A methodology has been presented for evaluating the partial safety factors on the sliding failure mode of vertical caissons of composite
breakwaters and for determining the cross sections of those by Level I reliability-based design method. Especially, a mathematical
model has been suggested for the sake of a consistency of code format as well as convenience of application in practical design, for which
the uncertainties associated with buoyancy and its own weight can be taken into account straightforwardly. Furthermore, design criteria
equation has been derived by considering accurately the effect of uplift pressure, so that the cross sections of caissons can be assessed
which must be safe against the sliding failure. It has been found that cross sections estimated from partial safety factors proposed in this
paper are in very good agreement with the results of Level II AFDA and Level III MCS under the same target probability of failure.
However, partial safety factors of the Technical Standards and Commentaries for Port and Harbour Facilities in Japan and Coastal
Engineering Manual in USA tend to estimate much bigger or smaller cross sections in comparison to the present results. Finally, many
reliability re-analyses have been performed in order to conform whether the stability level of cross section estimated by Level I
reliability-based design method is satisfied with the target probability of failure of partial safety factors or not.