| With the implementation of the strategic development plan of China’s"Land Silk Road Economic Belt and 21st Century Maritime Silk Road Economic Belt",the South China Sea region will become an important node of China’s Maritime Silk Road.Most of the islands and reefs are far away from the mainland,and the engineering construction materials are limited.At present,under the guidance of the concept of"green engineering and ecological islands and reefs",China is using calcareous sand as the main construction material to fill the islands and reefs in the South China Sea,which can effectively shorten the construction period and save land resources.The structure and characteristics of calcareous sand are obviously different from those of terrestrial sand,and its main component is calcium carbonate,and its complex composition also makes it porous,irregular in form,easily broken and easily cemented.The South China Sea is rich in oil and gas resources,coastal tourism resources,marine energy resources and port shipping resources.In addition to fulfilling the needs of military defense,it also assumes the demands of civil functions of safe navigation,service fisheries and tourism production.And the construction of islands and reefs in the South China Sea,as the basis for the development of each function,is becoming increasingly important in its strategic position.China is located between the Pacific seismic zone as well as the Mediterranean-Himalayan seismic zone,and the whole South China Sea is located in the area of plate subduction zone,where the crustal movement is very active,bringing challenges to the long-term stability of the islands and reefs.The calcareous sand,as the engineering foundation,is subjected to high stress levels,and the special properties of the engineering site,such as high compressibility and fracture deformation caused by the seismic action,bring challenges to the long-term stability of the islands and reefs.The engineering properties of calcareous sand are significantly different from those of terrestrial sand,and conventional engineering construction theories and techniques are inadequate.Therefore,it is imperative to conduct research on the mechanical properties of calcareous sand,taking into account the background of island construction in China,the needs of military and civilian functional engineering construction,and the understanding of the natural environment in the sea.In this study,the fractal properties,strength properties,shear deformation properties and changes of critical state of calcareous sand are investigated systematically by combining physical tests and discrete element numerical simulations.The main research contents and results of the thesis are as follows.1.The crushing evolution,fractal characteristics and creep characteristics of calcareous sand were analyzed by using physical test methods and fractal theory.The crushing characteristics of calcareous sand were analyzed by conducting a series of ring shear tests.The stress-strain relationships and deformation characteristics of calcareous sand under different shear displacements and different gradation conditions were studied,and the gradation evolution,relative crushing rate and fractal dimension of calcareous sand were discussed in the context of fractal theory of granular materials.The creep properties of calcareous sand were analyzed by one-dimensional consolidation tests with different stress levels on specimens of different gradations,and the fractal correlation was discussed.The physical tests combined with fractal theory show that:the ultimate particle crushing state exists in calcareous sand,when the shear displacement reaches a certain degree,the residual strain and axial displacement maintain a relatively stable state,the gradation curve basically remains unchanged,the fractal curve becomes more and more linear with the increase of displacement and gradually tends to the ultimate fractal dimension;the normal stress level has a positive promotion effect on the ultimate particle crushing,but in the test The normal stress level has a positive effect on the ultimate particle crushing,but its influence on the ultimate particle crushing decreases after the normal stress reaches a certain degree in the test range,and it cannot have a continuous promotion effect on the crushing.The change of stress-strain and deformation characteristics of specimens with different characteristic grain sizes are consistent.The larger the average particle size of the gradation group,the larger the fractal dimension after reaching the ultimate crushing,and the better the fractal characteristics,while the fractal dimension values obtained by the gradation groups with different characteristic particle sizes are basically the same,and the fractal cannot increase infinitely;the creep deformation of calcareous sand is related to the applied stress level,and the relative creep deformation decreases with the increase of stress,and the time-dependent nature of crushing during creep decreases with the increase of stress level.2.The simulation method of numerical simulation of crushed particles based on the discrete element method is improved to consider the mass conservation and volume conservation of the sub-particle self-organization mode,and the volume loss problem is discussed in depth.The crushing criterion of maximum point load considering local stress concentration is adopted and correlated with 14-ball Apollo-filled crushing self-organization mode and volume expansion conservation.The effect of volume effect on the fine view crushing condition of the specimen is considered,and then the response of its mechanical and deformation properties,gradation change and critical state are investigated,and the effect of interparticle action on macroscopic properties is analyzed from the fine view point.A high-pressure triaxial test comparison is applied to quantify the volume-induced verification of the rationality of the volume expansion method.It is shown that:the existence of the equilibrium point of the volume loss effect is related to the stress level to which the specimen is subjected,resulting in the difference of the bulk strain expansion and contraction phenomenon,and the stress development process is affected,resulting in the stress-strain difference;the variation of the particle number and the difference of the fragmentation rate indicate that the gradation deviation originates from the two-way effect of the volume loss on the particle fragmentation;the volume loss has no effect on the critical state stress ratio,but has an obvious effect on the stress variation during the shear process The volume loss has no effect on the critical state stress ratio,but has a significant effect on the stress change during shear,resulting in the decrease of critical state pore ratio increasing with the increase of the surrounding pressure;the rationality of sub-particle volume conservation is analyzed from the perspective of microscopic particle motion,displacement development and force chain evolution.3.The crushing of particles affects the behavior of calcareous sand,which is poorly reproduced by traditional physical test means.The crushing of calcareous sand is investigated by the discrete element numerical simulation method,which integrates the macroscopic properties and microscopic response of calcareous sand materials.The FRM volume expansion method was used to establish the discrete element model for the triaxial test,and the crushing and non-crushing numerical simulation tests were carried out on the specimens of different gradation groups with characteristic grain size to determine the influencing factors of crushing and their microscopic response,and the critical state of the crushed specimens was investigated.The results show that the shearing and swelling characteristics of each graded specimen are governed by the stress level,and the shearing and swelling characteristics decrease as the stress level increases;the differences in the macroscopic mechanical properties of the specimens of different graded groups originate from the degree of particle fragmentation and the different particle contact effects;the characteristic state stresses of each specimen with different characteristic particle sizes are linear in the p-q plane;as the content of fine particles decreases,the amount of fragmentation increases,and the critical state pore As the content of fine particles decreases,the amount of fragmentation increases,and the critical state pore ratio becomes larger in the downward shift and deflection;at low envelope pressure,particle fragmentation mainly occurs at the early stage of shear,while at high envelope pressure,particle fragmentation accompanies the whole shear process.4.Combining the fractal theory and numerical simulation methods,the crushing index considering the influence of initial gradation is proposed,and the influence of particle crushing on the critical state and the evolution law of the critical state are focused on.The initial grading and particle crushing problems are analyzed in detail by combining the physical test and numerical simulation test results,and the effects of particle grading and crushing on the evolution of critical state are studied based on the test data of triaxial simulated calcareous sand.The study shows that particle crushing has no effect on the critical state in the p-q plane;crushing state,particle size,density and grading conditions have obvious effects on the critical state in the plane.However,the critical state after crushing can still be linearly fitted;applying the grading state index I_G,the numerical test results are analyzed,and the concept of state index growth rate is proposed to distinguish the crushing amount from the crushing degree.A relative crushing index is proposed,and the absolute crushing and relative crushing of crushing are combined with the initial grading and surrounding pressure conditions,and a three-dimensional analysis is carried out;the immediate critical state line for the critical state point after crushing is analyzed,and a quadratic no-crushing critical state line that considers the correlation between grading and crushing state is proposed,which is a unified expression of the critical state line at the over crushing point,and the slope of this critical state line is always kept as The slope of the critical state line is always kept as a constant value,and the intercept decreases with the increase of the grading state index. |
