In situ testing of the strength of asphaltic revetments on sand dikes

Wichman B.G.H.M., , Huurman M. & Van M.A. (2012). In situ testing of the strength of asphaltic revetments on sand dikes; Proceedings 12th Baltic sea geotechnical conference 2012.

In the Netherlands 600 kilometers of the sea dikes are protected by an asphalt revetment.These asphalt revetments have to resist considerable repeated waves with a significant wave height of up to 4.5 meters.The subsoil is normally sandy, and the asphalt layer protects against erosion. The asphalt layer can fail as a result of fatigue due to repeated loading under storm conditions. In case of very high wave loads, the asphalt can fail already after a few large waves. The thickness of the asphalt layer varies between 15 and 30 cm. The asphalt material has a visco-elastic behaviour, whereas the subsoil can be described by various material models ranging from linear elastic to a more complex elasto-plastic model.The sensitivity of the asphalt revetment to weathering strongly depends on the void ratio, mainly for void ratios larger than 5. Due to weathering, the resistance against fatigue of the asphalt revetment decreases. Therefore, Dutch law prescribes to perform a monitoring every 6 years for asphalt older than 30 years (at this moment 75% of the 600 km asphalt revetments is older then 30 years). The monitoring consists of:
– falling weight deflection measurements (FWD),
– lab testing on small samples (3 point bending),
– Ground Penetrating Radar measurements,
– visual inspection and
– calculations with the so-called software program ´Golfklap´ (wave attack in English) in order to calculate the strength under storm conditions.
In this analysis the asphalt and the subsoil are considered to be linear elastic. This is a reasonable first approximation. The bearing capacity of the subsoil plays an important role in how the asphalt revetment deforms under wave loading. This is predicted with the results of the FWD measurements. For large loadings, however, it is still unsure how the soil will behave and if other failure mechanisms then fatigue may occur. In case the subsoil becomes saturated, the bearing capacity will reduce and also failure due to liquefaction might occur. Due to weathering, the revetment becomes more heterogeneous, because void ratio´s are locally increasing and the process of so-called stripping is accelerated. For safety reasons, 5% characteristic
values from laboratory testing are used for the analysis with the software program ´Golfklap´. This results in a safe but pessimistic estimate of the resistance of the revetment to wave loading, as the fatigue properties have a large effect on the overall uncertainty.Therefore, the current approach has to be validated in order to reduce the uncertainties.

 

Dutch: In deze paper wordt een in-situ validatie proef beschreven, waarbij het asfalt wordt belast met een mechanisch systeem dat de drukverdeling in een golfklap nabootst. Er zijn ook verkennende berekeningen aan de impact van golfklappen op asfalt op een met water verzadigde dijk opgenomen.