Rock quality, durability and service life prediction of armourstone

Rock armourstone is unusual amongst geomaterials because of the extremely wide range of intrinsic rock strengths that often have to be used and the range of end use conditions in which it must serve. The authors' experience has been distilled to formulate suitable guidance as demanded by practicing coastal and hydraulic engineers for the forthcoming update to the 1991 Rock Manual (CIRIA, CUR, CETMEF. in press. The Rock Manual. The use of rock in hydraulic engineering (second edition). C683, CIRIA, London.). The armourstone evaluation theme of earlier work by Lienhart, and abrasion testing and degradation modelling of armourstone by Latham are revisited. A general in-service degradation model for wear of armourstone, based on a progressive fractional mass loss with time, is redeveloped. The innovation is to introduce two new methods that are both practical and simple to implement. One method uses abrasion resistance, water absorption and block integrity in a system that couples the rock susceptibilities to various common site conditions. The other treats intrinsic rock properties from the quarry all together to achieve a global armourstone quality designation, AQD, of the rock, that is independent of the actual conditions of the proposed site of application. Depending on the assessment of site aggressiveness, the degradation with time on site is generated by retarding or accelerating the standard mass loss plot invoked from the AQD value, but adjusted by semi-qualitative engineering and/or scientific judgement of the rating geologist. Examples are used to illustrate and compare the two methods. Discussion is included on the need for more rigorous future research approaches based on breakage prediction and on the need for further case histories.