Arno Hefer Administrator Posts: 18 2/15/2017

Arno Hefer Administrator Posts: 18 When a multi-layered pavement system is set up for modelling using linear elastic mechanistic theory, the last layer of the geometric setup is a semi-infinite half space. Subgrade stiffness varies with depth due to influences such as stress dependency of the material, increased overburden pressure etc. Using a backcalculation routine and assuming a homogeneous semi-infinite subgrade will therefore result in under or over estimation of the stiffness in the critical subgrade zone, just below the pavement structure. Using stiffness values derived from such a model may obviously result in unrealistic pavement responses (stresses and/or stains) obtained during the pavement analysis stage. The number of available layers in linear elastic programs are normally limited. While many different approaches are used in backcalculation routines in attempts to realistically characterize the subgrade using a limited number of layers, the approach used in Rubicon Toolbox divides the subgrade into two layers, namely, the upper and the lower subgrade as illustrated below. The model allows a maximum of 6 layers (including the upper and lower subgrades). The stiffness result for the upper subgrade serves as an input to the pavement model. The upper subgrade therefore represents the critical subgrade zone just below the pavement structure. This layer should be sufficiently thick to represent the actual subgrade within the zone of wheel load influence, but not too thick as it will then recreate the original problem. A thickness of 800 to 1500 mm, and typically 1000 mm normally works well although features such as the presence of shallow bedrock may influence this choice. It is therefore important to utilise information such as test pit profiles, dynamic cone penetrometer data etc. to check subgrade conditions. A final note of caution. Although the lower subgrade stiffness result is normally omitted from the model setup for pavement design/response analysis (so-called forward calculation), this assumption is valid only when traditional strain-based subgrade failure criteria (or transfer functions) are used. If you are using a deflection based failure criterion, please make sure the exact same model setup used during the backcalculation process is transferred and used in the pavement response analysis. edited by arnoh@icon.co.za on 3/4/2017 edited by on 1/28/2023

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