Effects Of High Temperatures And Unloading Confinements On The Mechanical Behaviors Of Granite

High temperature rock mass geothermal development and utilization of high temperature rock mechanics involves the effect of high temperature on rock,In this paper,the physical and mechanical characteristics of granite after high temperature water cooling are analyzed and studied;In addition,during the development of geothermal resources,the stability of the geothermal drilling shaft wall is particularly important.Studying the stability of the geothermal drilling shaft wall is the basis for evaluating the safety and stability of the surrounding wall rock.This paper takes the granite treated with water cooling at different heat treatment temperatures as the research object.First,the physical properties of granite after high temperature heat treatment are studied.The influence of different heat treatment temperatures on the granite specimens is first observed,and then measured with tools such as balances and vernier calipers.The quality,volume,and density of the sample were changed.Finally,the longitudinal wave velocity and thermal conductivity of the granite sample were measured with the change of heat treatment temperature by the longitudinal wave velocity tester and the thermal conductivity tester;the temperature-seepage-stress-chemical The field coupling test system carried out 3 sets of conventional triaxial compression tests and unloading under different confining pressures(10MPa,20 MPa and 30MPa)and 5 sets of different temperatures(normal temperature,200?,400?,600? and 800?)on granite.The gas permeability test under confining pressure analyzes the mechanical characteristics of granite after high temperature water cooling.The main conclusions are as follows:(1)The degree of thermal damage to the granite samples had an increasing trend,which were pre-heated at different heat treatment and then water-cooled.At 25 °C to400 °C,there was no damage to the granite samples.However,when the heat treatment temperature is higher than 600 °C,the granite samples experienced a significant degree of damage,and when the heat treatment temperature reaches 800 °C,obvious micro cracks can be seen on the surface;(2)With the increase of heat treatment temperature,when the density of granite is between 25 °C-200 °C,the micro-cracks inside the sample are closed,the density of the sample increases,so that the dry density and saturation density increase,and the void ratio decreases;When the heating temperature is higher than 200 °C,as the the heat treatment temperature increases,the micro-cracks inside the sample increase,so that the sample density and saturation density decrease,and the porosity increases;(3)The longitudinal wave velocity and thermal conductivity of the granite sample obtained through longitudinal wave velocity test and thermal conductivity test gradually decrease with the increase of heat treatment temperature,and when the heat treatment temperature is higher than 400 °C,the longitudinal wave velocity and thermal conductivity have obvious change.(4)Under the prescribed treatment temperatures,the degree of failure of granite upon unloading confinements is different.The granite experienced the lowest reduction of confining pressure during the unloading process at 200 °C,suggesting that granite is most prone to failure at that temperature.(5)From 25 °C to 400 °C,the angle of internal friction decreases from 52.66° to50.86°,with a reduction of 3.42%,and from 400 °C to 800 °C the internal friction angle increases from 50.86 ° to 60.91°,with an increase of 16.5%;While from 25 °C to800 °C the cohesive strength is roughly reduced from 20.52 MPa to 8.28 MPa,with a reduction of 59.65%.(6)During unloading,as the temperature increases from 25 °C to 800 °C,the tangent modulus decreases gradually,with a decrease rate of 23.26% ? 49.80%,and the higher the temperature,the greater the decrease rate.(7)From 25 °C to 400 °C,the initial permeability of granite samples under unloading.confining pressure was almost the same.As the confining pressure is released,the gas permeability gradually increases.However,with increase of the heat treatment temperature,the initial permeability of the granite was clearly improved when the temperature reaches above 600 °C.As the confining pressure is released,the gas permeability decreases first and then increases.