Theory,technology and application of grouted bolting in soft rock roadways of deep coal mines

The grouted bolt,combining rock bolting with grouting techniques,provides an effective solution for controlling the surrounding rock in deep soft rock and fractured roadways.It has been extensively applied in numerous deep mining areas characterized by soft rock roadways,where it has demonstrated remarkable control results.This article systematically explores the evolution of grouted bolting,covering its theoretical foundations,design methods,materials,construction processes,monitoring measures,and methods for assessing its effectiveness.The overview encompassed several key elements,delving into anchoring theory and grouting reinforcement theory.The new principle of high pretensioned high-pressure splitting grouted bolting collaborative active control is introduced.A fresh method for dynamic information design is also highlighted.The discussion touches on both conventional grouting rock bolts and cable bolts,as well as innovative grouted rock bolts and cables characterized by their high pretension,strength,and sealing hole pressure.An examination of the merits and demerits of standard inorganic and organic grouting materials versus the new inorganic–organic composite materials,including their specific application conditions,was conducted.Additionally,the article presents various methods and instruments to assess the support effect of grouting rock bolts,cable bolts,and grouting reinforcement.Furthermore,it provides a foundation for understanding the factors influencing decisions on grouted bolting timing,the sequence of grouting,the pressure applied,the volume of grout used,and the strategic arrangement of grouted rock bolts and cable bolts.The application of the high pretensioned high-pressure splitting grouted bolting collaborative control technology in a typical kilometer-deep soft rock mine in China—the soft coal seam and soft rock roadway in the Kouzidong coal mine,Huainan coal mining area,was introduced.Finally,the existing problems in grouted bolting control technology for deep soft rock roadways are analyzed,and the future development trend of grouted bolting control technology is anticipated.

Pore-pressure and stress-coupled creep behavior in deep coal:Insights from real-time NMR analysis

Understanding the variations in microscopic pore-fracture structures(MPFS) during coal creep under pore pressure and stress coupling is crucial for coal mining and effective gas treatment. In this manuscript, a triaxial creep test on deep coal at various pore pressures using a test system that combines in-situ mechanical loading with real-time nuclear magnetic resonance(NMR) detection was conducted.Full-scale quantitative characterization, online real-time detection, and visualization of MPFS during coal creep influenced by pore pressure and stress coupling were performed using NMR and NMR imaging(NMRI) techniques. The results revealed that seepage pores and microfractures(SPM) undergo the most significant changes during coal creep, with creep failure gradually expanding from dense primary pore fractures. Pore pressure presence promotes MPFS development primarily by inhibiting SPM compression and encouraging adsorption pores(AP) to evolve into SPM. Coal enters the accelerated creep stage earlier at lower stress levels, resulting in more pronounced creep deformation. The connection between the micro and macro values was established, demonstrating that increased porosity at different pore pressures leads to a negative exponential decay of the viscosity coefficient. The Newton dashpot in the ideal viscoplastic body and the Burgers model was improved using NMR experimental results, and a creep model that considers pore pressure and stress coupling using variable-order fractional operators was developed. The model’s reasonableness was confirmed using creep experimental data. The damagestate adjustment factors ω and β were identified through a parameter sensitivity analysis to characterize the effect of pore pressure and stress coupling on the creep damage characteristics(size and degree of difficulty) of coal.

Factors Influencing Proppant Transportation and Hydraulic Fracture Conductivity in Deep Coal Methane Reservoirs

The gas production of deep coalbed methane wells in Linxing-Shenfu block decreases rapidly,the water output is high,the supporting effect is poor,the effective supporting fracture size is limited,and the migration mechanism of proppant in deep coal reservoir is not clear at present.To investigate the migration behavior of proppants in complex fractures during the volume reconstruction of deep coal and rock reservoirs,an optimization test on the conductivity of low-density proppants and simulations of proppant migration in complex fractures of deep coal reservoirs were conducted.The study systematically analyzed the impact of various fracture geometries,proppant types and fracturingfluid viscosities on proppant distribution.Furthermore,the study compared the outcomes of dynamic proppant transport experiments with simulation results.The results show that the numerical simulation is consistent with the results of the proppant dynamic sand-carrying experiment.Under the conditions of low viscosity and large pumping-rate,a high ratio of 40/70 mesh proppant can facilitate the movement of the proppant to the depths of fractures at all levels.The technical goal is to create comprehensive fracture support within intricate trapezoidal fractures in deep coal and rock reservoirs without inducing sand plugging.The sand ratio is controlled at 15%–20%,with a proppant combination ratio of 40/70:30/50:20/40=6:3:1.Proppant pumping operations can effectively address the issue of poor support in complex fractures in deep coal formations.The research results have been successfully applied to the development of deep coalbed methane in the Linxing-Shenfu block,Ordos Basin.

Double-directional control bolt support technology and engineering application at large span Y-type intersections in deep coal mines

Under deep and complex geological conditions,severe deformation occurs at intersection points of Y-type roadways with large cross sections during engineering projects in coal mines,especially at junction arches.Based on in-situ investigations and theoretical studies,we have summarized typical forms of destruction and identified high stress and unrestricted support at both sides of junction arch as its main causes.In this study,we also presented double-directional control bolt support technology for a large Y-type span intersection,applied to deep intersection engineering in the Jiahe Coal Mine,which has proved effective.

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 geo-mechanical 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 dis-tribution 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.

Surrounding rock control mechanism of deep coal roadways and its application

Aiming at the surrounding rock control problem of mining and preparation entries in Xingdong mine with large mining depth, and the comprehensive control countermeasures including high pre-stress cable truss system, this study put forward powerful anchor support system and anchor cable adaption technology to surrounding rock deformation. Furthermore, the control measures possess the supporting performance with ‘‘primary rigid-following flexible-new rigid, and primary resistance-following yield-new resistance'', which suits deep roadway surrounding rock control. The mechanical model of truss anchor supporting roof beams was established, and the inverted arch deflection produced by the cable pre-stress with stress increment effect and roof beam deflection were obtained. And then the system working mechanism was illustrated. Finally, the surrounding rock support parameters were determined by means of comprehensive methods, and put into practice. The results show that surrounding rock deformation realized secondary stability after three months. The roadway sides convergence value was less than 245mm, and roof subsidence was less than 124mm. In addition, there was no expansion and renovation during service period.

Soft–strong supporting mechanism of gob-side entry retaining in deep coal seams threatened by rockburst

When gob-side entry retaining is implemented in deep coal seams threatened by rockburst, the cementbased supporting body beside roadway will bear greater roof pressure and strong impact load. Then the supporting body may easily deform and fail because of its low strength in the early stage. This paper established the roadside support mechanical model of gob-side entry retaining. Based on this model,we proposed and used the soft–strong supporting body as roadside support in the gob-side entry retaining. In the early stage of roof movement, the soft–strong supporting body has a better compressibility, which can not only relieve roof pressure and strong impact load, but also reduce the supporting resistance and prevent the supporting body from being crushed. In the later stage, with the increase of the strength of the supporting body, it can better support the overlying roof. The numerical simulation results and industrial test show that the soft–strong supporting body as roadside support can be better applied into the gob-side entry retaining in deep coal seams threatened by rockburst.

Research on space-time coupling action laws of anchor-cable strengthening supporting for rock roadway in deep coal mine

In order to obtain space-time coupling relationship of anchor-cable to improve supporting effect for deep coal mine rock roadway, FLAC3D was used to investigate into mechanical characteristics of the roadway whose crosssection shape was vertical wall and semi-circular arch when the roadway was supported by bolts and metal mesh. The results show that the extent of stress concentrations, the range failure zone, and the deformation at the roof center and two spandrels of roadway are greater than those at other positions, except at the floor. The reasonable positions of anchor-cable supporting are the roof center and two spandrels of roadway. The anchor-cable should be installed at good time with bolts supporting after roadway driving be- cause it can improve the stress states of deep surrounding rock around the roadway and control the roadway deformation effec- tively. The engineering practice has proven that the sustained deformation of deep surrounding rocks is effectively controlled when the anchor-cable supporting is adopted at reasonable positions of the roadway at good time.

Microstructure and its influence on CH_4 adsorption behavior of deep coal

In this paper we investigate the influence of microstructure on the CH4 adsorption behavior of deep coal. The coal microstructure is characterized by N2 adsorption at 77 K, scanning electron microscopy （SEM）, Raman spectroscopy, and Fourier transform infrared spectroscopy （FT-IR）. The CH4 adsorptions are measured at 298 K at pressures up to 5.0 MPa by the the volumetric method and fitted by the Langmuir model. The results show that the Langmuir model fits well with the experimental data, and there is a positive correlation with surface area, pore volume, ID/IG, and CH4 adsorption capacity. The burial depth also affects the methane adsorption capacity of the samples.

Evolution characteristics of precursor information of coal and gas outburst in deep rock cross‑cut coal uncovering

As mines become deeper,the potential for coal and gas outbursts in deep rock cross-cut coal uncovering is enhanced.The outburst precursors are unclear,which restricts the effectiveness and reliability of warning systems.To reveal the evolution characteristics of coal and gas outburst precursor information in deep rock cross-cut coal uncovering,briquette specimens are constructed and experiments are conducted using a self-developed true triaxial outburst test system.Using acoustic emission monitoring technology,the dynamic failure of coal is monitored,and variations in the root mean square(RMS)of the acoustic emissions allow the effective cracking time and effective cracking gas pressure to be defined.These characteristics are obviously different in deep and shallow coal.The characteristic parameters of gas outburst exhibit stepwise variations at different depths.The RMS and cumulative RMS have stepped failure characteristics with respect to changes in gas pressure.The characteristic parameters of coal failure are negatively correlated with the average in-situ stress and effective stress,but positively correlated with the lateral pressure coefficient of in-situ stress and the critical gas pressure.The transition characteristics are highly sensitive in all cases.The critical depth between deep and shallow coal and gas outbursts is 1700 m.The expansion multiple of acoustic emission intensity from the microfracture stage to the sharp-fracture stage of coal is defined as the outburst risk index,N1.For depths of 1100–1700 m,N1≥7 denotes a higher risk of outburst,whereas at depths of 1700–2500 m,N1≥3 indicates enhanced risk.

Study of a low-disturbance pressure-preserving corer and its coring performance in deep coal mining conditions

With the increasing depth of coal mining,the requirements for coring devices that maintain pressure are increasing.To adapt to the special environment in deep coal seams and improve the accuracy of testing gas content,a low-disturbance pressure-preserving corer was developed.The measurement of gas content using this corer was analyzed.The coring test platform was used to complete a coring function test.A pressurized core with a diameter of 50 mm was obtained.The pressure was 0.15 MPa,which was equal to the pressure of the liquid column of the cored layer,indicating that the corer can be successfully used in a mud environment.Next,a pressure test of the corer was conducted.The results showed that under conditions of low pressure(8 MPa)and high pressure(25 MPa),the internal pressure of the corer remained stable for more than 1 h,indicating that the corer has good ability to maintain pressure.Therefore,the corer can be applied at deep coal mine sites.The results of this research can be used to promote the safe exploitation of deep coal mines and the exploitation of methane resources in coalbeds.

RESEARCH ON DEEP COAL SEAM MINING FLOOR STARTA WATER BURSTING INFLUENCED FACTORS BASED ON ANALYTIC HIERACHY PROCESS

Deep coal seam mining floor strata water bursting is a complicate nonlinear system, whose factors are coupling and influencing themselves. It built the analytic structure model for deep coal seam mining floor strata water bursting, the judgment matrix was found by the expert scoring method, the contribution weights of the influenced factors were given out by the equation analytic process. The thirteen controlling factors and five main controlling factors were put award by analyzing weights, so the result was basically conform to the field practice. The expert scoring method and analytic process can convert the objective fact to the subjective cognition, so it is a method that can turn the qualitative into the quantitative. This can be relative objectively and precisely to study the question of many factors and grey box.

Performance Evaluation of Low-Carbon and Clean Transformation of China’s Coal Economy

In China,the oversupply of coal occurred in 2009,and from that year onwards,China’s coal economy began a low-carbon and clean transformation.Evaluating transformation performance is the research goal of this paper.The data collection for this paper includes data on deep processing of Chinese coal products from 2009 to 2020,as well as data on asset structure evolution and financial performance of 34 listed companies in the Chinese coal mining.Entropy value method is used to calculate the entropy value of low-carbon transformation,and the regression analysis is used to study the performance of cleaner transformation,the conclusion is as follows:(1)From 2009 to 2020,in China’s total energy consumption,coal consumption accounted for 71.6%in 2009 and 56.8%in 2020,the goals set by the state have been achieved.(2)The national goal of reducing the proportion of coal consumption and reducing carbon emissions has forced the transformation of deep processing of coal products.The transformation of coal enterprises towards low-carbon and clean production has achieved remarkable results.(3)From 2009 to 2020,the non coal industry income of 34 listed companies in China’s coal mining industry increased by 8.21%annually.At the same time,the asset structure was adjusted,and nearly 80%of the asset structure evolution showed an orderly development trend.(4)The regression analysis results show that the entropy value of coal deep processing products and the entropy value of asset structure adjustment are significantly related to transformation performance.The paper proposes to summarize the successful experience of China’s coal energy economic transformation,lay a foundation for achieving the carbon peak and carbon neutral goals in the future,further increase the intensity of coal deep processing,increase the proportion of clean energy in total energy consumption,and strive to control asset operation towards the goal of increasing the proportion of non coal industry income.

Ground stability of underground gateroad with 1 km burial depth： A case study from Xingdong coal mine, China

This paper presents an integrated investigation of the ground stability of a deep gateroad with a 1 km burial depth based on a field test, case studies, and numerical modelling. In situ stress measurements and mechanical properties tests were first conducted in the test site. Then, the deformation behavior, stress and yield zone distributions, as well as the bolts load of the gateroad, were simulated using FLAC3D software. The model results demonstrated that the soft rock properties and high in situ stress were the main factors for the deep gateroad instability, and the shear slip failure induced by the high stress was the primary failure model for the deep rock mass. In addition, the unsuitable support patterns, especially the relatively short bolts/cables with low pre-tensions, the lack of high-strengthen secondary supports and the unsupported floor strata, also contributed to the gateroad instability. Subsequently, a new combined supporting strategy, incorporating longer bolts/cables, yielding ring supports, and grouting measures, was proposed for the deep gateroad, and its validity was verified via field monitoring. All these could be a reference for understanding the failure mechanism of the gateroad with 1 km burial depth.

The effect of the density difference between supercritical CO_(2) and supercritical CH_(4) on their adsorption capacities: An experimental study on anthracite in the Qinshui Basin

Deep unmineable coals are considered as economic and effective geological media for CO_(2) storage and CO_(2) enhanced coalbed methane(CO_(2)-ECBM) recovery is the key technology to realize CO_(2) geological sequestration in coals. Anthracite samples were collected from the Qinshui Basin and subjected to mercury intrusion porosimetry, low-pressure CO_(2) adsorption, and high-pressure CH_(4)/CO_(2) isothermal adsorption experiments. The average number of layers of adsorbed molecules(ANLAM) and the CH_(4)/CO_(2) absolute adsorption amounts and their ratio at experimental temperatures and pressures were calculated. Based on a comparison of the density of supercritical CO_(2) and supercritical CH_(4), it is proposed that the higher adsorption capacity of supercritical CO_(2) over supercritical CH_(4) is the result of their density differences at the same temperature. Lastly, the optimal depth for CO_(2)-ECBM in the Qinshui Basin is recommended. The results show that:(1) the adsorption capacity and the ANLAM of CO_(2) are about twice that of CH_(4) on SH-3 anthracite. The effect of pressure on the CO_(2)/CH_(4) absolute adsorption ratio decreases with the increase of pressure and tends to be consistent.(2) A parameter(the density ratio between gas free and adsorbed phase(DRFA)) is proposed to assess the absolute adsorption amount according to the supercritical CO_(2)/CH_(4) attributes. The DRFA of CO_(2) and CH_(4) both show a highly positive correlation with their absolute adsorption amounts, and therefore, the higher DRFA of CO_(2) is the significant cause of its higher adsorption capacity over CH_(4) under the same temperature and pressure.(3) CO_(2) adsorption on coal shows micropore filling with multilayer adsorption in the macro-mesopore, while methane exhibits monolayer surface coverage.(4) Based on the ideal CO_(2)/CH_(4) competitive adsorption ratio, CO_(2) storage capacity, and permeability variation with depth, it is recommended that the optimal depth for CO_(2)-ECBM in the Qinshui Basin ranges from 1000 m to 1500 m.

Laboratory investigation of coal sample permeability under the coupled effect of temperature and stress

The stress and temperature sensitivities of coal reservoirs are critical geological factors affectingcoalbed methane(CBM)well exploitation;in particular it is important to reduce or eliminate their influence on coal reservoir permeability.To investigate coal permeability behavior at various effective stresses and temperatures,CH4 permeability tests were conducted on raw coal samples under a varying effective stress of 2.0-8.0 MPa under five different temperatures(25℃-65℃)in the laboratory.The results show that the permeability of the coal samples exponentially decreases with increasing effective stress or temperature,which indicates obvious stress and temperature sensitivity.Through a dimension-less treatment of coal permeability,effective stress,and temperature,a new stress sensitivity index S and tempera-ture index ST are proposed to evaluate coal stress and temperature sensitivity evaluation parameters.These new parameters exhibit integrality and uniqueness,and,in combination with stress sensitivity coefficient αk,tempe-rature sensitivity coefficient αT,and the permeability damage rate PDR,the sensitivities of coal permeability to stress and temperature are evaluated.The results indicate that coal sample stress sensitivity decreases with increasing effective stress,while it first decreases and then increases with increasing temperature.Additionally,coal sample temperature sensitivity shows a downward trend when temperature increases and fluctuates when effective stress increases.Finally,a coupled coal permeability model considering the impacts of effective stress and temperature is established,and the main factors affecting coal reservoir permeability and their control mechanism are explored.These results can provide some theoretical guidance for the further development of deep CBM.