How are the results of FWD analyses presented?
Software packages produce a range of display outputs, but most include options that can be transported either directly or indirectly into spreadsheets for subsequent graphing to suit individual project requirements.
The advantage of spreadsheet files is that FWD information can be readily supplied on diskette and viewed graphically to facilitate appraisal by the designer. A display can show the inferred moduli and relevant parameters as well as a comparison of overlay requirements or depth of basecourse stabilisation using the mechanistic procedures described in the New Zealand Supplement (Transit 1997). It is generally useful to compare the overlay design methods using both the AUSTROADS subgrade strain criterion and the two methods which use a past precedent strain criterion. The visual condition assessment and known performance of local materials must then be used as a check on the appropriateness of the preliminary analytical model. Any inconsistencies must be addressed, the layer thicknesses adjusted in accordance with the destructive test information and a final model developed.
An example of a final report presentation of parameters is given in the following figure, showing a number of parameters plotted against road chainage.
The lower graphs at the foot of the page give the layer thicknesses used in the model and the actual dynamic deflections (corrected to standard temperature for an 8 tonne equivalent design axle loading).
The overlying graphs are subgrade strain ratio and subgrade modulus non-linearity. The strain ratio is the strain at the top of the subgrade divided by the allowable strain (AUSTROADS or New Zealand Supplement) for the proposed traffic. (The original AUSTROADS strain criterion has been used in this case). The subgrade modulus non-linearity allows identification of likely soil type in the subgrade and an indication of whether poor subsurface drainage could be a factor.
The next graph shows the critical layer, ie the layer that governs the design life of the pavement according to the adopted strain criterion.
The next set of graphs show the design traffic (ESA) and results of the structural analysis, giving the moduli for each layer: basecourse (if unbound granular chip seal, or asphalt if structural), subbase and subgrade. The resilient modulus scale is shown on the left, while the equivalent CBR is shown on the right margin. Colour coding is used to allow the various layers to be identified readily.
The upper set of graphs provide the interpretation and design guides. For each point is shown the remaining life (AASHO method in bar graph and AASHTO structural number method as a line graph) and calculated overlay (AUSTROADS or New Zealand Supplement method as required). Where cement stabilisation of the existing basecourse is being considered, the necessary depth of stabilisation is shown using the tensile strain criterion given by the Transit NZ Supplement.
To analyse sensitivity to layer thicknesses, a separate back analysis will be required. To evaluate other changes, a forward analysis only is needed and this could be carried out using the attached spreadsheet (Appendix 2). This will allow consideration of variations in ESA, overlay modulus or thickness, alternative strain criteria, and basecourse stabilisation.
When a satisfactory model is obtained, the individual results should be grouped into structurally uniform sub-sections to show practical intervals for which individual forms of treatment may be specified for construction. This vital step ensures a cost effective approach to ensure the design life is achieved without superfluous overlay. The emphasis is placed on obtaining comprehensive in-situ test data so that sections which are structurally deficient can be clearly delineated from areas which require no strengthening, thus avoiding the over design that can result where a single form of treatment is applied to an extended length of pavement.
The above example was taken from a road in which shallow shear was the principal distress mode, ie the AUSTROADS strain criterion rather than precedent subgrade strain methods should be applied. (Examples with results for the NZ Supplement methods are given in Appendix 1). The ELMOD software was used in this instance, but EFROMD2 together with CIRCLY will produce the same set of parameters except for the subgrade modulus exponent (n).
The above road could be interpreted in 4 sub-sections, as in the following table.
The first section (up to Chainage 0.16 shows relatively high strength basecourse and subgrade. No surface distress was apparent. A four layer model (including the subgrade) was adopted. The subgrade strain ratio is much less than 1, ie strains are already much lower than required by AUSTROADS and hence no overlay is required.
The second section (to Chainage 1.39) shows much greater variability in the basecourse modulus and includes some very low values. Layer 1 is shown to be critical, ie the analysis indicates that in several places the basecourse will be experiencing higher strains than in the subgrade, ie there would be clear potential for shallow shear. (The latter was markedly evident from visual survey). Using the AUSTROADS strain criterion an overlay of 100 mm of unbound basecourse is required.
The third section (to Chainage 2.35) shows only minor structural deficiency and it is evident that the basecourse modulus is uniformly high. No structural overlay is needed. The subgrade strain ratio is slightly less than 1, ie strains are only marginally less than required. (For new pavements a subgrade strain ratio much less than 1 gives a measure of the overdesign incorporated.)
In the last section, analysis indicates that the subgrade CBR is lower than elsewhere on the section and the basecourse modulus is also poor and variable. The greatest strains are occurring in the subgrade in part and in the basecourse for the remainder (critical layers are 1 and 4). The subgrade modulus non-linearity exponent (n) is unusually low, suggesting that the potential for improving subsurface drainage should be checked here.
Where precedent subgrade strain information is required, the appropriate strain ratio can be selected from the graph for any subsection, and the actual precedent strains calculated directly from the AUSTROADS subgrade strain relationship.
At completion of deflection testing and visual assessment, all raw data and a preliminary interpretation should be reviewed by the designer, in order to assess the need for and location of destructive tests (coring, test pits and penetration tests).
In the above example, where shallow shear was evidently the principal distress mode in this road, the test points showing the lowest basecourse moduli (or where basecourse strains are higher than subgrade strains) should be selected for test pitting and CBR testing in accordance with Section 10.3 of the New Zealand Supplement. A test pit at about Chainage 1.0 would identify the weakest basecourse and also confirm the typical subgrade CBR for the first 3 sub-sections. For the last sub-section, basecourse CBR should be investigated around Chainage 2.75, but in this case, care would be needed to identify the more adverse areas visually as the results show marked fluctuation in stiffnesses. The subgrade CBR here could be expected to be significantly lower than at the first test pit site.
Re-analyses for final design are normally carried out to incorporate the destructive testing information. Finally, geometric constraints need to be considered (eg kerb and drain levels) and then comparisons may be made to determine the most cost effective treatment, ie local digouts, overlay, cement stabilisation or reconstruction. In this example, costs for overlays of 100 to 120 mm of M/4 would be compared with those for cement stabilisation of about 250 mm to give the same design life. However the example shows some points where very deep stabilisation would be required, ie the subgrade may be too weak for this option.