Displacements of bridge foundations on sedimentary soft rock: A case study on small-strain stiffness

Full-scale behaviour of the foundations for an 800m long suspension bridge, Rainbow Bridge, constructed on sedimentary mudstone about 1·5 million years old and completed in 1991 is described. The settlement of the foundation with strain levels in the soft rock of less than 0·1% was overestimated several times by finite-element method simulations based on the stiffness from conventional oedometer tests and plate loading and pressuremeter tests. Field seismic surveys and triaxial compression tests measuring locally axial strains performed after the construction provided very similar elastic stiffness to each other. Based on the elastic modulus while taking into account the strain non-linearity obtained from the triaxial compression tests, the full-scale behaviour of the foundations could be simulated very well.