The Influence of Buildings and Ground Stratification on Tunnel Lining Loads Using Finite Element Method
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The influence of buildings and ground stratification on tunnel lining loads using finite element method L'influence des bâtiments et de la stratification du sol sur les charges de revêtement du tunnel utilisant la méthode d’éléments finis Rezaei A.H. Ph.D. student, Faculty of Civil Engineering, University of Tabriz, Tabriz, Iran Katebi H., Hajialilue-Bonab M. Associate Professor, Faculty of Civil Engineering, University of Tabriz, Tabriz, Iran Hosseini B. M.Sc in Geotechnical Engineering ABSTRACT: Urban development and increasingly growth of population have been accompanied by a considerable growth in mechanized Shield tunnelling. Commonly precast concrete segments used as tunnel lining in mechanized tunnelling and include relatively considerable part of tunnelling cost. The optimum design of lining that decreases tunnelling cost needs to accurate evaluation of loads act on lining. In this study, the effects of soil stratification, building’s geometry, position and weight on lining loads were studied. A 2D finite element model was applied to simulate the conventional procedure of tunnel excavation and lining using Abaqus software (Ver 6.10). The geometry of tunnel, lining segments, injection grout and around soil properties were adapted from under construction Tabriz urban railway line 2 project. The results show that ground stratification and building properties (especially the position of buildings) have considerable effects on lining loads. From the viewpoint of structural design, the buildings effect on lining is critical when the surface buildings are unsymmetrical. RÉSUMÉ : Le développement urbain et de plus en plus la croissance de la population s'est accompagnée d'une croissance considérable dans le domaine de Tunnelier. Communément préfabriqué segments de béton utilisés comme revêtement de tunnel en tunnel mécanisé et inclure une partie relativement importante du coût tunnel. La conception optimale du revêtement qui diminue les coûts tunnel besoins d'une évaluation précise des charges agir sur la revêtement du tunnel. Dans cette étude, les effets de la stratification du sol, la géométrie du bâtiment, la position et le poids des charges de revêtement ont été étudiés. Un modèle par éléments finis 2D a été utilisé pour simulation de la procédure classique d'excavation du tunnel et revêtement en utilisant le logiciel Abaqus (version 6.10). La géométrie du tunnel, voussoirs, coulis d'injection et autour des propriétés du sol ont été adaptées au tunnel de métro de Tabriz (line 2) qui est en cours de construction. Les résultats montrent que la stratification du sol et les propriétés de construction (en particulier la position des bâtiments) ont des effets considérables sur les charges de revêtement. Du point de vue de la conception structurelle, l'effet bâtiments sur le revêtement est essentiel lorsque les bâtiments de surface ne sont pas symétriques. KEYWORDS: FEM, Abaqus, mechanized tunnelling, tunnel lining loads, surface buildings, stratification. tunnel lining adopted in the design might be greater than the 1 INTRODUCTION actual loads, particularly in case of good ground conditions Urban development and increasingly growth of population have (Mashimo and Ishimura 2003). been accompanied by a considerable growth in tunnel The effect of soil layering on the ground response to construction for subways, railway underpasses, and urban tunnelling has been investigated by several researchers (Grant highways and a continuous development of tunneling and Taylor 2000). The focus of these studies has been on technology in recent years. Besides conventional excavation tunnelling induced settlement and stability of the ground above methods such as the New Austrian Tunneling Method (NATM) the excavated tunnel. In comparison with ground movements, shield tunneling is now a well established method which allows little attention has been paid to the effects of the overlying strata for tunnel advances in a wide range of soils and difficult on the stresses developing in the tunnel lining. Therefore, more conditions such as high ground water pressures, soft soils or study about evaluation of loads act on lining is required and the small cover depths. It is well known that segments’ production insights obtained from this study can contribute to an cost accounts for a large amount of the total shield tunnel improvement of load consideration in shield lining design. The construction cost and one of the effective methods to reduce this objective of this study is to investigate the influence of surface cost is to design the segments more efficiently. buildings and ground stratification on lining loads. In the usual design method the earth pressure acting upon the In engineering practice different methods are often used to segment lining is calculated by the overburden pressure or calculate lining stresses (Nunes and Meguid 2009).In this study, Terzaghi’s loosening earth pressure according to the stratum a 2D finite element model is applied to simulate the condition and the overburden height (Official Report of the conventional procedure of tunnel excavation and lining International Tunnelling Association 2000). In this method only installation stages properly. The geometry of tunnel, lining the weight of surface buildings are considered as a uniform segments, injection grout and surrounding soil properties were surcharge pressure. Nowadays, the modern shield technology adapted from under construction Tabriz urban railway line 2 equipped with precise pressure control system at cutter face and project. simultaneous backfill grouting system makes it possible to build a tunnel without loosening the surrounding ground. Therefore, 2 NUMERICAL MODELLING the actual earth pressure cannot be correctly predicted by conventional methods (Hashimoto et.al 2002).Some of the field Tunneling is often modelled two-dimensionally although it is a measurement results have shown that the loads acting on the three-dimensional (3D) problem since a full 3D numerical 789 Proceedings of the 18th International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013 analysis often requires excessive computation resources (both 3 CHARACTERISTICS OF TABRIZ METRO LINE 2 storage and time). During tunnel construction, a volume of soil 2 squeezing into the opening creates deformations around and Tabriz with 160 km area and the population about 1,600,000 is above the tunnel, which cannot be simulated directly in 2D one of most populated and important cities in northwestern of finite element analysis. Hence, various methods have been Iran. TURL2 about 22 km in length will connect eastern part of proposed to take account of the stress and strain changes ahead the city to its western part. This line comprises a single tunnel of the tunnel face when adopting 2D plane strain analyses to which has been constructed using one earth pressure balance simulate tunnel construction (Karakus 2007). EPB-TBM with a cutting-wheel diameter of 9.49 m and a shield In this study, a two-dimensional finite element multistep with external diameter of 9.46 m in front of shield that induce simulation model for shield-driven tunnel excavation is overcut equal to 1.5 cm in each side of shield. For lining of the presented. The model takes into account all relevant tunnel, 35 cm-thick precast concrete segments with a length of components of the construction process as separate components 150 cm are installed just behind the shield. in model (including: soil and ground water, tunnel lining and Geologically, in central part of the route, based on conducted tail void grouting). The buildings were simulated by an elastic studies in the corridor of TURL2, soil is mainly silt with low beam at the surface of the models. Each surface beam has an plasticity (ML) and silty sand (SM) and water table is about 9m equivalent moment of inertia (I) and thickness (t) representing deep. Geotechnical specifications used for soil layers of the the associate building. The surrounding soil above the ground models are presented in Table1. Mechanical properties of tunnel water level was discretized by 4-node first order fully integrated liner and tail void grout, utilized in the numerical simulations, continuum elements (CPE4) and the tunnel liner and elastic are summarized in Table2. beams (representing buildings) simulated as 2-node linear Table1. Geotechnical specifications used for soil layers of the model Timoshenko beam elements. The under groundwater soil and Dry Wet Elastic Internal the grout material are modelled as saturated porous media using Cohesion Dialation ID density density modulus friction pore pressure elements (CPE4P). The time dependence of the 3 3 angle (KN/m ) (KN/m ) (kPa) angle grout material characteristics due to hydration is modelled in a (kPa) simplified manner by employing a time-dependent Young’s SM 16.25 20 40000 7 34 3 modulus and Poisson’s ratio. The soil behavior is assumed to be ML 16.8 20.35 25000 17 25 0 governed by an elastic perfectly-plastic constitutive relation based on the Mohr–Coulomb criterion with a non-associative In general the grout pressure value considered 0.5 bar more flow rule. than applied face support pressure. Therefore, the required face The behavior of lining concrete is assumed to be linearly pressure for each model calculated and in accordance with elastic with properties which are usual for C45/55 concrete calculated value, the grout pressure adopted for each model. (E=36000MPa, υ=0.2). For considering decrease of rigidity at segment joints, a transfer ratio of bending moment is Table 2. Material properties used in numerical simulations introduced. This aspect is transferred to numerical analyses with Unit Compressive Elastic Poisson Correction of the elastic modulus of the ring, according to Material weight strength modulus modification factor ζ=0.3: (KN/m3) (MPa) (MPa) ratio Tunnel liner concrete 25 40 25200 0.2 Ec ECLS 36000)3.01()1( 25200Mpa (1) Tail void grout (fluid) 18 0 5 0.47 Where Ec is the virtual modulus of the ring and Ecls is the Tail void grout (hardened) 18 3 20 0.3 concrete modulus.