Hulei Zhng,Min Tu,Hu Cheng,Yongzhi Tng

a Key Laboratory of Safety and High-Efficiency Coal Mining Ministry of Education,Anhui University of Science and Technology,Huainan 232001,China

b School of Resources and Environmental Engineering,Anhui University,Hefei 230039,China

c Huainan Mining Group,Huainan 232001,China

Keywords:Thin bedrock Key stratum Compulsive roof caving

ABSTRACT The key problem to be solved urgently is how to avoid the occurrence of support break-off and water inrush in the stoping of sandstone straight roof under the action of load transfer in unconsolidated aquifer.For this reason,taking the thin bedrock 1602 (3)working face of Huainan (the middle part of Anhui Province)Panyi Coal Mine as the engineering background,this study establishes the stope mining model by using the discrete element UDEC software and the mathematics mechanical model of the support load,and analyzes the reason of support crushing and decides to re-mining the working face by using the compulsive roof caving method.It is concluded that when the working face of sandstone straight roof is broken,the ‘‘voussoir beam”structure cannot be formed and acts on the support in the form of cantilever beam,but only when it falls to the high key stratum can the ‘‘voussoir beam”structure be formed and at this point,at this time,the bracket bears the weight of the rock layer in the range from the fractured sandstone layer to the lower critical layer.The working resistance of the support increases with the increase of the thickness and the breaking length of straight sandstone roof.When the breaking length of the roof reaches a certain extreme value,the support crushing accidents will occur.Managing roof with compulsive roof caving method can reduce the intensity of rock pressure in the stope,and the working face can be safely stoped,which provides a certain reference for similar conditions.

1.Introduction

Thin bedrock coal seam has the advantages of shallow burial,high degree of exploration,complete production system,low mining cost,etc.It is the main resource for the steady increase of production in new and old mines in Huaihe and Huaihe mining areas.However,the shallow seam of thin bedrock is affected by both mining pressure and high water pressure,and the working face is prone to occur calamity accidents such as support crushing and water inrush resulting in serious economic losses and threat to production safety,which has aroused the attention of scholars at home and abroad [1-3],and has made a series of achievement.However,there are still in urgent need of further study,for example,the research aspects of safe mining methods for straight(without straight roof)mining roof of thick hard rock layer under the action of load transfer in unconsolidated aquifer are seldom reported.For this reason,this paper takes the thin bedrock coal seam malmston straight roof 1602 (3)face of Huainan Panyi Coal Mine as the research background,uses the methods of theoretical analysis,numerical simulation and field test to determine the reasonable roof control methods.

2.General situation of working face

2.1.Geological overview

The 1602(3)face of Panyi Coal Mine in Huainan Mining Area is the working face of raised upper limit with minimum waterproof coal pillar of 40 m and the actual minimum waterproof coal pillar of 44.3 m.The thickness of coal seam is 1.4-4.9 m,with an average of 3.4 m,and the thickness of coal seam varies partially at the position of crossing faults;The coal seam structure is simple;the coal rock occurrence is 189-221°,and the dip angle of coal seam is 2-8°,with an average of 6.The working face is located between the hanging wall of the F5 reverse fault and the footwall of the F8 normal fault,and fifteen faults were exposed during the excavation.The roof and floor of the working face are shown in Table 1.

Table 1 Conditions of coal seam roof and floor.

2.2.Stoping overview

The working face is first stoped by comprehensive mechanized coal mining methods,and is installed with 100 sets of ZZ6400/18/38 hydraulic support (actual working resistance 7200 kN).When the working face is pushed to 44 m,the first pressure occurred in the main roof,the support pressure is 42 MPa,and relief valve opening pressure is 36 MPa.The central hydraulic support relief valve discharges liquid in large area,and the amount of support living column quickly reduced,after that the working face stagnates.There are 35 hydraulic supports which are damaged with varying degrees,the damage forms are front cylinder head damage,weldsopened of rear column holder,displacement of column and socket,etc.Using undercover measures,etc.However,the column is then crushed,resulting in support crushing.In order to ensure the smooth exploitation of the remaining coal resources on the working face,it is necessary to study the characteristics of the mine pressure in the working face in order to find a reasonable and safe mining method.

3.Numerical simulation analysis of overlying strata movement law in stope

3.1.The establishment of the model

According to the specific geological conditions of the 1602(3)working face,the numerical calculation model with the discrete element software UDEC is established.The typical Mohr-Coulomb elastic-plastic theoretical model is selected for the constitutive model.The parameters selected in the model are based onthe measured results of roof and floor coal and rock mass physical mechanics as shown in Tables 2 and 3.The whole calculation model range is 300 m×120 m (X×Y).In order to eliminate the influence of model boundary on calculation results,60 m coal pillars are left on both sides of the model.The vertical displacement of the bottom boundary of the model is fixed,as well as the horizontal displacement of the left and right boundaries.

Table 2 Physical and mechanical parameters of roof and floor rock.

3.2.Numerical results analysis

The roof breaking characteristics of the working face are shown in Fig.1,and the roof displacement curve is shown in Fig.2.

It can be seen from Figs.1 and 2 that the initial breaking step of the working face is 60 m,and the average periodic breaking step is 18.3 m.Since there is no direct roof on the working face,the main roof cannot be filled with full goaf after sinking,and a large free space is formed in the goaf.The low key layer (straight sandstone roof)will be unable to form a stable ‘‘voussoir beam”structure because of the large amount of rotation,as shown in Fig.1c,instead it is directly collapsed in the‘‘cantilever beam”structure,and only when the key stories are located at higher levels can a stable‘‘voussoir beam”articulated balanced structure be formed,as shown in Fig.1f.With the advancement of the mining surface,the ‘‘cantilever beam”structure periodically breaks and sinks,causing acceleration of roof subsidence in the working face,and the coal wall is severely supported,increasing the rib spalling of coal wall and load on the support,and then According to the load transfer effect of the loose aquifer,the rock layer above the top plate will fall sharply with the rupture of the lower key layer,and the periodic pressure will be more severe.

Table 3 Mechanical parameters of contact surface.

Fig.1.The macroscopic fracture of stope roof.

Fig.2.Dynamic displacement curve of thick and hard roof under different mining heigh.

4.Roof control technology under thick and straight roof conditions

4.1.Mechanical model of support

It can be seen from the above numerical calculation results that the sandstone straight roof plate acts on the bracket in the form of a cantilever beam,and the calculation model for establishing the working resistance of the bracket is shown in Fig.3.In the figure,Qlis the load of the main top in the fallen belt.

It can be seen from Fig.3 that since the key layer 1 is broken and a long cantilever is formed on the side of the goaf,the invert torque formed by the support force P of the bracket is difficult to balance the moment formed by the main top load of the fallen belt.Therefore,the rotation of the main top is inevitable to some extent [4].At this time,in order not to cause the main top to cut along the working face,the working resistance P of the bracket should satisfy the formula [5]

Fig.3.Mechanical model of support and surrounding-rock.

where B is the width of the bracket,m/frame;γ is the average bulk density of the direct top rock layer in the slump zone,kN/m3;Lzis the length of the main top rock beam,Lz=Ld+Lh+Lzx(Ldis the end face distance,Lhis the sum of the length of the bracket top beam and the front beam,Ld+Lhis the maximum control top distance,Lzxis the ultimate suspension length of the main top rock layer after the bracket),m;α is the coal seam inclination angle,°,K is the safety factor,comprehensive the working face is generally taken as 1.2;γiis the average bulk density of the i-th main roof and its additional rock layers in the slump zone,kN/m3;hiis the thickness of the ith main roof and its additional rock strata in the slump zone,m;Liis the length of the i-th layer of the main top rock block in the fallen zone,m(the length of the uppermost layered rock block is consistent with the periodic pressure step).

It can be known from Eq.(1)that the working resistance P of the bracket is related to Lz,h.When the main roof break length is longer and the mining height is larger,the working resistance is larger.Because the working face is thick and straight roof plate,once the roof plate is broken,the rock layer below the high key layer 2 will fall down as a whole,which leads to the imbalance of the mechanical system of the goaf,the support and the coal wall,and the pressure appears severe.The pressure directly acts on the top beam of the bracket,and the pressure of the roof exceeds the working resistance of the bracket,resulting in the automatic unloading of the safety valve (the pressure gauge on the field shows a pressure of 42 MPa and the opening pressure of the safety valve is 36 MPa).The support column cannot effectively support the pile,eventually leading to the support crushing.

4.2.Compulsive roof caving technology

After the failure of the initial mining pressure frame of the working face,the support is retracted.According to the research results,the design scheme of the working face is re-designed.In order to prevent the over long roof plate above the mining area,and reduce the influence of the main roof pressure on the working face.The method of loose blasting of the roof of the goaf was adopted to treat the suspended ceiling of the goaf on the basis of previous studies [6-9].

(1)Determination of reasonable compulsive caving step of thick hard roof

The size of the roof caving step determines whether the working face can safely pass the impact on the bracket during the pressure.The purpose of controlling the roof caving step is to make the force of the bracket not greater than the limit resistance of the selected bracket when the thick hard roof is broken.In the analysis and determination of the roof caving step,it is assumed that the thick hard roof rock beam is broken at the most dangerous position that the bracket can bear,that is,the thick hard roof is cut off above the coal wall.At this time,analyzes the force of the support so that the generated pressure is not greater than the maximum working resistance of the bracket,and its calculation model is shown in Fig.4.

Fig.4.Calculation model of reasonable caving step.

Supposing the support resistance of the bracket is P,the roof caving step is l,the bracket control roof moment is lk,the length of the thick hard top suspension part is ls,the thickness of the thick hard roof board is h,the self-weight of thick and hard roof above the top of support is Q2,the thick hard top suspension roof is Q1,the thick hard roof fracture angle (weak thin roof plate 70o-75o,medium hardness roof plate 60o-65o,hard roof plate 50o-55o)is β.For the overburden load q,the calculation process is as follows:where（qn）1-Taking into account the load acting on the thick hard roof slate on the n-th level.E1,h1-The elastic modulus and thickness of thick hard roof rock layers.En,hn-The elastic modulus and thickness of each rock layer above the thick hard roof.γ1,γn-Bulk density of thick hard roof and above each rock layer.

When the calculation yields （qn+1）1 ＜（qn）1,the load q on the medium-thick hard roof plate is calculated to be（qn）1.

According to the principle of balance of forces,the distance between the forces on the thick hard roof plate to point O yields:

In which p is the pressure on the bracket when the roof plate is broken.

The pressure of the thick hard roof plate is not greater than the maximum working resistance of the bracket,that is

A reasonable roof caving step l can be obtained by substituting Eq.(6)and substituting the corresponding parameters.

(2)On-site industrialization test

According to the above research results,when re-mining the working face of raised upper limit,the compulsive roof caving method is used to control the roof plate.From the perspective of controlling the top plate [10-15],the circulating shallow hole groove topping technology is adopted,and the roof caving effect can be guaranteed and it is easy to operate,therefore,this type of roof caving is chosen on site.

Combined with the cylindrical shape of the borehole,the elastic modulus,thickness and bulk density of the thick hard roof and its overlying rock layer are substituted into the Eq.(2)can get the load q=175 kN/m2,and then substitute bulk density of thick hard roof rock γ=25 kN/m3,thickness h=13 m,fracture angle of thick hard roof β=50°,Maximum control caving step of single bracket on working face lk=5.5 m,Maximum supporting resistance of column p=300 kN into the Eq.(6),the maximum caving distance l=50 m can be obtained.

The roof caving measures for the working face are:during the initial stage of the working face,the overhanging edge of the goaf is more than 3 m,and the inclined direction exceeds 5 m without taking time,during the normal mining period,the overhanging edge of the goaf is more than 2 m,and the inclined direction exceeds 5 m without taking time,the top of the goaf must be loosely blasted when the top end of the goaf is more than 2 m.The compulsive roof caving plan is shown in Fig.5.

5.Analysis of characteristics of working face weighting

5.1.The main roof to weighting step

After the compulsive roof caving of the working face,the first weighting step is 9.7 m,the average resistance of the bracket is 23.23 MPa,the continuous step is 6.6 m,and the average periodic weighting step is 12 m,the specific weighting is shown in Table 4.Through the statistical analysis of the working resistance of the hydraulic strut on the working face with the advancement step and the curve is shown in Fig.6.

It can be seen from Fig.6 that with the continuous advancement of the mining surface,the working resistance of the column exhibits obvious periodicity,reflecting the first weighting and periodic weighting characteristics of the working face,and the weighting strength is slightly greater when the first weighting is applied,and the coefficient is 1.45.The working resistance of the pillars is unevenly distributed along the mining surface,and the working resistance is not too high,which is far less than the rated working resistance of the working face pillars of 34.6 MPa,indicating that the first supporting force of the strut support is low or the stiffness of the supporting system is too big.However,considering the safety production of the working face,such support stiffness ensures the support effect of the working face and guarantees the safe production of the working face.

Fig.5.Forced caving blast hole layout.

Table 4 Characteristics table of main roof weighting in face.

Fig.6.The relationship between average working resistance and advance step.

Fig.7.Advance abutment pressure distribution curve in the tunnel.

5.2.Advance abutment pressure

Through the monitoring of the resistance of the advancing single pillars in the two sides of the working face,the statistical results are shown in Fig.7.

It can be seen from Fig.7 that the peak value of the leading support pressure of the wind tunnel is 4.0-7.6 m in front of the coal wall,and the maximum value of the peak pressure concentration coefficient is 1.7,which is consistent with the numerical calculation result(the average value of the supporting pressure peak position in front of the working face is 6.0 m).The maximum supporting pressure peak is 23.1 MPa,and the peak pressure concentration coefficient is 2.24,the peak value of the pressure and the concentration factor after the roof caving is reduced.

6.Conclusions

(1)When the sandstone roof of the working face is broken,the voussoir beam structure cannot be formed,and the cantilever beam will act on the bracket,and the voussoir beam structure can be formed only when it is fallen to the high key layer,at this time,the support bears the weight of the large rock strata beneath the fractured sandstone layer to the high key stratum.

(2)The working resistance of the bracket increases with the increase of the thickness and breaking length of the sandstone roof.When the breaking length of the roof reaches a certain extreme value,the hydraulic support column will have no stroke and thus support crushing occurs.

(3)The compulsive roof caving method with the determined roof caving parameters to control the roof plate is adopted when re-mining,that is the row&line space is 2.5 m×5 m.After the compulsive roof caving is adopted for the thick hard roof,the breaking distance of the straight-covered main roof is artificially shortened and the force on the lower support is reduced when the sandstone roof plate is broken,and the working face is safely stoped.

Acknowledgments

The authors are grateful to the National Natural Science Foundation of China (Nos.51574007 and 51604007).