Yeni Avusturya tünel inşa yönteminde sonlu elemanlar yöntemiyle tünel kaplaması hesabı

Abstract

Bu çalışmada Yeni Avusturya Tünel înşa Yöntemi (NATM) ve Sonlu Elemanlar Yöntemi ile tünel kaplamalarına gelen yük lerin hesabı incelenmiştir. Çalışmada önce eski tünel inşa yöntemlerine değinilmiş; bu yöntemlerde kullanılan iksa sistemleri incelenmiştir. Daha sonra NATM'nin gelişimi ve ana ilkeleri verilerek yöntemin üstünlükleri sergilenmeye çalışılmıştır. Yeni Avusturya Tünel înşa Yöntemi'nde kullanılan uygun hesap yöntemleri açıklandıktan sonra, bu yöntemlerden Sonlu Elemanlar Yöntemi ile tünel kaplamalarına gelen yüklerin hesabı üzerinde durulmuştur. PLAXIS adlı sonlu elemanlar paket programı bir tünel profilinde uygulanarak elde edilen sonuçlar değerlendirilmiştir. Sayısal analiz yöntemlerinin uygulanmasıyla tünel açıl dıktan sonra oluşan gerilme durumu ve deforraasyonlar be lirli bir yaklaşıklıkla tahmin edilebilmektedir.

Tunnels form an important part of everyday life. Whether their role is for rail transport, highway traffic, as a subway, for water conveyance, sewer system, for cable transmission or hausing generating plant or for general storage-disposal purposes as arise in civil engineering applications, they serve an important function. Tunnelling experience during the last few decades in Austria and elsewhere has proved the advantage of the "New Austrian Tunnelling Method" (NATM) over the other methods in every respect, particularly for tunnels in unstable rock. NATM has made a significant contribution in tunnelling technique worldwide. In this thesis the New Austrian Tunnelling Method and its main principles have explained. The construction process of the NATM, tunnells are analyzed using the finite element method. The Finite Element Method (FEM) is used in a parametric study of the excavation and the support of a shallow tunnel using the NATM. The New Austrian Tunnelling Method (NATM) constitutes a method, where the surrounding rock or soil formations of the tunnel are integrated into an overall ringlike structure. Thus, the formations will themselves be part of this support structure. The jSew Austrian Tunnelling Method was patented by A.Brunner in 1958 (Austrian Patent No: 197851) and introduced by Professor L.v.Rabcewicz and Professor Leopold Müller (1964) into practice. Its use, and the international discussion about its relative merits, have been increasing ever since. There are many in the tunnelling industry, who still do not quite understand the basic principles of the NATM. VI First the following principles must be observed: The geomechanical behaviour must be taken into account. » Adverse states of stress and deformation must be avoided by applying the appropriate means of support in time. Especially the due to completion of the tunnel invert gives the above mentioned ringlike structure the static properties of a tube. The supports should be optimised according to the admissible deformations. General control measurements and constant checks on the optimisation of the supports must be performed. While driving a tunnel the existing, primary balance of forces in the rock mass will be changed into a new, secondary and also stable state of balance. This can only be achieved through a succession of intermediate stages accompanied by various stress redistribution processes. The New Austrian Tunnelling Method aims at getting under control these transitional processes, while still taking into account economic and safety considerations. The method also aims to check rock deformation on the following lines: i) On one hand, deformation should be kept to a minimum so that the primary state of stability and the compressive strength of the rock are not weakened more than is inevitable. ii) On the other hand, deformation is actually wanted to the extent that the rock formation itself acts as an overall ringlike support structure, thus minimising costs for excavation and supports. Professor Müller has said that there are 22 principles which characterise the New Austrian Tunnelling Method. The method has the following fundamental principles: Careful and cautious excavation procedure: A central feature of the NATM is the high level of understanding and acceptance of the method and the co-operation in decision making and the resolution of disputes that it requires of owners, contractors and design and supervisory engineers. Choice of the best cross-section, while allowing for extensive adaptation to specific rockmechanical and geomechanical conditions such as strength, anisotropy and specific variation in rock and soil strata. Particular adaptation to the primary rock or soil pressure distribution. VII The driving method must be adapted to the rock or soil properties encountered. The stability of the face without supports, the right choice of pilot headings into the face and the rate of advance play an important part, when choosing an operational and economically feasible method. The tunnel system is to be conceived as a compound structure consisting of the rock or soil formation surrounding the cavity and the various r.eans of support such as shotcrete lining, anchors, rock bolts, steel ribs, etc. Whatever support system is used, it is essential that it is placed and remains in intimate contact with the ground and deforms with it. The three dimensional state of stresses and strains that is compatible with the geomechanical properties of the ground, should be maintained as far as possible. A damaging loosening of the original rock or soil structure should be avoided. Rock or soil sample testing should be carried out under laboratory conditions as well as in-situ examinations. The thus gained values of the rock-mechanical and geomechanical properties, their variability in particular long-term changes and also the effects of water inflow must be taken into account. The elasticity of the support structure is important. A certain slenderness of the lining without unnecessary and wasted means of support should be achieved. The supports and the anchors must be installed at the appropriate time and should form a compound structure with the surrounding rock. The timing of the placement of the support and of closing the initial shotcrete ring is of vital importance in controlling deformations and will vary from case to case. The period of the heading without supports, the timing of the completion of the invert arch must be considered as a function of the given rock or soil pressure distribution by taking into account the rheological characteristics of the ground and the advance rate of driving operations. Constant measurements and visual inspection of the ground as well as of the various means of support are an integral feature of the NATM. These measurements guarantee operational safety. Pre-calculated support dimensioning can be compared and if necessary re- dimensioned accordingly during the tunnel driving operation. Thus optimisation of the supports according to the admissible deformations can be VTTT achieved. These measurements not only allow optimisation, as far as operational aspects are concerned, but also provide an objective evaluation of the geomechanical documentation. The inner lining must be dimensioned so as to allow for long-term changes of rock or soil pressure phenomena and the weakening or total failure of the overall ringlike support structure. It is generally accepted that NATM has evolved as a result of experiences and innovation achieved in Austrian Alpine tunnelling conditions..It was originally developed for tunnelling in squeezing rock. The NATM has been recently introduced as an economical alternative to conventional shield techniques for construction of tunnels in cohesive soil. It was used in soils for the first time in West Germany on the Frankfurt Metro (1973). The project consisted of a double tunnel driven in stiff plastic clay. Since then, the method has been applied in a variety of conditions for soft ground tunnelling, most of which are related to metro systems. Design for a NATM project requires information on the moments and thrusts for the lining and settlements at the ground surface. A common analytical method that is used for predicting these parameters is a finite element analysis. In the Finite Element Method (FEM), the subsurface is predominantly modeled as a continuum. Discontinuities can be modeled individually. It entails that the underground structure is approximated by an assemblage of properly selected finite elements. The finite elements are considered interconnected at a finite number of nodal points or joints. These finite elements are discrete elements. Each element is finite, i.e. geometrically defined and limited in size. This characteristic makes for the name of the method, Finite Element Method. With the given joint loading, known geometric configuration, and assumed material properties of the finite elements, the joint node displacements and the internal stresses of each finite element are determined by the application of the Finite Element Method. This requires the determination of the stiffness matrix for selected finite elements that model the problem domaine. Highly complex underground conditions and tunnel characteristics can be analyzed. The capability of the Finite Element Method includes the simulation of complex constitutive laws, non-homogenetities, and the impact of IX advance and time dependent characteristics of the construction methods. Solving of the complex mathematical problem requires a large computer processing and storage capacity. Most Finite Element programs require more program and computer knowledge from the user than other methods do. Also, extensive output is: typically generated that makes comprehension of the results more difficult. In this work, the Finite Element Method is used in a parametric study of the excavation and support of a shallow tunnel using the NATM. The results are cast as graphical outputs and as tables. So they can be easily used and compared. The major assumptions are as follows: The tunnel has a circular shape The ground around the tunnel is uniform soil or rock that behaves as an elastoplastic material. Only short term conditions are examined, which means that both the trained and the time dependent behavior of the soil are not considered. The lining material is an elastoplastic material.