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Arno Hefer Administrator Posts: 16
3/13/2024
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Background
Pavement design catalogues often evolve over many years, strongly relying on a heuristic knowledgebase of pavement performance including certain materials and climatic conditions within a region or a country. This information is therefore a great source to develop and calibrate computational methods for pavement design, especially in the absence of formal long term pavement performance (LTPP) data. Experience of pavement performance together with heavy vehicle simulator (HVS) testing culminated in the development of the South African Mechanistic Design Method which also served as a more fundamental basis in the ongoing development and updates of the South African TRH 4 pavement design catalogues.
Whilst mechanistic-empirical methods incorporate a “mechanistic” component in an attempt to link performance to material related behavioral parameters, these methods inevitably incorporate and will always require empirical input. Due to the complexity of pavement behavior, routine mechanistic-empirical design methods may be viewed as “predictive” methods which do not necessarily capture the underlying mechanisms (or” modelling” based on causal relationships) accurately. Method applications often include assumptions based on perceived mechanisms, which are not always well documented. Application of any design method should always be aligned with the assumptions made during its development.
Layer thickness considerations
Evolution of pavement design methods (including catalogues) need to accommodate changing technologies such as vehicle suspension, tyre design and increasing tyre pressures, and construction equipment technologies. The South African TRH4 pavement design catalogue also include such considerations, especially with regards to cemented layer thicknesses.
The traditional granular and stabilised layer thickness specification of 150 mm originates from constructability considerations based on early static roller technologies. Nowadays, vibratory, and more advanced roller technologies enable compaction to much thicker layers. It should be noted, however, that constructability and therefore specification of these thicknesses – especially in the context of pavement rehabilitation projects – depend greatly on and should take cognisance of adequate available/in situ supporting stiffness conditions.
Mechanistic-Empirical Design considerations
In the latest TRH4 catalogue some cemented layer thicknesses were combined – for example 2 x 150 mm layers shown as a 300 mm layer – typically associated with heavier pavement traffic load classes and higher category roads. The catalogue also includes examples where cemented layers are not combined. When evaluated mechanistically, pavements analysed containing a thick layer (e.g. 300 mm cemented) perform better compared to an “equivalent” system incorporating two layers (e.g. 2 x 150 mm cemented).
Inspection of the catalogue reveals:
- Even where the possibility of two-layer construction is implied, a monolithic layer is always assumed during analysis when the same material (e.g. C3) features.
- Where the cemented materials differ (e.g. C3 upper and C4 lower subbase), the layers are designed (analysed) separately.
Analytical considerations
- Full friction between layers is assumed which means that the difference in performance between “equivalent” scenarios is not mechanistically related to a difference in load response parameters (e.g. horizontal tensile strain).
- The South African Mechanistic Design Method (SAMDM) uses a shift factor to relate the capacity based on the critical strain level to a capacity representing progression to an advanced state of block cracking – also known as an “equivalent granular state” which is simply a modelled state where the layer modulus is equivalent to that of a granular material. The shift factor is a function of layer thickness. This relationship is non-linear between approximately 100 mm and 420 mm, which means that two 150 mm layers will deteriorate quicker compared to one 300 mm layer.
- The SAMDM uses a phased approach associated with observed performance behavior of cemented layers. These phases are modelled using different layer moduli representing an initial fatigue phase (with higher modulus) followed by an equivalent granular phase (with lower modulus), respectively. In a two-layer cemented system, therefore, the modulus of the lower layer (which typically cracks first) is reduced, resulting in a higher strain level at the bottom of the top cemented layer which overall contributes to shorter life.
Conclusions
- Analysis of a system using the SAMDM that includes two cemented layers of the same material will result in a shorter life (capacity) compared to an analysis of the system with one thick cemented layer.
- The structural model assumptions of a combined or monolithic layer versus two separate layers represent two different deterioration mechanisms.
- Using the TRH4 catalogue as reference, thick cemented layers (combined system/ same material class) is typically associated with higher category roads, where the likelihood of satisfying the assumptions of good support and suitable equipment holds.
- For lower category or lighter structures including two cemented layers, the layers typically represent two different material classes (e.g. C3 bases, C4 subbase) and thus analysed as such.
edited by on 3/21/2024
edited by on 4/4/2024
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