True triaxial test on bearing damage characteristics of drilled coal samples in high gas environment

MA Yankun; TANG Dingzou; YUAN Liang; LIU Xiaofei; ZHAO Aohan; HUANG Qinhao

doi:10.13247/j.cnki.jcumt.20240004

Abstract：To study the load-bearing damage characteristics of coal samples with drill holes in high gas environments,five kinds of load-bearing damage tests of coal samples with drill holes under gas pressure were carried out by using a real triaxial mechanical test system with one-side visualization and gas sealing function,and image information of the damage process of the samples under direct observation was obtained,and the characteristics of the borehole dynam-ics damage in the process of load-bearing and the evolution of the cracks on the surface of the samples were analyzed by combining with the DIC and acoustic emission monitoring technolo-gy.The results show that three types of typical dynamic damage phenomena,such as particle ejection,coal body throwing and debris spalling,will gradually appear in the process of drilling damage,and the whole damage process shows discontinuous and intense dynamic damage char-acteristics,with particle or debris ejection appearing in the drilling wall first,followed by in-tensive coal body throwing and debris spalling,and collapsing of the wall in the late stage of the damage.The existence of gas gas weakens the intensity of damage,and with the increase of gas pressure,the damage of the borehole gradually changes from intense particle ejection(coal body throwing)to debris spalling,and the average ejection velocity of particles(debris)gradually decreases in the range of test gas pressure(0～1.2 MPa),from 0.345 m/s to 0.197 m/s,with the decrease of about 42.9％.The emergence and expansion of cracks on the surface of coal samples coincide with the sudden change points of stress,strain and acoustic emission signals,and the damage process of coal samples can be divided into four stages:Accumulation of strain energy,damage of coal body above the drill hole,damage of coal body below the drill hole and crack penetration.In the test gas pressure range(0～1.2 MPa),the length of the strain energy accumulation phase decreased gradually with the increase of gas pressure,from 127 s to 84 s,with a decrease of about 33.6％,and the cumulative acoustic emission energy and counts were negatively correlated with the gas pressure,with a decrease of 92.2％and 98.5％,respectively.Cracks began to emerge around the borehole under the influence of shear,and gradually appeared to arch under the composite effect of tension shear,followed by crack penetration.Under the effect of shear,the cracks started to sprout around the drill hole,and gradually showed arch shape under the compound effect of tensile shear,and then the cracks gradually increased under the effect of tensile shear,and the coal body in the area of the drill hole showed the overall instability.The damage cracks on the surface of the coal samples showed the arch shape,and the largest area of the arch surrounded by cracks showed negative correlation with the gas pressure,which was reduced from 748.27 to 394.9 cm2,with a reduc-tion of about 47.2％.The strain localization phenomenon on the surface of the coal samples gradually intensified with loading.The phenomenon of strain localization on the surface of the coal samples gradually intensified with the loading,and the effective variance of the strain field accelerated,forming"X"shaped horizontal strain localization zone,"butterfly"shaped shear strain localization zone and"strip"shaped vertical strain localization zone on the surface of the coal samples.The area of the shear strain concentration zone corresponding to the region of the largest arch crack is negatively correlated with the gas pressure,which decreases from 47.89 to 27.90 cm2,with a decrease of about 41.7％.