Constructing a large diameter open caisson in soft clay

This paper presents the design, construction and monitoring of a 28 m internal diameter caisson located in the flood plain of the River Shannon in Athlone, Ireland. Under a self-weight exceeding 5000 tonnes the caisson ultimately penetrated through over 10 m of soft alluvial clays and silts with undrained
shear strengths between 7.5 and 28 kPa. The caisson then passed through strata of coarse-grained soils before partially bearing on limestone bedrock. The challenging ground conditions necessitated a robust design approach to address the risk of premature bearing capacity failure and excessive movements of the structure during construction. To address these concerns, the caisson interior was surcharged with 3000 tonnes of clean stone, and two complementary analytical methods were employed to predict soil-structure performance. The first employed an empirical bearing capacity analysis accounting for a tapered caisson toe in clay [1], and the second was a 2-D finite element analysis. Both approaches predicted the structure would penetrate the clay before the 15.4 m high, 1.6 m thick wall construction was complete. The Eurocode 7 observational method was adopted to monitor construction and was informed by real-time instrumentation data. The structure did not penetrate the clay during caisson construction as predicted; this may be attributed to the selection of conservative design parameters, natural variability in the soil profile and soil strengthening due to consolidation induced by the clean stone surcharge. This case study highlights the challenges of predicting soil-structure interaction in soft ground and provides practical insights on the design, construction and monitoring of large-diameter caissons in complex geological settings.