Field behaviour of very soft clays at the Athlone embankments

The approach road embankments to the new Shannon bridge rise up to 8·6 m above the marshlands of the River Shannon to the north of the town of Athlone in the Irish Midlands. These embankments are located on very soft glacial lake clay, highly compressible organic clay and calcareous marls, the geotechnical properties of which were largely unknown at the commencement of design in the early 1980s. Comparisons between compressibility, consolidation and shear strength parameters derived during the site investigation and those measured by monitoring the works are presented in this paper. It was found, for example, that the installation of vertical drains can significantly alter consolidation behaviour, that field compressibility was greater than that expected from laboratory testing, and that lateral movement monitoring was a useful indicator of embankment instability. Data from the construction of the Athlone embankments also confirm that the field vane strength underestimates the operational strength of the clays, owing to a combination of anisotropic and vane insertion disturbance effects. Good estimates of the operational strength are given by the piezocone. Details of some investigative work carried out on the soils beneath the embankments, some nine years after completion of construction, are also presented. It was shown that this testing confirms the findings of Mesri (1989) that the increase in shear strength is approximately equal to 0·22Δσ′_v. Also, the piezocone is capable of defining zones of anisotropy induced by embankment loading.