Monitoring the construction of a large-diameter caisson in sand

Large-diameter open caissons are a widely used construction solution for deep foundations, underground storage and attenuation tanks, pumping stations, and launch and reception shafts for tunnel boring machines. The sinking phase presents a number of challenges during construction, including maintaining caisson verticality, controlling the rate of sinking and minimising soil resistance through the use of lubricating fluids. This paper describes the instrumentation and monitoring of a large-diameter caisson on a UK construction site. The caisson was instrumented for the measurement of settlement and tilt, soil-structure interaction contact stresses and structural performance. A key objective for the monitoring project was to provide real-time feedback to the site engineering team to inform the construction process. The monitored data revealed the occurrence of complex soil-structure interactions during sinking that are not readily captured by existing prescriptive design approaches. This case history provides valuable information for the development of an improved basis for design as well as an important frame of reference for future monitoring projects.