Basınç tipi zemin ankraj davranışının sayısal analizler ve saha deneyleri ile incelenmesi

Investigation of compression ground anchor behaviour through numerical analyses and field tests

Tez No: 882773
Yazar: GÖKALP DEMİRCİ
Danışmanlar: PROF. DR. RECEP İYİSAN
Tez Türü: Yüksek Lisans
Konular: İnşaat Mühendisliği, Civil Engineering
Anahtar Kelimeler: Belirtilmemiş.
Yıl: 2024
Dil: Türkçe
Üniversite: İstanbul Teknik Üniversitesi
Enstitü: Lisansüstü Eğitim Enstitüsü
Ana Bilim Dalı: İnşaat Mühendisliği Ana Bilim Dalı
Bilim Dalı: Zemin Mekaniği ve Geoteknik Mühendisliği Bilim Dalı
Sayfa Sayısı: 155

Özet

Artan nüfus ve hızlı şehirleşmenin bir sonucu olarak ortaya çıkan çok bodrumlu yüksek katlı yapıların inşası, derin kazıların yapılmasını gerekli kılmaktadır. Derin kazılar sırasında ortaya çıkan kazı yüzeylerini destekleyen yapılara iksa sistemi adı verilmektedir. Derin kazıların güvenli bir şekilde yapılabilmesi açısından kazı yüzeylerini destekleyecek iksa sistemlerinin tasarımı geoteknik mühendisliğinin önemli konularından biri olmaktadır. İksa sistemleri düşey ve yatay destek elemanlarından oluşmakta olup yatay destek elemanı olarak genellikle eksenel çekme kuvvetine çalışan zemin ankrajları kullanılmaktadır. Eksenel basınç kuvvetine çalışan zemin ankrajları yatay destek elemanlarına bir alternatif olarak karşımıza çıkmaktadır. Çekme tipi zemin ankrajı serbest bölge ve kök bölgesinden oluşurken basınç tipi zemin ankrajı ise kök bölgesi yerine uç kısımda yer alan yapısal bir malzemeden oluşmaktadır. Basınç tipi zemin ankrajlarında kullanılan çelik halatlar, çekme tipi zemin ankrajlarının aksine kılıf ile kaplanarak çimento ile aderans halinde olmaması sağlanmaktadır. Böylece basınç tipi zemin ankrajlarına uygulanan öngerme yükü, çelik halatlar vasıtası ile doğrudan uç bölgede bulunan yapısal elemana aktarılmaktadır. Yapısal elaman deplasman yaptıkça ankraj deliğindeki enjeksiyona basınç kuvveti uygulamakta ve böylece ankraja yük taşıtılmaktadır. Geoteknik mühendisliğinde özellikle ülkemizde uygulaması yaygın olmayan bu tür ankrajlar, ankraj deliği içerisine enjekte edilen çimentonun çekmeye göre basınca daha dayanıklı olmasından dolayı daha yüksek öngerme yükü uygulanabilmesi, ankraj deliğindeki çimentoda çatlak oluşma ihtimalinin az olmasından kaynaklı korozyona karşı daha dayanıklı olması, destek sistemlerinde gereksiz olduğu durumda halatların etrafına sarılmış olan kılıftan dolayı ankraj deliği sonunda yer alan yapısal elemandan sökülebilmesi ve tekrar başka projede kullanılabilmesi yönleriyle üstünlüklere sahiptir. Bu çalışmada, basınç ankrajlarının çalışma prensibi, imalat yöntemi ve çekme ankrajları ile taşıma kapasitesi açısından karşılaştırılması hedeflenmiştir. Bu amaç doğrultusunda önce sonlu elemanlar yöntemi kullanılarak yapılan sayısal analizlerle kil ve ayrışmış kaya ürünü olan zeminlerde imal edilmiş basınç tipi zemin ankrajının yük alma kapasitesi ve davranışı incelenerek aynı zemin koşullarında ve halat boyunda tasarlanan çekme tipi zemin ankrajıyla karşılaştırılmış ve basınç ankrajlarının iksa sistemleri üzerindeki etkisi araştırılmıştır. Bu yapılan analizler elde edilen sonuçlardan basınç tipi ankrajların kullanıldığı iksa sisteminde yer alan düşey destek elemanı yanal deformasyonu ve düşeyde her sırada bulunan ankraj enjeksiyon gövdesinin maksimum deformasyon değerleri çekme tipi ankrajın kullanıldığı sisteme kıyasla daha az elde edilmiştir. Sistemin toptan göçmeye karşı güvenlik sayısı daha fazla hesaplanmıştır. Sayısal analizlerde iksa sisteminde yer alan çekme ve basınç tipi ankrajlar üzerinde kademeli yük arttırımı yapılarak davranışları incelenmiş ve nihai yükleme durumunda çekme tipi basınç ankrajının kök bölgesinin tamamında zemin nihai taşıma kapasitesine ulaşırken basınç tipi ankrajda tüm ankraj uzunluğunun yaklaşık 1/4'ü kadar kısmında zemin nihai kapasitesine ulaşmıştır. Ayrışmış kayada modellenen basınç tipi ankraj boyu kısaltılarak çekme tipi ve aynı boydaki basınç tipi ankraj ile karşılaştırması yapılmıştır. Elde edilen sonuçlara göre boyu kısaltılmış basınç ankrajlı sistemin ve ankraj enjeksiyon bölgesinin deformasyon değerleri diğerlerine kıyasla fazla olmasına rağmen mertebeleri aynı düzeyde olup yönetmelikte verilen sınır değerler içerisinde kalmıştır. Sahada uygulama esaslarını belirlemek, karşılaşılabilecek problemleri tespit edebilmek, zemin-ankraj etkileşimini incelemek, ankraj taşıma kapasitesine etkiyen faktörleri belirlemek amacıyla sonlu elemanlar analizi gerçekleştilen derin kazıların yatay yönde desteklenmesi için basınç ankrajlarının güvenli elemanı olarak kullanılabileceği gösterilmiştir. Bu çalışma kapsamında aynı sahada ve benzer zemin koşullarında imal edilen eksenel çekme ve basınca çalışan ankrajlar üzerinde 10 adet arazi deneyi yapılmış, elde edilen sonuçlar geoteknik mühendisliği açısından değerlendirilmiştir. Gerçekleştirilen saha deneylerinde ankraj deliği sonunda bulunan yapısal elemanlara bağlanmış kılıflı çelik halatlardan oluşan basınç ankraj tipi ve bu sisteme enjeksiyon-zemin arasındaki dirence ilave olması amacıyla yapısal elemana kaynaklanmış çelik borulu basınç ankraj tipi uygulaması yapılmıştır. Herhangi bir katkı malzemesi kullanılmadan imal edilen ankrajlar üzerinde yapılan yükleme test sonuçlarına göre basınç ankrajları özellikle yapısal elemana kaynaklanmış çelik boruların sağladığı direnç sayesinde çekme ankrajlarıyla aynı yük mertebelerine çıkabilmiştir. Bu çalışmada, basınç ve çekme tipi zemin ankrajlarının iksa sistemlerindeki performanslarını karşılaştırılarak basınç tipi ankrajların çekme tipi ankrajlara göre birçok avantajı gösterilmektedir. Basınç tipi zemin ankrajlarında kullanılan çelik halatlar, çekme tipi zemin ankrajlarının aksine tüm ankraj boyunda kılıfla kaplanarak çimento ile aderans halinde olmadığından inşaat sonrasında bağlı olduğu yapısal elemandan söküllebildiğinden şehir merkezlerinde ve nüfus yoğunluğunun fazla olduğu bölgelerde basınç tipi ankrajların kullanılması önerilmektedir. Basınç tipi ankraj davranış ve performansının daha iyi incelenebilmesi için daha fazla saha deneyi yapılarak uzun dönem etkileri gözlemlemek gerekmektedir.

Özet (Çeviri)

The continuous growth of population and rapid urbanization have led to a significant development in urban areas. However, this development has also resulted in a decrease in available construction sites within cities. Consequently, there is a need for more efficient use of existing land through the construction of multi-basement and high-rise buildings. For these types of projects, it is essential to maintain the stability of excavation surfaces during deep excavations, especially in urban areas where infrastructure, roads, and buildings surround the construction site. Ensuring the stability of these surfaces is crucial to prevent adverse effects on the construction area and surrounding structures caused by ground movements and stresses. To achieve this stability, it is necessary to control the ground movements induced by excavations and keep them within permissible limits. Retaining systems, designed to ensure the safe continuation of construction during deep excavations, play a vital role in this context. The construction of retaining systems has become a significant topic in geotechnical engineering, particularly for deep excavations near property boundaries and in densely urbanized areas where forming slopes is impractical. The most economical way to perform deep excavations is through open excavation with appropriate slopes. However, factors such as proximity to property boundaries and high urbanization levels make it necessary to support excavation surfaces. Retaining systems, subjected to horizontal earth pressures, generally consist of vertical and horizontal elements. Common vertical elements in retaining systems include bored piles, mini piles, diaphragm walls, shaft walls, sheet pile walls, and shotcrete. Horizontal support elements typically include prestressed ground anchors, soil nails, and steel pipe struts. In Turkey, the most commonly used horizontal support element in retaining systems to counteract earth pressures is the prestressed ground anchor working in axial tension. These anchors are preferred due to their lower cost compared to other horizontal support elements and their applicability in all soil and rock types. Anchors, properly connected to vertical support elements, contribute to keeping deformations within allowable limits. In tension-type ground anchors, the cables located in the root zone are bonded with cement, while those in the free zone are sheathed to prevent contact with cement within the anchor hole. This design ensures that the load applied to the cables is transferred to the root zone, theoretically a non-moving region, through the system's slip wedge. The adhesion forces between the cable-cement and cement-soil interface provide stability to the system but also lead to the anchors' service life being filled over time due to adherence within the soil. With advancements in anchor technology, there is growing interest in removable ground anchors, which can be extracted when no longer needed. These anchors are referred to as compression-type ground anchors. In compression-type ground anchors, the cables within the anchor hole are not bonded with cement, allowing the prestressing load to be transferred directly to a structural element at the end of the hole via the cables. As the structural element displaces towards the vertical element, the frictional force between the cement and soil helps the ground anchors achieve their designed service loads. The sheathing of the entire cable length in compression-type anchors enables the cables to be removed from the anchor hole if they become unnecessary in the retaining system. Despite their potential advantages, compression ground anchors are not widely applied in geotechnical engineering. One significant advantage is their ability to withstand higher prestressing loads due to the higher resistance of cement to compression compared to tension. Additionally, the lower risk of crack formation in the grout makes them more resistant to corrosion. The presence of a sheath around the cables allows for the structural element at the end of the anchor hole to be removed and reused in other projects if it becomes unnecessary in the current support system. These features present compression ground anchors as a versatile and durable option for deep excavation support. The main objective of this study is to evaluate the application potential of compression-type ground anchors by comparing their performance with tension-type ground anchors used in deep excavations through numerical analyses and field experiments. In this context, the load-bearing capacities, deformation behaviors, and safety factors of anchors used in support systems were determined, and the efficiency and safety of the anchor systems were examined in light of these data. This study aims to provide a comprehensive analysis of compression ground anchors, focusing on their working principles, manufacturing methods, and load-bearing capacities. To achieve this, numerical analyses using the finite element method were conducted. These analyses involved calculating the capacities of compression ground anchors of varying lengths and comparing them with tension ground anchors of the same length. The finite element method was employed to determine the practical applications, potential problems, and soil-anchor interactions, as well as to identify the factors affecting anchor load-bearing capacity. Furthermore, field experiments were conducted on both axial tension and compression ground anchors manufactured under similar soil conditions at the same site. The experimental results were evaluated to provide insights into the practical performance of compression ground anchors. The findings from the numerical analyses and field experiments were integrated to offer a comprehensive understanding of the behavior and advantages of compression ground anchors in geotechnical engineering. In this study, the excavation support systems used on excavation surfaces with a depth of 11.55 meters were modeled using the finite element program. In the models, clay and decomposed rock were used as soil profiles. The load-bearing capacities and deformation behaviors of tension and compression-type ground anchors were compared under the same soil conditions. Additionally, field experiments were conducted to evaluate the performance of these anchors under real-world conditions. Numerical analyses have shown that the load-bearing capacity of tension-type anchors is generally lower than that of compression-type anchors. In analyses conducted in clay soil, the load-bearing capacity of tension-type anchors was determined to be 780 kN, while the load-bearing capacity of compression-type anchors was found to be 1040 kN. This difference indicates that compression-type anchors can carry higher loads more safely. In decomposed rock soils, the load-bearing capacity of tension-type anchors was measured at 1040 kN, whereas for compression-type anchors, this value was 1200 kN. These results highlight that the load-bearing capacity of compression-type anchors is significantly higher depending on the soil type. Evaluations in terms of safety factors revealed that systems using compression-type anchors achieved higher safety factors. For example, in clay soil, the safety factor in a system using tension-type anchors was 1.49, while in a system using compression-type anchors, this value was 1.51. This demonstrates that compression-type anchors are safer even under higher loads. In decomposed rock soils, the safety factor in a system using tension-type anchors was determined to be 1.91, while in a system using compression-type anchors, the safety factor was found to be 1.95. These safety factors indicate that compression-type anchors are more stable under soil and load conditions, offering a safer alternative for deep excavation projects. Comparisons of deformation behaviors revealed that compression-type anchors have lower deformation values. In clay soil, the maximum deformation in a system using tension-type anchors was measured at 1.83 cm, while in a system using compression-type anchors, this value was found to be 1.51 cm. This indicates that compression-type anchors produce less deformation during excavation, enhancing environmental stability. In decomposed rock soils, the maximum deformation in a system using tension-type anchors was measured at 1.18 cm, whereas in a system using compression-type anchors, this value was determined to be 1.12 cm. These lower deformation values suggest that compression-type anchors are more stable within the soil, improving the performance of support systems. Detailed analyses have provided a better understanding of how deformation behaviors vary with anchor type and soil type. For example, in clay soils, the deformation values for each row of tension-type anchors were found to be 1.83 cm, 1.87 cm, 1.49 cm, and 1.62 cm. In contrast, for compression-type anchors, these values were measured at 1.51 cm, 1.53 cm, 1.48 cm, and 1.16 cm, respectively. These values demonstrate that compression-type anchors show less deformation in each row and have a more minimal impact on the soil. In analyses conducted in decomposed rock soils, the deformation values for tension-type anchors were determined to be 1.18 cm, 1.12 cm, 1.05 cm, and 1.04 cm. For compression-type anchors, these values were determined to be 1.12 cm, 1.05 cm, 1.04 cm, and 0.94 cm, respectively. These results indicate that compression-type anchors exhibit superior deformation behavior compared to tension-type anchors and offer more stable performance with less displacement. In this study, various field experiments were conducted to evaluate the performance of tension and compression-type ground anchors under real-world conditions. These experiments detailed how each type of anchor behaved in actual field conditions, revealing their load-bearing capacities and deformation characteristics. In field experiments on tension-type anchors, the design ensured that the cables in the root region adhered to cement, while in the free region, the cables were sheathed to avoid contact with cement. This design aims to transfer the load to the root region through the cables. In the field experiments, the maximum load-bearing capacity of tension-type anchors was measured at 1040 kN. The deformation values increased as the load increased. For example, under a load of 100 kN, the deformation was 0.14 cm; under 200 kN, it was 0.33 cm; under 300 kN, it was 0.59 cm; and under 400 kN, it was 1.04 cm. These values indicate that tension-type anchors exhibit significant deformations under high loads, which could affect the stability of the system. In compression-type anchors, cables sheathed along their entire length were used, and the prestress load was directly transferred to the structural element at the end of the anchor hole. This design ensures a more homogeneous distribution of stresses within the soil. In the field experiments, the maximum load-bearing capacity of compression-type anchors was measured at 1200 kN. The deformation values showed a more controlled increase as the load increased. For example, under a load of 100 kN, the deformation was 0.12 cm; under 200 kN, it was 0.30 cm; under 300 kN, it was 0.55 cm; and under 400 kN, it was 0.90 cm. These values indicate that compression-type anchors exhibit less deformation compared to tension-type anchors and remain more stable under high loads. During the field experiments, both types of anchors were loaded at specific intervals, and deformations were recorded. The deformations of tension-type anchors showed more pronounced increases, especially under high loads. For instance, under a load of 400 kN, a deformation of 1.04 cm was observed in tension-type anchors, while the deformation was 0.90 cm in compression-type anchors. This difference indicates that compression-type anchors exhibit lower deformation even under higher loads, offering a more reliable performance. In experiments conducted in clay soils, the maximum deformation for tension-type anchors was measured at 1.83 cm, while for compression-type anchors, it was 1.51 cm. In decomposed rock soils, the maximum deformation for tension-type anchors was 1.18 cm, whereas for compression-type anchors, it was 1.12 cm. These results demonstrate that compression-type anchors exhibit lower deformation in both soil types and are therefore more stable. The economic and practical advantages of compression-type anchors are also noteworthy. One of the most significant advantages of compression-type anchors is their removability. This feature allows the systems to be reused when they become unnecessary, leading to long-term cost savings. Additionally, the manufacturing and application of compression-type anchors are simpler and faster compared to tension-type anchors. This facilitates the completion of construction projects in a shorter time and reduces costs. Therefore, it is recommended to use compression-type anchors, particularly in city centers and densely populated areas. By comparing the performance characteristics of compression and tension ground anchors, this study aims to highlight the potential of compression ground anchors as a viable alternative. The research underscores the importance of considering both anchor types in designing safe and efficient retaining systems for deep excavations. Through detailed numerical and experimental investigations, this study contributes to the advancement of geotechnical engineering practices, offering practical solutions for the challenges associated with deep excavation support. The obtained results have demonstrated that compression-type ground anchors offer various advantages over tension-type anchors. The higher load-bearing capacity, lower deformation values, and higher safety factors of compression-type anchors make them preferable for deep excavation projects. Furthermore, the removability and reusability of compression-type anchors provide significant economic and environmental benefits. Therefore, it is recommended to use compression-type anchors in deep excavation projects, especially in city centers and densely populated areas. In summary, this research elucidates the working mechanisms, advantages, and practical applications of compression ground anchors. By providing a thorough comparison with tension ground anchors, it aims to enhance the understanding and utilization of these innovative anchoring systems in deep excavation projects. The findings emphasize the need for further exploration and adoption of compression ground anchors to improve the safety and efficiency of retaining systems in urban construction environments. In the future, experimental and numerical studies examining the performance of compression-type anchors in different soil types should be conducted, and long-term field observations should be made to collect more data on the corrosion resistance and longevity of these anchors. These studies are necessary to evaluate the broad range of potential uses for compression-type anchors and to develop safer, more economical, and environmentally sustainable excavation support systems. Consequently, the broader use of these innovative anchor systems in geotechnical engineering applications will enhance the safety and efficiency of construction projects.

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