Kazıklı radye temellerin düşey yükler altında davranışının sonlu elemanlar yöntemiyle iki boyutlu ve üç boyutlu olarak parametrik incelenmesi
Parametric study on piled raft foundation behavior under vertical loads by 2D and 3D finite elements method
Yazar: BATUHAN ÇOLAK
Danışman: DR. ÖĞR. ÜYESİ ASLI YALÇIN DAYIOĞLU
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / İnşaat Mühendisliği Ana Bilim Dalı / Zemin Mekaniği ve Geoteknik Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2023
Konu: İnşaat Mühendisliği = Civil Engineering

Özet:
Geleneksel yöntemde kazıklı temeller için üstyapıdan gelen tüm yükün kazıklar tarafından taşınıldığı düşünülmekteydi. Gerçek durumda ise çoğu zaman zeminle teması bulunan radye kısım yük taşımada etkili rol almaktadır. Zaman içinde yapılan çalışmalar sonucu radyenin yük taşımadaki rolü kanıtlanmış ve yeni bir temel türü olan kazıklı radye temeller ortaya çıkmıştır. Bu temel türüyle beraber, kazıkların sayısı geleneksel yönteme göre azalmış ve daha ekonomik tasarımlar mümkün kılınmıştır.
Genel olarak betonarme kazıklardan ve kazıkların bağlandığı bir betonarme plak sisteminden oluşan kazıklı radye temeller, yeterli taşıma kapasite sağlamak ya da ortalama ve farklı oturmaları azaltmak için kullanılmaktadır. Derin temel elemanları olan kazıklar, üstyapıdan gelen yükü daha derin tabakalardaki güçlü zeminlere taşımaktadırlar.
Kazıklı radye temeller, yapısal elemanları arasındaki etkileşime ek olarak zeminle de etkileşim halindedirler. Oldukça karmaşık olan bu etkileşimleri (kazık – zemin, radye – zemin, kazık – radye ve kazık – kazık) kazıklı radye temellerin analizi sırasında dikkate almak, hesapların temel sisteminin gerçek davranışına yaklaşmasını sağlayacaktır. Zaman içinde araştırmacılar tarafından geliştirilen ve bu etkileşimlerin tamamının ya da bazılarının dikkate alındığı bir çok yöntem mevcuttur. Basitleştirilmiş hesap yöntemleri, yaklaşık sayısal analiz yöntemleri ve ileri sayısal analiz yöntemleri olarak üç ana grupta toplanan yöntemlerden ilk ikisi genelde ön analiz aşamasında kullanılırken, üçüncüsü daha detaylı analizlerde kullanılır. Günümüzde ileri sayısal analiz yöntemlerinden en yaygın olarak kullanılanı sonlu elemanlar yöntemidir. Bu tez kapsamında yapılan tüm çalışmalarda sonlu elemanlar yöntemi kullanılmıştır.
Kazıklı radye temellerin giderek daha popüler olması, temel davranışının tam ve detaylı analizlerini gerektirmiştir. Temel sisteminin performansını etkileyen faktörlerin belirlenmesi, daha ekonomik ve güvenilir tasarımlar yapılmasına olanak tanımaktadır. Bu nedenle tez kapsamında bir dizi sonlu elemanlar analizi gerçekleştirilmiş ve kazıklı radye temellerin davranışı incelenmiştir.
İlk olarak iki adet vaka analizi çalışması yapılmıştır. Birinci çalışmada, saha ölçümleri yapılmış 5 adet kazıklı radye temel sistemine sahip yapının (Londra'daki Hyde Park, Frankfurt'taki Messeturm, Torhaus, Westend ve Skyper kuleleri) üç ve iki boyutlu modelleri oluşturulmuştur. Kıyaslama sonucunda üç boyutlu analizlerin, hem saha ölçümleriyle hem de literatürdeki çalışmalarla oldukça iyi bir uyum içinde olduğu gözlemlenmiştir. Ancak iki boyutlu analizlerin sonuçları, oturma değerlerini olduğundan daha fazla hesaplamıştır.
Vaka analizlerinin ikincisinde, saha ölçümleri yapılmış olan Torhaus yapısının kazıklı radye temel sistemi irdelenmiştir. Yapılan çalışmada yapının temel sistemin kalınlığı, temel sistemindeki kazıkların çapı, adedi, yerleşimi ve boyu gibi parametreler değiştirilerek kazıklı radye temel sisteminin değişen performansı orijinal halindekiyle kıyaslanmıştır. Çalışma sonucunda, kazık yerleşimini ve adedini değiştirmek yerine, kazıkların bireysel hacimlerinin sabit tutulup boy ve çaplarının değiştirilmesinin daha iyi sonuç verdiği gözlenmiştir. Kazık boylarının arttırılması ile kazık çapları azalsa da oturma değerlerinde bir miktar iyileşme sağlamıştır. Torhaus yapısının orijinal temel sistemini az bir miktar iyileştirmek mümkün olsa da, kazı maliyetleri açısından incelendiğinde orijinal temel sistemi hala tercih edilebilir olmaktadır.
Tez kapsamında yapılan son çalışmada, radye kalınlığının, kazık aralığının ve geometrisinin değişiminin 350 kPa yük taşıyan 32m x 32m boyutlarında bir kazıklı radye temel performansı üzerindeki etkileri incelenmiştir. Analizlerin tamamı Plaxis yazılımı yardımıyla gerçekleştirilmiştir. Aynı zamanda bu çalışma sert, orta sert, yumuşak ve çok yumuşak olmak üzere 4 farklı kil zeminde tekrarlanmış ve zeminin temel performansı üzerindeki etkileri de incelenmiştir.
Sonuçlar, zemin tipi, kazık aralığı, kazık adedi, kazık boyu ve radye kalınlığı parametrelerinin hepsinin temel sisteminin oturma performansı üzerinde etkisi olduğunu göstermiştir. Sert zeminlerin elastisite modülü de diğer zeminlere göre yüksek olacağından, oturma performansları da diğer zemin türlerine göre daha düşüktür. Buna uygun olarak, elastisite modülü en yüksek zeminle oluşturulan modeller diğer zeminlerdekine göre düşük oturma değerleri almıştır. Sonuçlardan elde edilen veriler ışığında, kazıklı radye temel sisteminin sert killi zeminlerde daha iyi performans gösterdiğini söylemek mümkündür.

Abstract:
Increased housing demand, urbanization, and the decreasing availability of suitable construction areas have led to a growing number of high-rise buildings. As a result, pile raft foundations have become a popular choice as the primary foundation system. While it is now understood that each element of the pile raft system (piles and raft) plays a role in transferring loads from the superstructure to the soil, this understanding has developed over time.
In the traditional approach, it was believed that piles carry the entire load from the superstructure. However, in reality, the raft, which is often in contact with the ground, also plays a significant role in load transmission. Through various studies conducted over time, the role of the raft in load-bearing has been proven, leading to the emergence of a new foundation type known as pile raft foundations. With this foundation type, the number of piles has decreased compared to the traditional method, allowing for more economical designs.
Pile raft foundations, generally consisting of reinforced concrete piles and a reinforced concrete slab system to which the piles are connected, are used to provide sufficient load-bearing capacity or reduce excessive settlements and differential settlements. Piles, as deep foundation elements, transfer the load from the superstructure to the stronger soil layers at greater depths. Piles are used as elements to reduce settlements for pile raft foundations that have sufficient load-carrying capacity but exceed acceptable limits in terms of settlements. In such cases, the emphasis is on improving settlements rather than the load-carrying capacity of the piles. While piles are often used for this purpose in stiff clay and dense sandy soils, pile raft foundation is not suitable for use in soils with soft clay and compressible soil layers near the surface.
Pile raft foundations interact not only with the structural elements but also with the soil. Taking into account these complex interactions (pile-soil, raft-soil, pile-raft, and pile-pile) during the analysis of pile raft foundations will help ensure that the calculations approximate the actual behavior of the foundation system. Over time, several methods have been developed by researchers, considering all or some of these interactions. While some of these analysis methods are used solely to test the feasibility of pile raft foundation systems, others allow for detailed analysis of the system's behavior. Simplified calculation methods, approximate numerical analysis methods, and advanced numerical analysis methods are the three main groups of methods. The first two are generally used in the preliminary analysis stage, while the third is used for more detailed analyses. The finite element method is the most commonly used advanced numerical analysis method today. In all the studies conducted in the scope of this thesis, the finite element method has been employed.
The three-dimensional finite element method is a numerical analysis method used for the analysis of complex structures. This method allows for a more detailed and realistic analysis by dividing a problem into numerous parts (elements). Widely used in the field of civil engineering, this method has been defined by Katzenbach et al. (1998) as the most realistic modeling method. Numerous studies comparing the results of three-dimensional finite element analyses with field measurements (Reul and Randolph 2003; Small 2008; Sales 2010; Abdel-Azim 2019) have confirmed the success of the method. The obtained results, such as load-settlement behavior and load-sharing ratio, closely match the field values.
The increasing popularity of piled raft foundations has necessitated a comprehensive and detailed analysis of their behavior. Identifying the factors that affect the performance of the foundation system allows for the development of more economical and reliable designs. Therefore, a series of finite element analyses were conducted within the scope of this thesis to examine the behavior of piled raft foundations.
Initially, two case study analyses were performed. In the first study, field measurements were carried out, and two-dimensional and three-dimensional models of five structures with piled raft foundation systems (Hyde Park in London, Messeturm, Torhaus, Westend, and Skyper Towers in Frankfurt) were modeled. All of these structures are located on over-consolidated clay soils (London clay - Frankfurt clay). To better model the behavior of such soils, a hardening soil model was chosen. While the piles were modeled using the embedded beam option, the raft was modeled as a plate element. The results of the three-dimensional and two-dimensional analyses were compared with the field measurements in terms of settlement values, pile loads, and pile load ratios. Additionally, the results from the three-dimensional analyses were compared with similar studies previously conducted in the literature. The comparison revealed that the three-dimensional analyses exhibited a good agreement with both the field measurements and the previous studies. However, the results of the two-dimensional analyses generally overestimated the settlement values. The accuracy of the results in two-dimensional analyses is influenced by the pile arrangement and the dimension of the foundation system. Consequently, three-dimensional finite element analysis proved to be highly effective in evaluating the behavior of piled raft foundations. Although the two-dimensional finite element method overestimated settlements, it exhibited a performance close to that of three-dimensional finite element analysis in predicting the maximum bending moments in the raft and the load-carrying ratio of the piles.
In the second case study, the piled raft foundation system of the Torhaus structure, where field measurements were conducted, was examined. The structure was built on two identical foundations with a distance of 10 m between them. Since the interaction between these two foundations needed to be considered in the analyses, the foundations were modeled together. By varying parameters such as the thickness of the foundation system, pile diameter, arrangement, and length, the performance of the piled raft foundation system was compared to its original state. While changing the geometric properties of the piles, it was ensured that the total cross-sectional area of the piles remained constant. Additionally, models were created where the volumes of the piles were equal for both the original and the newly modified piles. This way, the concrete volume required for the original piles and the newly created piles would be equal. The results indicated that increasing the thickness of the raft increased total settlements but reduced differential settlements. However, since the minimal settlement values were expected for this foundation system, the original thickness of the foundation was used for further analyses. Rather than changing the pile arrangement and quantity, better results were obtained by maintaining the individual volumes of the piles and modifying their lengths and diameters. Increasing the lengths of the piles improved the settlement values to some extent, even when the pile diameters were reduced. Although it was possible to slightly enhance the performance of the original foundation system of the Torhaus structure, the original system remained preferable considering excavation costs.
In the final study conducted within the thesis, the effects of changes in the raft thickness, pile spacing, and geometry on the performance of a piled raft foundation supporting a 32m x 32m structure carrying a load of 350 kPa were examined. All analyses were performed using the Plaxis program. Additionally, this study was repeated on four different clay soil types: stiff, medium stiff, soft, and very soft, to investigate the effects of the soil. The pile length was examined as 10m, 30m, and 50m, while the raft thickness was investigated as 1mm, 2m, and 3m. Furthermore, the pile spacing was varied as 2.5m, 3.75m, and 5m. All clay soils were modeled using a hardening soil model. While the piles were modeled using the embedded beam option, preliminary studies indicated that modeling the thicker raft (3m) as a volume element instead of a plate element provided more accurate results. The results demonstrated that the type of soil, pile spacing/quantity, pile length, and raft thickness all had an influence on the settlement performance of the foundation system. Increasing the pile length and reducing the pile spacing also resulted in reduced settlements. However, as the pile length increased, the effect of the other parameters diminished. Since the elastic modulus of stiff soils is higher than that of other soils, the settlement performances in stiff soils were also lower compared to other soil types. As a result, the models created with the soil with the highest modulus of elasticity exhibited lower settlement values than the other soils. Based on the obtained results, it can be concluded that piled raft foundation systems perform better in stiff clay soils.