جستجوی مقالات مرتبط با کلیدواژه "underground excavation" در نشریات گروه "عمران"

Selecting the appropriate model size is a challenging issue in the numerical modelling of groundwater inflow into underground excavation. This issue was studied in this paper by presenting a methodology for selecting appropriate domain size for numerical modeling of groundwater inflow into tunnel that is excavated inside of semi-infinite aquifer. To reach this goal, first, a dimensionless factor, so-called normalized rate of inflow variation (NRIV), was defined in cooperation with its limit value, so-called acceptable level of variation (ALV). Then, the appropriate or suitable domain size (SDS) of numerical model was determined based on the NRIV and ALV. The applicability of suggested methodology was evaluated for the results of wide range geometrical parameter of tunnel (including different tunnel radiuses and depths) and different flow domain sizes. The results of this study indicate that the required domain size for numerical modelling of groundwater inflow into tunnel increase nonlinearly for larger and deeper tunnels. Moreover, the required domain size increases to 1.8 times by decreasing the level of ALV from 0.0005 to 0.0001, where the relative accuracy of results has only increased up to 4%. Since the larger domain size requires much computational difficulties and insignificant accuracy, the ALV in the level of 0.0005 is suggested for practical numerical modelling of groundwater inflow into tunnels.

Computer method is one of the ventilation network design methods in underground excavations. Computer method has been designed based on mathematical approximate methods. Methods such as Newton-Raphson method, Hardy Cross method and its modified versions, critical path, linear analysis, non-linear programming and optimization techniques are considered as some mathematical approximate methods. Newton-Raphson method is one of the methods of solving nonlinear equations in numerical calculations. This method is designed based on the definition of derivative and its correction. Using Newton-Raphson method for the analysis of ventilation networks in underground excavations has been common. However, so far, perfect investigation of this method has not been carried out. The purpose of this paper is to investigate Newton-Raphson method and its improvement in the analysis of ventilation networks in underground excavations. The analysis indicates that this method in some models is unable to find the final true answer because instead of convergence, it diverges. Thus, the improvement of this method seems to be necessary. Ventilation design of underground excavations is done based on preparing underground excavation map, identification of branches and injunctions in ventilation network, calculation of mine resistance for any of branches, calculation of air flow quantity for any of branches, calculation of pressure loss for any of branches, calculation of natural ventilation, network adjustment, selection of regulator doors and selection of main fans. Various methods have been presented such a manual and computer methods for the analysis of ventilation networks of underground excavations. The choice of analysis method depends on the purpose of ventilation network analysis.
One of the methods for solving nonlinear equations in numerical calculations is Newton-Raphson method. This method is designed based on the definition of derivative and its correction. In this method, the error amount of the initial guess is calculated for purpose function, and then, the initial guess is corrected. In this method, based on the equation  and injunction rules, supposition flow quantity for any of branches.is supposed. Based on the ventilation network fans and mathematical approximate equations, the error of any ring in the ventilation network is calculated, and then, supposition flow quantity is corrected. Based on the mathematical equations, correction operations of air flow quantity are repeated until the calculation accuracy becomes equal to or smaller than the calculation error. Newton-Raphson method is one of solution methods in ventilation networks analysis. Convergence to the final solution in this method depends on supposition flow quantity, the direction of supposition flow and the arrangement of ring selection. If the direction of the hypothetical flow in the network is in accordance with the actual flow direction, the fastest method for reaching the final solution in the analysis of underground ventilation networks will be the use of Newton-Raphson method. However, if the direction of the hypothetical flow in the network is not in accordance with the actual flow direction, the validation of this method will indicate that this method in some models becomes divergent. Therefore, improvement of Newton-Raphson method is necessary. The improvement of Newton-Raphson method in this paper is made in two stages.  The first stage is the identification of negative flow intensity, and then, the flow direction is reversed. In the second stage, the correction of the equation  is performed, and then, the calculations are repeated.

Generally, the ground is compressed by natural stresses. Any disturbance such as tunneling, changes the stress distribution and causes some deformations. If such displacements exceed threshold values, they will sometimes develop important problems related to serviceability of the buildings on surface and subsurface structures. This paper includes comprehensive investigations and analysis using a finite element software named Plaxis. The objective of this paper is to study the relationship between deformations and ground surface settlements by means of parameters like soil density, elastic modulus, tunnel diameter and depth of tunnel axis. According to the results, it is observed that these parameters have significant effects. It also shows that to what extent these parameters affect the soil behavior. Also by estimating surface settlements of the second line of City of Tabriz subway in the location of BH-10 borehole, it can be concluded that ground surface settlements, to a large extent, are dependent on contraction, as a significant parameter.