Analysis of structural interaction in tunnels using the covergence–confinement approach

The rock-support interaction in tunnels is studied through the use of the convergence–confinement method. The equations that characterize the behaviour of the most important support types are given together with a set of conceptual interaction schemes. As far as the behaviour of the support is concerned, reference is made to the ultimate limit state concept, which is widely used in civil engineering. This approach is linked to the classical convergence–confinement method. The interaction between the temporary support system and the final lining is dealt with, and the noteworthy case of presupport ahead of the face, followed by a further internal support (usually steel sets and shotcrete) is also included. Finally, the ‘ground reaction curve of the reinforced tunnel’, which allows one to analyse the interaction between the reinforcement around the tunnel and supports, is introduced.     

Case history of the response of a longwall entry subjected to concentrated horizontal stress

The National Institute for Occupational Safety and Health (NIOSH) Pittsburgh Research Laboratory (PRL), RAG Pennsylvania and Strata Control Technologies of Australia collaborated in an intensive study of ground behavior, reinforcement performance, and stress redistribution at the Emerald Mine in Southwestern Pennsylvania. The study site was a longwall tailgate subjected to a severe horizontal stress concentration. Field measurements indicated that the stresses applied to the study site nearly doubled during longwall mining, resulting in roof deformations extending to a height of 4.8 m (16 ft) above the entry. A computer simulation of the field site was conducted using FLAC-2D, incorporating a broad range of rock behaviors and failure mechanisms. Comparison between the measurements and the simulation showed that the model was able to capture the most significant aspects of the roof and support system behavior, particularly, the extensive slip along bedding that created a partially destressed “softened” zone in the immediate roof. The model also showed that supplementing the normal roof bolt support pattern with cable bolts would allow the entry to survive a further 20–25% increase in the applied horizontal stress. Such information could have very practical application to the design of roof support systems for coal mines.     

Effect of strata properties and panel widths on chock performance

The selection of optimum chock (support) capacity is very crucial for a successful longwall mining. The selection of chock capacity depends on the site-specific geotechnical parameters, constraints and longwall panel geometry, which are generally not known in detail in priority. Hence, based on the field and laboratory data, various possible combinations should be analyzed to cater for the unforeseeable mining conditions. This paper discusses the use of numerical model for selecting an appropriate chock capacity based on the site-specific geological and geotechnical information and longwall panel geometry. The fracture mechanisms of immediate and main roofs are also discussed for various panel widths and support capacities. For the models considered, the chock convergence is predicted to increase by about 33% due to the increase in face width from 100 to 260 m. Similarly, the massive roof strata are found to yield higher chock convergence compared to bedded strata.     

Ground reaction curves for tunnels excavated in different quality rock masses showing several types of post-failure behaviour

Rock mass behaviour model selection and, in particular, rock mass post-failure behaviour are key issues in analysing tunnel stability, in particular in terms of the correct application of design techniques such as the convergence–confinement method and also numerical modelling. Three different quality rock masses (good, average and poor) were defined in which simulated standard tunnels were excavated. Different behaviours – including elastic perfectly plastic, elastic brittle and three strain softening behaviours – were modelled for each type of rock mass and increasingly realistic parameters were calculated, along with the corresponding ground reaction curves. The results obtained demonstrate the importance of adequate post-failure behaviour model selection for tunnel analysis. Also assessed are the effects of the standard support and reinforcement.     

急倾斜三软厚煤层留小煤柱沿空护巷技术

在急倾斜三软厚煤层走向长壁俯伪斜采煤条件下实施留小煤柱沿空护巷十分困难,煤柱稳定性和巷道围岩变形极难控制。针对这一难题,提出了包含煤柱小角度锚固法和十字护顶方法的留小煤柱沿空护巷技术,有效解决了煤柱易沿顶底板剪切破坏并向巷内搓动的问题,降低了巷道软弱围岩的破碎程度和变形量。现场试验结果显示,留设小煤柱的完整性保持较好,其中相较于原支护方式顶底板移近量减少了40%,两帮收敛量则减少了42%,巷道围岩变形得到了有效控制。与此同时,还得到工作面前后方回采巷道的矿压显现呈现明显的6个分区,分别为工作面前方无影响区、工作面前方矿压显现影响区、工作面前方矿压显现强烈区、工作面后方顶板激烈活动区、工作面后方顶板活动减缓区和工作面后方基本稳定区。其中,工作面前方矿压显现强烈区和工作面后方顶板活动激烈区的范围明显大于缓斜近水平煤层,这为分区制定围岩控制措施提供了有利依据。所得研究成果可为我国急倾斜走向长壁俯伪斜工作面沿空护巷技术研究提供一定的补充。     

Effect of strata properties and panel widths on chock performance

The selection of optimum chock (support) capacity is very crucial for a successful longwall mining. The selection of chock capacity depends on the site-specific geotechnical parameters, constraints and longwall panel geometry, which are generally not known in detail in priority. Hence, based on the field and laboratory data, various possible combinations should be analyzed to cater for the unforeseeable mining conditions. This paper discusses the use of numerical model for selecting an appropriate chock capacity based on the site-specific geological and geotechnical information and longwall panel geometry. The fracture mechanisms of immediate and main roofs are also discussed for various panel widths and support capacities. For the models considered, the chock convergence is predicted to increase by about 33% due to the increase in face width from 100 to 260 m. Similarly, the massive roof strata are found to yield higher chock convergence compared to bedded strata.     

Prediction of caving behavior of strata and optimum rating of hydraulic powered support for longwall workings

A reliable prediction of the caving behavior of strata and support capacity requirement for longwall workings has always been a challenge for rock mechanics researchers and mine planning engineers. It is very important for successful planning and implementation of high capacity longwall projects to have proper selection of site and compatible roof supports, to ensure safe operation of longwall face especially in massive and difficult to cave strata conditions. This paper illustrates a numerical modeling based integrated approach for predicting the progressive caving behavior of strata and optimum capacity requirement of powered support for longwall working in a given geo-mining and strata condition. A set of design criteria for selection of optimal capacity support integrating the field experience and the numerical modeling results of longwall panels, operated in a widely varying strata conditions in different coalfields of India. Case studies of two typical longwall workings are presented to assess the caving behavior and support requirement for safe operation of longwall working under the site specific regular and en masse caving strata conditions.     

The elasto-plastic response of underground excavations in rock masses that satisfy the Hoek–Brown failure criterion

This paper is intended to illustrate the relationship between the Hoek–Brown parameters describing the strength of rock masses and the mechanical response of underground openings.A formulation of the elasto-plastic behavior of rock in terms of the Hoek–Brown criterion is presented. The analysis assumes that the joint system present in the rock mass has no preferred orientation so that the medium can be considered to behave as an isotropic continuum. It is shown that appropriate scaling of the Hoek–Brown parameters leads to considerable simplification in defining the elasto-plastic response of the rock mass.The classical case in which the excavation process is treated as a uniform reduction of internal pressure in symmetrically loaded cylindrical and spherical cavities is considered. Closed-form expressions are given for the extent of plastic behavior and the related stress and displacement fields. A dimensionless graphical representation of these solutions is provided that allows accurate estimates of the response of excavations in Hoek–Brown materials to be made quickly and easily. Examples are given to illustrate the use of the graphs.Illustrative applications of the derived closed-form solutions are also described. The construction of ground reaction curves for the design of cylindrical tunnels according to the convergence–confinement method and a case study of stability analysis of spherical cavities produced by underground nuclear explosions in French Polynesian atolls are discussed.     

Distributions of airflow in four rectangular section roadways with different supporting methods in underground coal mines

A study has been carried out in four rectangular section roadways with different supporting methods in Yuwu Coal Company (a longwall mine), by measurements of airflow velocities in cross-section of the roadways. The asymmetrical distributions of airflow in each roadway section was obtained. The paper analyzes the low airflow velocity region of roadways through the drawing of the distributions of airflow in each roadway section. The supporting methods influence the low airflow velocity region around the roof and wall of roadways. It is shown that the low airflow velocity region increase with surface roughness of the roof and wall. The high airflow velocity region was located around the floor of the roadway with rough roof and wall. However, in the roadways with smooth roof and wall the high airflow velocity region was located around the center of section. The risk assessment should be carried out in the low airflow velocity region in the roadway with rough roof and wall. To ensure the safety of coal mining, higher volume of air intake or more smooth roof and wall of the roadways should be achieved in a dangerous zone.     

玲珑金矿主运巷塌陷治理区稳定性动态综合监测与评价

山东省玲珑金矿有新、旧2条相互平行、相距10m左右的主运巷（主运输巷道），其下方为一垂直穿过的脉群采空区。为保护主运巷安全，在主运巷与采空区之间留有保安矿柱，并对空区进行毛石充填。由于主运巷下方受到非法民采的破坏，包括保安矿柱被盗采，导致新、旧主运巷均发生急剧的塌陷和变形破坏。为维持安全和正常生产，首先采取一系列特殊技术对旧主运巷进行加固治理，以恢复其正常运输通行能力。旧主运巷加固治理完毕后，运输线由新主运巷移至旧主运巷。按原方案，下一步应采取措施加固治理新主运巷，但由于经费困难，此步未能实施。大规模的新主运巷塌陷空区必将对已恢复运行的旧主运巷的稳定性产生重大影响。为监测这种影响，保证主运输线的安全，采用多点位移计监测、断面收敛测线、水准测量、应力监测和声发射监测5种手段建立塌陷治理区稳定性监测网。通过对多种手段获得的监测结果进行多元信息耦合分析，证实玲珑金矿旧主运巷采用符合岩石力学原理的高压锚注技术进行加固治理所取得的突出效果，同时对已加固治理的旧主运巷和未加固治理的新主运巷共存情况下主运巷的稳定性状态及其发展趋势作出评价，从而为主运巷下一步加固治理提供依据。     

Mechanical behaviors of coal measures and ground control technologies for China's deep coal mines – A review

This paper reviews the major achievements in terms of mechanical behaviors of coal measures, mining stress distribution characteristics and ground control in China's deep underground coal mining. The three main aspects of this review are coal measure mechanics, mining disturbance mechanics, and rock support mechanics. Previous studies related to these three topics are reviewed, including the geomechanical properties of coal measures, distribution and evolution characteristics of mining-induced stresses, evolution characteristics of mining-induced structures, and principles and technologies of ground control in both deep roadways and longwall faces. A discussion is made to explain the structural and mechanical properties of coal measures in China's deep coal mining practices, the types and distribution characteristics of in situ stresses in underground coal mines, and the distribution of mining-induced stress that forms under different geological and engineering conditions. The theory of pre-tensioned rock bolting has been proved to be suitable for ground control of deep underground coal roadways. The use of combined ground control technology (e.g. ground support, rock mass modification, and destressing) has been demonstrated to be an effective measure for rock control of deep roadways. The developed hydraulic shields for 1000 m deep ultra-long working face can effectively improve the stability of surrounding rocks and mining efficiency in the longwall face. The ground control challenges in deep underground coal mines in China are discussed, and further research is recommended in terms of theory and technology for ground control in deep roadways and longwall faces.     

近距巷道平行开挖合理错距研究

矿井下运输巷道是矿业生产的重要通道。文中针对极松软地层近距双水平巷道掘进问题,以内蒙古上海庙矿区为工程背景,依据工程区地应力场的实测结果及岩石力学实验参数,运用FLAC^3D三维数值分析计算软件对开拓巷道的合理错距进行了数值模拟研究。通过分析巷道周边应力分布、塑性区变化、位移速率矢量场方向变化以及监测位移数据变化,研究了巷道在分步开挖过程中,不同的开挖错距对已开拓巷道监测端面的影响。结果表明在极松软地质条件下,巷道围岩周边高应力区分布范围较大,新开拓的巷道会对相邻已开拓巷道周边的应力、塑性破坏区及位移速率矢量场产生影响,双近距水平巷道合理开拓错距宜保持在50 m以上。     

A method for the design of longwall gateroad roof support

A longwall gateroad roof support design method for roadway development and panel extraction is demonstrated. It is a hybrid numerical and empirical method called gateroad roof support model (GRSM), where specification of roof support comes from charts or equations. GRSM defines suggested roof support densities by linking a rock-mass classification with an index of mining-induced stress, using a large empirical database of Bowen Basin mining experience. Inherent in the development of GRSM is a rock-mass classification scheme applicable to coal measure strata. Coal mine roof rating (CMRR) is an established and robust coal industry standard, while the geological strength index (GSI) may also be used to determine rock-mass geomechanical properties. An elastic three-dimensional numerical model was established to calculate an index of mining induced stress, for both roadway development and longwall retreat. Equations to calculate stress index derived from the numerical modelling have been developed. An industry standard method of quantifying roof support is adopted as a base template (GRSUP). The statistical analyses indicated that an improved quantification of installed support can be gained by simple modifications to the standard formulation of GRSUP. The position of the mathematically determined stable/failed boundary in the design charts can be changed depending on design criteria and specified risk.     

长壁孤岛工作面冲击失稳能量释放激增机制研究

煤矿开采中跳采形成的孤岛工作面由于容易产生应力集中，来压强度高，极容易发生冲击地压。基于唐山矿孤岛工作面的地质条件和周期来压步距的监测结果，通过数值分析的方法，研究孤岛工作面煤岩体能量释放的动态特征，揭示工作面前方能量释放激增机制，对比普通工作面和孤岛工作面能量场的区别，介绍冲击地压预警防治措施。数值模拟结果显示，长壁孤岛工作面回采时随着直接顶的随采随冒，采空区悬空面积的不断增大，使得老顶积聚大量的弹性能。若老顶发生周期性垮落，弹性能将瞬间释放，此时工作面和顺槽巷道极易冲击失稳。由研究结果可知，孤岛工作面周期来压时顶底板和煤层的能量激增可做为判断冲击失稳的前兆信息之一。因此，微震监测等手段可以根据此结论预测潜在的矿山动力灾害。针对老顶周期性断裂时积聚能量的突然释放规律，运用强制放顶、超前卸压孔、开切卸压槽和卸压爆破、煤层注水等技术可以提前释放煤层内积聚的弹性能，达到良好的冲击地压防治效果。     

在构造应力场中采动对底板运输巷道稳定性的影响

利用有限元软件，建立了底板运输巷道锚喷支护稳定性分析的三维计算模型，分析了在范各庄煤矿构造应力场条件下不同工作面回采顺序对底板运输巷道稳定性的影响。提出在构造应力场中，开采运输巷道上方的煤体使巷道产生拉伸形变是巷道破坏的主要根源。指出在保留上方煤柱使运输巷道处于近似双向等压条件下，以减轻或避免回采引起的支撑压力的强烈影响，为在高水平应力场条件下底板运输巷布置的重要原则。经开滦范各庄工程实践检验，输运巷道使用期间减少了维护量，并保持了巷道稳定。     

Auxiliary ventilation in mining roadways driven with roadheaders: Validated CFD modelling of dust behaviour

The production of dust when driving mining roadways can affect workers health. In addition, there is a decrease in productivity since Mine Safety regulations establish a reduction in the working time depending on the quartz content and dust concentration in the atmosphere.One of the gate roadways of the longwall named E4-S, belonging to the underground coal mine Carbonar SA located in Northern Spain, is being driven by an AM50 roadheader machine. The mined coal has a high coal dust content.This paper presents a study of dust behaviour in two auxiliary ventilation systems by Computational Fluid Dynamics (CFD) models, taking into account the influence of time. The accuracy of these CFD models was assessed by airflow velocity and respirable dust concentration measurements taken in six points of six roadway cross-sections of the mentioned operating coal mine.It is concluded that these models predicted the airflow and dust behaviour at the working face, where the dust source is located, and in different roadways cross-sections behind the working face.As a result, CFD models allow optimization of the auxiliary ventilation system used, avoiding the important deficiencies when it is calculated by conventional methods.     

Field investigations of high stress soft surrounding rocks and deformation control

Field investigations of high stress soft rock deformations show that the high stress soft rock roadway can slide with large deformation. Severe extrusion and floor heave can also be subsequently observed. The supported roadway can be locally damaged or completely fail, where the floor has a large deformation and/or is seriously damaged. The factors inducing large deformation of surrounding rocks in deep roadway are rock strengths, structure face cutting types, stress states, stress release, support patterns,and construction methods. Based on the deformation characteristics of high stress soft rock roadway, a comprehensive support scheme is proposed. The overall support technology of "step-by-step and joint,hierarchical reinforcement" for roadway is presented, and the anchor cable and bolt parameters to check the design methods are also given. Finally, the proposed comprehensive support method "bolt t metal mesh t U-steel arch t shortcrete t grouting and cable" is used in the extension section of east main haulage roadway at 850 m level of Qujiang coal mine. The 173-day monitoring results show that the average convergence of sidewalls reaches 208 mm, and the average relative convergence of roof and floor reaches 448 mm, suggesting that this kind of support technology for controlling large deformation of high stress soft surrounding rock roadway is effective.     

Exploitation of developed coal mine pillars by shortwall mining—a case example

The shortwall mining technique is similar to longwall mining but with shorter face lengths, ranging between 40 and 90 m, with the aim of controlling the caving nature of the overlying upper strata, the load on support and the overall operation of the supports applied at the face. Field observations and three-dimensional numerical modelling studies have been conducted for the longwall panel extraction of the Passang seam at Balrampur Mine of SECL to understand the caving behavior of the overlying upper strata. A large area of the Passang seam adjacent to the longwall panels has already been developed via bord and pillar workings. In this paper, numerical modelling studies have been conducted to assess the cavability of the overlying strata of the Passang seam in the mine over developed bord and pillar workings along with the support requirement at the face and in the advance gallery. The caving nature of the overlying rocks characterized by the main fall is predicted for varying face lengths, strata condition and depths of cover. The support resistance required at the face, the load in the advance gallery and its optimal obliquity were estimated for faster exploitation of the developed pillars in the Balrampur mine by shortwall mining.     

Innovation and future of mining rock mechanics

The 121 mining method of longwall mining first proposed in England has been widely used around the world.This method requires excavation of two mining roadways and reservation of one coal pillar to mine one working face.Due to considerable excavation of roadway,the mining roadway is generally destroyed during coal mining.The stress concentration in the coal pillar can cause large deformation of surrounding rocks,rockbursts and other disasters,and subsequently a large volume of coal pillar resources will be wasted.To improve the coal recovery rate and reduce excavation of the mining roadway,the 111 mining method of longwall mining was proposed in the former Soviet Union based on the 121 mining method.The 111 mining method requires excavation of one mining roadway and setting one filling body to replace the coal pillar while maintaining another mining roadway to mine one working face.However,because the stress transfer structure of roadway and working face roof has not changed,the problem of stress concentration in the surrounding rocks of roadway has not been well solved.To solve the above problems,the conventional concept utilizing high-strength support to resist the mining pressure for the 121 and 111 mining methods should be updated.The idea is to utilize mining pressure and expansion characteristics of the collapsed rock mass in the goaf to automatically form roadways,avoiding roadway excavation and waste of coal pillar.Based on the basic principles of mining rock mechanics,the“equilibrium mining”theory and the“short cantilever beam”mechanical model are proposed.Key technologies,such as roof directional presplitting technology,negative Poisson’s ratio(NPR)high-prestress constant-resistance support technology,and gangue blocking support technology,are developed following the“equilibrium mining”theory.Accordingly,the 110 and N00 mining methods of an automatically formed roadway(AFR)by roof cutting and pressure releasing without pillars are proposed.The mining methods have been applied to a large number of coal mines with different overburdens,coal seam thicknesses,roof types and gases in China,realizing the integrated mode of coal mining and roadway retaining.On this basis,in view of the complex geological conditions and intelligent mining demand of coal mines,an intelligent and unmanned development direction of the“equilibrium mining”method is prospected.     

Influence of the depth and shape of a tunnel in the application of the convergence–confinement method

One of the most widely used methods in tunnel support analysis and design is the convergence–confinement method (CCM). For its practical application, it is necessary to study the influence of the depth and cross-section of the tunnel and to confirm the calculations with two- or three-dimensional simulations carried out with finite elements or explicit finite differences programs. These simulations require elevated calculation times. In this paper, a modification of the CCM is proposed that directly introduces the effect of depth and the shape of the tunnel cross-section in the determination of the radial displacement of the tunnel. To do so, a series of functions are determined that approximate the radial displacement at points situated on the perimeter of the cross-section of the tunnel, considering several cross-sections at different distances from the working face. Should the cross-section of the tunnel or its depth be modified, it will not be necessary to perform new numerical simulations in order to apply the CCM. It will only be necessary to use the calculated shape functions. It is thus possible to use the CCM in the analysis and design of the support elements in a quite precise and significantly faster way.