A semi-rigid ground improvement method using bamboo poles is used in some countries in the Southeast Asian region, to improve land underlain by weak clays. It is a technology often used in projects in some areas of Indonesia where soft soils are common. Although this approach is adopted as a common ground improvement technique, the design is largely based on local experience. The semi-rigid inclusion consists of several individual bamboo poles bundled together as a cluster. In some projects a prefabricated vertical (wick) drain is also attached to the bamboo cluster to facilitate a shorter drainage path length. Following the installation of bamboo clusters, a bamboo raft is placed over them as a load transfer platform. This ground improvement technique is known as Bamboo Pile Raft System (BPRS). Although its performance under vertical loading has been found satisfactory in several projects, its performance under combined loading (vertical and horizontal) has not been understood well. The performance of a breakwater under combined loading was reviewed through numerical analysis to understand the behaviour of the BPRS ground improvement method. The results indicate that transient loading during construction and the depth of improvement with bamboo clusters in relation to weak clay thickness is critical. The results also show the increased strength gain due to pore pressure dissipation through wick drains incorporated in the bamboo clusters which helps improve the BPRS performance.