Study On The Influence Mechanism Of High Temperature Action On Rock Thermal Cracking

In projects involving high-temperature environments such as underground coal gasification and mining,dry hot rock geothermal resource development,deep mineral mining,and geological deep burial treatment of radioactive nuclear waste,rock structures may undergo thermal cracking under the action of high temperatures,causing changes in their mechanical properties and permeability,and inducing the risk of potential geological disasters.Therefore,it is of great theoretical and engineering significance to study the mechanism of high temperature effects on rock thermal cracking.In this thesis,the most common sandstone,limestone,and granite in deep engineering are selected as the research objects.Firstly,by studying the micro macro morphological characteristics,porosity,mass loss rate,and mineral composition changes of the three types of rocks under the action of a unified temperature path,and combining the temperature and stress fields obtained from numerical simulation,the mechanism of thermal cracking of the three types of rocks under the action of high temperature is discussed;Taking limestone as an example,the micro-macro morphology characteristics,porosity changes of three sizes of limestone under the action of a unified temperature path,and the micro-macro morphology characteristics,porosity changes of a single size of limestone under the action of four temperature paths were studied.In these studies,the heating and cooling processes of limestone were numerically simulated,and the mechanism of size effects and temperature paths on rock thermal cracking was discussed.Finally,based on the above research,the influence mechanism of high temperature on rock thermal cracking is revealed.The main research results and conclusions are as follows:(1)A study on the thermal cracking characteristics of three different rock types found that in sandstone,thermal cracking mainly develops between sand particles and cementitious materials,followed by development in cementitious materials,and extends in the cementitious materials or along the edges of sand particles.It is rare to form transgranular crackings through sand particles,and these fractures are mostly curved in shape;In limestone,thermal cracking is easy to develop along the calcite filling zone.The thermal cracking outside the calcite filling zone is mainly developed between the fine mineral crystals of limestone,and the shape of these fractures is mostly linear;In granite,thermal cracking is mainly developed in the quartz feldspar that forms the main body of the granite.When cracks develop and encounter other minerals,if the thermal expansion rate of quartz feldspar is relatively high compared to other minerals,if the cohesion of the mineral is greater than the bonding force between the mineral and quartz feldspar,only intergranular cracks will occur between the mineral and quartz feldspar.If the mineral is soft,If the cohesion of a mineral is less than the bonding force between the mineral and quartz mineral,intergranular and transgranular cracks will occur between the mineral and quartz feldspar.(2)The influence of temperature on the thermal fracture of sandstone,limestone and granite is studied.It is found that the thermal cracking of sandstone at the heating temperature of 400℃-600℃ is mainly due to the reduction of the strength of cement due to the decomposition of magnesite,and the thermal stress also reaches the tensile strength of sandstone,leading to the development of thermal cracking;When the heating temperature is between 600℃ and 800℃,the massive decomposition of dolomite and calcite further significantly reduces the strength of the cement,leading to a significant development of thermal cracking with a small increase in thermal stress.At the stage of 200℃-400℃,thermal cracking of limestone occurs in the secondary calcite filling zone;During the stage from 400℃ to 600℃,due to the increase of thermal stress and the decomposition of magnesite,the main body(non filled zone)of limestone undergoes thermal cracking;During the 600℃-800℃ stage,the decomposition of dolomite and calcite dominates the formation of thermal cracking.At200℃-600℃,the thermal stress of granite is relatively small,and the mineral composition is stable.The thermal cracking is mainly developed along the cracking surface of granite;At the stage of 600℃-800℃,the mineral composition of the granite remains relatively stable,but the thermal stress is relatively large,resulting in significant thermal cracking development,and fracture development separated from quartz feldspar.(3)After studying the thermal cracking characteristics of three sizes of limestone,it was found that increasing the size of the rock significantly reduces the initial temperature of rock thermal cracking development.This is mainly due to the fact that the increase in rock size will lengthen the distance between the internal and external heat transfer of the rock,resulting in an increase in the temperature difference between the internal and external heat expansion of the rock,a worse consistency of the internal and external heat expansion,a greater thermal stress on the surface of the rock,and easier fracture of the rock.(4)After studying the thermal cracking characteristics of limestone under four temperature paths,it was found that after slowly heating and cooling the rock,thermal cracking tends to develop at the edge of the rock,and the direction of development and extension is mostly perpendicular to the rock edge;After rapid heating and cooling of rocks,thermal cracking tends to develop in the middle of the rock top surface,and after development and extension,the rocks are mostly directly broken into two pieces;Among the two cooling paths,rapid cooling is more likely to cause thermal cracking of rocks than slow cooling.