Slope stability calculation method for highwall mining with open-cut mines

Slope stability is a prominent problem for the efficient application and promotion of highwall mining technology, especially when mining residual coal under high and steep end-slope conditions. This study proposes the concept of target time pillar strength based on the required coal pillar service time. Creep tests were performed to measure the time-varying properties of coal shear strength parameters under different loads, and a time-varying function was established by regression. The highwall mining length is divided into three categories based on discontinuous structural plane theory, including goaf, yielding, and elastic zones, all of which are considered to have resistances against shear stress. The basal coal seam is prone to weakening owing to the weight of overlying strata, which may shift the slope failure mode from circular to sliding along the weak layer. Numerical modeling was used to study the influence of the bearing stress and target time strength on the development of the yielding zone at the coal pillar ribs. The coefficients of the three zones were determined, and the temporal and spatial evolution patterns of the shear strength parameters of the weak layer were acquired. A slope stability calculation method is proposed based on rigid body-limit equilibrium theory that can quantify the influence of highwall mining operations on slope stability, which is significant for popularizing highwall mining technology.

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.

Development of placer deposits in the North using highwall mining systems

The article studies various aspects of the highwall mining systems applicability for the development of productive sands in terms of the occurrence conditions as well as the mining and technical parameters of placer deposits. The harsh climatic conditions of the Arctic zone and the specific properties of frozen sands impose additional requirements on the operation of the mining complex. Positive and negative aspects of the practical application of mining complexes been analysed with reference to the development of precious metals and gemstones deposits. The most promising mining sites have been identified, where the use of highwall mining systems is technologically and economically feasible. Commercial mining of reserves in the boundary zones in high walls of depleted fields and in thin unconventional seams will substantially expand the mineral resource base of mining companies. A promising trend of this technology development is discussed that includes backfilling of the mined-out space in order to reduce the loss of mineral resources. The ice-rock mixture is proposed as the backfill material, which significantly reduces the unit cost of these operations. A practical case of this technology implementation is described for the development of substandard sands of a placer deposit in the Far North conditions. The possibility of developing the reserves of tin placer deposits on the Arctic shelf using the highwall mining systems has been identified as the most promising direction of scientific and practical research for the development of the mining industry in the region.

Slope Stability Calculation Method of Highwall Mining with Open Cut Mines as an Example

Highwall mining machines have been used to recover retained coal at the toe of highwalls and endwalls over the past few decades. However, there has not been a universal method to evaluate the slope stability using highwall mining while maximizing the recovery rate. Based on the required service time of coal pillars, this study proposes the concept of the target time pillar strength. To obtain time-dependent parameters for the coal, time-dependent shear tests were performed on specimens from an open-cut mine in Inner Mongolia. The highwall mining length was divided into three categories based on discontinuous structural plane theory: goaf, yielding, and elastic zones. The three zones were considered to all have resistances against shear stress. The basal coal seam is likely to become weak due to weight from the overlying strata, which may change the slope failure mode from circular to sliding along the weak layer. Numerical modeling was used to study the influence of the overlying strata and target time strength on the yielding zone development at the coal pillar ribs. The coefficients of the three zones were determined and substituted into the Mohr-Coulomb equation to obtain the time-dependent shear strength parameters. Subsequently, the influence of highwall mining on the slope stability was evaluated using the rigid body-limit equilibrium method (LEM). The optimized coal pillar width is determined to maximize the recovery rate without compromising the slope stability.

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.

Simulation Research for the Influence of Mining Sequence on Coal Pillar Stability under Highwall Mining Method

Highwall mining, which is referred to the technique of extracting coal from the bottom of an exposed highwall, features safety, efficiency, and economy. According to existing highwall mining methods, the mining sequence has a great influence on highwall stability. Based on a highwall mining project in Australia, this study adopted the FLAC3D numerical simulation method to investigate the stability of coal pillars with different mining sequences. The results show that different mining sequences of boreholes exert a great effect on highwall stability. Compared with sequential mining, the skip mining method achieves higher speed of highwall stabilization and smaller plastic zone of coal pillar with its maximum strength decreasing by 12%. By adjusting the mining sequence scientifically, the coal pillar failure and roof collapse caused by the deviation of mining angle can be avoided. The results may provide a new angle for the studies on the coal pillar layout and stability design in highwall mining.