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Morken, O. A. (Geological Survey of Norway) | Hermanns, R. L. (Geological Survey of Norway) | Penna, I. (Geological Survey of Norway) | Dehls, J. F. (Geological Survey of Norway) | Bhasin, R. (Norwegian Geotechnical Institute)
A rock slope failure occurred on August 13, 2016 from a 710 m high south facing slope in Dzongu valley, Sikkim, India. This failure was preceded by ten years of slope deformation. Since 2006, crack development was evident in satellite images of the eastern flank of the failure. Satellite data show deformation in the days prior to failure, while eyewitnesses attest that deformation and rock fall activity accelerated after the September 18, 2011 magnitude MW 6.9 earthquake, which had an epicenter ~50 km away. Reconstruction of the pre-failure surface on a post-failure DSM, indicates a failed volume of 8.7 million m3. The failure surface is roughly planar. However, one structure (100/48, dipdir/dip) cuts through the planar surface. The failure occurred late in the monsoon season, not coinciding with any registered earthquakes. Such rock slope failures are widespread in the deeply incised valleys of Sikkim: three such events have been reported here in the past 30 years and multiple deposits have been mapped.
Highly heterogeneous and anisotropic conditions in rock comprising double porosity and complex geometries of defects affect stability, stress conditions, groundwater vulnerability and recharge, drainage and dewatering practices, corrosivity, and integrity of infrastructure, to name a few examples. Movement of water at partial and highly variable saturation is very complex, depending on very smallscale variations (e.g. orientation, aperture, roughness, bridging, infill) in ground conditions as well as very subtle changes in moisture content (e.g. wetting vs drying). In contributing to this, a number of physical experiments were conducted in the laboratory or mimicked in the field. Experiments progressively assessed the influence of parameters, including assessment of the cubic law under smooth parallel plate models, fracture intersection, geometrical variation and verification with natural rocks at highly variable hydraulic head conditions. Some were subjected to differing gravitational accelerations to scale the vertical dimension. Studies contribute to flow regimes (laminar, turbulent, rotational, irrotational), mechanisms (droplets, rivulets, films, etc.) and scenarios (capillary barriered vs basally confined perched systems, etc.) of variably saturated rocks, as well as the interface between soil and rock. Where possible, a link is established between available theoretical understanding and empirical approaches to physical experiments and field verification experiments. Here, hydraulic parameters are estimated to improve the quantification of said parameters at discrete scale rather than assuming single values for bulk systems with the hope of eventually upscaling the models to larger representative elementary volumes, or to infer the behaviour under differing pressure conditions such as for pressure tests. Behaviour is inferred for fractures of changing orientation, changes in medium from soil to rock, and for alternation between wetting and drying of different media. Selected experiments are presented to highlight novel findings and the way forward. The incorporation of behaviour under variable saturation contributes to fundamental rock mechanics theory.
Underground support system using tendons has been one of the significant achievements in Civil and Mining engineering endeavours in facing challenges of ground control. However, shear failure of rock bolts is still one of the least monitored phenomenon in underground excavations with respect to seismic events. The understanding of the performance of rock bolts under dynamic loading condition requires a great deal of research. A series of tests were undertaken utilising a drop hammer mass of 600 Kg from a maximum height of 3.7 m over concrete blocks in the double shear box with chemical resin encapsulated a rock bolt to investigate the performance of rock bolts under dynamic shear load. Load cells, displacement laser and high speed camera were used to monitor the test. Results from the data analyses are presented in the form of displacement, hammer mass drop velocity, acceleration and force variation with time for all components involved in each test. The time factor was found to contribute 30 % of the shear load in static testing in comparison with dynamic; In particular, the force-displacement curve and energy absorption for the reinforcement system are presented to examine the performance of rock bolts and conclusion drawn.
Uotinen, L. (Aalto University) | Torkan, M. (Aalto University) | Janiszewski, M. (Aalto University) | Baghbanan, A. (Aalto University /Isfahan University of Technology) | Nieminen, V. (Aalto University) | Rinne, M. (Aalto University)
Characterization of Hydro-Mechanical (H-M) properties of rock fractures is the initial and important step in modeling of fully H-M coupled processes in fractured rock masses. Fluid flow in the fractured rock mass is an important aspect when evaluating the safety of geological disposal of high-level nuclear waste. Many attempts have been taken to measure and model fluid flow in rock fractures in different stress field conditions. However, still study about the scale effect of fracture properties and confinement stress on the conductivity of rough rock fractures remains a challenging topic of research. As a part of an ongoing research project about fluid flow modeling in fractured rock mass (RAKKA), and as an initial step one rock slab pair with sizes of 250 mm x 250 mm of Kuru grey granite halves was prepared. It has a horizontal mechanically induced tensile fracture. The surface roughness of the fracture was mapped using a conventional profilometer and structure-from-motion photogrammetry before each fluid flow test. The fractures were subjected to different normal stress and then fluid flow within the fractures was conducted linearly from edge to opposite edge with perpendicular edges sealed, and conductivity of the fractures under steady-state condition was measured. Then the test is repeated with all three sides open. The results show anisotropic behaviour in permeability. The diagonal components of the permeability matrix are significantly stress-dependent. Together the new fracture digitization method and the new three-way fluid flow test allow the contactless characterization of hydro-mechanical properties of rock fractures and the validation of the results.
The term rockburst is defined as a sudden and violent explosion of rock in or around an excavation. Rockbursts have been regarded as the least understood and the most feared mining hazard problem facing mining and underground operations all around the world. The paper studies the mechanism of blasting-induced pillar rockbursts in underground mining by considering rockbursts as a buckling (instability) problem of column structures. The stress wave from blasting is considered as an exponentially decreasing dynamic loading. The effects of static loads and dynamic disturbances are investigated. The perturbation effect and parametric resonance of dynamic loadings were also illustrated. These results provide a guideline to optimally design drilling and blasting, such that the blasting stress wave is judiciously avoided to have a frequency of approximately double the pillar’s natural frequency.
Recent years have seen reports of rock collapses and rock-falls from steep slopes, even if such points have not been extracted as those requiring inspection. In the maintenance of steep slopes, it is important to confirm the presence of rock-falls and source rocks corresponding to the outlined survey; however, it can be difficult to investigate from the target structure to the vicinity of the ridge because of cost concerns and vegetation overgrowth. It is important to obtain detailed geographical information for an efficient overview survey; therefore, laser measurement technologies with significant performance improvements have been used in the field of slope disasters including public surveys and the study of landslides, and it is expected that the utility of these technologies will further increase.
Herein, multiple combinations of airborne and ground-based laser measurement technologies were used to study steep slopes facing a river. Fluctuation analysis was performed using three-dimensional (3D) point cloud data obtained via laser measurements. As a result, airborne laser measurements deciphered 1.5-m high scarplets. Furthermore, by conducting displacement vector analysis, we grasped the tendency of fluctuation that gradually increased from the mountain to the river. In addition, for the overhanging blocks closest to the river, we detected an average variation of 141 mm via displacement vector analysis using a terrestrial laser scanner (TLS) with a 2-m mesh. We report the evaluated behavior of steep slopes depending on the type of topography and geology and the effectiveness of the technology as a periodic measurement method for maintenance purposes.
Japan has experienced frequent slope disasters caused by heavy rains and earthquakes in recent years. Although the geological conditions associated with slope disasters do not significantly change over short periods, statistics show that the probability of heavy rainfall over 80 mm/h has increased by 10% over the past 10 years (Japan Meteorological Agency 2017). In addition, large-scale earthquakes and epicentre earthquakes have been increasing.
For rectangular pillars or pillars of irregular shape, the perimeter rule is typically used for the calculation of pillar strength in bord-and-pillar layouts. This rule implies that the strength of a rectangular pillar can be estimated by the traditional empirical strength formulae by using an equivalent square pillar with an effective width. The application of this rule for the calculation of the strength of long slender or irregular pillars is questioned. This study applied a limit equilibrium model to various pillar shapes in order to compare the empirical strength value obtained from the formulae to the simulated numerical strength. For pillars with increasing slenderness, the perimeter rule implies an asymptotic value to the effective width. It is shown that the maximum effective width is equal to double the smallest dimension. It was found that in the limit where the slenderness of the pillar resulted in the maximum effective pillar width, the modelled strength of the pillar also presents an asymptote in terms of strength. Results from this study may impact the design methodology of layouts exploiting rib pillars for regional stability.
In the South African mining industry, the traditional design methodology used for pillar designs in bord-and-pillar layouts assumes a square pillar shape. In instances where the pillars are rectangular, use of the perimeter rule (Wagner, 1974) is recommended. This rule implies that the effective pillar width is a function of the pillar area and the pillar circumference. This approach also assumes the pillars are regular and no irregular shapes will be mined.
Salamon and Munro (1967) derived a pillar strength formula (Eg. 1) for square coal pillars, postulating that the strength is proportional to the pillar dimensions. This is given by:
Where Strengthcoal pillar Coal pillar strength
α, β Constants calibrated for coal
h Pillar height
The power law strength formula (Eg. 1) was subsequently adapted by Hedley and Grant (1972) for the uranium mines in Canada. They calibrated the constants α and β after studying a number of intact and failed pillar cases. Of interest was that these uranium pillars were rib pillars and not a square shape.
Xing, J. (Tongji university) | Zhao, C. (Tongji university) | Xie, D. (Tongji university) | Zhou, Y. (The University of Hong Kong) | Chihiro, M. (University of Miyazaki) | Ma, C. (China Fortune Land Development Co. Ltd)
A series of cylindrical intact and jointed rock-like specimens with single pre-existing cracks were prepared using high-strength gypsum by embedded thin sheet mica. Uniaxial compression tests, triaxial compression tests, and hydro-mechanical coupling triaxial tests (water pressure was applied inside the pre-existing cracks) were conducted to investigate failure behaviour of jointed specimens under hydro-mechanical coupling. Microscopic observation by scanning electron microscope (SEM) was employed to determine the crack nature, and X-ray computerized tomography (CT) scanning revealed the spatial geometric feature of internal cracks under hydro-mechanical coupling. The results suggest that the hydro-mechanical coupling significantly weakened the strength of the intact and jointed specimens. Nine types of cracks with various crack nature and spatial geometry in this experiment were summarized. Specimens under high confining pressure tended to shear failure, and specimens tended to tensile failure under high water pressure.
A series of events, including dam failures, landslides, and injection-induced earthquakes, were believed to result from hydro-mechanical coupling (Rutqvist & Stephansson, 2003), while the failure behaviour of rock mass under hydro-mechanical coupling has not been comprehensively understood.
In previous studies, researchers conducted a series of studies on the brittle failure of rock mass and obtained many valuable results(Bobet & Einstein, 1998; Cao et al., 2019; Hoek & Bieniawski, 1965; Petit & Barquins, 1988; C. Zhao, Zhou, Zhao, & Bao, 2018). Among them, Wong and Einstein (2009a, 2009c) investigated the cracking behaviour in rock-like and marble specimens with a single flaw under uniaxial and biaxial compression, using a high-speed camera recording the process, and seven different crack types initiated from the pre-existing flaws were identified. Yang and Jing (2011) analysed the cracking process and failure mode of sandstone specimens, and proposed nine different crack types based on their geometry and crack propagation mechanism (tensile, shear, lateral crack, far-field crack, and surface spalling). Under hydro-mechanical coupling, a series of experimental studies have also been carried out(Li, Liu, & Jiang, 2016; Wei et al., 2016; Xing, Zhao, Yu, Matsuda, & Ma, 2019; Cheng Zhao, Xing, Niu, & Ma, 2019; Zhou, Zhao, Zhao, Ma, & Xie, 2018). These studies have improved understanding of rock failure under hydro-mechanical coupling, while these studies are mainly based on planar problems, and the behaviour of cracks in three-dimensional states has not been carefully studied.
Baryakh, A. (Perm Federal Research Center of the Ural Branch of the Russian Academy of Sciences) | Tsayukov, A. (Mining Institute of the Ural Branch of the Russian Academy of Sciences) | Evseev, A. (Mining Institute of the Ural Branch of the Russian Academy of Sciences) | Lomakin, I. (Mining Institute of the Ural Branch of the Russian Academy of Sciences)
When mining with a room-and-pillar method, the urgent task is to estimate the critical state of interchamber pillars. An effective control method is to measure the horizontal convergence rate of stopes.
Physical modelling of the interchamber pillar fracture was carried out by uniaxial loading of salt specimens with a control of longitudinal and transverse deformations. Three-dimensional modelling was carried out by the finite element method in an elastoplastic statement considering the hardening effect. The solution region was divided into the first order isoparametric elements of the hexahedral shape. The solution of the incremental nonlinear finite element equations was carried out according to the modified Newton-Raphson scheme. For the numerical integration of plastic constitutive equations, an implicit scheme of return-mapping algorithm was used. A three-dimensional modification of the Botkin and Mirolubov strength criterion was adopted as a yield criterion. Calibration of the model was carried out according to the results of laboratory tests for the fracture of salt specimens of a cubic form.
The resulting mathematical model of fracture quite closely describes the experimental data: loading curve and distribution curves of transverse deformations over the specimen's cross section. Some differences between model and experimental data were revealed.
As part of the study of the deformation processes of the bearing elements of the room-and-pillar system at the Verkhnekamsk potash deposit, a number of full-scale and laboratory studies of the salt deformation process were carried out. One of the test run aimed at evaluating the deformation rates and determining the correlation of the longitudinal and transverse deformations of interchamber pillars included an experiment on uniaxial compression of large cubic specimens of saliferous rocks with control of longitudinal and transverse deformations. Such an experiment was carried out in laboratory conditions (Fig. 1). The control of specimen's deformations was performed using a system of deep and contour marks. The displacements of marks were recorded by observing a non-contact three-dimensional optical system that simulates the use of an extensometer on the specimen's surface. The experiment resulted in an average curve of the longitudinal load dependence on the longitudinal deformation (Fig. 2), as well as the transverse deformation distribution over the specimen's cross section (Fig. 3). The graph of the averaged loading curve shows that there are two modes of specimen deformation: elastic (up to about 1500 kN) and plastic (over 1500 kN). Conversely, transverse deformation diagrams show an essentially linear dependence throughout the entire deformation process. In this paper, a theoretical description of the obtained loading curve and distribution curves of transverse deformations is carried out using the rock strength criterion proposed in the work (Baryakh A.A. and Samodelkina N.A. 2017).
Defay, A. (Cerema Centre Est, Institut de Sciences de la Mecanique et Applications Industrielles(IMSIA, EMR 9219)) | Maïolino, S. (ENSTA ParisTech) | Maïtournam, H. (Cerema Centre Est) | Subrin, D. (Institut de Sciences de la Mecanique et Applications Industrielles(IMSIA, EMR 9219))
We propose here to develop a numerical method to simulate the behaviour of a rock mass during excavation. We use the Hoek-Brown criterion which is a good representation of the rocks and rock masses behaviour. We developed a steady-state algorithm which considerably reduces the calculation time so the plastic strains with the Hoek-Brown criterion can be solved in reasonable time.
In the steady-state algorithm, the referential of the observer is in movement with respect to the applied load and we suppose the steady-state is reached. The hypothesis are a constant advancement rate and a section far enough from the entrance. Plastic strain are calculated using a projection algorithm, and then transported from each Gauss point to the next along each parallel to the advancement direction.
We use a projection algorithm on the yield surface to obtain the plastic strains. The closest point projection is used for associated materials, and this method is updated with the dilatancy angle as a parameter to fit with non associated materials. We use 3-dimensions smooth versions of the Hoek-Brown criterion, where the initial hexagonal shape function is replaced by smooth shapes functions.
With this numerical simulation, we can obtain the convergence profile of the excavation which depends on the ground behaviour. With this curve we can compare the effects of the material characteristics and determine the ground state in a given distance after the tunnel face.
To design supported tunnels, one of the most common method is the convergence-confinement. With this method we can obtain the equilibrium state between the tunnel wall and the support with the intersection of two curves: the convergence curve which represents the wall behaviour and the confinement curve which represents the support behaviour.
The convergence curve is obtained from transversal sections. To represent the effect of the excavation a fictive pressure