It is arguable that the development of reinforcing roof bolting systems has largely stagnated in recent times, primarily due to the prevailing industry view that few, if any, further improvements can be made to what c...It is arguable that the development of reinforcing roof bolting systems has largely stagnated in recent times, primarily due to the prevailing industry view that few, if any, further improvements can be made to what currently exists.However, this paper contends that reinforcing roof bolting systems can be further refined by considering both the specific manner by which horizontally bedded roof strata loses its natural self-supporting ability and the specific means by which reinforcing roof bolts act to promote or retain this natural self-supporting ability.The Australian coal industry has insisted on minimising bolt-hole diameter to maximise load transfer and on targeting full-encapsulation by any means necessary for many years.This has led to a significant, albeit unintended, consequence in terms of overall roof bolting effectiveness, namely increased resin pressures during bolt installation and the associated potential for opening bedding planes that may have, otherwise, remained closed during the bolt installation process.Given that the natural self-supporting ability of roof strata is strongly linked to whether bedding planes are open or closed, logically, minimising resin pressures should be a significant benefit.This paper focuses primarily on three key issues that relate directly to the function of the roof bolting system itself:(1) the importance of proper resin mixing in the context of maximising load transfer strength and stiffness,(2) the importance of minimising resin pressures developed during bolt installation, and(3) the importance of maximising the effectiveness of the available bolt pre-tension.All mine operators should be invested in improving the individual effectiveness of each installed roof bolt, even by relatively small incremental amounts, so this is an important topic for discussion within the mining community.展开更多
In 2017,one of the international authorities on coal bursts,Mark Christopher,published a paper entitled"Coal bursts that occur during development:A rock mechanics enigma",in which several relevant technical ...In 2017,one of the international authorities on coal bursts,Mark Christopher,published a paper entitled"Coal bursts that occur during development:A rock mechanics enigma",in which several relevant technical issues were identified.This paper outlines what is considered to be a credible,first-principles,mechanistic explanation for these three current development coal burst conundrums by reference to early published coal testing work examining the significance of a lack of"constraint"to coal stability and an understanding of how very specific structural geology and other geological features can logically cause this to occur in situ,albeit on a statistically very rare basis.This basic model is examined by reference to published information pertaining to the development coal-burst that occurred at the Austar Coal Mine in New South Wales,Australia,in 2014 and from the Sunnyside District in Utah,the United States.The"cause and effect"model for development of coal bursts presented also offers a meaningful explanation for the statistical improbability for what are nonetheless potentially highly-destructive events,being able to explain the statistical rarity being just as important to the credibility of the model as explaining the local conditions associated with burst events.The model could also form the basis for a robust,riskbased approach utilising a"hierarchy of controls",to the operational management of the development coal burst threat.Specifically,the use of pre-mining predictions for likely burst-prone and non-burstprone areas,the use of the mine layout to avoid or at least minimise mining within burst-prone areas if appropriate,and finally the development of an operational Trigger Action Response Plan(TARP)that reduces the likelihood of inadvertent roadway development into a burst-prone area without suitable safety controls already being in place.展开更多
The method of determining coal pillar strength equations from databases of stable and failed case histories is more than 50 years old and has been applied in different countries by different researchers in a range of ...The method of determining coal pillar strength equations from databases of stable and failed case histories is more than 50 years old and has been applied in different countries by different researchers in a range of mining situations. While common wisdom sensibly limits the use of the resultant pillar strength equations and methods to design scenarios that are consistent with the founding database, there are a number of examples where failures have occurred as a direct result of applying empirical design methods to coal pillar design problems that are inconsistent with the founding database. This paper explores the reasons why empirically derived coal pillar strength equations tend to be problem-specific and should be considered as providing no more than a pillar strength ‘‘index." These include the non-consideration of overburden horizontal stress within the mine stability problem, an inadequate definition of supercritical overburden behavior as it applies to standing coal pillars, and the non-consideration of overburden displacement and coal pillar strain limits. All of which combine to potentially complicate and confuse the back-analysis of coal pillar strength from failed cases. A modified coal pillar design representation and model are presented based on coal pillars acting to reinforce a horizontally stressed overburden, rather than suspend an otherwise unstable self-loaded overburden or section, the latter having been at the core of historical empirical studies into coal pillar strength and stability.展开更多
Current coal pillar design is the epitome of suspension design.A defined weight of unstable overburden material is estimated, and the dimensions of the pillars left behind are based on holding up that material to a pr...Current coal pillar design is the epitome of suspension design.A defined weight of unstable overburden material is estimated, and the dimensions of the pillars left behind are based on holding up that material to a prescribed factor of safety.In principle, this is no different to early roadway roof support design.However, for the most part, roadway roof stabilisation has progressed to reinforcement, whereby the roof strata is assisted in supporting itself.This is now the mainstay of efficient and effective underground coal production.Suspension and reinforcement are fundamentally different in roadway roof stabilisation and lead to substantially different requirements in terms of support hardware characteristics and their application.In suspension, the primary focus is the total load-bearing capacity of the installed support and ensuring that it is securely anchored outside of the unstable roof mass.In contrast, reinforcement recognises that roof de-stabilisation is a gradational process with ever-increasing roof displacement magnitude leading to ever-reducing stability.Key roof support characteristics relate to such issues as system stiffness, the location and pattern of support elements and mobilising a defined thickness of the immediate roof to create(or build) a stabilising strata beam.The objective is to ensure that horizontal stress is maintained at a level that prevents mass roof collapse.This paper presents a prototype coal pillar and overburden system representation where reinforcement, rather than suspension, of the overburden is the stabilising mechanism via the action of in situ horizontal stresses.Established roadway roof reinforcement principles can potentially be applied to coal pillar design under this representation.The merit of this is evaluated according to failed pillar cases as found in a series of published databases.Based on the findings, a series of coal pillar system design considerations for bord and pillar type mine workings are provided.This potentially allows a more flexible approach to coal pillar sizing within workable mining layouts, as compared to common industry practice of a single design factor of safety(Fo S) under defined overburden dead-loading to the exclusion of other relevant overburden stabilising influences.展开更多
As per most other earth science engineering problems,the underground coal geotechnical environment and the way in which roof and rib support interacts with the rock mass are complex issues.It is therefore generally re...As per most other earth science engineering problems,the underground coal geotechnical environment and the way in which roof and rib support interacts with the rock mass are complex issues.It is therefore generally recognised that without prudent simplification,the complexity of the problem will overwhelm all current geotechnical methods of modelling,not least for the reason that a rock mass can never be characterised to a level that allows a"non-simplified"analysis.The fact that numerical models,which are commonly purported to be a"simulation"tool and the so-called epitome of advanced geotechnical engineering,always need to be"calibrated"to a known reality is taken to be conclusive proof of this statement.While the problem should not be oversimplified(i.e.the dominant failure mechanisms or critical data input parameters should not be ignored),without question judicious simplification is at the heart of all engineering design,to the point that it has a well-established name–"reductionism".The hypothesis addressed in this paper,is that horizontal and vertical stress-driven slender beam and column behaviour(which includes unstable Euler Buckling)are respectively the dominant(but not only)roadway roof and ribline behavioural mechanism that(if not controlled)can lead to excessive deformation,failure and eventual collapse.As a part of the Scientific Method,a hypothesis can only be tested via real-world observations,measurements and analyses in establishing it is a credible Theory.Utilising the Scientific Method,this paper demonstrates that slender beam/column behaviour is the dominant instability mechanism within a coal mine roof/rib subject to elevated horizontal/vertical stress conditions and therefore,must be representatively accounted for in any credible empirical,analytical,or numerical approach to coal mine roof/rib stability assessment and associated ground support design.展开更多
文摘It is arguable that the development of reinforcing roof bolting systems has largely stagnated in recent times, primarily due to the prevailing industry view that few, if any, further improvements can be made to what currently exists.However, this paper contends that reinforcing roof bolting systems can be further refined by considering both the specific manner by which horizontally bedded roof strata loses its natural self-supporting ability and the specific means by which reinforcing roof bolts act to promote or retain this natural self-supporting ability.The Australian coal industry has insisted on minimising bolt-hole diameter to maximise load transfer and on targeting full-encapsulation by any means necessary for many years.This has led to a significant, albeit unintended, consequence in terms of overall roof bolting effectiveness, namely increased resin pressures during bolt installation and the associated potential for opening bedding planes that may have, otherwise, remained closed during the bolt installation process.Given that the natural self-supporting ability of roof strata is strongly linked to whether bedding planes are open or closed, logically, minimising resin pressures should be a significant benefit.This paper focuses primarily on three key issues that relate directly to the function of the roof bolting system itself:(1) the importance of proper resin mixing in the context of maximising load transfer strength and stiffness,(2) the importance of minimising resin pressures developed during bolt installation, and(3) the importance of maximising the effectiveness of the available bolt pre-tension.All mine operators should be invested in improving the individual effectiveness of each installed roof bolt, even by relatively small incremental amounts, so this is an important topic for discussion within the mining community.
文摘In 2017,one of the international authorities on coal bursts,Mark Christopher,published a paper entitled"Coal bursts that occur during development:A rock mechanics enigma",in which several relevant technical issues were identified.This paper outlines what is considered to be a credible,first-principles,mechanistic explanation for these three current development coal burst conundrums by reference to early published coal testing work examining the significance of a lack of"constraint"to coal stability and an understanding of how very specific structural geology and other geological features can logically cause this to occur in situ,albeit on a statistically very rare basis.This basic model is examined by reference to published information pertaining to the development coal-burst that occurred at the Austar Coal Mine in New South Wales,Australia,in 2014 and from the Sunnyside District in Utah,the United States.The"cause and effect"model for development of coal bursts presented also offers a meaningful explanation for the statistical improbability for what are nonetheless potentially highly-destructive events,being able to explain the statistical rarity being just as important to the credibility of the model as explaining the local conditions associated with burst events.The model could also form the basis for a robust,riskbased approach utilising a"hierarchy of controls",to the operational management of the development coal burst threat.Specifically,the use of pre-mining predictions for likely burst-prone and non-burstprone areas,the use of the mine layout to avoid or at least minimise mining within burst-prone areas if appropriate,and finally the development of an operational Trigger Action Response Plan(TARP)that reduces the likelihood of inadvertent roadway development into a burst-prone area without suitable safety controls already being in place.
文摘The method of determining coal pillar strength equations from databases of stable and failed case histories is more than 50 years old and has been applied in different countries by different researchers in a range of mining situations. While common wisdom sensibly limits the use of the resultant pillar strength equations and methods to design scenarios that are consistent with the founding database, there are a number of examples where failures have occurred as a direct result of applying empirical design methods to coal pillar design problems that are inconsistent with the founding database. This paper explores the reasons why empirically derived coal pillar strength equations tend to be problem-specific and should be considered as providing no more than a pillar strength ‘‘index." These include the non-consideration of overburden horizontal stress within the mine stability problem, an inadequate definition of supercritical overburden behavior as it applies to standing coal pillars, and the non-consideration of overburden displacement and coal pillar strain limits. All of which combine to potentially complicate and confuse the back-analysis of coal pillar strength from failed cases. A modified coal pillar design representation and model are presented based on coal pillars acting to reinforce a horizontally stressed overburden, rather than suspend an otherwise unstable self-loaded overburden or section, the latter having been at the core of historical empirical studies into coal pillar strength and stability.
文摘Current coal pillar design is the epitome of suspension design.A defined weight of unstable overburden material is estimated, and the dimensions of the pillars left behind are based on holding up that material to a prescribed factor of safety.In principle, this is no different to early roadway roof support design.However, for the most part, roadway roof stabilisation has progressed to reinforcement, whereby the roof strata is assisted in supporting itself.This is now the mainstay of efficient and effective underground coal production.Suspension and reinforcement are fundamentally different in roadway roof stabilisation and lead to substantially different requirements in terms of support hardware characteristics and their application.In suspension, the primary focus is the total load-bearing capacity of the installed support and ensuring that it is securely anchored outside of the unstable roof mass.In contrast, reinforcement recognises that roof de-stabilisation is a gradational process with ever-increasing roof displacement magnitude leading to ever-reducing stability.Key roof support characteristics relate to such issues as system stiffness, the location and pattern of support elements and mobilising a defined thickness of the immediate roof to create(or build) a stabilising strata beam.The objective is to ensure that horizontal stress is maintained at a level that prevents mass roof collapse.This paper presents a prototype coal pillar and overburden system representation where reinforcement, rather than suspension, of the overburden is the stabilising mechanism via the action of in situ horizontal stresses.Established roadway roof reinforcement principles can potentially be applied to coal pillar design under this representation.The merit of this is evaluated according to failed pillar cases as found in a series of published databases.Based on the findings, a series of coal pillar system design considerations for bord and pillar type mine workings are provided.This potentially allows a more flexible approach to coal pillar sizing within workable mining layouts, as compared to common industry practice of a single design factor of safety(Fo S) under defined overburden dead-loading to the exclusion of other relevant overburden stabilising influences.
文摘As per most other earth science engineering problems,the underground coal geotechnical environment and the way in which roof and rib support interacts with the rock mass are complex issues.It is therefore generally recognised that without prudent simplification,the complexity of the problem will overwhelm all current geotechnical methods of modelling,not least for the reason that a rock mass can never be characterised to a level that allows a"non-simplified"analysis.The fact that numerical models,which are commonly purported to be a"simulation"tool and the so-called epitome of advanced geotechnical engineering,always need to be"calibrated"to a known reality is taken to be conclusive proof of this statement.While the problem should not be oversimplified(i.e.the dominant failure mechanisms or critical data input parameters should not be ignored),without question judicious simplification is at the heart of all engineering design,to the point that it has a well-established name–"reductionism".The hypothesis addressed in this paper,is that horizontal and vertical stress-driven slender beam and column behaviour(which includes unstable Euler Buckling)are respectively the dominant(but not only)roadway roof and ribline behavioural mechanism that(if not controlled)can lead to excessive deformation,failure and eventual collapse.As a part of the Scientific Method,a hypothesis can only be tested via real-world observations,measurements and analyses in establishing it is a credible Theory.Utilising the Scientific Method,this paper demonstrates that slender beam/column behaviour is the dominant instability mechanism within a coal mine roof/rib subject to elevated horizontal/vertical stress conditions and therefore,must be representatively accounted for in any credible empirical,analytical,or numerical approach to coal mine roof/rib stability assessment and associated ground support design.
