Microcosmic Failure Mode Test And Rockburst Tendency Analysis Of Yellow Sandstone Under The Influence Of Particle Size

During the construction of the new Tianheshan Railway Tunnel from He-shun County of Shanxi Province to Xing-tai City of Hebei Province,rock burst occurred in the Yellow Sandstone section,which seriously affected the construction safety and progress.In this paper,uniaxial compression and triaxial tests were carried out in laboratory.Combined with SEM,macroscopic failure mode and microscopic fracture phenomenon,the variation rules of the overall failure and peak strength characteristics were studied.Direct shear test and Brazil splitting test were carried out in the laboratory to study the change of cohesion,internal friction Angle and average tensile strength of yellow sandstone with different particle size.Using geological thin sections identification and SEM scanning microscope test,X-ray powder diffraction(XRF and XRD)and test means such as high pressure Raman spectroscopy test of three groups of yellow sandstone microscopic structure,mineral composition,compounds and lattice stability test of main rock forming minerals,studies of these factors on the influence law of strength parameters in the yellow sandstone.According to the above research ideas,the main conclusions and new understandings of this paper are as follows:(1)The relationship between uniaxial compressive strength and elastic modulus of three groups of yellow sandstone with different average particle size(from small to large particle size)and particle size change was studied.The rule of yellow sandstone affected by particle size change was that the larger the particle size was,the lower the peak strength and elastic modulus of yellow sandstone were.Triaxial compression tests were carried out on three groups of yellow sandstone.It was found that the peak strength of yellow sandstone in the same grain size group increased with the increase of confining pressure,and the confining pressure strengthened the peak strength of yellow sandstone under triaxial compression.(2)Through Brazil splitting test and direct shear test of yellow sandstone with unequal particle size,it is concluded that the average tensile strength of yellow sandstone decreases with the increase of particle size.Similarly,the cohesion and internal friction Angle of yellow sandstone decrease with the increase of particle size.(3)The geological thin section identification technique combined with SEM scanning electron microscopy was used to observe the internal microstructure of three kinds of yellow sandstone with different granularity.The results showed that quartz was the main mineral that constituted the skeleton grain of yellow sandstone,and other minerals together constituted the cement.The framework grains(quartz)of the fine-grained yellow sandstone are the densest,followed by the medium-grained yellow sandstone and the coarse-grained yellow sandstone.With the increase of grain diameter of yellow sandstone,the arrangement of skeleton grains becomes looser,but the content of cementing material increases continuously.The microfracture of yellow sandstone under the action of external force is mainly caused by the destruction of the cement with low strength,followed by the destruction of grain skeleton.(4)The strength parameters of yellow sandstone are related to the content of quartz in the internal skeleton particles.The more quartz content,the better the strength performance.The results of high pressure Raman spectroscopy test show that the crystal lattice parameters of the main minerals(quartz)in the three groups of yellow sandstone are stable,and there is no drastic phase transformation.The crystal lattice changes of the main mineral components(quartz)particles have little effect under high pressure.(5)Based on rock brittleness index B as the index for analyzing the rockburst propensity of the three groups of yellow-sandstone,combined with elastic strain energy parameters,it is found that the rockburst propensity of yello w-sandstone has an upward trend with the increase of particle size,but it is basically at a slight or moderate degree of rockburst.Combined with the actual rockburst situation of Tianheshan tunnel field investigation,the rockburst propensity results obtained from the laboratory test of fine-grained yellow sandstone taken from the field were verified.