Analysis on Failure Mechanism of Rock-Coal Two-Body Structure with Weak Cemented Rock and the Condition of a Gob Side Entry without Coal Pillar

Though the real tri-axial testing simulated by FLAC3D, the failure mechanism and deformation process of rock-coal “two-body” structure in the in-situ area, named crucial point, which bore the ultimate lateral abutment pressure and broke in the beginning to lead to the whole damage of the “three soft” ground-rock around the gob-side entry retaining was obtained, and the relationship between the ultimate strength and cohesion of rock was obtained. By field observation the condition of gob side entry without coal pillar was obtained: top old roof as the bearing body, when the height of direct roof was larger than 1.309m1 and soft enough, the half-arch structure could made by the upper strata of the direct roof, as a weak layer, which gave a force straight up against to the old roof overturning and sinking. That was the condition of gob side entry retaining without coal pillar.


Introduction
Gob-side entry retaining was a effective method to relief the production capacity, but several scholars had studied much on the coal pillar remained beside the tunnel as a supporting structure.In fact, there was a gob side entry without coal pillar successfully applied in the 12024 coal face of SDIC Teaching-third Mining.
The ground rock of coal face unconstrained due to the caving activities, so it fractured, damaged, deposited and arranged out-of-order randomly as scattered rocks, then the upper layer over the caved area collapsed layer by layer from the bottom to up, which caused a small structure over coal face [1] .In fact, the scattered rocks from the broken and the collapse of thick, soft, and enormous fractures developed direct roof above the caving space, might formed a small structure named half-arch structure above the tunnel to prevent big damage and made the stability of the surrounding rock [2][3][4] .
The half-arch structure, a kind of force chain structure caused by shearing action [5] , appeared in the discrete body more easily [6] .According to the half-arch structure's mechanical mechanism, the half-arch structure, as a mechanical protection, it's failure process and deformation mechanics is the key to the possibility of the gob-side entry retaining without coal pillar and just with simple supporting structures.The deformation and failure mechanics of rock-coal two-body structure which picked out from the in-situ area, named crucial point, bore the ultimate lateral abutment pressure beside gob-side entry retaining with thick coal seam, regarded as one skewback, was very important.While the upper parts of the direct roof, above the other side of gob-side entry retaining, were not caved and could be converted into the other part of the complete half-arch was another key.
Several scholars had studied much on failure instability of two-body rock structure.For example, by simulating and analyzing the structure's failure space position and it's impact inability omen, Ling Peng putted forward that the high localization of stress or strain might cause the model's macroscopic crack form [7] .But the failure progress of two-body structure was not studied in detail.Wang Xuebing [8] simulated rock's mechanics properties relating to it's failure mechanism by FLAC, without physical and geology properties.Deng Xubiao [9] got the relationship between impact tendency and mechanical properties: the more distinct between coal and rock, the impact tendency was more easily occurred.While the influence under the dynamic load was less studied on the two-body structure, that picked out from the surrounding nearby the interface between call seam and rock seam.So for obtaining the deformation and failure mechanics of two-body rock and coal beside gob-side entry retaining, the structure failure mold, stress-strain curve, effect time of shear strain and tension strain, and the relationship of strain hardening stage and soft stage with the model in uniaxial compression texts were studied.

Geological engineering survey
The 12024 coal face of SDIC Teaching-third Mining, located in the south west of Henan, which was designed to mine 21 coal seam with 515m depth in average, 975m length in strike direction in average, 95m length in inclination direction with 10.5° dip in average.The coal seam belonged to the instable coal seam, with sandy mudstone base roof which was 12.6m in average, and with sandstone immediate roof, which was 9.86m in average, and with sandy mudstone direct bottom, which was 1.07m in average, and with limestone base bottom, which was 4.92m in average.
In the process of mining, the anisotropic immediate roof could distinct mainly into sandstone, medium grained sandstone and fine siltstone, which were all weak cemented strata.Therefore, in the selection of rock specimen, the four lithology was picked as the objects.All samples of the specific physical characteristics were shown in the table below:  Coal-rock two-body specimen was in tandem relations in the structure, and now assuming that the sample complied with Hooke's law, and assuming that the elastic modulus of the whole two-body structure modulus of the whole two-body structure was E12, the elastic modulus of coal was E1 and the elastic modulus of rock was E2.The relationship within E1, E2 and E12 could be described with the equation numerically: x So the two-body structure's constitutive structure could described as the below equation.The formula shows that the whole elastic modulus of the coal-rock two-body specimen was higher than the elastic modulus of the single coal specimen significantly.And the whole elastic modulus of the coal-rock two-body specimen was increased with the growth of the elastic modulus of rock E2.However, due to coal's elastic modulus E1 is much smaller than E2 on numerical value, at the same time the rock masses' elastic modulus were not big relatively on coal sample value.so the integral elastic modulus of different sample structure changed a little.

Two-body uniaxial compression analysis 4.1 Influence of rock cohesion on two-body intensity
Through the calculation of the five groups of two-body structure, the five structures' stress-strain curves was obtained and shown in figure 3. From the diagram, the ultimate strength of coal-rock two-body specimen was higher than the single coal specimen, and the strength rise 1.118, 1.176, 1.217 and 1.224 for the sandstone, medium grained sandstone, fine sandstone and siltstone was joined respectively.So the ultimate strength of two-body structure increased with the increasing of the cohesion of rock mass.But the relationship was not linear, while it could be described with the equation y=10.233Ln(x)+5.14,R 2 =0.9869, and it's relationship curve was shown in figure 4.
The results showed though the stress-strain curve in figure 3.
From figure 3, strain value corresponding to different two bodies rock ultimate strength changed little, and with the growth of rock's elastic modulus the strain reduced.So it could be regarded that before the two body's failure the whole deformation is mostly the coal specimen's deformation.When calculating to the 432th step, equivalently 0.00703 strain, the stress-strain curves of the two body specimens (not including single coal specimen) began to separate from point A, the main reason was the difference of rock's elastic modulus, and that the two body was regarded to get into yield stage (AD).In yield phase, different from the single coal two-body specimen, there were obviously ductile deformation (BC) in the other specimens.this was similar to the results of lab rock under uniaxial compression, which was because of the uneven produced by inhomogeneous fractures.As cohesion increased, the yield stress and strain corresponding to the ductile deformation of stress-strain yield period was increased monotonically.From the strain contour map corresponding to the yield stage in figures.4 and figures.5 respectively, the strain contour was in symmetrical distribution in the profile face of the specimen.The structure's stress area increased and then decreased , and finally the strain contour corresponding to the Y point on the stress-strain curve was disconnect in the center of profile face, and the internal structure was broken.The rock sample joining the two body played a large role to the deformation of the structure.Comparatively speaking, in the whole yield stage (BD) coal sample's deformation was given priority to whole deformation, while the proportion of rock's deformation increased in CD stage.The strain rate contour map from left to right corresponded to B/C/D points on the stress-strain curve of medium grained sandstone.
4.2 Influence of rock's tensile strength on the stress softening of two-bodies From the Five groups stress-strain curves local figure of two structure of coal and rock in the figure.5, the rock sample joining the two body played a big role in the strain softening stage.Especially the medium grained sandstone and sandstone specimen whose tensile strength was bigger than coal.And there were two pieces of strain softening and a clear ideal plastic stage in the stress-strain curve (lateral extension EF section).As the tensile strength increased, the horizontal extension length corresponded to the ideal plastic stage was longer, while, at the same time, the corresponding extension inflection point (E) dropped with the decrease of tensile strength.However there was a plastic softening stage in the softening stage of the specimens whose tensile strength is equal to or less than the tensile strength of coal specimen.So the rock's tensile strength bigger than coal tensile strength was the determination to the lateral extension, and with the tensile strength increasing, the strain corresponding to the lateral extension was bigger.

single coal specimen failure mechanism
From the single coal body shear strain gradient figure in figure 6, there was 'X' shear crack at both ends of the specimen and there was tensile fracture developing from center to the ends mainly before the peak.At the point of ultimate strength, shear failure occurred at the ends of the model, while tensile damage occurred in the centre.After peak shear strain developed from centre to the ends, the contour retaining at the ends and no appearance in the centre ultimately.It showed that the peak tensile failure is given priority to the specimen.Compare to coal-rock two-body specimen its destruction form was simple, and large lateral deformation occurred mainly, which is the reason of rib spalling and support damage by forced deformation.From the shear strain contour corresponding to the stage BD, the shear strain contour changed larger in the coal-rock two-body specimen than coal specimen.And from the shear-strain gradient figure in figure 6, starting from point D, shear-strain contour was almost not changed in rock part.It represented the shear broken, especially in the coal section, was the most reasonable result in the stage BD.The failure shape was 'OX' in the coal part and 'V' in the rock one.Point D was the turning point.From the shear-strain gradient map in the period from D to F, the shear-strain contour was becoming more and more uneven.shear-strain contour area in coal section decreased obviously in DE, but the shear-train contour was not disconnected from top to bottom in the model, which shows that more fractures increased by the shear failure.The form of the fracture developed from the "OX" form to the vertical tensile fracture, leading to the reduction of the bearing capacity area.Shear-strain contour in coal section started to disappear in the EF period, which shown EF period was the transition stage of the broken form from shear failure to tensile failure.It proved by the relationship between deformation and tensile strength of in EF for the horizontal restraint in the specimen, the coal section was compressing into strain softening state.The changed curve slope of the curve after F described that there was mainly tensile failure in this period.In fact, when the coal section was broken by tension crack, the crack would extend into the rock section.So the whole structure broken into scattered rocks with big deformation.Shear strain contour was disconnected up to down in the period from E to F in figure.6,which showed obvious plastic deformation and strain strengthening.Comparing the single coal specimen and the rock coal two-bodies, the bigger tension, the lager softening stage and gentle bearing pressure was proved.With the working face advanced, under the influence of lateral abutment pressure, the inner coal body and the direct roof of the gob side entry retaining experienced the elastic stage, brittle failure, plastic softening and failure deformation stage as in the compression test.During the elastic stage, the coal body and the direct roof were not damaged.When the stress in the coal body and the direct roof gradually reached the peak compressive strength, which was mainly determined by the coal body performance, the support in the roadway was in the stage of higher supporting force.Therefore, the initial supporting force of roadway support must be greater than the peak compressive strength of coal and direct roof.When the working face continued to advance and the internal fissures of the coal body and the direct roof began to expand or continued to develop, the coal body and the direct roof entered into the plastic softening stage.According to the simulation results, at this time, the coal body and the direct roof would deform horizontally, and the old roof would begin to overturn and sink.While the subsidence and horizontal deformation was not big enough to break, but the lateral horizontal movement at the coal body side would generate lateral thrust on the support to the goaf direction.So adding inclined legs to the roadway support or providing a part of the retractable space was effective measures to deal with the surrounding rock deformation of the roadway at this stage.When the working face continued to advance lateral support pressure increased continuously and coal seam and direct roof above the roadway were greatly damaged and old roof was sharp overturning and sinking.Due to the lateral stress of the support, the coal body is in triaxial compression state, and its bearing capacity and compressive strength will be increased At this time, with no confining pressure, the upper direct roof would be crushed and extruded to the unconstrained side, and be piled up beside goaf side nearby roadway.So the direct roof with soft and fracture development was more likely to be broken.The greater the degree of breakage, the more fully the rock in the direct roof was broken and extruded.It did not or weakly borne the load imposed by the direct roof above, so that the surrounding rock stress of the roadway could be small, the roadway could be more stable, and it's maintenance was easier.
The big deformation stage of the coal side of gob-side entry retaining was the stage of strain strengthening in the coal section in EF.In other words, the bigger tension of the direct roof, it's horizontal deformation develop towards roadway was bigger and irreversible.That EF period was the important stage for large tension deformation in the direct roof above gob-side entry retaining, which made the coal body side of gob retaining roadway was under a released pressure.

5.2
The distribution characteristics of the waste rock in golf and the condition of the half-arch in direct roof.Shugang Li [10] proposed the distribution characteristics of the waste rock in golf as natural accumulation area, pressure crushing zone and compaction area, and the calculation method of rock mass expansion coefficient.
Resuming the natural accumulation area was completely filled by the waste rock, the calculation formula of rock mass expansion coefficient was reducible as below.According to the rock mass expansion experimental data achieved by Lihui Sun [11] , after averaging the coefficient of dilatancy of different particle sizes under the same pressure, the regression curve and it's equation of coefficient of dilatancy and pressure was achieved.
In the natural accumulation condition, the height of caving zone could be described as the equation below.

¦
So when the height of the direct roof was bigger than h ¦ , the upper strata of the direct roof stopped to caving, and converted to bending zone with a large number of bed separation crannies.So the half-arch structure could made by the upper strata of the direct roof, as a weak layer, which gave a force straight up against to the old roof overturning and sinking.Then the stress in old roof would be not centralized and the initial collapse step was larger.So the bigger height of the direct roof, the larger breaking length of the old roof, and the maximum lateral bearing pressure beside the gob side entry retaining would go deeper.In fact, though field measurement, the distance from the point of maximum lateral bearing pressure to the roadway side wall was 8.6m, which was larger than the distance in the hand direct roof [12] .So it did not need coal pillar to protect the tunnel.Under the protecting of the half-arch structure, due to the influence of mining, soft and weakly direct roof damaged into scattered rocks and moved, rolled into the space under the half-arch.So the pressure in direct roof transferred to upper roof until old roof in the vertical direction, and it transferred far and deeper in the horizontal direction.The surrounding rock around the tunnel is in a small deformation, whose stability was proved by engineering practice.
In microcosmic, under the impact of mining, the rock-coal two-body broken fully by "OX" or "V" form or broken along the joints or weakly surfaces into scattered rocks, and in macrocosmic the half-arch was made by upper strata of the direct roof was the reason for reduction of the stress of tunnel surrounding rock.
While the height of direct roof in the 12024 coal face of SDIC Teaching-third Mining was 8 meters and soft enough which was larger than 1.309m1 ,so the half-arch structure could made by the upper strata of the direct roof, as a weak layer, which gave a force straight up against to the old roof overturning and sinking.That was the condition of gob side entry retaining without coal pillar.

Conclusion
1) The rock-coal two-body strength increased with the increasing of the cohesion of rock mass.
2) the softening period of stress-strain curves were studied particularly, and the rock tensile bigger than coal tensile was the key to the lateral extension.With the increasing of the tension of rocks, the stain value corresponding to the extension stage was growing.
3) The failure of the rock-coal two-body was transferred from 'OX' shear fracture to vertical tensile fracture in Microcosmic.Macroscopically, it was mainly tensile strain.And it happened mainly in the period of the lateral extension of the stress-stain curve (EF stage).
4) When the height of direct roof was larger than 1.309m1 and soft enough the half-arch structure could

Figure 1 .
Figure 1.The shape of surrounding rocks of gob-side entry retaining

3
The constitutive relation and the calculation model Geometry and boundary condition of the coal-rock two-body calculation was shown in figure.1.It's radius was 1, and it's height was 4. It was divided into two same samples, with coal specimen lower with blue and rock specimen upper with red.The model was divided into lots of elements.Load velocity v = 2.5×10 -5 m/step on the up surface and fix the down surface simulating uniaxial compression.

Figure 4 .Figure 5 .
Figure 4.The relationship between the ratio of two-bodies' ultimate strength and coal's and rock cohesion

Figure 6 .
Figure 6.Single coal two-body specimen shear strain gradient figure 4.4 coal-rock two-body failure mechanismFrom the shear strain contour corresponding to the stage BD, the shear strain contour changed larger in the coal-rock two-body specimen than coal specimen.And from the shear-strain gradient figure in figure6, starting from point D, shear-strain contour was almost not changed in rock part.It represented the shear broken, especially in the coal section, was the most reasonable result in the stage BD.The failure shape was 'OX' in the coal part and 'V' in the rock one.Point D was the turning point.From the shear-strain gradient map in the period from D to F, the shear-strain contour was becoming more and more uneven.shear-strain contour area in coal section decreased obviously in DE, but the shear-train contour was not disconnected from top to bottom in the model, which shows that more fractures increased by the shear failure.The form of the fracture developed from the "OX" form to the vertical tensile fracture, leading to the reduction of the bearing capacity area.Shear-strain contour in coal section started to disappear in the EF period, which shown EF period was the transition stage of the broken form from shear failure to tensile failure.It proved by the relationship between deformation and tensile strength of in EF for the horizontal restraint in the specimen, the coal section was compressing into strain softening state.The changed curve slope of the curve after F described that there was mainly tensile failure in this period.In fact, when the coal section was broken by tension crack, the crack would extend into the rock section.So the whole structure broken into scattered

Figure 7 .
Figure 7. Shear strain gradient figure 5 Analysis on the dynamic broken of surrounding rock of gob-side entry retaining 5.1 The deformation and failure mechanics of coal-seam and direct roof beside gob-side entry retaining.With the working face advanced, under the influence of lateral abutment pressure, the inner coal body and the direct roof of the gob side entry retaining experienced the elastic stage, brittle failure, plastic softening and failure deformation stage as in the compression test.During the elastic stage, the coal body and the direct roof were not damaged.When the stress in the coal body and the direct roof gradually reached the peak compressive strength, which was mainly determined by the coal body performance, the support in the roadway was in the stage of higher supporting force.Therefore, the initial supporting force of roadway support must be greater than the peak compressive strength of coal and direct roof.When the working face continued to advance and the internal fissures of the coal body and the direct roof began to expand or continued to develop, the coal body and the direct roof entered into the plastic softening stage.According to the simulation results, at this time, the coal body and the direct roof would deform horizontally, and the old roof would begin to overturn and sink.While the subsidence and horizontal deformation was not big enough to break, but the lateral horizontal movement at the coal body side would generate lateral thrust on the support to the goaf direction.So adding inclined legs to the roadway support or providing a part of the retractable space was effective measures to deal with the surrounding rock deformation of the roadway at this stage.When the working face continued to advance lateral support pressure increased continuously and coal seam and direct roof above the roadway were greatly damaged and old roof was sharp overturning and sinking.Due to the lateral stress of the support, the coal body is in triaxial compression state, and its bearing capacity and compressive strength will be increased At this time, with no confining pressure, the upper direct roof would be crushed and extruded to the unconstrained side, and be piled up beside goaf side nearby roadway.So the direct roof with soft and fracture development was more likely to be broken.The greater the degree of breakage, the more fully the rock in the direct roof was broken and extruded.It did not or weakly borne the load imposed by the direct roof above, so that the surrounding rock stress of the roadway could be small, the roadway could be more stable, and it's maintenance was easier.The big deformation stage of the coal side of gob-side entry retaining was the stage of strain strengthening in the coal section in EF.In other words, the bigger tension of the direct roof, it's horizontal deformation develop towards roadway was bigger and irreversible.That EF period was the important stage for large tension deformation in the direct roof above gob-side entry retaining, which made the coal body side of gob retaining roadway was under a released pressure.

K 1 m
was the expansion coefficient in the natural accumulation area, was the height of coal seam; h ¦ was the height of caving zone.

Table 1 .
coal and rocks physical property