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Collaborating Authors
Maji, V. B.
Abstract Tunnel Boring Machines (TBMs) are widely utilised for mechanical rock cutting in rock tunnelling. Design optimization and performance estimates play a key role in the success of mechanical rock cutting. For designing TBM operational parameters, important aspects such as torque and driving force are important together with the rock mass parameter. Accuracy in the estimation of forces acting on the TBM disc cutter is also important. To understand the complex interaction during the rock cutting process, researchers mostly used a linear cutting machine test (LCM) performed to find the forces working on a unit disc cutter. The outcomes are then used for TBM design on the same rock. In the present study, LS-Dyna software is used to simulate the LCM test in this study for a definite cross-sectional cutter. The Lagrangian method is used as the solution method where the accuracy of the outcomes depends on the failure parameters defined for element erosion. The simulation results are then compared to the laboratory test data from literature. To find the width of the damage area in the targeted rock, the RHT material model will be used in this study. It is important to accurately estimate the width of the damage area so as to determine the effect of the spacing of TBM disc cutters on the forces acting on them. Introduction In the process of underground excavation, mechanized drilling plays a vital role. It is considered as a typical feature in underground excavation. It is significant in the construction of underground systems like tunnels and various mining applications. Design optimization and performance estimation are two of the key aspects in the effective application of mechanised drilling. Tunnel boring machine performance prediction may seem large scale for the dimensional reasons. However, it can become small scale when it is looked from the interaction of the cutting disc with a rock that eventually cause disc cutter wear and rock fragmentation. The primary tool used by TBMs for cutting rock are disc cutters that undergo wear during rock cutting.
- Asia > Vietnam (0.20)
- North America > United States > Colorado (0.16)
- Energy > Oil & Gas > Upstream (1.00)
- Machinery > Industrial Machinery (0.81)
ABSTRACT: Fracture initiation and propagation modelling of poroelastic medium has a vast practical application such as extraction of oil and natural gas by hydraulic fracturing, dam failure due to high fluid pressure and a lot more. This has led to an increased attention among the researchers to model fracturing in geomaterials. However, modelling this process in standard FEM is a challenging task as the rock deformation and pore fluid pressures are coupled, it involves a lot more degrees of freedom per node compared to a deformation analysis of dry rock. On the other hand, fracture propagation with standard finite element method is a highly computationally expensive technique, which involves the remeshing of the fracture domain after every time increment. This paper adopts the XFEM technique to study the effects of rock stiffness on the behaviour of fracture propagation in a saturated rocks mass, and the pore pressure variation in to the crack tip region. The finding shows a negative pore pressure distribution in to the fracture process zone, and increased length of fractures for stiffer rocks. 1. Introduction The fluid-driven fracture propagation in the saturated porous medium is being studied for a long time. This technique is mostly used in hydrocarbon extraction from subsurface rocks. Analysing the stress-deformation conditions deep underground through field and laboratory investigation is often not feasible and therefore numerical techniques preferred to be adopted. The pore fluid plays an essential role in the mechanics of geomaterials. Therefore the numerical formulation, in this case, should also consider the coupling between deformation and pore pressure. Solutions for these sort of coupling equations can be achieved possibly in four different ways namely fully coupled, iteratively coupled, explicitly coupled and loosely coupled solutions (Armero, 1999, Minkoff, 2006, Massimiliano et al., 2010, and Kim et al., 2011). Among these solutions, the fully coupled solutions technique is unconditionally stable but with a higher computational effort. The finite element is one of the most suitable and widely accepted numerical methods for solving hydromechanically coupled domain problems (Zienkiewicz et al., 1999, Khoei, 2014). The accuracy of this method depends upon the polynomial approximation for the degrees of freedom (DOF). In the case of fracture propagation problems, when there are discontinuities in the field variable (displacement) or singularities in the crack tip zone, getting an optimally accurate solution may need the re-meshing of the domain after every time step of fracture evolution in the standard finite element method, therefore, the system matrix will be reconstructed every time, which will make this a computationally expensive method to adopt. On the other hand with the extended finite element method (XFEM) this problem can be resolved by choosing an extra set of DOF which will take care of the displacement part due to discontinuity. The number of DOF will be increased for those nodes whose basic function is cut by the crack. However implementing this technique into the already written standard finite element codes (e.g. Abaqus) are not easy, as the degrees of freedom per node will be varying, some nodes with enriched DOF and some without, different element matrix will have different dimensions (Belytschko et al., 2013). To overcome this issue, the enrichment DOF are added to some additional nodes which are called phantom nodes (Song et al., 2006). Only the elements with the fracture cut will have these phantom nodes activated. In this present work, an attempt is made to study the effect of stiffness on the pore pressures at the near crack tip zone considering the cohesive zone model (CZM) with maximum principal stress criterion as the fracture propagation mechanism. It has been tried to incorporate the XFEM technique for fracture propagation in the hydromechanically coupled rock.
- Europe (0.46)
- North America > United States > Texas (0.28)
Abstract The behaviour of jointed rock mass is highly dependent on the properties of rock joints. One of the most important property used in identifying the behaviour and performance of rockmass is the joint stiffness and the deformation behaviour of tunnels in jointed rocks is dependent on this joint stiffness. As the stiffness parameter rises to a very high value, the joint attains the strength comparable to an intact rock and behaves as a welded joint. Many studies under static as well as in dynamic conditions have been conducted to understand the effect of joint stiffness on rock mass behaviour. In this study, the wave propagation characteristics and transmission coefficient have been studied on single jointed rock mass under the influence of varying joint stiffness using Universal Distinct Element codes (UDEC). The analysis has been further extended to understand the effect of joint stiffness on the tunnel in jointed rocks with shake table model testing and subsequent comparison with dynamic loading with a sinusoidal input. A parametric study is performed to understand the effect of joint normal stiffness and joint shear stiffness on the performance of jointed rock mass is investigated. It is found that the tunnel deformation is linearly varies with the joint normal stiffness under static conditions but it varies exponentially under dynamic loads. 1. Introduction Rock mass encountered in the field is usually found with joints. These joints being unavoidable, needs special attention in the execution of big rock engineering projects. The presence of joints reduces the strength of rocks and the joint strength becomes the factor determining the performance of the rock mass. The main factor affecting the strength of the joints is the joint stiffness parameter. Unlike many other rock parameters, the estimation of joint stiffness is difficult mainly due to the absence of any proper specifications or guidelines (Kutalilake et al., 2016). A lot of research has been done on laboratory experiments to identify the joint displacement behaviour (Goodman, 1974, 1976; Bandis, 1983; Barton and Bandis, 1980, Jing et al., 1994). Joint normal stiffness (Goodman, 1968) can be defined as the ratio joint normal stress to the normal displacement. Finding the joint stiffness in a rock mass medium is considerably tough and studies established the importance of insitu stresses on the joint normal stiffness. Many linear and nonlinear models were developed to define the relationship between joint stress and joint displacements. Study by Hsuing et. al. (1993) highlighted the importance of normal stress in joint normal stiffness and joint shear stiffness and emphasized on the importance of normal stiffness on shear displacement. Most of the studies being done under laboratory conditions to understand the joint behaviour, Barton (1981) gave an equation correlating joint stiffness to the rock mass modulus and intact rock modulus.
- Energy > Oil & Gas > Upstream (1.00)
- Government > Regional Government > North America Government > United States Government (0.46)
Abstract The post peak behavior of rocks has a significant influence on its strength and deformational characteristics and is crucial in many applications like underground coal mining. A clear understanding of post peak behavior of coal is necessary for economical and safe underground coal extraction. In this study, laboratory experiments and corresponding numerical modeling is done to understand the post peak behavior of a coal and coal pillar. A series of laboratory tests are conducted using coal samples and also artificially prepared gypsum samples with varying material strength. A numerical model is developed in FLAC3D and simulations are run with strain softening model to capture the post peak responses of the material. This model allows to attain the peak strength following the Mohr-Coulomb behavior and once it attains the peak, the strength parameters are softened with respect to plastic strain that the material experienced using a piecewise linear functions. The softening parameters are selected in such a way that a realistic behavior could be achieved. With the validated model, several parametric studies such as influence of dilation, confinement, and friction angle are performed. Understanding the influence of the post peak parameters gave a full extent of the usage of material performance in the numerical model for better design of underground excavations. The numerical model is subsequently extended to design coal pillar for a underground mines is briefly discussed. 1. Introduction Design of supporting rock pillars in underground excavations specially applications like mining is based on the maximum pillar strength as well on the post-failure behavior. The complete stress-strain behavior of the pillars play an important role for those pillar stability. Around the underground structures there is possibility of formation of plastic regions. Hence, the design steps together with support systems expected to be accommodative in accordance with the existence of such regions. To estimate the parameters related to the post-failure of supporting pillars, large scale in-situ compression tests are needed to be conducted, which is quite difficult and expensive. In this study, post peak characteristics of coal samples and artificially prepared gypsum samples are obtained using MTS servo controlled testing machine available in the Department of Civil Engineering, IIT Madras. The same experiments are also numerically modeled in FLAC3D and attempt was made to capture the post peak response. Simulation were done by moderating various parameters namely cohesion, friction angle and dilation. Mohr-Coulomb Strain Softening model is used as the decay of strength parameters with respect to the plastic strain to obtain the softening behavior (Itasca FLAC3D manuals, 2008). With the validated model, parametric studies are done in order to understand the influence of various parameters on post peak responses.
- Asia (0.49)
- North America > United States (0.47)
ABSTRACT: The effect of joints on wave propagation in rocks has always been a matter of concern with regard to their transmission and attenuation characteristics. Experimental studies are conducted to analyze the wave attenuation characteristics in single and parallel fractures at varying angles considering non welded interfaces in rocks. After validation with the ultrasonic pulse velocity tests, numerical models using UDEC were developed to simulate the wave propagation in jointed rocks. The numerical study was extended for multiple fractures over a range of ultra sonic frequencies. The effect of the fracture angle and number of cracks on the transmitted P wave velocity and corresponding travel time ratio were investigated. This study was conducted on blocks of artificially prepared rock specimens casted with gypsum plaster, introducing artificial joints at various angles. The variation in P wave velocity and travel time ratio for different joint angles and multiple joints have been studied. 1. INTRODUCTION
- Research Report > New Finding (0.35)
- Research Report > Experimental Study (0.34)
Abstract Large infrastructure projects like tunnel excavations, underground constructions face innumerable risks. For successful completion of these projects, it is essential that we foresee and understand risks that could occur during different phases of the project. One of the risks affecting underground excavation projects is the inaccurate performance prediction of TBMs. As this could severely affect the project, resulting in lower advance rates and could also result in changing the excavation machine or method, due to difficulties in excavation, thereby increasing the overall cost of tunnelling. This makes performance prediction of the TBMs as one of the crucial aspects in tunnelling, as a precise estimation of its performance is required for planning and estimation of time and cost of the tunnelling projects. This performance prediction of TBM is a challenging geotechnical problem that is intricate and complex in nature, this is possibly due to the fact that TBM performance prediction involves understanding the rock fragmentation process in wide range from micro-scale (i.e. the interaction of surface of rock material and cutter tip) to macro-scale (including the interaction of rock mass and TBM). Thus in order for the performance prediction model to be efficient, parameters of both TBM specifications and ground conditions have to be considered. Various models have been proposed by researchers since the early phases of TBM application, these studies resulted in the development and improvement of numerous penetration prediction models. Most of these models consider the geotechnical parameters that are input to the model to be consistent along the TBM drive and neglect the uncertainties in parameters that occur. And so to improve the predictive capability of these models and to incorporate the above uncertainties, the penetration rate is modeled as a stochastic variable depending on the geotechnical and TBM parameters, its statistical distribution is determined and is used to characterize the penetration rate instead.
Abstract Rock under natural conditions experiences different stress environment and it fails when it crosses its threshold value. Understanding their failure will not lead to determine the strength and deformational characteristics but also will be useful for safe and economic design of structures which are built in rock. To know their mechanism of failure many failure theories have been developed and some of them most commonly used were Mohr-Coulomb and Griffith theories which are found to be applicable for rock. Based on these theories many researchers have done experiments to verify those failure mechanisms when it is been subjected to different stress condition and later extended with some other conditions which influence their behavior in failure. But even though some researchers have found these theories and experiments does not fully capture the complete mechanism of failure and they have their advantages and limitations especially when rock having discontinuities like cracks, joints, flaws, plane of weakness etc present in it. And by considering their limitations recent decades researchers have adopted numerical tool like finite element, finite difference methods to overcome these limitations. The present study focuses on the numerical verification of recently developed failure criterion and implementing it Finite element software ABAQUS to understand their failure mechanism when subjected to uniaxial loading conditions how their existing crack will gets initiated and propagated further ends with failure. 1. Introduction The nature of the rock is said to be inhomogeneous, anisotropic and inelastic because of the presence of discontinuities such as plane of weakness, cracks, flaws, joints etc. Rock under natural conditions experiences different stress environment. Based on the type of external load configuration and discontinuities present in it, the rock will have different failure modes. When the rock is subjected to higher stresses it fails at a certain point when it crosses the threshold value. Understanding the complete mode of failure will provide useful information for safe and economic design of structures and foundations involving rock. In order to understand the complete failure mechanism of rocks, many researchers have developed theoretical criteria based on experimental observations in accordance with the field. Most of experimental studies have considered rocks as isotropic and intact (unjointed) which is rarely found in nature. Some of the researchers have considered rock as a transversely isotropic and anisotropic material by implementing the numerical tools like finite element method (FEM), finite difference method (FDM), etc. They have verified those failure modes obtained from numerical analysis with the failure modes found from experiments. The present paper is an attempt to understand the different failure modes of brittle natured rock when subjected to compressive stress.
ABSTRACT: An attempt is made to exhibit the shear strength dependency of the strain using finite difference based package FLAC. Stability of the slope is a function of the shear strength and the development of failure strain reflects the potential failure zones of slope. The shear strain developed in the slope increases with reduction in the shear strength and is reflected in the analysis. The concept of failure ratio (Rf) is incorporated in shear strength reduction technique and is demonstrated. Relationships between the critical shear strength reduction ratio and the safety factor are examined. The variation of shear strain with shear strength reduction ratio for different values of failure ratio (Rf) is also studied. 1 INTRODUCTION In shear strength reduction technique the factor of safety (FOS) of a slope traditionally defined as the ratio of the actual shear strength to the minimum shear strength to prevent failure. The method was first used by Zienkiewicz et al. (1975) and later by Naylor (1981), Matsui and San (1992), Ugai and Leschinsky (1995), Griffith and Lane (1999),Dawson et al. (1999), Lechman and Griffiths (2000), Zhang et al. (2009) and many other researchers. In this method, the failure surface is automatically defined unlike conventional limit equilibrium. In the shear strength reduction technique it is assumed that failure mechanism of a slope is directly related to the development of the shear strain and the existence of the shear strength dependency of the strain. As the shear strain developed in the slope increases with reducing the shear strength, the existence of the shear strength dependency of the strain is also related with the stability of slope (Matsui and San, 1992). It has been demonstrated by many researchers using laboratory tests that the failure shear strain zone usually coincided with the rupture surface (Roscoe, 1970).