TUNNELING & UNDERGROUND SPACE ENGINEERING
Year:2018 | Volume:7 | Issue:1 |Papers Count:6

Summary In this study, modeling of groundwater inflow into underground excavations was studied based on the stochastic continuum theory. To reach this goal, the two dimensional FNETF computational code was modified based on the stochastic continuum theory to model groundwater inflow into underground excavations. The accuracy of the computations in this code was evaluated from validation point of view. Introduction All geological formations show random variation (or spatial nonuniformity) in the values of hydrogeological parameters leading to a considerable amount of uncertainty in the hydrogeological models. Therefore, it is essential to characterize the uncertainty of groundwater processes that can be achieved through the implementation of stochastic continuum theory. This method can be used to estimate the most likely range of groundwater inflow into underground excavations and assess the uncertainty of estimates. Methodology and Approaches In this paper, the FNETF computational code was modified based on the stochastic continuum theory, and then, the accuracy of computations in this code was evaluated from validation point of view. The results of analytical and numerical (by performing phase2 software) solutions for hydraulic head distribution around the circular tunnel, and also, the groundwater inflow rates were used to validate the accuracy of FNETF computational code. Then, the role of hydraulic properties of rock mass was investigated through parameter study. Results and Conclusions The results of this study for modification and validation of FNETF computational code show that the outputs of this code for ideal media (having near zero standard deviation of hydraulic conductivity) in terms of both hydraulic head distribution around the tunnel and the groundwater inflow rates are appropriately in good agreement with those obtained from ideal analytical and numerical solutions. Therefore, this computational code can be successfully applied for modeling groundwater inflow into underground excavations by the application of the stochastic continuum theory. The results of parameter study and sensitivity analysis also show that the maximum, minimum, and average values of groundwater inflow into the study tunnel decreases by increasing the standard deviation of the rock mass hydraulic conductivity. For very heterogeneous rock masses, both the analytical solution and deterministic numerical methods overestimate (i. e. with one or more order of magnitudes) the groundwater inflow into the tunnel in comparison with the stochastic continuum method.

Summary Dynamic Compaction is one of the best methods of deep soil improvement. Dynamic compaction operations generate large vibrations; Therefore it should be designed and implemented in a way that does not damage adjacent underground spaces. In this research, for a tunnel with constant diameter, four different locating depths and three different impact energies are modeled numerically using finite element code ABAQUS. In this way six impact distances from tunnel axis are considered. In order to determine safe distances of dynamic compaction from tunnel axis, peak particle velocity (PPV) values in tunnel which is defined by reliable standards are considered. Introduction Proposed method consists of repeated dropping of a heavy weight tamper in a predetermined pattern on the weak ground that is going to be compacted. Ground vibrations caused by dynamic compaction can damage adjacent underground spaces such as tunnels and buried structures. At First, by numerical modeling of dynamic compaction, critical zone of the tunnel was determined then variations of maximum PPV location for different depths of tunnel and different impact energies were indicated. Finally safe impact distances from tunnel axis for different depths of tunnel with different impact energies in critical zone were determined by allowable PPV that defined by reliable German, British and Swiss standards. Methodology and Approaches Dynamic compaction has been modeled by many researches with different numerical methods such as finite element. In this research, ABAQUS finite element code in three dimensional space was applied in three steps. In the first step, gravity analysis was performed to exert initial stresses. In the second step, excavation of the tunnel and installation of lining were performed. And in third step, Dynamic implicit analysis for drop of tamper on soil in ten number of blows with time duration 60s for each impact were considered. Results and Conclusions This research shows that the critical zone of tunnel is located at the first quarter of tunnel in side of dynamic compaction. Increase in depth of the tunnel, safe impact distances from tunnel axis were reduced. By considering constant impact energy and impact distance by, maximum PPV location at the end of critical zone will move up by increase in depth of the tunnel. Also, assuming constant depth of the tunnel and impact distance, maximum PPV point location was fixed with decrease in impact energy.

Summary Consequently, horse-shoe tunnels have better performance. Recently, public places and underground transport facilities such as subways and urban transport tunnels and routes have been the targets of terrorists. In this regard, there have been reports about explosion in these tunnels. Twin tunnels are built because of their structural strength and ease of construction. In this study, the behavior of tunnels with different cross-sectional geometries was examined numerically using the FLAC software under the impact of explosive forces. Thus, the performance of twin tunnels with three different shapes under internal explosion was studied in this research work. The results suggest that the twin tunnels with horseshoe cross section compared to rectangular cross section have better performance. Introduction Sadegh azar et. al. (2009) studied the impact of surface explosions on tunnels in different conditions. Ghaemi (2011) explained the effect of emission of missile compressive waves and their impact in continuous and discontinuous environment. Liu (2012) presented the nonlinear response of tunnels buried under explosive loading. Lee (2006) studied the stability of a tunnel, which was adjacent to an exploded tunnel. Tian (2008) studied the dynamic response of multi-story buildings affected by explosion in one of the twin tunnels. The effect of cross-sectional shape of twin tunnels that have not been investigated in these studies, is examined in this paper. Methodology and Approaches The shapes of twin tunnels were adopted from the code 161 of Ministry of Road and Urban Development. In this study, four types of twin tunnels have been considered. The primary section of the twin tunnels is the combination of 2 circles with the radii of 12. 5 and 5. 8 m in each tunnel. The first suggested section for the twin tunnels has a rectanglar shape with 10 m width and 6 m height in each tunnel. The second section for the twin tunnels is in the form of a circle with the radius of 12. 5 m and a line with the length of 10 m in each tunnel. The third section of the studied twin tunnels is a combination of 3 circles with the radii of 12. 5, 6. 4 and 2. 9 m in each tunnel. The FLAC software was used for this study. In this study, Mohr-Coulomb failure criteria for soil was considered. Results and Conclusions Bending moments and axial force in the crown of the damaged tunnel and adjacent tunnel were obtained for four different cross-sectional shapes. The results showed that the stress caused by the detonation was much lower than the compressive strength of concrete, while bending moments are high. Therefore, when an explosion occurs, rectangular tunnels have poorer performance compared to horseshoe tunnels. Among the tunnels with horseshoe section, the second section shows a good performance against explosion.

Summary Feasibility studies of pumped storage power plant in Azad dam has been carried out. This project includes underground spaces such as tunnels, shafts and two big caverns including power house and transformer. Thus, in this research, long term stability of the support system of Azad dam is studied numerically. To determine the time-dependent behavior of the surrounding rock mass of the cavern comprising of sandstone layers with phyllite interlayers, instrument curves have been used. To simulate the cavern time-dependent behavior, numerical modeling by employing Flac3D software has been used. Long term stability analysis for 100 years was reviewed. Data which obtained from convergent pins of access tunnel was used for conformity to numerical results. The results of the time-dependent analysis show that the cavern will be stable for a 95-year period with a safety factor of 1. In general, the results of the numerical modeling indicate a good agreement with the results of convergent pins of the access tunnel. Introduction In this research, due to importance of the subject and the sufficient knowledge of conducted studies in similar cases, the long-term behavior of the main cavern in Azad dam pumped storage has been investigated. Hence, identification and description of time-dependent behavior of the surrounding rock mass is important, as it is possible to calculate and analyze time dependent deformation and, consequently, long term stability of the cavern. To understand such behavior, creep parameters should be calculated by creep tests or using instrument curves obtained from the installed convergent pins in the underground space. Finally, considering the time-dependent analysis, the stability of the cavern for different time periods can be examined. Methodology and Approaches In this study, time-dependent behavior of the surrounding cavern rocks including sandstone with interlayer phyllite, have been studied using the data from the instrumentation installed in one of the access tunnels. Viscoplastic model (CVISC) was selected for the rock mass periphery of the cavern. To simulate the CVISC model, the finite difference software (FLAC3D-V5. 01) has been used. The results of the numerical method were verified with the instrumentation data. Moreover, the capacity diagrams of the support system were drawn. Results and Conclusions The results of the numerical model are in good agreement with the results of convergence pins data implying the accuracy of the numerical model. According to the long term stability analysis, the powerhouse cavern structure will be stable for a period of 95 years with safety factor of 1.

This paper aims to use pushover analysis for performance-based seismic assessment of linings of shallow tunnels constructed in soil that are subjected to vertical shear waves. Pushover analysis is a nonlinear static analysis that works based on pushing laterally two-dimensional (2D) numerical nonlinear model of soil with tunnel statically. This analysis considers just ovaling/racking deformation of a lining and compared to the other existing seismic analysis approaches, it has the advantage of using directly a standard acceleration response spectrum as seismic demand. Initially in this paper, responses of a typical tunnel due to four earthquakes were calculated using pushover analysis. Then, the approach of employing a typical standard acceleration response spectrum as seismic demand was presented using the building standard spectrum of FEMA 302 provisions. All the resultant performance points of pushover analyses were then evaluated by carrying out nonlinear dynamic time history analyses and the method was verified. However, further studies are required to propose an acceptable response spectrum for the geotechnical nature of soil deposits containing shallow tunnels as their seismic demand.

An analytical solution for the evaluation of dynamic response of a tunnel in infinite isotropic elastic porous media is presented. Tunnel is considered as a circular cavity. Two groups of complex functions for solid skeleton and pore fluid in a two-dimensional (2D) complex plane are introduced in order to solve the Biot equations. Stress, displacement and pore pressure fields induced by incident and scattered waves in the medium and especially in the vicinity of the cavity are evaluated in this complex plane. The validation of the proposed solution is shown by various numerical examples. A parametric study including the effects of fluid compressibility changes, shear modulus and permeability variations, several wave numbers and wave types (fast, slow and shear waves) is performed.