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Über dieses Buch

For many years the Austrian tunneling industry has demanded that research is urgently required to establish a theoretical basis for the New Austrian TUnneling Method and to assist site engineers in the often difficult day to day decisions. In particular it was felt that numerical models need to be improved considerably in order to be able to act as useful tools in predicting soil/rock mass behavior during tunneling. The required improvement not only refers to the quality of the models but also to their ease of use. As long as an experienced modeler is required to spend days in preparing the input and in interpreting the results the models will not be useful at the tunnel site. It is heartening therefore that a group of scientists in Austria has come together to attempt to tackle this challenging task. The initiative has been supported in a number of ways by the Austrian tunneling industry. All Aus­ trian companies associated with tunneling sent representatives to the man­ agement advisory board, which ensured that the research carried out in the project, was of benefit to the industry. The Austrian Geomechanics Society sponsored the project with a considerable amount, which was mainly used to cover site costs. HL-AG and OSAG, as well as the joint ventures allowed access to tunnel sites thereby making it possible to test new developments.



Chapter 1. Introduction

Tunnelling plays an important role in the construction industry. Great challenges await us in the new millennium. For example the construction of tunnels through the Alps in order to allow the environmentally friendly transport of goods from the northern to the southern part of Europe involve high overburdens and difficult ground conditions. The planned City of Tomorrow will involve underground space for the transport of people and goods as well as for housing. Some very large projects which are in the concept stage are a tunnel through the Andes in South America with extremely challenging ground conditions and undersea tunnel projects to connect Korea and Japan and Russia and America. In addition there will be increased use of underground space for the disposal of hazardous and nuclear waste.
Gernot Beer

Chapter 2. Tunnelling - the need for technological development and innovation

Tunnelling at a larger scale started in the 19th century with the development of the railway systems. In the beginning the practices used in mining for excavation and support were adopted also for the traffic tunnels. In contrast to tunnelling, in mining only a rather small portion of the underground structures is designed for permanent use. The rock mass conditions in mines are usually well known due to the long experience gained during the lifetime of a mine. This experience of excavation under the same or similar conditions explains also why in mining the layout of the excavations and the required support is mainly determined in an empirical way.
Wulf Schubert, Gernot Beer

Chapter 3. Improved site investigation Acquisition of geotechnical rock mass parameters based on 3D computer vision

Acquisition and evaluation of geotechnical data are integrated parts of subsurface and surface construction works. Geotechnical data serve as input for decision making processes during all phases of projects, ranging from feasibility studies to construction and maintenance. The present system of data acquisition, specifically applied during underground construction works, has a number of constraints. Sampling bias may be caused by the “human factor” of individual capabilities, inaccessibility of the rock exposure and time limitations. In most cases data are irrecoverable when excavation proceeds or support has to be applied. Data processing and evaluation is time consuming so that input data for numerical calculations cannot be provided on a daily basis. To overcome the listed shortcomings a digital stereoscopic colour imaging system has been developed which enables the evaluation of a large number of geotechnical data by interactive two and three dimensional image analysis. Among others the data can be used for the innovative modelling of the rock mass structure, for the provision of geometrical input data for numerical simulations performed on site as well as for a descriptive visualisation of complex structural conditions. The developed hardware and software components have been tested in different environments and on different rock mass types to investigate their general suitability and effectiveness. It was found that digital stereoscopic imaging and image evaluation are suitable for a cpmprehensive and reproducible documentation of the structural inventory of rock surfaces, and are most effective for acquisition of geotechnical data.
Andreas Gaich, Alfred Fasching, Wulf Schubert

Chapter 4. Advanced pre-processing methods

A user friendly computer program is presented for the automatic generation of finite and boundary element meshes. The program is designed to be used by professionals that are relatively inexperienced in the use of numerical simulation software.
Thomas Reichl, Bernhard Mathis, Gernot Beer

Chapter 5. Thermochemomechanical material model for shotcrete

Employing a thermodynamic framework, thermochemomechanical couplings for shotcrete are treated in this chapter. A material model based on multisurface thermochemoplasticity is presented. It accounts for hydration kinetics, chemomechanical couplings related to strength growth, stiffness properties, and to autogeneous shrinkage in early-age shotcrete. Creep is modeled by means of two mechanisms: stress-induced water movement in the capillary pores of shotcrete, and a relaxation mechanism in the nanopores of the cement gel. The underlying material functions are intrinsic, i.e., independent of field and boundary conditions. They are determined from standard material tests. As for the numerical treatment of the constitutive equations of the material model, an extended form of the return map algorithm is presented. Microcracking is considered by means of a Drucker-Prager failure surface for the compressive load regime and by means of Rankine surfaces for tensile brittle failure.
Christian Hellmich, Roman Lackner, Herbert Mang

Chapter 6. Material model for soil and applications

This chapter deals with the development of a realistic material model for the simulation of the mechanical behavior of granular soil. The material model is formulated in the framework of multi-surface viscoplasticity, involving a Mohr-Coulomb criterion for the simulation of compressive loading and a tension-cut-off for capturing the response of soil under tensile loading. Whereas the tension-cut-off is assumed to be ideally-plastic, friction harding is considered in the Mohr-Coulomb criterion. The commonly observed change in the dilational behavior of granular soil during plastic deformations is accounted for by a non-associative flow rule. After presentation of the algorithmic formulation of the proposed material model, its applicability for investigations of real-life structures is demonstrated by means of numerical analyses of two Austrian tunnels, focusing on (i) the effect of ground improvement by means of jet grouting and (ii) the safety of tunnels under fire load.
Roman Lackner, Yvonne Spira, Christian Pichler, Herbert A. Mang

Chapter 7. Damage mechanics of jointed rock in tunnelling

In this chapter, an anisotropic damage model is established in strain space to describe the behavior of jointed rock masses under compression-dominated stress fields. The research work focuses on rate-independent and small-deformation behavior during isothermal processes. It is emphasized that the damage variables should be defined microstructurally rather than phenomenologically for jointed rock masses, and a secondorder “fabric tensor” is chosen as the damage variable. Starting from it, a one-parameter damage-dependent elasticity tensor is deduced based on tensorial algebra and thermodynamic requirements; An equivalent state is developed to exclude the macroscopic stress/strain explicitly from the relevant constitutive equations. Finally, some numerical results are worked out to illustrate the mechanical behavior of this model. In order to eliminate the complexity and arbitrariness in the formulation of phenomenological anisotropic model, systematic micromechanical analysis has been done on damage elasticity, damage evolution laws and micromechanics of different types of microdefects.
Qiang Yang, Gunter Swoboda, Dean Zhao, Franz Laabmayr

Chapter 8. Parameter identification and its application in tunneling

This paper focuses on discussing the parameter identification techniques in geotechnical engineering. Firstly, the general formulation of the parameter identification process is presented. Secondly, the problem of identifiability is discussed and illustrated by an example of identifying the initial damage parameters of a damage model for jointed rocks. Then the algorithms of designing the optimal layout of displacement measurements are proposed, based on the analyses of the well-posednesses of the parameter identification processes with the Gauss-Newton method and the Complex method, respectively. The validities of these algorithms are proved by some academic and applied engineering examples. Finally, the advantages and drawbacks of the gradient-type methods and the direct-search methods are carefully compared.
Zhihai Xiang, Gunter Swoboda, Zhangzhi Cen

Chapter 9. Soft-computing-based parameter identification as the basis for prognoses of the structural behaviour of tunnels

A parameter identification (PI) method for determination of unknown model parameters in tunnelling is presented. The PI method is based on measurement data provided by the construction site. Model parameters for finite element (FE) analyses are identified such that the results of these calculations meet the available measurements as well as possible. For the determination of the unknown parameter set, use of an artificial neural network (ANN) is proposed. The network is trained to approximate results of already performed FE simulations. A genetic algorithm (GA) uses the trained ANN to provide a prognosis for an optimal parameter set which, finally, must be assessed by an additional FE analysis. In contrast to other gradient-free methods requiring a large number of FE simulations, the proposed PI method renders back analysis of model parameters feasible even for large-scale models. Finally, the performance of this PI method as the basis for prognoses of the structural behaviour of a tunnel is demonstrated.
Bernhard Pichler, Roman Lackner, Herbert A. Mang

Chapter 10. Quantification of stress states in shotcrete shells

As reported in Chap. 3, monitoring of deformations which occur during the excavation of tunnels is essential for the success of the NATM. Nowadays it is performed at every NATM construction site. The combination of these monitored displacements with the material model for shotcrete outlined in Chap. 5 has led to the development of a hybrid method for the analysis of closed shotcrete tunnel shells by means of nonlinear Finite Element (FE) analyses. It allows determination of the stress state in the shotcrete shell. Knowing this stress state a level of loading can be computed, amounting to 0% for the unloaded shell and to 100% for shotcrete (locally) loaded up to its compressive strength. The analysis of segmented tunnel shells required further development of this hybrid method. The desire for real-time monitoring of the level of loading led to the development of a shell-theory-based hybrid method avoiding time-consuming FE analyses.
Jürgen Macht, Roman Lackner, Christian Hellmich, Herbert A. Mang

Chapter 11. Compressed air tunnelling - determination of air requirement

In shallow tunnelling below the groundwater table compressed air can be used for preventing water inflow into the tunnel. When using this method air loss takes place through both the unsupported tunnel face and shrinkage cracks of the shotcrete lining. Until today it has been very difficult to correctly estimate the amount of air loss during the design phase of a project, although it could be a significant factor concerning the total costs of the tunnel construction. For solving this problem the multi-phase flow in the soil above the tunnel has to be considered. The aim of the project was to develop a new approach, based on existing design principles and unsaturated soil constitutive models. At the Institute for Soil Mechanics and Foundation Engineering in Graz, large scale laboratory tests were conducted to simulate the air-permeability of the shotcrete lining and the soil. Additionally, the experimental data are compared with results of numerical models. The models are based on existing constitutive laws to describe the mechanical behaviour of unsaturated soils. In this contribution results of the tests are discussed and a methodology is presented to estimate the amount of air loss during tunnel advance.
Stephan Semprich, Yannick Scheid, Jens Gattermann

Chapter 12. A coupled FE-model for tunneling by means of compressed air

This chapter deals with the development and the application of a coupled numerical model for tunnelling below the groundwater table, taking into account compressed air as a means for displacing the groundwater in the vicinity of the tunnel face. The coupled solid-fluid model is characterized by treating the soil as a three-phase material, consisting of the deformable soil skeleton and the fluid phases water and compressed air. It contains a number of special cases, which are of interest in geotechnical engineering. In particular, a two-phase formulation for dewatering of soils under atmospheric conditions and a two-phase formulation for fully saturated conditions, applicable to consolidation problems, are included in the complete model.
Gerhard Öttl, Rudolf F. Stark, Robert Stelzer, Günter Hofstetter

Chapter 13. A multilaminate model for finite element analysis of tunnel excavation

A constitutive model formulated within the framework of the multilaminate concept for soils is presented. It is shown that anisotropic material behaviour is easily accounted for with these types of models. The potential of the formulation to predict plastic volumetric strains purely caused by rotation of principal stress axes is demonstrated. Furthermore the model is able to capture the formation of shear bands due to an enhanced strain softening formulation including both frictional and cohesive softening behaviour. Due to a simple regularisation technique mesh independent results are obtained with sufficient accuracy for practical purposes. The latter feature will be highlighted by solving the practical problem of a shallow tunnel excavation constructed using the principles of the NATM. It is shown that the development of plastic shear strains leading to a failure mechanism that involves shear banding is realistically predicted by the proposed formulation.
Helmut F. Schweiger, Hartmut Schuller

Chapter 14. Advanced post-processing methods

This chapter describes a visualisation system, which is specially designed for tunnelling. New visualisation-techniques for the improved perception of results of numerical simulations and measurements are presented. A requirement is that the characteristic properties of the data must not be changed by the display-methods used. In this chapter the implementation of a real-time system as well as the theoretical bases for visualisation is beeing described. Parameter studies are also presented to ascertain the display quality. The aim is to develop a system which is intuitive to use and that can display the results of a finite- or a boundary element calculation as well as data from measurements. To be applicable on a tunnel site, the system has to display the data in a way, that makes it easy to compare it to the measured data, which are available. The Tunnelling Visualisation System (TVS), which has been developed in the visualisation-project of the Austrian Joint Research Initiative “Numerical Simulation in Tunnelling” enables the user to perform a virtual walk through a tunnel. Navigation can be done using the mouse or a spaceball, which is a three-dimensional input device. With the help of this easy navigation is possible. Special hardware makes it possible to see a three-dimensional picture.
Gernot Opriessnig, Gernot Beer

Chapter 15. Application of numerical simulation at the tunnel site

Since the tunnel engineer often has to respond quickly to unexpected ground conditions, rapid results from numerical simulations performed on site could serve as a tool, which assists important decisions. One object of the project was to evaluate the efficiency and applicability of 2D and 3D finite element methods (FEM) and boundary element methods (BEM) to simulate tunnel advance under different geotechnical conditions. Another task to be performed during this project was to make site data available to other projects within the JRI.
The finite element and boundary element code BEFE was used ([1]). The simulations were occasionally performed on site in order to gain experience with the practical applicability and efficiency of the numerical tools. Modelling techniques for excavation sequence and support were investigated, as well as the model influences on the results. The modelling approach was evaluated using criterion such as the time spent for model set-up, calculation and processing the results. Improvements in methods of data evaluation of results for a direct comparison with monitored data are presented. The calculation time and disk storage requirement using a Conjugate Gradient Solver (CG) are compared to a Frontal Solver for different model sizes. The development of longitudinal displacements and the orientation of displacement vectors are presented for homogenous and heterogeneous ground conditions. The influence of different primary stress conditions on the displacement pattern was investigated as well. Finally, the practical application of the numerical models is demonstrated by case histories.
Harald Golser, Wulf Schubert

Chapter 16. Summary and outlook

In this book the main results of the Austrian joint research initiative Numerical Simulation in Tunnelling have been presented. The main aim of this research work was to advance the reliability and efficiency of existing numerical simulation methods. To achieve an increase in reliability of the methods, i.e. to ensure that predictions made agree well with actual behaviour a significant amount of development had to be made with respect to material models for rock/soil and shotcrete as well as with respect to the material parameters used. To increase the efficiency of the methods and to make the methods usable for tunnel engineers the input of data to the simulation program has to be made automatic to a great degree. Meshes that are necessary for the numerical simulation need to be generated completely automatic. An essential part of this development was improvements in the acquisition of geological data. The new method of data acquisition developed means that the information can be obtained in digital form and fed directly into the pre-processing software for the numerical simulation program.
Gernot Beer
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