1 EVALUATION OF THE ACTIVE DEPTH OF FOUNDATIONS UNDER A CAISSON BREAKWATER SUBJECTED TO IMPACT WAVES Miguel Esteban 1 , Hiroshi Takagi 2 and Tomoya Shibayama 3 The mode of failure of a caisson breakwater subjected to impact waves is characterised by a progressive sliding and tilting movement along the rubble mound foundation (Kim & Takayama 2004). For the sliding evaluation the model of Shimosako & Takahashi (2000) can be used, and the tilting can be approximated using the model by Esteban & Shibayama (2006). This model uses a classic stress-strain soil mechanics approach to determine the vertical displacement at the shoreside of the caisson. However, uncertainties in the active depth of the foundations (the deepest level of the foundations to which there are movements in the gravel) prevent an accurate estimation of the vertical displacement. The present paper shows how the traditional soil mechanics theory developed by Bousinesq appears to be valid to estimate the active depth of foundations in the top region of the rubble mound foundation. INTRODUCTION To successfully carry out a risk assessment of a caisson breakwater it is crucial to understand how reliable the structure is against different types of wave attacks. Shimosako and Takahashi (2000) proposed a Level 3 design method for caisson breakwaters referred to as the “deformation-based reliability design”. In this approach the expected sliding distance of the caisson is a statistical average of the sliding distance over the service lifetime of the structure as computed by a Monte-Carlo type simulation. The European Report PROVERBS (1999) offers a comprehensive review of the state-of-the art probabilistic breakwater technology. It includes guidance on how to design breakwaters by using a probabilistic approach and recommendations on how to calculate the various forces acting on the breakwater and foundations. More recently research by Kim and Takayama (2003) and Takagi and Shibayama (2006) have proposed different improvements to the basic model of Shimosako and Takahashi. However, in all these models the displacement caused by a certain wave pressure is assumed to stay constant throughout the caisson’s life. Kim and Takayama (2004) modified this model to take into account the effect of caisson tilting on the computation of sliding distance. In this method an assumption needs to be made regarding the final angle of tilting of the caisson, as Kim and Takayama provide no way of estimating it. By using simple soil mechanics consolidation theory Esteban and Shibayama (2006) calculated the amount of settlement at the heel of the caisson, thus allowing for 1 Department of Civil Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-Ku, Yokohama, 240-8501, Japan. 2 Department of Civil Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-Ku, Yokohama, 240-8501, Japan 3 Department of Civil Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-Ku, Yokohama, 240-8501, Japan