Numerical Modeling Of Issues Related To The Coupling Of Elastoplastic Damage In Concrete And Seepage

The height of dams in China has been increasing steadily with the country's increasing scale of hydraulic and hydropower engineering constructions.A high tendency for seepage damage and failure is possible in concrete dams under high seepage pressure,especially at the heel of high dams.Therefore,numerical analysis of the coupling of seepage,stress,and damage is necessary.However,we have yet to obtain a sufficiently accurate understanding of the constitutive relationships in concrete.Furthermore,research on the mechanisms of seepage damage and concrete failure is still incomplete.Hence,several subject matters in this field remain worthy of research.This study used numerical analyses and focused on key issues in the numerical modeling of the coupling between elastoplastic damage and seepage in concrete.The main issues addressed in this study are as follows:(1)Several conflicting reports on the plastic yield function in existing concrete elastoplastic damage models are currently presented,and a systematic description has yet to be obtained.The flaws and interaction between the tension and compression damage variables under multi-axial conditions remain under the description of the “unilateral effect” of concrete materials.First,a concrete elastoplastic reinforcement model based on the unified strength theory is established based on continuum damage mechanics and irreversible thermodynamics by applying a modified explicit constitutive integration algorithm.Second,the tension and compression damage variables are defined,and a damage evolution rule is determined based on the energy release rates of the compressive and tensile elastoplastic damages.In addition,the influence factors of reverse loading are introduced to reflect the unilateral effect of concrete materials and the interaction of the tension and compression damage variables under multi-axial conditions.We establish a double-scalar elastoplastic damage model that can more accurately reflect the unilateral effect of concrete materials.The model validity is preliminarily verified by the numerical simulations of the uniaxial and multi-axial tests on concrete.Meanwhile,the model's ability to perform a nonlinear structural analysis is verified through a numerical simulation of the three-point bending test on simply supported beam with type I crack and a gravity dam.The results lay the foundation for the numerical analysis of the concrete elastoplastic damage under high seepage pressure.(2)Presently available seepage-stress coupling theories fail to explain the seepage anisotropy after the occurrence of damage and cracks and other issues,such as the fracture direction and crack opening width.This study establishes coupled equations that determine the coupling between the permeability coefficient of concrete materials with stress and damage and the effect of cracks under different stages of stress.In the case where damage has yet to occur,the coupling relationships between the permeability coefficient of the concrete materials and stress are directly established based on the existing research results.We then establish a concrete seepage-elastoplastic stress coupling model based on this relationship and verify its validity using an illustrated example.In the case where damage has already occurred,we amend the results obtained from a previous research and directly establish the coupling relationships between the permeability coefficient of concrete and damage variables,using the damage variables as a bridge.Furthermore,a sensitivity coefficient is introduced to modulate the coupling between seepage and damage.Meanwhile,in the cases where macro-cracks have appeared,we introduce the concept of damage-fracture thresholds,where an area with damage values greater than the threshold is defined as a crack,and the cracking direction is defined as the main direction of the damage tensor.Similarly,the cracking width is defined as the damage-related elastoplastic deformation perpendicular to the direction of the damage(crack)based on the ideas of crack band models.Finally,we establish a coupling relationship between the permeability coefficient and the cracking width after the appearance of cracks based on the cubic law.As a result,we are able to improve the theories on the coupling between seepage and stress.(3)We address the difficulties in implementing the concrete seepage-elastoplastic damage coupling models and their flaws.First,based on the elastoplastic damage model established in this study,we construct a seepage-elastoplastic damage coupling model that can describe all the processes involved in the seepage-induced damage and fracture in concrete by using the built-in seepage analysis methods in the ABAQUS program and the fully coupled seepage-stress program that we have developed.This model is used to model the hydraulic fracture damage in the Longtan Dam,which is a roller-compacted concrete gravity dam,under different stages of stress.The numerical analysis results show that the concrete gravity dam suffers damage under high seepage pressure.Moreover,high seepage pressures have significant effects on the field variables within the local regions of damage at the dam heel,including seepage fields,stress fields,plastic deformations,and damage.The damage at the dam heel gradually evolves into macro-cracks with the continuous rise of the water level behind the dam,thereby causing the pore pressure from the hydraulic head of the dam to advance along these cracks and enhancing the effects of hydraulic fracture,eventually leading to a further expansion in the damaged and cracked areas.