The Aging Properties And Tensile Strength Attenuation Law Of Compound Geomembrane In South-to-North Water Transfer Project

The South-to-North Water Transfer Project pass through the four major river basins of the Yellow River,the Yangtze River,the Haihe River and the Huaihe river,which involved more than ten provinces,municipalities and autonomous regions.This large scale project with wide span of water transmission lines,and generate very great economic benefit.It composed of the culvert,pre-stressed concrete cylinder pipes,reservoirs,tunnels,canals,siphons,river courses,channels,lakes,dams,pumping stations,aqueducts and other water conservancy facilities,the extremely huge investment project will definitely bring profound influence to our country’s development.Under the guidance of the Party Central Committee and the State Council related institutions and departments conducted an overall investigation and demonstration before the project can be implemented.After careful investigation and rigorous design,the detailed planning finally can be drawn up about fifty years later.Three destination water transfer lines are designed to be located in the west,middle and the east of China separately.Thus,the water diverted from the upper reach,the middle and lower reaches of the Yangtze River.The middle route of South-to-North Water Transfer Project which flow through Henan province is a very important part of the whole project.It can relieve the water shortage of the Yellow River plain,the Haihe River plain and the Huaihe river plain effectivelly,and also can optimize the water resources allocation.The project plays a significant influence to the development of Beijing,Tianjin and the Henan province.Canal is the integrant part of the water diversion project,the safety and reliability when operating and the economical in energy saving can affect the reliability,safety and maintainability of the project directly.Compound geomembrane is the one of Geosynthetics which made from macromoleclar polymer.The material is composited with plastic film and non-woven fabric,in which the function of the plastic film is to prevent seepage.Because of the characteristics of strong extensibility,strong ability to adapt deformation and low specific gravity,it is widely used in large scale water conservancy projects and other engineering fields.Because the compound geomembrane has advantage of simple construction,reliable quality,high work efficiency,it becomes an important measure to prevent canal seepage,and also be widely used in the South-to-North Water Transfer Project.Compound geomembrane as the major material to prevent the seepage of canal,it will be aged suffer from some environment facts,such as ultraviolet light,oxygen,temperature,humidity and so on.The process of material aging is slow with the fact of the interaction between temperature and aqueous solution which result in the variation of physical and chemical properties with the canal operating.In the aspect of material mechanical strength is the tensile strength of the material decreases gradually and the coefficient of permeability of the material increase with the influence of external environment,which will generate the variation of stress and strain of the compound geomembrane,and also affect the normal operating of the canal ultimately,shorten the service life of the canal,and will intimidate the safety of the whole water conveyance project eventually.Therefore,according to the practical application environment of project,taking research on the variation law of the tensile strength of the compound geomembrane has great significance to the safe operating of the whole project.Using concrete as the protective lining in canal structure of the middle route of South-to-North Water Transfer Project,the ultraviolet light irradiation won’t play a decisive role in the aging of the compound geomembrane,thus,the critical reason causing the compound geomembrane aging is the interaction between temperature and humidity.Therefore,this paper is based on the practical application environment of South-to-North Water Transfer Project,taking the attenuation law of tensile strength of compound geomembrane as the chief line of research,using the air oven aging tester,high and low temperature synthetic test chamber and tensile tester in labortary,doing some experimental study on the aging of compound geomembrane under different environmental conditions(thermal aging,hygrothermal aging and natural aging),And the aging properties and tensile strength attenuation law of the material under the three different environmental conditions are analyzed;Meanwhile,a model for predicting the aging life of compound geomembrane materials used in the South-to-North Water Transfer Project was established based on the failure criterion of compound geomembrane in engineering.Meanwhile in order to estimate the safety influence on the water project because of the aging of compound geomembrane material by using the numerical analysis method of quantitative analysis,Which will provide the technical support for effective operation of water transfer project and the benefits of water supply.The major research contents and results of this paper are as follows :1.According to the investigation of the application of compound geomembrane in the representative section of the middle route of South-to-North Water Transfer Project,combined with the local environmental factors,climatic factors,etc.,Considering the various factors which influencing the aging of compound geomembrane materials and the practical application environment of project,two factors,which are the temperature and the humidity,are used as the aging factors for the aging of compound geomembrane materials,and the aging test scheme of compound geomembrane material for the middle route of South-to-North Water Transfer Project is also be designed.2.Select the compound geomembrane which used in the representative section(Hebi section)of the middle route of South-to-North Water Transfer Project,two external factors,temperature and humidity,were selected as the aging factors to accelerate tests,three contrast tests in laboratory including the thermal aging test,the hygrothermal aging test and the natural aging test were carried out in order to analyze the aging properties and the variation of mechanical strength of compound geomembrane under different environment conditions.3.Under the three contrast tests in laboratory,the chief mechanical property indexes of the compound geomembrane has the trend of decline with the aging time increase,such as the tensile strength,elongation and the tear strength both in longitudinal direction and in transverse,In contrast,the modulus of elasticity remain unchanged with the aging time variation.But the coefficient of permeability does not change significantly with the increase of aging time,and it is kept at the same order of magnitude.And which also can meet the requirements of engineering seepage control.4.The decline rate of tensile strength,elongation and the tear strength both in longitudinal and transverse direction of the compound geomembrane has a remarkable variation with test temperature variation under the thermal aging accelerating test.In the three different temperatures,higer the temperature was,faster of the index rate declined,and faster the attenuation of mechanical properties was.On the contrary,lower the temperature was,slower of the index rate declined,and slower the attenuation of mechanical properties was.In addition,the decline rate of tensile strength in longitudinal direction is the most obvious index of all the mechanical strength of compound geomembrane under same temperature condition..5.The decline rate of tensile strength,elongation and the tear strength both in longitudinal and transverse direction of the compound geomembrane has a remarkable variation with the test temperature and the test humidity variation under the hygrothermal ageing accelerating experiment.In the three different temperatures and humidity,higher temperature and humidity were,faster of the index rate of declined.And faster the attenuation of mechanical properties was.On the contrary,lower temperature and humidity was,slower of the index rate declined and slower attenuation of mechanical properties was.6.With the same humidity condition in experiment,the higher the temperature is,the faster the tensile strength,elongation,tear strength of compound geomembrane material decreased,the aging rate which describe the aging performance of the compound geomembrane increment with the temperature increase when humidity stay same.In addition,with the same condition of temperatures and humidity,the decreasing rate of tensile strength in longitudinal direction is greater than transverse direction.While with same temperature condition,the higher the humidity is,the faster the tensile strength,elongation,tear strength of compound geomembrane material decreased,the aging rate which describe the aging performance of the compound geomembrane increment with the humidity increase when temperature stay same.7.Based on the results of accelerating tests in laboratory,the tensile strength attenuation law of compound geomembrane materials under the condition of thermal aging accelerating experiment and hygrothermal aging accelerating experiment was established by using mathematical analysis.In addition,the service life of the compound geomembrane was predicted in the practical application environment of the South-to-North Water Transfer Project,which based on the failure criterion of the tensile strength was decreased to 50% of the initial strength.8.In order to verify the attenuation law of the tensile strength under the hygrothermal aging condition with the practical engineering,the estimated value of the sample,which get from the test chamber of South-to-North Water Transfer Project under natural aging condition,and the measured value of the tensile strength in longitudinal direction of the sample,which get from 5 year test section of Xi-Xia Project,both are used to compared with the predicted value of the prediction model for tensile strength under hygrothermal aging condition,It can be concluded that the model prediction reliability can meet the engineering requirements,and also show that the model has the value of reference and application,if compound geomembrance material used in similar environmental conditions though with different specifications.9.In order to estimate the safety influence on the canal slope with the variation of parameters of material,which owing to the compound geomembrane material become aging by using the numerical analysis method of quantitative analysis.It can be concluded that the variation of the parameters of compound geomembrane(modulus of elasticity in longitudinal direction and in transverse direction;the aging degree of compound geomembrane)have weak effect on the distribution of displacement and stress of the slope;The maximum tensile stress of compound geomembrane in the canal is far less than the tensile strength of compound geomembrane material.Additionally,the slide of the slope is small when the canal operating normally.Then the canal slope keeps stable overall.