ABSTRACT: A series of compression tests were carried out in the laboratory on twelve (12) specimen groups of three (3) rock types, i.e. Stanstead Grey Granite, Indian Buff Limestone and Danby/Olympia White Marble, and four (4) different sizes, i.e. 51, 63, 81 and 99 mm outer diameter, to determine their nominal strength and investigate scale effects in regards of test configuration and size of test specimens. Specimens were loaded either diametrically to determine their splitting tensile strength using the Brazilian test configuration, either axially at five (5) levels of confinement, i.e. unconfined, 2, 4, 8 and 15 MPa, to determine their peak compressive strength and compute the parameters of the strength envelope. Triaxial confined tests were continued beyond the peak to determine the residual strength of formerly intact rock specimens. Test specimens were then slowly unconfined at a rate of 0.02 MPa/sec, while keeping the specimen into a continuous failing mode and recording both axial and confining pressures. Both sets of properties, i.e. peak and residual, are represented using the Mohr-Coulomb strength criterion for modelling rock behavior, expressed in terms of cohesion and friction angle. Results show very minimal if any scale effect when specimens were tested under axial and/or triaxial compression, for the size of specimen tested. However, a well-defined scale effect is observed with specimens tested in tension, confirming the importance of both the rock fabric and the mode of loading on the behavior of homogeneous rock materials at failure, as pointed out by most of the authors in literature. Testing methods are described and results are presented. In most cases, results are showing a very high degree of linearity and very low variability, leading to very consistent observations regarding all aspects of the testing program. © 2017 Copyright reserved, Natural Resources Canada.
A sound knowledge of intact rock and rock mass properties is required to proceed with the design and ensure the performance of structures either founded on or built in rock. This statement applies to every stage of construction, starting with site investigation, sampling and testing rock materials, and asserting that the set of properties then determined is properly suited for the application foreseen. The main objective of all testing carried out over the last two decades at the CANMET Rock Mechanics Laboratory of Natural Resources Canada, located in Ottawa, Ontario, within the capital region, was to satisfy the needs of two large groups of clients, which are the mine industry and the energy sector. However large the range of applications foreseen might have been, the laboratory has focused its effort on highly specialized services in relation with first the properties of intact rocks at failure, i.e. material strength and rock deformation, and second, their evolution once the material has failed and properties are reaching their minimal, residual value [1-7]. Applications foreseen for such properties include short and long term stability of drillholes, wellbores, shafts, tunnels, caverns, mine openings, etc, used by both mine and energy sectors, to proceed either to mineral, oil and/or gas extraction (and/or storage), or to nuclear waste disposal, where stability is most suited to ensure the safety and serviceability of excavations [8-12]. A second category of applications requiring a good knowledge of laboratory rock properties concerns the capacity of the rock to either collapse and/or to flow naturally, as required with operations using mine caving and/or stoping as their main extraction method [13-15].
ABSTRACT: In order to understand how the rock mass evolves with extraction in the particularly highly-stressed environment of a sill pillar in a hard rock mine in North America, we investigate the utility of seismic tomography as an interpretive tool. We use a rich dataset of blasts to highlight the compressional wave variations in the rock. Although tough there are areas within the sill pillar that are prone to smearing, we are able to resolve subtle variations in velocity structure. By considering different time periods, we image temporal changes in seismic velocity that we relate to the stress state and damage in the rock. Specifically, we consider data recorded during three sequential time intervals associated with the excavation of a stope during the start of the second interval. We observe a high-velocity anomaly in the first interval that is located to the edge of the future stope. After mining, the high-velocity regions migrate to the other side of the stope and then disperse to the edges of the resolvable area. Equating high-velocity anomalies with high-stress gives us the ability to characterize the evolving stress state in the mine and potentially an approach that can be used to avoid hazardous situations.
Underground excavations in mines entail a detailed understanding of the stress state to mitigate the risk of rockbursts and other seismicity that may cause damage to infrastructure and potentially casualties to personnel. Depending on the overall mining methodology, the highly-stressed areas become more focused in sill and crown pillars. As a mine reaches the end of its life-cycle, the need to safely excavate these areas becomes critical, and techniques that constrain the stress state are essential to enable the safe extraction of the resource.
The rock mass responds to variations in the stress state of the mine; high-stress areas have the effect of closing microcracks in the rock mass and, thereby, increasing seismic velocities. By mapping the velocity variations as a function of space and time we can start to understand the stress variations associated with extraction. Tomographic imaging of mines has been used to relate to the stress distribution in mines in several studies (e.g. Young & Maxwell 1992; Maxwell & Young 1996; Silver et al. 2007; Ma et al. 2016). A number of studies have shown promise in using these techniques to monitor the stability of block caving operations (Westman et al. 2012; Mercier et al. 2015).
ABSTRACT: We propose an interfacial contact/damage model for simulating dynamic fracture in rocks. An interfacial damage parameter, D, models the evolution of damage on fracture interfaces, while relative contact and contact-stick fractions model contact-separation and stick-slip transitions. The damage rate is determined by an effective stress, written as a scalar function of the normal and tangential components of the Riemann traction solution for assumed bonded conditions. We propose alternative definitions of the effective stress that generate failure criteria that resemble the Tresca and Mohr-Coulomb criteria for compressive stress states, and we compare their compressive strengths and fracture angles under a compressive loading. We adopt a stochastic Weibull model for crack-nucleation in which cracks nucleate at points where the effective stress exceeds the probabilistic fracture strength. We implement the nucleation model with an h-adaptive asynchronous spacetime discontinuous Galerkin (aSDG) method that captures accurately the complex fracture patterns that arise under dynamic loading conditions. Numerical examples illustrate the effects on fracture response of varying the stochastic nucleation parameters and the alternative definitions of the effective stress.
Understanding the stress states that cause rock failure is critical to the reliable analysis and safe design of structures in rocks. In situ rock is typically subjected to compressive stress fields, and experimental observations indicate that the compressive strength of rock increases with increasing confining pressure. Failure occurs by shearing along planes oriented at a rock-type-specific angle, θ, defined relative to the direction of maximum compressive stress . Failure criteria describe the variation of compressive strength with confining pressure and, in general, the stress states at which rock fails.
A number of failure criteria have been proposed in rock mechanics. The Tresca criterion assumes that a material fails on planes with maximum shear stress. While it is sometimes used for failure analysis of rock , the Tresca criterion is more appropriate for ductile materials as its corresponding shear strength is independent of the confinement pressure. The Mohr-Coulomb (MC) failure criterion depends linearly on the normal and shear stress components. This implies a linear relation between confinement pressure and compressive strength. However, this straight-line relation does not always fit experimental data , and the extension of the linear relation into the tensile loading regime generally over-predicts the tensile strength of rock. Experiments show that the rates of increase of the shear and compressive strengths decrease as confinement pressure increases. In fact, beyond a certain confinement pressure, rock reaches a critical state at which the shear strength no longer increases, similar to the constant shear strength of the Tresca model . Beyond the limitations associated with linearity of the Mohr-Coulomb model, various studies demonstrate that fully three-dimensional failure criteria are required to capture the influence of the intermediate principal stress [5-8]. However, due to its simplicity and the challenges involved in calibrating the more advanced models, the Mohr-Coulomb model is still the most popular and widely used-failure criterion for rock.
Esterhuizen, G. S. (The National Institute for Occupational Safety and Health (NIOSH)) | Tulu, I. B. (The National Institute for Occupational Safety and Health (NIOSH)) | Bajpayee, T. S. (The National Institute for Occupational Safety and Health (NIOSH))
ABSTRACT: Stress-driven instability of roof rocks in coal mine entries can result in large roof falls with associated safety hazards. Failure may be related to spalling, cutter formation, and delamination of bedding of the roof strata. The mode of failure is similar to failure of brittle rocks observed in hard rock environments. Additionally, coal mine roof rocks and coal materials can exhibit brittle failure characteristics when tested in the laboratory. A brittle failure criterion that considers both extensional fracturing under low confinement as well as shear failure under increased confinement is applied in numerical models of coal mine entries. The results show that such models can replicate field-monitored rock mass deformations and depths of failure. The results further show that the predicted zone of extension failure resembles the “softening zone” that is described in coal mine roof support literature. The authors conclude that modeling the presence of a reduced strength extension failure zone produces a satisfactory simulation of rock response around coal mine entries.
Roof instability is a leading cause of injuries and fatalities in underground coal mines. Each year more than 400 reportable roof-fall events occur in coal mines in the United States (MSHA, 2016). Improved understanding of the rock failure mechanics and development of methods to evaluate likely rock mass response for given geologic conditions can assist in designing ground support systems. Observations of rock damage and roof failures in the bedded roof of coal mines demonstrate that brittle spalling and delamination of beds contribute to the onset and propagation of roof instability (Hill 1986, Gadde and Peng 2005, Hasenfus and Su 2006). The mode of rock failure is observed to be similar to the extension-type brittle fractures encountered in hard rock excavations under low confinement described by Stacey (1980), Martin et al. (1997), Diederichs (2003), and others.
ABSTRACT: The contact area and stress distribution along rock wall joints, play an important role in the behavior and mechanical properties of the rock mass in mining and tunneling. Although properties of rock fractures such as aperture, roughness, filling, contact area, and orientation are highly significant, limited effective methods exist for their investigation at a fundamental level. In this study, laboratory contact electrical resistance profiles are generated for limestone samples, and correlated with uniaxial compression stress measurements. Further, the observed paired resistance-stress laboratory results are simulated using an artificial neural network. Methods, results and analyses are presented to provide insights about complex processes involved in rock wall joint behavior.
1. INTRODUCTION AND BACKGROUND
Rock joints are important factors influencing rockmass properties, and significant experience and knowledge is required to make estimates of geologically disturbed rock mass properties (Hopkins, D. L., 2000). Irregular discontinuities, especially mining-induced fractures behind excavations, can pose large hazards toward mine or tunnel safety. However, there are currently limited approaches used effectively to detect the rock wall joint behavior beyond the exposed surface.
Electrical resistivity measurements are widely used as an important tool for exploring minerals, fluid content, porosity and degree of water saturation in rocks and soils (Loke et al. 2013). In mining and tunneling area, great research efforts have been conducted on the electrical properties of rocks, and the influencing factors of electrical such as water content, stress, fractures and temperature had been recognized (Brace et al., 1966, 1967, Brown, S.R. 1986). For instance, an order of magnitude of changes was observed when saturated rock samples were stressed to fracture from a wide variety of rocks (Brace and Orange, 1966). Moreover, resistivity of saturated rock samples under compressive stress rises or falls very slightly before half of the peak stress, and it falls rapidly after samples reaching 90% of the peak stress. This large decrease of resistivity results from new-created porosity in a saturated rock (Brace and Orange, 1968). Water content is the dominant factor affecting electrical resistivity of rocks, thus additional efforts on electrical resistivity of rocks can be done to correlate electrical resistivity with porosity, water content and conductivity of rockmass.
ABSTRACT: During the life span of a well, the cement sheath may fail to provide zonal isolation and micro-annuli may develop between the cement-casing and cement-rock interfaces. Multiple factors during well construction and injection processes may have individual or combined influence on the micro-annuli formation. This paper presents a staged finite element modeling approach to simulate the well construction processes and injection cycle. Loads from the in-situ stress field, mud/slurry pressure, and periodic temperature changes are incorporated in the model. A “realistic” bottom-hole state of stress is generated in the model and microannuli generation is simulated by the tensile debonding of the cement-formation interface. The simulation results indicate the generation of micro-annuli is highly affected by the temperature difference between injection fluid and formation and the time span of the fluid injection cycle. Resulting debonding apertures in the models are in the order of 10−5m which corresponds with previous studies. The modeling results of this study indictates once debonding occurs, the debonding aperture increases quickly and may exceed the ranges considered hazardeous.
Gas migration (GM) or wellbore leakage is a significant problem that the oil and gas industry has encountered for decades in old well abandonment (Calvert and Smith, 1994), gas well production (Feng et al. 2016), and geologic CO2 sequestration (Nygaard et al. 2014). The leakage occurs as a result of wellbore integrity loss during drilling and injection processes during which the wellbore cement sheath may fail to isolate different zones. Wellbore failure and associated leakage pathways are shown in Fig. 1 (Celia et al. 2005). Except for cement cracking under extreme conditions, the development of micro-annuli is the main reason for gas migration in most scenarios.
Micro-annuli are defined as systematic and inter-connecting sets of fractures/tiny gaps between the cement-casing and the cement-rock interfaces, which provide possible pathways for gas migrating (Wang and Taleghani, 2014).
ABSTRACT: This paper presents a novel study on geomechanics of fluid injection from a fully penetrating vertical wellbore into a weakly consolidated formation confined with soft rocks. For the first time, impacts of vertical confinement are incorporated to evaluate: flow-induced poro-elasto-plastic stresses, failure mechanism/s, and failure planes. A new fully-coupled numerical model is developed where the response of the injection layer in the plane perpendicular to injection flow is simulated through adopting “interface” – a plane on which sliding or separation can occur – analogous to the Winkler model. An assessment of pore pressures, stresses, and failure planes confirms two types of induced behaviors: dilation in the well vicinity; and compaction, a main cause of physical clogging which impacts competence of the operation. Numerical results describe multiple distinct zones evolving with time around the injection well: (1) liquefied domain, (2) multiple plastic domains, (3) elastic region. Inner plastic domains are prone to occur.
The geo-environmental consequences of injecting large volumes of fluids into geological strata – a common practice in enhanced oil recovery, geothermal exploitation, aquifer storage and recovery, and deep waste disposal operations – remain a topic of dispute. The very essence of a safe and sustainable operation is to be able to quantify and control the induced subsurface flow and deformations, specifically to ensure preservation of sealing integrity of the confining rocks. This involves a realistic grasp of the interactions between the injection layer and the sealing strata, which entails a comprehensive understanding of the involved geomechanical processes.
Numerical computation of the injection problem in a confined weakly-consolidated formation is challenging due to the following: strong coupling between the diffusional-mechanical processes; high fluid-solid matrix stiffness contrast; plasticity and parting, where the strength and stiffness of the medium become effectively zero. This results in numerical instabilities and a considerably long run time of simulations. A common simplifying assumption in most available work in the literature is plane strain conditions perpendicular to the injection flow. This assumption is most appropriate for a case where a weakly consolidated reservoir formation is confined with stiff sealing rocks. In cases where the sealing rocks are of a softer nature, or when the stiffness of the reservoir layer increases over time during prolonged injection cycles, the plane strain assumption is no longer appropriate and the overall response of the injection layer is indeed governed by the behavior of the confining strata.
ABSTRACT: Rock fracture toughness testing has been widely studied in the past forty years. Most previous testing used pillar-shaped rock specimen as a result of the inconvenience and difficulties in processing rock into other geometry. The development of diamond-impregnated wire saw for rock processing provides us the opportunity to get large quantities of notch-precisely-machined specimens, whose shape are Double-Cantilever-Beam planks. The study here used Double Cantilever Beam (DCB) testing to obtain highly repeatable values of rock fracture toughness. Particle Flow Code 5.0 2D (PFC 2D) was used to simulate the whole testing process, on the other hand, the testing results were used to calibrate the PFC parameters in turn. Conclusion shows that model I fracture toughness of Longmaxi formation outcrop is around 1.55MNm-3/2. Critical tensile force has positive correlation with width of notch/crack tip. The study also indicates that in PFC simulation, micro tensile strength among balls (pb_ten) determines both model I fracture toughness and uniaxial compression strength. The study may guide the drilling process in recognizing the strength of rock fracture and provide experiment-based fracture toughness for the simulation of fracturing process.
Fracture toughness of rock was scholars’ highly concern as most issues of rock mechanics are closely related to failure process, such as mining process and hydraulic fracturing, etc.
Lots of work has been done since the first suggested method of International Society for Rock Mechanics for rock fracture toughness testing1 was published. Since then, the testing method mentioned above is regarded as the standard method for static fracture toughness testing of rock. As the availability of pillar-shaped rock specimen, almost all the suggested methods used rock pillar to conduct fracture toughness testing. The machined specimen is shown in Fig. 1 (Atkinson 1987). In order to obtain the intrinsic fracturing property of rock, the notch of the specimen is machined to the shape of V or chevron. Displacement and force used to calculate fracture toughness is recorded after the fracture propagates a small distance to form a natural sharp crack, and that is what the shape of notch used to. This elaborate design eliminates the influence of machined notch width on fracture toughness because that the ‘natural sharp crack’ is thought to be a parameter merely related to rock property.
Goteti, R. (Aramco Services Company: Aramco Research Center - Houston) | Agar, S. M. (Aramco Services Company: Aramco Research Center - Houston) | Brown, J. P. (Red Sea Exploration Division) | Sibon, H. J. (Red Sea Exploration Division) | Zuhlke, R. (EXPEC Advanced Research Center)
ABSTRACT: Layered evaporite sequences have been documented from various rifted margins, including the South Atlantic and the Red Sea. The intervening sedimentary layers in such sequences can undergo large deformation and present drilling hazards associated with high pore fluid pressures and/or rubble zones. Although physical and numerical models provide insights to the deformation of such layers, the former are limited in terms of scalability of material failure parameters to natural examples, while the latter predominantly focus on massive salt and adjacent frictional-plastic sediments. In this paper we present a 2D evolutionary large strain finite element model of a salt diapir in an idealized layered evaporite sequence (LES). Gravitational loading and sedimentation provide the driving force for halokinesis. Salt is assigned a temperature-dependent non-Newtonian rheology, whereas the sediments are assigned a non-associative cap-plasticity model that supports both compaction and shear localization. The model results suggest that mechanical stratification plays an prominent role in the evolution of a LES. Stresses and strains in the sediment layers evolve in a complex manner and are predominantly controlled by their structural position. The presence of multiple salt layers in a LES decouples the deformation at different depths such that poly-harmonic folds can develop near the salt diapir. Structural dip and position, in addition to curvature, impact the deformation within the sedimentary layers. Geomechanical forward models also provide directional guidance on the likely variations in in-situ stresses and in well planning in LES settings.
A Layered Evaporite Sequence (LES) is a compositionally stratified heterolithic sequence comprising salt(s) and sediments. Some examples of salt basins containing layered evaporites are the Brazilian salt basins (Fiduk and Rowan, 2012), Levant Basin (Cartwright et al., 2012), North Sea (Strozyk et al., 2012), South Oman salt basin (Li et al., 2012) and Lower Permian Basin (Raith et al., 2016). The stratification in a LES could be due to spatial variations in salt composition (e.g., halite, anhydrite, tachyhydrite, carnallite) or inter-layering of salt with sedimentary inclusions or ‘stringers’. During geological loading, evaporites in a LES could undergo viscous deformation while the inter-layered sediments deform by frictional-plastic processes. The bulk mechanical response of a LES to geological loading can be significantly different, than when the salt is compositionally homogeneous, such as in the massive allochthonous halite structures in the Gulf of Mexico.
ABSTRACT: The Marble Shear Block is a rockslide located between the spillway and tailrace on the right bank of the Revelstoke Dam, British Columbia. The Marble Shear is a thin graphitic foliation shear that forms the base of the Marble Shear Block. Failure of the Marble Shear Block could result in damage to the spillway and the tailrace area. The stability of the Block was reassessed for higher piezometric pressures and the design seismic event. The geological model was updated using additional geological and piezometric information from recent site investigations. An extensive instrumentation data review found that piezometric pressures close to the head of the slide had increased; however, this did not result in corresponding increases in slope movement. Instrumentation data showed that the 175 m high concrete dam moved downstream during reservoir filling and annually rocks back-and-forth due to thermal loading. The initial movement of the dam and this cyclic rocking motion was found to correlate with the movements observed in the Marble Shear Block. A finite element analysis showed that the movements of the concrete dam causes displacements along the centimetre-thick Marble Shear (which forms a continuous feature from underneath the concrete dam to the Marble Shear Block) and likely contributes to the downslope movement of the Marble Shear Block. Updated stability analyses also showed that the Marble Shear Block is currently stable and is more sensitive to piezometric pressures at the toe of the slope than the head of the slope.
The Revelstoke Dam is located on the Columbia River, approximately 5 km upstream of the City or Revelstoke, British Columbia Canada. The dam construction and reservoir filling was completed in 1984. The Revelstoke Dam consists of a concrete gravity dam in the Columbia River canyon with a maximum height of 175 m and a zoned earthfill dam with a maximum height of 125 m (Figures 1 and 2).