Roadway roof support design in critical areas at Anglo American Metallurgical Coal\u27s underground operations

In order to ensure the stability of roadways Anglo American Metallurgical Coal (AAMC) has developed an advanced roof support design methodology that integrates analytical, numerical and empirical modelling. This methodology currently is based on a deterministic approach (a single factor of safety against failure is calculated). However, an improved methodology, based on stochastic modelling technique, has also been developed and currently being evaluated. The main advantage of this methodology is that as the design is based on probability distributions of input parameters, the outcome is based on a distribution of factors of safety rather than a single factor of safety. Evaluation of factors of safety may also be used to determine the likelihood of failure which in turn may be utilised to determine and evaluate the associated risks quantitatively in decision making process. This methodology has been evaluated at Grasstree and Moranbah North Coal Mines in the designs of roof support in various critical areas and has been proven to be successful and a better way of determining the roof support requirements. A demonstration of application of this methodology from Moranbah North Mine, where the “world’s highest rated longwall” has recently been installed, is presented in this paper

Investigations into the effect of size and width to height ratio on the strength of the laboratory sized coal specimens

A dissertation submitted to the Faculty of Engineering, University of the Witwatersrand, Johannesburg, in fulfilment of the requir tents for the degree of Master of Science in Engineering. Johannesburg 1996.The design of bord and pillar working in South African collieries is based on the pillar strength formula developed by Salamon and Munro1967 and which has been used widely since then for designing pillars. This formula is based on the statistical analysis of 27 collapsed and 98 intact coal pillar cases from collieries located in the Transvaal and the Free state. The main objective of this study is to establish the difference in the strength of the coal material in ditferent seams by means of laboratory testing. In this manner, some 753 coal samples from 10 collieries from 4 seams were tested. The size and width to height ratio effects on strength were analysed. The size effect showed that the difference between the seams was obvious, with a difference of 59,4 per cent between the strongest and weakest coal. The statistical re-analysis showed that the strength of the six blocks from the No 2 seam, Witbank Coalfield occurred in a fairly tight strength range; and that laboratory coal strengths from individual seams or mines could deviate to a significant although relatively small extent from the overall average.AC201

Evaluation and design of optimum support systems in South African collieries using the probabilistic design approach

This thesis addresses the problem of designing roof support systems in coal mines. When designing the roof support, it is necessary to account for the uncertainties that are inherently exist within the rock mass and support elements. The performance of a support system is affected by these uncertainties, which are not taken into account in the current design methodologies used in South Africa. This study sets out to develop a method which takes all uncertainties into account and quantitatively provides a risk-based design. Despite the fact that the roof bolting is probably one of the most researched aspects of coal mine ground control, falls of ground still remain the single major cause of fatalities and injuries in South African collieries. Mainly five different support design methodologies have been used; namely, analytical modelling, numerical modelling, physical modelling, design based on geotechnical rating systems and field testing. As part of this study, it is shown that there are many elements of a support system that can impact the support and roof behaviour in a coal mine and the characteristics of these elements as well as the interaction between them is complex and can vary significantly within a short distance. These variations account for uncertainties in coal mine roof support and they are usually not taken into account in the above design methodologies resulting in falls of ground and/or over design of support systems. The roof and support behaviour were monitored at 29 sites at five collieries. It is found that there was no evidence of a dramatic increase in the stable elevations as experienced in some overseas collieries. A roadway widening experiment was carried out to establish the critical roof displacements. The maximum width attained was 12 m at which stage 5 mm displacement was measured. During the monitoring period no roof falls occurred at any of the 29 sites and road widening experiment site, even where 12 mm displacements were measured. The in situ monitoring programme was continued in additional 26 monitoring stations in 13 sites with the aim of establishing the effect of unsupported cut-out distance on roof and support performances. The results showed that the lithological composition of the roof strata plays a major role in the amount of deflection that was recorded. Bedding separation was seen to occur at the contacts between different strata types. It is concluded that the roof behaved like a set of composite beams with different characteristics. It is also found that the amounts of deflection corresponded with the deflection that would be expected from gravity loaded beams. During this monitoring programme variable nature of roof and support systems are also demonstrated. As many mines use different geotechnical rating systems, an evaluation of the currently used classification techniques were conducted to determine their effectiveness in design of roof support strategies. It is found that currently used systems cannot quantitatively determine the required support system in a given geotechnical environment. Impact splitting tests are found to be the appropriate system for South African conditions. It is however concluded that the roof lithology, stress regime and roof characteristics can change within meters in a production section. Therefore, in order to predict these changing conditions many boreholes are required for a section, which would be costly and time consuming. An in-depth study into the roof support elements was conducted for the purpose of obtaining an understanding of the fundamental mechanisms of roof support systems and developing guidelines for their improvement. All of the currently available roof bolt support elements and related machinery were evaluated using in situ short encapsulated pull tests. The results showed that, on average, bond strengths obtained from the roof bolts supplied by different manufacturers can vary as much as 28 per cent. The test results conducted on different resins showed that the strength of resin currently being used in South Africa is adequate. Differences between commonly used bit types were established. It is concluded that the 2-prong bit outperforms the spade bit in sandstone and shale rock types. In addition, the effect of hole annulus was also investigated as part of this study. The results show that an annulus between 2.5 mm to 3.8 mm resulted in the most effective bond strengths. The effect of wet and dry drilling was noted. It is found that bond strengths and overall support stiffnesses are greater with the use of the wet drilling in all resin types. The results from the tests in different rock types highlighted the very distinct differences between bolt system performances. Quality control procedures for compliance with the design, support elements and quality of installation are presented. Recommendations for improving the quality control measures and for developing testing procedures for bolt system components, installation quality and resin performance are provided. Finally, a roof support design methodology that takes into account all natural variations exist within the rock mass and the mining process has been developed and presented. This was achieved by adapting a probabilistic design approach using the well established stochastic modelling technique. This methodology enables rock engineers to design roof support systems with greater confidence and should result in safer and economic extraction of coal reserves.Thesis (PhD)--University of Pretoria, 2008.Mining Engineeringunrestricte

A strain-based failure criterion for pillar stability analysis

Strain-based failure criteria have several advantages over stress-based failure criteria: they can account for elastic and inelastic strains, they utilise direct, observables effects instead of inferred effects (strain gauges vs. stress estimates), and model complete stress-strain curves including pre-peak, non-linear elasticity and post-peak strain weakening. In this study, a strain-based failure criterion derived from thermodynamic first principles utilising the concepts of continuum damage mechanics is presented. Furthermore, implementation of this failure criterion into a finite-element simulation is demonstrated and applied to the stability of underground mining coal pillars. In numerical studies, pillar strength is usually expressed in terms of critical stresses or stress-based failure criteria where scaling with pillar width and height is common. Previous publications have employed the finite-element method for pillar stability analysis using stress-based failure criterion such as Mohr-Coulomb and Hoek-Brown or stress-based scalar damage models. A novel constitutive material model, which takes into consideration anisotropy as well as elastic strain and damage as state variables has been developed and is presented in this paper. The damage threshold and its evolution are strain-controlled, and coupling of the state variables is achieved through the damage-induced degradation of the elasticity tensor. This material model is implemented into the finite-element software ABAQUS and can be applied to 3D problems. Initial results show that this new material model is capable of describing the non-linear behaviour of geomaterials commonly observed before peak strength is reached as well as post-peak strain softening. Furthermore, it is demonstrated that the model can account for directional dependency of failure behaviour (i.e. anisotropy) and has the potential to be expanded to environmental controls like temperature or moisture

Numerical analysis of strain energy density at development and longwall face

Strain energy stored within coal mass is one of the main energy sources of coal bursts. Damage caused by a coal burst event can be attributed to the magnitude of strain energy accumulated around excavations. In this study, strain energy density (SED) within coal seam is examined around excavation boundaries during development and longwall retreat. Several numerical models are generated to investigate SED distributions for mining depths ranging between 100 m and 1000 m. For both development and longwall retreat, the maximum SED area migrated deeper into excavation boundaries with increasing mining depth. When the mining depth increased from 100 m to 1000 m, the maximum SED around development increased from approximately 6 kJ/m3 to 780 kJ/m3, while the maximum SED at longwall face increased from approximately 102 kJ/m3 to 1710 kJ/m3. The maximum SED around roadway ribs was lower than that at longwall face at the same mining depth. The sensitivity analyses presented in this study can provide guidance to geotechnical engineers to better understand and evaluate associated risks for different mining conditions

Design combined support under arbitrary impulsive loading

Rock bolts and cable bolts are usually considered to experience static loads under relatively low-stress conditions. However, in burst-prone conditions, support elements are subjected to dynamic loading. Therefore, it is important to understand cable bolt behaviour under dynamic loading conditions, particularly their energy absorption capacity. Rock bolts and cable bolts as well as steel mesh are widely used as permanent support elements in tunnelling, underground excavations and surface slope stability. This paper aims to determine the amount of the dissipated energy which can be taken into account to design combined yielding supports when subjected to dynamic loading. A ground support approach is suggested for underground excavations undertaking a range of mining-induced coal burst. A bench mark based on the largest expected impact loading is considered to conclude the level of coal burst risk and select an appropriate approach, whether quasi-static or dynamic, for the mine support

Numerically and Analytically Forecasting the Coal Burst Using Energy Based Approach Methods

Coal burst is referred to as the violent failure of overstressed coal, which has been recognised as one of the most critical dynamic failures in coal mines. This chapter aims to analytically and numerically evaluate the energy transformation between the different strata and coal layers. An accurate closed-form solution is developed. Due to the complexity of the causes and mechanisms contributing to the coal burst occurrence, 3D finite element modelling as well as discrete element models will be developed to validate the suggested analytical assessments of rock/coal burst occurrence. The energy concept is emphasised in order to improve the understanding of the underlying mechanisms of coal burst. Only with enhanced understanding of the driving mechanisms, a reliable coal burst risk assessment can be achieved

The optimisation of ground consolidation practices in longwall mining

The Australian longwall mining industry spends millions of dollars per year on both proactive and reactive ground consolidation methods from time to time across most of the underground operations currently in production. The design and effectiveness of the applications and products used are often variable and based on past experience at each site or what was considered successful at other sites in similar situations. This paper discusses the current product testing data and proposes an industry-standard testing regime that will provide mine site-based geotechnical engineers with a standardised set of data for product selection and geotechnical design. In addition, the preliminary results of an underground in situ testing program designed to provide quantitative data relating to the improvement in rock mass condition as a result of polymeric ground consolidation are presented and discussed. The results provide a measurement of changes in permeability, the material strength of the coal seam, and the migration of the injected material

A Review of In Situ Stress Measurement Techniques

Changes in stress orientations and magnitudes can have a significant adverse impact on mining conditions such as increasing the risk of violent failures. Knowledge of these changes can indicate the high-risk zones within a mine sites, which will enable mine operators to implement appropriate controls. At deep underground excavations, there are some difficulties in collecting reliable data at reasonable costs and majority of methods provide point measurement per test only. Thus, the utilisation of borehole techniques has received more attentions. In this paper, traditional stress measurement techniques are reviewed, including their pros and cons. Under specific geological conditions, some methods have significant advantages over others. Following the illustration of benefits and shortcomings of these techniques, the development potential of an in situ stress measurement technique using borehole breakout is briefly addressed in conjunction with the future research plan

Floor heave monitoring using floor instrumentation

Several underground coal mines in Australia have recently reported and anticipated significant floor heave in gateroads during longwall retreat. Floor heave on longwall retreat is typically attributed to a stress notch. To further understand the mechanisms of floor heave, in situ floor heave monitoring was conducted using floor instrumentation at two coal mines. This paper provides the field monitoring results along with the process of selecting the instruments and monitoring sites. To grasp the whole picture of the deformation of floor strata, the instruments for both the vertical and horizontal movements of floor units were considered. For the horizontal floor deformation, the strain gauged shear strips were used in both mines. For the vertical displacement of floor units, a remote reading tell-tale (RRTT) was chosen in Mine A, while a GEL extensometer specifically designed for this floor monitoring was selected in Mine B. As Mine A had negligible floor heave at the monitoring sites, no significant movement of the floor was captured. Although the level of floor deformation was minimal, there were indications of bedding separations as the longwall face approached the sites. In Mine B, minor floor heave was observed at the monitoring location. The data from the instruments showed that the horizontal movement of the floor strata occurred greater than approximately 4 m below the floor surface which may suggest the depth of floor failure. While several practical issues were identified from the field studies, the field monitoring results facilitated better understanding of the failure mechanisms