ASSESSING THE EFFECT OF ARMOURSTONE SHAPE AND WEAR

Abstract

Impacts, abrasion and physico-chemical weathering which sometimes result in rapid changes of armour shape and size can have disastrous consequences for armour stability. This paper draws together the background ideas and some new techniques which have been developed to tackle the prediction of weight loss and shape change of rock armourstone during the life of a coastal structure. The framework proposed is to measure separately the wear resisting material properties of a rock type using a tumbling mill simulation of prototype block degradation. Time on the structure is then related to mill time by a factor which accounts for initial block size and the site specific environmental conditions. A field site in Scotland is used to demonstrate the new image analysis methods for measuring rounding of prototype blocks. The shape analysis results can also be used to calibrate the equivalent wear factor for conditions at that site. Discussion of this framework is extended to dynamically stable design concepts where abrasion losses are faster. Preliminary flume testing in collaboration with H. R. Wallingford indicates that losses in stability due to rounding could be considerable.