Experimental Study On Creep Characteristics Of Uniformly Graded Calcareous Sand Based On Fractal Theory

With the deepening and continuous consolidation of the construction of China’s strategic projects on South China Sea islands and reefs,the calcareous sand in the South China Sea has become an source material for artificial islands and reefs and natural foundation materials in the construction of islands and reefs.However,the special biogenesis and the complex marine environment cause the calcareous sand has special engineering mechanical properties that is different from land-sourced materials,such as complex,diverse and irregular particle shapes,low strength,easy breakage,and easy cementation.Under the long-term action of external factors such as the additional stress of structures on island reef,and the internal catalysis of the special engineering properties of calcareous sand,the calcareous sand foundation is at risk of continuous creep and other disasters,which will affect the safety and long-term operation of island reef and infrastructure.Understanding the long-term creep characteristics of calcareous sand is a necessary condition for reliable estimation of the final creep deformation of island reef calcareous sand foundation,and controlling the long-term creep deformation of the calcareous sand foundation is one of the key issues to ensure the long-term safety,stability and sustainable development of the construction and operation of the South China Sea island reef project.Therefore,in-depth research on the long-term creep characteristics and creep mechanism of calcareous sand has important practical and strategic significance.This paper takes the calcareous sand from Hainan Island and reefs as the research object,through the one-dimensional compression creep test with different creep times,in-depth exploration of the creep characteristics of the calcareous sand.Using macroscopic laboratory tests and microstructure observation methods and introducing fractal theory to conduct macro-micro correlation analysis,further explore the medium and long-term creep characteristics of calcareous sand.The main research contents and conclusions are as follows:(1)Through 2 days,7 days,14 days,28 days and 56 days one-dimensional compression creep tests on calcareous sand,it was found that the calcareous sand had significant deformation timeliness under long-term load,and exhibits typical nonlinear attenuation creep characteristics.The creep process includes a rapid deformation stage and a stable attenuated creep deformation stage.The strain-time curve can be described by a power function.(2)Based on the results of creep tests with different creep times and the particle size distribution curve data before and after creep,On the basis of summarizing the development law of particle crushing in the creep process,the form of fractal crushing and the influencing factors,it was believed that the creep of calcareous sand is mainly caused by the development of frictional slip between particles to particle crushing.Different graded calcareous sands tend to have a self-similar fractal distribution under different stress levels.The speed of development is related to the amount of broken particles,which can be described by gradually increasing fractal dimensions.The isochronous curves of the relative particle crushing rate and stress level of different graded calcareous sands have good self-similar characteristics.The crushing degree of the calcareous sands of the small particle size group has a greater influence with the creep time.(3)Both the time effect of the particle breakage rate and the creep deformation in the creep process of calcareous sand show attenuation characteristics,Through scanning electron microscope analysis and Image-Pro Plus software for image processing,it is found that the size and distribution of pores on the surface of the calcareous sand sample change significantly during the creep process,and the particle shape parameters are not sensitive to the time response.Through the macro-micro cross-scale correlation and multi-scale analysis,it can be seen that the long-term creep characteristics of calcareous sand are mainly related to the particle crushing mode at each deformation stage in the creep process.