Backbreak is one of the undesirable phenomena in open-pit mines and causes several adverse hazards, such as lanslide, rock falling off and bench instability. Backbreak is influenced by many factors, such as rock properties, blasting design and local geology, so it is very difficult to assess and evaluate backbreak accurately. Therefore, controlling and accurate prediction of backbreak distance are crucial tasks to reduce hazards in open-pit mines. For this, soft computing-based techniques are considered to be an effective means, as they can integrate various sophisticated factors into a function to predict and evaluate backbreak distance. So, in this study, support vector regression (SVR) based techniques and three different types of bio-inspired meta-heuristic (BIMH) algorithms, such as chicken swarm optimization (CSO), whale optimization algorithm (WOA) and seagull optimization algorithm (SOA), are used to develop backbreak distance prediction models. The support vector regression is used as a regression tool and BIMH algorithms are used to optimize the hyper-parameters in the support vector regression. Four different types of evaluation metrics are utilized to assess the model performance, namely coefficient of determination (R2), mean square error (MSE), mean absolute error (MAE) and variance account for (VAF). An integrated evaluation system is adopted to provide overall performance for each backbreak prediction scenario. It can be indicated that CSO-SVR based backbreak prediction models can procure the best comprehensive performance and also show the best calculation efficiency. Detailed results include R2, VAF, MSE and MAE equal to 0.99475, 0.034, 99.477 and 0.1553 for a testing set and 0.97450, 0.1633, 97.466, and 0.1914 for a training set which can be said to be an excellent prediction result. By doing this, the hazard risk induced by backbreak can be indirectly assessed. In addition, it is also found that some superior performance can be obtained in some evaluation metrics compared with previous studies which utilized the same backbreak dataset for prediction.
Rockburst, characterized by a sudden and violent rock failure resulting in the expulsion of rock from its surroundings, poses a significant threat to the safety of tunnel excavation operations, often causing property damage and injuries to workers. Buckling has been identified as a critical mechanism leading to rockbursts. Seismic events or blasting can induce rockbursts when stress waves reach the free surface of underground openings. This paper aims to investigate the induced mechanism of tunnel rockbursts based on the dynamic buckling of rectangular rock plates. As a rock stress wave approaches a tunnel sidewall, it decomposes into perpendicular and parallel component loads relative to the free surface. The perpendicular stress reflects off the free surface, forming a rectangular thin plate of rock. The parallel stress triggers parametric resonance in the plate, resulting in a tunnel rockburst. An illustrative example of tunnel sidewall rockbursts in Jinping II hydropower project, China, is provided to study the effects of stress wave amplitude and frequency, static and dynamic components, rock damping, multiple frequencies, and vibration modes. Based on this mechanism analysis, recommendations are proposed to mitigate the risk of tunnel rockbursts. The research offers a plausible explanation for the heightened frequency and severity of rockbursts in Tunnel Boring Machine tunnels compared to New Austrian Tunneling Method tunnels at the Jinping II project for the first time.
Underground mining often faces the threat of roof fall disaster. As a new type of supporting technology, thin spray-on liner (TSL) has gained an increasing attention in underground mining due to its notable tensile strength, elongation capability, and bond strength with rock surfaces. To evaluate the roof fall support performance of TSL based on a novel ultra-high tensile-strength polyurea, tensile adhesive strength between polyurea and rock substrates were tested under different thicknesses, curing conditions, substrate strength, primer and coating method. Meanwhile, this study proposed a new testing method for dynamic tensile adhesive strength between TSL material and rock. The results indicate that the adhesive strength is inversely proportional to the square root of the coating thickness. When the curing time exceeds 7 days, the adhesive strength remains relatively constant. As the curing temperature/humidity increases, the adhesive strength gradually increases. But when the humidity exceeds 70%, the adhesive strength significantly decreases. Since the soft rock has the tensile strength that even lower than the adhesive strength, polyurea-based TSL is more suitable for harder rock from the perspective of adhesive strength. The application of a primer significantly improves the tensile adhesive strength more than 10 MPa. When the coating thickness is less than 2 mm, the adhesive strength of sprayed polyurea is significantly higher than that of brushed polyurea. Dynamic adhesive strength exhibits an insignificant loading rate effect with DIF ranging from 1.05 to 1.34. Based on the adhesion results, a supporting model was established, assessing the capability of supporting roof loose rock mass by polyurea-based TSL.
A brief overview of the basic principles of geomechanics of highly compressed rocks and masses is presented. The historical path of formation of this new scientific branch of the classical geomechanics is shown. The scales and structural levels of the geomedium failure are identified. The issues of adequate mathematical models at various geomedium structural levels developing, as well as methods for determining the parameters of these models are considered. The object, subject, methods and principles of geomechanics of highly compressed rocks and masses are formulated as a complex discipline at the intersection of classical geomechanics and mesomechanics.
Investigating rock damage behavior is crucial for understanding the formation mechanisms of fractured slopes in earthquake-prone areas. However, the current understanding of the nonlinear damage processes and mechanisms of rocks under cyclic loading is insufficient. This study investigated the damage behaviors of metamorphic sandstone, granite, and phyllite under cyclic loading using acoustic emission (AE), infrared thermal imaging, and digital image correlation (DIC) techniques. The experimental results demonstrated that the damage variables based on AE counts, infrared radiation temperature variance (IRTV), and surface deformation variance (SDV) increased with increasing cycles and stress levels. The temperature variation was influenced by lithology and the types of original pores and microcracks. The lag ratio and average lag time of the SDV effectively evaluated the progressive damage process. Specific damage mechanisms were identified, including the “compaction-embedment effect” in metamorphic sandstone, the “crystal incompatible deformation-fracture effect” in granite, and the “defective fracture effect” in phyllite.
The stability and management of waste dump slopes are pivotal research topics in geotechnical engineering and have long posed central challenges in coal mine ecological restoration. Controlling the stability of waste dump slopes is challenging due to the influence of time, rainfall, stacking height, and angle on the consolidation degrees of coal gangue. On the basis of those mentioned above, the study focuses on the waste dump slope of Dongpo Coal Mine as the primary research subject. A consolidation model is developed to dynamically adjust key parameters of coal gangue based on varying degrees of consolidation, burial depth, and stress. The results of coal gangue solidification experiments was used to further develop the constitutive model of FLAC3D software. The characteristic of coal gangue with varying consolidation degrees in response to burial depth and stress variations can be realized. The stability of the waste dump slope under different slope angles, continuous rainfall, and various support measures were simulated. The threshold values for slope angle and rainfall duration that precipitate slope failure were determined. The relationship of slope angle (θ), rainfall duration (t), and slope safety factor (Fs) was elucidated and a comprehensive comparative analysis of four prevalent slope support measures was performed. The research findings indicate that with an increase in slope angle, the slope safety factor demonstrates an inversely proportional trend of decline, suggesting that the slope angle of Dongpo Coal Mine waste dump should be reduced from the original 42°-33°. At a slope angle of 33°, the slope reaches the instability threshold after five days of continuous rainfall, with the slope safety factor steadily decreasing and stabilizing as the rainfall duration prolongs. Finally, a combined support measure of three-dimensional grid planting and slope protection piles for waste dump slopes were proposed, which has yielded excellent results in field applications. This research offers valuable insights for the evaluation and management of waste dump slope stability across different consolidation degrees.