TUNNELING & UNDERGROUND SPACE ENGINEERING
Year:2015 | Volume:5 | Issue:1|Papers Count:6

If layout of underground storage of energetic materials such as explosives، gases and petroleum designed inappropriately، an unexpected explosion can result in transmission and spread of the explosion to other adjacent underground spaces and causing catastrophic events both in surface and underground. In this paper a calibrated elasto-plastic numerical model in FLAC3D software is used to simulate the underground storage explosion. The peak particle velocity (PPV) damage criterion and the plastic deformation criterion were adopted to study the extent of damage zone around the explosion. The results show that the extent of damage measured based on the PPV criterion is larger than the plastic deformation criterion. Investigating responses of concrete lining supports shows tensile ruptures in the concrete due to reflection of stress waves from inner walls، which can cause transmission of explosion without direct contact of damage zone to the nearby storage chamber. Finally، in this paper the safe separation distance and embedment depth is proposed for three underground explosives storage chambers of Bakhtiari Dam project.Summary: In this paper، the explosion of underground storage facilities are modeled numerically using Flac3D software and it is tried to find the optimum layout of the underground facilities based on the extension of the explosion damage zone in numerical simulations.Introduction: The main purpose of this research is the use of numerical modeling in prediction of safe separation distance and embedment depth for energetic materials underground storage chambers، to prevent transmission of explosion between chambers.Methodology and Approaches: In this paper، major features of chambers and surrounding rock mass have been numerically simulated using 3D finite difference method. The damage extension around the exploded chamber is evaluated using two criteria of critical PPV and Mohr-Coulomb failure criterion. The results of numerical simulations are compared with international standards. Results and Conclusions: The results have proved that due to explosion stress waves reflection from inner walls of adjacent chambers that causes tensional cracks in concrete supports and adjacent rock mass it is necessary to simulate the presence of adjacent chambers in the numerical models. Results of simulations in different rock masses show that in strong rock masses، the extension of the damage zone is lower than week rocks، while due to lower attenuating ability of strong rock masses، the energy of stress waves propagating with in it is high and vice versa.

U7 station as a backup and service center has been situated in the middle of northern-southern Tehran’s metro line 7 project, in which is being excavated using an Earth Pressure Balance (EPB) machine. In this paper, the most appropriate stabilization method for the exterior portal has been nominated, and afterward the influence of tunnel excavation with TBM upon the stability of the above-mentioned portal has also been explored in the excavating procedure onset using numerical method. As a result, soil substitution with plastic concrete which can also be categorized as one of the ground improvement methods has been selected to mitigate difficulties more effectively. In addition, optimized length of plastic concrete used to minimize tunnel face displacement and to restrict prospect instability has numerically been yielded.Introduction: In today’s world, development in infrastructure facility systems such as subways is in the public eyes more than ever. These underground openings are situated in urban and/or residential areas equated with soft soils and alluviums. Furthermore, providing a stable span for the TBM operations in station would be of great importance to avoid encountering any potential hazard like settlement, economic limitation, and even human casualties. Consequently, in EPB mechanized tunneling, fairly proper stabilization method of any station portals is found to be influential.Methodology and Approaches: In this study, the FLAC3D software has been utilized to carry out 3D numerical modeling to investigate the influence of plastic concrete on the face stability. In addition, the Mohr-Coulomb criterion has been taken into account for geo-mechanical behavior of soil material surrounding the tunnel. Since EPB machine is not capable of producing fairly enough pressure against the tunnel face to maintain stability at the beginning of the excavation up to 1.5 meter, hence plastic concrete length required for face stability is found to be 1.5 meter. Accordingly, five plastic concrete of 2, 3, 4, 5, and 10 meter long has been inserted to a numerical model. Finally, optimized length and lateral expansion of plastic concrete have been designed generally based on result driven from numerical analysis such as tunnel crown and face displacement, raptured zones around the tunnel face and wall. Results and Conclusions: Three dimensional modeling has revealed that required length to fulfill tunnel face stability is 4 meter long with respect to face displacement magnitudes and also raptured zones. Lateral expansion of plastic concrete, however, should not exceed 1 meter. Results have demonstrated that plastic concrete plays a significant role in preventing tunnel crown from enormous displacement.

Due to the population growth and transportation problems in urban areas, the use of underground structures such as subways and tunnels are widely increasing in developing countries and populous cities. The passage of tunnels and subways under existing buildings reveals the important impact of these underground structures on the ground seismic response. In this paper, the effects of two long unsupported parallel tunnels aligned horizontally on the seismic response of the ground surface are studied using the boundary element method in the time-domain in this paper. The medium is assumed to have a linear elastic constitutive behavior subjected to vertically propagating incident SV in-plane waves. The proposed algorithm is validated by comparing it with the previous researchers' solution. Finally, the presented results have shown that the seismic interaction between twin tunnels with close spacing distance in comparison with a single tunnel could create more seismic amplification on the horizontal and vertical component of ground surface motion.Introduction: Underground structures due to population growth and transportation problems are one of the most substantial infrastructures of developing countries. Hence, in recent years many tunnels have been designed or constructed in urban areas in order to develop or extend underground transportation systems. Based on the experience gained from past earthquakes, it is evident that underground long structures, such as subway and tunnels and also surface structures above them have great potential for destruction and disruption during earthquakes. The shortage of considering the effect of underground cavities and tunnels, especially urban subsurface structures such as twin-parallel tunnels, on the seismic amplification of the ground surface is observed in building codes and seismic microzonation studies. Methodology and Approaches: The effects of the two long parallel tunnels aligned horizontally on the seismic response of the ground surface are studied. The elastic medium of the proposed model has been used that subjected to vertically propagating in-plane SV incident waves. In order to analyze the seismic response of twin-parallel cavities, unlined tunnels and its effect on the ground surface, a computer code named SAMBE (Seismic Analysis of Multiple Boundary Element) is implemented based on time-domain boundary element method.Results and Conclusions: The ground surface amplification potential above underground twin tunnels was strongly influenced by the wavelength of the incident wave, depth, spacing ratio and also tunnel shapes of the twin tunnels. Moreover, the conducted study can be a step forward in seismic design of buildings and seismic microzonation of the regions with underground structures.

Analysis of stresses and displacements around circular opening excavated in rock mass has been one of the most important problems in tunneling. Plastic zone is formed around underground opening as a result of high stress magnitudes. The ground response curve is one of the best methods for understanding of tunnel stability that describes the relationship between the decreasing of inner pressure and the increasing of radial displacement of tunnel wall. In recent years, several methods have been suggested for analysis of ground response curve by many researchers, however, most of the analytical solutions that have been presented, are relevant to elastic-perfectly-plastic or elastic-brittle-plastic behavior of rock. But, the real behavior of plastic zone is strain-softening with dilation. For strain-softening rock masses the attempts at elasto-plastic analysis are limited. This may be due to the difficulty in defining the material behavior and in obtaining the closed-form solutions.Summary: In this study, it is attempted to develop the earlier methods and present a new algorithm with considering quality of rock mass and variable dilatancy, the real behavior of rock mass be applied in analysis of ground response curve. The results based on the proposed analytical solution was in agreement with Numerical method and measured wall convergence in the Ghomroud tunnel. The results indicated effect of dilation parameter on convergence tunnel wall. With the increasing of rock mass quality (geological strength index),post-peak behavior of rock mass converge to elastic-brittle-plastic. Introduction: Due to loading of underground excavation and redistribution of stresses, the plastic zone form around tunnel. The real behavior of plastic zone is strain-softening with dilation. This paper proposes a new algorithm to calculate the distribution of displacements and stresses around tunnel excavated in strain softening rock masses with variable dilatancy. Also, Effect of various parameters including dilation, geological strength index, and critical softening parameter was studied on proposed algorithm.Methodology and Approaches: In this study, a new proposed algorithm was used for calculating ground response curve. In order to investigate the verification of proposed algorithm, the results of this analytical solution was compared with numerical method. For showing the applicability of the proposed algorithm, part of Ghomroud tunnel radius convergence was calculated by this proposed analytical solution and was compared with measured tunnel wall convergence at site. The results based on the proposed analytical solution was in agreement with the measured tunnel convergence.Results and Conclusions: In proposed algorithm, strain softening behavior, variable dilatancy and geological strength index applied through algorithm, which the results indicated that proposed algorithm was in good agreement with numerical and field results. The results of calculation via proposed algorithm shown importance of dilation in estimation of tunnel convergences. Using of constant dilation estimate the displacement of tunnel wall excessive. So, Analysis with constant dilation is conservative.

In this study, a methodology to determine an optimal support pattern and for the design of a tunnel is introduced based on risk analysis. It can be confirmed quantitatively that the more the tunnel is supported, the higher the reliability index becomes and the more stable the tunnel is predicted to be. Also an optimal support pattern can be determined quantitatively by performing a risk analysis considering the construction cost and the expected cost of losses that can occur due to the collapse of a tunnel.Summary: The main goal of this research is presenting a process for optimizing the support pattern for Shahriyar Dam water diversion tunnel utilizing quantitative risk analysis which take into account the existing uncertainties in the internal friction angle, cohesion, deformation modulus, joint cohesion and joint internal friction angle.Introduction: Construction of underground tunnels plays an important role in development of modern cities. The most important factors affecting safety and economic justification of underground structures is convenient support system. Presentation of a solution to deal with engineering errors and designers experience in the support system designing is the main challenges. Underground excavation done in an environment with high uncertainty and always are high risk. Geotechnical risks in tunneling generally stand for hazardous geotechnical conditions that could unfavorably affect a tunnel project and might – in the worst case – cause human fatalities.Methodology and Approaches: In this study, a methodology is introduced to determine an optimal support pattern. To this end, a risk analysis was performed to consider the uncertainty of ground properties based on the Monte Carlo Simulation (MCS) technique, which is used to obtain the probability distribution of safety factors of a tunnel. For this purpose, discrete element method (3DEC software) and reliability analysis respectively used to estimate the factor of safety (FOS) and expected costs of the failure of the tunnel and also Taguchi method used with the aim of sensitivity analysis of uncertainty factors on the support system safety factor from 3DEC.Results and Conclusions: According to the results, the optimal support system is recommended includes injection of rock bolts to 4 meters in length, spaced 1.5 × 1.5 m with 120 mm of shotcrete with index reliability, probability of failure and risk, respectively, 2.112, 1.733% and 70880.32 dollars at 95% confidence level. According to The Taguchi sensitivity analysis, increase and decrease of the rock mass quality based on rock mass classification (RMR), respectively, to increase theinfluence of parameters on tunnel stability will be spacing of discontinuities and Young's modulus.

Summary: One of the most important problems during the tunnel construction in weak grounds with difficult condition is providing efficient support and prevent from failures and deformations in tunnel crown and face. In order to regulate the rigidity of the advance core and to thereby create the appropriate conditions for complete control of the deformation response of the grounds with poor mechanical ground and therefore, in the final analysis, for complete stabilization of the tunnel in the long and short term the use of pre-confinement methods is unavoidable. In this study the application of some pre-confinement methods for stabilizing the Alborz tunnel passing the Kandovan fault was investigated by means of numerical modeling. It was concluded that a combination of mechanical precutting and fiber glass face bolts could effectively improve the stability of the tunnel along the weak fault.Introduction: Ground improvement techniques in order to overcome difficulties of excavation of tunnels in the past decade has grown considerably and the use of them is consistent with development in technology. Increasing advances in electronics and mechanical engineering has led to the ability of excavate a tunnel under complicated Geological structure and to have confidence in a safe and affordable condition. In order to regulate the rigidity of the advance core and to thereby create the right conditions for complete control of the deformation response of the grounds with poor mechanical ground and therefore, in the final analysis, for complete stabilization of the tunnel in the long and short term the use of pre-confinement methods is unavoidable.Methodology and Approaches: The finite difference based numerical modeling tool FLAC3𝐷 was used in this study to model the process of the Alborz tunnel passing through the Kandovan fault. Mechanical precutting as one of the effective pre-confinement methods is simulated in this study. Furthermore, the effect of fiber glass bolts on the tunnel stability has been investigated independently and in combination with mechanical precutting method.Results and Conclusions: Based on the results of this study, the mechanical precutting in combination with fiber glass face bolts can successfully reduce the plastic zones around the tunnel and its face, resulting is a more stable ground for tunneling. Therefore the combination of precutting and fiber glass face bolts was recommended for pre-confinement of Alborz tunnel passing through the Kandovan fault.