İstanbul Grovaklarına Soketlenen Fore Kazıkların Taşıma Kapasitesi

Abstract

Soketli kazıklar, zayıf kaya ve tabakaların bulunduğu zemin ortamında, daha derindeki sağlam kaya içinde delgi yapılarak imal edilen kazıklardır. Bu çalışmada kazığa gelen yükün büyük ölçüde kazığın kaya içindeki soketi tarafından taşındığı, İstanbul grovaklarına soketlenmiş, yerinde dökme betonarme kazıkların yük-deformasyon davranışı ayrıntılı bir biçimde incelenmiştir. Haliç Metro Geçiş Köprüsü temel kazıklarının imali öncesinde ve sırasında teşkil edilen test ve proje kazıkları üzerinde uygulanan dört adet Osterberg Hücresi deneyinin sonuçları kullanılarak, İstanbul grovaklarına soketlenen fore kazıklarda tasarımcılar için güvenli kaya soket sürtünmesi değeri önerilmiştir. Genel manada kayaya soketli kazıkların taşıma gücü kapasitesi iki bileşenden meydana gelir. Bunlardan birincisi soket tabanından elde edilen uç mukavemeti, diğeri ise soket yüzeyi boyunca mobilize olan çevre sürtünmesidir. Bu iki halin varyasyonu, soketli kazıklar projelendirilirken değişik durumlar gösterir: Sadece uç mukavemetinin mobilize olduğu, sadece çevre mukavemetini mobilize olduğu ya da iki bileşenin birlikte alındığı durum. Bu çalışmada; İstanbul Grovaklarına soketli yerinde dökme betonarme kazıkların tasarımında, soket sürtünmesi ile tasarım yapılması gerektiği ortaya konmuştur. Kaya soket ucunun yeterince temizlenmemesi durumunda veya kazığın küçük oturma değerlerinde uç dayanımının mobilize olmayacağı dikkate alınarak kayaya soketlenen kazıklarda sadece soket sürtünmesiyle tasarım yapılması tavsiye edilmiştir. Diğer yandan üzerinde çalışılan formasyon olan İstanbul grovakları jeolojik ve geoteknik karakteristikleri açısından incelenmiş, litolojik özellikleri, jeolojik geçmişi, başlıca fiziksel ve geoteknik parametreleri literatür ve bu çalışma kapsamında ortaya konan verilerle izah edilmiştir. Böylece pratikte sıklıkla beraber bulunduğu diğer zemin formasyonları (kiltaşı, silttaşı, çamurtaşı, şeyl) ile karıştırılan grovaklar için, tasarımcıların dikkate alması önem arz eden ayırdedici özelliklerinin sunulması amaçlanmıştır. Çapları 0,02-2 mm. arasında olan kum ta¬nelerinin doğal bir çimento ile birleşmesiyle “Kumtaşı” oluşur. Grovaklar; Ayrık Tortul Külteler grubunda yer alan “çimentolu” tortul kültelerden kumtaşının bir çeşididir. Literatürde grovakların 300-320 milyon yıllık sedimanlar olduğu tespiti yer almaktadır. İstanbul grovakları, “zayıf veya orta sağlamlıkta kaya” sınıfında değerlendirilebilir. İstanbul grovaklarının kritik yapılar sözkonusu olduğunda, yapısındaki süreksizlikler ve çatlaklar açısından “zayıf kaya” sınıfında değerlendirilmesi emniyetli tarafta kalacak bir çözüm için gereklidir. Grovaklar için sadece bağıntılar kullanılarak proje yapmanın yanlışlığı da ortadadır. Çünkü, Çizelgelerin verdiği ortalama dayanım değeri ile maksimum ve minimum değerler arasındaki salınımın genişliği, ampirik bağıntılarla elde edilecek dayanım değerinin hesaplarda esas alınması durumunda oluşacak riskin büyüklüğünü de göstermektedir. Herhangi bir projede, temel sisteminin inşa edileceği lokasyon ve derinlikteki gerçek durumun, bu istatistiğin en düşük değerine yakın olması halinde önemli stabilite sorunları ile karşılaşılması kaçınılmaz olacaktır. Orta sağlam-sağlam kaya ortamda mobilize olması beklenen ve literatürde yaygın olarak karşılaşılan 1000 kPa mertebesindeki kaya soket sürtünmesi değerlerine, bu çalışma kapsamında yapılan kazık yükleme deneylerinin bazılarında rastlanmıştır. Grovak içerisindeki magmatik orjinli diyabaz-andezit gibi sokulum kayaçlarıyla karşılaşılan seviyelerde, 300-400 mikrometre, diğer bir deyişle 1mm’nin yarısından az bir kazık yerdeğiştirmesi için soket sürtünmesinin 500 kPa, 1mm’lik kazık yerdeğiştirmesi için ise 1000 kPa değerini aştığı görülmüştür. Bununla birlikte, dayk ve sil şeklindeki bu sokulumların düşey ve yatay düzlemlerde sürekli olmadığı bilinmektedir. Dolayısıyla, temel sistemindeki hangi kazıkların, hangi soket boylarının bu sokulumlar içinde yer alacağını önceden kestirmek mümkün olmayacaktır. Diğer taraftan, aynı temel sistemindeki bazı kazıkların kaya soketleri için 300-400 kPa gibi alt sınır değerlerle hesap yaparken, bazı kazıklar için 1000 kPa dolayında üst sınır değerler seçilmesi kazık boylarında veya kazık çaplarında farklılıklar ortaya çıkaracağı için temel tasarımında istenmeyen bir durumdur. Bu sebeple, İstanbul grovakları ve benzeri geçiş kayaçlarında alt sınır dolayındaki nihai soket sürtünmeleriyle hesap yapılması durumunda yeterli güvenliğe sahip kazık tasarımları yapılmış olacaktır. Sınırlı yerdeğiştirme değerleri için seçilen güvenli kaya soket sürtünmesinin 300 kPa – 400 kPa bandını aşmayacak şekilde seçilmesi doğru bir yaklaşım durumundadır. Diğer bir deyişle, grovak kayasının çok parçalı ve çok zayıf yapısı, yüksek soket sürtünmelerinin her yerde mobilize olmasına imkân tanımamakta, bir anlamda grovakların, zemin ile kaya arasındaki geçiş kayacı özelliğini doğrulamaktadır.

Rock socketed piles are piles generally socketed into the rock layers below the soil layers present on the top levels of the soil profile. This study deals with the load – deformation behavior of cast in-situ reinforced concrete piles socketed into Istanbul Graywackes. Literative information on various pile types is given at the beginning of this study. This definiton part is followed the bearing capacity of a single axial loaded pile in order to create a base for understanding of general bearing capacity of piles before investigating to rock socketed piles philosophy. Pile design methods for piles socketed into rock layers and pile test methods are given in the following paragraphs of this study. Comprehensive information on design of piles socketed into rock layers, theoretical aspects on their load–deformation behavior are presented. This is followed by brief information on various pile test methods. Geological and Geotechnical characteristics of Istanbul Graywacke’s were examined in the first part of this study. The coverage of Graywackes in whole Istanbul soils is determined. A comparison between in-situ and laboratory findings of Graywackes is studied from literature in order to show it’s dramatically changeable characteristics. Physical and engineering characteristics of this rock strata is also determined from literature, and data investigated and provided during this study. This part includes also facts on the general geology of Istanbul City and its close vicinity. Lithological characteristics, geological history, major physical features and Geotechnical parameters set forth in the literature are illustrated. A certain part of the literature review is assorted to the advantageous and disadvantageous of pile load testing with Osterberg Cell. Instrumentation used in this bi–directional test is explained briefly. Finally, comparison of Osterberg test method with other pile test methods is made. Load transfer behavior of rock socketed bored piles or shafts (large diameter piles) is presented in this study in detail. A compressive force apllied to the top (head) of rock socketed drilled pile is transferred to the ground through shearing stress that develops at the concrete-rock interface along the side of the shaft and the compressive nomal stress that develops at the horizantal interface between the base of the shaft and the underlying rock. Upon initial loading, shearing stress develops the vertical shaft-rock interface. There is no relative diplacement between the concrete shaft and surrounding rock and the system may be modeled as being lnearly elastic. With increasing load, the shear strength along some portion of the socket sidewall is exceeded, initiating relative diplacement at the socket-rock interface. The load-diplacement curve becomes nonlinear. A greater proportion of the applied load is transformed to the shaft base. If loading is continued to a diplacement sufficient to cause failure of rock mass beneath the base, a peak compressive load may be reached. In practice, design of socketed piles in rock requires consideration of deformation limits and geotechnical and structural capacity (strength limit states). Rock socketed piles can be designed to carry compressive loads in sidewall shear only, or end bearing only, or a combination of both. Load distribution between sidewall and end depends on the factors as follows; Geometry of socket, elasticity modulus of pile and rock materilals, socket roughness, and strength, maximum applied load, and construction method. This study’s practice presented that, bearing capacity of large diameter borad piles socketed in Istanbul graywackes can be analysed sidewall shear strength only. Methods to determine the pile bearing capacity are based on several theoretical formulas or site tests. Generally, theoretical calculation methods are insufficient in manners of accuracy due to the heterogeneous structure of rock or soil layers. On the contrary, site tests demonstrate the exact behavior of piles in place. For this reason, several researchers have generated empirical formulas based on site tests. Nevertheless, it should be noted that these formulas are not applicable in each and every situation. The soil type for which the related empirical approach was generated plays a major role in obtaining the desired result. Empirical methods may cause over design or lack of safety. In order to provide suitable solution to create cost-safety balanced projects, to use only empirical methods is not sufficient. The safest method to determine the pile bearing capacity is the conduction of pile loading tests for every particular project. Performing the pile load test as a full scale test and in place mitigates the risk of a design based on erroneous pile bearing capacities. Before performing the project, comparision between design stage assumption and actual soil data which are provided by pile loding tests, prevents unexpected results and events at site, when project is being performed. This thesis studies the axial load tests performed on test and service piles of The Golden Horn Metro Bridge. Prior to evaluation of the pile design and pile test results, the soil investigation works performed at the inspection areas are summarized. The Geotechnical parameters and theoretical aspects on which they were based are constituting the major part of this summary on the soil conditions of the inspection area. Analysis of the load-diplacement behaviour of a socketed pile is an essential step in a raitonal design. Design of most sockets is governed by the requirement to limit settlement to a specified allowable value. The problem of predicting vertical diplacement at the top the rock socket has been studied through theoretical and numerical analyses along with results from full-scale field load testing. Load testing results and their evaluation the most important part of the design. Pile tests can be summarized as in seven groups. These are static loading test, Osterberg Cell test, dynamic pile test, semi static test, seismic continuity test, sonic continuity test, and instrumented tests. In this thesis, these test methods are presented with their advantageous and disadvantageous points from different aspects. Osterberg Cell Method is the most advantageous way to test piles when test load is relatively high. The other methods need more and bigger equipments. Establishment of relatively high load tests is very difficult with other conventional pile load test methods. Osterberg Cell Method also gives more safe and easy applicable test condition, usage of smaller areas, and quick results. In total four tests were performed by means of the Osterberg Method. Two of these pile tests were performed on test piles. These piles have been loaded till collapse. Maximum load performed on this piles were 36.80 MN, and 53.60 MN respectively. One of these piles has 1.0 m diameter. This pile was located on the shore and was socketed 11.0m into the rock layers with a total length of 41 m. The second test pile was constructed close to the shore of the opposite site. This pile has a 1.6 m diameter. The total length of this second test pile was 62.7 m and its rock socket was 12.25 m. The additional two tests were performed on the 2.22 m diameter service piles on which the piers of the Metro Bridge are resting. The maximum test loads applied to these piles was limited to 1.5 times of the service load. Maximum load performed on this piles were 47.00 MN, and 66.60 MN respectively. The length of the piles are 84.5m and 100.25m, while their rock sockets are 12.2 m and 10.25 m, respectively. The instrumented tests made possible to read the socket friction that has been mobilized on pile shafts socketed into the Graywacke’s, which are the members of the Thracian Formation. Details on instrumentation utilized for each pile tests are presented in the related paragraphs of this study. Strain gauges, LVDT’s and other equipment to record the load – deformation behavior of the test piles are placed inside the rock socket. One or two levels are placed above the socket border to show how the friction is damped at the pile shaft above the rock socket. Instrument levels are chosen above and below the Osterberg Cell, so that the contribution of tip resistance and shaft friction could be determined separately. Pile bearing capacities of the rock socketed piles, on which the piers of the Metro Golden Horn Bridge are resting, rely on socket friction. The contribution of tip resistance was neglected in the calculations. The aim of this study was to verify this assumption on that only shaft friction is mobilized for piles serving under limited displacement. Furthermore, the accuracy of the socket friction value that was utilized in the design was verified by evaluating the data obtained from the site tests. Pile load tests revealed that the tip resistance is mobilized only after certain amount of displacement of the pile, while the shaft friction starts to contribute on displacements in micrometer range. Test results showed that the friction values vary between 300 kPa – 350 kPa and slightly over 1000 kPa. It was also seen on the test the shaft frictions reaching the top values are generally mobilized when the piles are allowed to mobilize 20 mm – 25 mm. In other words, for designing a foundation system with limited settlements, the shaft friction to be utilized in the design has to be kept within a range of 350 kPa – 450 kPa. The design value utilized in the design of Golden Horn Metro Bridge foundations is 360 kPa. Although performed test gave for some levels more than 1.000 kPa shaft friction values, which are generated by dikes and sills exist in some levels in vertival and horizantal directions in rock, the design should not be developed on these extreme values. Assumption for socket friction value for sensitive superstructure on graywacke strata, could be close to minimum level of friction instead of average or maximum ranges. According to this study’s results, it is obtained that, the rock, graywacke does not allow to mobilize the highest shaft friction value obtained at any level of socket, because of it’s weak and much fractured structure. In order to keep design and application at safe side, Istanbul Graywackes should be considered as ‘weak rocks’.