Design of rubble mound slope protection for cellular cofferdam breakwater retrofitting

Abstract

A rubble mound slope structure was designed to protect a decaying cellular steel sheet pile cofferdam breakwater which has been partially damaged. Reparation of this decaying cofferdam breakwater to its original form is considered uneconomic. Considering three basic design requirement, i.e. economic feasibility, environmental sustainability, and structural reliability and constructability, the rubble mound slope design has been proposed as a retrofit structure against 4.7m height of a 100yrs return period design wave. In this design, the existing vertical cofferdam stands originally at the harbour side, whereas the new additional rubble mound structure is attached to the cofferdam at the sea side creating together a new horizontal composite breakwater. The existing cofferdam supports the rubble mound structure at the harbour side, whereas the rubble mound structure provides protection against direct wave attack. This retrofit design of structure requires less 25% of materials volume and costs 30% cheaper than a full standalone rubble mound breakwater type. This less requirement of materials will consequently have positive impact on environment and natural resource conservation. The design of rubble mound slope that is directly attached to the existing breakwater will enable its easy construction by using vehicles that stand on the top of the existing breakwater. This article describes the design consideration including discussion on the structural reliability of the retrofitted breakwater.