Pavement analysis can be simply defined as the determination of the thickness of various pavement and subgrade layers to achieve the design traffic life. The layer thickness is either based on an empirical approach or by a mechanistic analysis using a layered elastic analysis program, such as CIRCLY. For light traffic roads consisting of a granular pavement material, Figure 8.4 of the Austroads pavement design guide is commonly used and as traffic levels exceed 10 million ESAs the mechanistic approach is considered for the pavement analysis.
When a pavement analysis is carried out, it is based on laboratory determined material properties or assumptions for each layer. For stabilised materials, a material can be considered as bound or unbound, or modified, as detailed in Table 1.
Table 1: Degree of stabilisation in terms of unbound, modified or bound materials.
(Ref: Austroads Guide to Pavement Technology Part 4(d): Stabilised Materials Austroads Project No: TP1089, Sydney, NSW., 2006).
|Category of stabilisation||Indicative laboratory
strength after stabilisation
|Subgrade (subgrades and formations)||CBR* > 5%|
|Granular (subbase and|
|40% < CBR* < +100%|
|Modified (basecourse)||0.7 MPa < UCS** < 1.5 MPa|
|Bound (basecourse)||UCS** > 1.5 MPa|
* Four day soaked Californian Bearing Ratio (CBR).
** Unconfined Compressive Strength (UCS) values determined from test specimens stabilised and prepared using standard compactive effort, normal curing for a minimum 28 days and 4 hour soak conditioning.
The design of a pavement configuration consisting of one or more pavement layers on a subgrade is typically based on applying standard wheel loads to the surface of the model and establishing the maximum strains under the loading such that these strains may be used in a material design predication model, such as a fatigue equations for cemented and bituminous materials.
In Australia both the empirical and mechanistic analysis approaches are used. Figure 8.4 of the Austroads pavement design guide (see below) is a simple empirical approach to establish the base and subbase layer thicknesses of granular materials subject to different traffic loads and subgrade strength.
Figure 1 Granular pavement layer thickness for different subgrade support strengths (CBR) and traffic levels (DESAs)
For bound pavements using cementitious binders, AustStab has developed a technical guide to assist engineers with their interpretation to the information in the Austroads pavement design guide. For more information click here.
Over the last 10 years a gradual increase in the design and performance information for foamed bitumen stabilisation has resulted in the continuous development of a mechanistic approach to the thickness determination of these pavement layers under traffic and subgrade conditions. For more information on the structural design of foamed bitumen layers, click here.
Progress also continues on the design of bitumen emulsion layers and no design approach has been documented at this stage.
Many of the chemical binders when used in stabilisation do not produce a bound layer and the structural design approach is to treat the material as a granular unbound material.
AustStab has published a guideline to provide the design thickness of stabilised local roads using an empirical approach. The thickness of the stabilised layer is based on experience from a range of local government practices in Australia and includes the material mix design procedure using cementitious binders.
Figures 2 and 3 show the typical configurations of insitu stabilised base layers of local roads. When the existing road has less than the required pavement material, up to 30% of the subgrade material has been used in projects to get sufficient mixing depth and strength.