Pressure-Arching Characteristics of Fractured Strata Structure during Shallow Horizontal Coal Mining

It is an important problem for the alternated strong and weak roof weighting to threat the safety of working face during shallow coal mining and the total thickness breaking of the thin bedrock to cause the serious ground subsidence. To reveal the mechanism of the abnormal mining damage, the pressure-arching rule in overlying strata was studied. Based on the monitoring data of the typical shallow coal working face, the mechanical models of the symmetrical stress arch, the squeezed arch and the hinged structure of the fractured strata were established, and the difference of the load bearing capacity between the structures and the influencing factors was analysed by the deduced formula calculation. Then the evolution characteristics of the pressure-arch in the fractured strata were revealed by the numerical simulation analysis. The results show that the global pressure-arch of multilayer strata always exits in the surrounding rock and moves forward with continuous mining. The single pressure-arch and hinged structure are formed in each stratum under the global pressure-arch. The pressure-arch enables the fractured strata to carry load efficiently, and the instability of the pressure-arch can cause strong roof weighting and ground subsidence. These conclusions provide a theoretical reference for the stability control of the overlying strata structure under shallow coal mining

INVESTIGATION INTO MINE PILLAR DESIGN AND GLOBAL STABILITY USING THE GROUND REACTION CURVE CONCEPT

Pillars form an important support structure in any underground mine. A bulk of the overburden load is borne by the mine pillars. Thus, the strength of pillars has been a subject of detailed research over more than 6 decades. This work has led to the development of largely empirical pillar design formulations that have reduced the risk of pillar failures and mine collapse. Current research, however, has drawn attention to the fact that some of the assumptions used in the development of conventional pillar design methodologies are not always valid. Conventional pillar design methodology assumes that the pillars carry the dead weight of the overburden. This conventional method treats the pillars as passive structures. The limitation of this approach is that the self-supporting capacity of the overburden is not incorporated in pillar design. This suspension theory of pillar design treats the strata-pillar interaction problem as a classic case of static equilibrium, without detailing the interactions of the two structures. Globally, multiple pillar design methods have been developed, based on this suspension theory. Each of these methods approaches the calculation of pillar stability a little differently with respect to material properties, excavation geometries and stress conditions. Most of these design methods are derived empirically and lack a mechanics-based approach. Moreover, there is a lack of a unified pillar design methodology that can be used to design all types of mine pillars using a mechanics-based approach. The Ground Reaction Curve has been used as a means of correlating strata displacements to stress conditions. In addition, the Support Reaction Curve has been used in modeling the response of a support system under load, as a function of support properties and installation time with respect to opening development. In comparing the Ground Reaction Curves and Support Reaction Curves for different support systems, one can evaluate the effectiveness of installed support systems in maintaining the integrity of the excavated area(s). This approach has been widely used in designing secondary (artificial) support systems in both civil tunneling and the mining industry. Encouraged by the successful use of this single method in designing secondary support systems, this research revisits this concept for mine pillar design. This research investigates the utilization of the Ground Reaction Curve and Support Reaction Curve for the design of mine pillar support systems with respect to anticipated pillar loading and opening convergence. In addition, a conceptual three-tier solution to the pillar design problem, using a proper combination of numerical, analytical and data-driven methods is suggested, and a flowchart for the pillar design methodology is proposed. At the focus of this proposed method lies the Ground Reaction Curve (GRC) Concept. This research effort tries to verify the proposed pillar design flowchart using in-mine instrumentation and numerical modeling. For the purpose of this research, a deep longwall coalmine is instrumented to measure changes in pillar stress and associated roof convergence, due to mining activity. Subsequently, numerical models were developed in FLAC3D to model the geomechanical effects of underground longwall mining. The numerical modeling results are validated and calibrated using instrumentation data and a surface subsidence profile. The calibrated numerical models are further used to generate the Ground Reaction Curve for the overburden and Support Reaction Curve for the coal pillar. The comparison of both curves gives a detailed view of the overburden stability with respect to the mine pillar loading, in a more mechanics-based sense. The developed numerical approach can be used in future research and further development of this methodology for various mine types and different pillar support systems

Improved Understanding of Coal Pillar Behavior and Bump Potential through the Ground Response Curve

Continued depletion of easier coal reserves has necessitated development at deeper overburdens. At greater depth, operations often encounter more difficult ground conditions due to higher stresses and potential multiple seam interactions. Pillars which are left intact as the primary support mechanism experience an increase in loading. Mine design improvements are often incorporated to combat increased loads, principally by increasing pillar size. However, the potential for coal bumps, which are a rapid and violent failure of coal pillars, has increased due to these higher stresses and the use of larger width-to-height (W/H) ratio pillars. Many efforts have been made to predict coal bumps; however, coal is a naturally occurring, inhomogeneous, and discontinuous geologic material. As a result, the best means for understanding coal pillar bursts are not efforts to predict the events themselves, but to advance knowledge of the associated environmental factors including geologic influences, stresses, and mining method. These factors have a tremendous impact on the loading distribution and resulting behavior of coal pillars. Of particular importance is the post-failure behavior of coal pillars which influences the mechanisms and functionality of pillar failure. Unfortunately, understanding of the post-failure behavior of squat coal pillars and the recognition of functional pillar strain has been limited. The Ground Response Curve (GRC) has traditionally been used to evaluate the behavior of rock mass to the mining process by comparing the ground response/convergence curve to the support (e.g. pillars) response curve. The GRC has been employed in an effort to improve understanding of squat coal pillar behavior for numerous case studies with varying geologic and geometric conditions. The relationship between the GRC and individual pillar deformation has been examined using numerical modeling techniques. Using these widely accepted methods, a range of typical coal geologies and mining geometries was investigated, seeking to establish relationships between pillar performance, energy release, and the resulting mode of failure. The physical and dynamic properties of the rock and rock mass for coal and surrounding strata, geometric considerations, and pillar interface properties have been determined to be important indicators of squat coal pillar behavior and ultimately bump potential. As a result, new understanding of post-failure ground response has been developed and improvements have been made towards enhanced classification of mine-specific bump criteria, or bump “red zones”

Failure of a Large Circular Excavation

A circular excavation, 117 feet (36 m) in diameter by 90 feet (27 m) deep, was designed by an experienced engineering firm and construction was performed by an experienced contractor. Nevertheless, the excavation support system suffered a complete collapse long before the maximum depth was reached. The failure is described and its causes are discussed

Applications of Surface and Subsurface Subsidence Theories to Solve Ground Control Problems

The stability of the underground mine openings largely depends on the surrounding ground conditions, such as stress concentrations, geological conditions and support intensities etc. In particular, the ground control stability associated with large movements and deformations of the strata is much more complicated and could induce much more severe safety problems. A ground control failure could endanger the coal miner\u27s safety not only directly by roof, pillar, floor and/or rib failure, but also by ground cracks induced methane and water inundations indirectly. This study is aimed to develop comprehensive models to simulate the ground response to mining and solve the ground control problems associated with it.;During the last four decades, many research works have been conducted on the ground control study, and numerous models, including analytical, empirical, numerical and hybrid models, were developed to facilitate ground control and support design. If a model is to be used as a common mine design tool, the simplicity of the model itself and the consistency between actual in-mine and modeled ground response to mining are essential. For the study of the ground control stability associated with large movements and deformations, the key is to know the movements and deformations of the subsurface strata. The subsidence prediction models can determine the movements and deformations very accurately as proven by plenty of surface subsidence survey data. In this study, the subsidence prediction models are employed to analyze the stability of some subsidence related ground control problems based on the subsurface strata movements and deformations.;In this dissertation, an innovative approach, employing the influence function method while considering the hard rock layers, is applied in the development of an enhanced subsurface subsidence prediction model. This improved model is then applied in analyzing three specific subsidence related ground control problems. An analytical model, employing dynamic subsurface subsidence theory and considering the roof support interaction, is developed to analyze the stability of pre-driven longwall recovery room. The mechanism of the ground control stability problems as well as the potential safety problems associated with multi-seam mining interactions is discussed. Multi-seam mining subsidence prediction methods are re-examined based on the multi-seam mining interaction analysis. The redistribution of the stresses and strains in overburden is also able to affect the surface and subsurface water bodies in various degrees. Mathematical models are developed to link longwall induced overburden strata permeability change and subsurface deformations. A ground water flow model is used to assess the longwall mining impacts on surface and subsurface hydrological systems.;This study provides a greater understanding of the mechanism of the subsidence-related ground control problems. Innovative methods are developed to derive stress, strain and permeability change, and quantify the subsidence effects on mine structure stability and the hydrological system sustainability. The developed models are coded and incorporated into a software suite to provide an easy-to-use tool for the mine planning and designing of all subsidence related issues

Static and Dynamic Discrete Element Modelling of Slender Coal Pillars

Highwall mining is a mining method used in surface coal operations that involves driving a series of parallel entries into the exposed coal seam at the highwall face under an unsupported roof leaving behind a series of long, but very slender coal pillars. Highwall mining often occurs simultaneously with production blasting taking place in other areas of the mine. Although no failures of highwall pillars have been attributed to nearby blasting, numerical modelling presents an inexpensive means of investigating the possible effects of strong ground motion on the stability of these pillars. This thesis documents the development of a discrete element rock mass model and its application to the simulation of both static and fully dynamic highwall pillar simulations. The approach is geared toward parameter analysis and mechanism identification rather than exact prediction. Some conclusions are made regarding the potential effects of blast vibration on highwall coal pillars and general excavations in rock. The limitations of the modelling approach are discussed and suggestions for future research are proposed

Inverted Shell Foundation Performance In Soil

The use of shells in foundation structures over traditional forms has grown steadily since their inception in the early nineteen–fifties. Shell foundations outperform conventional flat footings and are reputable performers especially when heavy superstructural loads are to be transmitted to weak bearing soil. The geotechnical performance of shells in an elastic continuum concerns their bearing capacities and settlement behaviour, whose study has been trailing behind that of their structural performance. Bringing contact pressures closer to uniformity at the soil–shell structure interface is essential in developing a viable behavioural response under vertically concentric and monotonic loading conditions. This study encapsulates the development of new shell foundation geometries employing shell inversion under such loading conditions. Experimental investigation involves validation of the numerical phase in a comparative study following a two–dimensional analysis of shell models using commercially available geotechnical software with finite element analysis. New inverted triangular footings embedded in sand composed of ultra–high performance iShell Mix concrete using fiber–reinforced polymeric (FRP) microfibers are analyzed. A parametric analysis examines key sensitivity elements including shell angle and shell thickness in granular soil for both upright shells and their inverted counterpart. Linearly–elastic behaviour of concrete material is assumed while soil media is modeled under nonlinear elastic perfectly–plastic conditions following the Mohr–Coulomb yield criterion for loose, medium and dense sand states. Theoretical modeling was developed to generate inverted shell bearing capacity factors to predict ultimate bearing capacities of the shell footings. Simulation efforts scrutinized reveal comparable performance with bearing capacity increase of 3 – 5% for the inverted shells over upright shell models and notable improvements of 42 – 45% over conventional flat footings. The developed models investigated represent forefront configurations of superior performance signifying that shells in foundations be highly regarded and fully exploited whenever feasible

Pressure-Arching Characteristics in Roof Blocks during Shallow Coal Mining

To reveal the performance of the stepped subsidence and the strong roof weighting during shallow coal mining, taking the fully mechanized mining face with large mining height in the Shendong mining area in China as the engineering background, theoretical analysis and numerical simulation were used to analyze the pressure-arching effect of the hanging roof blocks. Three typical pressure-arch models of the roof structure were proposed, such as the symmetrical pressure-arch of two key blocks, the step pressure-arch of multiple key blocks, and the rotative pressure-arch of multiple key blocks. Results indicate that the horizontal stress displays a nonlinear distribution at the abutments of the symmetrical pressure-arch, and there is a linear distribution of horizontal stress with a higher peak value at the midspan of the pressure-arch. The high horizontal stress at the arch abutment is necessary to form the rotative pressure-arch of multiple key blocks. The horizontal stress is relatively less at the arch abutment of the step pressure-arch structure. The main key block is easier to slide in this structure as the boundary horizontal stresses display the nonlinear distribution. The results are of instructive significance for roof weighting forecast and strata control during shallow horizontal mining for a thick coal seam