Study On Distribution And Characteristics Of Gypsum Bearing Strata And Geological Environment Effect In Chengdu

As the urbanization degree of ground space in Chengdu is higher and higher,affected by the limitations of ground space,Chengdu tends to develop into underground space,which virtually requires more in-depth and extensive research on the geological environment of underground space.Gypsum and Glauber’s salt soluble minerals are generally contained in the strata of the xiafuguankou formation in Chengdu,which is highly corrosive to the concrete and reinforcement of the underground structure.Due to the dissolution of gypsum and Glauber’s salt in the underlying rock mass,the underlying bearing layer forms a cavity,changes its stress effect,and brings potential safety hazards to the project.Therefore,the research on the hydrochemistry of gypsum and mirabilite minerals in the underlying gypsum bearing strata in Chengdu and their impact on engineering construction is of great significance.The main research work and conclusions are as follows:（1）From Cretaceous to Neogene,the interior of Sichuan Basin shrank,and the sedimentary environment gradually became closed,that is,it was in a semi closed closed state,with strong evaporation under subtropical climate conditions.Under the action of evaporation,various solutes in the brine formed by concentration precipitate in turn according to their solubility to form evaporite.The genesis of gypsum and glauberite involves continental genesis,marine genesis,lacustrine sedimentation,metamorphism,hydrothermal process and karstification.Sulfate and other salt sources around the lake basin and arid climate are two important factors for the formation of gypsum rock.Besides marine genesis,gypsum and glauberite in Sichuan Basin are involved.Anhydrite and gypsum formed in continental environment are often symbiotic with red terrigenous mudstone,and gypsum is widely distributed in Chengdu.（2）In this paper,the research scope of the distribution law of gypsum and glauberite in the underground space of Chengdu is 100.0m below the ground.Within this scope,gypsum or glauberite is exposed in other areas of Chengdu except zone I（Wenjiang District,Qingyang District and Pidu District）.The areas with large thickness of gypsum rock in Chengdu are zone II（Wuhou District,Gaoxin District）,zone III（Tianfu New Area）,some Zone V（Jinjiang District,Longquanyi District,Xindu District）and Zone VI（Xinjin District）.Moreover,the distribution depth of gypsum bearing strata in Chengdu is basically more than 100.0m below the ground.Gypsum bearing strata are generally initially seen in the range of 8.0～40.0m under the bedrock,and the gypsum salt content is less in the upper part of the gypsum bearing strata（generally within the range of 8.0～20.0m under the boundary of gypsum bearing strata）,accounting for 5%～10%;The gypsum salt content in the lower part of the gypsum bearing stratum is high（generally 20.0m higher than the boundary of the gypsum bearing stratum）.In particular,thick glauberite rocks are distributed below 86.0m in Wuhou District and 53.4m in Gaoxin District.The proportion of gypsum and salt can reach 80%～85%.（3）This paper analyzes and summarizes the physical and mechanical properties of gypsum rock stratum（gypsum salt is mainly gypsum and glauberite）in Chengdu underground space.The main mechanical parameters include uniaxial compressive strength,uniaxial compressive strength,uniaxial compressive strength,internal cohesion,etc.（4）The results of indoor rock sample corrosion test show that the action area is different due to the different contents of gypsum and glauberite in the rock stratum.Generally,the paste content is small,slightly soluble in water,and the dissolution rate is slow.For rock samples containing glauberite,the content of glauberite is usually high,and glauberite is easily soluble in water,with fast dissolution rate and complete disintegration after immersion.When the concentration of SO₄^2-in groundwater is small,the greater the concentration,the stronger the dissolution of gypsum bearing strata.When the concentration of SO₄^2-increases to a certain extent,it can inhibit the dissolution of gypsum salt.After adding iron flakes to sodium sulfate solution,the dissolution rate of rock is faster than that of pure sodium sulfate solution.Under the same soaking time,the uniaxial compressive strength of rock decreases more.After adding iron flake and sludge to the sodium sulfate solution at the same time,the reduction of uniaxial compressive strength of rock is further increased compared with the test results of pure sodium sulfate solution and adding iron flake to the pure sodium sulfate solution,indicating that the sulfate contained in the sludge and the sulfuric acid produced by microbial metabolism will increase the SO₄^2-concentration rapidly in a short time and enhance the dissolution of gypsum containing rocks.（5）According to the corrosion test results of indoor concrete samples,the compressive strength of concrete samples decreased significantly after the chemical reaction between sodium sulfate solution and cement in concrete samples.The analysis of the test results after adding iron sheet into sodium sulfate solution also conforms to the law that the compressive strength of concrete decreases with the increase of sulfate ion concentration.When the concentration of sulfate radical increased to a certain extent,the effect of increasing the concentration on the chemical reaction was not obvious.By comparing the test results in pure sodium sulfate solution and the test results after adding iron sheet and sludge in sodium sulfate solution,the uniaxial compressive strength of concrete with only iron sheet in sodium sulfate solution is the highest at all concentrations.It shows that the presence of iron in sodium sulfate solution can inhibit the corrosion of concrete,and the sulfate contained in sludge and the sulfuric acid produced by microbial metabolism can promote the corrosion of concrete.