Experimental Study On The Gas Permeability Of Granite Under Cyclic Loading And Unloading

Underwater tunnel fires not only threaten people's safety of lives and property seriously,but also the great damage to the surrounding rock of the tunnel caused by high temperature and the subsequent cooling process,such as cracks,bursts,even landslide.The influence of high temperatures and subsequent rapid cooling processes on the mechanical properties and permeability of the surrounding rock is crucial for the assessment of the long-term safety of underwater tunnels.On the other hand,the thesis also considers that the permeability of rock mass is affected by temperature and excavation disturbance during the geothermal development of high-temperature rock masses.In this thesis,we used granite which treated at different temperatures(25??400??900?)as the research object,carried out the conventional triaxial compression test and the gas permeability test of granite under cyclic loading and unloading at different confining pressure(0MPa?5MPa?10MPa?15MPa?20MPa?30MPa)by THMC multifunctional test system.We analyzed the basic physical and mechanical properties of granites after high temperature and water cooling from the macroscopic and microscopic perspectives,the variation of physical parameters of granite,and the evolution of strength parameters and permeability with heat treatment temperature of granite after high temperature water cooling under cyclic loading and unloading.The main conclusions are as follows:(1)After high temperature treatment and rapid cooling,with the increase of heat treatment temperature,the dry density and saturated density of granite show a decreasing trend,the porosity and permeability increased;Through X-ray diffraction and SEM scanning experiments,it was found from the microscopic analysis that the high temperature caused thermal cracks inside the granite,the higher the temperature is,the more thermal cracks are produced,and the content of granite mineral components changes caused by high temperature;The specific surface area of BET has a similar rule to the volume of BJH: the specific surface area and pore volume did not change significantly after 400°C heat treatment,but the surface area and pore volume of granite are significantly reduced after heated treatment at 900°C;(2)With the change of heat treatment temperature,the regularity of peak strength and elastic modulus of granite after high temperature water cooling is consistent:With the increase of heat treatment temperature,the granite peak strength and elastic modulus showed a decreasing trend.After heated treatment at 400 °C,the peak strength and elastic modulus of granite have no obvious change;after heated treatment at 900°C,the peak strength and elastic modulus of granite are obviously reduced,and the destructive form changed from brittleness to ductility;(3)In the process of cyclic loading and unloading after heat treatment,granite's residual deformation formed a unclosed hysteresis loop,it showed that the loading and unloading process is not a completely elastic process,and there is plastic deformation.With the increase of confining pressure,the unloading modulus of granite increased significantly;With the increase of heat treatment temperature,the unloading modulus of granite decreased;The volumetric compressive strain of granite after water cooling at 400 °C was almost unchanged,but increased significantly after heat treatment at 900 °C;(4)With the increase of confining pressure,the gas permeability of granites decreased during cyclic loading and unloading.With the increased of heat treatment temperature,the gas permeability of granite increased during the process of loading and unloading,and got an obviously increased at 900°C.