The city of Adapazari, which is mostly located on a deep alluvial basin in the near field of the ruptured fault, experienced substantial damage during the 17 August 1999 Kocaeli (Izmit), Turkey earthquake (M-w 7.4). Besides over 2800 collapsed and heavily damaged buildings in the city, numerous cases of displacements of various forms and levels occurred at the building foundations. The undrained shear and deformation behavior of silt-clay mixtures encountered at the sites with significant foundation displacements are investigated through a series of standard and rapid monotonic, and stress-controlled cyclic triaxial tests conducted over anisotropically consolidated natural soil samples. Test results show that, while the dynamic stiffness and strength of these soils do not degrade under means of loading comparable to that of 17 August earthquake, their plastic strain accumulation characteristics critically depend on the mode of loading as well as the relative levels of applied load with regard to the monotonic strength. Implications of these results are consistent with the factors observed to be effective in the occurrence and severity of foundation displacements throughout the city. (C) 2004 Elsevier Ltd. All rights reserved.