Research On Mechanical And Macro-meso Structure Characteristics Of The Weakly Cemented Sandstone In Shendong Coal Field

The depth of main coal seam is shallow in Shendong coal field,usually around200m,and the bedrock is thinner,usually around 30~80m.However,in the process of mining,there are often slaving,roof fall,impact pressure,large deformation of laneway,ground step subsidence and so on.Shendong mining area coal measures strata containing weakly consolidated sand large,the rock diagenetic late,poor cementation,low strength,easy weathering,large plastic deformation and strong anisotropy.The special properties of weakly cemented rock is one of the main reasons of Shendong mining area occurred spalling and roof fall,step sinking,large deformation of roadway,rock burst disaster.But the phenomenon of strong strata behaviors,column has no itinerary and shields damage etc.Therefore,in this paper,a large number of weak cemented sandstones are collected by the method of core drilling by ground drilling.Then the microstructure,porosity and pore distribution are studied by scanning electron microscopy and low field magnetic resonance.The mechanical properties and acoustic emission of the weakly cemented sandstone are studied under the indirect tensile,uniaxial compression and thriaxial compression by the RMT-150C mechanical test machine.The damage theory model is established with the acoustic emission parameters and porosity parameters.Finally,the microstructure and mechanical response characteristics of the weakly cemented sandstone are studied by using the PFC~2DD particle flow numerical simulation software.The main research results are as follows:(1)It is found that there are obvious differences in microstructure,porosity,pore distribution and fractal characteristics of weak Cemented Sandstone and conventional sandstone.There are few cementation around the skeleton particles of the weakly cemented sandstone,the pores between the particles are very large and the particle size is relatively large.The porosity of weakly cemented sandstone is large,and the average porosity of weakly cemented sandstone is 34.03%,which is 1.73,3.96,10.63 and 17.91times of coarse grained sandstone,medium grain sandstone,fine sandstone and siltstone.The conventional sandstone is mainly small pores,but the number of small pores and medium pores in the weak Cemented Sandstone is equivalent.(2)It is found that there are obvious differences in density,wave velocity,disintegration,tensile strength,uniaxial compressive strength,uniaxial compressive strength,elastic modulus,cohesion and thriaxis residual compressive strength of weak cemented sandstone and conventional sandstone.But,there is no significant difference in internal friction angle.The average density of weakly cemented sandstone is 2071kg/m~3.Compared with coarse-grained sandstone,medium grain sandstone,fine grained sandstone and siltstone,the density of weakly cemented sandstone decreases by 7.1%,11.6%,13.1%and 13.9%,respectively.The average wave velocity of weakly cemented sandstone is 775 m/s.Compared with coarse-grained sandstone,medium grain sandstone,fine sandstone and siltstone,the wave velocity of weakly cemented sandstone decreases by 65.7%,66.9%,67.8%and 72.5%,respectively.The water does not occur after the disintegration of conventional sandstone,but weakly consolidated sand water will soon collapse.(3)The average tensile strength of weakly cemented sandstone is 0.376MPa.Compared with coarse-grained sandstone,medium grain sandstone,fine grained sandstone and siltstone,the tensile strength of weakly cemented sandstone decreases by66.7%,78.3%,92.5%and 92.2%,respectively.The uniaxial compressive strength of the weakly cemented sandstone is 5.318MPa,and the modulus of elasticity is 0.723GPa.The uniaxial compressive strength of coarse grained sandstone,medium grain sandstone,fine grained sandstone and siltstone are 6,7.9,11.1 and 11.9 times that of weak Cemented Sandstone,respectively.The elastic modulus of coarse grained sandstone,medium grain sandstone,fine grained sandstone and siltstone are 12.4,14.3,13.2 and19.2 times that of weak Cemented Sandstone,respectively.The failure of weakly cemented sandstone mainly occurs along the pores between particles,and the particles are relatively slip.The I damage is the main cause of the failure.The cohesive force of weakly cemented sandstone is 5.41MPa.Compared with coarse-grained sandstone,medium grain sandstone,fine grained sandstone and siltstone,the cohesive force of weakly cemented sandstone decreases by 83.3%,77.4%,74.5%and 81%,respectively.(4)There is significant difference of the acoustic emission characteristics between the weak cemented sandstone and the conventional sandstone.The peak acoustic emission count of the conventional sandstone is about 20 times the peak acoustic emission count of the weak cemented sandstone.There is a certain difference of the the cumulative acoustic emission count-time curve between the weak cemented sandstone and the conventional sandstone.The acoustic emission cumulative count curve of conventional sandstone can be roughly divided into three stages,and the cumulative count curve of acoustic emission of weakly cemented sandstone can be roughly divided into four stages.(5)A new damage model is established based on the parameters of acoustic emission and the parameters of porosity.