Proceedings Transport 98, 19th ARRB Conference, Sydney, Australia, 7-11 December 1998.

LAYERED ELASTIC PAVEMENT DESIGN-
RECENT DEVELOPMENTS

Leigh J. Wardle
Director, Mincad Systems Pty. Ltd.

Bruce Rodway
Pavement Consultant

Abstract

Early versions of CIRCLY performed only the basic task of calculating the stresses, strains and displacements within a layered elastic system. CIRCLY4 is a pavement design program that does this, then uses the strains to perform the design calculations, and also plots the results. Granular materials are automatically sublayered in accordance with either the method detailed in the Austroads Guide, or that used by the US Army Corps of Engineers. The damages to each pavement material (asphalt surfacing, cement-treated layer and subgrade) due to the nominated design traffic are computed simultaneously. The pavement is then designed automatically by adjusting the thickness of any pavement layer selected by the designer. The program plots, in either 2 or 3 dimensions, any user-nominated computed values (strains, stresses, deflections, strain energy, strain energy of distortion) at any selected depths below the surface.

The user can specify all design inputs, including material properties, each material’s performance relationship and any wheel loading configurations and tyre pressures. This flexibility and the program’s transparency are intended to provide the experienced designer with easy access to and control of the full capabilities of a layered elastic ‘mechanistic’ method. The program’s computational speed coupled with the ease with which all problem inputs can be changed allows the designer to assess the sensitivity of the design to each input and to each design assumption.

INTRODUCTION

The pavement design method described in the Austroads Pavement Design Guide (Austroads, 1992) uses a layered elastic computer program CIRCLY (Wardle, 1996) to calculate load-induced elastic stresses, strains and deflections in pavements. Two critical strains are used to design pavements. The maximum vertical compressive strain at subgrade level is related to the repetitions to cause surface rutting failure. The maximum horizontal tensile strain at the underside of the asphalt or cemented layers is related to repetitions to cause fatigue cracking of those layers.

The ‘mechanistic’ design method involves calculating pavement damage from these critical strains using empirical equations called ‘failure criteria’ or ‘performance relationships’ of the form:

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

 

The empirical parameters k and b are determined by calibrating the design method against observed performance of test pavements or of pavements in service. The method by which the AUSTROADS rutting criterion was derived will be described to illustrate the process. Neither measurements of rut depths nor measurements of subgrade strains was involved. For many years many unbound road pavements had been designed in Australia using the empirical CBR design chart shown in Figure 1 which reproduces Figure 8.4 of the Austroads Design Guide. The design traffic is plotted in terms of equivalent standard axles (ESAs), the standard axle being defined as a single axle with dual wheels that carries a load of 8.2 tonnes. A survey of users indicated that roads designed using the chart appeared to perform satisfactorily in about 80 to 90% of cases. It was decided, therefore, to accept Figure 8.4 as a record of pavement performance and to derive a rutting criterion from it. Jameson (1996) has detailed the origins of Figure 8.4, and the method used in the desktop study to produce the rutting criterion. This calibration determined that k = 0.008511 and b = 7.14.

Figure 1. Austroads design chart for granular pavements with thin bituminous surfacing.

Cumulative Damage Factor

If a range of different vehicles traffic a pavement a range of different strains will be induced. The Cumulative Damage Factor (CDF) concept is then needed to sum the damage. The Damage Factor for the i-th loading is defined as the number of repetitions (ni) of a given strain divided by the ‘allowable’ repetitions (Ni) of the strain that would cause failure. The Cumulative Damage Factor is obtained by summing the damage factors over all the loadings in the traffic spectrum using Miner’s hypothesis.

Cumulative Damage Factor =

The pavement is presumed to have reached its design life when the cumulative damage reaches 1.0. If the CDF is less than 1.0, the pavement has excess capacity and the CDF represents the proportion of pavement life consumed by the design traffic.

This approach allows analyses to be conducted by directly using a mix of vehicles with different axle types, axle loads and tyre pressures. In practice, however, the Austroads design method involves converting all magnitudes and types of vehicle loads (single, double and triple axles of different axle loads) to equivalent standard axles at the outset. CIRCLY computations of subgrade strains and tensile strains are then made for each pavement structure of interest using an ESA. The standard 8.2 tonne axle loading is represented in CIRCLY by uniform vertical stress of 500 to 1000 kPa applied over two circles of equal area separated centre to centre by 330mm. Figure 2, which reproduces Figure 8.2 from the Austroads Design Guide, shows the Austroads pavement model for mechanistic design.

Figure 2. Austroads pavement model for mechanistic design procedures.

CIRCLY Version 4

Early versions of CIRCLY performed only the basic task of calculating the stresses, strains and displacements within a layered elastic system. CIRCLY4 is more properly described as a pavement design program that uses the layered elastic program CIRCLY to calculate strains, then uses the strains to perform the design calculations, and which also plots the results. This development to a pavement design system followed the earlier development of the Aircraft Pavement Structural Design System (APSDS) which has been described elsewhere (Rickards, 1994, Wardle and Rodway, 1995, 1998). CIRCLY Version 4 is similar to APSDS but it does not cater for the vehicle wander that must be taken into account in aircraft pavement design.

Several users of these earlier versions of CIRCLY wrote programs to facilitate the entry of data and to sort and manage the results that were generated. These programs are no longer needed. Version 4 runs under Windows 3.1, 95, 98 and Windows NT. It incorporates databases for material properties, loadings and empirical failure criteria, thus eliminating the need to re-key the data.

CIRCLY4 automates a number of the design tasks as follows:

The initial pavement for the design example illustrated by the computer screen shown in Figure 3 consisted of 110 mm asphalt over 400 mm granular base over CBR 3 subgrade. The design traffic was 10 million ESAs.

Selection of ‘Calculate damage factors’ and running the program produces CDFs of 1.35 for the asphalt and 0.50 for the subgrade. This indicates that the pavement would fail by asphalt fatigue after 10,000,000/1.35 = 7,400,000 ESAs.

The designer can then decide to either increase the asphalt thickness or increase the granular layer. If the first option is taken by highlighting the asphalt layer and ticking ‘Design thickness of layer highlighted below’ the program increases the asphalt thickness to 125 mm and computes new CDFs of 1.01 for the asphalt (the designed layer) and 0.28 for the subgrade. The reduction from 0.5 reflects the extra protection given to the subgrade by the additional asphalt thickness.

If, however, the designer had highlighted the granular layer and requested that its thickness be designed, the results screen would be as shown in Figure 3. The granular layer has been increased to 452.67 mm which has reduced the CDF of the 110 mm asphalt layer from 1.35 to 1.00 indicating that the pavement will fail by fatigue of the asphalt layer at the design traffic of 10,000,000 ESAs. The extra granular thickness will have reduced the CDF of the subgrade from 0.5 to 0.18 indicating that rutting is not the critical failure mechanism for this pavement.

Figure 3. Sample CIRCLY4 screen

Figure 4. Sample CIRCLY4 three-dimensional plot

Unbound granular materials

Unbound granular pavement materials such as graded crushed rock basecourse and natural gravels require special attention because their elastic stiffness depends upon the stress state at each point in the material. The layered elastic method cannot fully deal with stress dependence. This is because of the important limitation of the method that elastic moduli must be constant within each horizontal layer. But stress diminishes with distance from the wheels so the modulus will also change with distance from the wheels, both in the horizontal and vertical directions.

However, the layered elastic method takes stress-dependence into account to some degree by dividing granular layers into sublayers and assigning moduli to each sublayer. This allows the modulus to change with depth. CIRCLY4 automatically subdivides granular layers and assigns moduli in accordance with the method specified in the Austroads design guide. Previously this was carried out manually by the designer. Alternatively the designer can choose to have CIRCLY4 automatically sublayer granular materials in accordance with the US Army Corps of Engineers method devised for heavy aircraft loadings. It is intended that future versions of CIRCLY will include other published sublayering systems such as that used in the Shell pavement design method (Shell, 1985). It is important to realise that each sublayering method is linked to a particular design method, as discussed in the next section.

Failure criterion dependence on pavement model

Each design method uses its own representation of the pavement structure (called a pavement model) to compute the strains and to derive the failure criteria. The sublayering method used to assign moduli to granular materials is an important characteristic of the pavement model.

Each failure criterion is an inseparable part of a particular pavement design method. It should not be extracted and applied outside the context of that method. To do so is to fracture the vital empirical link between the new pavement and the past pavement performance data that was used to calibrate the design method. Any design produced using the failure criterion from one design method with the sublayering system of another method would be invalid.

The necessary links to guard against this error are forged at the time data is first entered into the CIRCLY4 database.

Concluding remarks

CIRCLY4 is not a ‘black box’ approach to pavement design. The user can specify all design inputs, including material properties, each material’s performance relationship and any wheel loading configurations and tyre pressures. This flexibility and the program’s transparency are intended to provide the experienced designer with easy access to and control of the full capabilities of a layered elastic ‘mechanistic’ method. By utilising available performance data and material properties the user is able to apply customised treatment to individual pavement situations. For example, within a localised area, perhaps even as localised as a single site, quality pavement performance data including good estimates of past traffic may be available. In such a situation the designer can use the data to ‘calibrate’ the program to reflect this local pavement experience. In other words, the designer can generate site-specific performance relationships to replace the general rutting and asphalt fatigue relationships published by Austroads, Shell and others. The tool is then available to compute the effects of future traffic and new pavement materials for the local area with a high degree of confidence.

The program’s computational speed (typical runs take less than a minute on Pentium PCs) and easily changed problem inputs (wheel loads and spacings, tyre pressures, repetitions, pavement layer thicknesses, material properties, failure criteria) allow the designer to assess the sensitivity of the design to any input or design assumption.

References

AUSTROADS (1992). A guide to the structural design of road pavements. Austroads, Sydney.

BARKER, W. AND BRABSTON, W. (1975). Development of a structural design procedure for flexible airport pavements. Report No. S-75-17. US Army Corps of Engineers, Waterways Experiment Station, Vicksburg, Miss.

JAMESON, G.W. (1996). Origins of AUSTROADS design procedures for granular pavements. ARRB Transport Research Report ARR292, Melbourne.

RICKARDS, I. (1994). APSDS. A structural design system for airport and industrial pavements. Ninth AAPA International Asphalt Conference, Surfers Paradise, Australia.

SHELL INTERNATIONAL PETROLEUM COMPANY (1985). Addendum to the Shell pavement design manual. London.

WARDLE, L.J. (1996). CIRCLY Users’ Manual, Version 3.0, MINCAD Systems Pty Ltd, Richmond, Australia.

WARDLE, L.J. AND RODWAY, B. (1995). Development and application of an improved airport pavement design method. ASCE 1995 Transportation Congress, San Diego.

WARDLE, L.J. AND RODWAY, B. (1998). Recent developments in flexible aircraft pavement design using the layered elastic method. Third Int. Conf. on Road and Airfield Pavement Technology, Beijing.

Author biographies

Dr. Leigh Wardle is the author of the leading pavement analysis programs, CIRCLY and APSDS. His research interests include layered elastic analysis, mechanistic pavement design and development of pavement design methods for airports and heavy duty loads. He is a member of the International Civil Aviation Organization's Aircraft Classification Number Study Group.

Bruce Rodway has extensive experience (35 years) in the design, construction and maintenance of aircraft pavements, gained initially with the Commonwealth Department having engineering responsibility for Australia’s civil and defence aerodromes and, from 1989 as Chief Engineer-Pavements for the Federal Airports Corporation until its closure in 1998. Duties involved design, construction, maintenance, evaluation and load rating of runways, taxiways and aprons at the Corporation’s 22 airports throughout Australia, which included 8 International Airports. Now a private consultant.

Bruce is the current Australian representative on the International Civil Aviation Organization’s committee for load rating of new and future large aircraft. He is a past member of pavement research advisory committees of the Australian Asphalt Pavement Association and AUSTROADS (the association of Australasia’s state and federal roads authorities) and current aircraft pavement consultant to the Royal Australian Air Force.