ISPITIVANJE UTJECAJA PRUŽANJA DISKONTINUITETA NA REGIONALNI PROTOK FLUIDA U ŠUPLJIKAVOJ STIJENI UPORABOM HIBRIDNE METODE KONAČNIH VOLUMENA I MREŽE DISKRETNIH PUKOTINA TE SIMULACIJOM STRUJNICA

Understanding the fluid behaviour in rock masses is of great importance in various rock mass-related engineering projects, such as seepage in tunnels, geothermal reservoirs, and hazardous waste disposal. Different approaches have been implemented to study the flow pattern in fractured porous rock masses. Laboratory experiments can provide good information regarding this issue, but high expenses aside, they are time-consuming and suffer the lack of ability to study field scale mediums. Numerical methods are beneficial in simulating such mediums with the Discrete Fracture Network (DFN) method in terms of costs and time as they offer sufficient flexibility and creativity. In this paper, a Matlab code was extended to study the flow regime in a Dual Permeability Media (DPM) with two point sources in the right and left side of the model as an injector and a producer well, respectively. A high permeability discontinuity with different angles was embedded in a very low-permeability limestone matrix. Pressure equations were solved implicitly with a two-point flux approximation scheme of the Finite Volume Method (FVM). Streamlines were traced in the medium and used to analyse the model’s hydraulic behaviour with the aid of Time Of Flight (TOF) for each point. The results show that the FVM-DFN hybrid method can be used as a fast method for fluid flow in DPM with the aid of streamline simulation to study the fluid flow in a large model with discontinuity.Razumijevanje ponašanja fluida u stijenskoj masi izrazito je važno kod različitih inženjerskih projekata kao što su procjeđivanje u tunelima, geotermalna ležišta te odlaganje opasnoga otpada. U proučavanju obrasca protoka fluida kroz raspucanu, šupljikavu stijensku masu korišteni su različiti pristupi. Laboratorijska istraživanja mogu pomoći u izučavanju takvih problema, međutim, osim što su skupa, zahtijevaju puno vremena i teško ih je primijeniti u makrostrukturama. Numeričke simulacije mogu opisati takve prostore metodom mreže diskretnih pukotina smanjujući troškove i vrijeme jer nude dovoljnu prilagodljivost i kreativnost. Ovdje je prikazano proširenje koda u Matlabu s ciljem izučavanja protoka u stijenskome prostoru s dvostrukom propusnošću, tj. s izvorima fluida na desnoj i lijevoj strani modela koji predstavljaju utisnu i proizvodnu bušotinu. Vrlo propusni diskontinuiteti s različitim kutovima smješteni su unutar slabopropusnoga vapnenačkog matriksa. Jednadžbe tlaka aproksimiraju izravan shematski tok između dviju točaka metodom konačnih volumena. Strujnice su praćene kroz simulirani volumen te je njima analizirano hidrauličko ponašanje modela izračunom brzine protoka u svakoj točki. Rezultati pokazuju kako hibridna metoda konačnih volumena i mreža diskretnih pukotina mogu biti korištene kao brz način opisivanja protoka fluida u prostoru s dvostrukom propusnošću, tj. uz pomoć simulacije strujnica unutar makromodela s diskontinuitetima

Cumulative Fatigue Damage Under stepwise Tension-Compression Loading

Rock structures are subjected to cyclic tension-compression loading due to a blasting, earthquake, traffic and injection-production in underground storage case. Therefore study the fatigue behavior of rock samples under this type of loading is required. In this study, the accumulated fatigue damage for a Green Onyx rock sample which consisted of only one mineral composition with two-step high-low sequences of loading levels was investigated under completely reversed loading condition. New apparatus based on the R.R. Moore fatigue test machine is designed to assess this type of loading. A comparison between the predicted behavior of Linear Damage Rule and experimental data was conducted and a new damage model was proposed based on the experimental observation. The results showed a good agreement between the proposed damage model and experimental data

Rock Slope Stability Analysis in the Left Abutment of Bakhtiary Dam, Iran

In this research, directions of in-situ stresses in the rock slope in the left abutment of Bakhtiary dam (Center of Iran) are defined taking advantage of geological history, tectonic evolution of the area, and in-situ tests. To that end, the study draws on the kinematic analysis, limit equilibrium and numerical methods. It is of note that there is no possibility for toppling failure if kinematic analysis is used to study the stability in left abutment of Bakhtiary dam. The plane failure analysis indicated that there is a possibility of failure in the middle and upper walls based on joint set J1. Also, from geological perspective, wedge failure in the middle and upper walls is possible due to the intersection of bedding planes and Joint set J1. In the analysis of the slope stability using limit equilibrium, the least value of the safety factor obtained for plane failure belongs to joint set J1 in the upper wall, indicating that the left abutment is stable. Numerical analysis indicated that this slope needs support requirements

Prediction of strength and deformability of an interlocked blocky rock mass using UDEC

The accurate prediction of strength and deformability characteristics of a rock mass is very challenging. In practice, properties of a rock mass are often estimated from available empirical relationships based on the uniaxial compressive strength (UCS). However, UCS does not always give a good indication of in-situ rock mass strength and deformability. The aim of this paper is to present a methodology to predict the strength and deformability of a jointed rock mass using UDEC (universal distinct element code). In the study, the rock mass is modelled as an assemblage of deformable blocks that can yield as an intact material and/or slide along pre-defined joints within the rock mass. A range of numerical simulations of UCS and triaxial tests were conducted on rock mass samples in order to predict the equivalent mechanical properties for the rock mass under different loading directions. Results are compared against the deformability parameters obtained by analytical methods

A fractal model for characterizing fluid flow in fractured rock masses based on randomly distributed rock fracture networks

A fractal model that represents the geometric characteristics of rock fracture networks is proposed to link the fractal characteristics with the equivalent permeability of the fracture networks. The fracture networks are generated using the Monte Carlo method and have a power law size distribution. The fractal dimension DT is utilized to represent the tortuosity of the fluid flow, and another fractal dimension Df is utilized to represent the geometric distribution of fractures in the networks. The results indicate that the equivalent permeability of a fracture network can be significantly influenced by the tortuosity of the fluid flow, the aperture of the fractures and a random number used to generate the fractal length distribution of the fractures in the network. The correlation of fracture number and fracture length agrees well with the results of previous studies, and the calculated fractal dimensions Df are consistent with their theoretical values, which confirms the reliability of the proposed fractal length distribution and the stochastically generated fracture network models. The optimal hydraulic path can be identified in the longer fractures along the fluid flow direction. Using the proposed fractal model, a mathematical expression between the equivalent permeability K and the fractal dimension Df is proposed for models with large values of Df. The differences in the calculated flow volumes between the models that consider and those that do not consider the influence of fluid flow tortuosity are as high as 17.64-19.51%, which emphasizes that the effects of tortuosity should not be neglected and should be included in the fractal model to accurately estimate the hydraulic behavior of fracture networks

Fracture mechanism simulation of inhomogeneous anisotropic rocks by extended finite element method

The vast majority of rock masses is anisotropic due to factors such as layering, unequal in-situ stresses, joint sets, and discontinuities. Meanwhile, given the frequently asymmetric distribution of pores, grain sizes or different mineralogical compounds in different locations, they are often classified as inhomogeneous materials. In such materials, stress intensity factors (SIFs) at the crack tip, which control the initiation of failure, strongly depend on mechanical properties of the material near that area. On the other hand, crack propagation trajectories highly depend on the orthotropic properties of the rock mass. In this study, the SIFs are calculated by means of anisotropic crack tip enrichments and an interaction integral are developed for inhomogeneous materials with the help of the extended finite element method (XFEM). We also use the T-stress within the crack tip fields to develop a new criterion to estimate the crack initiation angles and propagation in rock masses. To verify and validate the proposed approach, the results are compared with experimental test results and those reported in the literature. It is found that the ratio of elastic moduli, shear stiffnesses, and material orientation angles have a significant impact on the SIFs. However, the rate of change in material properties is found to have a moderate effect on these factors and a more pronounced effect on the failure force. The results highlight the potential of the proposed formulation in the estimation of SIFs and crack propagation paths in inhomogeneous anisotropic materials

Effect of Workpiece Orientation, Lubrication and Media Geometry on the Effectiveness of Vibratory Finishing of Al6061

Vibratory Finishing is a commonly used process for deburring and polishing. The process effectiveness is difficult to quantify due to a nexus of variables relating to workpiece, media, compound and machine. In this paper, effect of orientation of rectangular Al6061 workpieces is considered with respect to the motion of ceramic media using water as lubricant. It is found that parallel orientation of workpiece to media motion and higher lubrication level lead to higher degree of sliding, thereby leading to smoother finish. Experiments with different ceramic media showed smoother finish with the geometry more susceptible to rounding. The aforesaid findings can be employed in applications requiring superpolished surfaces

Scale and Stress Effects on Hydro-Mechanical Properties of Fractured Rock Masses

In this thesis, the effects of size and stress on permeability, deformability and strength of fractured rock masses are investigated. A comparison study was carried out to examine the effects of considering, or not considering, the correlation between distributions of fracture apertures and fracture trace lengths on the hydro-mechanical behavior of fractured rocks. The basic concepts used are the fundamental principles of the general theory of elasticity, Representative Elementary Volume (REV), the tensor of equivalent permeability, and the strength criteria of the fractured rocks. Due to the size and stress dependence of the hydro-mechanical properties of rock fractures, the overall effective (or equivalent) hydro-mechanical properties of the fractured rocks are also size and stress-dependent. However, such dependence cannot be readily investigated in laboratory using small samples, and so numerical modeling becomes a necessary tool for estimating their impacts. In this study, a closed-form relation is established for representing the correlation between a truncated lognormal distribution of fracture apertures and a truncated power law distribution of trace lengths, as obtained from field mapping. Furthermore, a new nonlinear algorithm is developed for predicting the relationship between normal stress and normal displacement of fractures, based on the Bandis model and the correlation between aperture and length. A large number of stochastic Discrete Fracture Network (DFN) models of varying sizes were extracted from some generated large-sized parent realizations based on a realistic fracture system description from a site investigation programme at Sellafield, UK, for calculating the REV of hydro-mechanical properties of fractured rocks. Rotated DFN models were also generated and used for evaluation of the distributions of directional permeabilities, such that tensors of equivalent permeability could be established based on stochastically established REVs. The stress-dependence of the permeability and the stress-displacement behaviour were then investigated using models of REV sizes. The Discrete Element Method (DEM) was used for numerical simulation of the fluid flow, deformability properties and mechanical strength behavior of fractured rocks. The results show significant scale-dependency of rock permeability, deformability and strength, and its variation when the correlation between aperture and trace length of fractures are concerned, with the overall permeability and deformability more controlled by dominating fractures with larger apertures and higher transmissivity and deformability, compared with fracture network models having uniform aperture. As the second moment of aperture distribution increases, a fractured rock mass shows more discrete behavior and an REV is established in smaller value of second moment with much larger model size, compared with the models with uniform fracture aperture. When the fracture aperture pattern is more scattered, the overall permeability, Young’s modulus and mechanical strength change significantly. The effect of stress on permeability and fluid flow patterns in fractured rock is significant and can lead to the existence or non-existence of a permeability tensor. Stress changes the fluid flow patterns and can cause significant channeling and the permeability tensor, and REV may be destroyed or re-established at different applied stress conditions. With an increase in the confining stress on the DEM models, the strength is increased. Compared with the Hoek-Brown criterion, the Mohr-Coulomb strength envelope provides a better fit to the results of numerical biaxial compression tests, with significant changes of the strength characteristic parameters occurring when the second moment of the aperture distribution is increased.QC 2010070

An analytical model for estimating rock strength parameters from small-scale drilling data

The small-scale drilling technique can be a fast and reliable method to estimate rock strength parameters. It needs to link the operational drilling parameters and strength properties of rock. The parameters such as bit geometry, bit movement, contact frictions and crushed zone affect the estimated parameters. An analytical model considering operational drilling data and effective parameters can be used for these purposes. In this research, an analytical model was developed based on limit equilibrium of forces in a T-shaped drag bit considering the effective parameters such as bit geometry, crushed zone and contact frictions in drilling process. Based on the model, a method was used to estimate rock strength parameters such as cohesion, internal friction angle and uniaxial compressive strength of different rock types from operational drilling data. Some drilling tests were conducted by a portable and powerful drilling machine which was developed for this work. The obtained results for strength properties of different rock types from the drilling experiments based on the proposed model are in good agreement with the results of standard tests. Experimental results show that the contact friction between the cutting face and rock is close to that between bit end wearing face and rock due to the same bit material. In this case, the strength parameters, especially internal friction angle and cohesion, are estimated only by using a blunt bit drilling data and the bit bluntness does not affect the estimated results. Keywords: Analytical model, Rock strength parameters, Small-scale drilling dat