There have many open pits in China threatened by cavities under open pit benches because of former unreasonable underground mining.Shape and size of these cavities were greatly changed through many years mining activity.There have no clear figures or data to show how these cavities occurred.This would greatly restrict high-efficient and safe mining.Thus,the reasonable safety isolation layer thickness of cavity should be properly calculated,and the feasible stability monitoring method for this kind of cavity should be employed to well ensure safety of personnel and equipments on open pit benches.Based on the present situation of Sandaozhuang open pit of Luoyang Luanchuan Molybdenum Group Inc.,which has lots of cavities under its benches,the safety isolation layer thickness corresponding to different roof spans,which can provide a primary evaluation for cavity stability,was obtained through numerical modeling.Acoustic emission technique was used to monitor the cavity stability.Monitoring data of four types of cavity,i.e.unstable cavity,stable cavity,cavity with drill and cavity after blasting,which are all the typical ones under open pit benches,were listed.The signals of acoustic emission of those cavities were analyzed to obtain acoustic emission characteristics of typical cavities under open pit benches.This will provide reference for following accurate stability evaluation of cavities under open pit benches that are monitored by acoustic emission technique.Thereupon,the efficient safety guarantee system of open pit benches with cavity would be established through primary evaluation according to safety isolation layer and stability monitoring by the acoustic emission technique.
Abstract Slabbing failure and strain rock burst are the main failure patterns during the excavation and construction phases of deep tunnels in hard and brittle rock, which cause unexpected equipment damage and casualties. This study presents a numerical simulation with an unloading central hole within a hard rock specimen in the laboratory scale. A combined finite element/discrete element method (FEM/DEM) reproduces the crack initiation, propagation, and coalescence around the central hole during the entire failure process. The influence of sectional shapes, unloading rates of holes, and in situ stresses on the failure characteristics and mechanical response of typical hard and brittle rocks were investigated by analysis of the failure pattern, displacement distribution (average velocity) of monitoring elements, and released strain energy value. The numerical results indicate that the sectional shapes, unloading rates, and in situ stresses have a significant impact on the severity of destruction and failure range in hard rock under the excavation unloading conditions. Slabbing failure (stable failure) is always the dominant failure pattern around a circular hole, which shows higher bearing capacities and self‐stabilization. With the increase in unloading rates, more visible cracks are generated around the holes, and the displacement and average velocity of discrete blocks are further raised. This is particularly evident in rock specimens with holes having vertical walls, leading to intense unstable failure (strain rock burst) accompanied by a large amount of strain energy released. In situ stresses affect considerably the stability of the surrounding rock during the excavation unloading process. With constant in situ stress, the order of destruction severity around the central hole according to its sectional shape is cube > trapezoid > U‐shape > ellipse > circle. The destruction intensity is further aggravated with the increase in lateral pressure coefficient, and the failure regions are always observed in the roof and floor of the central hole. This study confirms that strain rock burst tends to be induced in hard and brittle rock tunnels with polygonal roadway section under high unloading rates and lateral pressure coefficients.
The validity of the dynamic tensile strength in rock materials is problematic in dynamic Brazilian splitting tests. The purpose of this study is to understand the failure law and effectiveness of the dynamic Brazilian disc (BD) test for five different rock types under three typical loading modes, including platform loading (mode-I), steel bar loading (mode-II) and arc loading (mode-III). Based on the digital image correlation (DIC) technique and high-speed camera, the deformation localization and rupture evolution of the BD specimen under impact loading were obtained, and two rupture characteristics of the specimen were summarized. The loading mode significantly affects the crack initiation location of the disc in the dynamic BD test. The experimental results show that loading mode-III is the most consistent with Brazilian test theory in the dynamic BD test, while loading mode-II violates the test principle. At the same incident energy, the loading rate and dynamic tensile strength are functions of loading mode and lithology, which are highest in loading mode-III, followed by loading mode-I, and the lowest in loading mode-II. The strain and displacement field after peak load were extracted by DIC, and it was found that there were four typical failure modes in dynamic BD tests.
The occurrence of rockburst can cause significant disasters in underground rock engineering. It is crucial to predict and prevent rockburst in deep tunnels and mines. In this paper, the deficiencies of ensemble learning algorithms in rockburst prediction were investigated. Aiming at these shortages, a novel machine learning model, deep forest, was proposed to predict rockburst risk. The deep forest combines the characteristics of deep learning and ensemble models, which can solve complex problems. To develop the deep forest model for rockburst prediction, 329 real rockburst cases were collected to build a comprehensive database for intelligent analysis. Bayesian optimization was proposed to tune the hyperparameters of the deep forest. As a result, the deep forest model achieved 100% training accuracy and 92.4% testing accuracy, and it has more outstanding capability to forecast rockburst disasters compared to other widely used models (i.e., random forest, boosting tree models, neural network, support vector machine, etc.). The results of sensitivity analysis revealed the impact of variables on rockburst levels and the applicability of deep forest with a few input parameters. Eventually, real cases of rockburst in two gold mines, China, were used for validation purposes while the needed data sets were prepared by field observations and laboratory tests. The promoting results of the developed model during the validation phase confirm that it can be used with a high level of accuracy by practicing engineers for predicting rockburst occurrences.
Ore bodies incubating within fault zones are a common phenomenon in geological strata and pose a huge challenge for underground mining. To effectively exploit mineral resources, the layout of the mining roadway and the interaction between the roadway and geological fault must be considered. In this paper, a bonding fracture was formed on granite samples to simulate a closed fault, under which a circular hole was fabricated to simulate the roadway of the gold mine. We performed a biaxial compression test at a true-triaxial electrohydraulic servo testing system for granite samples with a combined fracture-hole structure. It is worth noting that the fracture inclination β and relative distance between fracture and hole L were taken into account. The digital image correlation (DIC) technique was used to observe the displacement and strain field evolution around the fracture-hole structure. Our results demonstrate that (1) the strength of the granite sample decreases with increasing bonding fracture dip angle β, and the displacement drops between the hanging wall and foot wall raised in both the horizontal and vertical displacement directions. Macroscopic cracks become dense, and the failure degree becomes severe around simulated fault areas. (2) With the increase in the distance L, the strength of the granite sample increases, the influence of the hole on the slip of the fracture plane is weakened, and the discontinuity of displacement becomes less obvious. (3) The maximum principal strain field quantitatively reveals the details of the crack initiation, propagation, and coalescence around the fracture-hole structure, and displacement nucleation is observed in the vertical displacement field.
