Study On Hydration Characteristics Of The Rocks Containing Gypsum In A Railway Tunnel

As a kind of evaporating sedimentary rock,the rocks containing gypsum are widely deposited in sea basins or lake basins,and developed in almost every geological age,especially in North China and Southwest China.The rocks containing gypsum have special engineering geological characteristics,which are prone to diseases during the early construction or later operation and maintenance of the project,and bring huge potential safety hazards to the project.The rocks containing gypsum include anhydrite,gypsum,hydrated anhydrite,gypsum（mud）limestone,gypsum dolomite,gypsum solution breccia,etc.When anhydrite meets water,it will undergo hydration reaction,which volume will expand and strength will decrease.Gypsum rock is a kind of soluble rock.It dissolves under the action of groundwater and produces SO₄^2-.When SO₄^2-accumulates to a certain concentration,groundwater will produce corrosiveness and erode the surrounding concrete structure.This paper relies on the special geological treatment project of a railway tunnel,and conducts a detailed study on the hydration characteristics of the rocks containing gypsum through field investigations,laboratory tests,and theoretical analysis.On the basis of previous studies,this paper focuses on the identification method of the rocks containing gypsum,the identification method of the degree of hydration of anhydrite rock,the evaluation method of the rate of hydration of anhydrite rock and the new method of modifying the classification parameters of surrounding rock of the tunnel in gypsum rock series,It provides necessary technical support for the treatment of the special geological disaster emergency engineering of a railway tunnel and necessary reference for the investigation and design of similar the rocks containing gypsum tunnel engineering.（1）Through the sampling and surrounding rock identification of the whole tunnel,the distribution characteristics of the surrounding rock of the tunnel have been found out,especially the distribution characteristics of the gypsum rock.The relationship between tunnel disease and surrounding rock have been analyzed.The rocks containing gypsum account for 18.1%of the total length of the tunnel,in which the rocks containing gypsum diseases account for 39%of the total length of the rocks containing gypsum.The development of the rocks containing gypsum is stratified,discontinuous and heterogeneous.（2）In the study of the expansion characteristics of anhydrite,the theoretical expansion amount of anhydrite is calculated,and the free expansion rate,expansion rate and expansion force of anhydrite are tested.The results show that the hydration and expansion process of anhydrite is very slow,and the test results and determination indexes of expansion rock in the current test specifications are not applicable to the determination of the expansion of anhydrite.（3）According to the identification test of anhydrite hydration degree,the TG curve weight loss ratio has a good linear relationship with mineral content under the condition of continuous temperature rise,which can be used for quantitative analysis and calculation of anhydrite hydration degree.The wave velocity test of anhydrite rock shows that the higher the degree of anhydrite rock hydration is,the lower the wave velocity is.According to the law of wave velocity changing with time,the time required for anhydrite complete hydration is predicted.The hydration depth of anhydrite increases logarithmically with time.With the increase of hydration time,the hydration depth becomes smaller and tends to 0.Increasing the contact area between anhydrite particles and water can accelerate the hydration expansion reaction of anhydrite,and SO₄^2-can accelerate the hydration reaction of anhydrite obviously.There is a good linear relationship between the hydration rate and the uniaxial compressive strength of anhydrite.The higher the hydration degree of anhydrite,the lower the uniaxial compressive strength.（4）The hydration degree of anhydrite rock exposed in the tunnel site of nanlvliang mountain tunnel is different,the residual expansion amount and time of anhydrite rock in different sections are different,and the influence on the tunnel structure is different.When considering the classification parameters of surrounding rock and tunnel design parameters,reasonable parameters should be provided according to the degree and rate of hydration of anhydrite in different sections,not according to the same lithology.