Controls of size and shape of natural armourstone

Abstract

Summary The sizes and shapes of armourstone blocks for rubble breakwaters are primarily determined at the quarryface, and later modification then occurs in situ in the breakwater environment. The principal controlling parameters in the quarry include: rock type, rock joint frequency and orientation, size and type of blasting charge and the orientation of the blast's fractionating power. The majority of existing data on this subject has been obtained from detailed studies of quarries principally concerned with aggregate production in Britain, but extensive studies have recently been carried out on a variety of rock types in quarries that are currently producing primary armourstone for breakwater structures in the United Arab Emirates and the United Kingdom. These studies combine to permit the relative importance of the various controlling factors which operate during quarrying, transport, placement and subsequently in service to be evaluated and are reported here. The quarry-orientated study is supplemented by detailed studies of block modification and deterioration in the several different environmental zones of breakwater structures. The principal factors controlling this shape modification are micro-fracture pattern, mineralogy and fabric, grade of weathering of the original rock, grain interlock and intergranular cement type. Changes in shape will modify the stability factors of in situ armour blocks and alter void ratios and slope angles, which together are perhaps the most important factors controlling the dissipation of wave energies on rubble mound breakwaters. The importance of long-term abrasion effects should not be underestimated; to monitor these effects, damage assessments have been used to relate the shape modification to slope.