Failure Mechanism Of Concrete Dam Induced From Freezing And Thawing In Extremely Frigid Area

Dams, like other massive concrete structures exhibit significant cracking, which allow water intrusion into or through the structures. These cracks are the result of a variety of phenomena, including thermal gradient, ice loading, restrained concrete shrinkage and cycles of freezing and thawing. In extremely frigid area, seasonal temperatures vary very widely. There are many circles of freezing and thawing in a year. Deterioration of dam mechanical properties may result from the destructive combination of harsh weather conditions and ice loads. The study of the mechanism of dam failures induced from freezing and thawing is very meaningful for the rehabilitation of hydraulic structures. The main investigation contents are listed:1. Using the Fengman dam as a case study, based on the temperature records and geological condition of Fengman concrete dam, a numerical method with a transient thermal model and a transient stress model is developed to predict the freezing and thawing behavior of concrete dams. Appropriate heat transfer boundary conditions in the dam body are used for air and reservoir temperature. The research indicates that the tensile stresses due to freezing and thawing will exceed tensile strength of concrete on the down stream surface in December, January and February. So it may crack on the down stream surface.2. A simulation technology is applied to research on space-time distribution rules of the temperature and thermal stress of concrete gravity dam in harsh climate region during its construction and operation period. Based on the former temperature damage experiment in laboratory, principle that the damage changed with the temperature is analyzed. A thermal damage model is established. Damage finite element, which can be calculated by damage finite element program is applied to concrete dam to calculate damage field and stress field with damage character considered. The damage counter lines for50years during operation period are given. The results show that maximum damage and maximum thermal stress are at the same place of the dam down stream surface. So the dam down stream surface may crack easily.3. The seepage problem is analyzed on the basis of the observation information of the Fengman dam. The saturated zone and the wet surface are calculated. A micromechanics simulating model is established which considers the dam concrete body aggregated by two kinds of different attribute materials of aggregate concrete and cement mortar. In the micromechanics numerical model, these two parts are simulated by contact elements to analyze the feature of thermal transfer and crack formation problems under temperature load and influence of seepage. Stress distribution of concrete with cracks is calculated. Concrete cracking process induced from temperature is simulated. The research indicates that the stress of the concrete in saturated zone is far larger than it in unsaturated zone. The concrete in saturated zone will crack easily.4. The parameters of Duncan-chang E-B constitutive model of the Pushihe Dam is determined using genetic algorithm on the basis of triaxiality compress experiments of rockfill materials. The main influencing factors of concrete face deformation are analyzed. The deformation characteristic under these factors is studied. A3-dimension finite element model of Pushihe CFRD is developed and Duncan-chang E-B constitutive model is selected to denote the stress-strain character of rockfill. The deformation characteristics of rockfill in construction are analyzed. The deformation and stress distribution of the concrete face which vary with temperature are simulated. The results show that the concrete face bears tensile stress in February and compressive stress in August. The concrete face may crack in February.