سید رحمان ترابی

Summary:

Almost all of the proposed models of Discrete Fracture Networks (DFN) embedded within rock masses are discontinuities with zero tension strength. While, potential discontinuities and weak surfaces such as rock bridges, veinlet, and schistose surfaces are a candidate for breakage under stress and have also a significant effect on rock mass strength. Simultaneously with geometrical parameters, this geomechanical heterogeneous nature of fractures is crucial for understanding rock mass behavior and characteristics. This paper focuses on the probabilistic effect of potential discontinuity properties on the rock mass strength using Potential Discrete fracture networks (PDFN)-bonded block models (BBM) framework. The cohesive crack model is used to define block contact behavior. A comparison of the results indicates the importance of the proposed model for the assessment of realistic rock mass strength instead of traditional DFN.

Analysis of discrete fracture network (DFN) employing the complex nature of fracture patterns plays an important role in understanding the micro and macromechanical behavior of rock mass. Rock mass behavior analyses have traditionally been undertaken for discontinuity with zero tension strength along DFN to calculate mechanical properties. In point of this view, such methods regardless of geometrical parameters generally assume the same possibility of failure for all fractures in the rock mass. The distribution of the DFN strength parameter in Potential DFN can result in important changes in the strength of rock mass and remains a challenge in rock engineering design. The term of PDFN is used for those fracture network with tension and shear strength. Detailed analysis of the employed PDFN which elaborately highlights the role of the distribution of input strength property of each fracture that controls the real strength of rock mass has been presented in this research.

Methodology and Approaches:

In the BBM-DEM, the Voronoi tessellation scheme is employed and the material is simulated as assemblies of several particles bonded together at their contact areas. In order to define contact law of BBM, a cohesive crack model (CCM) is implemented in the UDEC. The calibration process is carried out to obtain contact properties. DFN is written in c++, in which the probability distribution of cohesion, friction angle, and tension strength is considered. The sensitivities of the rock mass strength calculated using SRM e.g., UDEC-DFN to the variability of the input parameter are investigated. A discussion of results is then made base on a reference simulation performed without a distribution method. 4 different P21 are investigated.

The cohesive crack model is implemented in UDEC to define contact law among generated BBM through the Voronoi tessellation technique. In order to assess the effect of the distribution of mechanical parameters in PDFN on the strength of rock mass, DFN is written and this feature is added. The results indicate the importance of the proposed model for the assessment of realistic rock mass strength in engineering applications.

Computation of fluid flow in fractured rocks is very important. The rock-mass is consisted of intact rock and fractures. Number and connectivity pattern of the fractures are two key factors controlling the fluid flow in the rock-masses. One of the most accurate methods to model geometrical structure of the rock-masses is discrete fracture network (DFN). Anisotropy and heterogeneity of the rock masses often affects the computations of the flow, therefore, three-dimensional DFN has been more desirable in literatures. Numerical calculation of the fluid flow requires solving a large system of equations which are generated by discretization schemes. Solving these systems are not usually straightforward and it needs more special and complex methods to converge the result. One of the best methods in this regard are Krylov subspaces methods. Evaluation of different Krylov subspaces methods which have been validated in comparison with a direct method and 3DEC modeling, has been considered in this research and the most optimized methods have been determined using a series of sensitivity analyses. Therefore, CG, CR and IOM have been characterized as the most accurate and fastest Krylov subspaces methods. The provided results in this research can be a sufficient guideline for the researchers who want to study the fluid flow in fractured rocks.

Nowadays, earth pressure balance tunnel boring machines (EPB-TBMs) are favorably used in urban mechanized tunneling projects. The availability of these machines is rather low, so their performance, time planning, maintenance scheduling and cost control may need further improvements. The reliability, availability and maintainability (RAM) of any machine could be analyzed by evaluating the actual performance of that machine or system and thereby minimizing maintenance costs. In current research work, RAM analysis of the electrical system of the EPB-TBM in line 1 of Tabriz metro in Iran is performed. For this investigation, failure and repair data were collected during about 26 months of machine operation. Statistical tests including trend and serial correlation tests indicated that the data are independently distributed, consequently the statistical technique was used for modeling. The data analysis also revealed that the time between failures (TBFs) and time to repairs (TTRs) data fit to the Lognormal (3P) and Gamma (3P) distributions, respectively. Reliability analysis showed that there is approximately 80.1% chance for the electrical system of TBM machine to be operational for 15 h without any failure. Moreover, the maintainability curve indicated that most of the failures have been repaired in less than 1 hour and that it is more likely that 80% of these failures would be repaired in 1.88h. Also the availability of electrical system of this machine is found to be equal to 97.8%.

Prediction of TBM performance is a crucial issue in mechanized tunneling projects، since machine performance has an influence on duration and costs of the projects. Rock mass parameters have significant effects on TBM advance rate as well as influence of these parameters on each other. In order to model the effect of geological conditions on TBM downtimes using Rock Engineering Systems (RES)، a thorough database based on three long water conveyance tunnels in Iran has been developed. Advance Rate index (ARi) has been calculated using Rock Engineering Systems method and the predicting relationship between the ARi and advance rate has also been obtained using regression analysis in quadratic equation form.

Excavation of a tunnel can cause changing stress concentration and development of probable displacements in the surroundings of excavated area. In the excavation process، excavation of tunnel head is the most critical step and if excavation of this part is not performed properly، then possibility of instability will be increased. In the Top-heading and Bench operation of tunneling، in addition to geo-mechanical properties، extension of plastic zone depends upon the operation circumstance. In this regard، advance rate of the tunnel head and bottom are very effective in the tunnel stability. To determine these parameters، numerical and experimental approaches can be employed. At present، numerical methods are successfully utilized in the stability analysis of the tunnels and for this purpose، three dimensional numerical modeling has given more realistic results in simulation of tunnel face and its effect on the support system. In this paper، stability analysis is performed for a tunnel which is excavated with Top-heading and Bench method having advance rate of 1 m and 2. 5 m for the tunnel head and bottom، respectively. Software was employed for stability analysis and the results are compared to the experimental methods RMR and Q. On the basis of the obtained results، it was concluded that for preventing instability and reducing pressure on the support systems، it is necessary to consider advance rate of 0. 5 m for both the tunnel head and bottom. Considering the obtained results، it was practically observed that the amount of displacements and destructive effects on the support systems were noticeably reduced in the case study tunnel.