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

Analysis of Soil Nailed Walls Under Harmonic Dynamic Excitations Using Finite Difference Method

Soil nailing is an efficient method to stabilize different soil structures. The method has been extensively used for improving stability of slopes. The construction process of Soil nailed walls commonly involve three basic sections: excavation, nail installation and face stabilization. The nail bars are inserted into ground by either drilling or grouting and are usually arranged in both horizontal and vertical directions. Present research intends to understand Soil-nailed wall behavior under dynamic excitations. Employing finite difference method a three dimensional model has been developed in the proper finite difference code. Soil constitutive behavior for dynamic analyses is predicted taking into account soil hysteresis behavior. To simulate nail bars cable structural elements are employed and also liner structural elements will be utilized for shotcrete facing. Dynamic excitation incorporated as semi-seismic harmonic loading is applied at the bottom of the model where represents soil subgrade. The boundary conditions are considered to be antisymmetric during dynamic analyses. Effects of different crucial factors are monitored during investigations. Some parameters such as, input motion frequency, nail inclination, nail length as well as soil strength properties have been examined

Seismic Response of Structures with Underground Stories Considering Non-Linear Soil-Structure Interaction

Most of the research conducted for soil-structure interaction analysis of structures are assuming the linear behavior of soil. It is well known that during strong ground excitations the soil adjacent to the structure behaves highly non-linear. The nonlinear soil behavior affects the soil-structure interaction in a complex way especially because of the inadequacy in modeling the unbounded soil medium. In the case where an elastic soil behavior is assumed, the surface motion will be amplified proportionally to the input motion. However, in reality the amplitude and frequency content of the response are modified due to the soil’s stiffness degradation and higher energy dissipation. The present work deals with the influence of soil non-linearity, introduced by hysteretic behavior of near-field soil, on the soil-foundation-structure interaction phenomena. The objective is to reveal the beneficial or detrimental effects of the non-linear SSI concerning both the drift and settlement of structures with underground stories. To examine the effect of non-linear soil-structure interaction a realistic non-linear soil model is incorporated into the finite difference FLAC software. To better understanding the non-linear dynamic SSI, interface elements are also used between the near-field soil and basement walls. For a practical structure throughout a parametric study, some non-linear seismic analyses are performed to demonstrate the effectiveness of the affecting parameters in response of the structure. The results showed much difference on seismic response of structure such as drift, settlement and developing pressure around the basement walls when the non-linear soil-structure interaction is considered

Nonlinear Seismic Analysis of Buried Pipelines During Liquefaction

The safety of buried pipelines during earthquakes has involved a great deal of attention in last few years. Important characteristics of buried pipelines are that they cover large areas and can be subjected to a variety of geotectonic hazards. Earthquake damages to buried pipelines can be attributed to transient ground deformations (TGD), permanent ground deformations (PGD) or both. PGD occurs as a result of surface faulting, liquefaction, landslides, and differential settlement from consolidation of cohesionless soil. To evaluate seismic behavior of buried pipelines subjected to large values of permanent ground deformations, appropriate non-linear cyclic stress-strain relationship should be implemented in any numerical method. Among the phenomena, which cause permanent ground deformations, the settlement and lateral spreading induced by liquefaction are considered as the main cause of damage in buried structures. Therefore, this study is aimed to take into account the potential of liquefaction during an earthquake into the numerical analysis of buried pipelines using FEM. During the earthquake, the soil volume and also pore-pressure water is changed and therefore as saturated loose sands undergo simple shear deformations, the stiffness at any time is changed as the function of mean normal effective stress. In this study, a hypo-elastic model is adopted for the soil to evaluate changes in the pore pressures and also effective stresses during the excitation. In a finite element modeling, for the areas not expecting the liquefaction to occur, the pipe is modeled using beam elements and soil is modeled by some bi-linear springs; while for liquefied areas, the pipe is modeled by shell elements and solid elements are used to model the surrounding soil

Investigating the Effect of Geocell Changes on Slope Stability in Unsaturated Soil

The purpose of this research is to investigate the performance and efficiency of reinforced slope in the stability of geocell layers in unsaturated soil conditions. Slope reinforced with geocell acts like a beam in the soil due to the geocell having a height (three-dimensional). Due to its flexural properties, it has moment of inertia as well as bending strength, which reduces the displacement and increases the safety factor of the slope. Taking into consideration unsaturated conditions of soil contributes a lot to making results close to reality. One of the well-known models among elastoplastic models for modeling unsaturated soils is Barcelona Basic Model, which has been added to the FLAC2D software by codification. Changes in thickness, length and number of geocell layers are remarkably effective on slope stability. The results show that the geocell\u27s reinforcing efficiency depends on the number of layers and depth of its placement. As the depth of the geocell\u27s first layer increases, the lateral and vertical side elevation of the upper part of the slope increases with respect to the elevation. Load capacity increases with increasing geocell length. By increasing the length of the geocell layer, the joint strength, the mobilized tensile strength, and the bending moment are increased. At u/H = 0.2, an increase in the bending momentum of about 20% occurs with increasing geocell thickness. In u/H = 1, the increase in bending momentum is 10.4%. In addition, by increasing the thickness of the geocell, the Value of moment of the inertia increases and, as a result, the amount of geocell reinforcement bending moment increases

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

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

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 Micro-Structure on Fatigue Behavior of Intact Rocks under Completely Reversed Loading

Rock formations and structures can be subjected to both static and dynamic loadings. Static loadings resulting from different sources such as gravity and tectonic forces and dynamic forces are intermittently transmitted via vibrations of the earth’s crust, through major earthquakes, rock bursts, rock blasting and drilling and also, traffic. Reaction of rocks to cyclic and repetitive stresses resulting from dynamic loads has been generally neglected with the exception of a few rather limited studies. In this study, , two crystalline quarry stones in Iran; (Natanz gabbro and Green onyx) and one non-crystalline rock (Asmari limestone) are used to evaluate the effect of micro-structure of intact rock on fatigue behavior. These rocks have different mineral compositions and formation conditions. A new apparatus based on rotating beam fatigue testing machine (R.R.Moore), which is commonly used for laboratory fatigue test in metals, is developed and fatigue behavior and existence of the endurance limit were evaluated for the mentioned rocks based on stress-life method. The obtained results in the variation of applied amplitude stress versus loading cycle number (S-N diagram) followed common relationship in other materials. In addition, the endurance limit is perceived for all tested rocks. The results also illustrated that the endurance limits for all types of tested rocks in this study are ranged between 0.4 and 0.6 of their tensile strengths. The endurance limit to tensile strength fraction of green onyx and Natanz gabbro were approximated in a higher value compared to the Asmari limestone with non-crystalline micro-structure

Numerical probabilistic analysis for slope stability in fractured rock masses using DFN-DEM approach

Due to existence of uncertainties in input geometrical properties of fractures, there is not any unique solution for assessing the stability of slopes in jointed rock masses. Therefore, the necessity of applying probabilistic analysis in these cases is inevitable. In this study a probabilistic analysis procedure together with relevant algorithms are developed using Discrete Fracture Network-Distinct Element Method (DFN-DEM) approach. In the right abutment of Karun 4 dam and downstream of the dam body, five joint sets and one major joint have been identified. According to the geometrical properties of fractures in Karun river valley, instability situations are probable in this abutment. In order to evaluate the stability of the rock slope, different combinations of joint set geometrical parameters are selected, and a series of numerical DEM simulations are performed on generated and validated DFN models in DFN-DEM approach to measure minimum required support patterns in dry and saturated conditions. Results indicate that the distribution of required bolt length is well fitted with a lognormal distribution in both circumstances. In dry conditions, the calculated mean value is 1125.3 m, and more than 80 percent of models need only 1614.99 m of bolts which is a bolt pattern with 2 m spacing and 12 m length. However, as for the slopes with saturated condition, the calculated mean value is 1821.8 m, and more than 80 percent of models need only 2653.49 m of bolts which is equivalent to a bolt pattern with 15 m length and 1.5 m spacing. Comparison between obtained results with numerical and empirical method show that investigation of a slope stability with different DFN realizations which conducted in different block patterns is more efficient than the empirical methods