<p>Structural failures of pile-supported excavations can serve as valuable lessons for the planning and execution of future deep excavation projects. This study presents a case of sudden failure that occurred five days after the completion of an 8.0-meter-deep excavation in stiff clay, supported by a cantilever pile wall with local struts. A forensic investigation was undertaken by reviewing site investigation records, construction procedures, field monitoring data, and 3D numerical modelling. The failure was triggered by elevated pore water pressures resulting from intense rainfall, combined with design-induced deficiencies in the structural detailing of the capping beam and piles. A comprehensive 3D numerical back-analysis reconstructed the failure process and guided the design of remedial measures. These measures, which included structural strengthening and adjustments to the construction sequence, were implemented and their effectiveness verified through post-intervention monitoring. This case study highlights the importance of integrated geotechnical and structural assessment and offers practical insights to prevent similar structural failures in pile-supported excavations.</p>