Post-cyclic settlement and tilting potential of mat foundations

Abstract

A simplified procedure for the estimation of cyclic shear stresses on foundation soil layers due to interactions of seismic excitation, foundation mat and overlying structural system, incorporating both kinematic and inertial aspects was described earlier in Cetin et al. [1,2]. This simplified procedure is now used, along with laboratory-based cyclic shear and volumetric strain relationships for the assessment of cyclically induced settlement and tilting potential of mat foundations. The proposed methodology is calibrated by well-documented seismic foundation performance case histories of 3–6 story, relatively rigid residential structures with no basements. Immediately after 1999 Kocaeli and Duzce, Turkey earthquakes, foundation settlements of these case history buildings were carefully mapped relative to available elevation benchmarks, such as relatively rigid concrete pavements, drainage pipes and ditches, and entrance stairs, located in the immediate vicinity of the buildings. Tilt angles of these buildings were also mapped in orthogonal multi-directions. Relative settlement and tilt angle values vary in the range of 0–60cm and 0–5.5rad, respectively. As part of subsurface characterization studies, SPT and seismic CPTU were performed. For the retrieved disturbed and undisturbed samples, an intensive laboratory testing program including sieve analysis, Atterberg Limit, hydrometer, oedometer, static and cyclic triaxial and cyclic simple shear tests was executed. The foundation soil profiles generally consist of silty soils, sand–silt mixtures and silt–clay mixtures of SPT-N values varying in the range of 2–5 blows/30cm in the upper 5m and gradually increases up to a maximum value of 25 blows/30cm beyond depths of 5–8m. As part of the proposed framework, displacement potential indices, defined as the product of induced-cyclic strains and the thickness of soil sub-layers, were estimated, the sum of which produce settlement and tilting potentials. Then, these settlement and tilting potentials were calibrated against recorded settlement and tilt values through a statistically based calibration scheme. The proposed simplified procedure is shown to reliably and precisely capture both deviatoric and volumetric components of post-cyclic settlements, as well as tilting responses of mat foundations.