ABSTRACT: Characterization of unstable rock slopes can pose a high level of risk toward the engineer/geoscientist in the field due to inaccessibility and safety issues. During recent decades, rapidly developing remote sensing (RS) techniques, including Terrestrial Laser Scanning (TLS), Terrestrial Digital Photogrammetry (TDP), and Unmanned Aerial Vehicle Structure-from-Motion (UAV-SfM) are being progressively employed for landslide investigation and risk assessment. These methods allow acquisition of three-dimensional (3D) data sets from previously inaccessible terrain with sub-centimeter accuracy. We present an innovative approach to investigate the preliminary engineering geological characterization of a large (~5.5 Mm 3 ), destructive landslide that occurred on August 2 nd , 2014 near Jure village, ~70 km northeast of Kathmandu, Nepal. We conducted traditional field surveys (Geological Strength Index, scanline, etc.), RS techniques and preliminary 2D/3D numerical modelling with the objective of understanding conditioning factors, slope failure mechanisms, and future hazards. With four years of RS data, analysis of strength degradation and progressive weakening of the rock mass has been analyzed by linking erosional and depositional processes using 3D change detection algorithms. Results of 2D/3D rock engineering mapping and modelling have been integrated within an interactive 3D virtual/mixed reality (VR/MR) geodatabase model of the Jure field site, enabling an immersive and enhanced engineering 3D geovisualization experience. We demonstrate how VR/MR techniques can be employed to conduct/compare discontinuity mapping on virtual outcrops and advance the understanding/communication of landslide investigation and hazard/risk assessment.

ABSTRACT: Horizontal well drilling and hydraulic fracturing have been widely applied to enhance the shale oil production in the Bakken Formation. Previous research for Bakken Formation primarily focused on reservoir characterization, hydraulic fracturing treatments, and well completion design. However, few studies have been done on the simulation of hydraulic fracture propagation under real Bakken Formation's mechanical properties and field stresses. The objective of this study was to investigate numerically the effects of the in-situ stresses and formation properties on the geometry and pressure of hydraulic fracture. A fully coupled lattice model, based on the distinct element method (DEM) and synthetic rock mass (SRM), was used for the simulations. XSite, a new, lattice-based software by Itasca was used in this study. The model input data was taken from 1D mechanical earth models (MEM) built and the results of lab test on 240 core samples acquired from eight wells drilled into Upper, Lower and Middle Bakken. The results indicated that the hydraulic fracture which is initiated at the center of the Middle Bakken Formation can penetrate into both the Upper and Lower Bakken formations due to the high Young's modulus of both formations. Sensitivity analyses of the formation properties and in-situ stresses on hydraulic fracture propagation were carried out.

ABSTRACT: The purpose of this paper is to present a numerical code for rock mechanics modelling of discontinuities such as fractures, joints and faults. We implement this in an open-source Matlab Reservoir Simulation Toolbox designated for rapid prototyping of geomechanics, reservoir modelling and simulation problems. Appropriate and explicit modelling of such discontinuities become a central element of the scientific discussions. In particular, we contemplate processes when fractures open, close and slip due to stress changes in the subsurface. Existing numerical codes use either long-syntax programming languages where details of implementation are not explicitly shown or commercial simulators that often lack the flexibility for implementing new ideas. We use the recently developed Virtual Element Method (VEM) and solve contact problems that allow fractures to dynamically interact with the surrounding rock mass and each other, transferring contacting stress across faces due to closure and friction. This method has advantages over standard Finite Elements or Finite Volumes in solving the contact problem because it handles complex, not necessarily convex grid geometries and hanging nodes. For the contact problems, we implement contact detection, normal and tangential contact interaction algorithms. We provide numerical examples and match to analytical solutions in order to validate employed algorithms.

ABSTRACT: As new development moves into the vicinity of large granite boulders, new factors including vibrational loads caused by construction equipment or seismic shaking, loss of downslope support, and human activity begin to play a role in the stability of precariously balanced boulders. This study presents a practical reliability-based approach, using a Monte Carlo simulation, nine simple field measurements, and three “in house” estimates to determine the stability of a granite boulder, with respect to both sliding and rocking motion. The addition of a contact percentage parameter practically augments the widely accepted joint roughness coefficient (JRC) to account for the three-dimensional effects of real natural rock discontinuities. The Monte Carlo simulation allows for the uncertainty of the field measurements and rock strength properties, obtained using traditional geologic field tools, to be incorporated into the determination of the reliability, or probability of boulder movement. The method of analysis transforms the complexity of estimating potential boulder movements into a simple procedure, which can be conducted using a typical spreadsheet allowing for immediate implementation into geological/geotechnical engineering practice. A case study site is also presented, for which an active mitigation approach using rock bolts is employed.

ABSTRACT: Accurate modeling of height growth in hydraulic fracture models is always a challenge, especially in laminated rocks. In this paper, we study fracture height growth in layered formations by accounting for the effect of shear slippage of crossed interfaces. The boundary of the shear activated zone along a weak interface is calculated based on a previous weak interface model (referred to as the FracT model). The effect of shear slippage along the interface on the subsequent height growth after the fracture crosses the interface is modeled using the 2D displacement discontinuity method (DDM). The reduction in stress intensity factors at the fracture tips resulting from the corresponding shear slippage length and shear displacement discontinuities are presented. The improved height growth model that accounts for the effect of shear slippage at weak interfaces is integrated in a pseudo 3D hydraulic fracture model. The impact of the shear slippage at weak interfaces on the fracture height growth will be demonstrated through a parametric study.

ABSTRACT: The Rock Engineering System (RES) approach provides a framework for geotechnical engineering design, and has been used successfully to support a wide range of rock engineering. RES solutions require the identification of the principal parameters that play a role in the process under examination. The relationship between these parameters and other related variables are then assessed to form an interaction matrix that represents the system. Previous studies have considered blast optimization using an RES approach. However, these approaches consider few details regarding the existing fracture network. How a rock mass reacts to a blast is dependent on the rock itself and on the discontinuities within the rock mass. A set of discrete fracture network (DFN) realizations generated and assessed using MoFrac are used to derive inputs for a RES fragmentation interaction matrix. Parameters representing block size and rock mass integrity derived from fracture network models are identified that can be incorporated into a RES solution. A new parameter for inclusion in existing solutions measuring fracture intensity variability is also presented. From analysis of a fracture network, estimations of in situ block size distribution, fracture set intensity, and variability are presented that can be integrated in a RES approach to blast optimization.

ABSTRACT: The Valley of the Kings (also known as King's Valley, KV) in Egypt is recognized as one of the richest archeological world heritage sites. Thousands of tourists visit the valley each day and its rock-cut tombs, of which many were excavated close to or into the steep cliffs of the valley. The rock mass properties and environmental conditions of the area were gathered from literature to conduct a preliminary stability assessment of the fractured cliff above the 18 th Dynasty tomb KV42 constructed for Hatshepsut-Meryet-Ra, the wife of Thutmose III, reused by a mayor of Thebes, Sennefer, and his family members. A 2D numerical model was developed in RS2® using the Shear Strength Reduction (SSR) modelling method, which indicates the importance of vertical fractures on the cliff stability. Preliminary results suggest that failure could be more influenced by the orientation of vertical joint sets rather than bedding planes. We installed a monitoring station to record the local environmental parameters such as temperature and relative humidity along with crack meter readings and discuss the impact of daily and seasonal climatic variations on vertical discontinuity opening and closing cycles. A coupled thermal and geomechanical model is recommended to be developed in further stages of this research to validate the effects of climatic variables on the extension of fractures and cliff stability at KV42.

ABSTRACT: Shear strength along discontinuities plays a crucial role in the stability of rock structures. The development of geophysical methods to remotely monitor and assess changes in shear strength is essential to the identification of rock hazards that can lead to the loss of life and failure of civilian infrastructure. In this study, compressional and shear ultrasonic waves were used to monitor slip along discontinuities (with different surface profiles) during shearing. A series of laboratory direct shear experiments was performed on two gypsum blocks separated by a frictional discontinuity. The gypsum blocks had perfectly matched contact surfaces with a half-cycle sine wave profile that spanned the central third of the discontinuity, surrounded by planar surfaces. The amplitude of the half-cycle sine wave was varied and ranged between 2 to 10 times the height of the asperities. Compressional, P, and shear, S, ultrasonic waves were continuously transmitted and recorded throughout the shearing process, while Digital Image Correlation (DIC) was used to capture surface displacements. At low normal stresses, distinct maxima in the normalized P and S wave transmitted amplitudes occurred before shear failure in regions where dilation was observed. Where dilation was not detected, an increase in transmitted wave amplitude was observed, even after the peak shear stress was achieved. At high normal stresses, dilation was suppressed, which was associated with an increase in wave amplitude with shear stress until the peak, and then a decrease in amplitude. Monitoring changes in transmitted wave amplitude is a potential method for the detection of dilation along rock discontinuities.

ABSTRACT: This paper presents a rock-slope kinematic analysis using a robust workflow that combines geologic reconnaissance with UAV-based photogrammetric methods. The photogrammetric workflow generates high-density, georeferenced, three-dimensional point clouds in true-color, enabling us to virtually map large numbers of discontinuity features such as fractures, bedding planes, and joints; features that would otherwise be inaccessible in a safe manner. Volumetric measurements can be made on discrete hazardous blocks to support rock fall simulation studies. This workflow was applied at a hazardous road-cut on the north shore of the Monongahela River in Pittsburgh, Pennsylvania (180 meters long by 40 meters tall). Analysis of the road-cut identified distinct lithologic units that experience different modes of failure; resistant Morgantown Sandstone undergoing planar failure and weaker Clarksburg Claystone undergoing wedge failure. Hybrid failure is possible in the lithologic units between the above-mentioned layers. The workflow was optimized to improve upon existing procedures that typically utilize distant ground-based cameras and stereo-imagery. Flight plans were designed to maximize visual coverage to the rock face, eliminate image distortion across the outcrop, and generate cloud point-spacing of 1.25 cm or less. This project provides one example of improving a fundamental practice in rock slope stability with an intelligent application of technology. The increased density of high quality data leads to more informed conclusions.

ABSTRACT: In analyzing Moment Tensor Inversion (MTI) of microseismic data recorded during hydraulic fracturing, a large number of events have been found to have moment tensors consistent with either vertical or bedding plane slip. We are endeavoring to verify the occurrence of bedding plane slip events via analog experiments. The interaction between a natural fracture and a hydraulic fracture has been studied extensively, both experimentally and numerically to better understand the potential for crossing and arrest of a hydraulic fracture intersecting a natural fracture. However, the actual mechanical interaction between a hydraulic fracture near a natural fracture or a bedding plane discontinuity has not been studied, particularly under triaxial stress and hydraulic fracturing conditions. In this paper, we present the results of some lab-scale experimental work, demonstrating HF/NF interaction with emphasis on slippage of a discontinuity surface. Injection pressure, stress applied, and the sample deformation are monitored during the test. Acoustic Emission (AE) technology is employed to record the AE signals generated during fracture initiation, propagation and during the sliding of the joint. In addition, strain gauges are used to measure the slippage on the natural fracture. The tests are carried out on 101.6 mm diameter cylindrical samples of PMMA and shale. The calculated displacement based on the recorded strain clearly shows a jump at the breakdown point, which is accompanied by increased AE activity. Analysis of the data clearly shows the occurrence of slippage on the joint in response to an approaching hydraulic fracture. Expectedly, the degree of shear slip varies with natural fracture dip, and friction angle.

ABSTRACT: The impact of rock falls on people, manmade infrastructures, and inhabited areas is considered in this paper, with examples taken from practice and personal experience. Both conventional and advanced investigation methods to study and characterize the rock mass and define the likely modes of rock instability initiation and failure are used. The rock fall phenomenon with rock block detachment and trajectories of blocks along the slope is investigated. Two case studies of high, steep, and jointed limestone rock cliffs in Italy, characterized by the presence of rock volumes of different shape and size, in limit equilibrium conditions, are described. In both cases, the studies undertaken for analyzing the stability conditions are presented, together with the kinematic simulations of the rock falls down slope, and the stabilization measures (active and passive measures) implemented. In one case in particular, Terrestrial Laser Scanning (TLS) is adopted to obtain a high-resolution 3D Digital Elevation Model (DEM) of the rock mass.

ABSTRACT: Rockfalls constitute an abrupt type of landslide process with manifold impact at local level, on economy and mostly on a human modified environment. Rapid automated and accurate mapping of rockfalls’ characteristics are crucial in order to identify areas of potential risk and propose mitigation measures. Simultaneously, urban expansion in hazardous regions in combination with climate change variations had played an important role on the increase of rockfall events. The current study presents a novel semi-automated approach to detect rockfall hazard from detailed 3D point clouds that were collected with an Unmanned Aerial Vehicle (UAV), right after a recent rockfall event, triggered by heavy rainfall that destroyed several houses in the base of a steep terrain. Block geometry measurements of sliding slope have been provided in relation with the geometrical and mechanical characteristics of the main discontinuity surfaces from the photogrammetric 3D point cloud. In addition, this study was contributed by field measurements and laboratory tests of the rock joint strength for the analysis of the slope stability of the rock blocks. A set of corresponding discontinuity factors have been detected and characterized in terms of point cloud analysis and provide a qualitative hazard system to assess rockfall probability by extensive characterization and parametrization of the joint systems. Results have demonstrated the capabilities of combining UAV platforms with computer-based methods for rapid and accurate identification of discontinuity parameters subjectively and even from inaccessible areas of the rockfall body.

ABSTRACT: Joint roughness directly affects the behaviour of discontinuities at the laboratory scale and the rockmass at excavation scales. As large infrastructure projects (e.g. deep base tunnels for transportation, large open pit and underground mines with complex geometries, and deep geological repositories for the storage of nuclear waste with unprecedented design life spans) are being constructed in more challenging environments, the need for sound understanding of the geomechanical behaviour of rock joints and in turn the roughness of joint surfaces is paramount. This paper presents practical methods for the analysis of joint surface roughness from three-dimensional Structure-from-Motion (SfM) photogrammetry models and two-dimensional cylindrical core scan photographs. In order to understand the variability of joint roughness on the surface of a discontinuity through drill core, hundreds of profiles were measured with JRC values calculated for each profile. Through this process, the variability of JRC values based on two-dimensional profiles across a surface is highlighted. Finally the effectiveness of cylindrical core scans is evaluated by comparing the roughness estimates to scanlines taken across the surface of the discontinuity.

ABSTRACT: Rock mass classification systems have been empirically developed based on the in-situ observation and user’s experience. These two are parameters that could potentially influence the results as the former faces inaccessibility issues, especially in steep mountainous terrains, and the latter includes significant uncertainty. GSI (Geological Strength Index) is such a classification system for which a quantification approach has been proposed by Hoek et al., 2013 including both structure and surface quality components. The current study demonstrates a powerful approach for the GSI rock mass characterization, based on the above-mentioned GSI quantification, utilizing both LiDAR and photogrammetry point clouds from several rock slopes. The proposed semi-automated methodology leads to the extraction of space-resolved GSI values for the entire slope, by automatically assessing the structure component of the system. On the produced maps, the different GSI-valued areas are easily identified along the slopes. The results are compared to the visual-based GSI assessment conducted in some cases in Greece, verifying the validity of the semi-automatically extracted results.

ABSTRACT: This work presents a numerical technique capable of handling evolving fractures in rocks that allows extending, in a simple manner, standard finite element programs developed for geological (continuum) media to deal with the presence of discontinuities. The proposed mesh fragmentation technique consists in introducing especial (high-aspect ratio finite elements) in- between the regular (bulk) finite elements. This methodology has been successfully used to model both, drying cracks in soils and fractures in concrete structures. The proposed approach is fully coupled and has been implemented in the finite element program CODE_BRIGHT, which was especially developed to tackle coupled thermo-hydro-mechanical problems in geological media. The paper presents the main components of the proposed approach and its implementations in CODE_BRIGHT. It also discusses the main advantages and shortcomings associated with this technique. The proposed method is verified and validated using available analytical and numerical solutions associated with the problem of hydraulic fracturing in rocks. The technique is also applied to solve engineering problems involving the formation and propagation of discontinuities in geo-materials. A very satisfactory performance of the proposed method is observed in all the analyzed cases.

ABSTRACT: In this paper, the algorithm of the hybrid finite-element meshfree method (FEMM) is first used to simulate three-dimensional thermal cracking in the context of the TOUGH-FEMM simulator. The TOUGH-FEMM simulator combines two existing codes sequentially, TOUGH2 and FEMM. The temperature distribution of the system is analyzed using TOUGH2, which is able to solve coupled problems of nonisothermal, multiphase, multicomponent fluid flow in complex geological systems. Recently, hydro-mechanical models based on FEMM and TOUGH2 have been coupled together for the analysis of hydraulic fracturing in fractured rock. The thermal stress, rock deformation and cracking induced by temperature change are modeled by FEMM. The finite element meshfree method (FEMM) is presented to model mechanical deformation and discontinuities. With the concept of partition of unity, the accuracy of global approximation can be improved without adding extra degrees of freedom (DOF), and various local approximation functions can be used for modeling strong or weak discontinuities. Linear elastic fracture mechanics (LEFM) with mixed-mode stress intensity factors are employed to determine the initiation and direction of crack propagation. Several benchmark examples, including stress intensity factor calculation, steady-state heat conduction, transient heat conduction, and deformation induced by thermal stress, are provided to validate TOUGH-FEMM in dealing with thermo-mechanical problems with cracking. Additionally, a numerical experiment of crack propagation induced by thermal stress is also given, and its results are compared with the experimental results. The simulation results agree well with the analytical solutions or experimental results, indicating that the proposed approach is effective for thermal cracking simulation.

ABSTRACT: Ubiquitous joint constitutive models have been available for numerical analysis for a few decades. However, mis-understanding of the true behavior of these models is still quite prevalent among users. The purpose and goal of addressing strength anisotropy through a ubiquitous joint model embedded in a constitutive model has been successful, but the deformations do not follow the intuitive behaviour in many cases. This paper dissects the constitutive formulation of the ubiquitous joint model and compares its behaviour against explicitly modelled joint networks to show the range of applicability of ubiquitous joint models and when they fall short of capturing the correct deformation behaviour. The paper will show that ubiquity does not equate to continuity and that ubiquitous joint models are not able to capture the correct behaviour for discontinuities of large extent, such as bedding or faults. The paper will also show that this behaviour is inherent in the incremental stress/strain formulation of constitutive models and not specific to any finite element or finite difference code.

ABSTRACT: The underground limestone mining industry has one of the highest fatality rates compared to all other types of mining in the United States (MSHA, 2016). Ground control issues heavily contribute to these fatalities. According to the National Institute for Occupational Safety and Health (NIOSH, 2011), structurally controlled instability is a predominant failure mechanism in underground limestone mines. Existing analysis methods, such as the limit equilibrium method, do not represent the real structure and behavior of a rock mass in-situ. The Discrete Element Method (DEM) when coupled with representative Discrete Fracture Networks (DFN) has been proven to provide accurate models of the actual response of a rock mass undergoing excavation. However, considering the statistical nature of DFN's, multiple simulations need to be run in order to obtain statistically significant results of the model. The following paper introduces a stochastic approach for analyzing rock block falls in underground excavations. This approach utilizes DEM (Itasca's 3DEC) and DFN's representing virtually mapped discontinuities obtained from terrestrial laser scans. The numerical models were carried out considering rigid blocks. Failed blocks were defined as those blocks that had displaced more than 2 cm and presented velocities indicating that the block was still in movement. This approach allows engineers to define the probability of block failure based on the geometry and weight of failed blocks formed by the intersection of discontinuities in the section of interest, as well as to define kinematics of the blocks. Such information can provide mining operators control measures to evaluate, map and mitigate risks associated with rock falls in underground mines, ultimately improving safety in the underground limestone industry.

ABSTRACT: Rock bridges are sections of an intact rock along otherwise through-going discontinuities, and the rock bridges play an important role in the geotechnical stability of underground excavations, surface slopes of mines, dams, foundations and other earth retaining structures. Numerical and analytical techniques for predicting the time-dependent degradation and failure of the rock bridges have been previously developed, and the results show a high sensitivity to the area percentage, shape, and distribution of the rock bridges along a discontinuity. Even a rock bridge percentage less than 1% can result in the stabilization of an otherwise unstable block in the roof of an underground excavation.

ABSTRACT ABSTRACT: Footwall slopes refer to bedding rock slopes in which the slope face is parallel to a set of persistent discontinuities, which exist extensively in opencast mine. Stability evaluation of the footwall slopes is significant for theoretical study and engineering application. In this paper, taking a certain footwall slope for the example, three calculation methods were compared for stability analysis of the footwall slopes. Firstly, calculation parameters of the rock masses and discontinuities were obtained through site tests and laboratory tests. Secondly, a strength reduction program was written using fish language and applied to the Ubiquitous-Joint Model in the finite difference method. Thirdly, a limit equilibrium method to calculate safety factor of the footwall slopes was proposed. In this case, the persistent discontinuities in footwall slopes were simulated by lamellar materials. Finally, comparison of the results, respectively, obtained by the finite difference method, the discrete element method and the limit equilibrium method were conducted. The results show that the safety factors determined by the three methods are consistent. 1. INTRODUCTION In the opencast mining, in which the strata have subjected to tectonic folds, slopes are often excavated parallel to the strata dip in synclinal and anticlinal structures forming extensive, high and unbenched footwall slopes (Stead and Eberhardt, 1997). Footwall slopes, also referred to as bedding slopes, are encountered in other engineering rock slopes (e.g., hydropower and highway) where instability is structurally controlled, typically by one joint set, fault or weak zone parallel or subparallel to the slope (Konietzky, 2004; Fisher, 2009; Alejano et al., 2011). How to establish a reasonable description of this instability has aroused the great concern of many scholars, and different numerical methods have been put forward, such as finite element method, finite difference method, discrete element method and limit equilibrium method. Zheng et al. (2013) considered the dominated structural plane of the steep rock as a macro-crack, and the extended finite element method (XFEM) was proposed to investigate the influence of the dip angle and position of the dominant structural plane on the deformation and fracture of the steep rock. Lu et al. (2011) used the software of the finite difference method (FLAC) to establish a three-dimensional calculation model of bedding rock slope, and the variation law of slope safety factor was obtained when the cohesion and friction angle of structural surface changed. Lei and Wang (2006) combined the software of Universal Distinct Element Code (UDEC) with the strength reduction method to analyze the stability of rock slopes with multi-structural planes. And compared with the results of traditional slice method, it shows that UDEC-based strength reduction method is a reliable and effective method. Fisher and Eberhardt (2007) carried out discrete element simulation together with limit equilibrium analyses to provide practical recommendations for the stability evaluation of footwall slopes.