Sunday the 22nd - Geosolve Ltd. -

In some cases the cause of premature distress in a pavement can be difficult to confirm from visual survey. However using visual condition data in conjuction with structural analysis provides a methodical tool for definitively establishing the mechanism of any distress.

The best technique in this instance is to have an observer working on site with the FWD operator, to ensure the geophones are effectively located on intervals of distress where the severity is essentially the same, ie avoid positions where the spread of geophones is partly on intact pavement and partly on distressed. The spread is usually 1.5 to 1.8 m, therefore constant distress severity over at least a 2 m length of wheelpath is necessary. The observer than moves along with the FWD, stopping the driver at meaningful positions and recording the nature and severity of distress for each test. Categories are usually rutting, cracking and shoving. Roughness may also be recorded if relevant lengths of constant severity are present. Severity must also be recorded. We recommend a simple ranking of 0-4. (None, Initial, Advanced, Terminal). Diagnosis software is available that allows the severity of distress to be related to any parameter, eg cracking severity may be plotted against the modulus of the surfacing to see if poor quality surface materials are implicated. Rutting severity may be plotted against modulus of each layer in turn, to determine if any one layer is the likely source of excessive deformation.

Correlation of severity against subgrade modulus exponent is often a guide to poor compaction at the top of the subgrade, or the need for drainage improvement.

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