Stability Analysis of Rib Pillars in Highwall Mining Under Dynamic and Static Loads in Open-Pit Coal Mine

The highwall miner can be used to mine the retained coal in the end slope of an open-pit mine. However, the instability mechanism of the reserved rib pillar under dynamic and static loads is not clear, which restricts the safe and efficient application of the highwall mining system. In this study, the load-bearing model of the rib pillar in highwall mining was established, the cusp catastrophe theory and the safety coefficient of the rib pillar were considered, and the criterion equations of the rib pillar stability were proposed. Based on the limit equilibrium theory, the limit stress of the rib pillar was analyzed, and the calculation equations of plastic zone width of the rib pillar in highwall mining were obtained. Based on the Winkler foundation beam theory, the elastic foundation beam model composed of the rib pillar and roof under the highwall mining was established, and the calculation equations for the compression of the rib pillar under dynamic and static loads were developed. The results show that with the increase of the rib pillar width, the total compression of the rib pillar under dynamic and static loads approximately decreases in an inverse function, and the compression of the rib pillar caused by static loads of the overlying strata and trucks has a decisive role. Numerical simulation and theoretical calculation were performed in this study. In the Numerical simulation, the coal seam with a buried depth of 122 m and a thickness of 3 m was mined by the highwall miner. According to the established rib pillar instability model of the highwall mining system, it is found that when the mining tunnel width is 3 m, the reasonable width of the rib pillar is at least 1.3 m, and the safety factor of the rib pillar is 1.3. The numerical simulation results are in good agreement with the results of theoretical calculation, which verifies the feasibility of the theoretical analysis of the rib pillar stability. The research results can provide an important reference for the stability analysis of rib pillars under highwall mining.

Parametric Analysis of Rib Pillar Stability in a Longitudinal Sublevel Open Stoping Operation in an Underground Copper Mine in Southern Africa

The pillar stability factor (PSF) is calculated in three different mining stages for a sublevel open stoping mining project located in northern Botswana. Several three-dimensional finite element models were developed by varying the stope span. Pillar strength was estimated using the Lunder and Pakalnis equation and pillar stress was obtained from the numerical models. As mining progresses, both the first and second mining stages meet the rib pillar stability factor requirement for safe extraction. Geometrical improvements are suggested in the mining layout for the third mining stage to achieve the required PSF, which is based on international practices.

Pillar Fracturing Production Enhancement Results for Mature, Clay Sensitive Reservoir in Ecuador

This paper discusses pillar fracturing technique application along with customized fluids formulation in a mature oilfield (low reservoir pressure and high permeability) where complex mineralogy limited the use of traditional stimulation practices. Integrated reservoir analysis, laboratory tests (fracturing gel, chemical consolidation resin) and hydraulic fracture modeling performed to obtain a major productivity increase (up to 16x increase) by a combination of tip screen out (TSO) and pillar fracturing techniques. The combination of clay sensitivity, low pressure and high permeability requires a careful planning stage for pillar fracturing (PF) application. The first step is to evaluate PF feasibility by a candidate selection factor using geomechanical parameters such as closure stress, net pressure, etc. The next step is to customize the fracturing gel to sustain high shear stress during TSO and guarantee a complete gel break. Pillar stability is supported by confined stress developed by the surface modification agents mixed on the fly with proppant. This stage requires laboratory tests based on resin hardener ratio at reservoir temperature and time. Clays such as kaolinite, chlorite, etc., limits the applicability of traditional acid stimulation blends on this reservoir. Completion brine as well as fracturing gel requires the addition of a quaternary amine to temporally avoid fines migration during workover operations before and after fracturing. Without this customization, conventional or even pillar fracturing will perform below expectations. Not all reservoirs are candidates for pillar fracturing, candidate selection is a critical step in the planning process. Two types of candidates are documented on this paper, new fracturing as well as re-fracturing jobs. For both cases a numerical gridded fracture simulator is used to understand fracture geometry, diagnose and match previous treatments. Pillar fracturing is designed and executed using pulsed or cycled proppant fracture stimulation, providing infinite acting conductivity for enhanced hydrocarbon production. It significantly reduces screen out tendency leading to higher proppant concentration, as well as total proppant mass reduction when compared to conventional TSO fracture design. The use of surface modification agents (SMA) improves pillar stability and reduces proppant flow back risk if adequate compressive strength is developed during curing time after fracturing operations. Production results show up to 16 times increase, exceeding expected production by conventional fracturing. A complete workflow to characterize, design and simulate a pillar fracturing job using proprietary geomechanical candidate selection criteria is presented. The combination of TSO and pillar fracturing yields a significant production increase over conventional fracturing and acid stimulation. The use of gridded 3D simulator significantly improves the understanding of previous fracturing jobs helping to propose improvements on fracture initiation depth, polymeric load and pumping schedule for re-fracturing candidates.

Study on Web Pillar Stability in Open-Pit Highwall Mining

When highwall mining technology is applied to recover large amounts of residual coal left under the highwall of a big open-pit mine, reasonable coal pillar width is the premise for maintaining the stability of web pillars. By adopting the numerical simulation method, the characteristics of the abutment stress distributions in the web pillars under different slope angles and mining depths are studied, and the function of the stress distribution in the web pillar is established. The relationship between the abutment stress and the ultimate strength of the web pillar under different widths is also analyzed and used in combination with the failure characteristics of the pillar yield zone to explore the instability mechanism of the web pillar. The retaining widths of the web pillars are determined. Based on the modeling results, a mechanical bearing model of the web pillar is established, a cusp catastrophe model of pillar-overburden is constructed, and the formula for the web pillar instability criterion is obtained. By analyzing and calculating the ultimate strength of the web pillar, the formula for calculating the yield zone width at both sides of the pillar is achieved. Using the instability criterion of web pillars in highwall mining, a reasonable pillar width can be deduced theoretically, which provides significant guidance on the application of highwall mining technology.

Field and Numerical Investigation on the Coal Pillar Instability of Gob-Side Entry in Gently Inclined Coal Seam

In order to study the coal pillar stability of gob-side entry in gently inclined coal seam, a comprehensive method including theoretical analysis, numerical modeling, and field monitoring was applied to study its fracturing and instability mechanism. The results show that the uneven horizontal stress was the internal cause of entry asymmetric deformation and failure in inclined coal seam. In gently inclined coal seam, the rotation movement of the main roof and stress distribution were closely related to inclination of the coal seam. Based on the asymmetric deformation characteristics and mechanisms of entry, a collaborative control technology of roof cutting for pressure relief and support strengthening has been put forward. The research results have practical significance for revealing the mechanism of entry damage in gently inclined coal seam mining and proposing engineering measures to prevent coal pillar damage and disaster occurrence.

Assessment of coal pillar stability using principal component analysis and stepwise selection and elimination

Prediction of pillar stability is one of the most critical tasks in underground mining industries. This pillar stability analysis requires many input parameters and some of them are difficult to be determined. Various statistical based analysis is presented in literature for assessing pillar stability successfully. In the present work, the data from three mines had been to determine the factor of safety. A total of 63 pillar cases had been collected from the mines. Principal component analysis (PCA) and Stepwise selection and elimination (SSE) models were developed by using multi variate linear regression (MLR) on 45 data sets and subsequently the proposed models were validated on 18 different data sets. The value of coefficient of determination (R2) is 0.86 and 0.84 for PCA and SSE respectively. The root mean square error for PCA and SSE are found to be 0.112 and 0.123 respectively. On validation of the proposed model developed by PCA and SSE, the PCA model provided a better validation results. Hence, PCA is recommended for modelling pillar stability.