Theory And Hypergravity Experiments On Damage To Gravity Dams By Underwater Explosion Shock Waves And Bubble Pulsations

Dams are prime targets of wars and terrorist attacks owning to their significant political and economic importance,as well as the potential for severe catastrophic disasters in the event of failure.During the Russo-Ukrainian War,Gulf War,and World War II,multiple dams encountered underwater explosions(UNDEXs),causing failure to the dam structure and downstream flooding.China has the largest number of large dams worldwide,but their safety design normally only considers conventional loads,ignoring extreme loads such as UNDEXs,which may lead to anti-explosion safety hazards for most large dams in China.To scientifically mitigate damage from explosions and develop reasonable wartime alert plans,it is necessary to undertake systematic research into the damage characteristics and mechanisms of dams caused by UNDEXs.Our research group conducted a series of centrifugal model tests of UNDEXs in the early stage,which verified that the centrifuge test can simultaneously satisfy Mach and Froude similitudes such that the physical processes of UNDEX shock waves and bubble pulsations can be effectively reproduced.The centrifugal model tests were then successfully applied to investigate steel plates against UNDEXs.Based on these previous studies by our research group,the current study focuses on the subject of concrete gravity dams subjected to UNDEXs,utilizing multiple research approaches including centrifugal model tests,whole-process numerical simulations,and theoretical analyses.Firstly,the research systematically explores the damage characteristics and mechanisms of scaled models of gravity dams caused by UNDEX shock waves and bubble pulsations.Then,to speculate the results of the gravity dam prototype,the rate-dependent similarity theory for scaling the blast responses and failures of gravity dams is established.Based on these investigations and according to the damage effects of near-field large-equivalent UNDEXs on the prototype of a high concrete gravity dam with various reservoir water levels,anti-explosion and protection suggestions for gravity dams and wartime alert measures are proposed.The main research work and innovations are summarized as follows:(1)Centrifugal model tests of concrete gravity dams subjected to UNDEXs are conducted,which obtains valuable experimental data including pressures of UNDEX shock waves and bubble pulsations as well as dynamic responses and fracture patterns of gravity dams.Based on the hypergravity experiments,a fully coupled numerical model of the water-explosive-airgravity dam is established using the coupled Lagrangian-Eulerian method,forming a reliable numerical analysis method to simulate the whole process of gravity dams subjected to UNDEXs.Constitutive models of concrete materials under blast loading are developed,including an improved Kong-Fang elastic-plastic constitutive model in hydrocode with consideration of the free water effect,and a new cap elastic-plastic constitutive model that considers shear,tensile,and volumetric compaction damage mechanisms.The substantial differences between the two kinds of constitutive models,in terms of the constitutive theory,are specified,as well as their respective applicable scenarios.(2)Through hypergravity experiments and whole-process numerical simulations,the damage characteristics and mechanisms of scaled models of gravity dams by UNDEX shock waves and bubble pulsations are revealed.The research finds that the damage and failure of the gravity dam by shock waves are primarily governed by the overall bending-induced tensile failure mechanism.As a result,the gravity dam ultimately exhibits failure modes of breaches with a certain depth in the upper part,which is consistent with the failure patterns of typical dams hit by UNDEXs in historical wars.Moreover,this study presents the first systematic investigation into the secondary attack on the gravity dam by bubble pulsations on the basis of the shock wave response results.A novel finding from the mechanism perspective is that the horizontal local impact responses of the gravity dam caused by shock waves and bubble pulsations can be uniformly described by their peak pressures,whereas the vertical structural bending responses of the gravity dam caused by shock waves and bubble pulsations can be uniformly depicted by their impulses.The discovery of the failure mechanisms and response mechanisms mentioned above provides the foundation for the establishment of the ratedependent similarity theory later.(3)A rate-dependent similarity theory for scaling the blast responses and failures of concrete gravity dams is established,which solves the challenge of dissimilar strain-rate effects between scaled models and prototypes of gravity dams,and builds a bridge for the mutual transformation of the results between them.Firstly,the applicability and limitations of the dimensional analysis-based similarity theory in the field of gravity dams against UNDEXs are thoroughly evaluated,with influences of the strain-rate effect being clarified.Then,a dimensionless response number Dni reflecting the overall bending-induced tensile dynamic response mechanism of the gravity dam is theoretically derived.Based on Dni,a correction method of the rate-dependent similarity theory is innovatively proposed.It is found that the dimensional analysis-based similarity theory faces the problem of dissimilar strain-rate effects between scaled models and prototypes of gravity dams,resulting in significant differences in tensile stress responses and tensile failures between them.The proposed correction method of the rate-dependent similarity theory can effectively address this problem,making the damage and failure results of geometrically similar gravity dam models with a scaling factor of β =1～1/100,basically the same.The proposed rate-dependent similarity theory provides a theoretical basis for the validity of scaled model tests for gravity dams subjected to UNDEXs.(4)Based on the above research work and according to the damage effects of near-field large-equivalent UNDEXs on the prototype of a high concrete gravity dam(dam height 147 m)with various reservoir water levels,anti-explosion and protection suggestions for gravity dams and wartime alert measures are proposed.The dam head especially the position around the cross-section change is the vulnerable part and is suggested to be locally strengthened using composite materials such as steel fiber-reinforced concrete.The UNDEX loads should be considered for dam safety design,by doing so,the connection between the gravity dam and the foundation should be reinforced,and the safety factors of overall anti-sliding and overall antioverturning of gravity dams should be enhanced.Moreover,it is suggested to lower the reservoir water level,to about 60% of the normal water level,in advance during foreseeable wars,which can effectively reduce the damage degree of the gravity dam.