Jan Nemcik

Micro- and macro-behaviour of fluid flow through rock fractures: an experimental study

Microscopic and macroscopic behaviour of fluid flow through rough-walled rock fractures was experimentally investigated. Advanced microfluidic technology was introduced to examine the microscopic viscous and inertial effects of water flow through rock fractures in the vicinity of voids under different flow velocities, while the macroscopic behaviour of fracture flow was investigated by carrying out triaxial flow tests through fractured sandstone under confining stresses ranging from 0.5 to 3.0 MPa. The flow tests show that the microscopic inertial forces increase with the flow velocity with significant effects on the local flow pattern near the voids. With the increase in flow velocity, the deviation of the flow trajectories is reduced but small eddies appear inside the cavities. The results of the macroscopic flow tests show that the linear Darcy flow occurs for mated rock fractures due to small aperture, while a nonlinear deviation of the flow occurs at relatively high Reynolds numbers in non-mated rock fracture (Re > 32). The microscopic experiments suggest that the pressure loss consumed by the eddies inside cavities could contribute to the nonlinear fluid flow behaviour through rock joints. It is found that such nonlinear flow behaviour is best matched with the quadratic-termed Forchheimer equation.RésuméLe comportement microscopique et macroscopique d’un fluide s’écoulant dans les fractures à parois rugueuses d’une roche a été étudié expérimentalement. La technique avancée sur les microfluides a été introduite afin d’examiner les effets visqueux et inertiels microscopiques de l’écoulement de l’eau dans les fractures d’une roche au voisinage des vides pour différentes vitesses, tandis que le comportement macroscopique de l’écoulement en fracture a été étudié en réalisant des tests triaxiaux sur (des) un grès fracturés, sous des pressions de confinement comprises entre 0.5 et 3.0 MPa. Les tests montrent que les forces inertielles microscopiques augmentent avec la vitesse, avec des effets significatifs sur le mode d’écoulement local près des bords. Avec l’augmentation de la vitesse, la déviation des trajectoires d’écoulement est réduite mais des petites turbulences apparaissent à l’intérieur des cavités. Les résultats des tests montrent que l’écoulement linéaire de Darcy se produit dans des fractures communicantes de la roche, en raison d’une faible ouverture, cependant que dans les fractures non communicantes une déviation non linéaire de l’écoulement survient pour des nombres de Reynolds relativement élevés (Re > 32). Les expériences microscopiques suggèrent que la baisse de la pression, dissipée par les remous à l’intérieur des cavités, pourrait contribuer à un écoulement non linéaire du fluide à travers les joints de la roche. On constate que c’est ce comportement non linéaire qui correspond le mieux au terme quadratique de l’équation de Forcheimer.ResumenSe investigó experimentalmente el comportamiento microscópico y macroscópico del flujo de un fluido a través de fracturas de rocas de paredes rugosas. Se introdujo la tecnología microfluídica de avanzada para examinar los efectos microscópicos viscosos e inerciales del flujo de agua a través de las fracturas de rocas en la vecindad de vacíos bajo diferentes velocidades de flujo, mientras que se investigó el comportamiento macroscópico del flujo de fracturas llevando a cabo pruebas triaxiales de flujo a través de areniscas fracturadas bajo presiones de confinamiento que varían de 0.5 a 3.0 MPa. Las pruebas de flujo mostraron que las fuerzas inerciales microscópicas se incrementan con la velocidad de flujo con efectos significativos en el esquema de flujo local cerca de los vacíos. Con el incremento de la velocidad de flujo, la desviación de las trayectorias de flujo se reduce pero aparecen pequeños vórtices dentro de las cavidades. Los resultados de las pruebas del flujo macroscópico muestran que el flujo linear de Darcy tiene lugar para fracturas de rocas apareadas debido a la pequeña apertura, en tanto que una desviación no linear del flujo ocurre para números de Reynolds los relativamente altos en fracturas de rocas no apareadas (Re > 32). El experimento microscópico sugiere que las pérdidas de presión consumida por los vórtices dentro de las cavidades podría contribuir al comportamiento no linear del flujo del fluido a través de las juntas de las rocas. Se encontró que tal comportamiento no linear de flujo se ajusta mejor por la ecuación cuadrática de Forchheimer.AbstraktMikroskopická a makroskopická chování proudění přes drsné stěny skalních lomenin byl experimentálně zkoumán. Vyspělá mikrofluidní technologie byla zavedena zkoumat mikroskopické viskózní a setrvačné efekty proudění vody různých rychlostni přes skalní lomy v okolí malych dutin, zatímco makroskopické chování toku v pukline byla sledována prováděním toku přes punlinu v pískovci pod napětím v rozsahu od 0.5 do 3.0 MPa. Průtokové experimentální studie ukazují, že mikroskopické setrvačné síly rostou s rychlostí proudění s významným vlivem na lokální proudění v blízkosti malych dutin. S nárůstem rychlosti proudění, je odchylka průtoku snížena, ale malé víry jsou vytvořene na vnitřní straně malych dutin. Ve výsledkach makroskopických toků testy ukazují, že k lineárním Darcy proudění dochází u zavřených kamenných lomenin kvůli malému otvoru, zatímco nelineární odchylka průtoku se vyskytuje při relativně vysokých Reynoldsovych číslel v rozsiřene kamenné pokliňe (Re > 32). Mikroskopické experimenty naznačují, že tlaková ztráta spotřebovana víry uvnitř dutiny by mohla přispět k nelineárnímu chování proudění tekutiny přes skalní puklinu. Je zjištěno, že takové nelineární chování se nejlépe da vyjadrit kvadratickou rovnicí, nazvanou Forchheimer rovnice.摘要对通过高低不平岩石断裂的液体流微观和宏观特征进行了实验调查。采用了先进的微流体技术来检验不同水流速度下孔洞附近水流通过岩石断裂的微观粘性和惯性影响,而通过进行围压应力(0.5 到3.0 MPa)下三维水流实验对断裂流的宏观特征进行了调查。水流实验显示微观惯力随着水流 速度的增加而增加,对孔洞附近的局部水流 模式产生较大影响。随着水流速度的增加,水流轨迹的偏差降低,但在孔洞内会出现小的漩涡。宏观水流实验结果显示,由于空穴很小,成双的岩石断裂会出现线性达西水流。而在非成双的岩石断裂(Re>32)中,水流的非线性偏差以高雷若数出现。微观实验表明孔穴内漩涡消耗的压力损耗可以有助于通过岩石节理的非线性液体流行为。发现这样的非线性水流行为与二次Forchheimer 方程最为匹配。ResumoFoi experimentalmente investigado o comportamento microscópico e macroscópico do escoamento de fluidos através de rochas com fraturas com paredes rugosas. Introduziu-se uma avançada tecnologia de microfluidos para examinar os efeitos microscópicos, resultantes da viscosidade e da inércia, do escoamento da água através de fraturas de rochas nas imediações de cavidades, utilizando diferentes velocidades de fluxo; por outro lado, o comportamento macroscópico do escoamento nas fraturas foi investigado através a realização de testes de fluxo triaxiais num arenito fraturado, sob tensões confinantes variando de 0.5 a 3.0 MPa. Os testes de escoamento mostram que as forças microscópicas inerciais aumentam com a velocidade do escoamento, com efeitos significativos sobre o padrão local do escoamento perto das cavidades. Com o aumento da velocidade do escoamento, o desvio das trajetórias de escoamento é reduzido, mas surgem pequenos turbilhões dentro das cavidades. Os resultados dos testes macroscópicos de escoamento mostram que o escoamento linear de Darcy ocorre em fraturas da rocha acopladas, devido à pequena abertura entre si, enquanto em fraturas de rocha não acopladas ocorre um desvio linear do fluxo para números relativamente elevados de Reynolds (Re > 32). Os ensaios microscópicos sugerem que a perda de pressão consumida pelos remoinhos no interior das cavidades pode contribuir para o comportamento do escoamento de fluido não-linear através das fraturas da rocha. Verifica-se que este comportamento do fluxo não-linear combina melhor com a equação quadrática de Forechheimer.

Behavior of fiber glass bolts, rock bolts and cable bolts in shear

This paper experimentally compares the shear behavior of fiber glass (FG) bolt, rock bolt (steel rebar bolt) and cable bolt for the bolt contribution to bolted concrete surface shear strength, and bolt failure mode. Two double shear apparatuses of different size were used for the study. The tensile strength, the shear strength and the deformation modulus of bolt control the shear behavior of a sheared bolted joint. Since the strength and deformation modulus of FG bolt, rock bolt and cable bolt obtained from uniaxial tensile tests are different, their shear behavior in reinforcing joints is accordingly different. Test results showed that the shear stiffness of FG bolted joints decreased gradually from the beginning to end, while the shear stiffness of joints reinforced by rock bolt and cable bolt decreased bi-linearly, which is clearly consistent with their tensile deformation modulus. The bolted joint shear stiffness was highly influenced by bolt pretension in the high stiffness stage for both rock bolt and cable bolt, but not in the low stiffness stage. The rock bolt contribution to joint shear strength standardised by the bolt tensile strength was the largest, followed by cable bolts, then FG bolts. Both the rock bolts and cable bolts tended to fail in tension, while FG bolts in shear due to their low shear strength and constant deformation modulus.

Effects of cyclic loading on the shear behaviour of infilled rock joints under constant normal stiffness conditions

The variation of the shear strength of infilled rock joints under cyclic loading and constant normal stiffness conditions is studied. To simulate the joints, triangular asperities inclined at angles of 9.5° and 18.5° to the shear movement were cast using high-strength gypsum plaster and infilled with clayey sand. These joints were sheared cyclically under constant normal stiffness conditions. It was found that, for a particular normal stiffness, the shear strength is a function of the initial normal stress, initial asperity angle, joint surface friction angle, infill thickness, infill friction angle, loading direction and number of loading cycles. Based on the experimental results, a mathematical model is proposed to evaluate the shear strength of infilled rock joints in cyclic loading conditions. The proposed model takes into consideration different initial asperity angles, initial normal stresses and ratios of infill thickness to asperity height.

Numerical Modeling of Fully Grouted Rockbolts Reaching Free-End Slip

AbstractIn this study, numerical simulation was conducted to model the behavior of rockbolts with free-end slip in tension. A nonlinear bond–slip relationship was input to the numerical model to represent the interaction mechanism of the interface between rockbolt and grout. The results of the model were validated against laboratory tests. The numerical model matched the experimental results in terms of load–displacement relationship, rockbolt strain distribution, and interfacial shear-stress distribution. This paper indicates that (1) the rockbolt element in the numerical model is capable of describing the strain-softening behavior of the rockbolt–grout interface; and (2) the pullout tests of rockbolts with free-end slip can be used to derive the bond–slip relationship, and thus provide insights into the mechanical behavior of rockbolts with free-end slip.

A New Equation for the Shear Strength of Cable Bolts Incorporating the Energy Balance Theory

The application of cable bolts as a secondary support system is an increasing trend in underground coal mines worldwide. The performances of cable bolts have been evaluated under both axial and shear loading conditions. Two methods of testing cables for shear, single and double shear, have been recognised. This paper examines the shear behaviour of a variety of cable bolts under different pre-tension loads by double shear testing. Plain, spiral and the combination of both cable types were used in this study. The initial axial load and the type of cable bolts are the main factors affecting their shear strength. By increasing the axial pre-tension load, the peak shear load occurs at lower shear displacement. The failure angle due to cable bending across the joint at different pre-tension loads varied between 41° and 49°. This demonstrates that the ratio of axial and perpendicular displacements is almost constant and on average the failure occurs at about 45°. A novel analytical model is proposed to evaluate the shear behaviour of pre-tensioned fully grouted cable bolts subjected to double shearing. Energy and Fourier Series methods were incorporated in the model to simulate the shear behaviour of cable bolts. The comparison of the experimental results with the proposed model shows a good agreement.

Effects of shear rate on cyclic loading shear behaviour of rock joints under constant normal stiffness conditions

The presence of joints and discontinuities within a rock mass can significantly affect its mechanical behaviour and therefore the stability of structures constructed at close proximity. Several studies have been carried out by previous researchers to understand the mechanical behaviour of joints under both constant normal load (CNL) conditions, in which the normal load remains unchanged during shearing, and constant normal stiffness (CNS) conditions to imitate the stiffness of the surrounding rock mass (Patton 1966; Ladanyi and Archambault 1969; Barton 1973, 1976; Seidel and Haberfield 1995; Indraratna and Haque 2000; Buzzi et al. 2008). The importance of CNS conditions to simulate the actual shear behaviour of rock joints in the field has been described by Johnston and Lam (1989), Skinas et al. (1990) and Indraratna et al. (1998). The above-mentioned studies focussed only on the monotonic loading shear behaviour of rock joints. The effects of cyclic loading on the shear behaviour of rock joints in earthquakes and blasting were investigated in detail by Plesha (1987), Hutson and Dowding (1990), Lee et al. (2001), Stupkiewicz and Mroz (2001), Grasselli and Egger (2003), Jafari et al. (2003) and Belem et al. (2009). As the shear rate might vary depending on the source of the load and rock media, Crawford and Curran (1981) carried out a series of experiments on artificial rock joints with various shear rates and normal stresses under CNL conditions. Based on the measured data, they concluded that the shear rate may influence the shear strength of hard and soft rock joints differently. In another study, Jafari et al. (2004) verified the results of Crawford and Curran (1981) for shear rates between 0.05 and 0.4 mm/min under monotonic loading. None of these researchers investigated the effects of shear rate on the cyclic loading shear behaviour of rock joints under CNS conditions, which is a critical issue in stability analysis of underground structures subjected to seismic events. Accordingly, three sets of cyclic loading shear tests with various shear rates and initial normal stresses were conducted on artificial triangular joints under CNS conditions. In this study, the experimental data are critically analysed.

Critical review on shear strength models for soil-infilled joints

An infilled rock joint is likely to be the weakest plane in a rock mass. The presence of infill material within the joint significantly reduces the friction of the discontinuity boundaries (i.e. rock to rock contact of the joint walls). The thicker the infill, the smaller the shear strength of the rock joint. Once the infill reaches a critical thickness, the infill material governs the overall shear strength, and the joint walls (rock) play no significant role. Several models have been proposed to predict the peak shear strength of soil-infilled joints under both constant normal load (CNL) and constant normal stiffness (CNS) boundary conditions, taking into account the ratio of infill thickness (t) to the height of the joint wall asperity (a). CNS models provide a more realistic picture of the soil-infilled joint behaviour in the field. This paper presents a critical review on the existing mathematical models for predicting the shear strength of soil-infilled rock joint and verifies the normalised peak shear stress model with further laboratory investigations carried out on idealised saw-tooth rock joints at the University of Wollongong. Based on the prediction of the experimental data, the normalised peak shear stress model is slightly modified by the authors. A simplified approach for using this model in practice is presented and a new expression for prediction of dilatation at peak shear stress is suggested.

The effects of installation procedure on bond characteristics of fully grouted rock bolts

The bond characteristics of fully grouted rockbolts installed in steel tubes were investigated by bolt push tests. Steel tubes were inserted in a mine roadway roof to represent the confinement of rock boreholes. Rockbolts were installed in tubes using the installation technique of Australian underground mines. These tubes, with rockbolts inside, were retrieved from the field and brought back to the laboratory to be cut into 100-mm sections, which were then push tested. It was found that each bolt section had a distinct load-displacement profile, and that bond strength varied significantly along the bolt length. The factors influencing the bond strength of rockbolts were identified. The influence of the installation procedure on the bond strength of bolts in tubes was investigated.

An elasto-plastic constitutive model for rock joints under cyclic loading and constant normal stiffness conditions

An elasto-plastic constitutive model is introduced for rock joints under cyclic loading, considering the additional shear resistance generated by the asperity damage in the first forward shear cycle and sliding mechanism for further shearing. A series of cyclic loading direct shear tests was conducted on artificial joints with triangular asperities and replicas of a real rock asperity surface under constant normal stiffness (CNS) conditions. The model was calibrated and then validated using selected data sets from the experimental results. Model simulations were found to be in good agreement with the rock joints behaviour under cyclic loading and CNS conditions both in stress prediction and dilation behaviour. In addition, dynamic stability analysis of an underground structure was carried out, using Universal Distinct Element Code and the proposed constitutive model.

Experimental study of the adsorption-induced coal matrix swelling and its impact on ECBM

Carbon dioxide (CO2) enhanced coalbed methane (ECBM) is an effective method to improve methane (CH4) production and this technology has already been used to increase gas production in several field trials worldwide. One major problem is the injection drop in the later period due to permeability decrease caused by coal matrix swelling induced by CO2 injection. In order to quantify the swelling effect, in this work, coal samples were collected from the Bulli coal seam, Sydney Basin and adsorption tests with simultaneous matrix swelling measurement were conducted. The adsorption and swelling characteristics were analyzed by measuring the adsorption mass simultaneously with the strain measurement. Then experiments were conducted to replicate the ECBM process using the indirect gravity method to obtain the swelling strain change with CO2 injection. The results show that the coal adsorption capacity in CO2 is almost two times greater than that in CH4, and nitrogen adsorption is the least among these gases. A Langmuir-like model can be used to describe the strain with the gas pressure and the swelling strain induced by gas adsorption has a linear relationship with gas adsorption quantity. Moreover, swelling strain increase was observed when CO2 was injected into the sample cell and the swelling strain was almost the sum of the strains induced by different gases at corresponding partial gas pressure.