Strata Behavior Research Articles

Structural Characteristics of the Overlying Strata in a Fully Mechanized Longwall Face with the Low-Position Thick-Hard Roof

Strata activity is enhanced in the condition of the thick seam and large mining height, which increases the difficulties in roof management. A detailed understanding of roof structure characteristics and its movement law is a key to control the strata behavior. Since the existing formula hardly applies to some specific cases of geological conditions, such as the condition of a fully-mechanized longwall face with the low-position thick-hard roof in Suancigou Mine, a new formula of distinguishing structural characteristics of key strata based on a mechanical model of rock beam in periodic motion was proposed. Considering the relationship between the allowed rotating space of key strata and its limited rotation, the judgment criterion whether a cantilever beam can be formed was established. Also, a method for calculating the reasonable working resistance of hydraulic supports under these special roof structure was deduced from the angle of preventing rotating instability of the main roof. A field application was performed at 6-105-2 working face, and the proposed judgment criterion indicated that the LPTHR would break in the form of a voussoir beam structure at the Z1 drilling area, while at Z2 and Z3 drilling area, the broken LPTHR would behave as the cantilever structures. Field monitoring results showed that the limit-rotation-based cantilever structure criterion could accurately discriminate the structural characteristics of LPTHR. According to the calculation formula of support resistance of hydraulic support, the reasonable working resistance of hydraulic supports in 6-105-2 working face was determined to be 16,245 kN.

Behavior Regularity of Dynamic Pressure based on Three-zone Structure Loading Theory for a Typical Mine

In terms of problems concerning safety production caused by strata behaviors in the process of digging on working face 31102 in a certain mine, the author tries to find its behavior regularity of dynamic pressure by theoretical study, field observation and practical analysis. Through dynamically analyzing deformation and failure process of surrounding rock and theoretically analyzing rule of broken and evolution of overlying rock and stress distribution based on three-zone structure loading theory, it is concluded that: First, when the thickness of immediate loading zone on 31102 working face is 60-70m, its influence distance in advance is 30-40m. The periodicity is short. When the height of delay loading zone is 280-320m, the distance is 150-170m. The periodicity is long. Second, The roof movement in low position of the working face determines the intensity of its strata behaviors. Third, the relation between digging speed of working face and energy release of overlying rock’s fracture is obtained, which is that as the digging speed gets faster, the energy and frequency of microseismic events appears liner growth. These research achievements have been applied to testing mines and the effect turns to be good.

Stress Observation and Strata Behavior in Fully-mechanized Coal Face of Shallow Coal Seam

In order to solve the problem of the strong ore pressure in the mining process of a shallow buried coal seam working face, the ore pressure law of a shallow buried coal seam working face was observed. According to the observation results of ore pressure, the law and characteristics of ore pressure on the working face of shallow buried coal seam are analyzed, the basic law and parameters of roof movement and the roof plate falling situation on the end face on the working face are obtained, and the rationality of the support is studied.

Study on Key Parameters of Roof Cutting and Pressure Release in Medium-Thickness Coal Seam

According to the characteristics of strata behaviors under the condition of roof cutting and pressure release, this paper takes the 5-200 working face of Dianping coal mine as the engineering background, and comprehensively adopts the theoretical analysis, numerical simulation and field measurement to study on key parameters of roof cutting and pressure release in medium-thickness coal seam. The results show that: (1) The theoretical analysis determines that the height of roof cutting is 8.3 m, the angle of roof cutting is 15°, and the distance between the blasting holes is 500 mm; (2) The peak value of stress under the condition of roof cutting is lower 38.8% than it been under the condition of non-cutting, and the maximum displacement of the roof is reduced from 2500 to 420 mm. The first weighting length of main roof on the side of cutting roof is increased by 6 m compared with the non-cutting side, the periodic weighting length is increased by 12 m, the peak stress is reduced by 29.3%, and the average stress is reduced by 20.9%. The size of the roadway is 2601 mm × 4673 mm, which is in line with the numerical simulation results. The roof cutting can significantly reduce stress, deformation, improve the retaining effect on roadway and enhance production efficiency.

Movement type of the first subordinate key stratum and its influence on strata behavior in the fully mechanized face with large mining height

The extraction height of a fully mechanized face with large mining height is large, so the mining-induced overlying strata failure area will be enlarged, which may result in various accidents such as water inrush accidents. In this paper, a combination of theoretical analysis, numerical simulations, and field observations was applied to investigate the movement types of the first subordinate key stratum and its influence on strata behavior in the fully mechanized face with large mining height. The results showed that there were two structural forms and six movement types of the first subordinate key stratum in the fully mechanized face with large mining height. Additionally, conditions favoring the formation of these movement types were presented. The two structural forms of the first subordinate key stratum were the cantilever structure and the stable hinged structure. The cantilever structure of the first subordinate key stratum was located in the caved zone and could present four movement types: direct caving movement of the cantilever structure, double-sided rotation movement of the cantilever structure, quadratic rotation movement of the cantilever structure, and alternative movement of the cantilever structure-hinged structure. The stable hinged structure of the first subordinate key stratum was located in the fractured zone and could present two movement types, namely, voussoir beam structure movement and short voussoir beam structure movement. Additionally, the movement law of each movement type of the first subordinate key stratum and its influence on the strata behavior of the fully mechanized face with large mining height were revealed. Finally, the correctness of the six movement types of the first subordinate key stratum and the corresponding formation conditions were verified using measurement data from four fully mechanized faces with large mining height in China. The outcomes may contribute to reasonable selection of supports and ensuring safe mining of the fully mechanized faces with large mining height.

Numerical simulation on strata behaviours of TCCWF influenced by coal-rock combined body

Due to top-coal and immediate roof as cushion layer connecting with support and overlying strata, it can make significant influence on strata behaviors in fully mechanical top-coal caving working face (TCCWF). Taking Qingdong 828 working face as engineering background, FLAC3D and UDEC2D were adopted to explore the influence of top-coal thickness (TCT), immediate roof thickness (IRT), top-coal elastic modulus (TCEM) and immediate roof elastic modulus (IREM) on the vertical stress and vertical subsidence of roof, caving distance, and support resistance. The results show that the maximum roof subsidence increases with the increase of TCT and IRT as well as the decrease of TCEM and IREM, which is totally opposite to vertical stress in roof-control distance. Moreover, although the increase of TCEM and IREM leading to the increase of peak value of abutment pressure, the position and distribution range have no significant change. Under the condition of initial weighting occurrence, support resistance has negative and positive relationship with physical parameters (e.g., TCT and IRT) and mechanical properties (e.g., TCEM and IREM), respectively.

Influence of Different Advancing Directions on Mining Effect Caused by a Fault

To study the correlation among advancing direction, strata behaviors, and rock burst induction, two physical models utilizing similar materials are established. Subsequently, the influence of advancing direction on the mining effect, caused by a fault, is studied. Moreover, the rock bursts affected by faults with different mining directions are compared and analyzed. The results show that the overlying structure varies notably, affected by fault cutting and fault dip, and the fault‐affected zone and the cause of induced rock burst differ with different mining directions. However, regardless of mining directions, the overlying structure of the hanging wall is stable and fault activation is not obvious, while that of the footwall is relatively active and fault activation is violent; the risk of rock burst on the footwall is larger than that of mining on the hanging wall. Finally, an engineering case regarding two rock bursts in panel 6303 is used to verify some physical simulation results to a certain extent. Study results can serve as a reference for face layout and prevention of rock bursts under similar conditions.

Experimental Investigation into the Bedding Plane Slip Effect on the Overlying Strata Behavior in Longwall Top Coal Caving of Soft Coal Seam

Bedding plane shear slip becomes more obvious in rock strata with quite different mechanical properties. A composite beam model considering the behaviors of the main roof and the immediate roof in the “three‐soft” coal seam is established based on physical similarity simulation experiments and the slip theory in this paper. The movement and failure of the overlying strata and the mechanism of the bedding plane slip at the 2211 working face are studied by experimental and theoretical analysis. The results suggest that the front abutment stress distribution occurs 50 m ahead of the working face, the initial caving interval of the main roof is 55 m, and the peak stress appears at a distance 20 to 32 m ahead of the working face. The bedding plane slip areas can be divided into the obvious slip area and the slight slip area along the mining direction. The range of the obvious slip area becomes wider and the range of the slight slip area grows to be narrower as the working face advances. The bedding plane slip becomes steady after gradual increase and leads to the subsidence of the overlying strata in the “three‐soft” coal seam. The observed initial caving interval of the main roof by field measurement is 51 m, which is consistent with the results of physical similarity simulation experiments and theoretical analysis. The results demonstrate that the beam slip model proposed in this paper is reasonable and able to describe the behaviors of overlying strata and bedding plane slip.

The Stability of Gob‐Side Entry Retaining in a High‐Gas‐Risk Mine

There are series of problems faced by most of the coal mines in China, ranging from low‐coal recovery rate and strained replacement of working faces to gas accumulation in the upper corner of coalfaces. Based on the gob‐side entry retaining at the No. 18205 working face in a coal mine in Shanxi Province, theoretical analysis, numerical simulation, and engineering practice were comprehensively used to study the mechanical characteristics of the influence of the width of the filling body beside the roadway and the stability of surrounding rock in a high‐gas‐risk mine. The rational width of the filling body beside the roadway was determined, and a concrete roadway‐side support with a headed reinforcement‐integrated strengthening technique was proposed, which have been applied in engineering practice. The stability of the filling body beside the roadway is mainly influenced by the movement of the overlying rock strata, and the stability of the surrounding rock can be improved effectively by rationally determining the width of the filling body beside the roadway. When the width of the roadway‐side filling body is 2.5 m, the surrounding rock convergence of the gob‐side entry retaining is relatively small at only 5% of the convergence ratio. It has been shown that the figure for roof separation is relatively low, and strata behaviors are relatively alleviated and gas density do not exceed the limit, which are the best results of gob‐side entry retaining. The results of this research can provide theoretical guidance for excavation of coal mines with similar geological conditions and have some referential significance to safety and efficient production in coal mines.

Comparison of a novel coupled hydro-mechanical model with typical analytical models in subsidence of coal seam gas extraction

Although the influence of conventional oil and gas extraction on surface subsidence has been widely recognised and studied, few studies are carried out on the surface subsidence in coal seam gas fields and its impact on surface infrastructure and the environment. In predicting land subsidence caused by coal seam gas extraction, the hydro-mechanical behaviour of geological strata are different and their hydraulic connections to the coal seams are not well-understood, which makes the analytical models are difficult to be applied in the prediction of land subsidence. This paper develops a coupled fluid flow-geomechanical model which can consider the interrelation of fluid flow and geomechanics of the ground. By comparison of dewatering and degassing with typical analytical models including the disc-shaped reservoir model and the uniaxial compaction model, the typical analytical models cannot estimate the potential pressure distribution and predict the real subsidence induced by coal seam gas extraction; however, the coupled fluid flowgeomechanical model is capable of describing the transport properties of coal seam, including water flow, gas flow and desorption and rock deformation. Therefore, the proposed coupled model can be better used in analysis of subsidence of coal seam gas extraction. [Received: April 15, 2017; Accepted: October 17, 2017]

Comparison of a novel coupled hydro-mechanical model with typical analytical models in subsidence of coal seam gas extraction

Although the influence of conventional oil and gas extraction on surface subsidence has been widely recognised and studied, few studies are carried out on the surface subsidence in coal seam gas fields and its impact on surface infrastructure and the environment. In predicting land subsidence caused by coal seam gas extraction, the hydro-mechanical behaviour of geological strata are different and their hydraulic connections to the coal seams are not well-understood, which makes the analytical models are difficult to be applied in the prediction of land subsidence. This paper develops a coupled fluid flow-geomechanical model which can consider the interrelation of fluid flow and geomechanics of the ground. By comparison of dewatering and degassing with typical analytical models including the disc-shaped reservoir model and the uniaxial compaction model, the typical analytical models cannot estimate the potential pressure distribution and predict the real subsidence induced by coal seam gas extraction; however, the coupled fluid flowgeomechanical model is capable of describing the transport properties of coal seam, including water flow, gas flow and desorption and rock deformation. Therefore, the proposed coupled model can be better used in analysis of subsidence of coal seam gas extraction. [Received: April 15, 2017; Accepted: October 17, 2017]

In Situ Studies on the Characteristics of Strata Structures and Behaviors in Mining of a Thick Coal Seam with Hard Roofs

The movements of overburden induced by mining a thick coal seam with a hard roof extend widely. The effects of breakages in the hard strata on the strata behaviors might vary with the overlying strata layers. For this reason, we applied a test method that integrated a borehole TV tester, borehole-based monitoring of strata movement, and monitoring of support resistance for an in situ investigation of a super-thick, 14–20 m coal seam mining in the Datong mining area in China. The results showed that the range of the overburden movement was significantly high, which could reach to more than 300 m. The key strata (KS) in the lower layer main roof were broken into a ‘cantilever beam and voussoir beam’ structure. This structure accounted for the ‘long duration and short duration’ strata behaviors in the working face. On the other hand, the hard KS in the upper layer broke into a ‘high layer structure’. The structural instability induced intensive and wide-ranging strata behaviors that lasted for a long time (two to three days). Support in the working face were over-pressured by large dynamic factors and were widely crushed, while the roadways were violently deformed. Hence, the structure of a thick coal seam with a hard roof after mining will form a ‘cantilever beam and voussoir beam and high layer structure’, which is unique to a large space stope.

The effects of the rotational speed of voussoir beam structures formed by key strata on the ground pressure of stopes

Underground coal mining leads to fracture and movement in the overburden and the occurrence of weighting events in stopes. There are certain key strata that play an important role in controlling overburden movements. After the key stratum breaking, the fractured blocks would form the voussoir beam structure. The rotational speed of the voussoir beam structures directly affects the intensity of the ground pressures in a stope. In this study, a mechanical model was constructed to describe the rotational motions of the voussoir beam structures formed by the key strata based on elastomechanical theories. The angular velocity equations for the rotational motions of the voussoir beam structures in the key strata were obtained. Moreover, the manner in which loads and block lengths affect the angular velocity was investigated. The rotational angular velocity of the fractured blocks was found to increase with greater overlying loads on the key strata and shorter block lengths. The main roof has a major influence on strata behavior in stopes, and increases in the angular velocity of its voussoir beam structures increase roof subsidence rates, the yield of powered supports within each coal mining cycle, and the intensity of weighting events in stopes. If one or more of the voussoir beam structures formed by key strata above the main roof have greater rotational speeds than those of their underlying key strata, additional loads are applied to the latter, causing their rotational speeds to increase. If this application of loads is transmitted to the main roof, there is a strong possibility that highly intense weighting events will occur as a result of increases in the rotational speed of the main roof's voussoir beam structures. The research result provided a rational explanation for the significant differences in the weighting events of the No. 52304 working face in the Daliuta Coal Mine, which was covered by thick versus thin Aeolian sands. The results may be greatly significant in evaluating the intensity of weighting events, predicting the scope in which the strata movement affects the weighting events in stopes and determining a reasonable powered support capacity.

Measurement of the Strong Pressure Appearance Laws with Hard Roof and Full Mechanized Mining Extra Thick Coal Seam

To master the laws of strong strata behavior of Tashan coal mine under Carboniferous coal mining process, the laws of strong strata behavior in 8107 working face was measured and analyzed. It was shown that the average initial weighting step of 8107 working face was 59.4 m. The average periodic weighting step of main roof was 16.2 m. The maximum working resistance during periodic weighting was 14,711.1 kN. The maximum working resistance during non-periodic weighting was 11,339.9 kN. The average dynamic load factor K during periodic weighting was 1.31. The stress of coal column on the side of the goaf could be divided into four zones (stress stabilization zone, stress slow-increasing zone, significant—increasing stress zone, stress reduction zone) along the strike of 8107 working face. There was a peak of lateral support pressure along the trend of 8107 working face. And the peak position was biased to the side of return airway roadway. With the increase of the distance from the down-side of return airway, the pressure peak of the inner coal body along the strike of 8107 the working face increased and the peak position decreased from the coal wall. The peak stress of coal column tended to be close to the up-side of return airway. And the distance from the down-side of return airway for the peak of inner coal was larger than that for the peak of coal pillar. The peak position of abutment pressure of hard roof was in the range of 10–25 m in front of 8107 working face under full mechanized mining extra thickness coal seam conditions. The relative stress concentration coefficient of k was 1.3–6.5. The range of 10–25 m from the front of the working face to coal wall was stress reduction zone. And the influence range of abutment pressure was about 80 m. It was of great significance to the control and practice of the surrounding rock of the stope for the mining of the hard extra-thick coal seam.

Strata Behaviors and Rock Burst–Inducing Mechanism under the Coupling Effect of a Hard, Thick Stratum and a Normal Fault

Strata Behaviors and Rock Burst–Inducing Mechanism under the Coupling Effect of a Hard, Thick Stratum and a Normal Fault

Characteristics of Strata Behavior during Thick Seam Mining by Fully-Mechanized Top Coal Caving in a Loess-Covered Gullied Region

This study systematically investigates the overburden movement and strata behavior at a fully-mechanized top coal caving (FMTCC) face in a thick coal seam in a loess-covered gullied region by field measurement and theoretical analysis. A comparative analysis was performed to examine how the attitude of surface gullies and the structure of overburden affect face support resistance. The characteristics and evolution mechanism of strata behavior during FMTCC under gully slopes in a loess-covered gullied region were discovered through a mechanical analysis of the roof structure and a comparison to the mining of a typical shallow seam under gully slopes. The study found that as the FMTCC face advanced, the strata behavior initiated in the middle part of the face and then continued to both ends, and its intensity was higher in the middle part than in the upper and lower parts. During mining under gullies, the increases in support resistance were generally less than 5% (except for the maximum increases of 7–9%), indicating that the occurrence of gullies on the surface had little influence on the strata behavior at the FMTCC face. Due to the increased burial depth and the number of key strata, the roof at the FMTCC face under gullies formed a stable load-bearing structure called a voussoir beam, which was the key difference from the strata behavior characteristics in the shallow mine under gullies.

Technology and Practice of Mechanized Backfill Mining for Water Protection with Aeolian Sand Paste-like

The technique of mechanized mining backfill with aeolian sand paste-like was invented. Aeolian sand was used as aggregate and alkali-activated fly ash as cementing agent in the backfill. The ratio was studied that the water-sand ratio was 1:1.3, the mass concentration was 72%, and the filling slurry’s initial fluidity reached 210mm. The filling capacity reached 360m3/h and the running energy consumption was low with adopting the filling station model of double pulping system and underground equipment pool. Industrial test showed that the technique met the requirements of water resources protection, based on that the filling rate was 98.5% in goaf, strata behavior appear slightly, roof water leaching was less than 2m3/d, and surface subsidence value was 38mm. Meanwhile, there were great breakthroughs in cost and production efficiency comparing with similar technology.

Stability analysis of surrounding rock and pillar design in roadway backfill mining method

Over the past few decades, the low recovery of resource coupled with the strong strata behavior has plagued the development of mines in Western China. To address these issues, an emerging and innovative technology- roadway backfill mining method was proposed in this paper on the basis of the compressive properties of backfill materials. A rational composition for backfill materials is obtained and the roadway layout and the backfill mining technology of this method are presented. Once placed, the backfill materials together with the barrier pillar have to satisfy certain mechanical stability requirement to ensure a safe underground working environment. In this respect, a mechanical model of roadway backfill mining method was developed using statics analysis. Formulas for predicting roof subsidence above mined out area and the stress distribution in barrier pillar are obtained. Furthermore, an instability criterion of coal pillars is proposed. Combined with the specific geological and mining conditions of 3101 working face in Changxing Coal Mine, the current study showed that the width of backfill mining roadway to be 7.0 m, the width of unexploited coal pillar to be not less than 3.0 m, and the elastic foundation coefficient of backfill body to be not less than 1.5 × 107 N/m3.

Strata behavior in extra-thick coal seam mining with upward slicing backfilling technology

Based on the character of upward slicing backfilling mining and the condition of Gonggeyingzi coal mine in Inner Mongolia, this paper describes the studies of the strata behavior and the stress distribution in the process of backfilling mining in extra-thick coal seams. This was achieved by setting up and analyzing the elastic foundation beam model using the ABAQUS software. The results show that: (1) With the gradual mining of different slices, the roof appears to bend continuously but does not break. The vertical stress in the roof decreases and the decreasing amplitude reduces, while the tensile stress in the roof grows with the mining slices and the maximum tensile stress will not exceed the allowable tensile stress. (2) The front vertical stress at the working face exceeds the rear vertical stress and both show a trend of decrease with decreasing amplitude of decrease. (3) The slices mined early have more influence on the surrounding rock than the later ones. Similarly, the strata behavior experiences the same trend. The field measured data show that the roof does not break during the mining process, which is consistent with the conclusion.

Pressure-Relief Mining of the Working Face Under the Coal Pillar in the Close Distance Coal Seams

Intensive strata behaviors are generated when the No. 8707 working face of the 8# coal seam in a coal mine is advanced by way of the pillars left over of the upper part of 7# close distance coal seam. The theoretical analysis, numerical simulation and filed measurement were utilized to obtain the rule of the stress change when the 8707 working face of the 8# coal seam passes the pillars left over of the 7# coal seam. Meanwhile, a pressure-relief mining (PRM) technology was put forward. According to the research results, when the 8707 working face in the 8# coal seam was advanced to the position that was 20 m in front of the pillar left over, the abutment pressure reached the maximum for 26 MPa and the stress concentration factor was 3.25, which was likely to give rise to the rock burst. With the advance of the working face, the abutment pressure was reduced slowly. As the 8707 working face advanced 15 m away the pillar left over, the transfixed shear failure region of 45° was found in the bedrocks of the upper and lower coal seams, which was readily to give rise to the shear rupture, leading to the rock burst. Based on the aforementioned research, this research carried out the PRM by applying the hydraulic fracturing technology on the coal roof and pillar, which can ensure the safety and efficient mining of working faces.