Fritz Jooste Administrator Posts: 6 1/21/2020

Fritz Jooste Administrator Posts: 6 The Rubicon Toolbox backcalculation algorithm uses a search methodology which employs heuristics and elements of the Secant-numerical search method. The heuristics component ensures that the search progresses from the lower layers towards to top using a search strategy that approximates the approach an experienced user would adopt when doing manual backcalculation. The Secant-search method ensures a near-optimal convergence rate. Seed values are simply the mid-points of the search ranges for each layer. This means the search could be made more intelligent and faster when a user selects an appropriate and relatively narrow search range. It is always recommended that a manual backcalculation be performed on a few typical bowls in the set so that the analyst can determine the behaviour patterns and likely deterioration mechanism in the pavement. This will help the analyst pick appropriate search ranges which in turn will ensure the backcalculation results are realistic. For the sub-stratum layer, the search is automatically limited between a lower stiffness value of 20 MPa (deep/weak/wet substratum) and 1500 MPa (strong sandy or shallow substratum). The initial seed value for the sub-stratum layer is again the midpoint of these two limits, thus 760 MPa. Note that a distinction is made between the sub-stratum and the upper subgrade. The sub-stratum is assumed to be semi-infinite whereas the upper subgrade is typically assigned a thickness of around 500 mm to 1500 mm depending on the type of subgrade. The user should decide on the most appropriate upper subgrade thickness to use, and experience and site information should be used to guide this decision. If, for example, the subgrade is known to consist of deep clay then the thickness of the upper subgrade should be set to 1500 mm or deeper. If, however, there are signs of a shallow subgrade such as rock outcrops, or if the subgrade consists of coarse-grained, stress-stiffening materials, then a shallower depth should be assumed. This is discussed in more detail in the post titled Subgrade Characterization. Note that the sub-stratum layer does not have to be included in design forward calculations. This is because the stresses-and-strains in the pavement structure are not influenced by the deeper subgrade. The deflections, however, are influenced by deeper subgrade conditions and therefore in backcalculation the sub-stratum needs to be included to mimic deeper subgrade conditions. edited by Fritz Jooste on 1/30/2020 edited by on 11/20/2020 edited by on 11/20/2020

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