Coal Side Research Articles

Comparative study of model tests on automatically formed roadway and gob-side entry driving in deep coal mines

Automatically formed roadway (AFR) by roof cutting with bolt grouting (RCBG) is a new deep coal mining technology. By using this technology, the broken roadway roof is strengthened, and roof cutting is applied to cut off stress transfer between the roadway and gob to ensure the collapse of the overlying strata. The roadway is automatically formed owing to the broken expansion characteristics of the collapsed strata and mining pressure. Taking the Suncun Coal Mine as the engineering background, the control effect of this new technology on roadways was studied. To compare the law of stress evolution and the surrounding rock control mechanisms between AFR and traditional gob-side entry driving, a comparative study of geomechanical model tests on the above methods was carried out. The results showed that the new technology of AFR by RCBG effectively reduced the stress concentration of the roadway compared with gob-side entry driving. The side abutment pressure peak of the solid coal side was reduced by 24.3%, which showed an obvious pressure-releasing effect. Moreover, the position of the side abutment pressure peak was far from the solid coal side, making it more beneficial for roadway stability. The deformation of AFR surrounding rock was also smaller than the deformation of the gob-side entry driving by the overload test. The former was more beneficial for roadway stability than the latter under higher stress conditions. Field application tests showed that the new technology can effectively control roadway deformation. Moreover, the technology reduced roadway excavation and avoided resource waste caused by reserved coal pillars.

Measure technology for loose circle and its application

Based on the 88,302 working face of Baode coal mine, in order to provide the basis for the auxiliary roadway maintenance of the working face, this paper introduces the test principle, test method, test process and result analysis of the loose circle. The analysis shows that the loose range of the two ribs of the auxiliary haulage roadway in 88,302 working face increases gradually with the reduction of the distance from the working face. Affected by mining, the maximum loosening range of roadway coal pillar side is 2.4 m, and the maximum loosening range of coal wall side is 2.2 m. It provides the basis for determining the reasonable bolt support parameters, and provides the practice for the loose circle test of the roadway under similar conditions.

Characteristic studies of heat sources and performance analysis of free-cooling assisted air conditioning and ventilation systems for working faces of mineral mines

High temperature heat hazard at mineral mine becomes more and more serious as the increase of mining depth. Heat sources at working faces of mineral mines are complex and are of different characteristics, presenting new challenges for air conditioning systems. In this paper, heat sources at four types of working faces are summarized and their characteristics are investigated. Based on this, simplified equations, which are linear with length of working faces, are proposed to calculate heat dissipation rates. So that the main heat sources of different working faces can be found, and cooling load of air conditioning systems can be calculated. Then, considering main heat sources of coal mines, a typical working face is designed to investigate performances of different ventilation systems and air conditioning systems. Simulation results show that segmented ventilation systems (SC) and heat shield assisted centralized ventilation systems (CCHS) can realize much better temperature distributions at working faces. However, cooling load can be greatly reduced for CCHS, when untreated air is supplied to the coal seam side. Based on this, free cooling assisted air conditioning systems are designed, and annual average energy efficiency ratio (EERann) of the systems are investigated and compared between direct evaporate cooling and indirect evaporate cooling (IEC). For SC, as compared with scenarios without free-cooling, IEC can increase EERann by 15%–23% and 22%–32% under Benxi and Datong ambient conditions, respectively. Besides, to ensure high EERann, CCHS is preferred and it is essential to increase thermal insulation of air ducts.

Numerical Analyses on Deformation and Failure Characteristics of Surrounding Rock of Gob-Side Entry Retention by Roof Cutting

Universal distinct element code (UDEC) is a simulation software based on the discrete element method, widely used in geotechnical mining. However, in the UDEC, when simulating large-scale excavation, the subsidence of the fractured zone is almost equal to the mining height, which makes the deformation value calculated in the study of gob-side entry retention too large. To solve this problem, in this paper, the double-yield constitutive model is applied to the whole caving zone to analyze the deformation and failure characteristics of surrounding rock along gob-side entry retaining by roof cutting. The results of the simulation are in good agreement with the result of drilling peeking (drilling observation by borehole televiewer) and field condition (observation and measurement in the field). Finally, by using this numerical method, the effects of roadway width, temporary support, and coal side support on the failure of the roof and the arc coal side are studied.

Numerical study of stability of mining roadways with 6.0-m section coal pillars under influence of repeated mining

This paper presents the results of a numerical study of the stability of mining roadways in double-roadway excavation with a 6.0-m coal pillar. This excavation method is used to alleviate the imbalance between mining and tunneling and improve the recovery rate of coal resources. The roadways on both sides of the 6.0-m coal pillar were excavated before the mining activities, resulting in a complex stress environment of the coal pillar. A meticulously validated numerical model was developed to investigate the dynamic evolution characteristics of the 6.0-m coal pillar vertical stress and plastic state. The results indicated that the coal pillar entered the residual state and that the solid coal side of the next panel became the main bearing body under the abutment load caused by the mining activity of the 8311 panel. The abutment load transfer process from the 8311 gob to the 8312 panel exhibited obvious non-instantaneity. During the mining of the 8312 panel, an L-shaped high-stress area was formed at the upper end of the working face, owing to the superposition of the abutment load of two adjacent gobs. When the coal pillar transitioned from an elastic state to a plastic state, large deformation and failure occurred. To ensure roadway safety during the service period, the combined control technologies of (1) roof cutting using dense drilling and (2) support strengthening were proposed. Field observations indicated that the stability of the roadway next to the 6.0-m coal pillar was maintained and that the severity of other ground control problems was reduced.

Three‐Dimensional Physical Simulation and Control Technology of Roof Movement Characteristics in Non‐Pillar Gob‐Side Entry Retaining by Roof Cutting

An overlying rock structure plays a key role in controlling the roof deformation of nonpillar gob‐side entry retaining by roof cutting. On the bases of the actual geological conditions of II 632 Haulage Roadway at the Hengyuan coal mine, a similar three‐dimensional simulation experiment of roof precutting is conducted. Thereafter, the caving characteristics and migration law of the roof strata in the strike and dip directions are obtained. Moreover, the roof of the retained roadway and key strata of the goaf can form a hinge structure of the key blocks. By monitoring the deformation of the surrounding rock and stress distribution of the roof, the skew deformation characteristics of roadway roof are obtained. By observing the borehole peeping technology, the roof subsidence near the goaf is determined to be greater than that of the solid coal side, and the roof subsidence of the gob‐side entry retained by roof cutting is greater than that of the floor heave and two sides approaching. Results of the three‐dimensional similar simulation experiment indicate that the mechanical structure model of the key block of the retained roadway roof is constructed, and the mechanical analytical solution of the required support resistance of the retained roadway roof is obtained. This study proposes the constant resistance and large deformation anchor cable reinforcement support method to control the roof deformation of the retaining roadway. Through engineering application, the maximum value of the roof and floor movement of the retained roadway is stable at approximately 650 mm. The retained roadway can meet the demand of the next mining face.

Mechanical Analysis of the Failure Characteristics of Stope Floor Induced by Mining and Confined Aquifer

Mining above confined aquifer has become an important task for water inrush prevention in China. To study the failure characteristics of stope floor along the strike, a mechanical model under combined action of mining and confined aquifer was constructed, and the distribution of vertical stress, horizontal stress, and shear stress was obtained. Based on the Mohr–Coulomb criterion, the failure range of the floor is determined and verified by the in situ test. The results indicate the following. (1) Both vertical stress and horizontal stress in the stope floor take the junction of stress increasing area and stress decreasing area as the dividing line, forming two groups of “convex arches” at the solid coal side and the goaf side, respectively. (2) The vertical stress gradient in the solid coal side is significantly higher than that in the goaf side, while the horizontal stress gradient in the solid coal side is similar to that in the goaf side. The shear stress distribution is divided into three regions by the boundary between positive and negative shear stress, which makes the stope floor in this area to show compression shear or tension shear failure. (3) According to the in situ test, the maximum floor failure depth of 41503 working face is 11.38 m, which is quite close to the theoretical calculation result of 9.68 m. (4) Applying the mechanical model to five other coal mines with different mining conditions and stress states, the maximum absolute error between the measured and theoretical values of floor failure depth is 1.1 m, the average absolute error is 0.8 m, the maximum relative error is 8.2%, and the average relative error is 6.5%. The study provides a certain mechanical basis and reference for the floor failure mechanism induced by mining and confined aquifer.

Study on the Migration Law of Overlying Strata of Gob-Side Entry Retaining Formed by Roof Cutting and Pressure Releasing in the Shallow Seam

A mechanical model of a hard roof was built to analyze the pressure relief roof cutting (RCPR) process for gob-side entry retaining (GER) and identify the critical stage of roadway stability control during RCPR. Based on the mechanical analysis of key parameters of automatic roadway with RCPR, the FLAC3D software was adopted to conduct a numerical simulation to investigate the influence law of height and angle of RCPR, to analyze the trend of variations in the vertical displacement of roadway surrounding rock stress and the roof under different conditions, and to verify that the optimal roof cutting height and seam cutting angle of the 12201 working face of Halagou Coal mine are 6 m and 20°, respectively. The effect of automatic roadway with RCPR has been well implemented through conducting the bidirectional cumulative blasting test on site. To impose effective roadway surrounding rock controlling measures on Halagou Coal mine in RCPR of hard, coal-bearing roof structures at a shallow mining depth, constant resistance large deformation anchor cables, in combination with a single hydraulic prop, joist steel 11#, and steel mesh reinforcement, could provide active surrounding rock support. In addition to the active support, surrounding rock control could be strengthened using grouting bolts. Based on the result, the stress in the roadway coal side and the vertical displacement of the roof can be reduced through increasing the roof cutting height, contributing to the stability of the roadway. Increasing of the roof cutting angle will lead to the increasing of stress in the coal side of the roadway and the increasing of roof displacement with a maximum angle of 20°. Meanwhile, the peak of stress concentrating on both sides of the extreme angle is decreased. Thus, increasing the cutting roof angle at random can be unfavorable to the management of roadway roof. To develop RCPR GER, roadway surrounding rock requires greater support when the mine face passes through a cutting slot. After industrial trials, these measures are proved to be effective in controlling surrounding rock movement and developing GER.

Research on Failure Characteristics and Zoning Control Technology of Thick-Soft Surrounding Rock for Deep Gob-Side Entry Retaining

Controlling the large deformation caused by bed separation failure of thick and soft surrounding rock in gob-side entry retaining is difficult. The deformation and failure modes of thick and soft surrounding rock are summarized and classified based on field research, theoretical analysis, laboratory tests, and actual measurements. Systematic research is conducted on the lithologic characteristics, failure characteristics, and control methods of the surrounding rock. The research findings are as follows: (1) The low strength, softening, and water swelling of thick mudstone, as well as its cataclasis, dilatancy, and long-term creep under strong mining and high stress are the objective reasons for large deformation of the surrounding rock. (2) Due to the weak stability of the surrounding rock-support structure and low collaborative roof side bearing capacity, no complete supporting structure is formed with the supporting system, causing the support body in each area to be crushed one by one, which is the subjective reason for the deformation and failure of the surrounding rock. (3) The deformation and failure characteristics of thick and soft surrounding rock in gob-side entry retaining are primarily divided into eight types: roof bending and sinking type, roof cutting along filling body, rib spalling type, roof fall type, filling body rotation type, filling body crushing type, roof step type, and roof cutting along the coal side. The initial points and key points for a chain reaction of each failure type are determined. (4) The surrounding rock is divided into 10 support zones at four levels, and control technology for “zonal support and overall reinforcement” is put forward. The mechanical effect of the support body in each zone and its role in maintaining the stability of surrounding rock are analyzed. This technology can ensure the integrity of the surrounding rock structure and improve the roof side collaborative bearing capacity.

Research on Plastic Zone Evolution Law of Surrounding Rock of Gob-Side Entry Retaining under Typical Roof Conditions in Deep Mine

To solve the control problem of the surrounding rock of gob-side entry retaining under typical roof conditions in deep mines, we conduct theoretical analysis, numerical simulation, and actual measurements. Starting from the plastic zone of the surrounding rock, the serious damage area, the degree and scope of damage, and the dynamic evolution process of the surrounding rock of the gob-side entry retaining are systematically analyzed under four typical roof conditions in deep mines; the expansion and evolution laws of the plastic zone of the surrounding rock are expounded; and a key control technology is proposed. The results indicate that (1) the plastic failure of surrounding rock was concentrated mainly on the coal side and on the floor, especially in the filling body. The plastic zone of the surrounding rock of the gob-side entry retaining with the thick immediate roof was widely distributed and deep, but the plastic failure of the filling body was not obvious. The plastic failure of the surrounding rock of the gob-side entry retaining with the compound roof was mainly concentrated on the roof, filling body, and floor of the filling area. (2) According to the typical roof conditions of the deep gob-side entry retaining, the order of the degree of damage to the surrounding rock was as follows: thick immediate roof, compound roof, thin immediate roof, and thick-hard roof. (3) A “multisupport structure” control system is proposed for the gob-side entry retaining in a deep mine, including measures for enhancing the bearing performance of the anchorage system, increasing the strength of the cataclastic coal-rock mass, enhancing the bearing capacity of the filling body, and increasing the bearing capacity on the tunnel side. The proposed technology was applied to the deep gob-side entry retaining project in the east area of Panyi Mine, and it effectively fulfilled the reuse requirements of gob-side entry retaining in deep mines.

Geomechanics model test research on automatically formed roadway by roof cutting and pressure releasing

The N00 mining technology is a new coal mining method of automatically formed roadway (AFR). The roadway is created by roof cutting and pressure releasing (RCPR). Compared with the traditionally longwall method, this new technology does not require excavating mining roadways and reserving protective coal pillars (no-pillar). In order to make an in-depth study on the law of the mining pressure behavior, the first working face of N00 mining technology around the world is taken as the engineering background in this manuscript. A three-dimensional geomechanics model test system is independently developed for the N00 mining technology and the model test is carried out. Combined with numerical comparative analysis, the law of surrounding rocks stress evolution and overlying strata movement of AFR is obtained. The variation law of roof and floor convergence deformation of AFR has obvious zoning characteristics, which can be divided into four significant stages. These stages are the pressure releasing stage, the dynamic pressure deformation rising stage, the dynamic pressure deformation stabilizing stage, and the compaction stabilisation stage, respectively. The overload test results indicate that the key damaged parts are the shoulder corner of solid coal side, and the gangue rib of AFR. The research results can be used as a reference for theoretical research, engineering design and field application of the N00 technology.

Failure analysis and control mechanism of gob-side entry retention with a 1.7-m flexible-formwork concrete wall: A case study

Gob-side entry retention (GER) improves coal resources recovery rate and solves the gas problem at the working face and is thus of great importance. In consideration of the existing GER results that lack a failure mechanism for GER with double roadways and roadside backfilling body (RBB) width below 2 m, taking the first test section of GER with double roadways of No. 3 coal seam of the Hexi mine as the research object, the uniaxial compressive strength of each age concrete is obtained through laboratory tests. Numerical simulation and theoretical calculations are used to comprehensively determine the optimal width of the flexible-formwork concrete wall of GER. The distribution and evolution of plastic zone and vertical stress of surrounding rock in different positions are simulated after mining for a certain distance. The results demonstrate that the plastic zone of surrounding rock of the roadway evolves morphologically as asymmetrical ellipse → asymmetrical circle → small semicircular arch → large semicircular arch. The maximum depths of plastic zone and the peak vertical stress in solid coal side are 3.56 m and 3.50 m, respectively, and the roof and solid coal present obvious asymmetric failure characteristic. The flexible-formwork concrete wall is completely in the plastic state, and the peak stress is mainly concentrated in the middle of RBB. Based on the above analyses, a zoning asymmetric coupling control technology named narrow flexible-formwork wall with steel bar reinforcement along single prop wall + strong double-row single props + high-strength and high-preloading asymmetric anchor cable is proposed, and its support mechanism is clarified. The on-site monitoring results demonstrate that the convergence of surrounding rock of GER is effectively controlled.

Effect of heterogeneous tar condensation on coking pressure dynamics – Qualitative numerical analysis

Abstract Having regard to the practical role of predicting the peak pressure generation inside a coke oven and the demand for recognizing the factors affecting the pressure levels, the transient one-dimensional of thermal and flow processes in a coke oven charge was developed. Tar clogging the cavities between coal particles is considered a factor additional to low permeability of softened coal that may have an effect on pressure increase, however not yet enough described. In order to analyze this issue, the phase transition of heavier pyrolysis gas components was accounted for in the proposed model. Obtained computation results provide the numerical evidence for partial coal impregnation, experimentally revealed by other researchers, clearly indicating the appearance of tar condensation zone in the area of parent coal that is adjacent to coal plastic layer. The progress in tar accumulation accompanying the softened stage layer migration towards an oven center as the process proceeds, was demonstrated. The applied approach allowed to illustrate the change in pressure distribution within the coal/coke bed, reproducing an effect of gas flow blockage on the parent coal side due to reduced permeability of both layers, that of softened coal and that occupied by condensed tar.

Coal pillar size design and surrounding rock control techniques in deep longwall entry

We combine experimental data, FLAC3D numerical simulation results, and on-site industrial tests to study the problem of coal pillar size design in longwall entry. The Zhaozhuang coal mine located in Changzhi City, Shanxi Province, China, is used as a case study. We use an inversion calculation of the double-yield constitutive model to improve the reliability of the simulation results. Calculated results are fitted using a theoretical value of the Salamon strength formula to obtain mechanical parameters of the rock mass in the goaf, which are used to numerically simulate its compaction-bearing characteristics. The stress distribution state and plastic zone failure characteristics of entry surrounding rock under different coal pillar widths during mining are obtained by numerical calculation. The results show that increased coal pillar size leads to a gradual shift of the peak stress from the solid coal side to the coal pillar side. Use of an 8-m coal pillar generates a lower stress environment and has sufficient bearing capacity to remain stable. Based on the results obtained here, we propose differentiated support techniques that address the differential deformation and stress distribution of surrounding rock both sides of the goaf-side entry. On-site industrial tests show that the proposed support technology effectively reduces the deformation variation and ensures safe entry use during servicing.

Combined Support Technology of Retained Entry in Large Mining Height Face with Double Roadways Layout

The stability of retained roadway plays an important role in the safety production of coal mine underground working face. Taking the 21,101 auxiliary haulage gate in Yaping Mine as the engineering background, the supporting technology was systematically studied. Firstly, based on the previous situation of pillar setting and surrounding rock failure and deformation in double roadway layout, the reasonable size of pillar between roadways was studied. The width of the broken zone was realized as 2.95 m by theoretical analysis, and then the width of pillar between roadways was determined as 7.95 m. On this basis, the law of stress evolution and retained roadway deformation with the pillar width of 6 m, 8 m, 10 m, 12 m, 15 m under the influence of two times mining were analyzed, comprehensive analyzed the above research results, 8 m in width of pillar between roadways was determined. Secondly, the failure rules of pillar between roadways, the failure rules and the failure process of retained roadways during two times mining were revealed: the range between 70 m in front of the work and 80 m behind the work was determined as the mining influence area according to the damage degree of coal pillar, the failure depth of surrounding rock in retained roadway during the two times mining was the largest in the pillar side, the second in the roof and the coal wall side, the smallest in the floor, and the damage degree was the largest in pillar side, the second in floor, the smaller in roof and wall, and the overall failure of roadway was asymmetric. Finally, on the basis of the above research results, considering all kinds of influencing factors and technical difficulties of retained roadway surrounding rock support, the system analysis method was used to determine the combined support technology with high strength allowable cables support + asymmetric cables support + bolts support system + concrete floor-allowable pressure non-repeated supports dynamic strengthening support. Practice shows that the combined support technology is scientific and reliable, which can meet the safety mining requirements in large mining height face with double roadways layout.

Squeezing failure behavior of roof‐coal masses in a gob‐side entry driven under unstable overlying strata

AbstractThis paper endeavors to reveal the squeezing failure mechanism of roof‐coal masses and its correlation with the movement of the roof strata. An integrated method incorporating field instruments and numerical modeling was conducted. The borehole camera detection revealed that the development degree and scope of cracks in the roof strata on the coal pillar side are significantly larger than that on the panel rib side, and the fracture line of the main roof occurred above the coal pillar. A rigorous modeling procedure verified the borehole camera detection results and also found that the vertical and horizontal displacement of the roof‐coal mass exhibited asynchronous development characteristics at various heights. Subsequently, a comprehensive control strategy was proposed to prevent the squeezing failure behavior of roof‐coal masses in DUOS gob‐side entry. This study provides a basis for understanding the squeezing failure behavior of roof‐coal masses, and the proposed control strategy can potentially be applied in other similar mining projects.

Mechanism and Prevention of Rockburst in Steeply Inclined and Extremely Thick Coal Seams for Fully Mechanized Top-Coal Caving Mining and Under Gob Filling Conditions

The steeply inclined and extremely thick coal seams (SIETCS) under the condition of gob filling frequently suffer from the occurrence of rockbursts. Figuring out the mechanisms of rockbursts under this condition for taking targeted measures to mitigate rockburst hazards in SIETCS is of great significance. Using the typical SIETCS with an average dip angle of 87° at Wudong Coal Mine (WCM) as a case study, a mechanical model and elastic deformation energy (EDE) function of a “steeply inclined suspended roof structure” was developed, and the influence factors were analyzed by theoretical analysis. Simultaneously, the rockburst risk assessment was carried out based on the theory of a rockburst start-up. The pressure relief measures are optimized by comparing the pressure relief effects of three kinds of destress blasting schemes. The results indicate that the damage characteristics of rockburst are mainly floor heave, the sidewall’s inward deformation and roof subsidence. The damage degree of headentry on the roof side is more severe than that of tailentry, and the resultant impacts showed the directionality from the roof side to the coal side. The steeply inclined and suspended roof breakage is one of the main causes for the occurrence of rockbursts. The EDE of the roof increases with an increasing dip angle of the coal seam from 0° to 72.6° and then decreases as the dip angle increases. Furthermore, that increase is accompanied by the decrease of the lateral pressure coefficient and the supporting force coefficient. The EDE stored in the roof is sufficient to cause roof breakage and induce rockburst after the complete roof exceeds a certain length. The mechanism of rockburst in SIETCS for fully mechanized top-coal caving mining under gob filling conditions was proposed, i.e., “high compressive stress concentration plus breakage of the suspended roof-induced stress” rockburst, and this is further verified by ground destruction, microseismic (MS) monitoring and numerical modeling. The results also indicate that alternate deep and shallow hole-blasting modes are more suitable for pressure relief in SIETCS.

Control mechanism of roof fracture in no-pillar roadways automatically formed by roof cutting and pressure releasing

The technology of automatically forming a no-pillar gob-side entry retaining by roof cutting and pressure releasing (RCPR) effectively cuts off the stress transfer in roofs by directional pre-splitting. Meanwhile, the rock mass of the gob roof falls and fills the gob, taking advantage of the work done by the mining pressure and the broken expand characteristics of the rock mass. Thus, roadways in mining areas are automatically formed, reducing the roadway excavation amount, eliminating reserved coal pillars, and preserving coal resources. The core of this technology is the formation of a roof-cut short-arm beam structure in the roof of the automatically formed roadway (AFR). The fracture location and morphology provide the fundamental basis for the design of the roof cutting and support parameters of the AFR. In this study, the dynamic evolution pattern of the surrounding rock of the roof in the no-pillar AFR by RCPR is analysed to clarify the roof fracture and control mechanisms. The Hoek-Brown failure criterion and the upper bound method are used to establish the fracture mechanics model and the fracture curve equation of the roof in the AFR by RCPR, and the influences of the fracture range of the roof-cut short-arm beam are analysed. The results show that with an increase in the strength of the surrounding rock, the pre-tightening force of the anchor cable, the gob-side roof support force, and the support force of the broken expand gangue or a decrease in the stress of the surrounding rock, the mining influence coefficient, and the cutting angle, the fracture range of the roof-cut short-arm beam gradually transfers towards the solid coal side and the stability of the AFR is enhanced. The findings from this study can provide a theoretical basis for the design of control schemes, including the cutting and support parameters of the no-pillar AFR by RCPR.

Influence of Rainfall Infiltration on Transient Volumetric Water Content of Coal Measure Soil Slope

With coal measure soil side slope in Jiangxi’s Pingxiang-Lianhua Expressway selected as the research object, GeoStudio, a finite element seepage analysis application, was adopted for numerical simulation on the variation pattern of volumetric moisture content in the side slope in rainfall as well as its spatial distribution characteristics. The results show that rainfall infiltration mainly influences the surface layer of the side slope and that its influence decreases with the increase of depth. The volumetric moisture content of undisturbed coal measure soil remains basically unchanged under the weathered layer. When soil mass permeability coefficient is less than rainfall intensity, the rainwater permeates quickly in the outermost layer of a weathering zone, but exerts a minor influence. When it is larger than the rainfall intensity, rainfall permeates slowly but exerts a major influence. When rainfall intensity reaches 8.33 ×10−7 m/s, soil moisture contents in the superficial layers of the slope toe and halfway up the slope surface become saturated. The saturation depth is 80% of the weathered layer. When the rainfall intensity reaches 1.1 ×10−6 m/s, it exerts a more extensive influence on the saturation area in the side slope, but the corresponding affected depth is only 50% of that in the weathered layer.

Research on the Deformation and Control Technology of Surrounding Rock in Entry Retaining along the Gob Side

This paper studies and introduces the successful case of gob‐side entry retaining technology and the typical mining pressure law in Luan mining area, which is the main mining coal seam in Qinshui coalfield. Qinshui coalfield has an estimated coal reserve of 300 billion tons, accounting for 9.58% of the total national coal reserve in China, especially anthracite, chemical coal, and coking coal. The methods of field investigation, theoretical analysis, physical experiment, and industrial test are adopted. Through the field investigation, theoretical analysis, physical experiments, and industrial test, the following conclusions have been drawn in this study: (1) A thorough engineering geological investigation was conducted on the entry retaining along the gob side on noncoal pillar mining working face, which covers multiple periods of mining process including the roadway excavation period, primary mining period, primary mining stability period, and secondary mining influence period. A series of analysis and tests were conducted such as core sampling, rock mechanics property testing, borehole detection, and flexible formwork support evaluation, which laid a foundation for identifying the mining pressure law of gob‐side entry retaining by using noncoal pillar mining. (2) The mining pressure law was studied through the collection of the field measurements taken from the entry retaining along the gob side on noncoal pillar mining working face. The keys to achieve the roadway surrounding rock stability through noncoal pillar mining are obtained. According to the study, the stability control of retained roadway surrounding rock mainly depends on the stability of top coal, coal side, and shoulder angle coal. (3) In this study, a roadway reinforcement scheme is proposed to improve the surrounding rock control technology for gob‐side entry retaining by noncoal pillar mining, whose effectiveness has been verified by a series of industrial test. Therefore, the wide adoption of the noncoal pillar mining method in Number #3 coal mine can significantly relieve the predicament of coal pressing under a large number of buildings in Luan mining area, which provides insightful guidance to the coal‐free pillar mining in the whole Luan mining area.