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The design and strength of pillars are important parameters while implementing the room-and-pillar mining method in a limestone mine. The process becomes complex with the presence of the karst voids which are frequent phenomena in a carbonate rock formation. The karst system is a network of cavities that along with an omnipresent fracture network may lead to ground control issues in a limestone mine. The objective of this study is to provide a safe and reliable alternative design for a pillar in a case study mine for safe and efficient ore recovery. This research is a continuation of a previous study aimed at studying the effect of karst void on the strength of the same limestone pillar. It was assessed from the calculated pillar strength and the estimated redistributed stresses that the pillar is capable of countering more stress than is currently imparted on it. Numerical modeling studies combining in situ stress determination and excavation sequencing were used to estimate the existing stresses on the pillar. Different possibilities of modification to existing pillar design were simulated using Distinct-Element Modeling (DEM) to determine a better design. Discrete fracture networks (DFNs) were included to imitate the effect of the presence of discontinuities. An effort is also made to understand the variation of pillar strength with height for karst-affected pillars using two different constitutive models. This study serves as a preliminary investigation in terms of improving the future design of pillars and also while recovering old pillars prone to karst formation.

Pillar collapses are events that due to their severe consequences can be classified as high risk. The design of pillars in underground room-and-pillar operations should migrate to risk-based design approaches. The authors of this work proposed a risk-based pillar design methodology that integrates stochastic discrete element modeling for pillar strength estimation, and stochastic finite volume modeling (FVM) for stress estimation. This paper focuses on the stochastic FVM component for stress estimation. The mining and geomechanical aspects of a case study mine (CSM) are described and pillar stresses are estimated by using three approaches: (1) analytical solutions, (2) 2D finite element modeling, and (3) 3D finite volume modeling. This operation extracts a 30° dipping deposit, which makes current underground stone mine design guidelines inapplicable for this CSM. This work compares results from each stress estimation approach and discusses uses the point estimate method as a simplified stochastic approach to evaluate the effect of rock mass elastic properties variability on pillar stress distribution. Results from this work show that the three estimation approaches lead to different estimations, possibly, due to the wide range of assumptions each estimation approach considers. It was also determined that the horizontal to vertical stress ratio has a significant impact on pillar stress magnitude. Therefore, it is recommended to perform in situ stress measurements, or assume worst-case-scenario values to account and reduce uncertainty due to this parameter. The stochastic stress estimation approach used in this paper provides results that can integrate a risk-based pillar design framework.

Underground limestone deposits are prone to karst void formations due to the dissolution of carbonate rocks. These deposits offer permeable conditions to groundwater flow leading to the formation of a network of void spaces or cavities. The cavities can accommodate unconsolidated sediments and groundwater. Interaction of these voids with a prevalent discontinuity network may lead to ground control problems during mining operations. This paper presents a numerical estimation of the effect of karst voids on the strength of pillars in an underground room-and-pillar limestone mine. It is shown that these voids may contribute to a considerable decrease in pillar strength. Mining advances into such formations may lead to a sudden in-rush of groundwater or unconsolidated sediments. Apart from posing a safety hazard, this may also disrupt the pre-planned mine operations and design of the headings and pillars. The procedure for estimation of pillar strength using numerical modeling may provide a viable approach to design the future pillars detected with karst voids. Previous studies performed at the case study mine used ground-penetrating radar (GPR) to map the karst boundaries inside the pillar. Apart from the LiDAR scans used to map the discontinuity network around the pillars, some approximations were made to account for the inadequate discontinuity characterization data. Distinct-element modeling (DEM) is used to simulate the presence of voids and discontinuities in the form of discrete fracture networks (DFN). An effort is made to understand the importance of pillar design and dimensions in an underground limestone mine with karst voids to prevent local instability related to pillar failures.

Mechanical and chemical characteristics of shales with different thermal maturities were investigated through Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and atomic force microscopy (AFM). The organic and inorganic components are identified through FTIR, and chemical structure variation associated with the maturation process is characterized preliminarily by Raman spectroscopy and FTIR analysis. Mechanical properties are characterized at the micrometer or even nanometer scale by AFM. Young’s modulus from AFM scans varies from 8.20 to 13.94 GPa for organic matter, with the softer organic matter (low modulus value) surrounded by stiffer organic matter (higher modulus value). Comparative analysis of the experimental data (Raman, FTIR, and AFM) indicates that higher modulus values may be attributed to a change in chemical structure during maturation. The growth of aromatic groups and decreasing CH2/CH3ratio may contribute to increased values for the mechanical properties of organic components.

This work focuses on the operational and safety issues associated with karst voids in large opening underground mines. Issues include water inrush, structural instability, and engineering uncertainty in these environments. Coupled with the fracturing prevalent in folded sedimentary rocks, karst voids are complex and challenging ground-control risks. Traditional methods of predicting karst void locations, such as probe drilling, are impeded by the inconsistent spatial distribution and variable sizes of the features as reported by Li et al. (J Rock Mechan Geotech Eng:232–242, 2010) and Hassan et al. (Procedia Chem 19:737–742, 2016). Ground-penetrating radar (GPR) is a geophysical technique that transmits radio waves into a medium and subsequently detects reflected waves via a receiver. The travel time and energy of received signals are then processed and interpreted. The difference in conductivity and dielectric permittivity between limestone and open karst cavities causes strong reflections. GPR is frequently used as a geophysical surveying technique in several industries; however, there is a lack of published research on underground mining GPR applications as reported by Zhao et al. (2015a; Geophys J Int:62–71, 2015b) and Eskelinen and Pellinen (Constr Build Mater:888–898, 2018). The purpose of this work is to demonstrate the use of GPR in an underground stone mine and to discuss the importance of karst void detection ahead of mining.

Terrestrial laser scanning (TLS) is a useful technology for rock mass characterization. A laser scanner produces a massive point cloud of a scanned area, such as an exposed rock surface in an underground tunnel, with millimeter precision. The density of the point cloud depends on several parameters from both the TLS operational conditions and the specifications of the project, such as the resolution and the quality of the laser scan, the section of the tunnel, the distance between scanning stations, and the purpose of the scans. One purpose of the scan can be to characterize the rock mass and statistically analyze the discontinuities that compose it for further discontinuous modeling. In these instances, additional data processing and a detailed analysis should be performed on the point cloud to extract the parameters to define a discrete fracture network (DFN) for each discontinuity set. I-site studio is a point cloud processing software that allows users to edit and process laser scans. This software contains a set of geotechnical analysis tools that assist engineers during the structural mapping process, allowing for greater and more representative data regarding the structural information of the rock mass, which may be used for generating DFNs. This paper presents the procedures used during a laser scan for characterizing discontinuities in an underground limestone mine and the results of the scan as applied to the generation of DFNs for further discontinuous modeling.

Finding an optimized adsorption model to estimate the true adsorbed quantity of methane in shale at reservoir conditions is fundamental for estimating the gas-in-place (GIP), and developing an accurate shale gas transport model. However, describing true methane adsorption behavior in shale is challenging because the density or volume of the adsorbed phase cannot be measured directly using current technology. There are several models available to describe the observed adsorption isotherms and extrapolate the true adsorbed quantity of methane, but a consensus model has not been reached by researchers. This work first revisits available absolute and Gibbs excess adsorption models for describing methane in shales. It then compares nine available adsorption models to assess the efficacy of each model in describing both high pressure and low pressure methane adsorption isotherms in shales. Three aspects of the adsorption model are compared: (1) the goodness-of-fit of each adsorption model, (2) interpretation of the observed test phenomena, and (3) predicted isotherms beyond test data. Comparison results show that even though the goodness-of-fit for each model is comparable, the dual-site Langmuir model is still superior to other available models in interpreting observed phenomena and extrapolating adsorption isotherms beyond test data. The successful application of the dual-site Langmuir model therefore lays the foundation for accurately estimating and extrapolating the deep gas resource and differentiating the accurate ratio of the adsorbed phase to bulk gas for use in shale gas transport models. This study also clarifies some inappropriate concepts and methods routinely used by the shale gas community.

•A dual-site Langmuir model can simulate high pressure scCO2adsorption in coal.•The crossovers of adsorption isotherms under different temperatures are interpreted.•Adsorption isotherms beyond tested temperature range can be predicted.•The isosteric heat of adsorption depends on both temperature and adsorption uptake.

Horizontal drilling and multi-stage stimulation technologies make enhance methane production and carbon dioxide (CO2) sequestration feasible for shale reservoirs. In March 2014, approximately 462 tonnes of CO2was injected into horizontal well in Morgan County, Tennessee over a 13-day period in order to examine the injectivity and the adsorption of CO2. The horizontal well was hydraulically fractured and stimulated in four stages. Due to the lack of microseismic data available for the well, the volume of the stimulated reservoir was unknown. Sensitivity analysis by using numerical modelling can help to determine the size of the stimulated reservoir volume (SRV). In this paper, grid refinement in fracture network, fracture spacing and fracture porosity, fracture network permeability, and fracture network volume were changed to identify the most important parameters for matching the gas and the bottom-hole pressure of the wellbore. [Received: January 7, 2015; Accepted: February 13, 2015]

Solid phase microextraction (SPME) is a solvent-free method of sample collection. SPME is an appealing method for sample collection because it is designed for the sampling of trace level analytes with short sampling times in a variety of environments. Additionally, SPME can be used to directly deliver a sample to a gas chromatograph (GC) for analysis by means of thermal desorption. In this paper, the performance of SPME under dynamic conditions was investigated. Additionally, the competence of SPME sampling for the simultaneous analysis of multiple trace analytes was also evaluated. This work is discussed in the context of underground mine ventilation surveys but is applicable to any industry in which ventilation circuits must be evaluated. The results of this paper showed that the performance of the 100μm PDMS SPME fiber was both precise and rapid under dynamic conditions. This SPME fiber was also able to simultaneously collect sulfur hexafluoride (SF6) and perfluoromethylcyclohexane (PMCH) with adequate sensitivity.