Zemin çivili iksa sistemlerinde öngermeli ankraj kullanılması

Abstract

Bu çalışma, derin kazı destekleme sistemlerinden olan zemin çivili iksa sistemlerinde öngermeli ankraj kullanılmasının mevcut sistem üzerindeki etkilerinin incelenmesi amacıyla hazırlanmıştır.Yapılan çalışmada, iksa sistemlerine etkiyen toprak basınçlarından bahsedilmiş, çeşitli zemin koşulları için öngörülen toprak basıncı dağılımlarına değinilmiştir. İksa sistemi olarak zemin çivili iksa tasarımı ele alınmıştır. Zemin çivisi uygulamasının kökeni ve tarihsel gelişiminden bahsedilmiştir. İksa uygulamaları öncesi yapılması gereken geoteknik incelemelere yer verilmiştir. Zemin çivili iksa sistemlerinin uygulanması için elverişli zemin koşulları anlatılmış ve iksa analizlerinde kullanılacak zemin parametrelerinin belirlenmesine yönelik çeşitli yaklaşım ve korelasyonlardan bahsedilmiştir. Zemin çivisi tasarım yaklaşımları ve tasarımda rol oynayan geometrik değişkenlerin belirlenmesi üzerinde durulmuştur. Zemin çivili iksaların iç stabilite, dış stabilite ve toptan stabilite problemleri ile ilgili bilgi verilmiştir.Çalışma kapsamında, zemin çivili iksa sistemlerinde, ilave olarak öngermeli ankrajların uygulanması durumu ele alınmıştır. Karma yapılar olarak adlandırılan bu iksa sistemlerinin çeşitli örneklerine yer verilmiştir. İki sistem inşaat aşamaları ve çalışma mekanizması açısından kıyaslanmıştır.Bir sonlu elemanlar yazılımı olan Plaxis bilgisayar programı ile İstanbul, Tuzla ilçesinde gerçekleştirilen örnek bir iksa uygulaması vaka analizi olarak incelenmiştir. Plaxis analizi yapılan zemin çivili ve ilave öngermeli ankrajlı iksa kesitlerinin farklı zemin modelleri kullanılarak analizleri yapılmıştır. Öncelikle zemin çivili iksa modelinin analizi yapılmıştır. Daha sonra mevcut zemin çivili modele ilave öngermeli ankrajlar uygulanarak sistem tekrar analiz edilmiştir. Buna göre, öngermeli ankrajın mevcut zemin çivili sistem üzerindeki etkileri değerlendirilmiştir.Öngermeli ankraj yatay aralığının deplasman üzerindeki etkilerinin incelenmesi amacı ile değişken ankraj yatay aralıkları için analizler yapılmıştır. Öngermeli ankrajların sık ve seyrek yerleştirilmesi durumunda, ankraj kademeleri ve bütün sistem için deplasmanda meydana gelen değişim gözlenmiştir. Ayrıca, iksadan uzaklaştıkça düşey deplasmanda meydana gelen değişim incelenmiştir. Değişik kazı kademelerinden alınan kesitler ile iksadan uzaklaştıkça azalan düşey deplasmanın derinlikle değişim grafikleri oluşturulmuştur. Plaxis analizinden elde edilen veriler sahada alınan inklinometre ölçümleri ile kıyaslanmıştır. Analizler neticesinde, sonlu elemanlar yöntemi ile hesaplanan deplasman değerlerinin, sahada ölçülen gerçek değerlere yakın mertebelerde olduğu görülmüştür.

This research is done to examine the effects of the usage of prestressed anchors with soil nailed retaining systems which is commonly used in braced excavations. First, information about lateral earth pressure distribution which plays an important role on shoring systems is given. The origin and the historical development of soil nailed retaining systems are mentioned. Then, the key part of the design of shoring systems; geotechnical site investigation, determination of soil conditions and soil parameters are explained. The design approach of some researchers and the effects of geometrical parameters on design are given. Besides, stability analysis of soil nail walls is presented in details such as internal, external and global stability analysis.Geotechnical site investigation plays an important role before any kind of construction project. It is needed for the first impression of field conditions, such as soil conditions, road and existing building situations. Design of safety and economic shoring system depends on the acceptance of correct parameters and load distribution.Field and laboratory tests can be used as a guide for the determination of soil parameters. Laboratory tests are done on samples taken from field that have no cracked surface or discontinuity. So that, because of representing real conditions better than laboratory results, it can be said that field tests are more trusted than laboratory tests especially for weathered rocks.During the decision of the type of shoring system, many parameters should be evaluated together. It is possible to guess, but not certain to determine the behaviour of the soil behind the wall. So that instrumental analysis play significant role on the observation of soil and related wall behaviour. As a result of the instrumental analysis, datas can be used to check the software analysis done for shoring system design.Soil nail walls are relatively flexible retaining systems and they give a more economical solution compared with the conventional retaining systems. Soil type plays an important role on the cost of the system. The most applicable soil conditions for soil nailed retaining systems are weathered rocks, dense to very dense granular soils and stiff to hard cohesive soils. To apply soil nailing on marginal soil conditions, such as very soft cohesive soils, organic soils or poorly graded cohesionless soils, will be too costly when compared with other techniques.It can be useful to compare soil nail wall with anchor wall which are based on top to down construction method. The differences between soil nail wall and anchor wall can be classified according to the installation equipments, load transfer and load distribution mechanism, design approach and wall behaviour.According to the installation equipments, soil nail walls have relatively rapid construction than anchor walls. Anchor walls require two step construction process; installation of vertical elements (soldier beams) at first, then anchors are installed and pre-tensioned to transfer loads. During the costruction of soil nail walls less construction materials are needed.According to the load transfer and load distribution mechanism, soil nails transfer load along its entire length, but anchors are designed to transfer load just behind the potential failure surface. Besides, load distribution is variable along the entire length of soil nail, while it is variable along the bonded length of anchor and constant in the unbonded length.Global stability analysis of soil nail wall and local stability at each stage of excavations are important during the design of soil nail wall, while the design approach of anchor wall depends on the determination of anchor capacity against to the lateral earth pressure acting on the wall at every excavation stages.If soil nail wall and anchor wall are compared according to the wall behaviour, it can be said that the lateral displacement of the wall is maximum at the midheight of the wall in anchor walls, whereas in soil nail walls it will be maximum at the top of the wall.The determination of geometrical parameters of soil nail wall such as nail length, the angle of nail with horizontal, the angle of the wall and the angle of slope behind the wall have a significant role on the stability of soil nailed retaining system. Especially, the angle of slope behind the wall is important for stability, because if the angle of slope increases, the active earth pressure acting on the wall will also increases.The soil ? nail interaction plays an important role on the design of the soil nail retaining systems. The ultimate bond strength of soil has an effect on the pullout capacity of nail from soil. There is no laboratory test to determine the ultimate bond strength of soil or rock. It can be predicted from field experiences of soil nail applications. Besides, soil nail tension test can be a useful guide in this case. If the adhesion between the nail and soil is not sufficient or nail length is not long enough, it will be inevitable to meet with stability problems.The horizontal displacement of soil nail wall, which is maximum at the top of the wall, decreases from top to down. If the displacement criterion is not provided, extending nail length may be a solution to decrease the displacement. However, the displacement is not with in acceptable limits, application of prestressed anchors especially on the upper part of the wall will be another solution.Within the scope of this research, the usage of prestressed anchors with soil nail retaining systems are examined. These two systems are compared according to their design concepts and working principles. To determine the effects of prestressed anchors on soil nail retaining systems, a deep excavation project in Tuzla, Istanbul which is supported by soil nails is analysed as a case study.With in this project, the depth of excavation changes between 4.5 meters and 18.5 meters. The analysed soil nail wall height equals to 18.5 m. Soil profile can be classified as weathered clay stone and silt stone in the upper zone and slightly weathered clay stone and silt stone in the lower zone.Before the analysis, first step is the idealization of soil profile according to the borehole logs and seismic tests done on the field. Idealized geotechnical model contains three different geological layers; first, filled soil approximately 2 m, then hard clay about 4 m which forms as a result of weathering of clay stone and finally clay stone ? silt stone. Determination of the soil parameters is the second step. The pressuremeter test can be a useful guide for that purpose because laboratory tests give high values that cannot be used in software analysis. On the other hand some correlations can be used for the determination of strength parameters of soil.Plaxis software, which is based on finite elements method, is used for the analyses. Soil nail wall and soil profile is modelled and analysed by Plaxis computer programme. Different soil models are used for the analyses and at two stages anchors are added to the model. As a result of the addition of anchors to the soil nail retaining system, it is seen that the lateral displacement of the wall decreases. The datas obtained from the analyses are compared with inclinometer measurements which represents the real displacements of soil nail wall on field.When the displacements obtained from Plaxis are compared with inclinometer measurements, it can be said that, the degree of the displacements are close to each other. Besides, it is determined that, the reduction in displacement can be seen more significantly from inclinometer results than Plaxis analysis. As a result of Plaxis analysis, the displacement continues decreasingly at the end of the wall. While, from inclinometer results, the displacement decreases significantly at anchor stages and approximately from 15 meters it is close to zero.To determine the effects of horizontal spacing of anchors on the displacement, for different horizontal spacings analyses are repeated. As a result of the analyses, it can be said that to design anchors close to each other decreases the displacement as it is expected. On the other hand, as a result of the analysis of the model in which anchor spacing equals to 6 meters, the degree of the displacement is calculated very close to the soil nail model without anchor. Therefore, it can be said that, from the horizontal spacing of 6 meters, prestressed anchors have lost positive effect on the displacement of the system.The relationship between the horizontal distance from the excavation and vertical displacement is examined as a result of Plaxis analyses. It is seen that, the vertical displacement decreases with depth as the horizontal distance to the excavation increases.The results of the analysis obtained from finite elements method can be useful to give an estimation to the designer before construction. In addition to this, it should not be forgotten that the results depend on the parameters entered to the programme. So that it is important to compare the values obtained from software analysis with the instrumental analysis done on field. Because, to provide the safety of the excavation, instrumental analyses are essential.