Numerical analysis of circular foundations built on a layered soil

In this study, prepared as a master's thesis at Sakarya University, Institute of Natural and Applied Sciences, Department of Civil Engineering, Department of Geotechnical Science, the load behavior of circular foundations resting on soft soils of different thicknesses on the surface was analyzed with the two-dimensional PLAXIS finite element software. In the first part of the thesis, an introduction is presented, and in the second part, the foundations are classified, and brief information is given about wind power plants and silo foundations. The third chapter presents the principles of theoretical calculations of bearing capacity in shallow foundations according to different methods. The fourth section gives information about numerical methods and Plaxis software. The fifth section presents the results of the analyses made with Plaxis software. In the last section, general results are given in a summary, and suggestions on the subject are presented. Soils are formed due to the decomposition of rocks consisting of various minerals due to external influences and the sedimentation and compression of the resulting grains by being transported by external effects. Settlements with alluvial soils like Adapazari can suffer significant damage during earthquakes when their foundation systems are insufficient. During the 1999 earthquakes, many buildings in Adapazari were faced with exceeding the bearing capacity and settlement/toppling problems. After the destruction and loss of life, Adapazarı soils became notorious regarding soil conditions. In the following soil investigations carried out in Adapazarı, it was revealed that the soils of the city were formed by a meandering river and the existence of relatively dense/very dense layers at different depths. Sancio et al. (2003) stated that, according to the results of 46 drillings and 135 probings (CPT: cone penetration test) made in different parts of Adapazarı city center, there were four different types of soil profiles for the upper 15 m. In recent years, comprehensive and unique structures have started to be built with developing construction techniques and structural analysis methods. With the developments in construction techniques and construction technology, construction in cities has spread rapidly, and problems have arisen in finding suitable lands for the construction of buildings. At the same time, due to the increasing population concentration, especially in some metropolises, new construction must shift to places where carrying power and settlement problems are experienced. In the geotechnical engineering literature, approaches related to improving weak soils are frequently encountered with experimental and/or numerical methods. The classical approach, one of the main approaches in determining the bearing capacity and settlement characteristics of soils, is suitable for foundations with simple geometry. The Semi-Empirical approach is more suitable for more complex soil conditions. Kinematic methods are used to study the behavior of soil under load in more detail. It calculates the bearing capacity using mathematical techniques such as numerical methods, computer-based models, and the finite element method. This approach is widely used for complex ground structures. The finite element method is used in detailed analysis. The artificial neural network approach is used to estimate bearing capacity in large data sets. In the current situation in the city, the construction of structures that require circular foundations, such as silos, is also on the agenda. In this thesis, research was conducted on circular foundations. Geotechnical engineering, which is the main responsibility of foundation design, plays a key role in civil engineering projects that include all structures that sit on the ground or are built in the ground, and is vital in the evaluation of natural disasters such as earthquakes, liquefaction, sinkhole formation, rockfalls and landslides. Regarding such a multifactorial issue, Terzaghi's development of the limit equilibrium theory has gained momentum in the history of geotechnical engineering studies. Following Terzaghi's development of his theory, other approaches such as Meyerhof, Brinch Hansen, and Vesic followed. In the future, the most important development after these studies is to ensure that the finite element method is used more effectively with computer technology. Thanks to these developments in technology, comparative studies of theories have begun to be made. The literature states that the foundation diameter, the foundation embedment depth, the presence of relatively weak soil above and below, and the shear resistance of the soils affect the bearing capacity and formation of the collapse mechanism. In this study, the bearing capacity of circular foundations is examined, taking into account the presence of problematic soils in the upper or lower layers. For this purpose, analyses were carried out for different combinations using Plaxis 2D software based on the finite element method, which is widely used in geotechnical engineering. The effects of existing parameters on the formation of the collapse mechanism are also revealed according to the results obtained from the analyses carried out with Plaxis software. Models with foundation diameters of 1 m, 2 m, 4 m, 6 m, 8 m, 10 m and 20 m were created and comparisons were made. Since the study is axisymmetric, half of these values, 0.5 m, 1 m, 2 m, 3 m, 4 m, 5 m, and 10 m, were taken in the geometry. In this thesis, solutions were made on the 2-dimensional axisymmetric model, since the basic shape to be solved is symmetrical around the z-direction and the deformations and stresses are independent of rotation. In order to see the effect of the relatively soft ground thickness on the top, solutions were made by taking the thickness of the layer as well as 4m as well as 8m. To eliminate the effect of the foundation weight on the results, the foundation was considered unweighted in the calculations. In the selected sections, the settlements of the circular foundations were graphed, and the vertical displacements at the center and edge of the foundation were carefully examined and interpreted. While the increase in the load in the undrained state brings the foundation system closer to collapse, the deformations with drainage appear as consolidation settlement. In this case, there is no possibility of migration. In such cases, it is appropriate to examine the results in terms of seating limits. It has been shown that the underlying solid layer cannot contribute to the foundation-bearing capacity in both the short and long term, regardless of whether it is close to or far from the surface. This confirmed the situation that emerged in the 1999 earthquakes. Currently, in such environments, it is recommended to consider one of the following options, regardless of the number of floors of the building; • Excavation-back engineering filling, • Constructing the building with a basement and • Soil improvement/deep foundation construction.