Dairesel enkesitli şaftlara gelen yanal toprak basınçlarının sayısal analizlerle incelenmesi
Investigation of lateral earth pressure from circular section shaft by numerical analysis
Yazar: MEHMET ÇERİ
Danışman: PROF. DR. RECEP İYİSAN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / İnşaat Mühendisliği Ana Bilim Dalı / Zemin Mekaniği ve Geoteknik Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2020
Konu: İnşaat Mühendisliği = Civil Engineering

Özet:
Şaftlar, yatayla 45 ila 90 derece arası eğimi olan, diğer yeraltı yapılarına bağlantılı ve özellikle düşeydeki uzunluğu yataydaki uzunluğundan fazla olan yeraltı yapıları olarak tariflenebilir. Şaft yapıları, karayolu ve metro tünelleri, köprü ankraj blokları ve maden ocakları vb. mühendislik uygulamalarında geçici veya kalıcı olarak kullanılmaktadır. Şaft yapılarının tasarımı genellikle dikey kemerlenme etkisinden faydalanmak için dairesel enkesitte olmaktadır. Bu sebeple dairesel en kesitli şaftlarda yanal toprak basınçlarını plastik denge ile belirlemek için konik göçme yüzeyine dayalı çözümlerin yapılması daha gerçekçi sonuçlar verecektir. Fakat günümüzde özellikle kalıcı şaft kaplamalarına etkiyen yanal toprak basınçlarının hesaplanması için klasik toprak basıncı teorileri tercih edilmektedir. Bu teoriler düzlem gerilme koşulları altında bulunan sonsuz uzunluktaki duvarlar için geliştirilmiş olduklarından şaft kazıları için kullanılmalarının çok uygun olmadığı açıktır. Bu sebepten dolayı birçok araştırmacı dairesel şaft duvarına etkiyen yanal toprak basınçlarının hesaplanması için yaptıkları teorik ve deneysel çalışmalar sonucunda klasik toprak basıncı teorileri dışında yeni yaklaşımlar geliştirmişlerdir. Bununla beraber gelişen bilgisayar ve yazılım olanakları sayesinde sonlu elemanlar metodu kullanılarak yapılan çözümler sayesinde daha hızlı ve gerçek durumu daha iyi yansıtan değerler hesaplanabilmektedir.
Bu çalışmada dairesel en kesitli şaftlarda yanal toprak basınçlarını plastik denge ile belirlemek için konik göçme yüzeyine dayalı yapılan çözüm yaklaşımları kısaca açıklanmış ve akademik çalışmalardan yararlanılarak bu yöntemlerin birbirleri ile klasik yöntemlerle ve santrifüj testleri ile karşılaştırılmaları verilmiştir. Bu çalışmasının sayısal analiz kısmında hem kohezyonlu zeminler için hemde granüler zeminler için elastisite modülünün yanal toprak basıncına ve şaft duvarı arkasında meydana gelen yerdeğiştirmelere etkisini incelemek için sonlu elemanlar analizleri yapılmış ve elde edilen sonuçlar hem klasik toprak basıncı teorileri ile hem de eksenel simetrik koşullar için geliştirilen yöntemlerlerden elde edilen değerler ile karşılaştırılarak zemin kohezyonunun, kayma mukavemeti açısının ve elastisite modülünün dikey kemerlenme üzerindeki olumlu etkisi ortaya koyulmuştur. Bu yapılan çözümlemeler ve karşılaştırmalardan elde edilen sonuçlardan şaft duvarında meydana gelen yanal toprak basıncının mertebe olarak sukunetteki toprak basıncına çok yakın olduğu tespit edilerek, şaft yapısına ait yükseklik ve çap ile orantılı ampirik bir kemerlenme katsayısı çalışması yapılarak şaft duvarına etkiyen yanal toprak basıncını sonlu elemanlar çözümüne yakın mertebede veren bir denklem geliştirilmiştir. Bir sonraki aşamada sabit bir metro şaftı modeli için zemin koşulları belirlenerek, kazı destek sistemi elemanlarının özelliklerinde ve şaft geometrisinde yapılan değişikliklerin şaft duvarında meydana gelen yanal toprak basıncına etkisi incelenmiştir. Son olarak kazı destek sistemindeki yatay destek elemanlarının dikey kemerlenmeye katkısı incelenerek kuşak kirişlerin şaft kazı destek sistemlerinin tasarımına olan ekonomik etkisi ortaya koyulmuştur.

Abstract:
Shaft structures can be described as underground structures with a slope of 45 to 90 degrees from horizontal, with a minimum cross-sectional area of 10m2 connected to other underground structures, and in particular, the length in the vertical is more than its horizontal length. Shafts are used in many underground structures such as approach shafts of tunnel structures, staircase, elevator and material shafts of tunnel type subway stations, long-distance road and railway tunnels ventilation shafts, mines, hydroelectric power station pressure shafts and nuclear and chemical waste storage areas. While approach tunnels are used in projects in open areas outside of the city to descend to the tunnel level, it is necessary to descend to the tunnel level by vertical shafts in order to excavate the tunnels of metro and other transportation structures, especially in urban tunneling. In the design of these shafts, the sections are generally circular or elliptical in order to produce more economical solutions by making use of the vertical arching effect of the ground. The majority of shaft excavations are conventional excavations using mechanical excavation or drilling and blasting methods. Besides the classical excavation method, there are also excavations made with SBM (Shaft Boring Machine) in the world for deep shaft excavations. Circular excavations up to 1000 m can be done with these machines.
It is used as an excavation support system of vertical circular shafts, generally with vertical support systems formed with reinforced concrete bored piles, shotcrete and soil nail systems, as well as horizontal support systems consisting of reinforced concrete or reinforced concrete and steel composite beams formed together or in areas with water problems such as alluvial soils and seafill areas diaphragm wall excavation support systems are preferred. For both the design of the excavation support systems created with any of these systems and the design of the outer shear wall of the permanent reinforced concrete structures to be built after the excavation, the lateral soil pressures acting on the shaft wall are required to be determined correctly. In this study, the lateral soil pressure calculations affecting the walls of circular metro shafts made by conventional method will be examined. Many researchers have suggested using new approaches other than classical soil pressure theories as a result of their theoretical and experimental studies to calculate lateral soil pressures affecting shaft walls. The soil pressure in the cylindrical vertical wall in the literature has been studied by many researchers; The main of these researchers are Terzaghi (1943), Berezantzev (1958), Prater (1977), Abel (1979), Lade et al. (1981), Wong and Kaiser (1988), Fujii et al. (1994), Hagiwara et al. (1994), Fujii et al. (1996), Ueno et al. (1996) and Imamura et al. (1999), Cheng and Hu (2005), Cheng et al. (2008) Liu and Wang (2008), Liu et al. (2009) and Tobar (2009).
These researchers have proposed various design approaches and theories based on hypothetical slip lines, convergence-limiting method and boundary balance methods based on centrifuge tests However, with the developing computer and software possibilities, it has become possible to make ideal solutions by providing balance, stress-strain behavior, continuity and boundary conditions in the analysis made by the finite element method. Therefore, lateral soil pressures acting on shaft walls with circular and elliptical sections can be calculated quickly and closer to ideal.

In this study, the lateral soil pressure values calculated in the finite element solution made by creating a two-dimensional shaft model in the axially symmetrical situation were calculated very close to the lateral soil pressure value at rest. This is the natural result of the vertical arching effect due to the cross-sectional feature of the cylindrical shaft structures. Due to the arching effect occurring on the ground, the displacements occurring behind the shaft wall are at very small levels.
The aim of this study is to determine the lateral soil pressures used in the design of temporary and permanent walls of circular shaft structures in the most accurate way and to use them in analysis. In addition, another purpose of this thesis is to give geotechnical engineers an idea about the preliminary design of temporary excavation support systems to be used in circular shaft excavations, in which case, should be done in 3-dimensional numerical analysis and in which case, 2-dimensional numerical analysis should be done.
This study consists of five chapters. The first part is introduction part. In the second part of this study, the most common classical soil pressure theories used in calculating static soil pressure are examined by accepting plane strain. In the third section, the solution approaches based on the conical collapse surface for the lateral soil pressures in circular cross-section shafts, comparing these approaches with each other with classical methods and centrifuge tests were made using academic resources. In the fourth chapter, the finite element method and PLAXIS 2D 2019 program are introduced. In addition, in this section, soil models are given to model the ground in the most accurate way. In the fifth chapter, in the numerical analysis section, the classical soil pressure theories for a circular subway shaft with a depth of 30 m and a diameter of 10 m, soil pressure theories developed for axial symmetrical problems and the lateral soil pressure values affecting the cylindrical shaft wall with Plaxis2D 2019 finite element software were calculated and compared. In these studies, a two dimensional shaft model was created in the axially symmetrical situation, and the effects of the cohesion value of the ground, the angle of shear strength and the elasticity module on the lateral soil pressure on the wall and the displacements on the back of the wall were investigated.
These studies showed that the increase in cohesion, internal friction angle and modulus of elasticity increased vertical arching, therefore, lateral soil pressure affecting the shaft wall and displacement values behind the wall decreased. In the empirical study conducted by taking into account the displacement and soil pressure values in these studies, taking into account the vertical archification effect, the calmness was reduced in the soil pressure values and values close to Plaxis2D results were obtained. After this stage, the characteristics of the excavation support systems used for the shafts such as pile diameter, pile socket length, number of belt beams and the effects of the shaft radius on the lateral soil pressure were examined and lateral soil pressure was not affected by these changes unless the collapsing situation occurred. Also in this study, it was observed that although the lateral soil pressure affecting the circular shaft walls is in the degree of soil pressure in the calm, the ground around the shaft can keep itself highly dependent on the ground cohesion due to the vertical arching caused by its geometry. Therefore, it has been revealed that designs of shaft temporary excavation support coatings can be chosen for more economical but arching effect instead of normal and bending stiffness (EA, EI) high retaining elements.

When examined the articles and sample shaft reinforcement shear wall reports within the scope of this study, it has been observed that the lateral soil pressure values affecting the shaft wall are generally taken with active soil pressure acceptance, but since it can be seen from the analysis outputs, a lateral displacement value that will ensure that the active soil pressure is never formed on the grounds around the shaft has not been involved therefore, it would be a more correct approach to consider the permanent reinforced concrete wall of the circular shafts and the underground circular silos during the design. The formation of the numerical models of shafts, along with the shaft structure, is also very important for the tunnels to be manufactured. If the ground cohesion on which the shaft will be built is low and cannot provide sufficient vertical arching effect, this situation can be understood by the lateral deformations that occur during the 2D analysis, it will be a more realistic solution to design the shaft and the tunnels planned to be excavated together with the 3D analysis.