Study On The Microscopic Mechanism Of Rock Thermal Damage And The Evolution Characteristics Of Macroscopic Physical And Mechanical Properties

The study of thermal damage of rock contributes to thetheoretical reference significance and the engineering application for tunnel fire evaluation or repair,underground coal gasification,deep burialdisposal of radioactive nuclear waste,geothermal resources development and utilization of deep underground space.In view of the engineering background above,limestone,sandstone and granite are selected as the research objects,which are typical rocks in the earth's lithosphere and relating to human engineering closely.The physical and mechanical properties of these rocks after temperature treatment(25-900°C)are studied through laboratory experiment and data analyses.The changes of microstructure and composition of rocks with temperature are analyzed by using the techniques of scanning electron microscope,X-ray diffraction,X-ray fluorescence,mercury injection,differential thermal analysis and thermogravimetric analysis,the response characteristics of micro-macroscopic parameters and the micro mutation reason of macroscopic parameters are revealed by the research of the microscopic characteristic of rock thermal damage,such as the growth and propagation of crack,the variation of main minerals,quantitative analysis of mineral elements,pore character and thermal physical-chemical reactions.Based on the canonical correlation analysis algorithm,the canonical correlation variable group comprehensively revealing rock thermo damage and the representative parameters in micro and macroscopic scale is obtained.The theoretical critical temperature of thermal damage is studied through the renormalization group theory and the result showes that 300-500°C is the critical temperature interval of thermal damage of rock under different shape parameters.Finally,the similar physical model experiment of tunnel fire is carried out to study the influence of temperature on tunnel lining,whose results are in good agreement with the results of physical and mechanical experiment.The results of this paper can be used as a reference for the structural design,damage detection,post disaster assessment and repair of rock engineering involving high temperature environment.The main conclusions are as follows.(1)The influences of high temperature on micro-characteristic(i.e.microstructure,mineral composition,element content and porosity)of limestone,sandstone and granite and its microscopic mechanism are studied by the scanning electron microscopy,X-ray diffraction,differential thermal analysis,X-ray fluorescence analysis and mercury injection techniques.The results shoe that the main reason of thermo damage of limestone is the development of pore and small crack caused by mineral decomposition and thermal stress,especially above 300°C,the thermnal stress induced the gradual development of new cracks,while part of the minerals(magnesite and dolomite)gradual decomposied,resulting in the damage or destroy of micro structure and the increase of porosity.The damage increases sharply after 800°C caused by melting action.The damage of sandstone is small lower than 400°C,which mainly caused by the dehydration andthe slight expansion of the micro defects such as original cracks and voids.Above 400°C,the rapid development of thermal damage appears caused by the decomposition of laumonite and calcite(700-800°C),the dihydroxylation of kaolinite,and the rapid initiation,propagation and stepwise connection of new cracks.The thermal damage of granite can be neglected below 200°C.The micro cracks and holes begin to develop and the porosity begin to increase in the range of 200-500°C.Above 500°C,the rapid development of thermal damage appears with the rapid development of crack,the quick increase of porosity and damage or even destroy of crystal structure under the influences of thermal stress and phase transition.(2)The change law of physical parameters of limestone,sandstone and granite and critical temperature threshold of these parameters caused by thermal damage are obtained through the experimental study of the physical parameters,such as surface color,mass loss,longitudinal wave velocity,thermal conductivity,apparent resistivity,permeability and electromagnetic radiation.The results show that the surface color changes obviously with the above 400°C.With the heating temperature increase,the mass loss rate,apparent resistivity,cumulative ringing count,cumulative energy,permeability coefficient,electromagnetic radiation and other parameters gradually increase on the whole scale,while the longitudinal wave velocity,thermal conductivity,thermal diffusivity and intrinsic magnetic susceptibility decrease gradually.Compared with the change of the physical parameters with the temperature,it is founded that 300-500°C is the critical temperature threshold of the thermal damage of the rock,and most physical parameters are mutated in this temperature range.(3)The vary characteristic of mechanical property and acoustic emission signals of limestone,sandstone and granite after high temperature treatment is obtained through the mechanical tests,i.e.uniaxial compression test(simultaneous strain and acoustic emission),tensile strength test and the test of expansion,the research result reveals that the mechanical properties of these three kinds of rocks are gradually deteriorated with the elevated treat temperature,such as the gradual decrease of compressive strength,tensile strength,swelling stress,elastic modulus,Poisson's ratio,hardness and the gradual increase of peak strain,cumulative ringing count,cumulative energy.There is a sudden change point in the process of variation of the mechanical parameters,and it is found that most mutation points are between 300°C and 500°C through statistics.When the temperature reached or exceeds the threshold,the mechanical properties of rock are deteriorated rapidly.(4)Based on the renormalization group theory,the three-dimensional renormalization group model of rock thermal damage is established,and the damage combination and its probability under the condition of non-damage loss and damage probability transfer are analyzed and calculated.The critical thermal shreshold of different shape parameters are obtained,and its value is about 300-500°C,which is in good agreement with the abrupt temperature range of almost parameters in the macroscopic physical and mechanics test.(5)Based on the changes of micro-macro physical and mechanical parameters of limestone,sandstone and granite,the micro-macro response characteristics of thermal damage and their mutual influence mechanism are analyzed.A good consistent temperature mutation value is found in the main minerals and the rapid development of the defects.The parameters,such as wave velocity,compressive strength,thermal conductivity,etc.,are selected as the good response to the change of micro properties.(6)Based on the canonical correlation analysis algorithm,the typical variable groups representing mineral element content and macro parameters are got through the correlation between the contents of major mineral elements(oxides)and the macro parameters after different temperatures and their own correlation.Besides,the mineral composition affecting microscopic characteristics largely and the representative macro parameters responding to micro change well are obtained according to the weight of each parameter.The results show that the decomposition of magnesite and dolomite has a large influence on the micro characteristics of limestone.The influences on sandstone are followed by calcite,zeolite and feldspar from large to small.The influences on granite are feldspar,quartz and hornblende from large to small.Wave velocity has a best response of the macroscopic parameters to the microscopic properties of all these three kinds of rocks,and the second is thermal conductivity and strength parameters.(7)The qualitative and quantitative methods of rapid and effective identification of thermal damage of rock by apparent color observation and wave velocity test are proposed based on the micro-macro response characteristics and correlation analysis.(8)The damage of tunnel lining after high temperature treatment is obtained by the similar physical model test of tunnel fire,whose results have a good agreement with the variation of physical and mechanical properties of rock.