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Arno Hefer Administrator Posts: 18
3/13/2024
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The question of how to model pavements with cemented layers in Mechanistic-Empirical analyses is often asked, particularly in terms of the thickness of the layers and the phased approach to the analysis.
Thickness Considerations
Guidance on how to address the issue of two cemented layers in a pavement structure in the South African Mechanistic Design Method (SAMDM) can be found by examining how the Catalogues in TRH4 (1996) were developed. The basis of these catalogues were developed with SAMDM analyses, however the catalogues were calibrated with experience of pavement designs.
In the TRH4 (1996) catalogue, some cemented layer thicknesses were combined. For example, 2 x 150mm layers shown as a 300mm 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. 300mm cemented) perform better compared to an “equivalent” system incorporating two layers (e.g. 2 x 150mm cemented).
Inspection of the catalogue reveals:
- When there are two layers of the same material (e.g. a C3), a single layer of the combined thicknesses is always used in the analysis of the pavement structure.
- When there are two layers, but of different materials (e.g. a C3 upper and C4 lower subbase), the layers are analysed separately.
Analytical Considerations
The SAMDM uses a phased approach associated with the observed performance behaviour of cemented layers. These phases are modelled using different layer moduli. Phase 1 represents an effective fatigue phase, with a higher modulus representative of a lightly stabilised cemented layer. Phase 1 takes the cemented layer from the initial stabilised state to an advanced stage of cracking and the start of what is considered the “equivalent granular state”. Phase 2 follows where the layer is considered to be in the equivalent granular phase (with lower modulus similar to a granular material). It is essential to use both phases in a pavement capacity analysis when a cemented layers is present.
In a two-layer cemented system using the phased approach, 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 contributes to a shorter life overall than when a single, thick layer, is analysed.
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 cracking (“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 100mm and 420mm, which means that two 150mm layers deteriorate quicker than one 300mm layer. The shift factor calculation is included in the layer capacity in a Rubicon analysis.
Conclusions
The structural model assumptions of a single, thick layer versus two separate layers represents two different deterioration mechanisms:
- A SAMDM analysis 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. In this case, the catalogues were developed using one single layer. These pavements are typically associated with higher category roads.
- A SAMDM analysis that includes two cemented layers but of two different material classes, e.g. C3 base, C4 subbase, are analysed as two separate layers. These pavements are typically lighter structures for lower category roads.
References:
1. Theyse, H.L. 1995. TRH4 Revision (1995). Phase II: Mechanistic Design Analysis of the Pavement Structures Contained in the TRH4 (1995): Pavement Design Catalogue, Transportek, CSIR, Contract Report I/PA/14/95.
2. TRH4. 1996. Structural Design of Flexible Pavements. Technical Recommendations for Highways. Draft. ISBN 1-86844-218-7. Pretoria.
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