Research On Creep Propertiesand Constitutive Model Of Anisotropic Layered Sandstone After High Temperature

With the development of geothermal resources in China,the underground mining and underground engineering,the three oil extraction,the nuclear waste disposal project and the underground coal gasification project,the surrounding rock has been involved in the high temperature environment,because of the physical and mechanical properties have changed since the rocks have cooled at high temperatures,The mechanical properties of the natural state can not be applied to the engineer ing construction,which has a great impact on the safety of the project.Therefore,in order to ensure the long-term safety of the project and provide the relevant parameters for the design and construction,it is particularly important to study the physical and mechanical properties of the layered rock after high temperature.In this paper,a series of tests on layered sandstone such as high temperature heating test,acoustic test,triaxialtest,triaxial cree p test,triaxial cree p test after high temperature,X-ray diffraction and microscopic thin slice test are carried out,the author intends to study the creep mechanical properties of anisotropic layered sandstone after high temperature from the angles of macroscopic and microscopic,and establish the anisotropic constitutive model of layered sandstone.The following results were obtained:(1)With the increasing of heating temperature,the appearance of sandstone samples gradually transits pale yellow to red brown,lower quality,volume increases,density decreases,the longitudinal wave velocity decreases gradually,and at400?decreased significantly,showing a trend decreases gradually with the increase of Angle,shows that the acoustic wave in the batch sandstone has anisotropic characteristics.(2)In the triaxial test,with the increase of the angle of bedding,the peak stress?the elastic modulus andpoisson ratio decreases first and then increases;The crack stress and expansion stress show the trend of decreasing first and then increasing with the increase of the angle.(3)The creep test results after different high temperature show that,with the increase of bedding angle,peak strength and long-term strength decreasefirst and then increase,the steady creep rate is?M?type,the peak strain increases first and then decreases,and the elastic modulus decreases first and then increases;With the increase of temperature,the peak strain of the different bedding angle increases,the instantaneous elastic deformationcorresponding to thehigher the load level when it tends to be stable,before 400?,the instantaneous strain and creep strain do not have obvious change,steady creep rate fluctuates,creep modulus increase gradually,after400?,the instantaneous strain and creep strainincrease significantly,the steady creep rate decrease rapidly,and the creep modulus decrease linearly.As temperature increases,the peak strength and long-term strength increase gradually and reached the maximum at 600?,when the temperature is over 600?,strength begin to decrease,after different temperature,the value of?/?_schange from 0.75 to 0.87,and the value increaseds first and then decreases with the increase of temperature.(4)in the normal temperature,the degree of anisotropy of the layered sandstone is relatively high,and the degree of anisotropy is between low anisotropy and moderate anisotropy.With the increase of temperature,the anisotropy parameter k_T decreases first and then increases and then decreases,indicating the degree of anisotropy is affected by temperature,at 20?~400?,the degree of anisotropy decreases,at 400?~600?,it increases,andwhen the temperature higher than 600?,the anisotropic degree lower again.(5)There is an obvious angle effect in the failure mode of layered sandstone,when the bedding angles are 0°,45°,60°,and 90°,Shear failure which is skewed towards the bedding planes mainly appears in rock samples,the sample mainly occurs shear failure which is along bedding planes or small angle oblique layer shear failure with the angle of 30°,it shows that the angle of 30°is the most unfavorable angle of mechanical properties,with the increase of temperature,the main failure mode has changed from shear failure to tensile shear failure mode.(6)The basic minerals of the sandstone are quartz and feldspar,and the proportion of quartz reached 93%~95%,which do not contain clay minerals,the mineral content do not change significantly before the temperature of 800?.The results showed that the high temperature treatment had no effect on the mineral composition of the sandstone.Through the microscopic thin slice test analysis,the sandstone are calcareous quartz sandstone,,which were formed in shallow sea facies,after high temperature,the microstructure of the sandstone has changed,the iron is strengthened,and the micro cracks appear and develop.(7)the reason of Increasing the peak strain after high temperature is due to the combined effect of the evaporation of internal water and the internal micro fracture,which is produced by differential expansion,the reason for the change of peak stress after high temperature are the mutual extrusion of mineral particles,The time effect of creep test results in the increase of the density of rock,Some micropores and microcracks were repaired,however,when the temperature is over 600?,the repair effect can not counteract the damage caused by the high temperature,and the peak strength is reduced,and the overall properties are deteriorated.(8)The damage after high temperature changes little at 200?,nd with the increase of temperature,the damage value increases gradually,and then increases rapidly at the temperature of 400?,the variation characteristics of different bedding angles are consistent,which shows that the damage values of sandstone samples with different bedding angles after high temperature are less affected by the bedding angle,which is closely related to the temperature.(9)Based on the Kachanov creep damage theory,the constitutive model of the layered sandstone at normalemperature andafter high temperature are established,according to the experimental data of different temperature,the model parameters were calculated,andcomparingwith the traditional Nishihara model,The results show that the constitutive model has a better fitting degree of creep test curves,It can effectively reflect the three stages of rock creep,which proves the reliability andcorrectness.