A geotechnical investigation was performed to develop information needed for an evaluation of the seismic vulnerability of a series of interstate highway bridges that cross a thick peat deposit in Washington. The research focused on estimation of dynamic pile stiffness, characterization of dynamic properties of peat, and prediction of ground motions.
A series of free vibration and forced vibration tests were performed on an 8-in. diameter pipe pile installed in the peat. The results of these tests were used to develop methods for estimation of the dynamic stiffness of other piles. A three-phase laboratory testing program was undertaken to investigate the dynamic properties of the peat. Ground response analyses were performed to investigate the influence of the peat on seismic ground motions.
The results of the analyses indicated that the ground response would be expected to vary along the alignment of the bridges in accordance with the variation in peat thickness. The peat exhibited very low stiffness in the field and laboratory tests, and is not expected to transmit large accelerations, particularly in the central portions of the bridges where it is thick.
The investigation has improved understanding of the dynamic response of peat and its implications with respect to seismic ground motions, but has also revealed the need for further research in this area.