Search

AbstractWith the increase in coal mining depth, many rock bursts triggered by fold structures have been observed, but the disaster mechanism is not clear. This research aims at investigating the influence of upright fold structure(UFS) on rock burst mechanisms. The mechanical model and numerical model of the UFS are constructed, and the mining stress evolution law and rock burst mechanism of working face in a UFS area are studied by utilizing engineering practice. The research shows that the occurrence of rock burst disasters in the fold structure area has obvious regional characteristics. The horizontal and vertical stress of the UFS in the horizontal distribution is similar to the periodic variation characteristics of the sine (cosine) curve. The stress state of the UFS area is divided into five areas. The horizontal and vertical stress concentration factors increase first and then decrease with the working face gradually approaching and away from the upright fold syncline axis. A roadway impact risk index I0 is proposed to determine whether the roadway is damaged, and the mechanism of inducing ground pressure during mining in UFS areas of deep mines is expounded. The control plan effectively reduces the stress levels of surrounding rock in working faces.

AbstractWith the increase of mining depth and mining intensity of the large inclined extra-thick coal seam (LIETCS), the number of rock bursts increases significantly. In this paper, based on a rock burst during upper roadway driving of the LIETCS, the distribution characteristics and fracture mechanism of microseismic (MS) events are analyzed using data statistics and seismic moment tensor inversion. By means of mechanical analysis and numerical simulation, this paper studies the static load distribution characteristics and dynamic load action effect of the LIETCS under upper slice mining condition. Based on the theory of dynamic and static combined load, the mechanism of rock bursts is proposed. The results show that the coal and rock mass of the LIETCS are mainly destroyed by shearing. The “shear-clamping” stress produced by the joint action of the roof and floor is the static load source. In upper slice mining, the shear stress and energy of the coal pillar behind the slice and the coal body below the coal pillar always remain high. Under the superposition of floor dynamic load, the regional stress environment changes, which induces rock bursts. This study provides a theoretical basis for rock burst prevention under similar conditions.

Dynamic disasters such as rockbursts have different characteristics with different geological structures. The rockburst mechanism and the influencing factors of a typical composite fault-fold structure were explored by theoretical analysis and numerical simulation. The results showed that: (1) the influence of the composite fault-fold structure on the roadway group rockburst was mainly caused by tectonic stress disturbance; (2) the stress peak value and the influence range of the hanging wall of the fault were larger than those in the footwall, and the stress of the syncline axis was larger than that of the anticline axis; (3) the tectonic stress of the composite fault-fold structure was the highest, the stress value reached to the peak when the fault dip angle was 65°, and the tectonic stress was directly proportional to the elastic modulus of coal and first increased and then decreased with the increase in coal seam thickness; and (4) before rockburst, seismic activities concentrated within 200 m from the footwall and 0–250 m in the hanging wall of the fault, mainly concentrated in 0–20 and 0–40 m, respectively. The spatial distribution coincided with the high static stress area. HighlightsThe asymmetry of stress and microseismic events of composite fault-fold structure reveals the asymmetry of rockburst potential. The rockburst potential of the hanging wall of the fault is greater than that of the footwall, and the rockburst potential of the syncline is greater than that of the anticlineBased on the analysis of stress characteristics and fault activation, it is revealed that the rockburst of roadway group is mainly affected by the static tectonic stress of composite fault-fold structure.Four influencing factors: structure type, fault dip angle, coal seam thickness, and coal elastic modulus are analysed, and the causes of stress concentration of composite fault-fold structure are revealed.

The variation of hard roof thickness is an essential contributor in triggering rock bursts during longwall mining. Case analysis and numerical modeling were used to study the stress and energy characteristics of the coal and rock mass and its fracture behaviour in the roof thickness variation zone (RTVZ). The results show that the coal seam has higher initial stress if overlain by a thicker hard roof, whose stress monitoring value is 1.8–2.6 times that of the thin zone. The increasing variation in the roof thickness or the roof properties causes a greater initial stress change in the coal seam. In the thick roof zones, the superposition of the advanced abutment pressure and the increased initial stress will result in a high-stress concentration area, where the stress mutation coefficient value can be up to 1.08–1.15. A higher rock burst risk might thus present in the roadway near the longwall in the thick roof zone, where more intensive elastic energy was released in the coal/rock mass. Also, it is more likely to have a significant dynamic load in the thin roof zone due to the higher possibility of roof breakage, and the total microseismic energy can reach 1.8–3.2E + 08J. HighlightsCase analysis and numerical modelling were used to study the formation mechanism of stress anomaly in coal seams, energy evolution characteristics of the coal/rock mass and its fracture behaviour in the RTVZ.The mechanism of rock bursts induced by coal mining in the RTVZ is determined. The thick roof zone has high-stress concentration, where more intensive elastic energy is released in the coal/rock mass. Due to easier roof breakage, it is more likely to have a significant dynamic load in the thin roof zone.The prevention and control method of rock bursts in the RTVZ is put forward. The rock bursts can be relieved by reducing the initial stress increased in the thick roof zone. Strengthening roadway support can reduce the influence of dynamic load on the roadway.

Seismic energy analysis is the basis of coal burst risk assessment. The prerequisite for utilizing seismic energy to quantify coal burst hazards is the accurate understanding of seismic energy attenuation. Thereby the characteristics of seismic energy attenuation were first systematically investigated in this paper. Based on the seismic data and 24 coal burst records during the period of sixteen months, a risk assessment method including three indexes (Static Intensity Index (SII), Dynamic Intensity Index (DII), and Risk Assessment Index (RAI)) was derived from seismic energy attenuation. In consideration of the static and dynamic response, the superposition effects of seismic energy were proposed to improve the performance of risk assessment. Results show that residual seismic energy (RSE) is relatively correlated with seismic energy, source radius, and energy attenuation coefficient; the higher seismic energy, source radius, and lower energy attenuation coefficient would result in higher coal burst risk. The case study demonstrated that SII could effectively predict the damaged area, but its prediction efficiency of high-magnitude events (HMEs) is relatively low; DII could predict damaged areas and HMEs, but may lead to risk overestimation; and RAI could efficiently predict the damaged area and HMEs. The sensitivity of the monitoring system and seismicity activeness could be the two main restrictions of the proposed method, leading to low efficiency or even false risk assessment. This paper sheds light on seismic energy utilization to risk prediction in underground coal mines. HighlightsThe attenuation response characteristics of seismic energy to different factors including seismic energy, source radius, and attenuation coefficient were investigated.A new assessment method based on seismic energy attenuation calculation was proposed to assess coal burst risks. Three seismic-based indexes considering static and dynamic response and their superposition effects were proposed to assess coal burst risk.Sensitivity of monitoring system and seismicity activeness could be the two main restrictions leading to low accuracy or even false in risk assessing, which implies that the proposed method could also have false judgements in the place where rare seismicity is monitored.