Investigating the stability of tunnels and other underground structures due to seismic loads

Abstract

Underground facilities are an integral part of modern society and are used for many applications, including subways and railway lines, highways, material storage, and water and sewage transportation. Underground facilities built in areas affected by earthquake activity must withstand both earthquake and static loading. By reviewing the historical cases of earthquake effects on such structures, it can be seen that their failure rate is lower than daily mine structures. At the same time, in recent earthquakes such as the 1995 Kobe earthquake in Japan, the 1995 Chi Chi earthquake in Taiwan, and the 1999 Kocaeli earthquake in Turkey, underground structures have suffered major damage.

Given that less studies have been done on the comparison of the seismic performance of single and twin tunnels, in this study, using the finite element method, the seismic performance of single and twin tunnels during an earthquake in strain conditions The page has been checked. One of the remarkable things about the seismic response of underground structures is to consider various parameters that affect this phenomenon, such as geotechnical properties and the depth of the tunnel, in order to have a comparison of the deformation and internal forces created by the tunnels. The results of the comparison of the seismic performance of single and twin tunnels show that the values ??of the forces created in single and twin tunnels are close to each other, for this reason, we cannot be sure about the internal forces and also the change of location at the depths of 15 and 30 meters with different geotechnical characteristics in the static phase in single tunnels is less than that of twin tunnels, which is due to the release of stress and weakening of the soil due to the digging of the twin tunnels, while the change of location at the depth of 10 meters with different geotechnical characteristics of the soil in the static phase in single tunnels is more than that of twin tunnels, which is the probability of this The trend can be due to the influence of the placement depth. The change of location in the depths of 10, 15 and 30 meters with different geotechnical characteristics of the soil in the dynamic phase is more in single tunnels than in twin tunnels, which can be caused by the action of twin tunnels. Keywords: twin tunnels, single tunnel, seismic, dynamic analysis. Chapter one:

Generalities of the research

1-1- Statement of the problem

Due to the increasing development of urban areas and the consequent increase in the needs of intra-city transportation, in recent years, underground transportation has received special attention. Tunnels dug for transportation can be used for subway, public transportation buses or private cars. The construction of tunnels can be done in two ways, single or twin. That is, in many cases, it is necessary to dig two tunnels with a smaller width at a short distance from each other instead of one with a large width.

Fewer studies have been done on the comparison of the seismic performance of single and twin tunnels. The most important researches that have been done on the comparison of the vibrations of single and twin tunnels have been numerical studies. These studies have been written using finite element or finite difference numerical modeling software. Among these studies, we can mention the 2D finite element analysis performed with FLAC by Nader Ghassimpour and Majid Kiyani, as well as the finite element analysis performed by Mohammad Reza Momenzadeh, Mohammad Reza Mansouri and Armin Azimi using ABAQUS software. The earthquake is felt more than before.  Due to the fact that structures are built with a round cross-section, the deformations and stresses created in the ground due to earthquakes have many changes with the depth change and ultimately provide more complex conditions. The design of underground facilities against earthquakes has very different aspects compared to the seismic design of surface structures. Earthquake force on conventional surface structures is mainly caused by the effects of inertia on the structure, but in underground structures, it focuses on the deformation of the ground and its interaction with the structure, with emphasis on the displacement. The problematic point here is that so far there have been fewer studies on the comparison of the seismic performance of single and twin tunnels with a circular cross-section, and a small number of influential parameters have been investigated in the discussion of the comparison of the seismic performance of single and twin tunnels with a circular cross-section.The problematic point here is that so far there have been fewer studies on the comparison of the seismic performance of single and twin tunnels with a circular cross-section, and a small number of influential parameters have been examined in the discussion of the comparison of the seismic performance of single and twin tunnels with a circular cross-section, and significant shortcomings are observed in this field. It is noteworthy about the seismic response of underground structures in considering different parameters affecting this phenomenon such as geotechnical properties and depth of tunnel placement. Due to the fact that few studies have been done on the effect of the mentioned parameters on the response of large underground structures with a round cross-section, this need is felt in order to have an almost clear idea of ??the behavior of underground structures during and after an earthquake by considering these parameters. It is one of the special issues that should be paid special attention to. In this research, using the finite element modeling method, we have modeled the soil and the tunnel in PLAXIS software to compare the forces created on the lining of single and twin tunnels against seismic loads in order to have an estimate of the forces and deformations created during the design. In the field of seismic behavior of underground structures

2) considering a suitable behavioral model to model the soil paste behavior under earthquake loading

3) simulating the underground structure using PLAXIS software

4) interpreting the results obtained from the comparison of numerical modeling results

5) summarizing and compiling the report

Chapter Two:

Overview of the conducted research

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2-1- Introduction

Underground facilities are an integral part of modern society and are used for many applications including subways and railways, highways, material storage and water and sewage transfer. takes Underground facilities built in areas affected by earthquake activity must withstand both earthquake and static loading. By reviewing the historical cases of earthquake effects on such structures, it can be seen that their failure rate is lower than daily mine structures. At the same time, in recent earthquakes such as the 1995 Kobe earthquake in Japan, the 1995 Chi Chi earthquake in Taiwan, and the 1999 Kocaeli earthquake in Turkey, underground structures have suffered major damage. In general, seismic design for underground structures is described in terms of deformations and strains imposed on the structure by the adjacent soil and often due to the interaction between the two. On the other hand, underground structures are designed for inertial forces caused by ground acceleration.

The simplest method is to ignore the interaction of the underground structure with the surrounding soil. Deformations of the free zone soil due to a seismic event are measured and the underground structure is designed according to these deformations. This method is satisfactory when the vibration is low or the underground facility is located in semi-hard soil. Other methods that consider the interaction between the structure's supports and the surrounding soil will be described. In the quasi-static analysis method, the deformation of the ground is entered as a static load, and the soil-structure interaction does not include dynamic effects and wave propagation. In the dynamic analysis method, a soil-structure interaction is implemented using numerical analysis tools such as finite element and finite difference methods.

Underground structures have characteristics that distinguish their seismic behavior from most underground structures, notably (1) complete enclosure in soil or rock and (2) their considerable length (tunnels). Therefore, the design of underground structures to withstand seismic loading is very different from the design of surface mine structures. Bulky underground facilities that are commonly used in urban areas include deep tunnels, slow and covered structures, and buried structures (Figure 1-2).  

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Thick tunnels are longitudinal underground structures in which the length is much greater than the cross-sectional diameter.