Experimental Study On Creep Characteristics Of Calcareous Sand Based On Particle Crushing Evolution Law

With the proposal of China's strategy of becoming a powerful marine country and the active promotion of the construction of islands and reefs in the South China Sea,the reclamation of islands and reefs in the South China Sea has become a strategic project in China,and calcareous sand is an important source material for the reclamation of islands and reefs and a foundation material for the structures of islands and reefs.However,calcareous sand is a kind of rock and soil material with special structure and mechanical properties formed by marine biological deposition,which has the characteristics of low strength and easy breakage.As the foundation of island reef structures,there is a risk of settlement.In order to ensure the long-term operation and durability of the South China Sea reef structures and to ensure the sustainable development of the strategic functions of the South China Sea reef projects,controlling the long-term deformation of calcareous sand foundation has become one of the key issues to ensure the long-term stability and sustainable development of South China Sea island reef projects.Therefore,it is of great practical and strategic significance to study the compressive creep characteristics of calcareous sand.In this paper,calcareous sand from an island reef in the South China Sea is taken as the research object.Through the research methods of literature investigation,laboratory tests and theoretical analysis,one-dimensional compressive creep test of calcareous sand has been carried out based on the study of its physical and chemical properties.The main research contents and conclusions are as follows:(1)The main physical properties and material composition of calcareous sand are analyzed.The calcareous sand used in the tests is unconsolidated loose body and belongs to poorly graded medium sand;The main types of particle composition are broken shells,coral limbs and reef limestone fragments.Through scanning electron microscope observation of its microstructure,it is found that there are many holes distributed unevenly.Mineral analysis shows that the main mineral components are aragonite and calcite,of which the equivalent CaCO3 content is 96.51%.(2)The deformation characteristics of calcareous sand are studied by onedimensional compressive creep test.Calcareous sand has strong deformation timeliness,and presents nonlinear attenuation creep characteristics in the compression process,which has gone through instantaneous deformation,accelerated deformation and gradually stabilized slow deformation stages respectively.The strain-time curve and stress-strain isochronous curve of calcareous sand can be well described mathematically by power function.On this basis,an empirical creep model of stressstrain-time for calcareous sand is established.(3)Based on Weibull distribution theory,the law of particle breakage evolution of calcareous sand after creep is studied.The crushing probability p of calcareous sand particles with a single particle size has nothing to do with the particle size but has a positive correlation with the stress state.The whole particle crushing state of the sample can be well described by a two-parameter Weibull distribution function.The smaller the Weibull distribution parameters a and b,the higher the overall crushing degree of the sample after creep,and the larger the final creep deformation.(4)The creep mechanism of calcareous sand is revealed on the basis of studying the creep fracture evolution law of calcareous sand and its microscopic morphology change characteristics.Crushing calcareous sand particles has certain timeliness,and creep deformation occurs due to the combined effect of inter-particle movement recombination effect and particle crushing effect.This test stress range(50kPa~ 800kPa),under the condition of low stress level,creep deformation is mainly the movement and recombination between particles,while under the condition of high stress level,creep deformation is mainly the sliding filling of pores with crushed and refined calcareous sand particles.