| In recent years,with the extension of oil and gas exploration to the depths of strata and the acceleration of the development and utilization of geothermal energy,the rock engineering problem of high temperature environment has naturally become one of the key factors restricting the development of rock engineering in the depths.More and more engineering activities require us to grasp the changes of physical and mechanical properties of rock after high temperature,such as evaluation and repair of tunnel fire,deep geological treatment of nuclear waste,underground coal gasification mining and so on.at the same time,the transformation of physical and mechanical properties of rock after thermal damage and its genetic mechanism should also be considered.Therefore,the study of the mechanical response and genetic mechanism of soft rock after thermal damage is of great significance to improve the theory and engineering application of soft rock mechanics.In this paper,the real original rock samples in the white sandstone area of Zigong City,Sichuan Province are selected as the test object for thermal damage treatment.with the help of scientific research instruments,the physical and mechanical properties of white sandstone and the variation law of long-term strength are explored and its genetic mechanism is analyzed.the main research results are as follows.(1)The physical properties,fracture appearance and mineral composition of white sandstone specimens after different degrees of thermal damage were analyzed and explored.The characteristic color of white sandstone gradually changes from grayish white to brownish red,deepens and then fades;the mass loss rate and average volume increase with the increase of thermal damage degree;with the increase of thermal damage degree,the wave velocity loss rate and the thermal damage factor based on wave velocity gradually increase,and the porosity increases gradually with the increase of thermal damage degree.The meso-appearance of white sandstone fracture is mainly in sharp triangular shape,and most of them are potholes,and the surface roughness of mineral crystals is high.(2)Uniaxial compression test and Brazilian splitting test were carried out on white sandstone with different degrees of thermal damage.The uniaxial compressive strength increases at first and then decreases with the increase of the degree of thermal damage.on the whole,thermal damage can strengthen the uniaxial compressive strength of sandstone,and 600℃ is about the threshold temperature for strong uniaxial compressive strength and weak transformation of uniaxial compressive strength.Thermal damage treatment will enhance the self-healing of white sandstone to a certain extent,and the elastic modulus increases at first and then decreases with the increase of thermal damage.The uniaxial compression of white sandstone is mainly in the form of typical shear compound failure.the higher the degree of thermal damage,the more serious the crushing degree of white sandstone and the increase of grain and powder detractions.with the increase of the degree of thermal damage,the failure mode of white sandstone changes from monoclinic shear failure to multi-shear plane failure,and the splitting failure mode of white sandstone shows obvious brittle characteristics after thermal damage.The tensile strength and residual tensile strength decrease monotonously with the increase of thermal damage.700°C can be regarded as the threshold of rapid deterioration of tensile strength and residual tensile strength of white sandstone.(3)Uniaxial creep tests at stress levels of 30% σ,45% σ,60% σ,75% σ and 90% σwere carried out on white sandstone treated with different degrees of thermal damage.The total strain of the specimen with the same thermal damage degree increases with the increase of the stress level,and the total strain of the specimen with the same stress level increases with the increase of the thermal damage degree.The instantaneous strain of the specimen with the same degree of thermal damage increases with the increase of stress level,which is a typical mechanical behavior of elastic-plastic materials;at the same stress level,the instantaneous strain increases gradually with the increase of thermal damage.The steady-state creep rate of the specimen with the same degree of thermal damage increases monotonously with the increase of stress level,and the steadystate creep rate increases monotonously with the increase of thermal damage degree at the same stress level.The change law of steady-state creep rate can be shown by the improved Carter model,which is suitable for the creep law of white sandstone specimens.(4)Based on the uniaxial creep experimental data of white sandstone,the long-term strength of white sandstone after different degrees of thermal damage is determined by transition creep method,isochronous stress-strain method and steady-state creep rate intersection method,a simple model for predicting the thermal damage degree and longterm strength of white sandstone is established.The smaller the stress level difference selected by the transition creep method is,the higher the accuracy of the long-term strength range is,and the long-term strength of white sandstone obtained by isochronous stress-strain method increases at first and then decreases with the increase of thermal damage degree.the long-term strength / failure strength ratio is about 0.55-0.75.The long-term strength of white sandstone obtained by creep steady-state rate intersection method increases at first and then decreases.The difference of long-term strength is caused by fewer data points after thermal damage at 300℃.The long-term strength /failure strength ratio ranges from 0.45 to 0.75.Only when the thermal damage treatment exceeds a certain threshold temperature,the thermal damage will have a deterioration effect on the long-term strength,and the effect of high temperature thermal damage on rock strength is a long-term effect accumulated over time. |
