Study On Physical-mechanical Degradation Characteristics Of Sandstone Under Acid Attack

With increasing acid sedimentation and industrial polluted water,the acidic environment causes damage to the rock structure to varying degrees,seriously threatening the stability and durability of the project.Through the physical and mechanical properties of sandstones corroded by hydrochloric acid and sulfuric acid solutions with different pH values,the thesis analyzes the change of mechanical parameters with the corrosion time.Combined with the scanning electron microscope test,the microscopic characteristics of the sandstone before and after corrosion are analyzed.The acid corrosion mechanism of sandstone was discussed and a chemical-stress coupling damage constitutive model wad established based on chemical kinetics and chemistry thermodynamics,mineral dissolution mechanism,energetics,and damage mechanics theory.The main research contents and results are as follows:(1)A comparative test study on the physical and chemical properties of sandstones attacked by HCl and H₂SO₄with different pH values was applied.The results show that the quality change rate,cationic dissolution rate,elastic modulus and the compressive strength decreases with increasing corrosion time.The pH value compression section and peak strain increase with increasing corrosion time.Smaller the pH greater the variation;The sandstone is transformed from the original massive dense structure into a loose structure.granular with the help of SEM analysis.(2)Based on the chemical kinetics theory,the mechanism of acid corrosion of sandstone was studied from the mechanism of mineral dissolution.It was found that feldspar and calcite can react with the acid solution through quantitative description of the dissolution rate of minerals composed of sandstone.The reaction is more violent when the pH value is less than 3,the dissolution rate is higher and increases with decreasing the pH value;when the pH value is greater than 3,the dissolution rate is low.At the same time,the influence of pH,temperature,specific surface area,flow velocity and time on the dissolution rate of sandstone minerals was discussed.(3)From the perspective of energetics,by calculating the energy at each characteristic point in the uniaxial compression of acid-etched sandstone,the dynamic evolution of energy in the whole process of sandstone uniaxial compression is studied.The results show that:the total energy,elastic strain energy,dissipated energy and energy increment of each stage of uniaxial compression of the corroded sandstone are higher than that of the original sandstone.The energy storage and energy release properties of the sandstone are enhanced;The dynamic evolution of energy in the entire process of the corroded sandstone is quite different from that of the original sandstone.The elastic strain energy and dissipation energy both show an increasing trend before the peak point.In the compaction section,elastic section and crack propagation section,the proportion of elastic strain energy increases with increasing load,and the proportion of dissipation energy decreases.In the unstable crack propagation stage,the ratio of elastic energy drops and the proportion of dissipated energy increases.(4)Based on the principles of continuous damage mechanics,statistical theory,and chemical kinetics,the statistical damage constitutive model of acid-corroded sandstone was established.The chemical-load coupling damage variable was introduced,and the constitutive model considering the compaction section of the stress-strain curve of sandstone was established.The segmented statistical damage constitutive model can better reflect the deformation characteristics of the acid-corroded sandstone.The stress and displacement of the tunnel during different corrosion periods are analyzed through the establishment of the acid-corroded tunnel finite element model.The results show that after 30 years of tunnel corrosion,the vertical and horizontal displacements around the vault are 2.73 and 2.90 times of the initial state respectively.The lateral stress,vertical stress and shear stress around the vault increase by 17.0%,4.1%and 9.6%respectively compared with the initial state.