Zayıf jeolojik ortamlarda (İstanbul metrosu) sığ ve çoklu yeraltı açıklıklarının neden olduğu yüzey deformasyonlarının kestirilmesi

Abstract

Artan kent nüfuslarının, kentlerdeki ulaşım problemleri üzerinde büyük etkisi olduğu açıktır. Bu durumu çözebilmek için pek çok gelişmiş kentte toplu ulaşım çözümleri yeraltına taşınmaktadır. Bu sayede daha fazla kişi uzak mesafelere hızlı ve ekonomik olarak kent trafiğini zorlamadan taşınırken ulaşım konforu da beraberinde arttırılmaktadır. Ancak, metro hatlarının inşaatı sırasında birbirine yakın çoklu kazıların yapılması kaçınılmaz hale gelmektedir Özellikle sığ derinliklerde zayıf ortam koşullarında açılan kentiçi yeraltı kazıları, kent altyapısında ve yüzeydeki yapılarda hasarla sonuçlanan sorunlara neden olabilmektedir. Bu sorunlar, proje maliyeti ve çevre sağlığı ve güvenliği açılarından ciddi yükleri de beraberinde getirmektedir. Bu riskleri en aza indirmek amacıyla, son yıllarda modern teknolojik imkanlar kullanılarak, kazı aynasında ve kazı çeperindeki örtü yükünü dengeleyen makinalı (EPB-TBM) kazılar yapılmakta, kazıların yüzeye yansıyabilecek etkileri önceden belirlenen noktalarda yapılan periyodik ölçümlerle denetlenmektedir. Kazı bağımlı oluşan düşey deformasyonların, tekil kazılar ile olan ilişkisi değişik jeolojik ortamlar için literatürde geniş olarak ele alınmıştır ve konuyla ilgili ampirik ve analitik çözümler önerilmiştir. Ancak uygulamada kent içi ulaşımına yönelik çift hatlı, birbirine paralel (özellikle metro tünelleri) ve yakın mesafelerde açılmakta olan tüneller için, bilinen yaklaşımların geçerli olmayabileceği sıkça yaşanan vakalardan anlaşılmaktadır. Tünellerin açımı sırasında çoğu zaman teorik açıdan benzer ikiz tünel kazılardan biri önden diğeri arkadan yürütülmektedir. Proje karakteristikleri nedeniyle kazılar arası yatay mesafe olabildiğince sınırlı tutulmaktadır. Dolayısıyla bu tür çoklu kazılar birbiri ile gecikmeli de olsa etkileşmektedir. Yapılan çalışmalar, önde yürütülen kazının zayıf ortamlarda örselenmeye neden olduğunu, dolayısıyla arkadan gelen tünelin açıldığı ortam aynı olsa bile, teorik olarak önden ilerletilen kazı nedeniyle örselendiği, bu nedenle de ortamın jeomekanik parametrelerinde negatif yönde bir değişimin olduğunu göstermektedir. Arkadan gelen tünelin önde yürütülen tünelin etki alanı içerisinde kalması durumunda daha zayıf bir ortamdan geçmesi söz konusu olacaktır. Bu durum tünel kazılarına bağlı gelişen yüzey oturma formları üzerinde de etkiye sahiptir. Genel şekliyle birinci tünelin kazısı sonrası yüzeyde oluşan form Gauss eğrisi ile yakın benzerlik göstermektedir. İkinci tünelin kazısından sonra bu form,eğrinin birinci tünele yakın olan tarafı ile ikinci tünele yakın olan tarafı üzerinde farklılaşarak asimetrik bir görünüme kavuşmaktadır. Bu sonuç, tünellerin yüzeyde oluşturdukları etki alanlarının birinci ve ikinci tüneller için farklı bölgeleri etkilediğini göstermektedir. Tez çalışmasında, İstanbul ilinin Avrupa yakasında açılmış ve açılmakta olan Kirazlı-Olimpiyat-Başakşehir ve Ataköy-İkitelli Metro Hatları üzerindeki 6 farklı lokasyon incelenmiştir. Bu lokasyonların araştırma için seçilmelerindeki temel neden, bölgelerde zayıf zemin şartlarının geçerli olması ve sığ tünel kazılarının neden olacağı yüzey deformasyonlarının kentsel alanda hasar riski oluşturmasıdır. İnceleme çalışmasında, EPB makinaları (Earth Pressure Balanced Machine) kullanılarak, aynı hat üzerinde birbirine yakın ve sıralı olarak açılan ikiz tünel kazılarının yüzeyde neden olduğu deformasyonların zaman ve kazı aşamasıyla ilişkisi değerlendirilmiştir. Çalışmada bu hatlarda, yapılar açısından hasar riski yüksek, sığ yeraltı kazılarının yüzeyde neden olduğu oturmaların güvenilir düzeyde kestirimi için uygun yöntemin belirlenmesi ve yüzey oturma teknesi formunun ikincil tünel kazılarından sonra nasıl değiştiğinin ortaya çıkarılması amaçlanmıştır. Bu bağlamda, izleme verileri (projelerden elde edilen yüzey deformasyon verileri) ile iki ve üç boyutlu (2D, 3D) sayısal analiz yönteminden elde edilen sonuçlar birlikte değerlendirilmiştir. Tez çalışması kapsamında gerçekleştirilen tüm analizler birlikte değerlendirilmiş ve karşılaştırılmıştır. Sonuç olarak, birbirine yakın, sığ ve zayıf jeolojik ortamlarda aynı kotta açılan çoklu kazıların etkilediği alanların kısmen de olsa çakıştığı sonucuna varılmıştır. Birinci tünellerin ortamda yarattığı örselenmeden dolayı, ikinci tünellerin yüzeyde oluşturduğu deformasyon etki alanının büyüdüğü ve bu etki alanının oturma teknesi formunda değişikliğe neden olduğu anlaşılmıştır. Erişilen bu durum yeraltı kazılarının yüzeyde neden olabileceği yerdeğiştirmeleri izleme amacıyla oluşturulacak izleme projesi için teknik ve önemli bir temeldir. Ortaya çıkan bu negatif etkinin azaltılabilmesi için, özellikle kentiçi zayıf zemin koşullarının hakim olduğu bölgelerde gerçekleştirilen çoklu yeraltı kazılarında, proje güzergahının jeolojik/jeoteknik parametrelerinin dizayn aşamasından önce detaylıca incelenmesi ve belirlenmesi, tünelcilik yönteminin bu parametreler nazarında seçilmesi ve şekillendirilmesi, yeraltı kazılarına bağlı yüzeyde gelişebilecek olan etkileşim bölgesinin izlenmesi adına oluşturulacak olan sistemlerin detaylıca tasarlanması ve yeraltı kazılarının teknik parametreler dahilinde dikkalice takip edilip kayıt altına alınması gerekmektedir.

It is obvious that increasing city populations have had a significant impact on transportation problems in big cities. To solve this problem, public transportation infrastructure is being moved underground in many developed cities, and multi-line metro tunnel solutions are being reviewed. In this way, the comfort of transportation is increased and more people can travel long distances quickly and economically without forcing an increase in city traffic. Urban underground excavations, especially in weak geological conditions at shallow depths, can cause problems that result in damage to the city's infrastructure and surface structures. These problems are a serious burden to the city in terms of the budget and the health of living creatures existing within it. The structures located on the surface are subject to different movements depending on how the surface settlement trough deforms; these include extension and hogging over the convex parts of the settlement trough, or compression and sagging over the concave parts. In general, a structure located near a tunnel under construction can expect to be subjected to the following movements: uniform settlement (or heave), differential settlement (or heave) between supports, overall or differential rotation, overall horizontal displacement, and differential horizontal displacement in compression or extension. To minimize these risks, earth pressure balanced tunnel boring machines (EPB-TBM) have been employed in recent years using modern technology ; the face and wall loads are balanced and the effects of the excavations that are reflected on the surface are controlled by periodic measurements with the help of built-in monitoring points. With the use of such mechanized excavation equipment, it is possible to excavate and support long project lines more economically and quickly in urban areas. By using modern technology and geotechnical monitoring surveys, surface deformations that may occur as a result of the excavation can be kept within more controllable limits. In contrast to these advantages, the high initial investment costs due to the use of modern technology and the cost of maintenance repair works encountered during the operation phase are listed as disadvantages. The relationship between excavation-dependent vertical deformations and single tunnel excavations has been widely discussed in the literature under different geological conditions, and empirical and analytical solutions have been proposed. However, in practice, there are some cases where these known approaches may not be valid, such as double-tube tunnels, parallel tunnels (especially subway tunnels), and tunnels that are constructed at close distances for urban transportation. During the opening of twin tunnel excavations, theoretically, one of the excavations is usually advanced forward and the other behind. Due to the characteristics of the project, the horizontal distance between excavations is kept as limited as possible. Therefore, multiple excavations can interact with each other, albeit in a delayed manner. Studies show that the excavations carried out at the front can cause disturbances in weak geological conditions. Therefore, even if the second tunnel was opened in the same strata as the first tunnel, the environment could theoretically have been damaged by the forward excavation, and there could therefore be negative influences on the geomechanical parameters of the geological setting. The tunnel coming from behind would stay within the impact area of the forward tunnel and would therefore pass through a weaker environment. This situation also affects the development of surface settlement forms due to tunnel excavations. In terms of the general structure, the form of the settlement trough formed on the surface after the excavation of the first tunnel closely resembles a Gaussian curve. After the excavation of the second tunnel, the symmetrical form of settlement trough will transform on the side of the curve close to both the first and second tunnels, resulting in an asymmetrical shape. This result suggests that the areas that are impacted as a result of twin tunnel excavations are different for the first and second tunnels. In this thesis, six different locations on the Kirazlı-Olimpiyat-Başakşehir and Ataköy-İkitelli Metro Lines that have either been completed or are still under construction in the European side of the city of Istanbul were examined. The main reason for choosing these parts of the subway line for this Ph.D. thesis is that the weak ground conditions that dominate the region and the surface deformations caused by shallow tunnel excavations pose a risk of damage to the urban area. In the study, the relationship between the deformations on the surface induced by the twin tunnel excavations, which were opened close to each other and sequentially on the same line using EPB machines, was evaluated. It is aimed at determining an appropriate method for the reliable prediction of settlements induced by shallow underground excavations on these lines that pose a high risk of damage to urban structures. In this context, the monitoring data (surface measurement data) and the results obtained from numerical analyses were compared and evaluated together. The surface monitoring data consist of values obtained as a result of setting a series of measurement points on the project route and recording them at certain intervals. Within the scope of the thesis, the surface and building settlement bolts obtained from the examined projects were used in the deformation analysis. In addition to the surface monitoring survey, data were obtained from supplementary instruments such as extensometers, inclinometers, piezometers, load cells, and strain gauges, which are used in general tunneling applications. Numerical method analysis (finite element method) was carried out using software actively used by researchers in the design and analysis of underground structures modeled for a heterogeneous geological environment. The software can be grouped into two categories: two-dimensional or three-dimensional models. In two-dimensional studies, a planar model is created based on the horizontal and vertical axes. In three-dimensional model studies, it is easier to understand how the points determined on the x, y, z axes behave as a whole as a result of the excavation and support activities applied during the study of the model. These model studies require more expertise to operate. Both methods are based on the principle of calculating a created model by dividing it into smaller parts. When solving problems using models based on the finite element method (FEM), equilibrium conditions, conformity conditions, material structure behavior, and other boundary conditions must be taken into consideration. In this thesis, a series of modeling studies using numerical methods were carried out and analyzed in theoretical terms. In these studies, the settlement trough profiles formed on the surface of twin tunnels opened with the same diameter in different positions from each other and different positions to the surface. The effects of the change in the elastic modulus of the geological environment on the surface settlements were examined with the help of two-dimensional analysis, and the effect of twin tunnel excavation steps on each other was examined by three-dimensional analysis. In addition, the time-dependent change of the twin tunnel excavations above the groundwater level was examined in two dimensions. From a practical point of view, the most important outputs of this study were the following: in surface settlement troughs formed by the excavation of twin tunnels that are in different positions to the surface and each other, it is understood that the cumulative deformation reflected on the surface decreases as the distance between the tunnels increases. In addition, by changing the modulus of elasticity, it was observed that the settlement values of the surface increased when the elasticity modulus values of the geological strata decreased. Three-dimensional excavation modeling studies show that the disturbances induced by starting the excavation of the first tunnel can disrupt the geological strata of the second tunnel, even if the excavation of the second tunnel has not yet started. Analyses using the data obtained from the monitoring survey carried out during the thesis were evaluated by processing the section lines that were considered appropriate for the locations along the subway line. By using a two-dimensional model in conjunction with the section lines, the surface settlement trough forms were determined, and the inflection points (i) of the Gaussian curves and the surface trough width parameter (K) were estimated from the data analysis. Subsequently, tunnel excavations were carried out sequentially using FEM analysis in accordance with the site conditions. The surface settlement curves were obtained and the same parameters were determined from the surface settlement forms and compared to the values obtained from the real field values. As a result of the evaluations, it was determined that when Z0/D (Depth to tunnel axis level/Tunnel diameter) < 3.9, the effect that the twin tunnel excavations had on each other was more noticeable at shallow depths, and the value of K obtained for the first tunnel increased as the depth increased. The settlement profiles derived from the monitoring data were compared to the results from a two-dimensional FEM analysis. Although there were small differences between the trough width parameters of the FEM analysis and the monitoring data from shallow (Z0/D < 3.9) twin tunneling in weak subsurface conditions, changes in the asymmetrical shape of the settlement profiles were similar after the second tunnel had been excavated in both cases. It was also found that when the Z0/D value was 4.8, the reflection of the actual deformation due to tunnel excavation at the surface was less than that suggested by the results obtained using the FEM. It is recommended that this phenomenon is examined using additional datasets and that it be taken into consideration by researchers and designers during the project design phase. Collective evaluation of all the data showed that the FEM solution might be one of the most powerful tools for predicting the surface settlement form induced by multiple excavations at shallow depth. From our results, it was evident from the asymmetrical form of the settlement curve after the excavation of the second tunnel that the trough width parameter that corresponded to the right side (toward the first tunnel) of the second tunnel is higher than on the other side of the tunnel. At shallow depths, the areas affected by tunneling that are associated with multiple excavations at the same level and in weak geological strata can overlap partially in time. The expansion of the surface area that is affected by the second tunnel is considered to be an adverse reaction to the disturbance caused by the first tunnel. The form of the Gaussian curve that developed as a result of the tunnel excavations differed between the first and second tunnels, and this situation should be taken into account when predicting the extent of the area that is affected by tunneling. Considering that the excavations of subway tunnels in Istanbul, as well as in countries where similar conditions prevail, are likely to continue intensively in the future, it is highly beneficial that issues concerning potential surface settlements are evaluated during the planning stage. It is equally essential to consider the adverse effects of multiple underground excavations during the prediction of damage-risk zones and to establish a monitoring network.