Twin Tunnel Behavior Under Static and Dynamic Loads of Shiraz Metro, Iran1 R
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ISSN 1062-7391, Journal of Mining Science, 2016, Vol. 52, No. 3, pp. 461–472. © Pleiades Publishing, Ltd., 2016. _________________________________ GEOMECHANICS _______________________________ ____________________________________________________________________________________________________________________________________ ___________________________________________________________________________________________________________________________ Twin Tunnel Behavior under Static and Dynamic Loads of Shiraz Metro, Iran1 R. Shirinabadi and E. Moosavi* Islamic Azad University, South Tehran Branch, Tehran, Iran *e-mail: [email protected] Received February 14, 2016 Abstract—Safety during construction and long-term stability of tunnels is among important factors in the design and implementation of underground spaces. Since tunnels and underground spaces are under dynamic loads such as earthquakes and explosions during construction and operation stages, dynamic stability analysis of such structures is of great importance. In this study, the twin tunnels of Shiraz subway were numerically modeled under static and dynamic loads with the help of Universal Distinct Element Code (UDEC). This is a finite element method (FEM) software, is any of a family of numerical methods. Unbalanced forces increase after tunnel excavation and applying static and dynamic loads. Although the increase in unbalanced forces was higher under dynamic loads, under static loads, velocity and displacement changes in the ceiling of the tunnel were higher than the rest of the tunnel. To apply a dynamic load, a sine wave was applied to the lower boundary of the model. After applying the dynamic load, velocity and displacement changes of the tunnel floor were higher than the rest of the tunnel. According to modeling results, the twin tunnels are quite unstable under static and dynamic loads and need a support system. Keywords: Static and dynamic loads, finite element method, universal distinct element code software, twin tunnel of Shiraz metro, earthquake. DOI: 10.1134/S1062739116030669 INTRODUCTION According to recorded data, underground structures are safer than surface structures against dynamic waves, because surface structures are only connected to the ground from the lower surface and vibrate freely; but, underground structures are completely connected to the surrounding environment and thus are more resistant to vibration. However, there are reports of damage to such structures due to dynamic waves. This implies that underground structures are not absolutely immune against dynamic waves and are prone to damages. Thus, relative strength of underground structures against seismic waves does not guarantee an adequate resistance to dynamic loads, especially earthquakes. Accordingly, there is a crucial need for fundamental and continuous research on wave propagation in rock and soil environments. Most urban underground structures are constructed in shallow and soft grounds and thus are prone to seismic disturbances. Such structures, depending on their use, are of great importance in terms of long-term safety. If the construction site of large underground structures such as public transport tunnels is very soft or loose, their seismic behavior should be estimated. In the past decades, many studies were concentrated on the consequence of single tunnel excavation in terms of surface and subsurface movements [1–4]. On the other hand, the case of twin tunnels nowadays very common in many metro-line projects around the world because twin tunneling is particularly favored when developing underground transportation systems all over the world [5–13]. Interactions between closely-spaced tunnels were studied in the past using a variety of approaches: physical model tests [14–19], numerical modeling [20–27], empirical and analytical 1The article is published in the original. 461 462 SHIRINABADI, MOOSAVI methods [28–33]. The physical modeling to predict, as well possible different construction settings adopted during the second excavation, generally alters the expected ground movements due to the construction of a new one, often leading to a non-symmetric final settlement trough. Numerical methods demonstrate a valuable tool to analyze this class of problems, overcoming the limitations related to the empirical methods. In several cases, a detailed investigation can only be fulfilled adopting a three-dimensional (3D) solution, which permits to calculate for any construction scheme, including the case of twin tunnels, for any kind of surface structure and its relative position with respect to the tunnels’ axes. However, the validity of such methods is strongly affected by different factors [34], the correct simulation of the tunnel excavation sequence [35] and the detail of the structural modeling [36]. Extensive surveys of the papers submitted for the session on numerical and physical modeling of tunnels were provided by Jacobsz [37]. Do et al. [38] induced that due to the interaction of twin tunnels, an increase in the surface settlement can be expected compared to that induced above a single tunnel. A full 3-D finite element of tunnel–structure problem was performed [39] in which the weight and stiffness of a surface skeletal structure were considered. In that parametric study, the tunnel construction parameters and real structure stiffness were not properly considered. The interaction between the ground and tunnel lining during earthquake excitation studied by Pakbaz and Yareevand [40], they concluded effect of earthquake on tunnel–ground interaction depend on various parameters including peak acceleration, intensity and duration of earthquake and the relative rigidity between tunnel and ground. Liu and song [41] found that the increase in buried depth improved the safety of the underground structure against earthquake damage. Park et al. [42] simulated tunnel response under spatially varying ground motion, they found that the spatially variable ground motion causes longitudinal bending of the tunnel and can induce substantial axial stress on the tunnel lining. A new model study on the effects of input motion on the seismic behavior of tunnels via Cilingir and Gopal [43], they found that the magnitude of the maximum input acceleration plays a crucial role on the maximum and residual lining forces, which the tunnel experiences. Shong-loong and Meen-wah [44] ended that the deeper the location of the tunnel, the less the tunnel lining is affected by the effect of earthquake. Sahoo and Kumar [45] investigated seismic stability of a long unsupported circular tunnel and concluded that the failure zones around the periphery of the tunnel becomes always asymmetrical with an inclusion of horizontal seismic body forces. Gomes [46] examined the effect of stress disturbance induced by construction on the seismic response of shallow bored tunnels, he found that stress disturbance due to tunnel construction may significantly increase lining forces induced by earthquake loading. Sahoo and Kumar [47] concluded that an increase in the magnitude of the earthquake acceleration leads to a significant increment in the magnitude of internal compressive pressure. However, the above mentioned studies improved the knowledge of tunnel–structure interaction and the effects of operation parameters on the settlement, the interactions of twin tunnel construction parameters and adjacent structures were not considered simultaneously. In this paper, the interaction between twin tunnels from static and dynamic loads has been studied using numerical finite element method. The main purpose of this study was to provide a numerical model which would allow the behavior of the interaction of mechanized twin tunnels to be evaluated, in terms of structural forces induced in the ceiling and floor of the tunnel and ground displacement surrounding the two tunnels. The content of this paper is organized as follows. In Section 1, the effect of dynamic loading on twin tunnels discussed in sufficient detail. The problem statement is studied in section 2. In Section 3, at JOURNAL OF MINING SCIENCE Vol. 52 No. 3 2016 TWIN TUNNEL BEHAVIOR UNDER STATIC AND DYNAMIC LOADS OF SHIRAZ METRO, IRAN 463 first the numerical modeling of construction parameters effects on the speed and direction of loading applied on the tunnel was studied. In section 4, a full analysis of the interaction between tunnels and the static and dynamic forces, considering unbalanced force and bending moment variations in all structural members, was performed. Finally, the conclusions are given in section 5. 1. EFFECTS OF DYNAMIC LOADING ON TWIN TUNNELS Understanding the behavior of underground structures during earthquake events is one of the most interesting challenges in geotechnical engineering. While tunnels generally enforced better than above ground structures during earthquakes, damage to some of these important structures during previous earthquake events, that is, the 1995 Kobe, Japan earthquake, the 1999 Chi Chi, Taiwan earthquake, the 1999 Bolu, Turkey earthquake, the 2004 Baladeh, Iran earthquake, the 2008 Sichuan, China earthquake, the 2014 Valparaiso, Chile earthquake, and recently the 2015 Illapel, Chile earthquake, highlights the need to account for seismic loading in the design of underground structures. Earthquake effects on underground structures leads into two groups: (1) ground shaking, and (2) ground failures such as liquefaction, fault displacement, and slope instabilities [48]. Ground shaking implies to the vibration of the ground