Experimental Study On The Permeability Characteristics Of Calcareous Sand In South China Sea

Calcareous sand is widespread in tropical marine environment regions (including the South China Sea). Composed of coral, seaweed, shell and so on, it is a kind of special geomaterial whose principal chemical composition is calcium carbonate. Because of special physical, chemical and biological impact, calcareous sand has unique hydro-physical properties that are very different from terrigenous sand. The fundamental reasons are that calcareous sand has the characteristics of abundant pore, particles easily crushing, shape irregular and strong structure.In this study, calcareous sand collected from a coral island in the South China Sea has been initially analyzed by the way of grading analysis, energy spectrum analysis and scanning electron microscope analysis. Through the conventional constant head permeability tests, the effect of particle size, porosity and grading index (uniformity coefficient Cu and curvature coefficient Cc) on the permeability of the calcareous sand has firstly been investigated. In light of the unique shape of calcareous sand particle,ψ (the ratio between sphericity 0 and circularity X) was introduced to quantitatively describe the particle shape based on the SEM images. In order to explore the effect of particle shape on the permeability of the calcareous sand, an investigation has been performed by comparing with Fujian standard sand and glass bead with the same particle size distribution and relative compaction. Then, to model the complicate loading conditions in ocean environment like wind, wave and earthquake, a self-designed permeameter apparatus has been developed to investigate fluid flow behavior within porous media from Darcy to Post-Darcy regime. At the same time, two kinds of porous media seepage equations have been used to analyze the experimental results. Finally, the CFD-DEM method has been introduced to simulate the seepage process in granular materials, and the results have been compared with experimental results under the same conditions.The results of the conventional constant head permeability tests indicate that the permeability of the calcareous sand increases with the growth of particle size and square relation is found between permeability coefficient and average particle diameter. Similarly, the permeability increases with the growth of specimen’s porosity under the same particle size distribution, but the relationship between permeability coefficient and porosity is linear. The permeability of the calcareous sand increases with the growth of uniformity coefficients and curvature coefficients, and the relationships between permeability coefficient and two grading index are linear. Compared to the other two kinds of granular materials, the particle shape of the calcareous sand has significantly structural and nonuniform properties, which can significantly reduce the permeability of it. The results of the non-Darcy flow tests indicate that the seepage characteristics of calcareous sand are largely influenced by particles’various shape and abundant pore. The results of CFD-DEM flow-structural-interaction numerical method have a good match with experimental results, which indicates that CFD-DEM is an effective method of simulating seepage process in granular materials. What’s more, this numerical method could be used for simulating the seepage of complex shape of particles such as calcareous sand in the future.