Experimental Study On Static And Dynamic Mechanical Properties Of Thermally Damaged Marble Under Dry And Saturated Conditions

In deep rock engineering,rock can be caused different degrees of damage by temperature(from geothermal and artificial activity)and unloading.The change of pore fluid content after damage will have an important impact on rock mechanical properties.Therefore,the exploration of physical and mechanical properties of damaged rock under saturated condition is important to ensure the safety of rock engineering.In this paper,the Beijing Fangshan marble is taken as the research object,and the CT scanning experiment of thermal damaged rock is carried out.The equivalent pore concept of rock is proposed to show effect of temperature damage in three-dimensional space.The dynamic and static tests of thermal damaged rock are carried out at dry and saturated conditions.The physical and mechanical properties of rock under dry and saturated conditions of rock are analyzed,and the damage is analyzed by the CT scanning.The crack law of thermal damaged rock after failure be studied.The main work and research results in this paper are as follows:(1)By using CT and ultrasonic techniques,the damage detection experiments of Beijing Fangshan marble were carried out before and after thermal damage.The results show that with the increase of the temperature,the longitudinal wave velocity of rock increases first and then decreases.The maximum value of CT increases first and then decreases with the increase of the temperature,while the minimum value of CT decreases first and then increases.In the macroscopic characteristics,with increase of the temperature,the internal micro-cracks of damaged rock show that some areas are closed and some areas are open.(2)Since there is no obvious large crack in the rock after the thermal damage,the concept of the equivalent pore is proposed—that is the part with the gray value smaller than the dark area.With the increase of temperature,the equivalent porosity decreases first and then increases,and the volume of equivalent pore of damaged rock decreases first and then increases,which is consistent with the variation of microscopic pore obtained by longitudinal wave velocity.(3)By the uniaxial static test,after the thermal damage,the plasticity of the rock increases with the increase of the temperature,and the uniaxial compressive strength of the rock increases first,then decreases,the elastic modulus decreases.Both the strain and the radial strain increase,and the Poisson's ratio decreases first and then increases.The presence of water further reduces the strength of the thermal damaged rock,When the damaged rock is saturated,the uniaxial compressive strength decreases,the axial strain decreases,the radial strain increases,the difference of elastic modulus is not obvious,with temperature increases,the variation range of Poisson's ratio decreases.(4)The dynamic compressive strength of the damaged rock shows a significant rate correlation with the increase of the loading rate(increasing with the loading rate),and the rock after the thermal damage has stronger rate effect than the undamaged rock.When temperature is 105 C,there is little difference in the dynamic strength between drying and saturation.When the temperature reaches 450 °C,the dynamic compressive strength of rock under saturated state is higher than that of dry state.When temperature is 700 °C,with the increase of loading rate,after 1500GPa/s,the saturation strength is greater than that under dry conditions,shows a significant phenomenon of saturated water.With the increase of loading rate,the cracks appear from the outside to the inside,from less to more.