نشریه مهندسی تونل و فضاهای زیرزمینی
سال هفتم شماره 1 (تابستان 1397)

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 and for demand of modeling of groundwater inflow into underground excavations. The accuracy of computations in this code was evaluated from the validation point of view. All geological formations show random variation (or spatial nonuniformity) in the values of the hydrogeological parameters that lead to a considerable amount of uncertainty hydrogeological models. Therefore, it is becoming essential to be able to characterize the uncertainty of groundwater processes that can be achieved through implementation of continuum theory with stochastic hydrogeology. 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 the validation point of view. The results of numerical (by performing phase2 software) and analytical 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. The results of this study for modification and validation of FNETF computational code show that the outputs of this code for ideal media (with near zero standard deviation of hydraulic conductivity) in terms of both hydraulic head distribution around the tunnel and the groundwater inflow rates are appropriately matched 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 stochastic continuum theory. The results of parameter study and sensitivity analysis also show that the maximum, minimum, and average values of groundwater inflow into tunnel decreases by increasing the standard deviation of rock mass hydraulic conductivity. Therefore, for very heterogeneous rock masses, both the analytical solution and deterministic numerical methods will overestimate (i.e. with one or more order of magnitudes) the groundwater inflow into tunnel in comparison with stochastic continuum method.

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. 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. 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.

Recently, public places and underground transport facilities such as subways and urban transport tunnels and routes have been targets of terrorists. 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 by FLAC under the impact of explosive forces. The results suggested that the performance of tunnels with a horseshoe section performance will be better than rectangular. In recent decades, there have been reports about explosion in tunnels. In this article, the performance of twin tunnels with three different shapes under internal explosion was studied. Consequently, horse-shoe tunnels have better performance. Stevens et.al. have studied the impact of surface explosions on tunnels in different conditions. Ghaemi explained the effect of emission of missile compressive waves and their impact in continuous and discontinuous environment. Liu presented the nonlinear response of tunnels buried under explosive loading. Lee studied the stability of a tunnel which was adjacent an exploded tunnel. Tian studied the dynamic response of multi-story buildings affected by explosion in one of two twin tunnels. In the previous studies, the effect of cross-sectional shape of twin tunnels has not been investigated. The shapes of twin tunnels were adopted according to code 161 of ministry of road and urban development. Four types of twin tunnels were considered. The primary section is the combination of 2 circles with 12.5 and 5.8 meter radii for each tunnel. The first suggested section is a rectangle with 10m width and 6m height for each tunnel. The second section is a circle with 12.5 radius and a line with 10 meter length for each tunnel. The third section is the combination of 3 circles with 12.5, 6.4 and 2.9 meter radii for each tunnel. FLAC was applied. Mohr-Coulomb failure criteria was used for soil.
Results and 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 other types of the horseshoe tunnel. Among the tunnels with horseshoe section, the second section shows a good performance against explosion.

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. So, in this research long term stability of the powerhouse cavern is studied numerically. To determine the time-dependent behavior of the surrounding rock mass of cavern which includes sandstone layers with phyllite interlayers, was used instrument curves which obtained from one of the access tunnel. To simulate the cavern time dependent behavior, numerical method and Flac3D software have been used. Results of the numerical model showed a good agreement with results of convergent pins of the access tunnel. Long term stability analysis of support system of Azad dam main cavern was objective of research. The study was numerically and Flac3D software was used. Bergar’s time dependent model was considered for rock mass periphery of cavern. Numerical model of cavern was built. 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. Results of the time dependent analysis showed that the cavern will be stable for a 95-year period with a safety factor of 1. 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 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 which obtained from installed convergent pins in underground space. Finally, with time-dependent analysis, it can be examined the stability of the cavern for different time periods. In this study time-dependent behavior of rock surrounding cavern which consists of sandstone with interlayer phyllite, have been studied using instrumentation data that were installed in one of access tunnels. Viscoplastic model (CVISC) was selected for the rock mass periphery of cavern. To simulating CVISC model, finite difference software (FLAC3D-V5.01) is applied. Results of numerical method were verified with instrumentation data. Capacity diagrams of support system were drawn. Results of numerical model are in good agreement with the results of convergence pins data which show accuracy of the numerical model. According to long term stability analysis, powerhouse cavern structure will be stable for a period of 95 years with safety factor of one.

این مقاله قصد دارد که با استفاده از تحلیل بارافزون به تحلیل لرزه ای بر مبنای عملکرد پوشش تونل های کم عمقی که در خاک ساخته شده اند و تحت بارگذاری موج برشی قائم قرار گرفته اند بپردازد. تحلیل بارافزون یک تحلیل استاتیک غیرخطی است که بر اساس جابجا کردن جانبی مدل دو بعدی خاک و تونل عمل می کند. این تحلیل تنها مود تغییر شکل اعوجاج (رکینگ) مقطع عرضی پوشش تونل را ارزیابی می کند و نسبت به سایر روش های عددی تحلیل لرزه ای دارای مزیت استفاده از طیف شتاب پاسخ استاندارد نهشته ی خاکی به عنوان تقاضای لرزه ای می باشد. در ابتدا پاسخ یک تونل نمونه بر اثر چهار زلزله به طور جداگانه به وسیله تحلیل بارافزون محاسبه شد. در ادامه روش بکار گیری یک طیف استاندارد نمونه به عنوان تقاضا در تحلیل بارافزون بیان گردید که در آن از طیف استاندارد ساختمان فیما 302 استفاده شد. نتایج تحلیل بارافزون با انجام تحلیل دینامیکی تاریخچه زمانی غیرخطی مورد ارزیابی قرار گرفت و روش تحلیل بارافزون تایید شد. با این وجود برای بکار گیری این روش به مطالعات بیشتری جهت ارائه یک طیف استاندارد قابل قبول، که با طبیعت ژئوتکنیکی نهشته خاکی حاوی تونل کم عمق سازگاری داشته باشد، الزامی است.

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 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 solut ion 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.