Feasibility Study on In-Situ Driving Roadway along Goaf in Three Soft Thick Coal Seam

Uniaxial compression, triaxial compression and coal crushing experiment were tested to get the soft the three soft characteristics of coal seam and surrounding rock in the third teaching mine in western Henan. Considering the influence of water and argillaceous composition, the relationship between the waste rock expansion factor and pressure was obtained after correction. Introducing the relationship into the calculation of maximum lateral bearing pressure point and collapse characteristic distribution, its results were in good agreement with the measured and simulated results. Establishing the accumulation form factor, “v”, of gob waste rocks in trend, and the representation of “v” was obtained by theoretical calculation. After a lot of calculations to analysis the relationship of “v” and “N”, “q 1”, “Kp ”, “H”, “m”, “Rt ”, and a lot of numerical simulations by 3dec, the results were got below: When the thickness of the direct roof was large, 1<“v”,the lower park of direct roof was collapsed and accumulate naturally, and the upper park was formed an arch bearing structure. While, the roadway excavation in this situation was larger than the condition of “v”<1 in the early stage, but its excavation was easy with broken wall rock, and the roadway deformation and maintenance were smaller with the protection by the arch bearing structure. The more soft the coal seam, its deformation toward gob and the distance between maximum lateral bearing pressure point and coal wall were more larger, and driving along the goaf would recover a large amount of coal. Under truss support,roadway deformation decreased significantly and very similar to the experimented results in situ. So when the seam and direct roof was soft, thick, and 1<“v”, driving roadway along goaf in situ was in the low stress zone protected by the arch bearing structure formed above, and it was a good engineering conditions for the recovery of coal in the gob, and for the overall roadway excavation with one-time laying of the anchor rod and metal mesh. This was the reason why roadway excavation along the goaf was mainly used for mining medium thick or thick coal seams with gentle inclination, inclination and large thickness.


Introduction
Driving roadway along goaf was a roadway along the edge of the gob with a small pressure, where the overlying strata had collapsed and stabilized.It had many advantages, such as making full use of coal resources, reducing the impact of ground pressure, reducing roadway maintenance, accelerating roadway construction, and facilitating the smooth replacement of the working face.It was generally divided into roadway without coal pillar and roadway with small coal pillar.A lot of research [1][2][3][4][5][6][7][8] had

Geological conditions
The direct roof, basic roof and direct bottom of 15031 coal face of SDIC Teaching-third Mining were respectively medium-grained sandstone, sandy mudstone and sandy mudstone.The comprehensive histogram of 15031 working face was shown in Figure .1,and the mechanical parameters of surrounding rock were shown in Table 1 Through on-site sampling of direct roof and old roof, standard rock samples were made and uniaxial and triaxial compression tests were conducted to obtain the failure of rock samples, as shown in Figure .2. Uniaxial compression failure is mostly tensional shear failure, while the failure mode of triaxial compression is relatively simple, with shear failure.The uniaxial compressive strength of the basic roof strata was small, and the fracture development was obviously observed in the field.
Through the roof classification of gently inclined coal seam mining face, it's first weighting equivalent of basic roof was 804.78 less than 895 which belong to the class I roof.And the uniaxial compressive strength of direct roof was 58.65MPa less than 61.75 MPa, which was belong to the class 1b direct roof with obvious fractures.Though the "testing of the pounding method", the strength coefficient of coal was 0.7.The lithology of the floor was similar to that of the basic roof.Thus the 15031 coal face was in three soft conditions of roof, coal seam and floor.

Analysis on deformation mechanism of surrounding rock in upper section
Under the influence of mining, there was a process of big expansion deformation, crushing, caving, collapsing, filling and consolidating occurred in the direct roof.Li Shugang [15] divided the goaf of fully mechanized top coal caving face into natural accumulation area, pressure crushing area and compaction area as shown in Figure .3,and calculated it's expansion coefficient of each area.

The relationship between the expansion factor of the waste rocks from direct roof and stress
The compaction characteristics of waste rocks obeyed the quadratic function relationship by several studies researched by Sun Lihui [13] and Chu Tingxiang [16] .After sorting out sun Lihui's experimental data, averaging the expansion factor of different particle sizes under the same stress, and implementing the regression calculation between the expansion factor and stress, the smooth curve and regression formula of the expansion factor and stress as shown in Figure .4were obtained.
Relationship of the expansion factor and stress Because the basic bottom of the coal seam is a limestone aquifer, the waste rocks in the goaf is in the natural moisture state, or even in the water-saturated state.Ma Zhanguo [17] found that the broken rock deformation increased with it's moisture content increased.Chen Xiaoxiang [18] studied the expansion factor of broken rock mass under different moisture content, and proposed that, under natural water saturation, the residual expansion factor of mudstone was 1.031 to 1.180, and the factor of sandstone was 1.191 to 1.370, the factor of sandy mudstone was 1.163 to 1.329.Compared with the natural state, the decrease in the residual expansion factor of mudstone, sandstone, sandy mudstone were respectively 14.21%, 9.24%, 11.10% in average.Considering that the mudstone composition in sandy mudstone direct roof might be lost with the water in the goaf, and that the mudstone composition was generally regarded as 10%, by introducing water bearing weakening factor w K =0.85, the formula is corrected and revised below: 2 0.00051 0.032725 1.499315 ² 0.9976

Analysis on the characteristics of waste rocks crushing expansion and compaction in goaf
After the mining of the face in the upper section, the main roof overturned, sinked, and the direct roof collapsed, deformed and accumulated, to form a stable structure.Hou chaojiong and Li Xuehua [19,20] made a deep analysis on the stability of large and small structures of surrounding rock in gob side entry of fully mechanized top coal caving, and putted forward the reasonable reserved width of small coal pillar and the design basis of roadway bolt support structure。Subsequently, several scholars [1][2][3][4][5][6][7][8] made in-depth researches and promotion on the small coal pillar and support forms on the basis of large and small structure theory, which verifying and enriching its applicable conditions。According to the researches, the main roof in the compaction area overturned stably, and had the largest subsidence.The waste rocks in the pressure crushing and compaction area born the load of the main roof in a certain range, and the subsidence gradually decreased from the compaction area to the natural accumulation area. .the calculation model of the form factor of waste rocks accumulation The natural accumulation area was an area where the natural accumulation waste rocks could not completely fill the separation gap, and its subsidence in this area was a minimum relatively.The characteristics of the three zones could be established the accumulation model as shown in Figure .5.The author puted forward the form factor of waste rocks accumulation in the goaf on the side of the roadway, v, which was the ratio of the theoretical accumulation and partial or total compaction volume of the waste rock came from the direct roof collapsing to the gob space made up of main roof, floor and coal wall.This index could be characterized as the compaction factor in the pressure crushing area and the compaction area.It could be described as the formula below：   V " was the overturned subsidence volume of the main roof." 2 V " was the pre mining volume of collapsed direct roof before mining influence." 3 V " was the pre mining volume of mined coal before mining influence." t K " was the equivalent expansion factor corresponding to the initial support force at the tunnel." pc K " was the expansion factor of the compaction factor." p K " was the average expansion factor of the zone from the tunnel to the compaction area in golf." l " was the cantilever length of main roof turnover, " 0 w " was the maximum subsidence of the main roof，" 0 x " was the distance of the maximum lateral bearing pressure from pillar，" old h " was the thickness of direct roof ， " 1 h " was the distance from top point of the coal pillar to the main roof along " " direction." " was the fracture angle of direct roof." m " was the caving height at one times." N " was the height ratio of the direct roof and coal seam.
According to Li Shugang's research on the compaction area and natural accumulation area around the goaf and the theoretical calculation of its scope [15]  ,and the theory, which included that the pressure behaviors of goaf along the dip trend and strike trend were in similar basically, and the development process and occurrence time of the basic roof fracture were synchronous in dip and strike direction of the golf, putted forward by Song Zhenqi [21]  , the cantilever length of main roof turnover was taken according to its periodic weighting step distance as the follow equation: .Because the overburden load borne by the pressure crushing and swelling area and the natural accumulation area was small, the full distribution of stress would form a stress arch structure above the pressure crushing area and the natural accumulation area, and the two arch foots were located in coal seam and the compaction area of goaf [22]  .So there were two stress concentrations at the end of the main roof cantilever and at the point of maximum lateral support pressure, which made a arch-structure as shown in Figure .1.The stress concentration factor , c k , at the goaf end of the main roof cantilever was generally 1 to 2. Du Tailiang [23]  putted forward the proposed solution of direct top fracture angle for homogeneous soft rock slope through numerical calculation: pc K was the residual expansion factor of waste rocks.Some scholars had introduced the calculation formula of the peak position of the stress and lateral support pressure of the two sides of the coal seam roadway proposed by Hou chaojiong [24]  to the calculation of the lateral support pressure distribution in the goaf.In the process of application, the author found that there was a large gap between the calculation and the measured results in the calculation of the lateral support pressure distribution in the goaf.Song Zhenqi [25]  calculated the lateral support pressure distribution form through the support pressure volume and two-stage linear function, and gave the calculation formula of the maximum lateral support pressure point, which could described as the below equation.

MSEE-2023
Journal of Physics: Conference Series 2706 (2024) 012059 p was the ratio of the range of lateral support pressure to the distance from the maximum lateral pressure to the coal wall.
0 L was the face length, and 1  was the comprehensive moving angle.
The expansion factor of the compacted zone, pc K , and the expansion factor of the waste rocks beside the tunnel supports, t K , could be calculated respectively by the internal stress of waste in the compacted area, 2 q , and the initial support strength of supports, zc q .While 2 q could be calculated by the equation as 21  old q q h   .By introducing the distribution factor k, The expansion factor of pressure crushing area could be described as = ( ) / 2 k was the distribution factor of pressure crushing area.The k was taken as 1 for uniform distribution calculation temporarily.

Factors analysis
Through theoretical analysis, calculated with MATLAB, when 1  We know that there were close relationships between v and N, 1 q , K p , H, m , t R from the Figure .6.The v was in logarithmic relationship with N, and increased with the increase of N, but it was growth rate gradually decreased.The v was in power function relationship with t R , and decreased with the increase of t R , while it's reduction rate gradually decreased.The v was in quadratic function relationship with H, and increased with the increase of H.The v was in quadratic function relationship with m, and decreased with the increase of m, when the height of the direct roof was 4.2m.whileit's reduction rate gradually decreased.The v was in line function relationship with qz, and increased with the increase of qz.

Deformation characteristics of surrounding rock
According to the factor analysis, v was the most closely related to N.Under the given deformation condition of the main roof, when the height of the collapse zone was determined, if 0＜ v ≤1，the direct roof all collapsed and the gob couldn't be completely filled with falling collapsed waste rocks, so the natural accumulation area was large，and there were huge closed fractures exit in the compaction zone of gob and the call wall.At this time, the main roof, pressure crushing area, compaction area and the coal body beside the gob driving roadway form a bridge structure, to bear the upper stress.While the natural accumulation area was located in the pressure release area, and the supporting structure of roadway only beared the weight of rock mass in the overlying collapse zone of the natural accumulation area, which was the most favorable condition for driving roadway along goaf.For the v decreased with the increase of m, so the bigger the m, the more obviosely the nature accumulation zone, which proved that the driving roadway along goaf was mostly used for mining medium thick or thick coal seams with gentle inclination and large inclination.
The wide of the nature accumulation area and pressure crushing area could be described as 2 2 0 0 d l w x    , which was the favorable area for roadway excavation.Excavating roadway "in-situ" in the nature accumulation area , regarded as released area, was theoretical basis of "in-situ" roadway.However, when mining along the goaf in the next section, due to the insufficient compaction of waste rocks in the natural accumulation area near the roadway, it was unable to provide support stress for the upper part.The overturned main roof cantilever reversed flip to the roadway side and overall subsided.The roadway deformation was large, and it's maintenance was hard.
When 1＜ v , the collapsed waste rocks would filled the gob in theory, the nature accumulation area was not exist, and the pressure crushing area would compact gradually to be the compaction area with the increase of v.In fact, when 1＜ v , the overturning subsidence of old roof decreased with the increase of v , the collapse zone would decrease with the upper direct roof strata stopping collapse and being into the fractured zone.So when 1＜ v , The height of the collapsed zone gradually increased from the coal wall to the depth of the goaf, in the other word, the height of the collapsed area in the nature accumulation area was less than that in the pressure crushing area, and the height of the pressure crushing area less than that in the compaction area.Then the upper not collapsed strata above the pressure crushing area and the nature accumulation area had become a curved fracture zone, which formed the pressure bearing half-arch structure in Figure .1 under the pressure bearing action.The structure was similar to the "granular arch" structure of upper immediate roof in top coal caving proposed by Li hong-tao [27] .The formation of half-arch structure provided bearing stress against to the upper strata, and prevented the overlying strata continuing to collapse.Meanwhile the v in the lower collapsed zone was less than 1, which provided a foundation for small stress and deformation of roadway and a stable excavating space.When mining in the lower section, due to the arch bearing characteristics, the roadway deformation inside the arch structure was small, and the later maintenance was less。 Bring relevant data of the third teaching mine into the formulas above, the periodic weighting of the hard main roof, l , was 10.2787m, whose error with the field measured data, 9.8m [28] , was within 5%, so good agreement of theory and field measured results was obtained.Through inverse calculation, the height of the collapse zone, h  , in the natural accumulation area was 4.2m, which was less than the height of direct roof,8.02m.In the other word, the height of arch structured fractured zone above the nature accumulation area was 3.82m.The peak bearing pressure on the inner side of the coal body was 3.008m away from the side of the coal wall, and the distance of roadway could be arranged, d , was 7.0881m.

Study on numerical simulation of roadway excavation along goaf 3.1. Mining simulation of upper section
Using 3DEC to simulate the mining model of roadway excavation along the goaf to simulate the strike long wall one-time full height mining, the boundary conditions of the model were shown in Figure .7 the designed working face length was 140m, the dip angle of coal seam was 10°,the heights of coal seam, direct roof, and the old roof were 2.5m, 8m, 12m respectively.As the lateral abutment curves of the coal and roof shown in the Figure .8,the distance from the maximum lateral abutment pressure to the coal wall in the coal seam and old roof were 3 meters, which was similar to previous theoretical calculation result.The distance from the maximum lateral abutment pressure, -4.8e7Pa, to the coal wall in the lower part of the direct roof(0-4m) was 4.6 meters with, while the distance in the upper part(4-8m) was 6.3 meters with -6.7e7pa of the maximum lateral abutment pressure.So the upper part of the direct roof was the main bearing layer.

Figure 8. The lateral abutment curves of the coal and roof
As the lateral abutment curves of the bottom and top of the coal seam shown in the Figure .8,there were big downward vertical stresses in the range, which was in the gob from 7 meters away the coal wall to the deep gob.It proved that the compaction area was 7 meters away the coal wall, and had be bearing the up load.Comparing the stress curves of the direct roof (2m、4m、8m above the seam)and old roof (11m and 13m above the seam), in the direct roof, the trend of the stress curve of the strata 2m above the coal seam was obvious different with the trend of the curves of the strata 4m and 8m above the coal seam, while the stress curves of the stratum 4m and 8m above the coal seam were in similar trend with bigger abutment stresses.It proved that the upper part,(4-8m), of the direct roof was converted to a overall bearing structure, which was similar to the previous theoretical calculation result.While in the old roof, the abutment pressures of the stratum 11m and 13m above the coal seam decreased sharply.So the stratum from 4m to 11m above the coal seam, including the upper part of the direct roof and the lower part of the old roof, was the main bearing layer.
The thickness of the direct roof was taken as 2m ( v < 1) and 8m (1 < v ) respectively for simulation calculation, and the stress and displacement nephogram of coal seam strike slice was obtained as shown in the Figure .As shown in the Figure.9, the total range of natural accumulation area and pressure crushing area had little effect on the direct top height, which was mainly related to the nature of the main roof.The roadway space in v ＜1 condition was obvious bigger than that in the 1＜ v condition.When v ＜1, a beam structure was formed above the roadway with easily in-situ excavation.But the coal wall would be more broken due to the superposition of concentrated stress in the next section of mining, and the main roof would sink and overturn, which was extremely unfavorable for roadway maintenance.When 1＜ v , the lower part of the direct roof above the roadway was stacked in the form of collapse, and the upper part of the direct roof and the lower part of the old roof formed an arch bearing structure, which had the overall bearing characteristics of bending and sinking.Although a larger amount of roadway excavation was need in the roadway excavation, the formation of its arch bearing structure provides an important guarantee for the stability of the roadway in the next section mining.
Changing the mechanical parameters of hard coal soft coal, and simulating its deformation characteristics, the lateral abutment stress and displacement nephogram was obtained as shown in Figure .10: The mechanical properties of the coal had an obvious influence on the deformation and stress distribution.The harder the coal, the smaller the deformation, 1.5m, of the coal body towards the goaf was, but the closer the peak lateral support pressure to the coal wall, 0.8m, was.The softer the coal, the more developed the fractures were, and the worse the mechanical properties of the fracture were.Under the action of vertical stress, the coal body had underwent elastic, plastic, crushing, extrusion, large deformation processes, resulting in large transverse strain, with greater deformation towards the goaf side, 2.3m, and with farther peak lateral support pressure to the coal wall, 2.7m.Therefore, the softer the coal seam was, the greater the range of the deformed coal invaded into the goaf was due to the concentrated stress had transfered to the deep.The large deformation area with lower stress also created conditions for in-situ goaf driving, recovering the left coal, overall tunneling with laying anchor bolts and metal mesh one time.In this mining, the maximum deformation is 2.3m.The location of the maximum lateral abutment pressure in the coal seam was 2.7m away from the coal wall, which was a favorable area for roadway excavation along the goaf.

Soft coal
Hard coal Figure 10.The lateral abutment stress and displacement nephogram

Roadway excavation support design
Because the coal was soft and high side free along the goaf, the traditional shed support could not control the deformation of surrounding rock.The labor intensity of workers was high, the cost investment was high, and the safety effect was poor.BOUCHAUD J P, el., [29][30] proposed that when the rock mass in the collapse zone was disturbed stably, the coordinated action of surrounding rock and support would form a new "force field", namely "support load field", in which there was a support structure curve with the most reasonable stress.On this basis, Wu Yongping [31] based on the interaction between the support and the surrounding rock, proposed that the load field of the support along the goaf without roadway was a eccentric half arch structure, as shown in Figure .11.Its mechanical core was that the support could have variability characteristics, adapt to the original rock stress, and form a

Roadway support effect
Through the simulation calculations and field measurements under the conditions of no support and truss support, the position where the roadway deformation changed dramatically was within 10 meters beside the roadway as shown in Figure .12.The deformation of the roadway with no support within this range presented a characteristic of logarithmic function.The closer it was to the face, the greater the deformation and deformation rate were.The deformation of the roadway with truss support was significantly reduced.Comparing the measured results, the truss support could play a good role in the surrounding rock control of roadway driving along goaf in the next section of thick and soft coal seam minning, and also provided the support basis under the similar engineering and geological conditions.1. Uniaxial compression, triaxial compression and coal crushing experiment were tested to get the soft the three soft characteristics of coal seam and surrounding rock in the third teaching mine in western Henan。Considering the influence of water and argillaceous composition, the relationship between the waste rock expansion factor and pressure was obtained after correction.Introducing the relationship into the calculation of maximum lateral bearing pressure point and collapse characteristic distribution，its results were in good agreement with the measured and simulated results.
2. Establishing the accumulation form factor, "v", of gob waste rocks in trend, and the representation of "v" was obtained by theoretical calculation.After a lot of calculations to analysis the relationship of "v" and "N"," 1 q ", "K p ", "H", "m", " t R ", and a lot of numerical simulations by 3dec, we know that the v was in logarithmic relationship with N, and increased with the increase of N, but it's growth rate gradually decreased, and it was in power function relationship with t R , and decreased with the increase of t R , while it's reduction rate gradually decreased, and it was in quadratic function relationship with H, and increased with the increase of H, and it was in quadratic function relationship with m, and decreased with the increase of m, when the height of the direct roof was 4.2m, while it's reduction rate gradually decreased, and it was in line function relationship with qz, and increased linear with the increase of qz.
3. Through theoretical calculations and numerical simulations, the results were got below: "v" increases with the ratio of direct roof to coal seam thickness, and decreases with the coal seam thickness.In the other word, the thicker the coal seam，the greater the "v", when the "v"＜1，the natural accumulation area was great and the beam strata structure was created up the driving roadway along goaf.In this situation, the driving roadway excavation was easier, but the maintenance of the driving roadway was extremely unfavorable because of the main roof would sink and overturn in reverse during the next section mining.When the thickness of the direct roof was large，1＜"v"，the lower park of direct roof was collapsed and accumulate naturally, and the upper park was formed an arch bearing structure.While, the roadway excavation in this situation was larger than the condition of " v "＜1 in the early stage, but its excavation was easy with broken wall rock, and the roadway deformation and maintenance were smaller with the protection by the arch bearing structure.The more soft the coal seam, its deformation toward gob and the distance between maximum lateral bearing pressure point and coal wall were more larger, and driving along the goaf would recover a large amount of coal.Under truss support，roadway deformation decreased significantly and very similar to the experimented results in situ.So when the seam and direct roof was soft, thick, and 1＜"v" , driving roadway along goaf in situ was in the low stress zone protected by the arch bearing structure formed above, and it was a good engineering conditions for the recovery of coal in the gob, and for the overall roadway excavation with one-time laying of the anchor rod and metal mesh.This was the reason why roadway excavation along the goaf was mainly used for mining medium thick or thick coal seams with gentle inclination, inclination and large thickness.
4. The deformation of the roadway with truss support was significantly reduced.Comparing the measured results, the truss support could play a good role in the surrounding rock control of roadway driving along goaf in the next section of thick and soft coal seam minning, and also provided the support basis under the similar engineering and geological conditions.

Figure 3 .
Figure 3. Distribution of compaction form of waste rocks in Goaf

1 Figure 5
Figure 5. the calculation model of the form factor of waste rocks accumulationThe natural accumulation area was an area where the natural accumulation waste rocks could not completely fill the separation gap, and its subsidence in this area was a minimum relatively.The characteristics of the three zones could be established the accumulation model as shown in Figure.5.The author puted forward the form factor of waste rocks accumulation in the goaf on the side of the roadway, v, which was the ratio of the theoretical accumulation and partial or total compaction volume of the waste rock came from the direct roof collapsing to the gob space made up of main roof, floor and coal wall.This index could be characterized as the compaction factor in the pressure crushing area and the compaction area.It could be described as the formula below：

1 q
of direct roof, t R was the direct top's tensile strength.was the unit weight of overlying strata on the main roof, which was got by 1 qH   Kp, H, m , t R as independent variables respectively to calculate, then the following results were got:

Figure 6 .
Figure 6.The relationship between v and N, zq , H, m , t R

Figure 7 .
Figure 7.The boundary conditions of the model By simulating the mining process, the stress and displacement nephogram when the working face advances 140m was obtained.Due to the influence of coal seam dip angle, the stress and displacement nephogram of coal, direct roof and old roof showed overall reserve movement characteristic, with the stress nephogram showing the characteristics of overall movement in the inclined direction and the displacement nephogram showing the characteristics of overall movement in the opposite inclined direction.The range of the lateral abutment pressure in the lower side of goaf was bigger than that of the upper side of goaf along the dip direction of coal strata, while the maximum lateral abutment pressure of the lower and upper goaf showed the opposite characteristics.When the face putted forward 140 meters, the abutment pressures had little changed in the range of 40 meters to the 140 meters, which was regarded as stability range of surrounding rock after caved and it would be the initial state of the roadway driving along goaf for the next section.The lateral abutment curves of the coal and roof was shown in the Figure.8.As the lateral abutment curves of the coal and roof shown in the Figure.8, the distance from the maximum lateral abutment pressure to the coal wall in the coal seam and old roof were 3 meters, which was similar to previous theoretical calculation result.The distance from the maximum lateral abutment pressure, -4.8e7Pa, to the coal wall in the lower part of the direct roof(0-4m) was 4.6 meters

Figure 9 .
Figure 8.The lateral abutment curves of the coal and roof As the lateral abutment curves of the bottom and top of the coal seam shown in the Figure.8,therewere big downward vertical stresses in the range, which was in the gob from 7 meters away the coal wall to the deep gob.It proved that the compaction area was 7 meters away the coal wall, and had be bearing the up load.Comparing the stress curves of the direct roof (2m、4m、8m above the seam)and old roof (11m and 13m above the seam), in the direct roof, the trend of the stress curve of the strata 2m above the coal seam was obvious different with the trend of the curves of the strata 4m and 8m above the coal seam, while the stress curves of the stratum 4m and 8m above the coal seam were in similar trend with bigger abutment stresses.It proved that the upper part,(4-8m), of the direct roof was converted to a overall bearing structure, which was similar to the previous theoretical calculation result.While in the old roof, the abutment pressures of the stratum 11m and 13m above the coal seam decreased sharply.So the stratum from 4m to 11m above the coal seam, including the upper part of the direct roof and the lower part of the old roof, was the main bearing layer.The thickness of the direct roof was taken as 2m ( v < 1) and 8m (1 < v ) respectively for simulation calculation, and the stress and displacement nephogram of coal seam strike slice was obtained as shown in the Figure.9:

Figure 11 .
Figure 11.A eccentric half arch structure and the truss support construction picturesThe truss support composed of bolt, metal mesh, anchor cable and shotcrete could re reinforce the broken surrounding rock of the roadway.It has the deformation characteristics in coordination with the original rock stress, and it was easy to form a stable bearing structure.It had the advantages of fast construction progress, low support cost and good support effect.It could be used for the goaf support of soft coal seams to form the roadway at one time, ensure the return air section, and eliminate the safety hazards existing in the later repair, and reduce the labor intensity of workers.According to the mechanical form and the support forms were designed below as shown in Figure.11.

Figure 12 .
Figure 12. the deformation rate of the tunnel 10m in front of the face

Table 1 .
. Parameters for different rocks