Doygun olmayan zeminlere oturan yüzeysel temellerin taşıma kapasitesinin analitik ve sayısal analiz yöntemleriyle belirlenmesi
Determination of the bearing capacity of shallow foundations sitting on unsaturated soils using analytical and numerical analysis methods
Yazar: MOHAMMAD OSAMA FAWWAZ ALSMADI
Danışman: PROF. DR. AŞKIN ÖZOCAK
Sakarya Üniversitesi / Fen Bilimleri Enstitüsü / İnşaat Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2024
Konu: İnşaat Mühendisliği = Civil Engineering

Özet:
Bir temelinin taşıma kapasitesi, yapıların stabilitesi ve güvenliğini sağlamak için kritik öneme sahiptir. Ancak, doygun olmayan zeminler üzerinde yer alan temeller, bu tür zeminlerin karmaşık davranışları nedeniyle ayrıca değerlendirilmelidir. Araştırıcılar, bu temellerin taşıma kapasitesini belirlemek amacıyla analitik ve sayısal analiz yöntemleri geliştirmişlerdir. Bu yöntemler, doygun olmayan zeminlerin davranışlarına dair parametreler sağlayarak daha güvenilir tasarıma yardımcı olur. Bu çalışmada, doygun olmayan zeminler üzerinde yer alan sığ temellerin taşıma kapasitesini belirlemek için analitik ve sayısal yöntemleri incelenmiştir. Tez kapsamında bahsedilen yöntemlerin avantaj ve dezavantajları değerlendirilerek doygun olmayan zeminler üzerine inşa edilen yapıların güvenliğini ve uzun ömürlülüğünü sağlamadaki rollerini irdelemeyi amaçlamaktadır. Ayrıca, bu çalışmada Plaxis 2D'yi kullanarak, uniform zemin tabakaları üzerindeki şerit temellerin taşıma kapasitelerini analiz edilmiştir. Oturma ve yer değiştirme gibi hususlar da bu araştırmanın ayrılmaz bir parçasıdır ve farklı doygunluk seviyelerinde, farklı koşullar altında temel davranışının anlaşılabilirliğini artırmaktadır.
Kentsel nüfuslar genişledikçe inşaat yapılarına talebi önemli ölçüde artmıştır. Köprüler, kuleler ve barajlar, modern dünyamızı sürdürmek için gereken yaşamsal altyapı örneklerinden sadece birkaçıdır. Ancak, göz ardı edilmemesi gereken kritik bir unsur vardır: Yapı temeli. Uygun bir temel olmadan, bu yapılar çökme ve başarısızlık riski taşır. Temellerin titizlikle tasarlanması ve inşa edilmesi gerekir ki, bunlar yapılarımızın güvenliğini ve dayanıklılığını sağlamak için esastır.
Sonlu Elemanlar Yöntemi (SEM), matematiksel ifadelerle tanımlanan sürekli sistemleri çözmek için etkili bir tekniktir. SEM, sürekli sistemleri düğümlerle birbirine bağlı sınırlı sayıda bileşene veya elemanlara bölerek başarır. Dolayısıyla, sistem bu sonlu elemanlara bölünür ve her eleman için denklemler türetilir. Bu bireysel denklemler daha sonra sistem seviyesinde denklemler oluşturmak için birleştirilir. Bu yaklaşım, sürekli bir alana uygulanan orijinal diferansiyel denklemleri, yönetilebilir bir dizi doğrusal denkleme indirger.
Bu çalışmada, Plaxis 2D kapsamında 15 düğümlü elemanlar kullanılarak sayısal analizler yapılmıştır. Plaxis 2D, iki boyutlu geoteknik problemleri modellemek için tasarlanmış ileri bir yazılım aracıdır ve hem statik hem de dinamik analizleri destekler. Yazılımın doğrusal olmayan zemin malzeme modellerini kullanması, zemin içerisindeki herhangi bir noktada gerilme-deformasyon değerlerinin doğru bir şekilde belirlenmesini sağlar. Yazılımın zemin-yapı etkileşimini modelleme yeteneği, temel tasarımı ve inşaatı için kritik öneme sahiptir ve mühendislerin zeminlerin yük taşıma kapasitelerini değerlendirmelerine, taşıma kapasitesini belirlemelerine ve yapıların güvenli tasarımını sağlamalarına olanak tanır.

Abstract:
The bearing capacity of a foundation is crucial for ensuring the stability and safety of structures. However, shallow foundations on unsaturated soils pose challenges due to the complicated behavior of such soils. To address this issue, engineers have developed analytical and numerical methods of analysis, which can be used to determine the bearing capacity of these foundations. These methods provide valuable insights into the behavior of unsaturated soils and help design more dependable foundations. This research uses these analytical and numerical methods to determine the bearing capacity of shallow foundations on unsaturated soils. This study explores the advantages and disadvantages of these methods, offering a thorough understanding of their role in ensuring the safety and longevity of structures built on unsaturated soil. Furthermore, this study employed Plaxis 2D to analyze the bearing capacities of strip foundations on uniform soil strata. Considerations of settlement and displacement are also integral to this research, enhancing the understanding of foundation behavior under different conditions and different global level with different values in saturation.
As urban populations expand and the global population surpasses 8 billion, the demand for civil structures has increased significantly. Bridges, towers, and dams are just a few examples of the vital infrastructure required to sustain our modern world. However, one crucial element that cannot be overlooked is the foundation. Without a sturdy foundation, these structures risk collapse and failure. It is imperative that foundations are meticulously designed and constructed, as they are essential for ensuring the safety and durability of our built environment.
Unsaturated soil (UNS) mechanics is commonly considered a part of saturated soil mechanics, and saturated soil parameters are preferred in calculations. After many studies in the 1960s, the results found a big difference between saturated and unsaturated soils, if the soil has more than 98% voids filled with water and when the air bubbles are not directly connected, they are called saturated soil. If the volume is 95% or less, the soil loses its saturated characteristics, and as a result, it is referred to as unsaturated soil [9]. UNS commonly are assumed to be three stages. However, after many studies, the results have discovered a very thick layer formed between water and air, called the water-air interface. When conducting tests on the mass–volume relationships of UNS, the water-air interface is typically disregarded due to its minimal volume.
Engineers used to design projects based on bearing capacity until the 1950s. They stated that if the bearing capacity is sufficient, the foundation will not settle. However, Hough emphasized the need to consider both aspects together. In the design of engineering structures, the settlement rate is as important as the settlement amount. Especially in cases of excessive settlements, structural and/or non-structural damage may occur in situations where settlement is rapid or even if settlement is not rapid. Settlement (St) has three components: immediate settlement (distortion settlement) (Si), consolidation settlement (time-dependent) (Sc), and secondary settlement (time-dependent) (Ss) 
Researchers have identified three types of bearing capacity settlements (failures): general shear settlement, punching shear settlement and regional (partial, local). When it comes to shallow foundations, understanding bearing capacity is crucial. Fortunately, over time, experts like Terzaghi have developed theories and solutions for various load conditions, including the widely used theories: Meyerhof (1951), Brinch Hansen (1961), and Vesic (1975). However, continued research, particularly into the behavior of unsaturated soils and advanced simulation tools like Plaxis, is essential for further advancements in geotechnical engineering.
Both theoretical and empirical, are employed to investigate the behavior of soil. These approaches continuously evolve to simplify the intricate nature of the soil and its behavioral characteristics. However, With the emergence of computer and software technologies, numerical methods have become a valuable tool in solving geotechnical problems by offering more realistic and efficient solutions, similar to other engineering challenges. In these methods, differential equations describe the behavior of physical systems, which are analyzed through numerical techniques.
The Finite Element Method (FEM) is an effective technique for solving continuous systems described by mathematical expressions. FEM achieves this by breaking down continuous systems into a finite number of interconnected components, or elements, linked by nodes. Consequently, the system is partitioned into these finite elements, and equations are derived for each element. These individual equations are then combined to form system-level equations. This approach simplifies the original differential equations, which apply to a continuous domain, into a manageable set of linear equations.
This study employed Plaxis 2D version 24.01.00.1060, utilizing 15-node elements for numerical analysis. Plaxis 2D is an advanced software tool designed for modeling two-dimensional geotechnical problems, supporting both static and dynamic analyses. Its use of non-linear soil material models allows for the accurate determination of stress-deformation values at any point within the soil.
Plaxis 2D is recognized globally as a premier tool for stress-strain analysis, particularly in understanding how soil materials respond under various loading conditions and different saturation levels. The software's capability to model soil-structure interaction is crucial for foundation design and construction, allowing engineers to assess the load-bearing capabilities of soils, determine bearing capacity, and ensure the safe design of structures.
Determining the initial stresses is crucial in geotechnical engineering finite element analyses to approximate natural conditions. The initial stress represents the undisturbed soil's equilibrium state under its self-weight. Plaxis 2D offers different methods to determine initial stress based on the problem's characteristics. The K_0  procedure is appropriate when the ground surface, ground layers, and groundwater level are all parallel and horizontal. The gravity loading method determines initial stress in cases where the K_0 Procedure's idealized assumptions do not apply, such as non-horizontal ground surfaces, non-parallel ground layers, or non-conforming groundwater levels. The gravity loading method is used when the ground surface has slopes, inclinations, or non-horizontal features or when ground layers and groundwater levels deviate from parallelism to accurately account for real-world conditions in finite element analyses.
The load-bearing capacity of the strip foundation was determined by modeling the soil and the foundation in Plaxis 2D. Initially, the stress causing failure in a saturated condition was investigated, and a load of 100 kN/m² was selected for all models to ensure comparability of the results. The model was then subjected to the load using the parameters discussed earlier. However, it is essential to note that the results obtained from the Plaxis 2D model differed from those obtained from the experimental modeling test. This chapter will discuss the results from the fourth model. In each model, the difference between the results for the displacement and the pore pressure P_excess was observed.
In this study, the aim is to determine the bearing capacity of shallow foundations located on an unsaturated soil layer. The analyses conducted using Plaxis 2D software aim to achieve the following points:
Evaluate the behavior of unsaturated soils. Accurately estimate the bearing capacity of shallow foundations resting on unsaturated soils. Examine the behavior of unsaturated soil in sections with different groundwater levels and analyze the effects of the water level on the soil and displacement. Additionally, examine the changes in soil with different degrees of saturation (from 0.99 to 0.50) and the changes in displacement and pore water pressure.