Efficiency of mattress and geogrid on settlement reduction of rubble-mound breakwaters over thick soft clay layer

The construction of rubble mound breakwaters on deep soft clay is a significant geotechnical challenge due to excessive settlement and potential instability. Basal reinforcement, using either foundation mattresses or geogrids, is a common solution, yet a direct comparison of their effectiveness and clear guidelines for optimizing their geometric parameters are lacking. This paper addresses these gaps through a comprehensive numerical analysis based on the Al-Faw Breakwater case study. The comparative analysis reveals that the two systems function through fundamentally different mechanisms. Geogrids are highly effective in enhancing stability, with an increase in strength from 50 to 300 kN increasing the safety factor by 25%. In contrast, mattresses are more effective in controlling settlement during construction, with an increase in thickness from 0.5 m to 2 m reducing maximum settlement by 30%. For mattress design, an empirical formula is presented to estimate the required thickness based on breakwater height and sub-soil condition. For geogrid design, the results indicate that a single basal layer is the most efficient configuration, and an optimal anchorage length is proposed as 0.5 × H. These findings provide a quantitative basis for selecting and optimizing the most appropriate basal reinforcement strategy for coastal infrastructure on soft ground.