| The marine science and technology and China's Maritime Power and Strategy have been developed rapidly in recent years.Meanwhile,the construction of islands and reefs in the South China Sea has also attracted increasing attenion.The calcareous sand is widely used as buliding and foundation materials in the construction process of ocean engineering.The physical and mechanical properties of calcareous sands are different from that of terrigenous sands due to the special genesis and sedimentary environment.The conventional engineering theory and technology which is derived from terrigenous sands are diffcult to be utilized in ocean engineering.Therefore,it is necessary to further investigate the physical and mechanical properties of calcareous sands so as to provide theoretical support for the framework of ocean engineering construction theory and technology.There are still some rate-dependent engineering problems during the construction process,such as military anti strike,wave load,construction and long-term settlement.Hence,this study focuses on the rate-dependent macro and micro mechanical behavior of calcareous sand.The relationships between rate-dependent behavior and particle breakage of calcareous sand are analyzed by a series of experimental and numerical tests.The main contents are as follows:(1)The basic physical properties of undisturbed calcareous sands,including particle size distribution,specific gravity and relative density,are fisrtly analyzed.In addition,the characteristics of particle shape and internal pores of calcareous sand particles are also quantificationally characterized.Accodring to the results,the gravity of calcareous sands is2.79,which is larger than that of quartz sands.In addition,with the decrease of particle sizes,the shape of the sand particles and pores is more regular.Moreover,the isotropy of pores increases with the decreasing particle sizes.(2)According to the size effect,strain rate effect and cushion effect of single calcareous sand particles,the effect of particle sizes,loading rates and coordination number on particlce crushing strength and fragmentation mode are determined by using experimental and nuemrical tests.Based on the results of laboratory and numerical tests,with the incresing particle sizes,loading rate and coordination number,both the variability and breakage strength of particles increase,and the fragmenation mode is also significantly affected.Furthremore,the random equation incorprating the size effect,the strain rate effect and the cushion effect is established by using inverse function method,which can characterize the breakage strength of calcareuos sand particles in various operating conditions.(3)According to the particle crushing strength and the fragmentation mode of calcareous sand particles,the proposed random equation of particle crushing strength are introduced into numerical model,which resluting in a close relation between experimental and numerical samples at the particle scale.Furthermore,a novel particle crushing simulation method is proposed based on the discrete element method.The proposed was fully verified by comparisons of numerical and experimental results.Furthermore,the proposed approach is further used to simulate the marco-and micro-scaled mechincal properties of calcareous sands.Once the stress state of targeted particles fulfills the particle breakage criterion,the targeted particle is cut by a series of virtual cutting faces,which can effectively avoid the non-conservation of mass or volume and the produce of external stress.Besides,it also can describe the complexity of particle crushing mode.(4)A series of triaxial compression tests were carried out on calcareous sands.The rate-dependent shear behavior of calcareous sands under different relative densities and confining pressures is preliminarily analyzed.The resutls show that the shear strength increases with the loading rates,and the dilatation is more obvious under a higher loading rate.Additionlly,it can also observed that the particle breakage of samples is more obvious under a igher loading rate.In addition,the strength equation of single calcareous sand particle is introduced into the discrete element method to establish the numerical samples.It can be concluded that the rate depencency of calcareous sand samples is shown to results from the coupling effect of particle breakage and arragment.By comparing the rate dependency of the shear behavior of breakable and unbreakable samples,it can be found that the rate dependency of breakable samples is more obvious than that of unbreakable samples.Besides,the effect of particle breakage on rate dependency is quantitatively analyzed according to the experimental and numerical results.(5)The rate-dependent compression behavior of calcareous sands is explored by using the constant strain rate consolidation tests and the Split Hopkinson Pressure Bar(SHPB)tests.The typical compression behavior of calcareous sands under different strain rates is analyzed.Furthermore,the characteristics of compression curves and the rate-dependent behavior of yield strength of calcareous sands increases with the increasing loading rates.The particle breakage extent in samples decreases with the increasing loading rates in the low and middle loading rate intervals.However,the particle breakage extent in samples under a relative high loading rate slightly increases with the increasing loading rate.(6)The creep behavior of calcareous sands is investigated based on a series of one-dimensinoal creep tests.The delayed breakage model of calcareous sand particles is established according to the long-term strength equation of rock particles.In addition,the rate process theory is introduced into the discrete element method to simulate the creep behavior of unbreakable sands.The results show that the creep behavior of calcareous sands is nonlinear,which can be divided into three typical phases,namely initial near-linear phase,damping phase and steady phase.Moreover,the particle breakage mainly occurs in the damping phase. |
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