A high cantilever seawall was constructed in soft clay of 15 m to 32 m thickness and the enclosed water between the shoreline and the seawall was reclaimed. The area was 7 hectare and the reclamation height ranged from 10 m to 20 m. The reclamation methods were controlled and not intended to cause additional incremental deflection to the already high design deflection of -385 mm or 0.89% of H (H = 44 m is defined as rotating and not cantilever height). However the different methods of reclamation had a significant influence on the slurry clay flows and thus caused additional incremental deflection. The slurry clay flows pushed the deflection of seawall to an additional maximum magnitude of -353 mm or 0.80% of H. The maximum seawall deflection of -526 mm or 1.20% of H was based on inclinometer IW1903 readings. At such high seawall movement, the inclinometer deflection profiles indicate safety in the design and the long term deflection is stable. This reclamation experience indicates that it is difficult to control slurry clay flows in future high reclamation filling of 10 m to 20 m height above seabed with underlying thick soft clay of 15 m to 32 m. The deflection is far exceeding BS8002 guideline of 0.5% of H, which stipulates the geo-structural stability of the seawall. This long term stability at such a high deflection is attributed to the robust turning back capacity of the strong box. The turning back capacity of the strong box against the active force above the rotating point is from the integration of vertical bending capacity of the pipe piles, the rigid connection of the capping beam, and utilisation of the P-P interlock to produce the box action. Below the rotating point the adequate passive resistance into Old Alluvium maintains the external equilibrium.