Experimental determination of dynamic properties and site amplification analysis in low plasticity clays

The magnitude and characteristics of earthquake ground motion depend not only on source parameters but also on the conditions of local soil through which the seismic waves propagate. As seismic waves travel towards the surface, many parameters such as layer thickness, depth of the bedrock, soil type, and the dynamic properties of the soil cause changes in the amplitude and frequency content of the motion. This makes it essential to determine the site-specific soil behavior for the safe design of engineering structures, particularly in active seismic regions. This study evaluates the dynamic behavior of low-plasticity clay soils based on laboratory tests and analyses performed on undisturbed samples obtained from drilling operations within the boundaries of Sakarya province, located along the North Anatolian Fault Zone. The undisturbed soil samples collected from two different drilling locations were tested in laboratory conditions with the resonant column and torsional shear testing systems. As a result, shear modulus and damping ratio curves, which vary with the shear strain, were obtained. These data were used as input for the determination of site-specific dynamic parameters and in the numerical modeling. In the numerical analyses, both the original earthquake records and the data scaled using two different scaling methods defined in the 2018 Turkish Building Earthquake Code (TBDY 2018) were utilized. One-dimensional equivalent linear and nonlinear methods were applied in the analyses. For this purpose, 17 different earthquake records were scaled according to the two scaling methods outlined in TBDY 2018 as expressed before, and site amplification analyses were performed using DEEPSOIL and OPENSEES software. The spectral parameters obtained from the analyses were compared with the spectrum envelope proposed in TBDY 2018. In conclusion, this Ph.D. thesis presents a comprehensive approach that combines both experimental and numerical analyses, enabling reliable determination of the seismic behavior of low-plasticity clay soils. Site-specific soil analyses provide more comprehensive data for accurate evaluation of seismic behavior, enabling safer engineering designs and presenting fundamental parameters that must be considered especially in engineering projects located in settlement areas above active fault zones.