Experimental Study Of Bedding Structure Effect For Sandstone Strain Burst When Unloading Double Faces

In this paper,the experimental study on double faces strain burst and its bedding effect was carried out to simulate strain burst at the intersection of roadway.A strain burst test method for double faces rapid unloading in one direction that satisfies the on-site stress transformation process was proposed.The new hydraulic servo true triaxial rockburst test system was used to reproduce the ejection failure process of double faces strain burst.Secondly,the comparative experiments of single and double faces unloading were carried out,and the similarities and differences between the two types test were summarized.Finally,the bedding effect of the sandstone strain burst with bedding inclinations of 0°,15°,30°,45°,60°,75° and 90° was obtained,when unloading dobles faces.The main contents include:The sample size,initial stress level,and loading path were designed to conduct strain burst tests.Brazilian splitting,three-point bending and direct shear tests of sandstone were conducted to obtain the essential parameters to analysis strain burst,including the critical slope of the tensile and shear micro-cracks in the average frequency-rise time/amplitude coordinates,the energy consumed by shearing and tension per unit area,the critical value of the variation coefficient of the image gray value that distinguishing shear or tensile failure.Comparative results of single and double faces strain burst tests indicate that the similarities of above two types strain bust contain ejection failure process,the evolution of the mean velocity of the free face,failure stress state,dissipation energy due to crack propagation,the evolutions of acoustic emission(AE)activity,and the energy release.Additionally,the ratios of kinetic energy to the releaseable strain energy both are very low.However,the strain burst intensity for unloading double faces is larger than that for unloading single face,which manifested as the larger initial ejection velocity,lower peak frequency of AE signals occurred during yield and burst stages(i.e.scale of micro-cracks is larger).Typical phenomena of double faces strain burst includes crack propagation,fragments splitting,particles ejection,surface bulging,bending,burst,and burst pit.According to the evolution of the average velocity of free surface,the ejection process can be divided into three stages that stage ? is particle ejection or fragments splitting,stage ? is rock block bending,and stage ? is the integral ejection.There are obvious yield points in the stress-strain curves which manifested as strain softening,strengthening and maintenance constant after yielding.According to the type ? and ? cracks in the fracture mechanics,the surface cracks in the direction of the intermediate principal stress of the specimen were divided into shear and tensile cracks,and the tensile cracks include buckling and splitting cracks.The geometric characteristics of surface cracks including the length of various cracks and the shear and tensile fracture angles,and the buckling expansion angle.The strain energy can be divided into five parts that contains the energy consumption due to plastic deformation before yielding,the consumed energy by shear and tensile micro-cracks before and during strain burst,the elastic strain energy that can be released,and the residual strain energy.Additionally,the ejection fragments were divided into two categories according to the shearing and tension failure mechanism,and the tension and shear energy consumption of the ejection fragments were estimated.According to the acoustic characteristics of the double faces unloading strain burst,the initial velocity of the ejection,the maximum principal stress drop rate,and the failure mode,the intensities of strain burst were divided into four grades:none burst,slight,medium,and strong strain burst.The corresponding classification indicators were also given.In the end,the four subareas of the strain burst fracture zones were summarized that the shear,buckling,splitting and ejection zones.Moreover,the evolution process of the fracture zones was also given.The AE activity,cumulative release energy,amplitude-frequency characteristics,ratios of tensile and shear micro-cracks and spatio-temporal evolution of shear,tensile and mixed mode micro-cracks for sandstone with different bedding dip angles were analyzed.The results indicate that unloading enhances AE activity and new AE signals with higher amplitude,lower frequency,releasing a small amount of energy,and shear micro-cracks are dominant.Additionally,stronger AE activity was concentrated in the yield and burst stages which corresponding to the intense release of energy.Moreover,the new generated AE signals had the highest amplitude and lower frequency,and was also dominated by shear micro-cracks.The locations of shear,tensile and mixed mode micro-cracks on Face2 have a good correspondence with surface macroscopic cracks.Most of the cracks had been formed before yielding,and only partially exacerbated during the yield and burst stages.The proportion of shear micro-cracks decreases after entering the yield and burst stages which is contrary to the variation of tensile micro-cracks.Furthermore,the accumulated damage of the shear micro-cracks is larger than that for the tensile micro-cracks.Additionally,the density of AE events,for the location of micro-cracks with greater energy release alwasys,is small.Finally,bedding effects of double faces unloading strain burst are as follows:the peak stress,maximum principal stress drop rate,maximum initial velocity of ejection fragments,maximum average velocity of free surface of double faces strain burst and dissipated energy during the last stage can be expressed as a quadratic curve with respect to the bedding dip angles.As far as the difficulty of strain burst,it is most likely to occur when the dip angle is 60°,which intensity is weakest.The strain burst is the least likely to occur in horizontal bedding,If it occurs,it is strong.The difficulty of strain burst from 0° to 60° gradually decreases,and gradually increases from 60° to 90°.After strain burst,the depth of burst pit,shear and tensile fracture angles,buckling expansion angle,fractal dimension of coarse fragments,and consumed energy by tensile for ejection fragments can be expressed as the 3rd curves of bedding dip angles that all decrease firstly and increase and then decrease with the increase of the bedding dip angles.The ratios of shear,tensile and mixed mode micro-cracks to the total micro-cracks are 6:3:1,which is less affected by the bedding dip angles.