Study On The Degradation Mechanism Of Sandstone And Mudstone Under The Action Of High-temperature And High-humidity

During the summer,the rock formations in Chongqing are exposed to high-temperature and high-humidity for a long time.Therefore,Relying on Chongqing Changshou Second Yangtze River Bridge Project,aiming at the more developed sand and mudstone rock formations in southwestern China,the impact of high-temperature and high-humidity on the degradation of sand and mudstone will be studied.Firstly,the laboratory test method was used to analyze the macro,micro and mechanical characteristics of sand and mudstone,and the mechanism of deterioration of sand and mudstone under the action of high temperature and high humidity was obtained.Then,the mechanical damage model of sand and mudstone under the action of hightemperature and high-humidity was constructed and verified according to the mechanical test.Finally,through numerical simulation,the depth of the surface layer of sand and mudstone under the action of high-temperature and high-humidity and its influence on the displacement of the sand layer impact,and make suggestions for the protection of foundation pits under the action of high-temperature and high-humidity.The main results of the paper are as follows:(1)The high-temperature and high-humidity tests on sand and mudstone were conducted for 15 days and 7 days.Then the macro porosity,expansibility,water absorption(sandstone)and disintegration(mudstone)tests of the sandstone and mudstone after high-temperature and high humidity.And the mechanical uniaxial and microscopic mineral analysis tests and the mechanical triaxial and microscopic scanning electron microscopy tests were carried out respectively,and the changes of various property parameters of sand and mudstone with the increase of high temperature and high humidity duration were analyzed.For sand and mudstone,the macro characteristic parameters increase with the duration of high-temperature and high-humidity;the mechanical characteristic parameters decrease with the increase of the duration of high temperature and high-humidity;The micro-fractal dimension has a negative correlation with the triaxial strength parameters;The proportion of albite content in the microscopic characteristics of sandstone is positively correlated with the uniaxial strength parameter;The proportion of albite in the microscopic characteristics of mudstone is positively correlated with uniaxial compressive strength,and the proportion of clay mineral content is negatively correlated with uniaxial compressive strength.In addition,the confining pressure suppresses the reduction of the mechanical properties of sand and mudstone after high temperature and high humidity,and the suppression is limited under low confining pressure.(2)According to the changing rules of sand and mudstone laboratory test parameters,define and calculate the degradation degree of sand and mudstone under the action of high tempperature and high-humidity.The analysis shows that the macro and micro degradation effects affect each other,and the two at the same time aggravate the mechanical degradation effect.Then,through the normalization and differentiation of the fitting results of the deterioration degree of all parameters,the sequence of the influence of high-temperature and high-humidity on the deterioration of the sand and mudstone properties is obtained.According to this,the degradation stage of sandstone and mudstone under the action of high-temperature and high-humidity is defined,and the degradation mechanism of sandstone and mudstone under the action of high temperature and high humidity is obtained.For sandstone,the deterioration degree of mechanical compressive strength is about 19%,and the deterioration degree of shear strength is about9%;and the main sequence of high-temperature and high-humidity effects on sandstone degradation is:macro parameters>mechanical deformation parameters>mechanical strength parameters>microscopic characteristic parameters.The degradation process of sandstone is divided into four stages:heat absorption stage,damage transition stage,damage expansion stage and damage stability stage.For mudstone,the deterioration degree of mechanical compressive strength is about 75%,and the deterioration degree of shear strength parameter is about 41%;and the main degradation sequence of high-temperature and high-humidity on mudstone is the same as sandstone's.The degradation process of mudstone is divided into three stages:heat absorption stage,damage surge stage and macro rupture stage.(3)Based on the conventional triaxial compression test of sandstone and mudstone under the action of high-temperature and high-humidity,using the strain equivalent assumption and staistical strength theory,the mechanical damage constitutive equation of sandstone and mudstone under the action of high-temperature and high-humidity is derived.Using the elastic modulus and fractal dimension under the action of high-temperature and high-humidity to characterize the damage variable caused by high-temperature and high-humidity and the micro-element damage variable caused by the load under high-temperature and high-humidity respectively,construct the damage evolution equation of sandstone and mudstone.Then,based on the nonlinear elastic constitutive relationship,the two damage evolution equations are derived to obtain the sandstone and mudstone damage constitutive equation under the action of high-temperature and high-humidity.Compared with the test curve,the theoretical fitting curve has a better reduction degree,which can reflect the mechanical damage of sandstone and mudstone under different high-temperature and high-humidity durations.(4)Firstly,the effectiveness of Ansys software in simulating the effect of high-temperature and high-humidity was verified by acoustic test.Then,using Ansys software,construct a solid model of the foundation pit,and analyze the depth of the effect of different high-temperature and high-humidity time periods on the degradation of the surface layer of sandstone and mudstone.The 15-day high-temperature and high-humidity sandstone degradation effect depth is0.308 m,and the 7-day high-temperature and high-humidity mudstone degradation effect depth is 0.252 m.Based on the simulation results of the depth of degradation,the effect of hightemperature and high-humidity on the displacement of the anchorage side of the foundation pit was analyzed.The displacement of the anchor side of the sandstone foundation pit under the action of high-temperature and high-humidity for 15 days increased by 7.2% compared with the natural state.Based on the conclusion of the effect of high-temperature and high-humidity on the degradation of sand and mudstone,the protection suggestions for foundation pit construction under high-temperature and high-humidity are proposed.