نشریه مهندسی تونل و فضاهای زیرزمینی
سال چهارم شماره 1 (تابستان 1394)

In this paper, an analytical solution for ground reaction curve is presented, under drained conditions considering pore pressure effect. A theoretical solution for seepage forces due to groundwater flow under steady-state flow is derived. Afterward, an elasto-plastic model based on a linear Mohr–Coulomb yield criterion is adopted and the ground reaction curve is evaluated based on plastic potential function for the non-associated flow rule. A simplified relation in elasto-plastic conditions is proposed for calculating the radius of plastic zone and a sensitivity analysis on the parameters affecting the ground reaction curve is also made. The ground reaction curve is in fact the relationship between the support pressure and the radial displacement of tunnel wall, which is used for an optimum design of the support system of the tunnels under ground water table influenced by seepage forces. A simplified and pervasive analytical solution to calculate stresses and displacements on the tunnel wall has not yet been suggested. In this paper, the approximation of calculating plastic radius is eliminated and a method to obtain its precise value, by a direct relation and a theoretical solution for the ground reaction curve, is proposed. Methodology and Approaches: In this paper, it is assumed that the seepage forces due to groundwater table are based on steady-state flow, the tunnel is circular, a plastic zone appears around the tunnel considering the Mohr-Coulomb failure criterion, and a drained condition is governed. Common stress-strain and pressure equations, and differential equations are used. The radial displacement for a circular tunnel can be calculated by means of the elasto-plastic theory. The plastic strains can be represented using the plastic flow rule. The strain in the plastic zone is the sum of elastic and plastic strains. Simplified relations, which the ground reaction curve would be expressed by these relations, are obtained. Results and Sensitivity analysis shows that by increasing the water height above tunnel crown, the ground reaction curve becomes more critical and the necessity of adequate tunnel wall will further be required. Moreover, by increasing the water height above tunnel crown, the influence of seepage increases as the effect of drainage is major in lower depths.

Nowadays, the necessity of implementing the underground structures in the urban areas has been increased due to the continuous increase of the spaces needed for intra-urban transports. The road and metro tunnels are among the underground spaces mostly considered in the cities. As the cities are mainly located on the soft and alluvium grounds, some difficulties associated with implementing underground spaces in these grounds may be arised. The most important difficulties in this regard are the settlement of the ground surface during construction and its effect on the surrounding structures. The construction of large section underground spaces such as metro lines stations is essentially important. One of the factors in this regard that should be considered in the urban areas is to select the method of excavating the spaces. This fact has significant effect on the stability of surrounding tunnel spaces during excavation, surface settlement due to the excavation and long term stability of excavated space (e.g. metro station). Methodology and Approaches: In this research, three methods including pre-supporting concrete arc, side galleries and German method have been studied and evaluated for implementing the underground stations concerning the stability of space, surface displacement of the tunnel and the settlements due to the excavation. In the other words, this research mainly focuses on the effect of using different excavation methods (with or without supporting structure) on the superficial displacements of the tunnel, settlement of ground surface and other parameters. Then, the proper method is suggested for excavating the considered station. The numerical modeling of different methods for implementing the space has been conducted using finite element method and Plaxis 3D software. Results and The results obtained in this research indicate that the maximum settlement of the ground surface is seen in the side galleries and its minimum in the pre-supporting arc methods. Moreover, at the end of implementation of the first floor of the station, the settlements of ground surface have been very similar in the concrete arc and German methods. It means that these two methods can be used in the one- floor stations.

In this paper, Given the high importance of joint persistence characteristics on the mechanical behavior of the rock mass, geometric-stochastic joint network model, based on Veneziano method, has been developed by considering the statistical characteristics of joint size. Using surveyed data in the access tunnel of Rudbar Lorestan dam & hydropower plant and estimation of the best probability distribution function on geometric characteristics of existing joint sets in the study area, three-dimensional (3-D) geometric model of joint network has been presented. Nowadays, modeling is increasingly put forward as a method of determination of rock mass strength characteristics. An accurate description of the rock mass structure does provide a better starting point for modeling of rock mass. Besides, there will always be some random variation in the geometric properties of joints by virtue of rock mass heterogeneous nature. Therefore, it is necessary to describe the ordered properties stochastically and to use in rock mass modeling. 3-D stochastic joint network modeling technique represents the most optimal choice for simulating the probability nature of joint geometric properties. Methodology and Approaches: Using field measurements and statistical analysis, stochastic parameters of discontinuities are detemined. In this paper, collected data is obtained using a linear surveying. Data processing including matching different probability distribution function on geometric properties of surveyed joint sets has been performed using Dips and EasyFit software packages. In order to calculate volumetric joint density, joints, which have been visible to the naked eye in the field, have been counted within an area of 1 m2 using a square frame (1×1 m) with grid lines at 10 cm intervals. Using the areal joint density and existing equations, the volumetric joint density has been calculated for each joint set, separately. Thus, to prepare the geometric-stochastic model, the required inputs,mentioned above, have been used. The generated joints by the developed model, are coincided on the ordered probability distribution function of joint persistence. Results and In this paper, a 3-D geometrical- stochastic discontinuities network model for the access tunnel of Rudbar Lorestan dam has been presented. In order to model implementation, a computer code written in C++, named DFN-FRAC3D, has been developed to represent the joint network in different directions and to generate digital outputs. The results of this paper can be a useful input for numerical stability analysis and hydraulic behavior studies of rock mass.

Prediction of tunneling induced settlement in soft grounds is of particular importance for estimating potential risks to pre-existing structures. Considering soil complex behavior, particularly granular materials, empirical methods are of special significance. In this regard, a study using physical modeling has been conducted in Soil and Rock Laboratory of Department of Civil Engineering at Sahand University of Technology. The observed surface and sub-surface settlements have been recorded and compared with the results of the most common empirical method used for this purpose. Tunnel excavation causes relaxation of in-situ stress, and we expect some deformation at tunnel depth that in soft grounds may reach to ground surface. Prediction of these settlements is of particular importance for estimating potential risks to pre-existing structures. In practice, the prediction of ground displacements is based mainly on empirical relationships. The most common empirical method is Peck’s method. In this method for prediction of settlement, it is generally supposed that ground loss at tunnel depth is thoroughly transferred to surface. This assumption may be correct in clay, however, for drained condition such as one in sands, soil volume does not remain constant and shear deformation causes dilatation or contraction of soil leading to ground loss at surface, which is not exactly the same as that at the tunnel depth. Therefore, settlement in sand needs more consideration. In this paper, we present a physical model to monitor soil displacement due to mechanized tunneling. The obtained results from tests on sandy soil are then interpreted. Methodology and Approaches: Ground loss at tunnel depth has been used to model tunneling effect on surrounding soil. Front wall of the physical model is transparent and soil layers movements have been measured by image processing technique. Sand sample movement during tests has been recorded by digital camera. Then, frames have been extracted and processed by means of particle image velocimetry (PIV) and observed surface and sub-surface settlement curves have been compared with the Peck curve. Since sand behavior varies in respect to its relative density, two different states have been considered. Results and The obtained results show that surface and subsurface settlement curves in the dry sand sample follow Gaussian curve with acceptable accuracy. During excavation in drained sand, ground loss at depth may not be the same as that at the surface, and therefore, the use of ground loss at tunnel depth may lead to incorrect prediction of surface settlement. Moreover, the results indicate that the magnitudes of ground movements in looser sand increase and, the shape of the settlement trough becomes narrower.

In this paper, train induced tunnel-structure interaction problem is analyzed using the coupled scaled boundary finite element (SBFE)-finite element (FE) method. Scaled boundary method is a relatively novel approach, which enables us to model bounded and unbounded domains accurately. Effects of adjacent buildings and effects of double tunnel on the train-induced vibrations are evaluated using this method. Construction and development of underground transportation lines like metro tunnels in urban areas is a suitable choice to reduce volume of the traffics especially in large cities. Train induced vibrations in these tunnels can affect adjacent structures. Train induced vibrations can make some damages to the adjacent structures and these structures can also affect vibrations of the tunnels. To analyze train induced vibration problems numerically, a special numerical method should be used to model unbounded soil media. The numerical methods, used for this purpose, are grouped in two classes of global and local. Local methods cannot model radiation-damping effect of unbounded soil media completely and accurately. SBFE method is a global approach with semi analytical formulation, which can model unbounded media precisely. In this paper, this novel approach is used to analyze tunnel-structure interaction problem. Methodology and Approaches: In this paper, the coupled SBFE-FE method is used to model train-induced vibrations in tunnels. The semi analytical SBFE method is used to model unbounded soil media where conventional finite element method is used to model near field soil media and contained structures. A MATLAB code, based on the coupled SBFE-FE method, has been developed in this research work. The written code is verified by comparing its results with the results of the finite element-boundary element method mentioned in the available published papers. As train induced vibrations cannot make large displacements, linear elastic behavior is considered for both soil and structures. Results and It is shown that the buildings in the vicinity of train induced tunnels can make an increment on displacement time history of tunnels and it can reduce radiation-damping effect of the considered soil media. In the case of a double tunnel, it is also shown that adjacent tunnel can increase horizontal dynamic stiffness of the trapped soil between two tunnels. Furthermore, it is demonstrated that simultaneous movement of the second train can magnify vertical displacements of the domain. In the case of the horizontal displacement, it is indicated that displacements of the domain between two tunnels are decreased but displacements of the other areas are magnified.