Deep Underground Coal Mines Research Articles

Effect of faulting on coal burst – A numerical modelling study

Coal burst occurrence on roadways has always been a major concern in deep underground coal mines, especially under complex geological conditions. To evaluate the effect of faulting on coal burst, the stress concentration in the vicinity a reverse fault was analysed considering the geological history of the fault formation where high horizontal stresses led to the initiation and propagation of the reverse fault. Various in situ stresses and mechanical parameters of the fault, including the ratio of horizontal stress to vertical stress were used to analyse the state of fault. Numerical modelling was conducted using two and three dimensional distinct element models (UDEC and 3DEC) based on a geotechnical conditions of an Australian underground coal mine. The formation process of reverse fault was simulated to evaluate the stress characteristics in the coal seam and the immediate roof and floor near the fault. The results show that, both the horizontal and vertical stress in footwall were higher than those in hanging wall after the formation of the reverse fault. The stress condition near fault was complicated due to complex geology in the coal measures, and the vertical stress peaked in the footwall at a distance of about 160 m from the fault. When a roadway was excavated, stress concentration occurred at both the roadway face and ribs, which reached as high as 38 MPa in the ribs at a depth of 500 m. This will significantly elevate the risk of dynamic instability of the roadway such as coal burst. The stress concentration zone in the footwall can be considered as a hazardous zone near the reverse fault. This study provides a general reference for analysis of roadway stability affected by faults.

A Numerical Study of Stress Distribution and Fracture Development above a Protective Coal Seam in Longwall Mining

Coal and gas outbursts are serious safety concerns in the Chinese coal industry. Mining of the upper or lower protective coal seams has been widely used to minimize this problem. This paper presents new findings from longwall mining-induced fractures, stress distribution changes in roof strata, strata movement and gas flow dynamics after the lower protective coal seam is extracted in a deep underground coal mine in Jincheng, China. Two Flac3D models with varying gob loading characteristics as a function of face advance were analyzed to assess the effect of gob behavior on stress relief in the protected coal seam. The gob behavior in the models is incorporated by applying variable force to the floor and roof behind the longwall face to simulate gob loading characteristics in the field. The influence of mining height on the stress-relief in protected coal seam is also incorporated. The stress relief coefficient and relief angle were introduced as two essential parameters to evaluate the stress relief effect in different regions of protected coal seam. The results showed that the rock mass above the protective coal seam can be divided into five zones in the horizontal direction, i.e. pre-mining zone, compression zone, expansion zone, recovery zone and re-compacted zone. The volume expansion or the dilation zone with high gas concentration is the best location to drill boreholes for gas drainage in both the protected coal seam and the protective coal seam. The research results are helpful to understand the gas flow mechanism around the coal seam and guide industry people to optimize borehole layouts in order to eliminate the coal and gas outburst hazard. The gas drainage programs are provided in the final section.

Design of multi-layer coupling support and span of setup entry roof at depth

The setup entry is driven at the starting line of a longwall panel and used to set up equipment, from which mining activities begin. The roof stability of setup entry roof with large span is a major concern in deep underground coal mines, especially when the surrounding strata are weak and fractured. Large-span setup entry roofs at depth induce a serious danger of roof fall and difficulty in the determination of the rational span. We determined a reasonable span for such a situation by studying the 1105 large mining-height coal face setup entry of Zhaogu No. 2 coal mine. A multi-layer coupling support method, led by long-extension bolt and medium length cable, was proposed. The setup entry was designed for four layers of coupling support. The setup entry’s reasonable span was determined and the reliability of the coupling support system was analyzed. These measurements allowed us to set up separate roof depth-displacement monitors and bolt (cable)-condition sensors in the mined area and enabled us to remove secondary instrumentation. The monitoring results show that unstable setup entry roof collapse occurs as roof-fractionated instantaneous fracture within 0 to 6–10 m. The size of the hanging roof varies between 11.7 and 13.6 m at the first fracture, and the hanging roof varies between 13.0 and 14.0 m at the time of the second fracture. For complete roof instability, the reasonable span should be ~ 12.4 m. The deformation of each horizon’s roof and the roof fall achieve effective control in the anchorage range before roof fracture, and the support effect of the core level is significant with a long-extension bolt (20-mm diameter × 5000 mm) and cable (21.6-mm diameter × 12,000 mm). The roof’s rupture depth did not develop further, which indicates that the multi-layer coupling support can adapt to control the large-span roof of setup entry at depth.

Failure mechanism of surrounding rock with high stress and confined concrete support system

Surrounding rock control is a big challenge in roadway with high stress in deep underground engineering. To address this issue, Zhaolou coal mine – a typical kilometer deep well is taken as engineering background to monitor and analyze the original support scheme. The analysis shows that the following main reasons lead to the support failure in roadway with high stress in deep underground coal mine. Roadway deformation is large and lasts longer with a large extent of damage; all anchor bolts are in severely damaged surrounding rock zone without reliable adhesive basis; and U-shaped steel arch unevenly contacts with surrounding rock without sufficient bearing capacity. With summary of support failure mechanisms on-site, a concept of “high-strength and integrity support” is put forward; a support system with square steel confined concrete (SQCC) is developed; and a full scale in-door comparative test is conducted on its core component – SQCC arch and also on the conventional U29 arch. The research shows the bent and damaged legs of U29 arch result in dramatic reduction of bearing capacity of arch. However, the SQCC arch with similar steel content has a bearing capacity of 2.15 times of that of U29 arch and still has a higher post bearing capacity even after the arch is deformed dramatically. Quantitative evaluation index is established such as vault displacement control rate and bolt strength utilization rate; comparative analysis is made on surrounding rock control mechanism and the mechanical properties of support components under the influence from the four factors – arch strength, geostress, lateral pressure coefficient and surrounding rock mechanical parameters; and good control effect on surrounding rock of SQCC support is verified. Comparative tests are conducted on site. The monitoring results show the roadway surrounding rock deformation on the test section of SQCC support is only 21.2% of that on test section of U29 arch support; and no yield and damage occurs on the test section of SQCC support. Therefore, the SQCC support system is proved to be effective on surrounding rock control.

Optimal pre-conditioning and support designs of floor heave in deep roadways

In order to reduce deformation of roadway floor heave in deep underground soft rockmass, four support design patterns were analyzed using the Fast Lagrangian Analysis of Continua (FLAC)3D, including the traditional bolting (Design 1), the bolting with the backbreak in floor (Design 2), the full anchorage bolting with the backbreak in floor (Design 3) and the full anchorage bolting with the bolt-grouting backbreak in floor (Design 4). Results show that the design pattern 4, the full anchorage bolting with the bolt-grouting backbreak in floor, was the best one to reduce the deformation and failure of the roadway, the floor deformation was reduced at 88.38% than the design 1, and these parameters, maximum vertical stress, maximum horizontal displacement and maximum horizontal stress, were greater than 1.69%, 5.96% and 9.97%. However, it was perfectly acceptable with the floor heave results. The optimized design pattern 4 provided a meaningful and reliable support for the roadway in deep underground coal mine.

Wireless real-time sensing platform using vibrating wire-based geotechnical sensor for underground coal mines

Continuous, real-time and remote measurement of stress in underground coal mines plays a vital role in the safety of the mines. Vibrating wire-based geotechnical sensors are widely used for stress measurement of strata in underground coal mines. These types of sensors have several advantages over other geotechnical sensors. However, the measurements from these sensors are taken in-situ using conventional readout unit manually till date even in the case of deep underground coal mines where the depth of cover is more than 300m. When sensor site moves to inaccessible places due to dynamic nature of mining, then the measurement is discontinued and instruments become abandoned. Also, sensors connected through wire get disconnected during movement of machinery, roof falls, and other associated hazards. Therefore, there is a need for wireless sensing platform for vibrating wire-based geotechnical sensors which can assess the stress on a real-time basis and obviate the impending mishap associated with the strata failure. Hence, in the present paper, authors have designed a prototype wireless real-time sensing platform for vibrating wire-based geotechnical sensors for continuous, real-time and remote monitoring of strata behavior. The laboratory trial and the field demonstration of the same have been carried out and compared with conventional readout. The results are in agreement.

Meso-characterization of the effective thermal conductivity of selected typical geomaterials in an underground coal mine

To comprehensively understand the bulk macro conduction characteristics of typical geomaterials existing in deep underground coal mines, i.e. coal, rock, and concrete (shotcrete), meso-scale composition (solid mineral and fluids such as water and air) and structure (particle and pore size, shape, and contact form) analyses were conducted via water absorbability and thermal conductivity measurements, scanning electron microscope, and X-ray diffraction technologies. The results indicate that coal, rock, and concrete's respective meso-scale composition and structures have different degrees of influence on their macro bulk effective thermal conductivity at various conditions. First, the fluid (water and air) composition significantly impacts thermal conductivity at a certain bulk effective porosity: coal < rock < concrete. The ranking of thermal conductivity shows two different forms: concrete < coal < rock in a dry state and coal < concrete < rock in a water-saturated state. Second, the surface morphology and face porosity obtained by scanning electron microscope imaging can explain thermal conductivity fluctuations at various moisture contents and magnification times. The face porosity as well as its quadrant anisotropy degree can also be achieved via scanning electron microscope digital image processing technology, and both of them have a certain relationship with magnification times (scale effect) and effective volumetric porosity, as obtained by water absorbability experiments. In addition, apart from the fluid composition, the mineral composition (species and content of mineral) of the solid skeleton or matrix also significantly influences the macro-scale thermal conductivity of geomaterials. Furthermore, a quantitative comparison analysis can be performed by X-ray diffraction, but further research is needed to improve its accuracy. This study's detailed meso-characterization of the macro effective thermal conductivity for typical geomaterials in an underground coal mine may be of reference value to establish the relationship between different properties of differing scales.

Application of the retained gob-side gateroad in a deep underground coalmine

Zhuji coalmine, a typical deep underground coalmine located in Huainan city, Anhui province of China, is characterised by rich gas, high in situ stress and low permeability. These geotechnical conditions cause serious problems including outburst, excessive gas emissions in the upper corner of the coal face, and low resource recovery resulting from large remaining coal pillars when using conventional mining methods. Retained gob-side gateroad (RGSG) is proposed to resolve these problems by eliminating the large coal pillars and extracting gas simultaneously. To achieve a scientific and reasonable application of the RGSG in the Zhuji coalmine, stress evolution in the surrounding rock of this roadway was studied using FLAC3D numerical simulation method. Then, a staged supporting strategy and corresponding parameters were proposed in terms of the characteristics of stress evolution. Finally, the effectiveness of the proposed method was discussed based on the field measurements and roadway maintenance was carried out for reuse during the adjacent panel excavation.

Application of a combined support system to the weak floor reinforcement in deep underground coal mine

More and more coal mines begin to exploit deep resource, leading to more serious supporting problems. Common supporting measures for shallow roadways can hardly apply to the deep geological conditions, which are mainly reflected by high ground pressure, more broken and weak surrounding rock. This paper takes a winch room in China׳s Gubei coal mine as an example to study a new kind of combined supporting system for weak floor under adverse geological conditions. The stability of the winch room is simulated by FLAC3D. The deformation characteristics and stress distribution of the surrounding rock is obtained. Then, a new kind of combined supporting system as well as monitoring measures for the floor is proposed. A new type of hollow grouting cable which has many advantages is introduced in the supporting system. The results show that the combined supporting system is satisfactory.

Coal Mine Roadway Stability in Soft Rock: A Case Study

Roadway instability has always been a major concern in deep underground coal mines where the surrounding rock strata and coal seams are weak and the in situ stresses are high. Under the high overburden and tectonic stresses, roadways could collapse or experience excessive deformation, which not only endangers mining personnel but could also reduce the functionality of the roadway and halt production. This paper describes a case study on the stability of roadways in an underground coal mine in Shanxi Province, China. The mine was using a longwall method to extract coal at a depth of approximately 350 m. Both the coal seam and surrounding rock strata were extremely weak and vulnerable to weathering. Large roadway deformation and severe roadway instabilities had been experienced in the past, hence, an investigation of the roadway failure mechanism and new support designs were needed. This study started with an in situ stress measurement programme to determine the stress orientation and magnitude in the mine. It was found that the major horizontal stress was more than twice the vertical stress in the East–West direction, perpendicular to the gateroads of the longwall panel. The high horizontal stresses and low strength of coal and surrounding rock strata were the main causes of roadway instabilities. Detailed numerical modeling was conducted to evaluate the roadway stability and deformation under different roof support scenarios. Based on the modeling results, a new roadway support design was proposed, which included an optimal cable/bolt arrangement, full length grouting, and high pre-tensioning of bolts and cables. It was expected the new design could reduce the roadway deformation by 50 %. A field experiment using the new support design was carried out by the mine in a 100 m long roadway section. Detailed extensometry and stress monitorings were conducted in the experimental roadway section as well as sections using the old support design. The experimental section produced a much better roadway profile than the previous roadway sections. The monitoring data indicated that the roadway deformation in the experimental section was at least 40–50 % less than the previous sections. This case study demonstrated that through careful investigation and optimal support design, roadway stability in soft rock conditions can be significantly improved.

Safety technologies for the excavation of coal and gas outburst-prone coal seams in deep shafts

An analysis of the stress distribution characteristics of shaft working faces in deep underground coal mines was performed using theoretical analyses and field tests. The analysis of the outburst risk within the studied shaft demonstrated that a funnel-shaped pressure relief zone existed below the working face, and a zone of concentrated stress surrounding the shaft was a critical area for a potential outburst disaster. We then divided the process of coal excavation within the shaft into four stages to manage and propose improved technical, drainage and protective measures as well as other requirements. When using the processes and requirements provided, the Haizi and Taoyuan coal mines, which are located in the Huaibei mining region and have depths of up to 1000m, realized safe and rapid excavation through outburst-prone coal seams. It was determined that the technical systems developed in this paper could provide for safe coal excavation in deep shafts, which could be established as new regulations for rapid and safe coal extraction in outburst-prone coal seams.

Mining-induced strata stress changes, fractures and gas flow dynamics in multi-seam longwall mining

This paper presents key findings from a recent comprehensive study of longwall mining-induced strata movement, stress changes, fractures, and gas flow dynamics in a deep underground coal mine in Anhui, China. The study includes field monitoring of overburden displacement, stress and water pressure changes at the longwall panel 1115 (1) of the Guqiao Mine. In addition, 3D modelling of strata behaviour at the longwall panel using a 3D finite element code and goaf gas flow simulations with a CFD code are carried out. This research has resulted in many new insights into the complex dynamic interaction between mining induced strata stress changes, fractures, and gas flow patterns. Based on the findings from the field monitoring and numerical modelling, a three-dimensional annular-shaped overlying zone along the perimeter of the longwall panel is identified for optimal methane drainage during mining. A practical method that helps define the geometry and boundary of this zone is proposed. This study provides a new methodology and a set of engineering principles for the design of optimal co-extraction of coal and methane.

Rock mechanical analysis of a Ml= 4.0 seismic event induced by mining in the Saar District, Germany

SUMMARY On 2008 February 23 a 4.0 magnitude seismic event occurred in close proximity to a deep underground longwall coal mine in the German Saar mining district. Preceding this event was a series of smaller seismic events, which can be traced back to geological structures. A seismic network was installed to observe and analyse the seismic events. Underground core drilling was performed to obtain rock samples for rock mechanical testing. Possible causes for the events, failure of rock mass or slip on unknown faults, are evaluated by numerical methods. Failure of rock mass is excluded as a cause since the stresses are well below the strength of the rock mass. Seismic events are of similar fault plane strike and are organized in several clusters, associated with local and regional tectonic elements. It was found from backcalculation that fault planes with friction angles as low as 8 ◦ must have been present. The role of pore pressure on the fault stability is discussed and found to play no role in generating the seismic events. Finally, the question of mining in a rock mass obviously close to failure is addressed and it was concluded that any mining activity would lead to a significant seismic response.

Distribution Laws of in-Situ Stress in Deep Underground Coal Mines

Abstract With depth and intensity of coal mining increasing, effects of in-situ stress on deformation and failure of the surrounding rock are increasingly growing. Through data independently measured and collected within 600-1500m, this paper summaries distribution laws of in-situ stress in deep underground coal mines of our country, main existing types of in-situ stress and its directional characteristics. Maximum horizontal stress, minimum horizontal stress, vertical stress and lateral pressure ratio variation laws with depth are obtained by using regression analysis. Generally, in-situ stresses increase with depth. With increasing depth the rate of increase in horizontal stresses decreases. A considerable scatter in the in-situ stress test data may be due to distinct differences in both the strength and deformation moduli of strata located in varying geological environments and different coal districts. Two types of stress field distribution have been noted with σHvh (σHmax>σv>σhmin) and σvHh(σv>σHmax>σhmin) found mainly in deep coal mines. The ratio of the maximum horizontal principal stresses to vertical stress is usually between 0.63 and 2.42 in the deep coal districts.

Models of methane behaviour in auxiliary ventilation of underground coal mining

Abstract Studies have been conducted in a roadway excavated in a coalbed of a deep underground coal mine located in Northern Spain (Hullera Vasco Leonesa SA), by both airflow velocity and methane concentration measurements. Modelling of ventilation and methane behaviour of that roadway by Computational Fluid Dynamics (CFD), through software Ansys CFX 10.0, is presented. A good correlation was found between the experimental data and the results predicted by CFD. Then a comparison of the results obtained by conventional methods with the results obtained by CFD 4D modelling, for the auxiliary ventilation in a dead-end roadway, was made. Although conventional methods give satisfactory results when considering methane dilution, this research proves that there may be some roadway zones in which methane concentration is higher than regulation values. This is owing to the airflow and methane behaviour in the roadway with the presence of dead zones, and the fact that only a part of the airflow provided by the fan reaches the working face. CFD modelling allows us to know in which zones of the roadway it may be necessary to reinforce forcing ventilation by additional systems, such as a forcing system with exhaust overlap, jet fans, spray fan systems or compressed air injectors.

Bursting liability indices of coal

Natural coal's ability to store and release rapidly elastic strain energy seems to be a fundamental condition for the occurrence of rock-bursts in deep underground coal mines. In Upper Silesia where a number of existing mines work below a critical depth, several indices have been proposed and their numerical values found by experiment in order to classify the potential liability of coal seams to create rock-burst hazards. Three of these indices are most popular, namely the Strain Energy Storage Index ( W ET ), the Bursting Efficiency Ratio (η), and the Rheologic Ratio (ϑ). The first is defined as the proportion of strain energy retained to that dissipated during a single loading-unloading cycle of uniaxial compression. The second is the ratio of the kinetic energy of chips thrown out on failure to the maximum elastic strain energy. The Rheologic Ratio is calculated as a dynamic resistance stress rate (failure velocity) divided by an average stress relaxation rate. W ET values may be obtained through laboratory tests or by direct (double-hole method), or indirect (rebound method) in-situ evaluations, while both η and ϑ values are obtainable in laboratory solely. Each of the indices quoted leads to classifications and/or criteria for the bursting liability of coal. When complete predictions of the rock-burst hazard for a specific location in a mine are to be made, data on in-situ stress concentrations must also be collected. Since direct stress measurements in coal are hard to recommend as a routine procedure in mines, a number of indirect methods are currently being practiced, drilling yield and seismic wave velocity methods giving the most reliable results.