ABSTRACT: The efficient harvest of geothermal energy, especially geothermal electricity generation, has been an international research hotspot and challenge for decades. However, the conventional geothermal exploitation technologies with water extraction bring on many issues such as high operation cost, low electricity generation efficiency and huge environmental impacts. In this paper, a novel downhole in-situ power generator system (DIPGS) for geothermal resources is firstly proposed to simultaneously extract geothermal heat and achieve electricity generation without fluid extraction. By making use of the unique advantages of a heat pipe, such as high thermal conductivity and phase transition property of working fluids, the thermoelectric generator (TEG) and magnetic levitation generator (MLG) are innovatively integrated into a large-scale heat pipe for multistage electricity generation in a single geothermal well. Following this technical conception, a laboratory-scale testing platform was designed and established to investigate the electricity generation efficiency of the in-situ power generator system in simulated geothermal reservoirs. Experimental results show that the electrical output power of the integrated system assisted by 16 BiTe-based TEGs and a turbine generator peaked at 28.6 W, with an electricity generation efficiency of around 2.55%, when the simulated geothermal temperature reached 195.6°C with a total input power of 1122 W. The preliminary performance of the laboratory prototype demonstrated the feasibility and potential of the in-situ power generation technology, and further system optimization and economical analysis should be conducted to enhance the power generation efficiency for practical engineering applications. 1. Introduction Geothermal energy, essentially the Earth's internal heat, is one of the most promising renewable energies with many competitive advantages, such as large reserves, wide distribution, low operation costs, and high energy utilization rate (Lund et al., 2020; Huttrer, 2020). The global geothermal resources within the upper 10 km of the Earth's crust are estimated to be 1.45×10 26 J, which is equivalent to 4.95 × 10 9 billion tons of standard coal. The energy contained in geothermal resources is 170 million times higher than the total energy released by all coal resources on the earth (Xie et al., 2020). Thus, geothermal energy is practically an infinite energy resource with a huge potential for direct use and power generation. Generally, geothermal energy exists in the shallow crust in the form of steam, hot water, hot dry rock, and magma (Xie et al., 2018;2019), among which hydrothermal resources especially hot water are the current mining objectives. The large-scale extraction of hydrothermal fluids, however, may cause serious environmental issues such as groundwater recession, land subsidence, and environmental pollution. As another prospective resource, hot dry rock is expected to be exploited by an immature technology namely enhanced geothermal systems (EGS), which inject water at high pressure into deep rocks to create artificial reservoirs and allow hot water or steam to flow into extraction wells (Chen et al., 2015). Since there have been many arguments about EGS during recent years, including high drilling cost (Sanyal et al., 2007), poor connectivity between wells (Lu et al., 2015), and frequent earthquakes induced by hydraulic fracturing (Ellsworth, 2013), the development of hot dry rock is still in the experimental stage.

ABSTRACT: Heat exchange and energy extraction from a low-permeability reservoir are important processes for EGS design. Cold water is circulated from a wellbore and carries warmer water up to the surface. Depending on the circulation velocity and temperature out to the surface, different extraction efficiency may be achieved. Such an efficiency also depends on the depth of the energy source, formation temperature gradient, hydraulic and thermal properties of the porous solid skeleton and fluid. Thus a model coupling the energy transfer from an injecting tubing, annulus and into/from the low-permeability formation must be established. A semi-analytical solution is obtained for energy transfer between tubing, annulus and porous formation. Temperature, fluid pressure and induced stresses can be calculated and heat extraction power can be evaluated by an integration over the circulating rate with annulus outlet temperature. One may evaluate and design a geothermal system based on the model proposed. 1 Introduction As one of cleaner and more economic viable resource, heat energy extracted from geothermal formations becomes increasingly attractive. Also decreasing cost of geothermal installations in deep hot dry rock (HDR), enhanced geothermal systems (EGS) have the potential to replace more costly and environmentally unfriendly technologies [Grasby 2011]. The advances in EGS are primarily motivated by the success and technologies developed in Enhanced Oil Recovery (EOR), staged hydraulic fracturing (SHF) of shale gas and shale oil formations with and without super-critical CO2 and enhancing methane while sequestrating CO2 permanently. While these technologies are gaining more attention for environmental purposes, energy extraction efficiency and economic evaluation are under serious considerations worldwide. Considering production and injection in a low-permeability HDR or shale formation, energy transfer and fluid flow, also carrying the energy, into a porous intact rock, which itself allows both fluid flow (Δp) and heat flow (ΔT) through are diffusive processes. HF can increase productivity and further stimulate energy transfer processes by creating a large surface area. Seeing the critical role of hydraulic fracturing (McLennan 1980) in EGS to achieve higher production efficiency, extensive studies are conducted [McTique, 1989; Karshige, 1989, Wang and Papamichos, 1994; 1999, Wu et al., 2015, 2017]. It has been pointed out that drastic thermal stress changes when massive cold water is injected at a lower temperature into a deep HDR formation contributes to the initiation and propagation of fractures in HDR, which can not be justified by traditional hydraulic fracturing theory and is therefore referred to as thermal fracturing [Clifford et al. 1991, Charlez et al. 1996]. Calculation of thermal stress changes that induces thermal fractures and their geometry changes including the thermal cracking processes requires a fully coupled thermal-hydraulic-mechanical (THM) model. This model consists of two mass balance equations for the fracture and a matrix systems diffusion process with small portion of free gas flow. A dynamic thermal cracking zone can be created and considered once the effective tensile stresses exceed the tensile strength near a wellbore or hydraulic fractures. In addition, two mass balance equations, in the fracture and matrix systems respectively, and one equilibrium equations are coupled to the two aforementioned energy equations. Parts of these equations are nonlinear and stress dependent. Therefore solving this set of equations is challenging, yet the solution is a key to practical completion design and consequently researchers and companies have allocated effort intensively to find answer to this challenge in different ways. Over the years, there have been quite a few attempts made to simulate thermal fractures [Abousleiman et al. 1996, Tarasovs and Ghassemi 2010, Hofmann et al. 2016]. Nevertheless, there are challenges yet to overcome at least from the following perspectives. At first, to accurately assess the thermal stresses, thermally induced pressures, and the thermal conduction/convection associated to the temperature change in the reservoir must be properly addressed [Wang and Dusseault 2003]; secondly, to evaluate the size of a potential zone surrounding a wellbore or fracture where thermal crack could be initiated and propagating, from these cracked zone larger surface areas are created and more heat energy exchanged; Thirdly, boundary element method has been proposed and a kernel function corresponding to a temperature change inside the hydraulic fracture is required. Developing such a function and determining the heat transfer process along a hydraulic fracture with thermal leakage term are critical. Fourthly, calculating the radius and evaluating the THM properties of a thermal cracking zone surrounding a wellbore and hydraulic fracture are extremely important for thermal energy extraction and heat exchange efficiency in the EGS. lastly, to predict the thermal fracture propagation in a fractured deep geothermal reservoir, THM perturbations due to the interaction from the natural fractures must be properly addressed [Wang 2017].

Abstract This paper investigates the applicability and operational characteristics of different partial pillar extraction methods for a room-and-pillar coal mining project located in the region of southern Africa. Three partial pillar extraction methods, i.e. the Pillar Splitting method, the Duncan method and the Lift Left-Right method are evaluated using three-dimensional finite element analysis. Multiple 3D models were developed for each method by varying the geometry of the remnant pillar during partial pillar extraction. Pillar strength was estimated using the Salamon and Munro equation and pillar stress was obtained from the numerical models. Surface subsidence was also calculated for each room-and-pillar panel. All three methods meet the stability factor requirement for safe secondary extraction. The Lift Left-Right method exhibits the highest stability factor while the Pillar Splitting method is the most favorable in terms of surface subsidence. Introduction The Room-and-pillar (R&P) method is a widely utilized underground mining method used in coal, limestone and metal mines. The basic premise is that a series of rooms (i.e., long horizontal openings) will be extracted, while leaving behind pillars of ore. Pillars constitute the main support elements of the underground openings, while this type of mining may require ground support systems, such as roof and/or cable bolting. The cross-sectional area of the pillars needs to be minimized in order to maximize the extraction ratio. In addition, the room dimensions (entry span) must be large enough to accommodate the traffic of mining equipment, but not so large that room stability is compromised. The coexistence of these two basic elements (i.e., rooms and pillars) makes it necessary to evaluate their safety jointly. Since coal is a nonrenewable resource, it is important to improve the initial extraction ratio by recovering pillars during a secondary mining stage. Pillar extraction (or recovery or retreat mining) is the practice of partially or fully extracting some or all of the pillars developed during the development stage of an R&P operation. Full or partial pillar recovery helps increase resource recovery, since any coal left in place that is not recovered at the time a particular mine section is mined, will most likely never be recovered.

ABSTRACT: Mining-induced fracture fields play important role in gas extraction of underground coal mines. Low concentration of extracted gas has been observed during the mining of gas burst-prone coal seams in Shaqu Coal Mine, Shanxi province, China. Theoretical analysis and field observation are used to quantity the fractures evolution of 4 # coal seam to get an optimal borehole design and enhance the efficiency of gas extraction. In this study, pixel extraction and gray value calculation of borehole images are developed using Matlab, which can be used to obtain the 3D gray value distribution for the borehole images. Based on the 3D gray distributions, the development of borehole fractures is identified and then a proper design of gas extraction borehole sets is determined. Based on the analysis, an area of high density of mining-induced fractures with relative stable overlying strata is found about 25 ~ 30 m above the working face of the protected coal seam. A proper gas extraction borehole depth is estimated and this in turn will assist in enhancing gas extraction efficiency and improving mine safety.

ABSTRACT: As an important supporting project of the “West-to-East Gas Pipeline project” of China, the construction of the underground gas storage (UGS) of salt cavern in Jintan is mainly to solve the problem of emergency gas supply. Due to the unbalanced gas supply caused by seasons, climate changes and the accidental failure of gas transmission pipeline. In this paper, the construction and operation stability of gas storage in layered salt rock are employed as the research topics. The optimal gas injection and extraction operation schemes are determined by studying the operating characteristics of the salt cavern gas storage. Based on the geological conditions of the Jintan gas storage and the measured data of sonar, a set of theoretical methods is used to determine the operating pressure interval. The finite-difference software FLAC3D was used to simulate the entire process of four gas injection and extraction scheme. The entire operation process can be divided into four stages: constant high pressure stage, step-down gas extraction stage, constant low pressure stage, and pressurized gas injection stage. The volume shrinkage, key point displacement, cavity deformation and plastic zone distribution are discussed and analyzed in a numerical way. The results show that the unstable gas extraction/injection rate and frequent gas injection/extraction are not conducive to the stability of UGS. By comparing with the simulated results of the constant average internal pressure, it is shown that the changes of gas storage during the gas injection/extraction phase can be roughly estimated based on the average internal pressure monitored at the site. The research results can provide reference for the design of UGS operation scheme and the later operation management.

ABSTRACT: A fully coupled thermal-hydraulic-mechanical (THM) model was developed to investigate the heat extraction process of an artificial enhanced geothermal system (EGS). The random fracture network in the stimulated geothermal reservoir was generated by the fractal theory. The local thermal non-equilibrium (LTNE) theory was adopted to simulate the heat exchange between the rock matrix and the injection cold fluid. Temperature-dependent fluid thermodynamic properties and pressure-dependent fracture/pore permeability were also incorporated into the thermo-poroelastic coupling model by some empirical formulas. The proposed multiphysics model was validated by several analytical solutions. The evolution of temperature, effective stress and reservoir permeability during heat extraction was analyzed in detail. The sensitivity of heat production performance to heat convection coefficient, injection rate, injection temperature, fracture network morphology was discussed. Results indicate that at the early stage, the interconnected large-scale fractures around wellbore dominate the mass and heat transport. Some scattered small-scale fractures contribute to uniformly propelling the cooling of the heat rock mass. The change in effective stress associated with the thermo-poroelastic effect may induce fracture shear dilation and pore expansion, resulting in the permeability enhancement of the overall reservoir. In some regions, where the temperature drop is insignificant and the pore pressure decreases, thereby inducing compressive stress to make fractures closure. It is more practical to use the LNTE theory to analyze the heat extraction process in fractured geothermal reservoirs. The heat extraction efficiency, thermal breakthrough time and service-life of an EGS are seriously affected by the hydraulic conductivity and connectivity of fracture networks under constant injection rates and injection temperatures. Generating a complex and scattered fracture network but without preferential channels is conducive to extracting more heat from geothermal reservoirs.

ABSTRACT: This paper evaluates the stability of drives excavated in paste-filled stopes that have been previously mined out. Stability of the drives is important as the vibrations generated during the drilling and blasting processes affect the stability of the drives in low-strength paste. The practical implementation of the process has been carried out at an underground lead-zinc mine in India. A similar detailed study for such a mining practice in India has not been conducted in the past. Simulation for effect of blasting vibrations and subsequent excavations of the drives on the paste-filled stope is carried out through dynamic numerical analysis in FLAC3D. Rayleigh damping conditions were applied to the model along with the application of dynamic loading conditions. Dynamic response of the blasting and excavation process is studied to evaluate displacement magnitudes and strength-to-stress ratio for the excavated drives. The numerical model is calibrated using the peak particle velocity data of blast vibrations obtained from the field test. The numerical results are in accord with the field observations in the mine. Limited damage to the drives is observed and the dynamic model confirms the same due to the effect of blast loads. Future scope of the study is to understand the stability of drives with changes in stress conditions as the mining advances to greater depths. It may also help to modify the blasting parameters to control the vibrations affecting the drives in paste-fill.

ABSTRACT: The Santo Niño ore body at Pinos Altos Mine has been in production since 2009 using both open pit and underground mining methods. Between 2015 and 2017 an extensive study was conducted to evaluate extraction of the crown pillar which remained between the open pit and underground mine workings. The project evaluation considered numerous factors influencing mine safety, operations and economics. This case study paper will describe the geomechanical aspects of the project evaluation and execution. 1. INTRODUCTION The Santo Niño ore body is one of several mineralized zones at Agnico-Eagle Mines’ Pinos Altos property. Pinos Altos is located in the state of Chihuahua in northwestern Mexico within the Sierra Madre mountain range (Fig. 1). Gold and silver production at Pinos Altos started in 2008 with the extraction from the Santo Niño and Oberon de Weber open pits. The Pinos Altos Complex produces over 2Mt per year from the main Santo Niño orebody and nearby satellites. Milling and heap leach are used to treat the underground and open pit ores. Underground mining was initiated simultaneously with the open pit production at the Santo Niño orebody with the first pyramid extending from Level 20 to Level 16 (Fig. 2). The mining strategy consisted of excavating and backfilling the upper most stopes up to Level 16 before the open pit reached its final depth. The recovery of the crown pillar between the pit bottom and Level 16 was not fully detailed in the initial mine planning. Following the technical and economic evaluations in 2015-2016, the crown pillar ounces between the pit bottom (elev. 2023 masl) and the top of the underground mining (elev 2000 masl) were incorporated into the Life of Mine Plan and contribute to the replacement of higher grade ore to the mill following depletion of the large Santo Niño open pit. Initial thoughts on crown pillar recovery focused on simply mining the open pit in the usual 7 m benches down to the top of backfill on Level 16. Open pit mining is less costly than underground extraction and geotechnical studies focusing on open pit slope stability indicated that deep slope instability was not expected with the removal of the crown pillar. Nonetheless, further analyses demonstrated that there were advantages to recovering the crown pillar from the underground; economic evaluations showed that resource recovery could be increased which made underground recovery more profitable (60% by open pit versus at least 85% from underground), slope deformations could be reduced hence the risk of localized slope instability was decreased and exposure of personnel in the pit ramp could be significantly reduced. A thorough risk analysis and review process was implemented by a team of key personnel from pit and underground operations, mine and corporate technical services, health and safety and mine management. The objectives of the exercise was to develop measures to control identified risks to personnel and machinery while optimizing the recovery of the reserves. This paper documents the geomechanical analyses and monitoring measures implemented to support the design and recovery of the high grade Santo Niño crown pillar.

ABSTRACT: Sandbox Mining (Sandbox) is developing instrumentation that will indicate load changes in a tensioned support tendon installed in an underground excavation. It is envisaged that this instrument will indicate, in real time, the load change and notify relevant persons as soon as the change in load is detected by the instrumentation. Testing of this instrumentation has been taking place since September 2010, and a unique opportunity has arisen to test this instrumentation in an actual underground monitoring situation for which this instrumentation is being developed. A test to simulate a cable anchor being loaded beyond its capability was simulated to detect whether or not the ELI could detect this occurrence as well as record the load loss in the cable once individual strands start failing. The Electronic Load Indicator (ELI) was tested to measure the load loss in a simulated cable anchor failure. This failure was simulated by cutting individual strands of a tensioned cable “truss” and measuring the loss in tension on the cable. An attempt was made to derive a load-deformation curve of an installed cable anchor using the ELI. The ELI successfully recorded the loss in load of a tensioned cable as strands were cut through.

ABSTRACT: The development of longhole stoping mining technique involves the mechanization and increased productivity of underground bulk mining operations. Longhole stoping is usually developed with the tunnel dimensions of 15m × 15m spans. However, when mining such large chamber in deep to ultra-deep gold mining, an appropriate support design need to be taken into consideration so as to maintain the stability of the excavation. This paper provides a detailed study on the support design for large chamber excavations in mechanized deep to ultra-deep gold mining in South Africa. To achieve the objectives of the study, empirical span determination using rock mass classification methods, stability graph method and numerical modeling were used. The assessment using Q system on the hangingwall and sidewall of all the case studies shows that the ground is in good condition. Assessment using stability number has gave 20.3 and 8.6 stability numbers for the sidewall and hangingwall of the excavations respectively. Hydraulic radius results have shown that the excavations have hydraulic radius ranging from 4.6 to 7.4, depending on the dimensions of the excavation. Stability analysis also indicated that both case study A and B are within the stable zone, meaning the excavation can be stable without the installation of support tendons. Results on case studies C, D and E indicated that the excavations were within the transition zone of supported excavation and unsupported excavation. As a result, the excavations required support tendons to improve their stability. Numerical analysis on ground closure indicated a ground closure of 400mm during the extraction of primary stopes, followed by deformation of about 600mm at the first extraction of secondary stope and eventually 800mm deformation was simulated after completion of extraction. It was, therefore, concluded that the large chamber excavation can be stable for a long time.

ABSTRACT: This paper presents a case study on the application of hydraulic fracturing to destress abutment pressures. The study site was an entry of a longwall coal panel at Yuwu Coal Mine, Shanxi, China. The main roof of the panel consists of a massive sandstone layer which is not easy to cave after the extraction of coal, leading to high abutment pressures applied on the tailgate and a bleeder entry and the subsequently large deformation. Hydraulic fracturing was performed on a trial basis to “soften” the massive sandstone so that it can cave after the longwall face passes by. Extensive monitoring system was placed to evaluate the destressing effort of the hydraulic fracturing including vertical stress in the chain pillar and longwall panel, abutment pressures, and deformations of the tailgate. Numerical modeling using distinct element method was carried out to examine the related mechanisms of using hydraulic fracturing to destress abutment pressures caused by longwall coal extraction.

ABSTRACT: Geothermal energy from naturally or hydraulically fractured reservoirs is extracted by cold water injection and circulation using injection and production wells. Well spacing, well pattern, and injection-extraction rate schedule are among the most important factors that control the efficiency of heat extraction. Aside from the heat transfer aspect of the process, the hydromechanics of the circulation and fracture stability also needs be considered. Previous work and analysis has been mostly restricted, assuming stationary fractures without the possibility of fracture propagation during the circulation period. This paper addresses these issues by focusing on the hydro-mechanical aspect of the circulation and the possibility of natural fracture propagation in geothermal systems with reference to the planned “EGS Collab” project’s circulation experiment. A fully coupled 3-dimensional model is developed for this study using the displacement discontinuity method along with a finite difference scheme. Fracture propagation is incorporated into the model by enforcing the critical stress intensity condition at the fracture boundary. Simulation results indicate that natural fracture propagation is likely to occur during circulation. Furthermore, fracture opening variation by propagation, in turn, controls the hydro-mechanics of the circulation as it affects the fracture conductivity and the production rate.

ABSTRACT: According to the studies on tight gas reservoirs, liquid CO 2 (L- CO 2 ) is much superior in hydraulic fracturing compared to conventional fracturing fluids. However, the applicability of this technique for hydraulic fracturing of coal seams has been hindered due to the lack of understanding. This paper investigates the superiority of L-CO 2 as a fracturing fluid for coal seam gas extraction, in terms of break-down pressure and acoustic emission (AE) energy release during fracture propagation. The results reveal that L-CO 2 induced break-down pressure is around 19.6% lesser than the water induced break-down pressure, whereas the time taken to break-down is 59.3% higher for L-CO 2 compared to water. Importantly, due to the low compressibility, water injection pressure showed an exponential pressure development closer to the break-down, resulting a higher break-down pressure within a short time period. Conversely, the highly compressible L-CO 2 exerted only a gradual increment in pressure in the sample over a considerable time, which caused a more controllable fracturing process. The observed AE suggests that the ability of low viscous L-CO 2 to penetrate through the tiny pores in the coal mass has the potential to create a stable and dense fracture network, instead of the uncontrolled unstable sudden failure occurred under the water based fracturing. This well-developed fracture network can significantly enhance the rock mass permeability and the reservoir gas extraction. Overall, it can be concluded that the combined characteristics of L-CO 2 (high compressibility, low viscosity) lead more controllable and effective coal seam hydraulic fracturing process compared with the water based fracturing.

ABSTRACT: Biological and engineering flow systems maximize their efficiency by following the path of minimum energy over the domain they are embedded in. This fact motivates the present research, since industrial fluid extraction and injection processes are designed to minimize the implementation cost (energy, materials) and maximize the volume of fluid injected (or withdrawn). This work presents a bio-inspired fluid flow model to optimize the path that connects resource-rich pores in a rock. We explain the commonalities between the equations governing flow in a porous medium and growth of slime mold, an organism that dynamically deploys tube-like structures and adapts them as a function of their contribution to the overall network. We perform several simulations to analyze the influence of the pore size distribution and of pore spatial distribution on the topology of the extraction network predicted by the slime mold growth algorithm. We discuss the suitability of the biomimicry model to design fracture patterns for optimal fluid extraction from a porous rock. 1. INTRODUCTION

ABSTRACT: In order to understand how the rock mass evolves with extraction in the particularly highly-stressed environment of a sill pillar in a hard rock mine in North America, we investigate the utility of seismic tomography as an interpretive tool. We use a rich dataset of blasts to highlight the compressional wave variations in the rock. Although tough there are areas within the sill pillar that are prone to smearing, we are able to resolve subtle variations in velocity structure. By considering different time periods, we image temporal changes in seismic velocity that we relate to the stress state and damage in the rock. Specifically, we consider data recorded during three sequential time intervals associated with the excavation of a stope during the start of the second interval. We observe a high-velocity anomaly in the first interval that is located to the edge of the future stope. After mining, the high-velocity regions migrate to the other side of the stope and then disperse to the edges of the resolvable area. Equating high-velocity anomalies with high-stress gives us the ability to characterize the evolving stress state in the mine and potentially an approach that can be used to avoid hazardous situations.

ABSTRACT: Fully grouted rock bolts are considered the most economical support system for exposed unsupported rock in various mining operations. The paper presents parametric study of an underground metal mine with the horizontal cut-and-fill method of mining at the depth of 150 meters from the surface through numerical simulation using Fast Lagrangian Analysis of Continua (FLAC3D). Four different height of extraction i.e. 3m, 5m, 6m & 7m has been excavated numerically to evaluate the induced stress value and its parametric variation with the different height of extraction. Factor of safety was calculated numerically for estimating the stability of different stope height. The performance of the bolts were studied by conducting series of in-situ pullout tests, and parametric analysis of stress along the length of the bolt at different extraction/stope height. The load-deformation curve of bolts indicates the de-bonding nature of bolt with axial deformation of 42-46 mm. From the parametric study, it can be apprehended that with lode width of 8m and average height of extraction/stope (5m) the underground working does not experienced any pronounced ground control problems. At extraction height more than 5m the induced stress values are more and the chances of roof fall and failure of rock bolts are higher.

ABSTRACT: Over the past decade, the mining industry has seen an increase in the application of the stope-and-fill mining method as conventional underground deposits have been depleted and operations have been forced to produce at deeper depths and in more challenging ground conditions. Through the utilization of cemented paste backfill (CPB) material for localized ground support, modern stope-and-fill mining operations have been able to increase productivity, effectively manage mine waste costs, as well as provide a safer mining environment. While the application of CPB has allowed for an increase in ore recovery, it is important that engineering and operations personnel have a clear understanding of the fill material as well as the limitations of the fill design to provide the most efficient, cost-effective, and safe extraction of underground reserves. Despite extensive use of CPB in mines around the world, many fundamental factors affecting the design of safe and economical fill structures are still not well understood. A critical issue in the design of backfilled stopes is the determination of the stress state within the fill material and surrounding rock mass.

Abstract: Legal license boundary restrictions and economic considerations for extraction of vein type iron orebody in Erzincan/Bizmisen area of Turkey limit the overall production by open pit mining. Based on overall Stripping Ratio (OSR) assessments, ore production is decided to proceed by open pit mining to a certain depth and continue with a cut and fill type underground mining operation. A crown pillar is to be designed in the transition region from surface to underground operations. Crown pillar dimensioning effects the overall ore recovery in such small-scale ore bodies. Proper dimensioning has to ensure the stability of open pit mine slopes and underground mine structural units as underground mining progresses to upper levels. Dimensional optimization of the crown pillar is performed by empirical, deterministic and numerical modeling approaches. First, scaled span method was employed to estimate the safe crown pillar thickness providing the required service life. Later, rigid analysis method was used to check shear type failure from the abutments. Finally, 2D finite element modeling analysis was used to ascertain the stable crown pillar thickness. Input parameters of numerical models were produced from geotechnical site investigation work and laboratory experiments. Disturbance effect of surface mine production blasting on the slopes of rock mass above the underground workings was taken into account in deciding on the rock mass properties. Introduction Reserve is the economically valuable ore mineralization that is host within the earth crust. Especially in low grade, low profit ores, mining cost outlines the feasibility conditions for reserve extraction. Surface mining is mostly preferred due to low mining cost, ease and speed of production. In spite of its advantages, surface mining may not be feasibly applicable for the complete orebody extraction mainly due to two reasons. The first reason is the feasible open pit layout. Economic depth of excavation is defined by comparing the Maximum Allowable Stripping Ratio (MASR) (see Eq. (1).) and Overall Stripping Ratio (OSR) (see Eq. (2).). (equation) (equation) where a is surface mining cost per ton of ore, b is overburden stripping cost per m 3 , c is underground mining cost per ton of ore, V is volume of overburden, and M is weight of orebody. V is calculated by subtracting the volume of a cone representing the surface mine from the orebody it includes. Economic open pit layout may not contain whole of the orebody. In this case, rest of the orebody is exploited by U/G (underground) mining.

Abstract: An underground iron mine in China has been used as a case-study to research the subsidence due to ore extraction and backfilling during open stoping operations. A 3-D discontinuum numerical model was built to scale incorporating geologic complexities including faults and interfaces between different lithologies, and the stoping and backfilling sequence adopted from the mine plans. The stoping was carried out in two vertically stacked horizontal layers, with a total of 16 stopes. Large displacements of up to 50 cm were observed along the roof of the stopes, and a maximum surface subsidence of 22.5 cm was observed at the surface. Backfilling was found to eliminate subsequent deformation and subsidence from occurring. The extraction of the upper orebody was found to influence deformations in the lower orebody. Finally, a subsidence profile was constructed to show the subsidence at all locations along the length of the surface and region of influence on the surface. Introduction Surface subsidence is a significant problem affecting infrastructure, settlements and forests over underground workings. In this paper, a discussion on ore extraction, backfilling operations and the consequential surface subsidence is presented considering the case study of the Luohe mine in China. Luohe is an underground pyrite holding of the Magang Steel Company in the Anhui province of China. The 250 m thick orebody lies at a depth of 400 m (Magang, 2012a), under a significant number of surface infrastructure (Fig. 1) which need to be protected from any adverse consequences of mining the iron deposits. To limit the subsidence, the mine has adopted backfilling operations post extraction in all stopes. In this paper, a section of the unmined orebody is considered and analyses have been performed to predict the amount of subsidence that the region is likely to undergo due to the extraction methodology being currently adopted in the mine. Because this study has attempted in incorporating the effect of in-situ stresses in the region, fault network, rock mass mechanical properties, interface features and backfilling operations on the subsidence, the discrete element method has been selected to carry out the analyses.

Abstract This paper describes a method for simulating the goaf of mechanized longwall panels at shallow depth in various coal mines in India for prediction of subsidence, abutment pressure and stresses over goaf and barrier pillars. Proper modelling of goaf is necessary for the accurate prediction of stress distribution around the face and subsidence at the surface. In this paper, the broken rock debris has been modeled as a non-linear stress strain characteristic. Two characteristic properties have been identified to describe the stress-strain behaviour. This study is undertaken to find out the goaf properties for seven previously worked longwall panels. A numerical model is developed in FLAC3D, a finite difference modelling tool and the modelling procedure has been described. The results obtained from modelling were compared with field observations in case of subsidence and theoretical values in case of stress distribution over the stabilized goaf and average load on barrier pillars. This paper provides the characteristic values for the goaf properties for the three different roof-rock type which may be used for simulating the goaf. 1. INTRODUCTION As the longwall face advances, the immediate roof overlying the coal seam, consisting of weak beds, readily cave behind the supports. The main roof lying above the immediate roof consists of strong rock beds and is selfsupporting within a certain roof span. This remains intact and exerts pressure on the barrier pillars and coalface. With the progress of mining, it eventually fails and caves in as main fall. As the size of the worked out area i.e. the goaf increases, it gets filled up with fragmented rock material that provides resistance to the lowering of the relatively intact strata. The amount of resistance offered by the goaf depends on its compaction and caving behaviour of the overlying roof rocks. Proper modelling of goaf is necessary for the accurate prediction of stress distribution around the face and the load exerted on the face support system for designing the parameters such as support density and resistance.