Kayaya soketli kazıkların farklı yöntemlerle taşıma kapasitelerinin belirlenmesi: Bir vaka analizi
Determination of the capacity of rock socketed piles by different methods: A case analysis
Yazar: CEREN AYDIN
Danışman: PROF. DR. RECEP İYİSAN
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / İnşaat Mühendisliği Ana Bilim Dalı / Zemin Mekaniği ve Geoteknik Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2022
Konu: Mühendislik Bilimleri = Engineering Sciences
Dizin: Kazık yükleme deneyi = Pile load test ; Kazıklar = Piles ; Sonlu elemanlar yöntemi = Finite element method ; Taşıma kapasitesi = Carrying capacity

Özet:
Hızla artan nüfus, barınma ihtiyacının artmasına yol açmaktadır. Özellikle büyük şehirlerde belli bir yer ölçümü ile maksimum insanın temel ihtiyaçlarını karşılayabilmek adına çok katlı yüksek yapılar inşa edilmektedir. Üst yapının güvenliği açısından bu çok katlı ağır yapıların dayanımının yanında üzerine oturduğu zeminin de dayanımının yeterli olması gerekmektedir. Zemin de doğada bulunan her malzeme gibi yük altında şekil değiştirmekte ve göçmektedir. Zeminin taşıma kapasitesinin belirlenmesi adına geoteknik mühendisliğinde birçok çalışma yapılmış ve hesaplama yöntemleri geliştirilmiştir. Bu taşıma gücü hesapları sonunda zeminin üst yapı yüklerini taşıyamayacağı ön görüldüğü takdirde zeminin taşıma kapasitesini arttırmaya yönelik ya da üstyapı yüklerini daha derinde bulunan sağlam tabakalara aktarmaya yönelik ek önlemler alınmaktadır. Bu ek yöntemlere örnek, zemin iyileştirilmesi ya da derin temel inşa edilmesidir. Kazıklar, derin temellerin uygulamada en yaygın kullanılan tipleridir.
Kazıklı temellerin tasarımı için günümüze kadar birçok farklı hesap yöntemi geliştirilmiştir. Bu hesap yöntemlerinde yapılan kabuller ve kullanılan malzeme özellikleri, derin temel sisteminin güvenliği, uygunluğu ve maliyeti açısından önem arz etmektedir. Saha koşullarının doğru modellenmesi ile kazık kapasitelerinin gerçeğe yakın bir şekilde hesaplanması, geoteknik mühendisliğinin en önemli konuları arasında gelmektedir. Bu nedenle sahada yapılan kazık yükleme deneylerinden elde edilen sonuçlar hem kazık kapasitesinin belirlenmesini hem de hesaplarda yapılan kabullerin kontrolünü sağlamaktadır. 
Kazıklı temeller; çok katlı yapılar, köprüler gibi ağır yapıların altında yaygın olarak kullanılsalar da yüksek olmayan yapılarda da zemin koşullarının elverişsiz olması nedeniyle uygulanmaktadır. Çok gevşek siltli, kumlu zeminlere inşa edilen alçak yapılar altında da derin temeller görünmektedir. Çöl, bataklık alanları, deniz dibinden taranarak elde edilmiş ve sıkıştırılmadan yığın haline getirilerek elde edilen dolgular bu tür zeminlere örnek olarak gösterilebilmektedir. Katar'da bulunan Doha Limanı da bu yöntemle elde edilmiş yapay bir dolgu üzerine kurulmaktadır. Limanın bir parçası olan Büyük Terminal projesinin temel kazıkları 16 metre kum tabakasından sonra zayıf ve parçalı kaya tabakasına soketlenmiştir. 
Projedeki kazıkların çapları sabit olduğundan, kazık yükleme deneylerinin kullanılabilir sayıda olmasından ve zemin profilinin neredeyse uniform dağılış göstermesinden, bir başka deyişle; değişken faktörünün azlığından dolayı Büyük Terminal Binası – Doha Limanı – Katar Projesi vaka analizi çalışması için uygun görülmüştür. 
Kazıkların kapasitelerinin belirlenmesinde, kaya tabakalarının tek eksenli basınç deneyinden elde edilen mukavemet değeri kullanılarak, ampirik yöntemlerden yararlanılmıştır. Buna ek olarak kazık yükleme deneyleri, bilgisayar ortamında Plaxis 2D ve Plaxis 3D ile modellenmiş ve sonlu elemanlar yöntemi kullanılarak yük altındaki şekil değiştirmeleri incelenmiştir. Elde edilen yük – şekil değiştirme grafiğinden teğet yöntemi kullanılarak taşıma kapasiteleri belirlenmiştir. Son olarak sahada uygulanan kazık yükleme deneylerinden elde edilen yük – şekil değiştirme grafiği üzerinde yorumlama yöntemlerinden Chin ve Decourt metodu uygulanarak elde edilen kazık kapasiteleri hem ampirik yöntemlerle hem de sonlu elemanlar yöntemi ile elde edilen kapasiteler ile kıyaslanmıştır. Ampirik yöntemler ve sonlu elemanlar yöntemi ile elde edilen kazık kapasiteleri de kendi içlerinde kıyaslanmıştır. Buna ek olarak göçme davranışı göstermeyen çevrimli yükleme deneylerinin yorumlanmasında yeni bir yöntem üzerine çalışılmış, bu yöntem diğer yorumlama yöntemleri ile karşılaştırılmıştır.
Yapılan çalışmalar sonucunda, yükleme arttıkça ampirik yöntemlerin ve sonlu elemanlar yönteminin yükleme deneyi sonuçlarına daha yakın sonuçlar verdiği, yükleme azaldıkça kazık lineer elastik bölgede şekil değiştirme davranışı gösterdiğinden, yorumlama yöntemlerinin daha az kapasite hesaplamaya neden olduğu sonucuna varılmaktadır. Ampirik yöntemlerden elde edilen tüm nihai taşıma kapasiteleri incelendiğinde, mertebe farkının çok fazla olduğu gözlendiğinden, ön tasarım aşamasında saha koşullarını yansıtan yöntemin seçilmesinin önemi bu çalışma ile bir kez daha vurgulanmıştır. Ayrıca kazıkların optimum tasarımının yapılması için öneriler sunulmuştur.

Abstract:
Rapidly increasing population leads to an increase in the need for housing. Especially in big cities, multi-storey high buildings are built in order to meet the basic needs of maximum number of people within a certain area. In terms of the safety of the superstructure, besides the strength of these multi-storey heavy structures, the strength of the ground on which they sit must be sufficient. Soil, like any material found in nature, changes shape and collapses under load. In order to determine the bearing capacity of the soil, many studies have been carried out in geotechnical engineering and calculation methods have been developed. If at the end of these bearing capacity calculations it is predicted that the ground will not be able to carry the superstructure loads, additional measures are taken to increase the bearing capacity of the ground or to transfer the superstructure loads to the deeper layers. These additional methods are soil improvement or deep foundation construction. Piles are the most widely used types of deep foundations in practice.
Many different calculation methods have been developed for the design of pile foundations. The assumptions made in these calculation methods and the material properties used are important in terms of the safety, suitability and cost of the deep foundation system. Accurate modeling of site conditions and calculation of pile capacities similar to real conditions are among the most important issues of geotechnical engineering. For this reason, the results obtained from the pile loading tests carried out in the field are extremely important in terms of both determining the pile capacity and controlling the assumptions made in the calculations.
Although pile foundations are widely used under heavy structures such as multi-storey buildings and bridges, they are also applied in low-rise structures due to unfavorable ground conditions. Deep foundations are also applicable under low structures built on very loose silty, sandy soils. Desert, swampy areas, fillings obtained by dredging from the seabed can be shown as examples of such soils. Doha Port in Qatar is also built on an artificial filling obtained by this method. The foundation piles of the Grand Terminal project, which is a part of the port, were socketed into the weak and fragmented rock layer after the 16 meters sand fill layer.
Since the diameters of the piles in the project are all the same, the number of pile loading tests are usable / enough number for a study and the soil profile is almostuniformly distributed, in other words; due to the lack of variable factor, the Grand Terminal Building - Doha Port - Qatar Project was deemed appropriate for the case study.
By using the strength value obtained from the uniaxial compression test of the rock layers, empirical methods were applied in the capacity calculation. In addition, pile loading experiments were modeled with Plaxis 2D and Plaxis 3D and strains under load were investigated using the finite element method. The bearing capacities were determined using the tangential method from the load-strain graph obtained. Finally, the pile capacities obtained by applying Chin and Decourt method, one of the interpretation methods on the load-strain graph obtained from the pile loading tests applied in the field, were compared with the capacities obtained by both empirical methods and finite element method. The pile capacities obtained by empirical methods and finite element method were also compared among themselves. In addition, a new method has been studied for the interpretation of cyclic loading tests that do not show collapse behavior, and this method has been compared with other interpretation methods.
In the application of the empirical methods, the strength value obtained from the uniaxial compression test of the rock layers was used. When the results obtained from the empirical methods are compared within each other, the extreme difference between the carrying capacities obtained from the method to the method draws attention. As a result of the calculation, the lowest capacity was determined by the Carter and Kulhawy method, and the highest capacity was determined by the Prokoso method. In the literature review part of the study, the methods of obtaining all empirical methods are summarized. Accordingly, during the application of empirical methods, the conditions under which all methods are valid should be examined and care should be taken in the selection of the method.
In the application of the finite element method, the pile loading tests were modeled with Plaxis 2D and Plaxis 3D, and the displacement was effected in the Plaxis 2D program to the extent that collapse was observed over the pile after the model was confirmed to be suitable for site conditions. In the Plaxis program, the load-strain graph of the middle top point of the test pile is output. The bearing capacities were determined using the tangential method from the load-strain graph obtained. The bearing capacities determined by the finite element method are given in Annex-B. The determined carrying capacities were compared with the capacities obtained from empirical methods. As a result of this comparison, it was observed that as the test load decreased, the similarity between the capacities obtained from the finite element method and the empirical methods decreased, and the method that gave the most similar results was the Meigh and Wolshi method.
For the interpretation of pile loading test results, 40 different methods in the literature have been mentioned. On the other hand, it was found appropriate to apply Chin and Decourt interpretation methods on the load-strain graphs obtained as a result of the static pile loading experiments. In addition, a new method has been studied in the interpretation of the static cyclic loading tests. When Chin and Decourt methods were compared with the new method, it was observed that the new method gave results closer to the Chin method. When the capacities obtained from all interpretation methods and the capacities obtained from the finite element method are compared, it is observed that the compatibility between the capacities obtained from all interpretation methods decreases as the test load decreases. On the other hand, the highest similarity rate was achieved with the new method. In addition, the average of the capacities obtained by using Chin and Decourt methods was taken and the similarity ratios were examined with empirical methods. As a result of the examination, it was observed that the method that gave the most compatible results with the loading test was the Carter and Kulhawy method.
As a result of all comparisons, it was observed that the similarity ratios between the results obtained from pile loading tests and empirical methods were considerably less than the similarity ratios between the results obtained from the finite element method and empirical methods.
In addition, as the maximum test load increases, empirical methods and finite element methods seem to give more consistent results with field experiments. It is seen that the similarity rate of the bearing capacity obtained by the Meigh and Wolski method, which is an empirical method in piles subject to maximum test load, is up to 97%. In the pile subjected to the minimum test load, the most similar result obtained by empirical methods was 38%. Similarly, while the compatibility with finite elements increased to 99% in the maximum loaded pile, 90% similarity could be achieved in the pile subjected to the minimum test load. Based on these results, it can be said that as a result of the interpretation of the loading test results, which show behavior in the linear elastic region of the load-strain graph, lower pile capacities than the current capacity are obtained. In the aforementioned case, both the empirical methods and the finite element method give higher final pile capacity.
The suggestions that emerged as a result of the scientific research are as follows.
1- With any of the methods mentioned in the study, the skin friction mobilized cannot be determined clearly. Therefore, before calculating the final pile capacity, it is recommended to determine the mobilized skin friction by performing a field load test with a two-way Osterberg cell.
2- Continuing to increase the load until the failure behavior is observed and placing displacement gauges in the pile makes it possible to determine the skin friction to be taken into account in the pile capacity by back calculation. This method is recommended in order to control the results obtained.
3- When all the final bearing capacities obtained from the empirical methods are examined, it is important to choose the method that reflects the field conditions at the preliminary design stage, since the difference is observed to be too large in different methods results. In the previous sections, ground conditions, number of tests and field information regarding the tests performed during the determination of empirical methods are given. Determining the method to represent the project ground in the light of these studies helps to obtain results close to the real situation.