Failure analysis and control technology of intersections of large‑scale variable cross‑section roadways in deep soft rock

In deep underground mining,achieving stable support for roadways along with long service life is critical and the complex geological environment at such depths frequently presents a major challenge.Owing to the coupling action of multiple factors such as deep high stress,adjacent faults,cross-layer design,weak lithology,broken surrounding rock,variable cross-sections,wide sections up to 9.9 m,and clusters of nearby chambers,there was severe deformation and breakdown in the No.10 intersection of the roadway of large-scale variable cross-section at the−760 m level in a coal mine.As there are insufcient examples in engineering methods pertaining to the geological environment described above,the numerical calculation model was oversimplifed and support theory underdeveloped;therefore,it is imperative to develop an efective support system for the stability and sustenance of deep roadways.In this study,a quantitative analysis of the geological environment of the roadway through feld observations,borehole-scoping,and ground stress testing is carried out to establish the FLAC 3D variable cross-section crossing roadway model.This model is combined with the strain softening constitutive(surrounding rock)and Mohr–Coulomb constitutive(other deep rock formations)models to construct a compression arch mechanical model for deep soft rock,based on the quadratic parabolic Mohr criterion.An integrated control technology of bolting and grouting that is mainly composed of a high-strength hollow grouting cable bolt equipped with modifed cement grouting materials and a high-elongation cable bolt is developed by analyzing the strengthening properties of the surrounding rock before and after bolting,based on the Heok-Brown criterion.As a result of on-site practice,the following conclusions are drawn:(1)The plastic zone of the roof of the cross roadway is approximately 6 m deep in this environment,the tectonic stress is nearly 30 MPa,and the surrounding rock is severely fractured.(2)The deformation of the roadway progressively increases from small to large cross-sections,almost doubling at the largest cross-section.The plastic zone is concentrated at the top plate and shoulder and decreases progressively from the two sides to the bottom corner.The range of stress concentration at the sides of the intersection roadway close to the passageway is wider and higher.(3)The 7 m-thick reinforced compression arch constructed under the strengthening support scheme has a bearing capacity enhanced by 1.8 to 2.3 times and increase in thickness of the bearing structure by 1.76 times as compared to the original scheme.(4)The increase in the mechanical parameters c andφof the surrounding rock after anchoring causes a signifcant increase inσt;the pulling force of the cable bolt beneath the new grouting material is more than twice that of ordinary cement grout,and according to the test,the supporting stress feld shows that the 7.24 m surrounding rock is compacted and strengthened in addition to providing a strong foundation for the bolt(cable).On-site monitoring shows that the 60-days convergence is less than 30 mm,indicating that the stability control of the roadway is successful.

三轴加卸载下层理砂岩的脆性特性

准确定量表征煤或岩石的脆性对有效估测深部工程的开挖效率具有重要意义。层理砂岩在全生命周期演化过程中所受到的应力演化特征必然会伴随有能量演化过程。因此,基于上述能量演化特征,本文提出“三阶段”三轴加卸载应力路径,获得损伤演化规律,并定义了脆性指数BI16。研究结果表明:1)围压可以提高层理砂岩的承载力和塑性变形抵抗能力。此外,加载方式和围压对塑性体积变形的抵抗能力存在显著影响,而倾角对其影响则较小。2)层理砂岩在常规三轴加载条件下的峰前损伤演化过程可划分为三个阶段。同时,层理砂岩在三轴加卸载条件下的峰前损伤演化过程则可划分为四个阶段。3)层理砂岩的整个变形过程会显著影响其脆性,且脆性是动态的、并非恒定的。上述研究结论可为准确估算深地工程的开挖效率提供依据。

Response characteristics of gas pressure under simultaneous static and dynamic load:Implication for coal and gas outburst mechanism

Coal and gas outbursts are dynamic disasters in which a large mass of gas and coal suddenly emerges in a mining space within a split second.The interaction between the gas pressure and stress environment is one of the key factors that induce coal and gas outbursts.In this study,first,the coupling relationship between the gas pressure in the coal body ahead of the working face and the dynamic load was investigated using experimental observations,numerical simulations,and mine-site investigations.It was observed that the impact rate of the dynamic load on the gas-bearing coal can significantly change the gas pressure.The faster the impact rate,the speedier the increase in gas pressure.Moreover,the gas pressure rise was faster closer to the impact interface.Subsequently,based on engineering background,we proposed three models of stress and gas pressure distribution in the coal body ahead of the working face:static load,stress disturbance,and dynamic load conditions.Finally,the gas pressure distribution and outburst mechanism were investigated.The high concentration of gas pressure appearing at the coal body ahead of the working face was caused by the dynamic load.The gas pressure first increased gradually to a peak value and then decreased with increasing distance from the working face.The increase in gas pressure plays a major role in outburst initiation by resulting in the ability to more easily reach the critical points needed for outburst initiation.Moreover,the stronger the dynamic load,the greater the outburst initiation risk.The results of this study provide practical guidance for the early warning and prevention of coal and gas outbursts.

Technical evaluation system of co-extraction of coal and gas

Coal and gas are two important resources in China,and it is an effective way to recycle them with the method of co-extraction of coal and gas.In view of actual situation of China's co-extraction of coal and gas,this research adopts the science evaluation of gas extraction of coal by the method of quantitative evaluation,and preliminarily establishes the technical evaluation system of co-extraction of coal and gas.Technical evaluation system of co-extraction of coal and gas includes safety evaluation,economic evaluation and resource recovery evaluation in the process of mining coal and gas.In addition,this paper results in the selected evaluation parameters and target functions that are used in the three evaluation methods.The establishment of evaluation system for co-extraction of coal and gas plays an important theoretical significance and guiding role in co-extraction of coal and gas for China's coal enterprises.

Experimental investigation on predicting precursory changes in entropy for dominant frequency of rockburst

Rockburst is a dynamic phenomenon accompanied by acoustic emission(AE)activities.It is difficult to predict rockburst accurately.Based on the fast Fourier transform(FFT)method and the information entropy theory,the evolution model of dominant frequency entropy was established.The AE energy,frequency and stress were synthetically considered to predict rockburst.Under the triaxial and the single-face unloading tests,the relationship between AE energy and the development of internal cracks was analyzed.Using the FFT method,the distribution characteristics of AE dominant frequency values were obtained.Based on the information entropy theory,the dominant frequencies evolved patterns were ascertained.It was observed that the evolution models of the dominant frequency entropy were nearly the same and shared a characteristic“undulation-decrease-rise-sharp decrease”pattern.Results show that AE energy will be released suddenly before rockburst.The density of intermediate frequency increased prior to rockburst.The dominant frequency entropy reached a relative maximum value before rockburst,and then decreased sharply.These features could be used as a precursory information for predicting rockburst.The proposed relative maximum value could be as a key point to predict rockburst.This is a meaningful attempt on predicting rockburst.

Three-dimensional spatial and temporal distributions of dust in roadway tunneling

To study the three-dimensional spatial and temporal distributions of dust in tunneling roadways,and to solve the problems of inadequate time and limited number of monitoring points,this paper designs a device for the real-time monitoring and storage of data on the concentrations of dust at multiple measuring points in the same section of a tunnel.The proposed device can measure the total concentration of dust and that of respirable dust in real time at different instances and locations,and using different working procedures.These measurements are used to study the temporal and spatial migration of dust.The results show that there was a sharp fluctuation zone 0–25 m from the heading face,about 25–40 m was high speed subsidence,beyond 40 m was gentle subsidence,The change of respiratory dust is much smoother.At different distances from the heading face,the total dust concentration exhibited a process of“violent oscillation–rapid descent–stable descent,”while the respirable dust exhibited a process of“fluctuating ascent–gradual subsidence.”Changes in the concentrations of total dust and respirable dust dust were consistent at different positions in the same section of the tunnel.The concentration of dust near the wall was low,and those along the sidewalk and air duct of the roadway were slightly higher than in the middle.The concentration of dust farther down the air duct decreased more slowly than that in the remaining lines of measurement.Small amounts of dust featuring large particles settled quickly.High concentrations of dust were observed to be intermittent,and the background value of dust concentration within 100 m of heading face was between 0.5 and 3 mg/m3.

Study on the relation between damage and permeability of sandstone at depth under cyclic loading

The damage and permeability evolution of rock under stress is of great significance to engineering safety.In this paper,the evolution law of rock damage and permeability is studied by means of acoustic emission (AE) seepage experiment on deep roof sandstone with cyclic loading.Characterization of damage uses the changes in acoustic emission fractal characteristics and compression parameter which based on elastic modulus.The experimental results show that the AE events has fractal characteristic,in which the AE b-value and correlation dimension can represent the damage of rock.When the fractal characteristic value of AE increases,it indicates that the rock is in the compaction stage and the damage is not obvious.When the fractal characteristic value of AE drops,it indicates that the rock was damaged,and the permeabilityincrease.Under the cyclic load increasing step by step,the elastic modulus first increases and then decrease.Introducing compression parameter C to characterize the state of compaction and damage,it is obtained that the rock damage state and hydrostatic permeability show a power law function relationship with porosity and have the same monotonicity.When compression parameter is less than zero,the evolution law of permeability and damage can be described by functional relationship between hydrostatic permeability K and compression parameter C.

The role of potential soil cavity on ground subsidence and collapse in coal mining area

Based on five basic assumptions, of the ground subsidence and collapse was using theoretical analysis method, the nature revealed from the mechanics point. Divided into four phases as groundwater level descent, soil cavity formation, soil cavity expansion, and ground collapse emersion, the whole process of ground subsidence and collapse was analyzed in detail. The study shows that ground subsidence and collapse is the macro- scopic performance and inevitable result of the soil cavity expansion and development, and the dynamic mechanics is the spalling force induced by the groundwater falling. The activities of underground water play a very important role in the process from the formation of soil cavity to the production of ground subsidence.

Theory of gas extraction from coal seams and its use

Gas extraction is one of the main measures of control and use of gas of coal mines. At present, the design method is under the experimental period and do not satisfy the need of practice. In this paper, the theory of gas extraction of coal seams based upon Darcy law was studied. Mathematical model of gas extraction of coal seams was established and two kinds of solv- ing approaches based on computer software and linear approximation were given. The rightness and the validities of the model were examined with a practical example. Results obtained can be used to determine and optimize the parameters related etc.

Undesorbable residual gas in coal seams and its influence on gas drainage

The definition of ‘‘residual gas" can be found in different scenarios, such as the ‘‘fast" and ‘‘slow" desorption methods of measuring gas content and the sorption hysteresis test and gas management of coal mines, however, its meaning varies a lot in different contexts. The main aim of this paper is to discuss the existence of truly undesorbable residual gas in coal seam conditions and its impacts on sorption model and gas drainage efficiency. We believe the undesorbable residual gas does exist due to the observation of the extended slow desorption test and the sorption hysteresis test. The origin of undesorbable residual gas may be because of the inaccessible(closed or semi-closed) pores. Some gas molecules produced during coalification are stored in these inaccessible pores, since the coal is relatively intact in the coal seam condition, these gas molecules cannot escape during natural desorption and then create the undesorbable residual gas. Based on the existing adsorption models, we propose the improved desorption versions by taking into consideration the role of residual gas. By numerically simulating a gas drainage case, the gas contents after different drainage times are studied to understand the influence of residual gas content on gas drainage. The results indicate that the influence starts to be obvious even when the total gas content is at a high level, and the impact becomes more and more apparent with increasing drainage time. Our study shows that the existence of residual gas will impede the gas drainage and the total amount of recoverable coal seam methane may be less than expected.

Research on stability of a slope due to underground mining

This paper will present a detailed analysis of the deformation mechanism and stability assessment of the slope through field investigations, numerical modeling and measurements. Field investigation indicated that three thin coal seams encountered large mined-out area at one side and free surface of hill slope at the other side, which lead to the caving of roof strata movement, ground movement and crown crack along the preferred orientations of joints. The three-dimensional numeri- cal modeling study on the case demonstrated that the plasticity failure occurred gradually along with the extension of mined-out area in depth. When the depth of mining reached the verge defined by the seismic prospecting method, a large mount of tension failure occurred on the crown of the slope. The factor of safety was 1.36 calculated by the shear strength reduction technique, which indicated the slope was in stable state. The measurement showed that the residual deformation occurred before 1998 and became stable subsequently, which indicated that the residual deformation almost finished and the slope is in stable state.

The whole process confirmation management and its application in coal mine safe production

After analyzing the reasons for coal mine accidents and the current management methods, effective actions for the implementation of safety management were put forward by carrying out the whole process confirmation management. The ba- sic content and the five implementation steps were described, and the implementation method and the program of every step were introduced. Some rules for the implementation of the whole process confirmation management in coal mine safety pro- duction were explained, such as during the process of preproduction, before descent, descent, after descent, walking in the roadway, post-operation, shift, and hoisting after work. The results show that the guardians and the executors should both con- firm the implementation, which can improve the workers＇ attention and self-awareness to avoid errors in detail and reduce the ＂three violations＂ phenomenon. To ensure the effect of the whole process confirmation management, relevant departments should designate a person-in-charge in the specific work in all stages, make a work plan, and strengthen the internal evaluation.

Variation in gas drainage rate from a coal seam during mining

Gas flow patterns during draining of gas from a coal seam during mining are discussed.The coal seam is treated as a dual medium with both pores and cracks.The seepage,diffusion,and desorption processes are treated using a gas flow equation that describes flow around drill holes.MATLAB is used to solve the differential equations.The permeability tracer test results from a mined coal seam are used to study the variation in gas drainage from a coal seam during mining.The results show that mining can increase the permeability of a coal seam,which then increases the gas drainage.There are inflection points in this variation over time.A close relationship between this variation and the rate of change in coal seam permeability is observed.

Prediction of methane emissions during the extraction of close-to-roof layer of a seam of large thickness

Prediction of methane emissions at the stage of longwall planning constitutes the basis for the determination of the appropriate method and parameters of ventilation and selection of prevention means including the methane drainage technol- ogy. The growth of methane saturation of coal seams with the extraction depth, with simultaneously increasing output concen- tration, contributes to the increase of the quantity of methane emitted into longwall areas. The subject matter of the article has been directed at the predicted quantity of methane emissions into planned longwalls with roof caving in the layer of seams adjacent to the roof of large thickness. The performed prognostic calculations of methane emissions into the longwall working were referred to two sources, i.e. methane liberated during coal mining by means of a cutter-loader and methane originating from the degasification of the floor layer destressed by the longwall conducted in the close-to-roof layer. The calculations of predictions allow to refer to the planned longwall, on account of the emitting methane, with possible and safe output quantity. Planning of extraction in the close-to-roof layer of a seam of large thickness with roof caving is especially important in con- ditions of increasing methane saturation with the depth of deposition and should be preceded by a prognostic analysis for de- termining the extraction possibilities of the planned longwall.

Experimental Study of the Adsorption-Induced Coal Matrix Swelling and Its Impact on ECBM

Carbon dioxide （CO2） enhanced coalbed methane （ECBM） is an effective method to im- prove methane （CH4） production and this technology has already been used to increase gas production in several field trials worldwide. One major problem is the injection drop in the later period due to permeability decrease caused by coal matrix swelling induced by CO2 injection. In order to quantify the swelling effect, in this work, coal samples were collected from the Bulli coal seam, Sydney Basin and adsorption tests with simultaneous matrix swelling measurement were conducted. The adsorption and swelling characteristics were analyzed by measuring the adsorption mass simultaneously with the strain measurement. Then experiments were conducted to replicate the ECBM process using the indi- rect gravity method to obtain the swelfing strain change with CO2 injection. The results show that the coal adsorption capacity in CO2 is almost two times greater than that in CH4, and nitrogen adsorption is the least among these gases. A Langmuir-fike model can be used to describe the strain with the gas pressure and the swelling strain induced by gas adsorption has a Hnear relationship with gas adsorp- tion quantity. Moreover, swelling strain increase was observed when CO2 was injected into the sample cell and the swelling strain was almost the sum of the strains induced by different gases at correspond- ing partial gas pressure.