Occurrence of displacements of shallow mat foundations resting on saturated silt-clay mixtures were reported in Mexico City during 1985 Mexico Earthquake, and in Adapazari during 1999 Kocaeli (izmit) Earthquake. Soft surface soils, shallow ground water, limited foundation embedments and deep alluvial deposits were the common features pertaining to such foundation displacements in either case. Experience shows, while uniform foundation settlements, even when excessive, do not limit post earthquake serviceability of building structures, tilting is particularly problematic. In this study, a simplified methodology is developed to estimate the seismically induced irrecoverable tilting potential of shallow mats on fine saturated soils.
The undrained shear and deformation behavior of silt-clay mixtures encountered at the Adapazari 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 shear strength of these soils do not significantly degrade under means of loading comparable to that of Kocaeli 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.
Based on the results of laboratory tests, the response of nonlinear soil-foundation-structure system is reduced to a single-degree-of-freedom oscillator with elastic-perfectly plastic behavior. The natural period of the system is expressed by simplified soil-structure-interaction equations. Pseudo-static yield acceleration, which is required to initiate the foundation bearing capacity failure when applied to the structural mass, is estimated by the finite-element method. Eventually, the tilting potential of the foundations is estimated utilizing inelastic response of the nonlinear oscillator. Response of the deep alluvium sites, which involves velocity pulses with periods consistent with the fundamental site period, is significant in determination of inelastic response of low bearing capacity systems.
Predictive capability of the methodology developed is tested with actual case data. The methodology is observed to predict irrecoverable tilting potential of foundations consistent with the observations, except for the cases with low seismic bearing capacity. Deviations are explained considering the sensitivity of low-strength systems to asymmetrical behavior and uncertainties involved in seismic demand.
| Identifer | oai:union.ndltd.org:METU/oai:etd.lib.metu.edu.tr:http://etd.lib.metu.edu.tr/upload/3/12605402/index.pdf |
| Date | 01 September 2004 |
| Creators | Yilmaz, Mustafa Tolga |
| Contributors | Bakir, Bahadir Sadik |
| Publisher | METU |
| Source Sets | Middle East Technical Univ. |
| Language | English |
| Detected Language | English |
| Type | Ph.D. Thesis |
| Format | text/pdf |
| Rights | To liberate the content for public access |
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