Overburden Strata Research Articles

Groundwater system for the periods of pre- and post-longwall mining over thin overburden

The groundwater loss is one of the primary issues for environmental destruction. Underground longwall mining methods can cause rapid subsidence of the overburden strata. Especially in shallow coal seam mining. Since the subsidence through the overburden to ground surface, the groundwater may be affected and drawdown into the lower levels above the longwall panels. This paper analysed the potential impact of longwall mining on groundwater over thin overburden. Four water wells were drilled down to the proposed deformation zone above two longwall panels to record the different water levels’ variations throughout the mining period. A numerical groundwater flow model (GGU-SS-FLOW3D) was performed to predict the water levels in the periods of pre- and post-mining conditions. To set-up model, local mining conditions and regional geology characteristics were ascertained in this study. The measurement data from the four water wells were adopted to calibrate the flow3D model. The field measurement and GGU-SS-FLOW3D model can help to better understand the variation of the groundwater system related to the ground subsidence from longwall mining over thin overburden.

Comparative Analysis of Overburden Strata Movement in Fully Mechanized Top Coal Caving Mining in Entire (Sub) Layer of Ultra Thick Seam

Reasonable determination of entire layer or sublayer mining method is the key factor of safe and efficient mechanized caving mining of 14.0～28.0m ultra thick seam , numerical simulation on the entire (sub) layer coal caving mining by the FLAC3D , studying the stress and displacement variation of overburden strata , combining the underground pressure theory , comparative analysis on the calculation results . The results show that entire (sub) layer mining roof stress , movement contours are presented in a semi-elliptical arch morphology , indicating the existence of a high arch structure overburden formations , mining process appears roof tensile destruction , but the entire layer caving tensile stress intensity is lower than sublayer caving . When advancing the same distance , entire layer caving puts a greater impact on the overburden strata than the sublayer caving , has some features such as big range of influence and extent , small roof pressure , lead abutment pressure forward from grand , short time of achieving full extraction , etc.

Mechanism of Roof Shock in Longwall Coal Mining under Surface Gully

The paper presents an interpretation on the abnormal roof shock in longwall coal mining under gullies using physical modeling, numerical modeling, and mechanical analysis. The modeling results show that the roof movement causes the shock load onto the stope in longwall coal mining under surface gully. The triggering mechanism of shock load depends on the direction of the face retreat with respect to the bottom of the surface gully. The slope tends to slide along the interface plane with a long periodical weighting intervals when mining towards the bottom of the gully (downslope direction), while the overburden strata may be split into blocks and tend to topple towards the free face of gully when mining away from the bottom of the gully (upslope direction). The mechanical models showed that, during the period of mining in downslope direction, planar sliding and key fragmental blocks cause a sudden roof shearing off which could result in shock load and, during the period of mining in upslope direction, the overburden blocks may become unstable due to shearing off which could resulted in large shock pressure onto the stope.

Movement of Overburden Stratum and Damage Evolution of Floor Stratum during Coal Mining above Aquifers

Based on coal mining above aquifers of Carboniferous limestone in a coal mine in North China Mining Area, the numerical method of CDEM (Continuum-based Discrete Element Method) was employed to simulate the movement of overburden stratum and damage evolution of floor stratum during coal mining above aquifers. The results indicated that the lithology, thickness, and structure of the impermeable strata and the pressure of confined water were the most pivotal elements which lead to the risk of water inrushes and damage evolution of floor stratum. In addition, the activities of overburden stratum, such as deformation, stratum separation, caving, also exhibited important effects. Compared to caving activities of the immediate roof and main roof, the critical upper strata with the larger thickness and strength showed more important influence on the damage evolution of floor stratum and water inrush risk, although it was relatively far from the coal seam. Before the initial caving of critical roof, the destruction extent of floor stratum and the risk of water inrush increased with the increase of the mined-out area and reached their peaks when the maximum caving interval was reached. When critical strata started periodic caving, the extent of destruction of floor and the risk of water inrush reached their peaks when hanging arch of critical strata reached the maximum value in this period. For caving interval in critical strata is larger than the mail roof and the stress pressure was mainly applied in the rear of the mined-out area other than the mining face, it is difficult to observe the rules of the activities of critical strata above coal seam through conventional mining monitoring ways. The activities of critical strata above coal seam should be studied to effectively prevent water inrush disaster in coal mine.

Analysis on Water Abundance of Loose Aquifer and Quality Evaluation of Overburden Strata during Mining under Loose Aquifer

The establishment of pillars is one of the most effective methods to keep mining safety under loose aquifer. Water abundance of bottom aquifer of loose strata,strength and structure of overburden strata are the main factors controlling type and height of coal pillar.Taking Xiyi area in Pansan coal mine as an example, hydrogeology characteristics of the bottom aquifer of quaternaty system and engineering geological properties of overburden strata of No.8 seam were analyzed.The characteristics of weathered rock was particularly analyzed.The results indicated that the type of the water abundance and permeability in this aquifer were medium.When the No.8 seam in Xiyi area was at shallow depths mining, water-proof pillar had to be designed.The structure of overburden strata of No.8 seam belonged to the type of upper soft and lower hard,which can restrain the failure of overburden strata.The thickness of weathering zone was thick and distribution was also stable.Weathered rock had good water-resisting property and crack resistance.

The Numerical Simulation of Overburden Strata Failure Law by Full-Mechanized Caving Mining in Extra Thick Coal Seams

Given the irrationality and limitations in thick coal seam of the empirical formula of height of water flowing fractured zone , this article reveals the special thick seam fully mechanized mining damaging rules of overlying rock and determines the height of water flowing fractured zone with 3D finite element numerical simulation,according to the complex geological conditions and the special thick seam fully mechanized mining methods of Laohutai ore mining working face E5400 as an example. Results show that the destruction of repeated mining area of the overlying rock will have superposition effect and the superposition effect will increase with the development of mining; the fracture sharp of water fracture zones is in close proximity to the "arch". Simulation results and the actual detection height fitting is better, so it assesses the range of overburden water flowing fracture zones and rationality of the height.

Research on the Mechanism and Capacity of CO<sub>2</sub> Stored in Abandoned Coal Mine Gob

To explore new pathways of CO2 geological storage, the mechanism of CO2 stored in abandoned coal mine gob was analyzed through the investigation of the gobs exploitation current situation. Methods to calculate the capacity to store CO2 was proposed based on establishing mathematical model. This paper presented suggestions about how to carry out research in the future. Research suggests that after the coal mine is abandoned, there are large underground spaces consisting of rock cavities of gob caving zone, rock fissures of fractured zone, and permanent residual laneways, which CO2 can be stored, and there will be tremendous sequestration potential. Compared with deep aquifers storage, the space has the advantages of larger scale porosity, simpler storage mechanism, easier injection, more abundant geological data, etc. At the same time, the space also remains problems such as lower seal strength and failure of overburden strata easily causing CO2 leaks, etc. This paper suggests that six aspects should be further researched in the future.

Mechanical Analysis of Shell Structure in Longwall Mining Face of Steeply Dipping Seam

Taking the longwall mining face area in steeply dipping seam as the research object, the stability of asymmetrical rock mass structure is analyzed using shell theory and masonry beam theory. The results show that asymmetrical shell structure is formed at the top strata of mining stope and its magnitude is mainly determined by the mechanical characteristic of overburden strata of working face, mining space size and so on. Localized instability of the shell structure is the main factor that leads to the overall structural instability in the stope. The instability modes includes: the shell roof instability dominated by tensile failure, shell shoulder instability initialized by compression-shear failure, and shell bases composite instability occurs in tensile and compression-shear modes. The shell structure stability dedicates rational strata control measures to choose, and the caving height of overburden strata is the key for shell structure instability.

Experiment research on overburden mining-induced fracture evolution and its fractal characteristics in ascending mining

The simulated material model was used to simulate the formation and distribution of overburden fractures induced by ascending mining. The fracture evolution law was revealed. The fractal geometry theory and MATLAB software were applied to analyze fractal characteristics of fracture network of overburden stratum. The results showed that the overburden failure evolution is trapezoidal, accompanied with collapse in layer group and the developing height of fractures discontinuously jumps; as the mining width increased, the fractal dimension of fracture network showed an overall ascending trend, presenting three stages, namely, fast dimension rising stage, slow dimension rising stage, and stable stage. In the caving zone, fracture zone, and bending zone, the development, propagation, and closure of fractures were asynchronous and inner similar. These achievements are important to roof roadway stability control and layout of gas drainage system and contribute to the simultaneous extraction theory of coal and gas by pressure relief.

Analysis of Three-Dimensional Numerical Simulationin on Overlying Strata’s Stress Field and Displacement Field in Thick Seam Mining

It takes the typical coal of mining in S2S9 face of Daping mine, using the 3-D numerical simulation to reveal on its face with the vertical characteristics of the stress field and displacement field .The study showed that the overburden strata migration features during the mining process in in thick seam mining of Daping mine.This research can be a reference for safety and high efficient coal mining of other parts of the soft overburden in extra- thick seam mining technology.

Study on Zoning of Overburden Strata and Roadway Deformation of Ascending Mining in Thin Coal Seam

In order to solve the problems appeared in mining process of upper seam in Xiaohezui coal mine, a technical scheme of ascending mining was put forward. UDEC, a discrete element numerical simulation soft was used to study the characteristics of zoning of overburden strata and the rule of vertical stress distribution of the upper coal, the results show that the upper coal is on the top of the equilibrium belt without step dislocation, the structure of roof stratum and floor stratum stay complete with slight separations, so ascending mining is feasible; the range of the upper coal influenced by the stoping of below working face is -85m~65m, the vertical stress reaches a maximum 15m ahead the below working face, and reaches a minimum behind it 25m. It is applied in the engineering practice, the roadway deformation discipline of the upper coal is similar to the distribution rule of vertical stress, provides a theoretical basis for guiding and improving the roadway support of ascending mining.

Evolution and distribution of macroscopic gas channels in an overburden strata

The evolution of gas bearing channels in the roof, and their spatial distribution, was studied. A complete consideration of gas flow changes through the stress–strain changes in the roof near a working face is made. The theoretical abutment pressure distribution using displacement monitors and borehole visual recording instruments allow a theoretical analysis. Field test research determined the conditions for formation of macroscopic gas channels. These appear along the working face roof, normally distributed to it. These results show that the coal rock stratification becomes a macroscopic gas channel boundary if its deformation is less than the lower layer, or greater than the layer above it. At the same time the stability is greater than the distance from the roof for hanging dew conditions. The working face advances and the roof gas channels experience a cycle of development. Microscopic channels dominate the initial stage then macroscopic gas channels form, develop, and close. The evolution of the macroscopic channels depends on the ratio between the distances from the new compaction area in the goaf to the initial stress area in front of the working face. The amount of daily advance of the face also affects channel development. The experimental observations in one mining area showed that the main gas channels are located about 2 and 6.2m above the lower surface of the roof and that they have an evolution period 7 to 11days long.

Field Measurement Research on Overburden Stratum Movement and Deformation

According to field measurement datum in disturbed overburden stratum of five drillings, disturbed overburden stratum movement characteristics were analyzed. Negative exponent function model of measurement points movement law was also constructed. Moreover, variation law of stratum subsidence speed and relation between overburden stratum subsidence factor and surface subsidence factor were deeply researched. The results show that starting-distance of panel is nearly 70m, bedrock and alluvium subside synchronically and key-stratum isn’t exist in overburden stratum. Also, displacement curve of measurement points in stratum submits negative exponent function model and mutagenesis of surveying points subsidence speed is the key position where face passed drilling 100m. Furthermore, subsidence factor of overburden stratum and surface submits exponent function model.

Study on Overburden Strata Rupture Development Height Rules in Repeated Mining of Thick Coal Seams

Take 3 coal repeated mining of Luxi colliery as research background, overburden strata rupture development rules in repeated mining of thick coal seam were studied by using the methods of field measurement. The results indicate that the height of overburden crack belt reduced along with the increase of goaf development time; the crack belt height of upper layer is 34.73m, the ratio of crack height and mining thickness is 15.1, the crack belt height of lower layer increased to 41.51m, but the increased trend declined, the ratio is only 12.14, the development height after up-layer was excavated is 83.6% of the height after two coalface were excavated. The results have important guiding sense for predicting overburden rupture rules in the similar mining conditions and improving the upper limit.

Study on the Deformation and Stability of Building Foundations in Mining Subsidence Areas

To enhance the effective of the land in mining subsidence areas, realize the development of new rural construction and urbanization. According to the geological and mining conditions, the scale, shape and regularity of the potential crack areas in the coal goafs, separated overburden strata, and the influencing factors which affect the deformation foundation of architecture and stability are studied. Probability integral method is used to establish the further deformations formula of the building foundations in mining subsidence areas. The monitoring data from the ground observation stations are used to invert the further deformation predicting parameters, the surface further deformations of each coal goafs, and the whole surface’s further deformations are calculated by the superposition method. The foundation further deformations, geological structures, etc, are comprehensively taken into consideration. The stability of the building foundations in mining subsidence area are analyzed and evaluated. Research results are very valuable to process, design, and plan the foundations of the buildings in coal mining subsidence areas.

Numerical Simulation of Study on Rupture Development Rules of Overburden Strata in Repeated Mining

Take 3 coal repeated mining of Luxi colliery as research background, overburden strata rupture development rules in repeated mining of thick coal seam were studied by using the methods of field measurement and numerical simulation. The results indicate that the height of overburden crack belt reduced along with the increase of goaf development time; the crack belt height of upper layer is 34.73m, the ratio of crack height and mining thickness is 15.1, the crack belt height of lower layer increased to 41.51m, but the increased trend declined, the ratio is only 13.84; the final crack height of numerical simulation is 39.4m, in good agreement with measurement results. The results have important guiding sense for predicting overburden rupture rules in the similar mining conditions and improving the upper limit.

New Viewpoint of Foundation Stability of Newly Building upon Goaf

Foundation deformation of new buildings upon old mined out area and the survival subsidence and deformation of old goaf had been studied by similar material model in the paper. When additional stress caused by building foundation reached a thick hard rock sandstone stratum, the vertical deformation was blocked while horizontal deformation expanded significantly. The actual influence depth of ground loading is obvious smaller than theoretical result. The spread depth of building load upon mined out area is related with the disturbed degree by mining, the ground loads spread depth is greater with the increase of disturbance degree. After passed through key strata and caused overburden stratum separation closed, the additional stress caused by ground foundation would quickly reach inside of goaf, caused movement and deformation on strata upon goaf. But the movement and deformation is relied to the situation of goaf, there would not significant activation in all mined out areas.

Research and Development of Solid-Gas Coupling Physical Simulation Experimental Platform and Its Application

Abstract Based on the characteristics of deformation and caving of overburden strata and the law of gas delivery, a solid-gas coupling physical simulation experimental platform was self-developed. The different problems which are studying on the law of relief gas with face advancing in lab will be solved. The experimental platform is used to research the rule of change of seepage velocity with face advancing. The result shows that change trend of seepage velocity with the face advancing which decreased first, then increased, later, decreased, increased again and finally kept unchanged

Numerical simulation of compound media coupling mechanism of deep mining overburden strata

Aiming at the regularity of deep mining strata movement, through the application of plate theory and discrete medium theory in establishing the coupling model of the deep mining strata composite medium, the continuum media and the non-continuum media were coupled into the compound media giant system, and the stress of compound layer and strain coupling relationship were established. The accuracy of forecasting surface subsidence in deep mining conditions was improved. The deep mining was simulated through 3-D numerical value by the FLAC3D finite difference software, and the coupling relationship and coupling layer in the strata composite layer were analyzed. The results show that, under the deep mining condition, the coupling zone is in the position of coal seam roof with the thickness of 15–20 times, on which, the stress-strain has much difference on the coupling zone. Considering interlayer effect of coupling zone can improve the prediction precision of surface subsidence.

Study on the Monitoring Method of Overburden Failure of Shallow Wongawilli Mining Working Face

Through the practice and improvement of Wongawilli Mining method in Shendong Mining Area, it created Shendong Characteristic short wall mechanize room mining method. Due to the shallow coal seam, the fracture zones throughout the bedrock directly after the mining, therefore the bending sinking belt didn’t exist; the conventional "three zones" theory couldn’t explain the overburden rock deformation damage rule of this situation perfectly. In order to monitor the damage of overburden strata, based on the analysis of coal pillars stability in Wongawilli working face, the paper puts forward 3D monitoring methods of overburden strata damage as follows: underground borehole stress monitoring, roof separated strata and layer's internal monitoring, surface subsidence monitoring. The set of monitoring method can monitor the instability of coal pillars, roof caving, production of separated strata in the roof, overburden rock damage and subsidence, the study is of reference and significance.