JOURNAL OF ROCK MECHANICS
Year:2017 | Volume:1 | Issue:2 |Papers Count:8

Summary Different tests with different practical results have been so far presented for determination of mode I fracture toughness of rocks. In this paper, amongst various methods of mode I fracture toughness determination, Single Edge Crack Round Bar Bending Test and Straight Notch Disk Bend Test are selected in order to investigate the difference of fracture toughness values. Results of investigations showed minor difference of 13. 5% for fracture toughness values between these two tests. In order to identify the main cause of this difference, numerical modeling of these two tests was applied with three dimension finite element method. Besides, another reason of different values of fracture toughness in these two tests was notch width to length ratio of the sample. In regards to the less ratio of this factor in Single Edge Crack Round Bar Bending Test and being more acceptable to define notch as a crack in these test, Single Edge Crack Round Bar Bending Test provides more accurate and reliable values of fracture toughness. Introduction Fracture toughness represents resistance to crack expansion and is one of the most significant factors of fracture mechanics in rocks and other solids. Different tests with different practical results have been presented to date for determination of mode I fracture toughness of rocks. In practice, however, the results of these methods have been very different. In this paper, amongst various methods of mode I fracture toughness determination, Single Edge Crack Round Bar Bending Test and Straight Notch Disk Bend Test are selected in order to define the difference of fracture toughness values. Methodology and Approaches Fracture toughness values were determined experimentally. Next, in order to analyze the main causes of the differences between values of fracture toughness in different method, these tests were modeled with 3D finite element method. In these models, the fracture process zone around the crack tip was calculated for all tests based on “ normal tensile stress” criterion. Also, these two tests were compared based on notch type, and ratio of notch thickness to the sample length. Results and Conclusions Results of investigation showed minor difference of 13. 5% for fracture toughness values between these two tests. Numerical modeling showed the equal volume of Fracture Process Zone of crack tip in these two tests. Therefore, it can be said that accuracy level of Linear Elastic Theory in these two tests is the same. Besides, another reason of difference in the values of fracture toughness between these two tests was ratio of notch thickness to the sample length. In regards to the less ratio of this factor in Single Edge Crack Round Bar Bending Test (SECRBB) and being more acceptable to define notch as a crack in this test, Single Edge Crack Round Bar Bending Test (SECRBB) provides more accurate and reliable values of fracture toughness.

Summary We have investigated the validation of different closed form solutions for seismic design of circular tunnel lining. To this end, we have used the Tehran metro line 6 as a case study. The finite difference commercial code (Flac2D) is used to simulate ovalling deformation due to earthquake loads. The influence of the deformation on structural behavior of tunnel lining under various ground conditions and soil-structure interactions is also evaluated. Obtained results show that Penzien method calculates very low axial force under no slip condition. However the axial load by Wang and Park methods is very near to the computed load by numerical method. The validated numerical model is conducted to investigate the pseudostatic design of circular tunnel lining under real ground-structure interaction Introduction Since Iran is located in high risk area which experienced the intensive earthquake, the seismic analysis of underground structures is very important. There are presented some closed form solutions for seismic analysis of tunnel lining under full slip and no slip conditions. To different results obtained from the analytical methods under no slip condition, there is necessary to conduct numerical investigation for validation of the methods. Hashash et al. (2001), Sedarat et al. (2009), Park et al. (2009) and Do et al. (2015) have conducted some research in this area. Methodology and Approaches In this research, the analytical solutions based on mathematical equations and numerical modeling based on finite difference equations are conducted. To simulate the ground-structure interaction under no-slip, full slip and frictional conditions, the interface element is used in numerical modeling. The constitutive model of ground and lining is considered by linear elastic behavior and the seismic load is modeled by shear load at upper boundary of model. The induced inner traction in tunnel lining (axial force and moment) is calculated by numerical model at different aforesaid conditions and compared by analytical results. Results and Conclusions The seismic analysis of circular tunnel lining of Tehran metro line 6 is conducted by numerical model and analytical solutions. Obtained results show that the calculated axial load by Penzien method is not valid under no-slip condition. However, results of Park et al. method in very close to numerical model under no-slip condition. Under full slip behavior, the results of three analytical methods (Wang, Park and Penzien) are almost same and have certain difference with numerical model.

In recent years, mechanized tunneling and using tunnel boring machines in different ground conditions has developed. Selecting the appropriate tunnel boring machine is one of the important and determinative steps in excavation advances. In this regard, according to excavation and tunnel boring machine properties, can be select the best option. In this study, in first step, criteria such as RQD index, uniaxial comprehensive strength, tensile strength, wall's instability, face instability, distance between joints, water inflow, squeezing, fault zones, karstic voids, tunnel diameter, tunnel slope and tunnel curvature radius was selected, until in next step, four proposed machine be evaluated, and the appropriate option be selected. In following, by using Fuzzy-TOPSIS method and expert's comments, appropriate machine for excavation the second part of Emamzade Hashem tunnel was selected. The results showed that, according to the above criteria, the Double Shield Universal TBM is the best selection to excavate the second part of Emamzade Hashem tunnel. Summary In this study, in first step, in order to select the appropriate tunnel boring machine among proposed machines, thirteen criterions was chosen. In next step, For the use of expert's Comments, Questionnaires was sent. After weighting to criteria, and the proposed alternatives by using the experts Comments, best option was selected. Introduction Nowadays Due to population growth, road traffics has increased. To overcome this problem, reduction in costs and decrease the journey time, tunneling and using the tunnel boring machines has increased. Selecting the appropriate tunnel boring machine can be done by using decision-making methods. In order to, the alternatives most be compared according to criteria with each other. Finally according to the decision-making method solution, the best option is selected. Methodology and Approaches In this study, for selecting the appropriate TBM, Fuzzy-TOPSIS decision-making method were used. In this method in first step after making decision matrix and weighting to criteria, Results and Conclusions According to the results, Double Shield Universal TBM selected as appropriate Option. Flexibility of machine against criteria, specially Geotechnical and geological hazards is the most important reason to achieve this result.

Summary In the design of foundations on rock masses, it is necessary to examine the rock mass from different aspects. One aspect, in addition to settlement and instability, is determining the bearing capacity of rock foundations. Rock foundations have more strength and rigidity compared to soil foundations due to their rocky essense. In dead this aspect leads rock foundation to show the required bearing capacity against many incoming loads. However the factors of rock foundation such as crushed rock masses, discontinuities, high weathering, karst cavities, faulting, etc. reduce their bearing capacity. Due to the deployment of many structures on rock foundation and problems caused by structures instability on the inappropriate rock foundations, today, it is so necessity to examine rock foundations and estimate the effective parameters related to their bearing capacity, accurately and comprehensively. Studies have shown that the discontinuities have a great influence on the bearing capacity of rock foundations. In fact studies also have approved that the bearing capacity and settlement of Khark Island are allowed and it is suitable for construction of tanks...

Summary Failure mechanism of rocks is one of the fundamental aspects to study rock engineering stability. The macroscopic deformation and failure of rock is a dynamic, gradual and cumulative process of nucleation, growth, penetration, coalescence of micro-cracks, which is a non-equilibrium, non-linear evolutionary process. In the present study, the effect of loading rate on rock failure mechanism was considered. For this purpose, some experimental tests were conducted on Brazilian disk specimens of a homogeneous and isotropic sandstone at six different loading rate (0. 3, 0. 6, 1. 2, 2. 4, 4. 8 and 9. 6 mm/min). During the tests, acoustic emission (AE) sensors were used to monitor the fracturing process. AE monitoring showed that micro-crack density induced by the applied loads during different stages of the failure processes increases as loading rate increases. Also, it is found that loading rate influences the mode of induced fracture, so that the number of tensile fractures decreases when loading rate increases.

Summary Usually, TBM performance and operational parameters and their variations are related to variations of geological conditions along the tunnel. This relationship is the base for developing many empirical TBM performance prediction models. By knowing the geological conditions along the tunnel alignment it is possible to predict the TBM performance using these models. It’ s obvious that in the reverse way by knowing the actual performance and operational parameters of the machine in the bored tunnel, the approximate geological conditions along the tunnel can also be recognized. This technique especially in the tunnels that being bored by shielded machines can be useful for recognizing the engineering geological properties of the excavated rock masses. In this research an attempt is made to detect exact location of the poorkan-vardij active fault in the tunnel alignment using the actual TBM performance parameters. Introduction Presence of active fault zones can be considered as one of the main geological hazards in mechanized tunneling projects that induces serious stability problems and direct faulting during tunnel excavation and operation. During tunnel excavation when shielded machines are used for tunnel completion, due to limitations in observation of tunnel face, recognizing the exact location of fault zones is very difficult. Therefore it is required to use the indirect methods like using the actual performance data collected during construction and analyzing them to probe the geological conditions of tunnel alignment. In the Karaj water conveyance tunnel project (Lot 2) after tunnel breakthrough, as a client requirement, it was needed to locate the poorkan-vardij fault zone in the tunnel, to design a scheme for reducing the damages that may occur if the fault is activated during the tunnel operation. Methodology and Approaches In this research to detect the exact location of poorkan-vardij fault zone, a part of the tunnel alignment (almost 1000 meter length) around the fault is selected, and variations of TBM performance and operational parameters were analyzed. Results showed that among the considered parameters, two parameters of Field Penetration Index (FPI) and Torque/Thrust ratio are good criteria to detect fault zones. Results and Conclusions Results of this study by considering the direct and indirect indices showed that the crushed zone caused by poorkanvardij fault is almost 400 meters in thickness and is located in the chainage 6050 to 6450 m of tunnel alignment. This section of tunnel must be considered as the most likely position of damage zone. Therefore, special design to overcome the damages of tunnel during fault activation must be considered for this section.

Summary In this paper, the effect of cutter edge shape on the failure mechanism of rock has been investigated using particle flow code in two directions (PFC2D). Particle flow code represents a rock mass as an assemblage of bonded rigid particles. The standard process of generating a PFC2D assembly to represent a test model involves: particle generation, packing the particles, isotropic stress installation (stress initialization), floating particle (floater) elimination and bond installation. In PFC2D circular disks are connected with cohesive and frictional bonds and confined with planar walls. The values assigned to the strength bonds influence the macro strength of the sample. Friction is activated by specifying the coefficient of friction and is mobilized as long as particles stay in contact. Tensile cracks occur when the applied normal stress exceeds the specified normal bond strength. Shear cracks are generated as the applied shear stress surplus the specified shear bond strength either by rotation or by shearing of particles. The tensile strength at the contact immediately drops to zero after the bond breaks, while the shear strength decreases to the residual friction value. For all these microscopic behaviours, PFC only requires selection of the basic micro-parameters to define contact and bond stiffness, bond strength and contact friction, but these micro-parameters should provide the macroscale behaviour of the material being modelled. The code uses an explicit finite difference scheme to solve the equation of force and motion, and hence one can readily track initiation and propagation of bond breakage through the system. For this purpose, two numerical models with different tensile strength of 5 MPa and 25 MPa were built and compressed by two different confining pressures of 5 MPa and 25 MPa, respectively. Eight disc cutters with different edge shape peneterate into the model at the rate of 0. 02 m/s till 4 mm of disc peneteration is reached. Totally 16 simulation has been done. The rock materials, below the cutters, show three different mechanical behavior i. e. failure, plastic and elastic behavior. The failure zone is fully fractured. The plastic zone is consisted of partially micro crack with several major fractures. The elastic zone is an undamaged zone. The shape of cutter edge has important effect on extension of three introduce zones. When tensile strength is 5 MPa, the failure stress resulted from penetration of convex-shape cutter is the lowest one, 5. 3 MPa while the number of total cracks is maximum one, 102. It means that the cutter shape controls the failure stress and failure extension when it cuts the weak rock. When tensile strength is 25 MPa, the failure stress resulted from penetration of different cutters is similar, 21 MPa, but the extension of failure is largest under Ushape cutter. It means that the cutter shapes has not any effects on the failure stress when it cuts the hard rock while the U-shape cutter produce the largest failure zone. The results show that convex-shape edge and U-shape edge cutters have the best performance when tensile strength of rock is 5MPa and 25 MPa, respectively. The results also showed that the failure stress increases with increasing tensile strength, while the failure extension decreases.

A new apparatus which allows laboratory Hydraulic fracturing experiments under true triaxial compression was developed at Shahrood University and laboratory hydraulic fracturing experiments were conducted to investigate the dependence of breakdown pressure upon two factors which could influence the use of the hydrofrac technique for in-situ stress determinations namely: the viscosity of fluid and borehole diameter. Therefore, in this study, by using the new apparatus was examined if the fracture pressure will change with viscosity of fluids and borehole diameter. In this regard, Some 300 mm cube specimens of concrete were made and used in this study. It should be noted that, three steel rods of 30, 50 and 80 mm outer diameter were buried for a simulated borehole in the specimens. Meanwhile, 3 types of Hydraulic oil (46H, 68H and 100H) were injected to boreholes of different diameters. In this study, to examine the effect of viscosity of fracturing fluid and borehole diameter on hydraulic fracturing breakdown pressure, some tests were conducted. Results show that breakdown pressure decreases with increasing borehole diameter and also there is an inverse relationship between breakdown pressure and viscosity of fracturing fluid. In other words, it decreases with increasing viscosity of fracturing fluid...