Full-Scale Bi-Directional Static Load Testing of Large-Diameter Hand-Dug Caissons Using Distributed Fibre Optic Sensing

In response to the increasing demand for high-resolution evaluation of deep foundations under extreme axial loading, this study presents an integrated application of bi-directional static load testing (BDSLT) and distributed fibre optic sensing (DFOS) on large-diameter hand-dug caissons. Although widely used in urban and access-constrained sites, caisson performance is often under-investigated due to the limitations of conventional load testing and point-based instrumentation. This study addresses this gap through full-scale testing of two caissons: one socketed into moderately weathered granite and the other terminating in residual soil, both constructed in Pahang, Malaysia, with diameters ranging from 2800 mm to 4000 mm. Each pile was instrumented with four DFOS cables for continuous axial strain profiling and tested up to 200% of the working load. The socketed pile mobilised peak shaft friction of 2586 kN/m², while the non-socketed caisson reached 516.8 kN/m². End-bearing remained linear, indicating that capacity was not fully mobilised. Settlements at peak load were 13.8 mm (socketed) and 25 mm (non-socketed), confirming the rock socket's role in enhancing foundation stiffness. DFOS enabled high-resolution detection of strain anomalies and load transfer patterns beyond the reach of discrete sensors. The results confirm the performance benefits of granite socketing and establish DFOS as a transformative tool for pile testing and deep foundation verification in complex geology.