dept. of Transportation. FAA Technical Center, New Jersey.

Federal Aviation Administration (1994). LEDFAA User’s Manual. US Dept. of Transportation. FAA, Washington, D.C.

Ho Sang, V.A. (1975). Field survey and analysis of aircraft distribution on airport pavements. Report No. FAA-RD-74-36. U.S. Federal Aviation Administration.

Monismith, C.L., Finn, F.N., Ahlborn, G. and Markevich, N. (1987). A general analytically based approach to the design of asphalt concrete pavements. Proc. 6th. Int. Conf. on the Structural Design of Asphalt Pavements, Ann Arbor.

Wardle, L.J. (1977). CIRCLY Users’ Manual, MINCAD Systems Pty Ltd, Richmond, Vic., Australia.

Wardle, L.J. and Rodway, B. (1995). Development and application of an improved airport pavement design method. ASCE 1995 Transportation Congress, San Diego, October, 1995.