Roof deformation and collapse of stamps with isolated grooves: a contact mechanics approach

This paper has revisited the roof deformation and collapse of stamps with isolated grooves based on a contact mechanics approach, with emphasis on establishing the non-adhesive and adhesive contact solutions for surfaces containing a shallow rectangular groove with the effects of applied load and interfacial adhesion taken into account. By solving singular integral equations and using the energy release rate approach, closed-form solutions are derived analytically for the deformed groove shapes, interfacial stress distributions and equilibrium relations between load and contact size, which reduce to the previously proposed solutions without adhesion or without applied load. Finite element analysis is performed to validate the non-adhesion solutions, while experiment results of stamp collapse reported in the literature are adopted to examine the adhesion solutions. By introducing the Johnson parameter a to represent a competition between surface energy and elastic strain energy of the groove, four kinds of contact behaviors of the groove roof can be characterized appropriately: non-adhesion, weak adhesion, intermediate adhesion and strong adhesion. Hysteresis loop and energy loss due to distinct load/unloading paths are revealed in the cases of intermediate and strong adhesion. We also provided the critical applied pressure to achieve roof collapse and the corresponding equilibrium contact size for full range of a.

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.

Dynamic Evolution Law of Roof Deformation in Continuous Mining and Continuous Backfilling Method

To investigate the dynamic evolution law of roof deformation in the continuous mining and continuous backfilling (CMCB) method, the alternating bearing characteristics of coal and the backfilling body were obtained through statistical analysis. On the basis of two-step mining and the elastic foundation beam theory, the mechanical model of the entire stage roof deformation was established, and the dynamic change law of roof settlement was obtained. On the basis of the working face of CMCB in Haoyuan Coal Mine, the deformation rule of the two-step mining roof was analyzed. In the first stage of mining, the coal pillar is the main bearing structure, the deformation of the roof is small, and the distribution is wavy. In the second stage, the backfilling body gradually becomes the main bearing structure, and the deformation of the roof significantly increases and exhibits a U-shaped distribution. The results obtained by numerical simulations reveal that four-step mining increases the width of the coal pillars and the roof deformation is effectively controlled during the filling process. After the filling is completed, the roof remains stable under the support of the filling body under different filling modes; therefore, its final settlement remains consistent. Through field measurement, it was found that the CMCB method can effectively control the roof subsidence in Haoyuan Coal Mine.

Novel Application of the Roof-Cutting-Type Gob-Side Entry Retaining in Coal Mine

In this study, the roof-cutting-type gob-side entry retaining is introduced, and its application in medium-thickness coal seams is studied. Based on the analysis of the construction procedure and principle, the mechanical model of the retained roadway structure and cantilever beam formed by roof cutting was established, and the support resistance and roof deformation were obtained. In addition, through technological design analysis and numerical simulation, the parameters of roof cutting were determined. The roof-cutting height and angle were designed to be 9 m and 15°, respectively. Flac3 D was used to analyze the stress evolution law under different mining conditions. The stress on the integrated coal side and roof subsidence was lower when the roof-cutting height was 8∼10 m and the cutting angle was 15°. Through field monitoring, the roof pressure, gob-side lateral gangue retaining pressure, anchor cable stress, and deformation of the surrounding rock eventually reached a stable state. This indicates that the roof cutting can effectively cut down the overlying strata over the gob and form a stable entry structure to meet the requirements of the next working face.

Numerical Investigation on Time-Dependent Deformation in Roadway

The roadway deformation normally relates to time especially for underground coal roadway. The strength of soft coal is low, and therefore the deformation increases gradually under constant stress with time, which is called rheology deformation. In this study, based on a field case, the rock mass properties and deformation data of the roadway were obtained according to the field test. A 3D numerical model was then established, and the rheological deformation including horizontal and vertical deformation of the coal roadway was systematically analyzed. The results showed that the rheological deformation of horizontal sidewall accounts for almost 30% of the whole deformation, and the stable time for such roadway is around 60 days after excavation. The tendency of the roof deformation is similar to the sidewalls, and however, the floor deformation is different. Then the related suggestions for maintaining the stability of such roadway were proposed, which is useful in-field application.

Fracture criterion of basic roof deformation in fully mechanized mining with large dip angle

To explore the structure evolution of overlying strata and pressure characteristics of coal mining with large dip angle, the basic roof mechanical model was established that based on the thin plate theory and the development characteristics of 1212 (3) working face of Panbei Mine. The formula was deduced that used for calculating the basic roof stress distribution in large dip angle coal seam. It revealed the mechanism evolution of mining stress and its influence on overburden deformation. Furthermore, it was also discussed that the effect of false roof on the failure of the basic roof. It showed that the false roof increases the differentiation of gangue’s filling rate in goaf and improves the evolution rate of basic roof fracture. It is the main influencing factor that the large dip angle leads to the “scoop” distribution of the stress and deformation in basic roof. It dominates the evolution of overburden fractures and the regional instability. The maximum deformation of the basic roof is located in the middle and upper part of the working face. This theoretical model is verified by means of numerical simulation and field monitoring.

Nonlinear Creep Model of Deep Gangue Backfilling Material and Time-Dependent Characteristics of Roof Deformation in Backfilling Mining

With the gradual increase in mining depth of coal resource exploitation, deep backfilling mining has effectively solved the impact of strong deep mine pressure and strong mining disturbances. However, after deep backfilling mining, the backfilling material is subjected to high stress for a long time, and its viscoelasticity has a significant impact on the roof control effect. This paper uses a large-scale bulk confinement test device to analyze the viscoelastic properties of gangue, establishes a high-precision fractional viscoelastic creep model, and identifies the gangue parameters. The established fractional viscoelastic model was used as the foundation model of the beam, and the roof model based on the fractional viscoelastic foundation was solved. The top deformation characteristics of elastic foundation and fractional foundation were compared and analyzed, and the time effect, viscoelastic effect, and order effect of the fractional order viscoelastic foundation beam were discussed. The results show that the viscosity of gangue increased under the action of deep high stress. As time increased, the roof deformation also increased. In order to more effectively control the long-term deformation of the roof, the viscosity coefficient of the backfilling material should be greater than 20 MPa. This research provides relevant guidance for the requirements of backfilling materials for deep backfilling mining and the prediction of long-term dynamic deformation of the roof in underground excavations.

Experimental and Theoretical Investigation of Overburden Failure Law of Fully Mechanized Work Face in Steep Coal Seam

In this study, both theoretical analysis and similar simulation experiment are employed to investigate the overburden failure law of fully mechanized face in the steep coal seam. By establishing the mechanical model of inclined rock beam, the deflection equation of overlying strata beam is obtained. Based on the geological conditions of Xiangyong coal mine in Hunan Province of China, the laws of roof deformation and failure in steep coal seam are obtained by similar simulation experiments. The results showed that the roof deformation of the goaf is relatively large after the working face advances along the strike, and the deformation mainly occurs in the upper roof of the goaf. The backward gangue in the immediate roof fills the lower part of the goaf, which plays a supporting role in the lower part of the roof and floor. The roof fracture of goaf is located in the middle and upper parts of the working face, which is consistent with the results derived from the mechanical model. After the roof fracture, a “trapezoid” bending fracture area and the secondary stability system area is formed, which is composed of four areas: the lower falling and filling support area, the upper strata bending fracture area, the fracture extension area, and the roof bending sinking area.

A study of the mechanical structure of the direct roof during the whole process of non-pillar gob-side entry retaining by roof cutting

The non-pillar entry (roadway) retained by roof cutting serves the two adjacent working faces. As compared with the conventional mining roadways, the roadway retained by roof cutting has a longer life cycle and receives more complicated influence from mining. Determining the location where the roof deformation and maximum deformation occur can provide an important basis for roadway support. Here, the direct roof of the roadway is studied by assuming it as an elastic deformation body. The stress features of the direct roof of the gob-side entry retained by roof cutting are analyzed, and the roof deformation is divided into five stages. The stress superposition principle is employed, and the equivalent concentrated load within the roadway is introduced. The mechanical model of the direct roof is established for the whole process of gob-side entry retaining by roof cutting. Next, the calculation formula for the concentration of direct roof at different positions is obtained for the whole process of gob-side entry retaining by roof cutting. The application scope of the calculation formula and the determination method of the key parameters are analyzed. The relationship between direct roof deformation of the roadway and stiffness of the support system is studied. The results show that the direct roof deformation has a symmetrical distribution about the midline. The maximum roof deformation occurs in the middle of the roadway, and it gradually decreases as the coal seam stiffness increases. During the example calculation, the maximum roof deformation is 280 mm for the gob-side entry retaining under primary mining. The measured maximum roof deformation is 320 m, and the error rate is 12.5%. It is then verified that the uniform mechanical model proposed in this study applies to the calculation of direct roof deformation in the gob-side entry retained by roof cutting.

Mine Pressure Behavior Characteristics and Control Methods of a Reused Entry that Was Formed by Roof Cutting: A Case Study

Noncoal pillar mining with automatic formation of a roadway is a new coal mining method that is tailored to improve the coal resource recovery rate and reduce the investment in roadway tunneling. Using this proposed method, a reuse entry is formed by roof cutting instead of tunneling. In this paper, the S1201-II working face of the Ningtiaota Coal Mine was used as a case study. The stress distribution of surrounding rock and the roof deformation characteristics of the reused entry during the mining process of the second working face were studied through FLAC3D numerical simulations combined with field measurements. The results indicate that the zone close to the reused entry led to higher stress in advance. If this stress is superimposed with the lateral pressure of the adjacent mined working face, it will be more difficult to maintain the reused entry. In the engineering case study described here, the reused entry created a stress increase zone and a severe deformation zone in the range of 0–80 m in front of the working face, and its range was approximately 37.5% larger than an ordinary entry. The stress peak in the stress increase zone increased by approximately 34.7% over that of an ordinary entry. The maximum amount of deformation within the severe deformation zone increased by 94.4% over that of an ordinary entry. To properly control the surrounding rock stress and deformation of the reused entry, a dynamic pressure bearing support in front of the working face with adaptability to the large roof deformation and high support strength is proposed here. Field application results showed that the final roof deformation with the dynamic pressure bearing support can be satisfactorily controlled within 110∼130 mm. These findings can provide a reference for researchers and field engineering technicians when engaging in the support work of reused entry.