Zemin ankrajlarının taşıma gücü ve imal yöntemlerinin taşıma gücüne etkisi
Soil anchor ultimate load capacity and construction methods effect on ultimate load capacity
Yazar: ANIL HELVACIOĞLU
Danışman: DOÇ. DR. AHMET OĞUZ TAN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / İnşaat Mühendisliği Ana Bilim Dalı / Zemin Mekaniği ve Geoteknik Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2015
Konu: İnşaat Mühendisliği = Civil Engineering
Dizin: Zemin ankrajları = Soil anchors ; İnşaat mühendisliği = Civil engineering

Özet:
Çalışma prensibi olarak, aktif ve pasif olmak üzere iki ana gruba ayrılabilen ankrajların taşıma gücü bu tezin kapsamında esas alınmıştır. Ankrajlar ile ilgili geniş bir literatür taramasına da yer verilen tezin içerisinde sırasıyla, tarihçesi, farklı ülkelerdeki tarihsel olarak gelişimi, yapısal sistemi, malzemesi-imalat yöntemi-çalışma prensibine göre ankraj tiplerinin incelenmesine yer verilmiştir. Tezin asıl kapsamını oluşturan ankraj taşıma gücünün, farklı tip ankrajların farklı tip zemin veya kaya içerisindeki hesapları ve hesap yaklaşımları ile ilgili geniş bir araştırmaya yer verilmiştir. Bununla birlikte, imalat yöntem ve tekniklerinin taşıma gücüne etkisi ile ilgili yapılan araştırmalar ve sonuçlarında bulunan tasarım varsayımları da bu tezin kapsamında incelenmiştir.
Bu tezin hazırlanması esnasında yapılan çalışmaların amacında, değişik karakterlerdeki zeminlerde imalatı yapılan ankrajların imalat yöntemi ve tekniklerinin taşıma gücüne yansıtacağı etkisi yatmaktadır. Tez kapsamında, uygulanması planlanan derin kazı projesi için sahada uygulanan germe deneyleri ile birlikte farklı yöntem veya teknik kullanılarak imal edilen ankrajların taşıma gücü tayinleri yapılmış ve karşılaştırmalı olarak değerlendirilmiştir. Bununla birlikte, Tezin literatür araştırması kapsamında incelenen taşıma gücü ile önerilen hesap yaklaşımları kullanılarak teorik taşıma gücü hesaplanmış ve saha sonuçları ile birlikte değerlendirilmiştir.
Sahada gerçekleştirilen uygulama, İstanbul'da yapılan projeye ait kazının desteklenmesinde kullanılacak olan zemin ankrajlarının taşıma gücünün tespit edilmesi amacıyla, imalatı yapılan deney ankrajlarına ait deneyleri kapsamaktadır. Yumuşak-Orta Katı, Orta Katı-Katı, ve Çok Katı kil tabakaları içerisinde gerçekleştirilen deneme ankrajları imalatında; geleneksel yöntem, ard enjeksiyon (Tube a Manchette) ve ard enjeksiyonlu tek delgi içerisinde yer alan çoklu ankraj (SBMA) teknikleri kullanılmıştır. Saha deneyleri sonucunda, mekanik özellikler açısından (Kayma Mukavemeti, Deformasyon Modülü vb.) düşük olan zeminlerde Tube a Manchette ve SBMA tekniğinin ankraj taşıma gücüne olan etkisi araştırılmıştır.

Abstract:
From past till today, different purposed structures have been built under the discipline of civil engineering. Bridges, highways, viaducts, dams, coastal and harbor structures, buildings and other related structures are the examples of such structures within the frame of civil engineering. These structures which have different intended use possess an important common property. They transfer the loads applied on them by statically, by earthquakes, by winds or other external forces safely to the soil or rock layers beneath them. Soil mechanics and Geotechnical Engineering come into stage at this phase and analyses the soil -structure relationship. Generally, loads transferred from super structures form compression forces on soil or rock interface, some may form tension forces. It is not possible for a soil layer or rock layer to resist applies forces directly. Although, change in geometry of foundation underneath a building overcome the tension forces; for long electricity transmission towers, wind turbines, high-rise buildings or dams tension forces which will react on soil is inevitable.
Anchor whose origin dates back to screw piles in 1800's, is a soil technology that has an important place in civil engineering world by transferring tension forces of structures to the soil. It was officially first used in Algeria to strengthen the Dam Cheurfas and has become widely used parallel to the developments in civil engineering era. Especially after Second World War, countries which had been affected from the war were rebuilding themselves and started to open excavation pits under ground level to meet the needs. In order to prevent damages of these pits to the near facilities such as buildings or highways and to construct safely, these excavation pits had to be supported. Generally steel-prefabricated systems which have been still used today were being utilized as a support system. This application which can also be named as internal support brought some difficulties and limitations regarding the progress of excavation. Moreover, it affacted the construction stages of super-structure and made the process more complicated.
Anchors which have high tensional capacity and are capable of transferring subjected tensional forces to soil or rock layers have been evaluated as an alternative method for the solution of support system in excavations. Particularly, when difficulties of internal supports and their unfavorable costs are considered anchors have become more attractive. In 1958, anchors used for a successful excavation in Munchen/Germany proved the performance and easy use of anchors compared with the internal support system and contributed a lot to the spreading to world. Germany, France, Austria-Switzerland and England are the main countries in Europe which have used and developed ground anchor technology parallel with developed machine-materials technology. In addition to developments on anchor production, same countries have also contributed to advances on ground anchor design, such as increasing anchor capacity and preparing of standards-regulations. Ground anchor technology which was an origin of West Europe, have been used by countries in different region of worlds; USA, Japan, Australia etc.
Ground anchors are load bearing elements which are made of steel tendons as a base material and can be installed in unsuitable soil conditions with any directions. In addition to this description, lots of definitions, terms are given in literatures by different researchers. In the third part of this thesis, ground anchors are examined with their definitions and parts which are anchor head, free length and fixed length. Moreover, terms of active and passive anchor are defined comparatively including also their advantages/disadvantages of each. Ground anchors are divided into two main groups according to their service life; temporary and permanent anchors. In addition to this, degree of risk after anchor will fail also affect definition of anchor as a permanent or temporary. In this part, definition of temporary and permanent anchors in accordance with different standards, regulations (PTI, SIA 191, TA'95, BS8081, EN 1537) are also considered.
In the scope of this thesis, bearing capacity of anchors which can be divided into passive and active with respect to their working principle are considered. By including a wide range of literature review, the content consists of historical development, structural system and researches to study anchor types changing with respect to material-production-working principle respectively. Calculation methods and approaches of anchor bearing capacity which constitutes the main scope of this thesis for different types of anchors and soil conditions are investigated. Furthermore, researches done for examination the effects of construction methods and techniques on bearing capacity and design assumptions derived from the results are also examined within the scope of this thesis.
In the first years which soil or rock anchors were began to be used, lack of specialized firms and experiences made it unavoidable to fail in this technology. This enforced countries such as America to prepare standards and specifications. At the beginning, these documents which had been made up in the lights of experiences of specialized firms were providing guidance. In the beginning of 1970's researches were mainly based on theoretical studies such as bearing capacity and load transfer mechanism of anchors. Results of these studies took their places on these standards and specifications and further provided a guidance on application methods and design of anchors. BS8081:1989, PTI 2004, DIN 4125 are the main ones which are examined under the scope of this thesis.
The studies in this thesis mainly focus on the determination of anchor capacities in different type of soils and effects of production methods on bearing capacities. The main indicative factor for bearing capacity of ground anchors is the grouting methods which are categorized in four types. Moreover, grouting methods are also determined in according to soil type where anchor is installed. Three main groups are examined for bearing capacity of ground anchors with regards of soil type; cohesionless soil, cohesive soil, and rocks. In these examinations, calculation or determination of bearing capacity of ground anchors with respect to different approaches including theoretical, statistical researches are considered.
There are four different failure system which can be available for ground anchors; failure of soil mass, failure of interface between soil and grout, failure of interface between grout and steel (bar or strand or wire), failure of steel or anchor head. Design of each issues will be performed with respect to safety factors defined in standards/regulations or by researchers. Moreover, service life of anchors (temporary or permanent) or degree of risk by failure of anchor are criteria which can affect safety factors.
In the fifth section of thesis, detail information about post grouting type anchors and special anchors (SBMA, single borehole multiple anchor) are given. Post grouting anchors can be preferred on soil types which are high plastic clay, marn, loose silt or highly fractured rocks. By application of post grouting, bearing capacity of ground anchors which are low for such soils, will be increased. In addition to this, post grouting can be preferred for anchors installed already and failed during stressing process especially when they do not pass the criteria as creep. Moreover, effect of post grouting to bearing capacity of ground anchors is also examined in this section by considering time of post grouting process. Advantage of post grouting process on bearing capacity of ground anchors by showing an example of site application in France is also shown in this section.
SBMA type anchors were invented based on non-uniform bond stress occurred during tensioning of anchors. Non-uniform shear stress between interface of grout and ground, mobilizes due to fact that soil-grout-steel do not show same elasticity behaviour under loading. Since 1970, a lot of investigations and researches about this issue have been performed and it has been concluded that shear stresses are not uniform such as assumption taken for conventional anchor design. While conventional anchors have single unit (bond), SBMA type anchors have multiple unit that is generally shorter than conventional's. At the stage of initial loadings, stresses are concentrated on top part of anchor bond which are near to discrimination of free and fixed part of anchors. By increasing of loading, stressed part of anchor bond is shifting to the end of bond and anchor bond will be failed at the end which is also described as progressive debonding. In according to investigations more than 60, conventional anchor design was developed by using efficiency factor which is depends on length of bond. Nowadays, it is possible to achieve higher loads; 1000-1500 kN by installation of SBMA type anchors in weak soils.
In the sixth section of thesis, the case study includes the experiments to determine anchor bearing capacities performed on trial anchors executed for the support of the excavation of a project located in İstanbul. In trial anchors performed in soft-medium clay, medium-hard and stiff clay layers, conventional method, post grouted (Tube a Manchette) and SBMA (Single Bore Multiple Anchors) methods were utilized. Mechanical properties of weak soils were determined as a result of site investigations (i.e. shear strength, deformation modulus etc.) and effect of Tube a Manchette and SBMA techniques on anchor bearing capacity is examined.
As a result of analysis done with case study given in this thesis, SBMA and post grouting type anchors provide significant advantage instead of conventional anchors in soft-medium clayey soil. It is also concluded that performing of trial tests are useful to determine design of ground anchor with theoretical calculations.