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ABSTRACT: When an immediate roof comprises competent ('massive') strata, caving is often delayed in total pillar extraction situations. The caving angle at the goaf edge, primarily depending on the roof type and depth of cover, is about 25 degree (or low) in a manner that a relative thin thickness (sometimes as low as 'razor sharp' edge) of rock suddenly may over-ride the breaker-lines. This phenomenon, known as the featheredging goaf-fall, is common in Australia (mostly in Westem and Northern Coalfields), South Africa and USA. These goaf-falls are major source of injury & fatality accidents and are reasons for burial incidents of continuous miners. To alleviate such feather-edging, the alternatives may be by use of Mobile breaker lines, longwall chocks and fully grouted bolted breaker lines at goaf-edges. This paper finds the last alternative as viable and practicable. The results and analysis of data fi:om "instrumented" bolts validate this proposition. INTRODUCTION: It is an operational requisite for "total" pillar extraction methods, that caving of the roof rocks must occur on the goaf side of a fender/stook. In presence of massive strata in immediate roof, the roof strata "bridge" across large spans without failing, and the 'main' caving (described in this paper) is delayed invariably. The "massive" (competent) strata such as sandstones are capable of storing consequential high energy build-ups leading to increase of abutment stresses at goaf-edges. When a goaf fall finally happens, sudden (without warning) and often dynamic failures may take place leading to unpredictability and loss of roof control, particularly at the goaf edges. The roofs at the goaf edges may fall as a thin wafer of rock, over-riding the common breaker-lines (timber). It may mn down and may affect adversely a working bord at distances of 15 meters or more (two pillar lengths in one reported case in South Africa) (UNSW 1993a,b). This phenomenon, known as feather-edgingoaf-fall, is a cause of concern and source of injury and fatal accidents in Australia, South Africa and USA (Galvin et al. 1991), wherever such conditions exist. Obviously, alleviation of feather-edging Goaf falls should be resorted to the extent possible. This paper takes a brief resume on this aspect after elaborating the associated caving mechanism. The mechanistic understanding of the caving presented here is analogous to the observations made at 6 th North panel, Blue Mountain Colliery, NSW, Australia. This colliery has been selected for such study because more than 90% of the goaf-falls in the panel had characteristic of feather-edging. Out of many alternatives, the bolted breakerlines are found to be viable and practicable to alleviate feather-edging. The results and analysis of "instrumented" bolts at the sites validate this proposition in the whole gamut of observed caving mechanics in such situations. The study presented in this paper is the first of its kind in an Australian pillar ('total') extraction mine. The bolted-breaker line has been in use since long in such situations (McCosh et al. 1989), nevertheless its adaptability as a pro-active and alleviating support member at a goaf-edge in an Australian pillar-extraction mine always warrants a validation. This paper aims to provide such validation which, moreover, enhances the confidence and reliability of its use to mitigate the feather-edging.
- Africa (1.00)
- North America > United States (0.70)
- Oceania > Australia > New South Wales (0.29)
ABSTRACT: There is much evidence that the mechanics of discontinuous media such as jointed rocks is neither tractable by conventional continuum methods, nor is readily amenable to intuitive analysis. Highlights from four case histories will illustrate the peculiar behavior of rock masses in both quasistatic and dynamic conditions. A brief summary is presented here. CRESTMORE MINE The first story (1966-1969) concerns the behavior of the roof at the Crestmore room-and-pillar marble mine (Heuze and Goodman, 1967; Heuze, 1983). Large chambers had been excavated, with a length of 60m, height of 7.5m, and width of 9m. Two of them were widened in successive steps to 12, 15, 18, and 21m, and the height finally extended to 21 m. Among the many measurements made during the mine-by of the first room, were those of tangential stresses at mid-span in the roof, measured by flat-jacks both transversely and longitudinally to the main axis of the chamber. The initial tangential stresses were compressive. Based on continuum mechanics, these tangential stresses should become less and less compressive as the span increases. On the contrary, the transverse stress became more compressive as the span went to 12 and 15m, stabilized at a span of 18m, and decreased very slightly at a span of 21m. The longitudinal stress increased all the way to a span of 18m and stabilized between 18 and 21m. Both stresses showed a significant reduction in compression when the bench was taken and the room height went from 7.5 to 21m. In 1985, a model of a beam roof with vertical joints was applied by Pender to the Crestmore data, upon the suggestion of the author (Pender, 1985). The analysis clearly showed how dilatant joint behavior could create the pattern of tangential roof stresses which had bee recorded years earlier. The suggestion was made on the basis of the work of Obert et al, who in 1976 had documented the large increase in the normal stress on joints during shear tests under controlled transverse stiffness, as opposed to shear tests under controlled normal stress (Obert et al, 1976).
ABSTRACT: The purpose of the work presented in this paper is to develop an integrated strategy for incorporating the rock mechanics and rock engineering aspects into site selection, repository design; construction, waste emplacement and closure for a radioactive waste disposal programme. The basic concept for deep geologic disposal and the role of bedrock in the design process are described in brief. In the overall disposal context, it is necessary to have a holistic approach and consider the complete system model of Geology (G), Thermal processes (T), Hydrogeology (H), Mechanical processes (M), Chemical processes (C) and Engineering activities (E). The strategy of incorporating rock mechanics data in five different stages of a waste disposal programme are presented. From a list of activities, existing gaps in the information and knowledge can be identified and research needs can be prioritised. INTRODUCTION Geologic disposal of radioactive waste and spent nuclear fuel has been the focus of scientific research for more than 30-40 years. At present, several countries have deposited lowand intermediate radioactive waste in geological formations. In Sweden spent nuclear fuel from the nuclear power stations is stored in an interim underground storage in hard rocks, an encapsulation facility is designed, an underground research station is in operation, and prospecting of a suitable site for disposal continues. A similar situation exists in Finland. Unlike the hazards of toxic materials such as mercury, lead, and arsenic, radioactive materials decay over time. Early efforts to study disposal options led to geologic environments that have remained stable for millions of years and are likely to remain so. Now, there is a world-wide scientific consensus that radioactive waste packed in robust, long-lived canisters and placed deep in stable geological formations could be isolated from the biosphere for the very long time periods necessary. However, some scientists argue that waste should be stored on surface for several generations to learn more about geologic disposal and to take advantage of new and better methods and technologies that may come in the future. One way to keep these options is to dispose wastes in a manner that allows their retrieval and/or final disposal.
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Geological Subdiscipline > Environmental Geology > Hydrogeology (0.34)
- Water & Waste Management > Solid Waste Management (1.00)
- Energy > Power Industry > Utilities > Nuclear (1.00)
ABSTRACT: This paper presents a bifurcation analysis of a thick-wall cylinder which is loaded with increasing radial internal or external pressure. Material behaviour is modelled by large strain elastoplastic deformation theory of the Drucker-Prager solid. Stability analysis is carried out along the primary equilibrium path to examine possible emergence of bifurcations that may lead to different failure modes. The model is used to investigate the effect of bifurcation modes on the initiation of a tensile fracture and to interpret the experimental results obtained from fracturing tests on thick-wall cylinders INTRODUCTION Knowledge of the insitu stresses is required in many phases of drilling and completion in petroleum engineering. One of the most reliable techniques for measuring the insitu stresses is micro-hydraulic fracturing. This technique consists of pressurizing an isolated zone of the borehole with pumped fluid in order to initiate and propagate a tensile fracture. The minimum stress, an is estimated from back-analysis of the pressure response. The estimation of the maximum horizontal stress in vertical wellbores, óH is sometimes calculated from the Hubbert and Willis (1957) equation [Equation available in full paper] where Pb is the breakdown pressure and T is the tensile strength. Equation (1) is based on the assumption that the material around the wellbore remains elastic and a tensile crack is initiated when the hoop stress exists the tensile strength of the rock. Fracturing experiments were performed in thickwall cylinders of Jurassic shale in order to investigate the effect of plastic yielding and the range of validity of breakdown equation (1). The first attempt to interpret the experimental results was based on a Mohr-Coulomb elastoplastic analysis of an internally pressurized cylinder (Papanastasiou et al. 1995). We found that when initiation of a tensile fracture occurs in the elastic regime, the breakdown pressure can be estimated from equation (1). When tensile failure occurs after plastic yielding, classical elastoplasticity predicts a breakdown pressure greater than the one predicted by equation (1). In addition, for high confining pressures, a tensile state of stress cannot be achieved directly during pressurization of the wellbore according to classical elastoplasticity. In contrast to the classical theory, significant lower values of breakdown pressure were observed in the experiments. Due to the inadequacy of the classical elastoplastic procedure to describe the low initiation pressures, we revisited the subject examining in this study possible emergence of bifurcations along the primary equilibrium path which may lead to different failure modes, like shear banding (Papanastasiou and Vardoulakis 1992). Lower initiation pressure can occur if a shear failure mechanism develops and the fluid penetrates the shear fracture which will further propagate in a tensile mode. The paper is organized as follows: Firstly we present in section 2 the elastoplastic solution for the primary path of a pressurized thick-wall cylinder. Section 3 deals with the formulation of the bifurcation problem. Computational results are presented and compared with the experimental data in section 4. Finally, we outline the main conclusions in the last section.
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock (0.36)
- Asia > Middle East > Kuwait > Jahra Governorate > Arabian Basin > Widyan Basin > North Kuwait Jurassic (NKJ) Fields > Marrat Formation > Upper Marrat Formation (0.98)
- Asia > Middle East > Kuwait > Jahra Governorate > Arabian Basin > Widyan Basin > North Kuwait Jurassic (NKJ) Fields > Marrat Formation > Sargelu Formation (0.98)
- Well Drilling > Wellbore Design > Wellbore integrity (1.00)
- Well Completion > Hydraulic Fracturing (1.00)
INTRODUCTION ABSTRACT: The motivation for this study has been to establish a methodology for estimation of stiffness and strength of weak sandstones based on sonic,measurements. A key element in this work has been to establish quantitative relations between static and dynamic moduli. The work was based on analyses of laboratory tests where both static moduli and sound velocities were measured simultaneously. The results of these analyses showed significant differences between the static and dynamic moduli even for dry rocks, under both hydrostatic and uniaxial loading. The differences were found to be strongly dependent on stress and strain. The relations between the static and dynamic moduli have been described by a set of mathematical formulas. These formulas allow for quantitative descriptions of the relations between static and dynamic moduli in weak sandstones, and give a foundation for predictions of how such materials will respond to changes in stress and strain. The elastic moduli of a material describe its ability to resist deformations due to external stresses. It is customary to differ between "static" moduli, which are the responses to static or slowly varying stresses with relatively large amplitude, and "dynamic" moduli, which are the responses to low amplitude oscillations - normally acoustic waves. It is well known that there may be large differences between the static and dynamic moduli of porous rocks, in particular at low stress levels (Simmons & Brace 1965, Walsh 1965, King 1970, Cheng & Johnston 1981, Fja?r et al. 1989, Jizba & Nur 1990). Figure 1 shows a typical stress-strain curve for a weak sandstone, under triaxial stress con- ditions. The figure also shows the static (tangential) Young's modulus - which is the slope of the stress- strain curve - and the dynamic modulus, which has been determined from acoustic measurements. A test with high confining pressure has been chosen in order to illustrate the effects clearly. The focus is here on initial loading as measured in such a test, and not on loading cycles, which yield higher static moduli (Plona & Cook 1995). It is seen in Figure 1 that the relation between the static and the dynamic modulus is not a constant shift or a constant ratio - the two moduli rather appear to change quite independently with increasing stress. At the peak stress point the static modulus vanishes, while the dynamic modulus is still nearly as large as in the steepest part of the stress-strain curve. The contribution of the pore fluid is often an important part of the difference between static and dynamic moduli even in partially saturated rocks, and it has been argued that the difference between static and dynamic properties can be corrected using Biot's theory (Montmayeur & Graves 1985). However, even in carefully dried rocks there is a significant difference between the static and dynamic moduli (King 1970). It has been argued that the presence of cracks in natural rocks is the origin of the difference between static and dynamic moduli, since the difference between the bulk moduli appears to diminish at high confining pressures (Walsh & Brace 1966, Jizba & Nur 1990). It has also been shown to be a correlation between the static/dynamic ratio and the non-linear elastic behaviour of rocks (Yale et al. 1995), and is has been suggested that this may be explained by grain contact adhesion and stick-slip sliding mechanisms (Tutuncu et al. 1995). It is now mostly believed that the difference in strain amplitude is the major cause for the difference between static and dynamic moduli in dry rocks (Martin & Haupt 1994, Tutuncu et al. 1994). The motivation for this study has been to establish a methodology for estimation of stiffness
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)