Long term monitoring of the foundation of a cable stayed bridge

The highway between Napoli and Formia, in Southern Italy, crosses the river Garigliano 65 km north of Napoli by means of a long viaduct with two intermediate cable stayed spans. In the alluvial plain of the river Garigliano; the soils consist of NC or slightly OC silty and sandy clays with a substantial organic content down to a depth of about 50 m, where a sand and gravel bottom layer is found. The foundation of the central pier is a thick raft resting on a rectangular group of 9x16=144 piles, arranged in a square grid with a spacing of 1,2 m axis to axis. The instrumentation installed since the beginning of the construction, started in 1991, was aimed to monitor the settlement of the pier and the loads taken by the piles. The loads were monitored by vibrating wire instruments, while precision geodetical survey was periodically carried out to measure settlement. The long term monitoring covers now nearly 30 years testifying an extraordinary long and accurate response of the large majority of the installed instruments. Important details of the measured performance will be described outlining the implications and lessons learnt in terms of soil-structure interaction. The settlement data obtained by geodetic surveys have been compared and integrated a posteriori with satellite-based measurements (ERS, ENVISAT satellites) for the first common monitoring period (1993-2004) which has been further extended until 2010 (ENVISAT satellite). Keywords: Foundation Monitoring, Long term performance, bridge foundatio