Park, H. (National Institute of Advanced Industrial Science and Technology (AIST)) | Ito, K. (National Institute of Advanced Industrial Science and Technology (AIST)) | Takahashi, M. (National Institute of Advanced Industrial Science and Technology (AIST)) | Osada, M. (Saitama University) | Smolnik, G. (Silesian University of Technology)
It is fundamental to understand the process of crack initiation and propagation of intact rock to clarify the coupled hydro-mechanical properties due to the stress change in underground. We have been working on the study of coupled shear-flow properties on sedimentary rock using pumice tuff, Japan. The changes of shear stress, normal stress, flow rate and process of fracturing on sample surface have been investigated by using a new type coupled shear-flow test apparatus. In this study, we designed a numerical model for the coupled shear-flow test using a discrete element code. We report the comparison between the experimental results and the simulation results. The porosity changes of model showed good agreement with the flow rate behavior of the test.
New fractures due to underground excavation or fault cause mechanical and hydrological problems in underground (Tsang et al., 2005, Mitchell and Faulkner, 2009). The fractures are generated through a variety of mechanisms with various scales (Bossart et al., 2002, Scholz et al., 1993). For the research of underground disposal or geological evaluation, it is necessary to investigate the coupled hydro-mechanical behavior of rock (Olsson and Barton, 2001, Li et al., 2008). We conducted an experimental study on coupled shearflow properties of sedimentary rock, and designed a numerical model for the coupled shear-flow test using a three-dimensional simulation tool, PFC3D (Particle Flow Code3D, Itasca Inc.). The purpose of this study is to examine the performance characteristics of the numerical model. The test system and model are briefly introduced, and the results are compared focusing on stress, permeability and crack propagation.
2 Particle Flow Code3D
PFC3D is classified as a discrete element code (Cundall and Strack, 1979, Potyondy and Cundall, 2004) based on the definition in the review by (Cundall and Hart, 1992). It allows finite displacements and rotations of discrete bodies (including complete detachment), and recognizes new contacts automatically as the calculation progresses. PFC3D can be considered as a simplified implementation of the DEM because of the restriction to rigid spherical particles.
Hartami, P. N. (Mining Engineering Department, Trisakti University) | Nugroho, B. (Mining Engineering Department, Trisakti University) | Mayangsari, S. (Economic Faculty, Trisakti University) | Nurhardono, _ (Department of Energy and Mineral Resources)
Generally small-scale mining does not pay attention to the Health and Safety (HSE) aspect. This is due to the fact that the knowledge and awareness of the miners is limited. The miners do not use personal protective equipment and they work in unsafe conditions, such as working in unsupported holes, digging and excavating below unstable slopes, which consequently causes many mining accidents. on the research site is a small-scale gold mine in Sekotong, West Lombok. In recent years, many people of Sekotong, a small village that is located about 40 kilometers from Mataram, West Nusa Tenggara (NTB), Indonesia, have become gold miners. The small-scale mining in this area is sporadic, disorganized, and used the underground mining method and simple equipment such as crowbars, hammers and chisels. The mining activities are carried out at a depth of approximately 40 meters below the surface with a very narrow entrance that sometimes only is large enough to fit a body. It is not surprising that there are many mining accidents. This paper aims to describe the contribution of rock mechanics to small-scale mining in Indonesia in order to decrease the number of mining accidents because people are not aware of good mining practices, safety and the behavior of rocks.
Indonesia is a country that has many natural resources and because of this mining activitities in Indonesia are not only done by big companies but also by individuals and small companies. However, actually, small-scale mining is often counter productive because it creates many problems. Usually, the operators of small-scale mines do not pay attention to the Health and Safety aspect due to their lack of knowledge as well as the lack of awareness of the miners. The miners do not use personal protective equipment and they work in unsafe conditions, such as working in unsupported locations or digging and excavating on unstable slopes. Therefore there are frequently many accidents which is some cases are fatal.
According to the report of N.D. Soemantri (2010), several mining accidents caused by underground collapses that resulted in dozens of deaths were recorded. Furthermore, there are many mining accidents that result in either injuries or fatalities that are not reported. Therefore, to date there is no accurate data regarding mining accidents in Indonesia.
Rock mechanics is a science that studies the characteristics and behavior of a rock mass that can be used for designing underground stability. The application of the principles of rock mechanics can assist miners in performing their activities safely. Using the case study from West Lombok, Indonesia as one of small-scale mining centers, it is expected that methodology and principles of rock mechanics can be applied to other small-scale mines.
Traditional design methods for mining, tunneling and public underground constructions use deterministic approaches. Reliability requirement for these constructions became very high and deterministic approaches lead to introduction of overpriced Factors of Safety (FS). It is more appropriate to select an optimal support based on the risk focused management approach. The Monte-Carlo method for statistical simulation is recommended for the design of underground constructions. As the first step we use simplified approach based on a plausible assumption that variation in the stress and the strength of rock mass, rock pressure manifestation, and support capacity can be described by the normal distribution law.
Karl Terzaghi was one of the first specialists who underlined the importance of risk management in the design of geotechnical structures (Terzaghi 1982). He considered risk management as a process of balancing economy and safety. During the last 50 years of quantitative analysis of probability failure for different constructions was developed and discussed in numeral workshops and international conferences (Whitman 1984, Baecher & Christian 2003, Chowdhury& Flentje 2008). However, application of modern probabilistic solutions in design practice is still limited because of rather poor current statistical estimates of the initial data and results. These solutions can not adequately describe the complicated multifactorial process of rock deformations around underground openings and interaction support with rock mass. Risk management for the design of underground construction shall take into account uncertainties associated with the stress and the strength of rock mass as well as uncertainties of construction and technology:
• Uncertainties of the initial geological, geomechanical parameters:
– strength and creep properties of the rock mass around opening– stress state in the rock mass– dip angle of the rock seams
• Uncertainties of the technological condition:
– dimension of the opening cross-section– rate of opening driving– opening direction in respect to strike direction– type of construction: drill-and-blast or TBM– distance and time support installation from the face– type of support: concrete, anchor or yielding support– contact condition: full contact or contact with rubblework behind contour– type of opening: separate, chamber, parallel or junction of the openings
• Uncertainties of the strength and deformation characteristics of the support
• Uncertainties of the design methods
The rocks encountered in civil and mining engineering applications are invariably jointed. The presence of joints in rock makes them anisotropic and weak in strength behaviour. One of the techniques for enhancing the rock mass strength is to provide rock bolts. Assessment of strength of reinforced jointed rock is a challenging task before the geotechnical engineers. The strength of reinforced jointed rock depends on characteristics of intact rock, joint and rock bolt. An experimental study has been conducted to assess the mechanism of strength enhancement in jointed rocks due to provision of rock bolts. NX size cylindrical specimens of natural rocks having natural joints at variable orientation have been tested under uniaxial loading condition. Tests were also conducted on the bolted specimens. The results have been used to come out with an expression, which correlates the strength and deformation of intact rock to that of jointed rock without, and with reinforcement.
Rock bolts are used as a primary support in many rock structures. Bolts increase the stiffness of the rock mass against external tensile or shear forces. In past, several studies have been conducted by researchers to evaluate the strength of jointed rock reinforced with rock bolts. The strength of reinforced rock has been reported to be dependent on various factors like strength of the parent rock (Egger & Zabuski 1991, Pellet & Egger 1996, and Sakurai 2010), joint orientation, angle of inclination between joint and bolt, and diameter of bolt (Bjurström 1974, Ludvig 1983, Pellet & Egger 1996, Grasselli et al. 1999, and Grasselli 2005). Effect of failure mode on engineering response of jointed rock has been emphasized by Singh (1997) and Singh et al. (2002) which is also an important factor in the case of jointed rocks. In present work, the effect of joint orientation on the strength and deformation behaviour of unreinforced and reinforced natural joint rock has been investigated.
Mine development imposes stress changes in the surrounding rock that typically induce or trigger seismicity with a wide range of magnitudes. Seismic monitoring can provide insight into the rock deformation and give critical feedback to the on-going operations. We have developed a passive seismic imaging algorithm based on earthquake seismology to jointly locate induced microseismic events and update the velocity of the rock model illuminated by the seismicity. We calculate travel-time based on the fast sweeping method to account for complex 3D distribution of velocity and use the adjoint method to transform the inverse problem to a forward problem which can also be solved by the fast sweeping method. In this paper, we demonstrate the efficiency and robustness of the fast sweeping and adjoint method based travel-time tomography in monitoring fracture evolution and dynamic velocity variations.
During the development of a mine site, monitoring of induced seismicity is generally used to provide insight into the rock fracturing and deformation processes and to give critical feedback to the on-going operations. Passive microseismic (MS) monitoring applications are often restricted to imaging induced fractures using source locations assuming a velocity model and ignoring the time-lapse variation of velocity with complex structures. We have adapted the seismic tomography technique from earthquake seismology (e.g. Thurber, 1983) and developed a passive seismic inverse algorithm to jointly locate induced MS events and update rock velocity model traveled by the ray paths. Honoring the coupling between the event location and the velocity model, passive imaging can provide a more realistic image of the rock velocity structure, its time-lapse variation and more accurately locate events. Recent advancement includes the double-difference tomography using S-P times in addition to both absolute and relative P- and S-wave arrival times (Zhang et al., 2009), and velocity tomography only using the absolute arrival time of P- and S-wave to derive 3D vp, vp/vs as well as quality factor QP (Tselentis et al., 2011). However, both methods calculate the travel times using conventional ray tracing which cannot treat complex velocity shapes and arbitrary discontinuities commonly observed in mines. We propose to use the fast sweeping method (Zhao, 2004) to account for complex 3D distribution of velocity. Furthermore, poorly covered region included in the model introduces additional unstable issues to the conventional linearized travel-time inversion. Damped least square method can stabilize the inversion but scarifies the model resolution. The adjoint method-based tomography (Huang & Bellefleur, 2012) formulates an inverse problem as a forward problem and only updates the region with adequate coverage. In the following section, we first briefly introduce the background of the fast sweeping and the adjoint method. We then demonstrate the performance of the fast sweeping and adjoint method-based travel-time tomography in locating MS events and recovering velocity variations using synthetic data based on field well logs and a sample of the seismicity induced during the caving development of Lift 2 of Northparkes mine. We demonstrate that the fracture evolution and dynamic velocity variations at Northparkes mine is well detected by the tomographic velocity model.
In this study; to get different approach to periphery mapping technique the application of tunnels opened by NATM (New Austrian Tunneling Method) method is aimed. Surface settlements due to excavation are important to keep under control in every stage of tunnels are opening by NATM method. During the progress of the tunnel, weak zones must be identified and spreading of weak zones should be estimated. As a result of detection of weak zones and other geological structures, stability can be achieved by applying additional support systems (additional rock bolt, cement and/or chemical injection, temporary invert concrete with strut etc.) before geological structure cause of the surface settlement. All geological structures encountered along the route are saved on the periphery map. Periphery map allows for three dimensional observation and estimation. Because of NATM is a gradual excavation method, periphery map allows to estimate of the geological structure of other stages. Dimensions and orientations of discontinuities (faults, beddings etc.) encountered along the tunnel progress is saved on the periphery map. Existing faults in progress of the tunnel can be estimated with the help of periphery map. Also, bedding orientation is an important parameter in tunneling works can be estimated with the help of periphery map. According to estimated fault and bedding orientation, additional precautions (AGF umbrella arc, fore pilling, face bolt, additional steel support, gradual excavation etc.) are taken to the stability of the excavation surface. It is important to hold to existing water level to keep under control of surface settlement in NATM tunnels. During the progress of the tunnel, periphery map is prepared to showing to status of water on tunnel surface. While excavations continuing, geotechnical values such as RMR, Q, GSI, are calculated with geotechnical data which was collected according to observational standards published by ISRM (International Society for Rock Mechanics) in 1981, are processed on periphery maps. So the areas, additional support measures to be taken, will be selected. NATM is a method of gradual excavation. The NATM tunnels which is designed to initially opening of the pilot tunnel, geological and geotechnical properties of the tunnel on the main section are determined with the help of periphery map prepared with pilot tunnel data. Also, after the excavation of the top heading, estimating the other stage’s (such as Upper Medium Heading UMH, Lower Medium Heading LMH, bench and invert) geological and geotechnical properties, collectively showing the concerned data to be provided.
Jiránková, E. (Institute of Geodesy and Mine Surveying, VSB-Technical University, Ostrava) | Waclawik, P. (Institute of Geonics, Department of Geomechanics and Mining Research, Academy of Sciences of the Czech Republic)
If coal seams are mined by longwall mining, the original stress balance in the rock mass is changed. The stress increases around the mined-out area. As a result, increased stress causes compression around the excavation. The compression around the excavation is shown by exact measurable subsidence. The impact of mining can be theoretically determined by a limiting impact angle. The paper deals with the assessment of mining in the ninth and seventh blocks at Karvina Mine Lazy Plant and its influence on the surface. The process of failure of the mining seam’s rigid roof in both areas will be explained. On the basis of spatio-temporal relationships of real surface subsidence, mined longwalls, and geomechanical events, the character of overlying rock deformations can be evaluated. The character of rigid overlying strata will be evaluated by means of an inflexibility coefficient which has the advantage of including the main factors of influence: the thickness of the seam and the rigidity and compactness of the overlying strata. It is possible to specify the time of breakthrough by synchronous evaluation of seismic events at the mining seam’s overlying strata because surface measurements are usually done once every half-year.
In every case, measured subsidence at the surface occurs as a result of higher stress, which arises in the surroundings of mined areas (Jirankova 2010). Determination of the value of subsidence at the surface with regard to the extent and thickness of mined longwalls is important for distinguishing situations where the deformation of solid overlying strata occurs (Mučková et al. 2010). In many cases a strutting vault forms over the mined area and breakthrough of the whole thickness of the intact firm overburden does not occur. At the origin of the strutting vault this can lead to an enormous concentration load of formation and the occurrence of geomechanical events. But in cases where the intact overlying strata break, the breaking cannot extend further. Incurred overhangs of intact firm strata by its restraint to the no undermine overburden contribute to considerable additional load on the affected areas.
Dimensioning of the mined-out area in which get in concrete conditions to breakthrough unyielding hanging rock strata has a very important meaning for consideration of the changes in the state of stress in rock massif. It is possible to obtain the proportions of the mined-out area at the time of breakthrough by backward evaluation of mine surveys and seismic observations in the locality in which they exist (Jirankova et al. 2012). Backward evaluation also provides a survey of the break overburden earlier dig for seam which has substantial meaning for the correct interpretation of the actual evaluation break overburden at the same time dig for strata.
Acoustic anisotropy in unconsolidated sediments and sedimentary rocks is a function of both the intrinsic character of the material and variability in the externally applied stress field. This paper reports measured intrinsic, acoustic anisotropy in unconsolidated sands from fold-thrust belts, from sub-salt settings, and for sands in extensional basin settings. In the latter, vertical effective stress has been the principal stress throughout the sand’s burial history. In contrast in thrust belt settings the principal stress is non-vertical for some portion of the burial history. Similarly, in sub-salt settings horizontal stress gradients arise due to rapid changes in salt thickness. Polar and azimuthal acoustic properties were measured under isostatic stress conditions at in situ stress. Azimuthal anisotropy in thrust belt sands averaged 15% in both crestal and flank structural positions. Polar anisotropies tend to be substantially lower than azimuthal values, averaging half the azimuthal anisotropy. In sub-salt settings the degree of azimuthal anisotropy in sands ranged from 5–10%. Polar anisotropies are low, typically less than 1%. In extensional basin settings, measured azimuthal anisotropy ranged from 0–3%. In these samples, polar anisotropy is larger than the azimuthal value, and varies with the compaction state of the sands. In all cases the level of anisotropy measured in the core plugs was directly tied to textural changes observed in thin section, and is in good agreement with log measures of anisotropy. These observations suggest that a TI medium assumption may be inappropriate in fold/thrust belt and subsalt settings.
Predicting reservoir properties ahead of the bit in fold- thrust belts and in sub-salt settings must account for the action of elevated horizontal stresses that are commonly active in these settings. Elevated horizontal stresses result in enhanced compaction of reservoir sands and bounding mudrocks, although the magnitude of these effects is not well understood. One method for estimating the magnitude of the layer parallel compaction is the measurement of directional acoustic properties. This paper reports on the development of laboratory techniques for measuring acoustic anisotropy (polar and azimuthal) on single vertical plugs taken from full diameter core. The laboratory measurements were made under isostatic stress conditions. This implies that any directional differences in acoustic properties are intrinsic to the material and not related to stress state.
The paper deals with formulation and numerical implementation of anisotropic strength condition for micro layered rock. Periodically layered two–constituent microstructure is considered. The failure criterion of the constituents is assumed to be governed by Drucker-Prager strength condition. The macroscopic criterion is derived based on the micro-mechanics approach. It appears that a microstructure failure function can be satisfactory described by the conjunction of Jaeger critical plane condition and Pariseau anisotropic criterion. The formulation derived is then implemented into the commercial codes FLAC and FLAC3D. Numerical integration scheme, i.e. an elastic predictor and a plastic corrector of the failure function proposed are discussed in details, in the paper. Efficiency and numerical stability of the model proposed are verified against a series of numerical examples. The proposed model is also compared with “Ubiquitous Joints”: a classical Jaeger critical plane approach available in FLAC environment. The presented results show that the model in some cases provides more precise description of micro-layered rock then Jaeger criterion.
In many rocks one can recognize a characteristic pattern: two or more constituents appear in a form of thin, periodically repeating layers. These rocks are sometimes referred as “micro layered”. The microstructure especially often occurs in sedimentary rocks: sandstone, claystone and schist are the most typical examples. The main consequence of the presence of micro layers in the material is its strong anisotropy in both elastic and inelastic range.
The micro layered rocks are usually considered in civil engineering practice as a problematic case for foundations. From the other hand necessity of structures foundation in such condition continues to grow. New structures are usually designed based on numerical calculations performed with some commercial codes. Identification and implementation of adequate, numerically efficient model of micro layered rock to one of these codes is an important and complex task.
A number of researchers focused on formulating a macroscopic anisotropic strength criterion for micro layered rocks based, mainly, on a phenomenological approach. Brief review of some of these criteria is presented in a work of Duveau et al. (1997). Two of these criteria are especially worth noting, i.e. Pariseau criterion (Pariseau 1972) which is an extension for the case of anisotropy of Drucker-Prager isotropic criterion, and Jaeger criterion (Jaeger 1960) which can be interpreted as a basis of a so-called critical plane approach.
The above mentioned Jaeger criterion is implemented as yield function for one of the plastic models available in FLAC and FLAC 3D codes. The model referred as “Ubiquitous Joints” is one of the few most popular models of micro layered rock.
Based on the classic visco-elastic Burgers model, a new visco-elastoplastic creep model is proposed in this paper, introducing a viscoplastic unit accounting the irreversible creep strain. The viscoplastic unit consists of a dashpot element and the Mohr-Coulomb slider element, which are connected in parallel. By connecting the viscoplastic unit and the viscoelastic Burgers unit in series, the new model can compute the elastic and plastic creep deformation of rock.
The governing equation of the proposed model is implemented in the numerical finite difference code (FLAC2D) using its built-in FISH language for constitutive models, and then applied on a real tunnel accounting for the delayed deformation of rock mass around tunnel. A discussion is presented comparing the numerical results and the monitoring data of the time-dependent convergence developed in the tunnel.
Regarding the creep behavior of geomaterials, various kinds of constitutive equations have been developed, following different assumptions and principals. Elastic or reversible deformation is one of the assumptions which are considered in classic models. These visco-elastic models have been used by researchers through analytical formulations to obtain tunnel convergence (Kontogianni et al. 2006, Dai 2004, Sakurai 1978, Pan & Huang 1994, Fahimifar et al. 2010). Subsequently, elastoplastic time-independent components have been added to visco-elastic models by other researchers (Minkley et al. 2001, Swift et al. 2003, Guan et al. 2008, Barla et al. 2010). Furthermore, considering the irreversible creep behavior of rocks, many rheological models have been developed accounting for visco-plastic behavior of rocks typically using slider unit and dashpot unit connected in parallel named as the Bingham unit (Tomanovic 2006, Erichsen & Werfling 2003, Malan 1999, Sterpi & Gioda 2007).
For an acceptable prediction of time-dependent feature of rocks and rock masses, a proper description of elastic and plastic creep feature of rock materials should be taken into account. As a matter of fact, a simultaneous elastic and plastic creep response, which can be exhibited in different stress levels in various types of rocks, should be assumed in a creep model. Thus, some modifications to previous rheological models are necessary. Regarding to this fact, on the basis of the classic visco-elastic Burgers model, a new visco-elastoplastic rheological model is proposed in this paper. The proposed model consists of the Burgers model and of a viscoplastic unit, which are connected in series. Thus, the reversible time dependent strains of Kelvin unit explain the primary stage of creep curve, and the strains associated with the secondary stage of rocks modeled by both elastic and plastic deformations.