Tilt displacement of caisson breakwater due to wave loading

Abstract

Tilting and overturning of caisson breakwaters due to wave loading is well documented. Although simplified methods of caisson analysis have been proposed, they are either fully elastic analyses that do not account for permanent displacement, or stability analyses that determine whether overturning will occur. In reality, a caisson can incur significant tilt without overturning: such a phenomenon cannot be replicated by either of the above analyses. This paper presents a simplified analysis that takes into account the elastic caisson response as well as the accumulation of permanent tilt with successive wave cycles. Elastic response was simulated using a lump-mass–spring model. Permanent caisson tilt was analysed based on the assumption of the caisson tilting over a circular slip surface beneath its base. A search algorithm was used to determine the radius and depth to the centre of rotation of the critical slip circle. Permanent tilt is generated by the difference between the overturning and stabilising moments during wave peaks. This allows permanent tilt to be accumulated over successive wave peaks. Comparison with centrifuge test data shows that the analysis is able to capture the progressive build-up of tilt reasonably well. Furthermore, comparison with a field case of caisson failure during a storm shows good correlation, although uncertainty over the number of wave cycles precludes a more direct comparison.