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Coal Burst Mechanism and Modelling - Mining Engineering Assessment Answers

November 14, 2017
Author : Julia Miles

Solution Code: 1DAA

Question:Mining Engineering

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The project title is "Coal burst mechanism and modelling"

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Solution:Coal Burst in Mining

  • Introduction

Coal Bumps, Shock Bumps, Coal burst or a Rock burst are same phenomenon called by different names earthquake in different regions. These phenomenon are the major challenges being faced by mining industries since its inception. In other words they can be under stood as artificial earthquake caused due to human activities. Mining excavation leads to seismic activities, coal burst and shock bumps and their frequency increases as the depth of excavation increases.

The process involves violent and sudden ejection of rock or coal and always occurs without any indication. (Mark, 2016). The source of generation of such stresses are still not understood properly and number of processes have been used to predict bumps.

  • Rock burst Accidents

The first rock burst dates back to 1900 in India, where Kolar gold mine suffered a huge loss due to the bump. In 1904 the Atlanta Copper mine suffered severe damages that led to its closure. Other rock burst included in USA and Canada followed the same suite. It was a time in 1975 in South Africa when around 73 workers were killed in 680 rock burst events in over 31 gold a span of nine years from 1984-1983, total number of deaths recorded were about 3275 all due to rock burst in gold mines in South Africa. Recently, in 2014 Auster Coal mine located in Hunter Valley NSW, a major rib burst took place in the mines. The accident took place at about 555m deep inside the mine where the roof cutter of coal seam was subjected to a higher stress. The construction of roof included support from ribs, steel wire mesh and bolts. (orr, 2014)

  • Factors involved

Fundamental factors responsible for such hazards has wide variety while some of them can be discussed below. As there can be a number of factors actually responsible for the coal burst which cannot be listed, but major factors responsible can be discussed. Some of the factors responsible can be stated as stiffness of strata, barrier size and shape, abutment stress and Strata’s caving characteristics. Some of the factors are

  • Vertical Stresses

It is the basic factor which triggers both dynamic and static stress components. Whenever the pillar is overburdened and is greater than 300m, it increases the chances of occurrence of coal burst sequences. The removing of supports from the excavated mines further adds on to increase the stress concentration. As the pillar stands from time, the strength of the pillar gets compromised resulting in increased stress factors. The extension of coal burst also relies on the pillars which are overburdened with more than 500m deep. However in every case it can be understood that the as the stress level is elevated they trigger the coal burst and sometimes becomes more violent. (Iannacchione Anthony T, 2016)

  • Lack of Appropriate size pillars while extracting

Barrier pillars which are accurately designed as per the requirement can hold the coal seam and the roof easily. There are many processes which can design this case such that the stress induced in minimum. There are works done on to design the problem using some empirical formulas. The sizing of barrier pillars depends on overburden and the thickness of the coalbed. Another major work done by Ashley represented a design protocol for retreat mining in the pillars. The design demonstrated in the papers are carried out generally for those barriers which were prone to stress conditions and rock bursts.

  • Stiff and Huge Strata

Burst occurrences can also be accounted to the strata strength surrounding the coal bed as they have high compressive and burst strength. When a mining has roof and floor made up of sandstone they always get connected with the rock burst. It can be better understood with an example which states that wherever either the young modulus ratio for roof to coal or floor to coal is more than 8 or floor to coal ratio (UCS) is greater than 4 the rock burst chances are maximum. Softer strata allows the deformation and fails very easily under any stress conditions whereas the stronger strata in turn becomes much stronger for the pillars to carry their load and bursting happens. Hence the strata must be stiff neither strong nor softer. Such strata which are suitable for mining are always defined with the help of its rock property strength and the rock layer thickness.

  • Abnormal caving of strata

The rocks that forms the roof for mining have always some discontinuity which helps in propagation of burst. Hence the characteristics of strata’s caving are also important and must be taken under consideration. When the pillars near the caving having such cracks are removed, they either propagate the crack due to higher induced stress for a long time, or puts an additional stress on the nearby pillar. The loads on the pillars can be relieved when the barrier pillar are allowed to push into the floor of the coal bed, as it relieves the stress and allows the stresses to be carried away to the floor. Non -yielding and Cantilevered rocks can accelerate the changes of rock burst.

  • Critical Size of pillar

When a study was carried out a number of pillars, they were found prone to the burst as their size was more than the critical value. The width to height ratio matters in such cases. If the ratio is less than 4 they are less prone to any violent rock burst. Any way the large barrier pillars can have width to height ratio greater than 20 and retain a stable core of pillar. These pillars are used as abutment pillars at the end of mines and are always thought to great stress absorbers. Such pillars can fail when the conditions of floor and roof bed shows different characteristics increasing the chances of violent rock burst. Their design matters because they normally can handle all other violent rock burst by transferring stresses to the nearby pillars. The critical sized pillars which can easily release strains have their width to height ratio in between 7 to 15. They can also accelerate the chances of failure when any rock burst happens in proximity. (Abthony T. Innacchione, 1994)

A number of researches were done to find the cause and remedy for the coal burst issues happening around the coal mines. Some articles presented ways to control the stress concentration while some focused on specific types of problems. The empirical relations were generated on the basis of seismic energy and stress concentration. Some predictive and forecasting methods were also advised by some other researchers. The literature review section studies some of the basic findings in the below section.

  • Literature Review

  • Destress Blasting

Destress blasting can be adopted for coal seams, floor rocks in order to manage any coal bump or coal heave. As stated by Petr Konicek in his state of the art review done on “Destress Blasting in coal mining” (Petr Konicek, 2011), the mining condition are some major part that influence stress concentration in rock mass. These condition can be listed as protective pillars in coal seams, advance mining rates, extraction thickness, unmined part lying overseas, size of the openings, advance direction of stress in overlying years in different parts. Mining within a number of coal seams lying in a range of 3m to 100m deteriorate the case further.

The main objective of destress blasting is to shift the excessive stress induced in the rock mass and provide a protective barrier. They can be used for avoiding floor heave, rock burst and the cutter roof failure.. Stress blasting (as mentioned by Petr Konicek in his de-stress blasting review) can be used as a safety tool for mining as there rises the increase of production rate by going more deep mines. Their careful consideration in regard of designing a new rock blasting pattern may become a necessary tool.

Destress blasting softens the rock layers by reducing their elasticity modulus and thus the extra induced stress releases. (Petr Konicek, 2011). The dilution of blast fumes induced determines the waiting time after destressing of rocks.

The number of fired boreholes, its diameter, length of charge, their location and spacing, total explosive charge in a well-designed destress rock blasting reduces the deformation and strength properties of rock mass which in turn leads to shifting of high stress concentrated region inside the rock farther. Effective Evaluation is done for destress blasting using a number of approaches. Like in a case of cutter failure, visual observation is the effectiveness evaluation approach. Monitoring in pre and post blasting for stress parameters have also been attempted for all type of mines for effectiveness.. (Toper AZ, 1994) (Konicek P, 2011)

The seismic effect calculation gives us the effectiveness of de (Konicek, 2009) (al, 1983 and 1985). The ratio of seismic energy released to the considered energy of particular charge gives the seismic effect of rock blast destress.

SE= ESeisK.Q. , where Eseis = Seismic Energy, Q = explosive charge weight, K = 2.1

They can be related as blasting can be calculated and is further classified as excellent, good, very good and insignificant. Geophysical methods to characterize destressing effects has also been tried in past. These methods have further helped in developing different empirical method to measure destress blasting effectiveness. (Andrieux PP, 2003). Andrieux presented the empirical methods and estimated it on a 0-1 scale after taking a rating of nine various operational and geotechnical parameters to analyze the total effectiveness of destressing method done on large scale. These practices are being followed in many countries like China, South Africa, Poland, and Czech Republic.

  • Directional Hydraulic Fracturing method used

One of the factor identified that can be main factor which induces rock burst is hard roof (Fan Jun, 2012) Fan Jun et al. discussed the control of hard roof rock burst using Directional Hydraulic Fracturing in his article in 2012. Water injection and deep hole blasting are some of traditional methods followed to deal with hard roof issues and had inherent defect (Yin DJ, 2001) (baneji A, 2004).the theoretical analysis and the numerical simulation was performed on the mechanism of directional hydraulic fracturing technology to prevent rock burst. The process was tested and optimized for all the procedures and following conclusions were drawn out

  • After using the method, the impacting effect on coal mass was weakened after the main roof fractured. The released elastic energy in total along with the released kinetic energy decreased significantly.
  • The process was divided into three section accordingly. Dramatically pressure-ascending stage for crack initiation, rapidly pressure-ascending stage for crack propagation and pressure levelling –off phase for communication process, respectively.
  • Sufficient emulsion is supplied and the drill holes are sealed whenever the pressure of pumping station is meeting the required parameters. It leads to the cracking of hard roof in the designed direction. The technology verified the automation and its effectiveness as it verified the fracture radius.
  • The directional hydraulic fracturing reduced the rock burst danger and notably reduced the abutment pressure and hence can be used as preventive method for rock burst situation.

  • Energy Equilibrium Studies

The article by Wang Jiong and Yan Yubiao discusses on Mechanism of energy limit equilibrium of rock burst. (Wang Jiong, 2011) As the mining depth increases, the phenomenon of rock burst in coal mining increases making the process more complicated. There were many theories which studied various mechanisms to control the rockburst process but there were hardly any reasonable explanation which could explain the whole mechanism. Wang in his paper tried to study the energy limit equilibrium or ELE. The coal seam was divided into two parts the Elastic zone and ELE zone. The positions of such rock burst were determined later in both roof cutters and floor seams. (Wang Jiong, 2011) Jiang et al. concluded his research with a difference in energy equation that gave a relationship between limit width of occurrence and ELE zone mechanisms.

Further conclusion drawn from his study can be listed as follows.

  • The energy source for any roof-coal-floor system lies at the top and bottom of the seam where the two energy zone is formed.
  • Before the Energy Limit Equilibrium zone is less than a particular value of limit width the rock burst phenomenon in coal mines occur. It further tells that the value of EDF increases as the width of ELE zone gets smaller and smaller leading to a frequent rock burst phenomenon.
  • Even when the zone reaches a balanced situation of energy equilibrium by distributing its energy, it does not stops the whole phenomenon.
  • The mechanism can be controlled when the width of ELE zone goes wider in its limit as the energy required to create that zone is larger. In such case the bursting does not happen and can be controlled.

  • Longwall destressing methods

Petr Konicek along with Kamil Soucek further observed the effect of Modern longwall destressing technology as normal destressing was not handy in the case of long wall coal seams. They studied the destressing mechanism in a seam thickness of 3.1 m to 5m at a total depth of around 700m. The average height of the long walls were in between 58 - 75 meters. A massive and strong strata of sandstone with conglomerate of uniaxial compressive strength was observed in an analysis of inner burden rock mass. (Kamil Soucek, 2013) A compact Conical Borehole Monitoring or CCBM measured the stress factor at number of instance after and before mining followed by a laboratory test which states a very high ratio of elastic and total deformation. The very high ratio of deformation indicated a storage of large energy bumps making the area prone to coal bumps and rock burst.

Under suitable mining condition in a destress blasting scenarios the blasting and long hole drill design is adopted to prefracture the strata component from every gate rods well in advance. It results in bump free extraction of whole panel after destress blasting is done. Here the seismic effect is calculated in terms of charged explosive weight to that of monitored data of different mining stages. The system overall resulted in successful extraction of longwall panel smoothly without any sign of rock burst or coal bumps. The research was carried out in the Lazi colliery, a Karvina Ostravia Coalfield located in the upper Silesian Coal basin. (Kamil Soucek, 2013)

The process not only helped in easily removal of long wall panels but also reduced induced stress concentration present in the longwall face. From a comparison of pre and post data of induced stress showed a large scale of variation of 50m from that of 93m in actual field measurement.

  • Mechanics of Pillar involved

Innacchione stated his review paper in a conference proceeding at US bureau about the pillar mechanics involved in coal burst mechanisms. They identified areas with high releasing energy with the help of Geomechanic field measurement and Micro - seismic sensors. They further helped in identifying all coal beds prone to failure in identified stress conditions (Abthony T. Innacchione, 1994) (Heasley KA, 1992). Three major factors such as mining methods, stress field and geology was categorized according to these environmental conditions. This also led to number of viable coal strategies not recognized earlier.

Three categories of burst was defined for the coal burst mechanism by Innacchione. They are

  1. Seismic Shock – this mechanism was brought into light by Rice in 1920’s. His theory for the coal burst was related to the presence of thick and massive strata on the coalbed, thus transmitting a shock wave to the amount of coal present below it. He also accounted the growth and sudden failure of strata for the creation of seismic waves. Other factor made responsible was massive volume impact of rock on the coal bed. Theoretically both of this phenomenon are enough for creating a sizeable shock seismic waves resulting in the coal bumps.
  2. Loss of confinement – this method was proposed by Babcock and Bickel in 1984. They were the first to suggest the happening by a thorough investigation in laboratories. Loss of confinement stated that the reduction in confinement of stress as the vertical stress was maintained within a load frame produced an abrupt failure if they were reduced dramatically. Although the theory was simple, still the laboratory investigation confirmed such interaction for coal burst.
  3. Excessive Pressure mechanism - Holland and Thomas extensively discussed about this theory in their paper in 1954. After a long research the burst mechanism was held responsible for excessive abutment pressure, whenever a large load was applied to mining structures. The shifting of massive loads occurs very fast as it can exceed the ultimate load bearing capacity of pillar. The cause of shifting can be accounted to bursting or shifting of adjacent pillars, thus creating a disbalance of pressure and making it prone to coal burst.

Other environmental characteristics that are related to this type of burst includes, Geology, Mining and Stress conditions.

  • Micro seismic monitoring

The micro seismic monitoring is a real time, dynamic and region based technology which forecasts many geological hazards including , mine tremor, gas outburst and rock burst. This monitoring can further prevent the disaster from happening and reduce the number. The study by CAO An-Ye and DOU Lin Ming presented that prerequisite for any seismic mine activity is the study of its focal mechanism. The monitoring technology has its advantage over predicting a large scale geological hazard. Using friction and pressure as a medium near any seismic source, a shear and compression wave can be emitted to induce shear fracture at the time of breakage. Using the data the rock burst or any sort of tremor can be forecasted and can be handy in preventing major accident happening due to coal bumps. Few points concluded from the paper was

  • The coal pillars are made usually for storing and transferring stress and energies from roof to floor whenever the stress is exceeded from the limit of the pillar the coal burst mechanism may happen. The difference in stresses and strength limit along with pillar’s stiffness release a large amount of energy, which becomes strong enough for the disaster to happen.
  • Shear fracture can be stated as root cause of failure because of friction and pressure acting between the pillar with its roof and floor. For study purpose the equivalent model is considered as a double couple point source model.
  • The micro seismic signals observed from different sources has different characteristics as well. In such cases focal mechanism to identify the origin of seismic wave can be identifying quickly and easily.
  • On discussion, when failure happens due to tremors and rock burst, they are companied by roof falls and tensile failure. In such situation the process of focal mechanism becomes a bit complex

  • Rock burst disaster prediction

This method of rock burst prediction was reviewed by Tong Bin Zhao and Yanchun Yin in 2014. It is a well-known fact that any charge inside a coal bed can be induced due to friction created between coal particles. For the purpose a microstructure with four different frictional coefficient according to the law was generated. The flow of frictional energy for a sample under uniaxial loading was studied was followed by electromagnetic radiation method. The method was then applied to study failure at Muchengjian Colliery by predicting its outburst potential in any coal pillar which was isolated. The result expressed that the friction coefficient decreased linearly as the axial loading increased. In further advancements the electromagnetic radiation signal went away from normal as the rock burst was onset. These electromagnetic signal predictions were tried on site and the results were fruitful. The radiation method hence developed by them was in turn able to predict the disaster well before.

Rockburst is a sort of falsely instigated catastrophe by mining unearthing, and it essentially includes a procedure of vitality aggregation, developing and sudden arrival of collected vitality. The vitality for rockburst event fundamentally originates from mining uncovering impelled aggravation. Consequently, the component of rockburst taking into account unsettling influence vitality incited by mining removal is a logical comprehension for designing practice. Two fundamental conditions for the event of rockburst exhibited in this setting have been proposed, which are experimental and valuable judging criteria for rockburst expectation in metal mines. For the unwavering quality reason, it is fundamental to gather solid information from different sources, for example, in situ stress estimation, topographical examination, rock mechanics tests, and site-particular mining format and process, which can give an exploratory premise to a quantitative investigation of rockburst event.

In correspondence to numerical demonstrating on "spatio-worldly quality" regularities of rockburst, a checking net of multi-parameter and multi-data identified with vitality discharge ought to be set up, where the observing focuses ought to be painstakingly chosen. The coupling examination of checking results with numerical investigation can give a sound premise to continuous forecast of rockburst. According to the expectation aftereffects of rockburst, the measures for anticipation and control of rockburst ought to be sufficiently chosen utilizing upgraded mining strategies, enhanced mining format, successful bolster measures, and removal grouping, to lessen stress circulation in rock mass.Through the association between mining progression and learning of seismology, hypothetical expectation of "spatiotemporal-quality" regularities of rockburst in metal mines can be quantitatively made. In any case, the significant issue at present is the absence of full grown methods for observing and forecast of rockburst. In such manner, further examinations concerning creating strategies and gear of high exactness and accuracy are required for clever and noticeable discovery purposes

  • GANTT Chart

The project was divided into following schedule and a following plan was accepted. The time frame was accepted and every possibility was considered. The project was divided into following major parts

  1. Study – this part will be including all sorts of studies done to understand the basics of Coal bursting mechanism. The Study part was further divided into three categories for Introduction, Analyzing Journals and then identification of problem statement. The main purpose of these step is get an in depth knowledge of the total proceedings done so far in the topics related to it, The reviewing of journal was as important as planning to do the project. The journals from all around the world shall be taken into consideration. They have been denoted with blue color in the chart.
  2. Planning – this step is very important for any project to be completed in time. The planning does not always includes time goal but the process sequence planning is also equally important. Planning is usually done to carefully and wisely allocating the job into the group members and then following them up. They have been denoted with green colour in the Gantt chart. The tasks which comes under planning are Process planning and Project time planning. The final check listing should also be a part of planning as the checklist prepares all the tasks attended and not attended wisely.
  3. Visits – in whole schedule a visit to nearby mine was also planned to collect all sorts of method utilized and practiced to predict the rock burst processes beforehand in the mining sites. The other purpose of organizing such visit is to get familiar with the approach, study their practices and difficulties being faced. The visits shall be done by each and every member of the group. Another visit might be done to verify the process developed and assessing it for better feedback. This part in the Gantt chart has been covered in grey color and shall be scheduled around 5th week to 8th week after initiating the project.
  4. Calculation and verification - the calculation and verification part in the Gantt chart has been denoted by pink color. Process verification and data verification are two different parts to be dealt with. Process verification will allow the process developed to be tested at the site or on software simulation and the data verification will be like evaluating the results obtained from numerical and empirical formulas and compare it with the
  5. Documentation – Documentation includes collecting of data for calculation and then segregating the data to form a formal report presenting each and every aspect of the processes followed in achieving the result. The formal documents includes the reports presentation and supporting files they have been scheduled in the 12th week for finalizing. The reports and data received for calculation must be solved within two weeks after the data is collected. The proceedings are to be monitored and followed up carefully. The documentation part involved in the Gantt chart is presented by orange colour.

Table 1 GANTT Chart of the tasks involved and completed

In this Gantt chart the part which is completed by weeks gets the cross off from the chart. The increase of color in the row shows the depth of tasks and time to be devoted in particular.

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