Rockburst means stress induced violent ejection of rockmass in tunneling and mining. Rockmass comes off under high energy release. To determine this energy, different approaches exist, depending on different countries and regions. Rockburst phenomena mostly occur in deep tunnels and mines but also in regions, where high stress concentrations appear. A major engineering challenge for mines experiencing significant seismicity is the performance of the support systems. The development of an adequate retaining system, such as dynamic rock bolts together with high-tensile steel wire mesh and their behavior during rockburst had been tested and quantified. The special large scale test facility was constructed for that purpose. It allowed the estimation of portions of energy transmitted by rock bolts and wire mesh.
As more and more near surface deposits are mined out, deeper mines are required in order to continue the exploitation of resources. This inevitably leads to the occurrence of more rock bursting, deformation problems and safety issues. Rock burst means stress induced loosening of parts of the rock mass under enormous energy release. It seems to be similar to damage from natural seismic phenomena. Rock burst events occur mostly in deep tunnels and mines but also in regions with high horizontal stresses.
There are different methods to mitigate rock burst risks thus reducing exposure of personnel. Changes include mine design, layout and extraction sequence (Potvin 2012). In addition the ground support needs to be chosen and designed in such a way that it can cope with the conditions. In deep mines the ground support is complex and cost is high. Static ground support is not satisfactory for such a demanding environment. The ground support consists of the rock reinforcement (e.g. bolt), a surface support (e.g. mesh) and the connection between the two (e.g. plate). For dynamic loads it is essential that these components fit and work together as a system (Cala & Roth 2007). The observations of Heal (2007) show that most of the damage done from rock bursting led to the failure of the surface support or the rupture of the reinforcement.
In order to investigate the dynamic behavior of reinforcement elements and surface support, extensive testing was carried out at various test sites. Some tests were done in South Africa, Canada and Australia. Hadjigeorgiou (2011) put the results of the various tests together and tried to make them comparable. However the boundary conditions of the test sites were different making comparison difficult. There are two active test sites at the moment, one is in Canada at CANMET and the other in Australia at WASM (Player et al. 2004). The WASM test site is the best instrumented facility and is based on the momentum transfer concept. It can test reinforcement, surface support and to a certain extent combinations of the two.
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.
This article describes consolidated drained triaxial tests to determine the acoustic and mechanical properties of heavy oil sandstone from shallow, high-temperature boreholes in the Eastern Venezuela basin. The purpose of this laboratory campaign is to create physical correlations between rock under in-situ conditions and the severe temperatures generated during steam injection. These tests will be used for modeling the impact of Steam-Assisted Gravity Drainage (SAGD), in unconsolidated sandstones.
For most oil and gas reservoirs, temperature effects on rocks during the life of the well are incorrectly thought to have aminimal effect, compared to the effects of hydrostatic or injected fluid pressures such as from completion fluids or drilling muds. However, this is clearly not the case where steam is injected for heavy oil reservoir stimulation. In these situations there is a tremendous elevation in the bottom hole temperature, resulting in changes in the rock structure and its elastic properties and rock strength.
Thermal properties of rocks are very complex.Their thermal parameters depend on mineralogy, density, and grain size distribution in addition toporosity and permeability and static and dynamic in-situ temperature effects that include the reservoir geothermal base temperature plus induced by the enhanced oil recovery (EOR) process. Steam-assisted gravity drainage (SAGD) processes generally disturb the stress field, stress direction and rock properties. High thermal conductivity results in a wide range of temperature distribution, improving the porosity, permeability and stiffness of the rock (Rabe et al. 2008).
Among the 9–13 trillion barrels of oil that presently constitute the world oil reserves, 30% correspond to conventional oil (greater than 22.3? API), 15% to heavy oil (between 22.3 and 10? API), and 55% correspond to extra-heavy oil, bituminous sands and bitumen (Alboudwarej et al. 2006). Many thermal processes have been developed to increase the recovery factor of heavy oil and extra-heavy oil reservoirs around the world. Chalaturnyk & Li (2004) concluded that steam stimulation is one of the most successful thermal processes that can affect the rock formations. Indeed, steam stimulation can reduce the effective stresses by varying the pore pressure, inducing oil sands to shear, generating compaction or surface dilation, reducing the strength of the formation around the steam chamber and occasionally causing in rock failure.To properly evaluate the impact of geomechanics on the SAGD process in the Faja del Orinoco study area, itwas necessary to understand the rock properties and porepressure behavior, the stress state as well as the fracture gradient during steam injection to predict wellbore instability, design the casing shoe, correctly locate the well pairs and predict accurately the recovery factor as well as the volume of heavy oil that could be extracted in the studied field (Rabe et al. 2008, Perdomo et al. 2010). The purpose of this paper is to provide a fundamental geomechanical data set of the studied area to optimize and mitigate risks in SAGD projects elsewhere.
Verma, A. K. (Department of Mining Engineering, Indian School of Mines-Dhanbad) | Singh, T. N. (Department of Earth Sciences, Indian Institute of Technology-Mumbai) | Bajpai, R. K. (Repository Programme Section, Bhabha Atomic Research Centre)
Due to increasing gap between energy demand and supply, India is planning to build significant number of nuclear reactors which will produce significant amount of High-level radioactive waste. It can generate considerable amounts of heat as a side effect of radioactive decay of nuclear wastes which will be disposed around 400 m–1000m below ground inside the underground nuclear repository. The rock chosen is granite and barrier near the canister is proposed to be clay. In this paper, thermo hydro mechanical (THM) analysis has been done to study the effect of heat on deformations, stresses and pore pressure variation in granite and clay barrier. For this purpose, finite difference method has been used. It has been found that both temperature and stresses at any point in the rock mass is below the design criteria which are 100?C for temperature.
Nuclear energy has been a source of unlimited cheap power and is based on non-renewable energy sources. Nuclear power produces high level and low level radioactive wastes with long half life time periods. The major problem is due to the generation of long lived high level nuclear wastes (HLW) and it’s safe disposal. Although the level of radioactivity of these wastes decay over time, it remains dangerously high that it must be isolated from the outer biosphere, until it has decayed to levels that pose less risk.
It is necessary to study the several processes at excavation stage in an underground repository laboratory (URL) and supplemented by surface based and traditional laboratory tests. A typical cross section of one such disposal tunnel with a disposal pit is shown in Fig. 1.
2 Regional Geology and Structure
Site near Bhima basin is found to be suitable for URL site (Fig. 2). Bhima basin is the smallest and youngest amongst the Proterozoic basins of Peninsular India. The basin receives it name after “Bhima River” a major tributary to the river Krishna. The Basin is exposed between lat. 16°:20'00"N–17°:35'00"N: long. 76°:15'00"E–77°:44'00"_E on the north western fringe of eastern of Dharwar Craton. The basin has a reverse sigmoidal array of outcrops between Tandur in the northeast and Muddebihal in the Southwest, for over a stretch of 160 km with a maximum width of 40 km across Sedam. The exposed area of the basin is about 5200 sq. km. Northern and northwestern extensions are concealed under Deccan traps.
The structure comprises of a major thrust plane striking NE-SW with granitic rocks riding over limestone.
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.
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.
Rocks are generally more or less anisotropic, depending on their structure at the scale of interest. The anisotropic rocks in civil and mining engineering projects cause non-symmetric deformation and their behaviour is unpredictable. The authors present the results of Brazilian Indirect Tensile (BIT) strength and Cracked Chevron Notched Brazilian Disc (CCNBD) tests for the determination of the fundamental tensile fracturing parameters of Brisbane phyllite specimens. In general, the influence of orientation angle (ψ) and foliation-loading angle (β) were found to influence both fracturing and the failure load.
Anisotropy is a characteristic of foliated metamorphic rock masses (gneisses, phyllites and schists), and intact stratified or bedded sedimentary rocks (coal, shales, sandstones, siltstones, limestones, etc.). At a larger scale, rock mass anisotropy is found in volcanic formations and in sedimentary formations consisting of alternating layers or beds of different rock types and in rock formations cut by one or several regularly-spaced joint sets. Since the deformation and fracture of these rocks is of importance to engineers concerned with the design of mining excavations or of foundations for civil engineering structures, it is obvious that research into the effects of anisotropy on rock behavior is necessary (Hoek, 1964; Chen & Hsu, 2001; Cho et al., 2012).
As in the case of the original Griffith criterion (Griffith, 1924), it is generally assumed that the specimen contains a sufficient number of randomly-oriented cracks for fracture to initiate from those cracks, which are inclined at an angle. If, however, the cracks are oriented preferentially, as in the case of a highly antistrophic material, it is necessary to consider the inclination of the cracks with respect to the applied stress system (Hoek, 1964). Brace (1961) presented evidence indicating that the cracks, from which fracture of the rock propagates, probably lie within the grain boundaries of the rock. Even in rocks of sedimentary origin, which exhibit marked foliation and planar anisotropy, the constituent bedding planes are made up of grains that are cemented together and hence randomly-oriented grain boundary cracks are likely to be present (Brace, 1960).
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
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.