Placed revetments under wave attack : study of structural mechanics

This report studies clamping phenomena in placed revetments on the Dutch dikes. One of the reasons of placing stones is that the stability of the single stones increases. Pull–out tests on single stones show a strength up to 10 times the single stone weigth. In present standardised design methods for placed revetments there is no significant contribution of clamping to the design stability of the single stone. The physical model study is based on a structural beam model. The beam consists of loose column or block elements on an elastic foundation. The joined elements act as a structure due to the normal (clamping) force from the gravity component along the dike slope. The elements are lifted by wave pressure differences, which occur during run-back and slamming of the waves on the relatively smooth slope. In analytical models it can be demonstrated that upward pressure up to say 2.5 times the element weight can be taken. For concentrated pressure peaks even higher clamping factors are found. The clamping factor is defined as the pressure load divided by the element weight, and can be interpreted as a factor of increased strength of the revetment. One of the most important threats for utilisation of these results is the uncertainty regarding the actual presence of normal force in the top layer of the revetment. Another thing is that there are always loose elements. Failure of these elements should not necessary lead to progressive failure of the top layer. This opens possibilities for utilising the clamping strength for resistance against extreme wave loads in the design of revetments.