Showing papers in "Journal of rock mechanics and geotechnical engineering in 2020"

Spatial gradient distributions of thermal shock-induced damage to granite

Abstract: In this study, we attempted to investigate the spatial gradient distributions of thermal shock-induced damage to granite with respect to associated deterioration mechanisms. First, thermal shock experiments were conducted on granite specimens by slowly preheating the specimens to high temperatures, followed by rapid cooling in tap water. Then, the spatial gradient distributions of thermal shock-induced damage were investigated by computed tomography (CT) and image analysis techniques. Finally, the influence of the preheating temperature on the spatial gradients of the damage was discussed. The results show that the thermal shock induced by rapid cooling can cause more damage to granite than that induced by slow cooling. The thermal shock induced by rapid cooling can cause spatial gradient distributions of the damage to granite. The damage near the specimen surface was at a maximum, while the damage inside the specimen was at a minimum. In addition, the preheating temperature can significantly influence the spatial gradient distributions of the thermal shock-induced damage. The spatial gradient distribution of damage increased as the preheating temperature increased and then decreased significantly over 600 °C. When the preheating temperature was sufficiently high (e.g. 800 °C), the gradient can be ignored.

Stability assessment of landslide-prone road cut rock slopes in Himalayan terrain: A finite element method based approach

Abstract: Large-scale slope destabilization could be aggravated due to swift urbanization and ever-rising demands of geoengineering projects such as dams, tunnels, bridges and widening roads. National Highway-58 connects Delhi to Badrinath in India, which passes through complex geomorphological and geological terrain and often encounters cut slopes susceptible to slope failures. In the present investigation, a detailed geotechnical appraisal is conducted along the road cut slopes from Rishikesh to Devprayag in the Himalayas. Twenty vulnerable road cut slopes were demarcated for detailed slope stability analysis using Phase2D finite element modeling simulator. Nonlinear generalized Hoek-Brown (GHB) criterion was adopted for stability analyses. Out of 20 slopes, five slopes (S6, S7, S18, S19 and S20) are unstable with factor of safety (FoS) less than or equal to 1, and thus needs immediate attention. The FoS values of four slopes (S2, S9, S13 and S17) lie between 1 and 1.3, i.e. marginally stable, and slopes S1, S3, S4, S5, S8, S10, S11, S12, S14, S15 and S16 are stable. Mohr-Coulomb (MC) criterion was also adopted to compare the slope stability analysis with GHB criterion. The FoS calculated from GHB criterion is close to that using MC criterion for lower values of FoS whereas for higher values, the difference is marked. For the jointed rock in the Himalayan region, the nonlinear GHB criterion gives better results as compared to MC criterion and matches with the prevailing field conditions. Accordingly, some suggestions are proposed to strengthen the stability of cut slopes.

Rockburst characteristics of several hard brittle rocks: A true triaxial experimental study

Abstract: Rockburst is a typical rock failure which frequently threatens both human life and construction equipment during highly stressed underground excavation. Rock lithology is a control factor of rockburst. In this paper, rockburst tests were conducted on rectangular prismatic specimens of six types of intact hard brittle rocks, i.e. granodiorite, granite, marble, basalt, sandstone and limestone, under one-free-face true triaxial loading conditions. With the use of high-speed cameras, an acoustic emission (AE) system and a scanning electron microscope (SEM), rockburst of different rocks was investigated. The results show that the strainbursts of granodiorite, granite and marble were accompanied by tensile splitting near the free face, and consequently were relatively strong with a large amount of fragment ejection and kinetic energy release. For basalt, sandstone and limestone, failure was primarily dominated by shear rupture. The strainbursts of basalt and sandstone were relatively small with minor fragment ejection and kinetic energy release; while no burst failure occurred on limestone due to its relatively low peak strength. Rock strength, fracturing and fragmentation characteristics, and failure modes of different rocks can significantly affect rockburst proneness and magnitude. The AE evolution coupled with SEM analysis reveals that the differences in the inherent microstructures and fracture evolution under loading are the primary factors accounting for different rockbursts in various rock types.

Prediction of TBM jamming risk in squeezing grounds using Bayesian and artificial neural networks

Abstract: This study presents an application of artificial neural network (ANN) and Bayesian network (BN) for evaluation of jamming risk of the shielded tunnel boring machines (TBMs) in adverse ground conditions such as squeezing grounds. The analysis is based on database of tunneling cases by numerical modeling to evaluate the ground convergence and possibility of machine entrapment. The results of initial numerical analysis were verified in comparison with some case studies. A dataset was established by performing additional numerical modeling of various scenarios based on variation of the most critical parameters affecting shield jamming. This includes compressive strength and deformation modulus of rock mass, tunnel radius, shield length, shield thickness, in situ stresses, depth of over-excavation, and skin friction between shield and rock. Using the dataset, an ANN was trained to predict the contact pressures from a series of ground properties and machine parameters. Furthermore, the continuous and discretized BNs were used to analyze the risk of shield jamming. The results of these two different BN methods are compared to the field observations and summarized in this paper. The developed risk models can estimate the required thrust force in both cases. The BN models can also be used in the cases with incomplete geological and geomechanical properties.

Determination of full-scale pore size distribution of Gaomiaozi bentonite and its permeability prediction

Abstract: Gaomiaozi (GMZ) bentonite is a potential buffer/backfill material for a deep geological disposal of high-level radioactive waste. It has a wide pore size distribution (PSD) with sizes ranging from several nanometers to more than one hundred microns. Thus, properly characterizing the pore structures of GMZ bentonite is a challenging issue. In this study, pressure-controlled porosimetry (PCP), rate-controlled porosimetry (RCP), and scanning electron microscopy (SEM) were used to investigate the PSD of GMZ bentonite. The results indicate that each method has its limitation, and a combined use of PCP and RCP is suitable to obtain the full-scale PSD of GMZ bentonite. Moreover, we also compared the full-scale PSD with nuclear magnetic resonance (NMR) result. It is found that there is no significant difference in the range of PSD characterization between NMR and mercury intrusion method (PCP and RCP). However, in a certain range, the detection accuracy of NMR is higher than that of mercury injection method. Finally, permeability prediction based on PCP and SEM data was conducted, and both of the two methods were found to be able to predict the permeability. The combined method is effective to obtain the full-scale PSD of GMZ bentonite, which is the key to estimation of the sealing ability of bentonite buffer.

Evaluation of empirical estimation of uniaxial compressive strength of rock using measurements from index and physical tests

Abstract: The uniaxial compressive strength (UCS) of rock is an important parameter required for design and analysis of rock structures, and rock mass classification. Uniaxial compression test is the direct method to obtain the UCS values. However, these tests are generally tedious, time-consuming, expensive, and sometimes impossible to perform due to difficult rock conditions. Therefore, several empirical equations have been developed to estimate the UCS from results of index and physical tests of rock. Nevertheless, numerous empirical models available in the literature often make it difficult for mining engineers to decide which empirical equation provides the most reliable estimate of UCS. This study evaluates estimation of UCS of rocks from several empirical equations. The study uses data of point load strength (Is(50)), Schmidt rebound hardness (SRH), block punch index (BPI), effective porosity (n) and density (ρ) as inputs to empirically estimate the UCS. The estimated UCS values from empirical equations are compared with experimentally obtained or measured UCS values, using statistical analyses. It shows that the reliability of UCS estimated from empirical equations depends on the quality of data used to develop the equations, type of input data used in the equations, and the quality of input data from index or physical tests. The results show that the point load strength (Is(50)) is the most reliable index for estimating UCS among the five types of tests evaluated. Because of type-specific nature of rock, restricting the use of empirical equations to the similar rock types for which they are developed is one of the measures to ensure satisfactory prediction performance of empirical equations.

Evaluation of new Austrian tunnelling method applied to Bolu tunnel's weak rocks

Abstract: Since the development of the new Austrian tunnelling method (NATM) in the 1960s, this technique has been applied successfully in many tunnels. However, opinions of NATM principles emerged till 2000, i.e. NATM is not a tunnelling method, but an approach covering all general principles of tunnelling. To investigate the general principles of the NATM, this study focused on tunnelling practises in the Bolu tunnel, and evaluated the conditions under which the NATM practises could be effective. The Bolu tunnel project was designed following the NATM principles. It is evident that practises adopted in this tunnel are important with respect to the NATM. In addition, it shows that the solutions to the problems encountered in this tunnel are consistent with the NATM principles. Finally, the study determines the ground types of the NATM principles and proposes associated updates.

Sugarcane press mud modification of expansive soil stabilized at optimum lime content: Strength, mineralogy and microstructural investigation

Abstract: This study attempted to investigate the potential of sugarcane press mud (PM) as a secondary additive in conjunction with lime for the stabilization of an expansive soil. The physico-mechanical properties of an expansive soil, such as plasticity, shrink-swell behavior, unconfined compressive strength (UCS), mineralogical and microstructural characteristics were investigated. The expansive soil was stabilized at its optimum lime content (7%) for producing maximum strength, and was modified with four different quantities of PM in small dosages (0.25%–2%). Cylindrical soil samples, 38 mm in diameter and 76 mm in height, were cast and cured for varying periods to evaluate the strength of the amended soil. The spent samples after strength tests were further used for determination of other properties. The test results revealed that PM modification led to a substantial improvement in 7-d strength and noticeable increase in 28-d strength of the lime-stabilized soil (LSS). The addition of PM does not cause any detrimental changes to the shrink–swell properties as well as plasticity nature of the stabilized soil, despite being a material of organic origin. Mineralogical investigation revealed that the formation of calcium silicate hydrate (CSH) minerals, similar to that of pure lime stabilization with only the type of mineral varying due to the modification of PM addition, does not significantly alter the microstructure of the LSS except for superficial changes being noticed.

Cement-lime stabilization of crude oil contaminated kaolin clay

Abstract: The present study examines the effect of stabilization on the geo-environmental properties of crude oil contaminated kaolin clay. Lime and cement were mixed in a ratio of 1:2 and added to the simulated crude oil contaminated kaolin clay at different percentages (5%, 10%, 15%, and 20%) as a stabilizing binder. Parameters investigated include consistency limits, unconfined compressive strength (UCS), and direct shear, and compressibility and leaching characteristics of the untreated and stabilized soils. The experimental testing reveals a decrease in the consistency limits with addition of the stabilizing binder. Maximum UCS values occurred for 15% cement-lime stabilized kaolin clay at different curing periods (i.e. 0 d, 7 d, 14 d, and 28 d). By increasing the cement-lime content from 5% to 15%, the UCS values of the stabilized clay increase from 185 kPa to 350 kPa and from 785 kPa to 1160 kPa for uncured and 28 d-cured samples, respectively. Both the compression and recompression indices of the contaminated kaolin clay from the consolidation test decrease by 40% and 50%, respectively, with 20% stabilizing binder addition. The leachability of the contaminated clay also reduces with incorporation of cement and lime. According to the scanning electron microscope (SEM) test, addition of stabilizing binder transforms the dispersed structure of contaminated kaolin clay into a knitted flocculated structure. The study shows the effectiveness of cement-lime mix in stabilizing the contaminated kaolin clay and the possible use of stabilized contaminated kaolin clay as an alternative construction material.

Mesoscale fracture behavior of Longmaxi outcrop shale with different bedding angles: Experimental and numerical investigations

Abstract: The mechanical properties and fracturing mechanism of shale containing beddings are critically important in shale gas exploitation and wellbore stability. To investigate the effects of shale bedding on crack behavior and fracturing mechanism, scanning electron microscope (SEM) with a loading system was employed to carry out three-point bending tests on Longmaxi outcrop shale. The crack initiation and propagation of Longmaxi shale were observed and recorded by taking photos during loading. The cracking paths were extracted to calculate the crack length through a MATLAB program. The peak load, fracture toughness and fracture energy all increase with the bedding angle from 0° to 90°. The crack length and energy were also found to increase with the bedding angle in the range of 0°–60° and then drop slightly. The fracturing mechanism of shale includes the main crack affected by the bedding angle and disturbed by randomly distributed particles. The main cracking path was accompanied by several microcrack branches which could form an interconnected crack system. When the main crack encounters larger sedimentary particles, it will deflect around the particles and then restore to the initial direction. A numerical technique using extended finite element method (XFEM) coupled with anisotropic cohesive damage criteria was developed, which is able to capture the dependence of crack propagations on bedding angle and sedimentary particles. This study sheds light on understanding and predicting mesoscale fracture behavior of shale with different bedding angles.

Effect of biopolymers on permeability of sand-bentonite mixtures

Abstract: Application of biopolymer-modified geomaterials in waste disposal practices is gaining wide acceptance due to their superior tensile characteristics and improved crack resistance. Permeability is an important design parameter which determines the suitability of a material as a liner for construction of engineered landfills. Given this, the permeability characteristics of sand-bentonite mixtures amended with biopolymers was studied using a modified-falling head permeability apparatus under an accelerated gravity environment. Both distilled water and synthetic leachate were utilized as permeant liquid to assess the role of biopolymer amendment on the permeation behavior of sand-bentonite mixtures. Experimental results indicate that addition of biopolymers causes aggregation of the clay platelets, which in turn enhances the permeation behavior of the biopolymer-modified sand-bentonite mixtures. These mixtures meet the regulatory requirement of the liner.

Influence of microwave treatment on mechanical behaviour of compact basalts under different confining pressures

Abstract: The realisation of microwave-induced fracturing of hard rocks has potential significance for microwave-assisted mechanical rock fracturing and stress release in deep rock masses. In this context, compact basalts were treated by microwave heating in a multi-mode cavity at a frequency of 2450 MHz, and then, we investigated the mechanical behaviour of basalt samples after microwave treatment under uniaxial compression and conventional triaxial compression (CTC) tests. After microwave exposure, cracks appeared on the surface and inside of the rock sample, and the temperature of the sample's surface was unevenly distributed. The results show that the conventional triaxial compressive strength (CTCS) of basalt samples decreased linearly with microwave exposure time, and the higher the confining pressure, the smaller the reduction in the strength of basalt samples after microwave treatment. Under uniaxial compression, microwave exposure greatly affected the axial deformation, suggesting that deformation resistance of the samples gradually decreases with increasing microwave exposure time. Under triaxial compression, some microcracks induced by microwave exposure closed due to the effect of confining pressure, resulting in the confining pressure inhibiting any rightward shift of the axial deformation curve. Furthermore, under uniaxial compression, the elastic modulus and Poisson's ratio of basalts also decreased in a quasi-linear manner with elapsed microwave exposure time. Under triaxial compression, microwave exposure has slight influence on elastic modulus and Poisson's ratio. After microwave treatment, the changes in rock strength and deformation mainly result from changes in between the mineral structures. Confining pressure results in the closure of microcracks produced by microwave exposure, so that effects of microwave treatment on strength and deformation decrease, thus reducing the influence on elastic constants. The cohesion decreases with increasing microwave exposure time and shows an approximately linear decrease over time. In the basalt samples, new microcracks in various directions generated by microwave exposure can increase the discreteness of test results, while the discreteness of test results caused by microcracks gradually reduces with increasing confining pressure.

Ground behaviour analysis, support system design and construction strategies in deep hard rock mining – Justified in Western Australian's mines

Abstract: Development of deep underground mining projects is crucial for optimum extraction of mineral deposits. The main challenges at great depth are high rock stress levels, seismic events, large-scale deformation, sudden failures and high temperatures that may cause abrupt and unpredictable instability and collapse over a large scale. In this paper, a ground control and management strategy was presented corresponding to the three stages of projects: strategic design, tactical design and operational design. Strategic design is results in preparing a broad plan and primary design for mining excavations. The tactical design is to provide detail design such as stabilisation methods. Operational design stage is related to monitoring and updating design parameters. The most effective ground control strategies in this stage are maintenance, rehabilitation, monitoring and contingency plan. Additionally, a new procedure for design of ground support systems for deep and hard rock was proposed. The main principles are: static and/or dynamic loading types, determination of loading sources, characterisation of geological conditions and the effects of orientation of major structures with openings, estimation of ground loading factor, identification of potential primary and secondary failures, utilisation of appropriate design analysis methods, estimation of depth failure, calculation of the static and/or dynamic demand ground support capacity, and selection of surface and reinforcement elements. Gravitational force is the dominant loading force in low-level stresses. In high stress level, failure mechanism becomes more complex in rock mass structures. In this condition, a variety of factors such as release of stored energy due to seismic events, stress concentration, and major structures influence on ground behaviour and judgement are very complicated. The key rock engineering schemes to minimise the risk of failures in high-stress levels at great depth involve depressurisation and quality control of materials. Microseismic and blast monitoring throughout the mining operations are required to control sudden failures. Proper excavation sequences in underground stopes based on top-down, bottom-up, centre-out and abutment-centre were discussed. Also, the performance of a ground support system was examined by field observation monitoring systems for controlling and modifying ground support elements. The important outcome of the research is that the proposed procedure of selecting ground support systems for static and dynamic situations was applied in several deep underground mines in Western Australia. Ground behaviour modes and failure mechanism were identified and assessed. Ground demand for static and dynamic conditions was estimated and an appropriate ground support system was selected and evaluated in site-specific conditions according to proposed method for ground support design at great depth. The stability of rock masses was confirmed, and the reliability of the design methodology for great depth and hard rock conditions was also justified.

Controlling effects of differential swelling index on evolution of coal permeability

Abstract: Coal permeability measurements are normally conducted under the assumption that gas pressure in the matrix is equalized with that in fracture and that gas sorption-induced swelling/shrinking strain is uniformly distributed within the coal. However, the validity of this assumption has long been questioned and differential strain between the fracture strain and the bulk strain has long been considered as the primary reason for the inconsistency between experimental data and poroelasticity solutions. Although efforts have been made to incorporate the impact into coal permeability models, the fundamental nature of those efforts to split the matrix strain between fracture and coal bulk remains questionable. In this study, a new concept of differential swelling index (DSI) was derived to theoretically define the relation among sorption-induced strains of the coal bulk, fracture, and coal matrix at the equilibrium state. DSI was a function of the equilibrium pressure and its magnitudes were regulated by the Langmuir constants of both the matrix and the coal bulk. Furthermore, a spectrum of DSI-based coal permeability models was developed to explicitly consider the effect of differential strains. These models were verified with the experimental data under the conditions of uniaxial strain, constant confining pressure, and constant effective stress.

Dynamic compression characteristics of layered rock mass of significant strength changes in adjacent layers

Abstract: Layered rock mass of significant strength changes for adjacent layers is frequently observed in underground excavation, and dynamic loading is a prevalent scenario generated during excavation. In order to improve the driving efficiency and reduce engineering accidents, dynamic compression characteristics of this kind of rock mass should be understood. The dynamic properties of a layered composite rock mass are investigated through a series of rock tests and numerical simulations. The rock mass is artificially made of various proportions of sand, cement and water to control the distinct strength variations at various composite layers separated by parallel bedding planes. All rock specimens are prefabricated in a specially designed mould and then cut into 50 mm in diameter and 50 mm in height for split Hopkinson pressure bar (SHPB) dynamic compression testing. The test results reveal that increasing strain rate causes the increases of peak strength, σp, and the corresponding failure strain, ep, while the dynamic elastic modulus, Ed, remains almost unchanged. Interestingly, under the same strain rates, Ed of the composite rock specimen is found to decline first and then increase as the dip angle of bedding plane increases. The obtained rock failure patterns due to various dip angles lead to failure modes that could be classified into four categories from our dynamic tests. Also, a series of counterpart numerical simulations has been undertaken, showing that dynamic responses are in good agreement with those obtained from the SHPB tests. The numerical analysis enables us to look into the dynamic characteristics of the composite rock mass subjected to a broader range of strain rates and dip angles than these being tested.

Physico-mechanical properties of granite after cyclic thermal shock

Abstract: Understanding rock mechanical behaviors after thermal shock is critically important for practical engineering application. In this context, physico-mechanical properties of Beishan granite, Gansu Province, China after cyclic thermal shock were studied using digital image correlation (DIC), acoustic emission (AE) monitoring, and microscopic observation. The results show that the peak strength and elastic modulus decreased gradually with increase in thermal shock cycle. However, the above two parameters showed no further changes after 10 thermal shock cycles. The loading stress ratio (i.e. the ratio of the current loading stress level to the peak stress in this state) corresponding to the occurrence of the uneven principal strain field and the local strain concentration zone on the surface of the granite specimen decreased with increase in thermal shock cycle. Three transformation forms of the standard deviation curves of the surface principal strain were found. For granite with fewer thermal shock cycles (e.g. no more than 2 cycles), the standard deviation curves exhibited approximately exponential growth in exponential form. With increase in thermal shock cycle, the S-shaped curve was dominant. After 10 thermal shock cycles, an approximate ladder-shaped curve was observed. It is displayed that AE activity was mainly concentrated around the peak strength zone of the granite specimen when the rock samples underwent fewer thermal shock cycles. With increase in thermal shock cycle, AE activity could occur at low loading stress levels. Microscopic observation further confirmed these scenarios, which showed that more microcracks were induced with increase in thermal shock cycle. The number of induced microcracks at the edge location of the granite specimen was significantly larger than that at the interior location. Finally, a continuum damage model was proposed to describe the damage evolution of the granite specimen after cyclic thermal shock during loading.

Searching for critical slip surfaces of slopes using stress fields by numerical manifold method

Abstract: This study first reviews the numerical manifold method (NMM) which possesses some advantages over the traditional limit equilibrium methods (LEMs) in calculating the factors of safety (Fs) of the slopes. Then, with regard to a trial slip surface (TSS), associated stress fields reproduced by NMM as well as the enhanced limit equilibrium method are combined to compute Fs. In order to search for the potential critical slip surface (CSS), the MAX-MIN ant colony optimization algorithm (MMACOA), one of the best performing algorithms for some optimization problems, is adopted. Procedures to obtain Fs in conjunction with the potential CSS are described. Finally, the proposed numerical model and traditional methods are compared with stability analysis of three typical slopes. The numerical results show that Fs and CSSs of the slopes can be accurately calculated with the proposed model.

Particle flow code simulation of intact and fissured granitic rock samples

Abstract: This study presents a calibration process of three-dimensional particle flow code (PFC3D) simulation of intact and fissured granite samples. First, laboratory stress–strain response from triaxial testing of intact and fissured granite samples is recalled. Then, PFC3D is introduced, with focus on the bonded particle models (BPM). After that, we present previous studies where intact rock is simulated by means of flat-joint approaches, and how improved accuracy was gained with the help of parametric studies. Then, models of the pre-fissured rock specimens were generated, including modeled fissures in the form of “smooth joint” type contacts. Finally, triaxial testing simulations of 1 + 2 and 2 + 3 jointed rock specimens were performed. Results show that both elastic behavior and the peak strength levels are closely matched, without any additional fine tuning of micro-mechanical parameters. Concerning the post-failure behavior, models reproduce the trends of decreasing dilation with increasing confinement and plasticity. However, the dilation values simulated are larger than those observed in practice. This is attributed to the difficulty in modeling some phenomena of fissured rock behaviors, such as rock piece corner crushing with dust production and interactions between newly formed shear bands or axial splitting cracks with pre-existing joints.

Numerical manifold method modeling of coupled processes in fractured geological media at multiple scales

Abstract: Author(s): Hu, M; Rutqvist, J | Abstract: The greatest challenges of rigorously modeling coupled hydro-mechanical (HM) processes in fractured geological media at different scales are associated with computational geometry. These challenges include dynamic shearing and opening of intersecting fractures at discrete fracture scales as a result of coupled processes, and contact alteration along rough fracture surfaces that triggers structural and physical changes of fractures at micro-asperity scale. In this paper, these challenges are tackled by developing a comprehensive modeling approach for coupled processes in fractured geological media based on numerical manifold method (NMM) at multiple scales. Based on their distinct geometric features, fractures are categorized into three different scales: dominant fracture, discrete fracture, and discontinuum asperity scales. Here the scale is relative, that of the fracture relative to that of the research interest or domain. Different geometric representations of fractures at different scales are used, and different governing equations and constitutive relationships are applied. For dominant fractures, a finite thickness zone model is developed to treat a fracture as a porous nonlinear domain. Nonlinear fracture mechanical behavior is accurately modeled with an implicit approach based on strain energy. For discrete fractures, a zero-dimensional model was developed for analyzing fluid flow and mechanics in fractures that are geometrically treated as boundaries of the rock matrix. With the zero-dimensional model, these fractures can be modeled with arbitrary orientations and intersections. They can be fluid conduits or seals, and can be open, bonded or sliding. For the discontinuum asperity scale, the geometry of rough fracture surfaces is explicitly represented and contacts involving dynamic alteration of contacts among asperities are rigorously calculated. Using this approach, fracture alteration caused by deformation, re-arrangement and sliding of rough surfaces can be captured. Our comprehensive model is able to handle the computational challenges with accurate representation of intersections and shearing of fractures at the discrete fracture scale and rigorously treats contacts along rough fracture surfaces at the discontinuum asperity scale. With future development of three-dimensional (3D) geometric representation of discrete fracture networks in porous rock and contacts among multi-body systems, this model is promising as a basis of 3D fully coupled analysis of fractures at multiple scales, for advancing understanding and optimizing energy recovery and storage in fractured geological media.

GPGPU-parallelised hybrid finite-discrete element modelling of rock chipping and fragmentation process in mechanical cutting

Abstract: Mechanical cutting provides one of the most flexible and environmentally friendly excavation methods. It has attracted numerous efforts to model the rock chipping and fragmentation process, especially using the explicit finite element method (FEM) and bonded particle model (BPM), in order to improve cutting efficiency. This study investigates the application of a general-purpose graphic-processing-unit parallelised hybrid finite-discrete element method (FDEM) which enjoys the advantages of both explicit FEM and BPM, in modelling the rock chipping and fragmentation process in the rock scratch test of mechanical rock cutting. The input parameters of FDEM are determined through a calibration procedure of modelling conventional Brazilian tensile and uniaxial compressive tests of limestone. A series of scratch tests with various cutting velocities, cutter rake angles and cutting depths is then modelled using FDEM with calibrated input parameters. A few cycles of cutter/rock interactions, including their engagement and detachment process, are modelled for each case, which is conducted for the first time to the best knowledge of the authors, thanks to the general purpose graphic processing units (GPGPU) parallelisation. The failure mechanism, cutting force, chipping morphology and effect of various factors on them are discussed on the basis of the modelled results. Finally, it is concluded that GPGPU-parallelised FDEM provides a powerful tool to further study rock cutting and improve cutting efficiencies since it can explicitly capture different fracture mechanisms contributing to the rock chipping as well as chip formation and the separation process in mechanical cutting. Moreover, it is concluded that chipping is mostly owed to the mix-mode I-II fracture in all cases although mode II cracks and mode I cracks are the dominant failures in rock cutting with shallow and deep cutting depths, respectively. The chip morphology is found to be a function of cutter velocity, cutting depth and cutter rake angle.

Three-dimensional distinct element modeling of fault reactivation and induced seismicity due to hydraulic fracturing injection and backflow

Abstract: This paper presents a three-dimensional fully hydro-mechanical coupled distinct element study on fault reactivation and induced seismicity due to hydraulic fracturing injection and subsequent backflow process, based on the geological data in Horn River Basin, Northeast British Columbia, Canada. The modeling results indicate that the maximum magnitude of seismic events appears at the fracturing stage. The increment of fluid volume in the fault determines the cumulative moment and maximum fault slippage, both of which are essentially proportional to the fluid volume. After backflow starts, the fluid near the joint intersection keeps flowing into the critically stressed fault, rather than backflows to the wellbore. Although fault slippage is affected by the changes of both pore pressure and ambient rock stress, their contributions are different at fracturing and backflow stages. At fracturing stage, pore pressure change shows a dominant effect on induced fault slippage. While at backflow stage, because the fault plane is under a critical stress state, any minor disturbance would trigger a fault slippage. The energy analysis indicates that aseismic deformation takes up a majority of the total deformation energy during hydraulic fracturing. A common regularity is found in both fracturing- and backflow-induced seismicity that the cumulative moment and maximum fault slippage are nearly proportional to the injected fluid volume. This study shows some novel insights into interpreting fracturing- and backflow-induced seismicity, and provides useful information for controlling and mitigating seismic hazards due to hydraulic fracturing.

Effect of fatigue loading-confining stress unloading rate on marble mechanical behaviors: An insight into fracture evolution analyses

Abstract: Rocks in underground works usually experience rather complex stress disturbance. For this, their fracture mechanism is significantly different from rocks subjected to conventional triaxial compression conditions. The effects of stress disturbances on rock geomechanical behaviors under fatigue loading conditions and triaxial unloading conditions have been reported in previous studies. However, little is known about the dependence of the unloading rate on fatigue loading and confining stress unloading (FL-CSU) conditions that influence rock failure. In this paper, we aimed at investigating the fracture behaviors of marble under FL-CSU conditions using the post-test X-ray computed tomography (CT) scanning technique and the GCTS RTR 2000 rock mechanics system. Results show that damage accumulation at the fatigue stage can influence the final fracture behaviors of marble. The stored elastic energy for rock samples under FL-CSU tests is relatively larger compared to those under conventional triaxial tests, and the dissipated energy used to drive damage evolution and crack propagation is larger for FL-CSU tests. In FL-CSU tests, as the unloading rate increases, the dissipated energy grows and elastic energy reduces. CT scanning after the test reveals the impacts of the unloading rate on the crack pattern and a fracture degree index is therein defined in this context to represent the crack dimension. It shows that the crack pattern after FL-CSU tests depends on the unloading rate, and the fracture degree is in agreement with the analysis of both the energy dissipation and the amount of energy released. The effect of unloading rate on fracture evolution characteristics of marble is revealed by a series of FL-CSU tests.

On the use of 1g physical models for ground movements and soil-structure interaction problems

Abstract: The paper focusses on the use of physical modelling in ground movements (induced by underground cavity collapse or mining/tunnelling) and associated soil-structure interaction issues. The paper presents first an overview of using 1g physical models to solve geotechnical problems and soil-structure interactions related to vertical ground movements. Then the 1g physical modelling application is illustrated to study the development of damage in masonry structure due to subsidence and cavity collapse. A large-scale 1g physical model with a 6 m3 container and 15 electric jacks is presented with the use of a three-dimensional (3D) image correlation technique. The influence of structure position on the subsidence trough is analysed in terms of crack density and damage level. The obtained results can improve the methodology and practice for evaluation of damage in masonry structures. Nevertheless, ideal physical model is difficult to achieve. Thus, future improvement of physical models (analogue materials and instrumentation) could provide new opportunities for using 1g physical models in geotechnical and soil-structure applications and research projects.

Factor of safety of strain-softening slopes

Abstract: Stability analysis of strain-softening slopes is carried out using the shear strength reduction method and Mohr-Coulomb model with degrading cohesion and friction angle. The effect of strain-softening behavior on the slope factor of safety is investigated by performing a series of analyses for various slope geometries and strength properties. Stability charts and equations are developed to estimate the factor of safety of strain-softening slopes from the results of traditional stability analysis based on perfectly-plastic behavior. Two example applications including an open pit mine in weak rock and clay shale slope with daylighting bedding planes are presented. The results of limit equilibrium analysis and shear strength reduction method with perfectly-plastic models were in close agreement. Using perfectly-plastic models with peak strength properties led to overly optimistic results while adopting residual strength properties gave excessively conservative outcomes. The shear strength reduction method with a strain-softening model gave realistic factors of safety while accounting for the process of strength degradation.

Interactive roles of geometrical distribution and geomechanical deformation of fracture networks in fluid flow through fractured geological media

Abstract: In this study, the combined effects of geometrical distribution and geomechanical deformation of fracture networks on fluid flow through fractured geological media are investigated numerically. We consider a finite-sized model domain in which the geometry of fracture systems follows a power-law length scaling. The geomechanical response of the fractured rock is simulated using a hybrid finite-discrete element model, which can capture the deformation of intact rocks, the interaction of matrix blocks, the displacement of discrete fractures and the propagation of new cracks. Under far-field stress loading, the locally variable stress distribution in the fractured rock leads to a stress-dependent variable aperture field controlled by compression-induced closure and shear-induced dilatancy of rough fractures. The equivalent permeability of the deformed fractured rock is calculated by solving for the fracture-matrix flow considering the cubic relationship between fracture aperture and flow rate at each local fracture segment. We report that the geometrical connectivity of fracture networks plays a critical role in the hydromechanical processes in fractured rocks. A well-connected fracture system under a high stress ratio condition exhibits intense frictional sliding and large fracture dilation/opening, leading to greater rock mass permeability. However, a disconnected fracture network accommodates much less fracture shearing and opening, and has much lower bulk permeability. We further propose an analytical solution for the relationship between the equivalent permeability of fractured rocks and the connectivity metric (i.e. percolation parameter) of fracture networks, which yields an excellent match to the numerical results. We infer that fluid flow through a well-connected system is governed by traversing channels (forming an “in parallel” architecture) and thus equivalent permeability is sensitive to stress loading (due to stress-dependent fracture permeability), whilst fluid flow through a disconnected system is more ruled by matrix (linking isolated clusters “in series”) and has much less stress dependency.

Effect of pipe characteristics in umbrella arch method on controlling tunneling-induced settlements in soft grounds

Abstract: Recent developments in tunneling have stimulated design practitioners to more effectively utilize the underground spaces. However, tunneling at shallow depth in soft grounds gives rise to concerns associated with tunnel instability. Umbrella arch method (UAM), as a pre-reinforcement approach of tunnels in complex geological conditions, is widely used to maintain the tunnel stability. Quantitative assessment of the impacts of the entire approach and forepoling pipe features on tunnel stability remains challenging due to the complex nature of the UAM application. This study aimed to assess the effect of pipe design parameters on reinforcing the tunnels excavated in soft grounds. This practical investigation considered the actual field conditions attributed to the tunneling procedure and UAM deployment. Then, the tunneling process was modeled and the tunnel excavation-induced settlements were calculated. The post-processed results confirmed that deploying the UAM substantially reduced the tunnel crown and ground surface settlements by 76% and 42%, respectively. Investigation of various design parameters of pipes underscored the significance of incorporating the optimum value for each individual parameter into design schemes to more effectively control the settlements. Additionally, contrasting the settlement reduction rates (SRRs) for pipe design variables showed that the tunnel stability is more sensitive to the changes in the values of diameter and length, compared to values of the installation angle and center-to-center distance of the pipes.

Particle breakage evolution of coral sand using triaxial compression tests

Abstract: Particle breakage continuously changes the grading of granular materials and has a significant effect on their mechanical behaviors. Revealing the evolution pattern of particle breakage is valuable for development and validation of constitutive models for crushable materials. A series of parallel triaxial compression tests along the same loading paths but stopped at different axial strains were conducted on two coral sands with different particle sizes under drained and undrained conditions. The tested specimens were carefully sieved to investigate the intermediate accumulation of particle breakage during the loading process. The test results showed that under both drained and undrained conditions, particle breakage increases continuously with increasing axial strain but exhibits different accumulating patterns, and higher confining pressures lead to greater particle breakage. Based on the test results, the correlations between particle breakage and the stress state as well as the input energy were examined. The results demonstrated that either the stress state or input energy alone is inadequate for describing the intermediate process of particle breakage evolution. Then, based on experimental observation, a path-dependent model was proposed for particle breakage evolution, which was formulated in an incremental form and reasonably considers the effects of the past breakage history and current stress state on the breakage rate. The path-dependent model successfully reproduced the development of particle breakage during undrained triaxial compression using the parameters calibrated from the drained tests, preliminarily demonstrating its effectiveness for different stress paths.

Incorporation of a nanotechnology-based additive in cementitious products for clay stabilisation

Abstract: The mechanical performances and water retention characteristics of clays, stabilised by partial substitution of cement with by-products and inclusion of a nanotechnology-based additive called RoadCem (RC), are studied in this research. The unconfined compression tests and one-dimensional oedometer swelling were performed after 7 d of curing to understand the influence of addition of 1% of RC material in the stabilised soils with the cement partially replaced by 49%, 59% and 69% of ground granulated blast furnace slag (GBBS) or pulverised fuel ash (PFA). The moisture retention capacity of the stabilised clays was also explored using the soil-water retention curve (SWRC) from the measured suctions. Results confirmed an obvious effect of the use of RC with the obtained strength and swell properties of the stabilised clays suitable for road application at 50% replacement of cement. This outcome is associated with the in-depth and penetrating hydration of the cementitious materials by the RC and water which results in the production of needle-like matrix with interlocking filaments – a phenomenon referred to as the ‘wrapping’ effect. On the other hand, the SWRC used to describe the water holding capacity and corresponding swell mechanism of clays stabilised by a proportion of RC showed a satisfactory response. The moisture retention of the RC-modified clays was initially higher but reduced subsequently as the saturation level increased with decreasing suction. This phenomenon confirmed that clays stabilised by including the RC are water-proof in nature, thus ensuring reduced porosity and suction even at reduced water content. Overall, the stabilised clays with the combination of cement, GGBS and RC showed a better performance compared to those with the PFA included.

Crack initiation of granite under uniaxial compression tests: A comparison study

Abstract: In this study, a combination of acoustic emission (AE) method (AEM) and wave transmission method (WTM) is used to investigate the behaviors of AE and ultrasonic properties corresponding to initial fracturing in granitic rocks. The relationships of AE characteristics, frequency spectra, and spatial locations with crack initiation (CI) are studied. The anisotropic ultrasonic characteristics, velocity distributions in different ray paths, wave amplitudes, and spectral characters of transmitted waves are investigated. To identify CI stress, damage initiations characterized by strain-based method (SBM), AEM and WTM are compared. For granite samples, it shows that the ratio of CI stress to peak strength estimated by SBM ranges from 0.4 to 0.55, and 0.49–0.6 by WTM, which are higher than that of AEM (0.38–0.46). The CI stress identified by AEM indicates the onset of microcracking, and the combination of AEM and WTM provides an insight into the detection of rock damage initiation and anisotropy.

A modified method of discontinuity trace mapping using three-dimensional point clouds of rock mass surfaces

Abstract: This paper presents an automated method for discontinuity trace mapping using three-dimensional point clouds of rock mass surfaces. Specifically, the method consists of five steps: (1) detection of trace feature points by normal tensor voting theory, (2) contraction of trace feature points, (3) connection of trace feature points, (4) linearization of trace segments, and (5) connection of trace segments. A sensitivity analysis was then conducted to identify the optimal parameters of the proposed method. Three field cases, a natural rock mass outcrop and two excavated rock tunnel surfaces, were analyzed using the proposed method to evaluate its validity and efficiency. The results show that the proposed method is more efficient and accurate than the traditional trace mapping method, and the efficiency enhancement is more robust as the number of feature points increases.