Article

Connected inner pore analysis of calcareous sands using SEM

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Abstract

Calcareous sand is a special kind of geotechnical sand which consists of much carbonate. Based on the remains of the original marine organisms, the calcareous sand grains have irregular shapes and contain inner pores. The inner pore structure has great effect on the mechanical properties of the calcareous sand. To investigate the distribution of inner pores, the calcareous sand samples from the South China Sea are divided into six groups according to grain size, and a series of scanning electron microscopy (SEM) tests are carried out on different shape grains from each group. MATLAB is used to process the images got from the SEM tests with proper threshold values. The effects of grain size and grain shape on inner pores of calcareous sands are analyzed. The results reveal that, for the sand grains with size less than 1 mm, the porosity increases with the growth of grain size and there is no obvious difference between the porosity of grains with different shapes. However, for the sand grains with size greater than 1 mm, the porosity decreases with the growth of grain size and the strip shape grains have the largest porosity while the slice shape grains have the smallest one. © 2017, Editorial Office of Chinese Journal of Geotechnical Engineering. All right reserved.

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... Meanwhile, studies on the particle shape of coral sand are reported minimally. Owing to its special marine biological origin, coral sand, which is often accompanied with cementation [47,48], features high porosity ratio [49,50], easy breakage [51,52], and other physical and mechanical properties that are significantly different from those of terrigenous sediments. However, it has not been confirmed by experimental data whether the above research results are applicable to coral sand. ...
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珊瑚砂是岛礁陆域吹填唯一的材料,因其特殊的海洋生物成因和孔隙结构特征,珊瑚砂颗粒在常规工程应力水平下就会发生破碎。通过对取自南海某吹填岛礁的珊瑚砂开展室内三轴固结排水剪切试验,研究珊瑚砂强度、变形参数和颗粒破碎程度随相对密实度、围压的演变规律,并与其他研究成果中的南海珊瑚砂强度参数进行对比分析。研究结果表明:珊瑚砂应变软化及剪胀特征随有效围压的增大、密实度的减小而逐渐减弱;在常规应力范围内,得到珊瑚砂峰值摩擦角和临界状态摩擦角的取值范围分别为33o~58o和28o~47o,且均随有效围压的增大而减小;建立了珊瑚砂割线模量E50与相对密实度、有效围压间的关系式;珊瑚砂峰值摩擦角与修正相对破碎指数Br*间的关系可用指数为负数的幂函数关系式拟合,且当颗粒破碎程度较高时,峰值摩擦角随修正相对破碎指数的增大而减小的趋势变缓;珊瑚砂修正相对破碎指数随塑性功的增大近似以双曲线的形式增长,密实度对两者间的关系影响不大。该研究成果可为南海岛礁基础设施的安全设计提供参考。
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Coral sand is the only material for island land reclamation. Due to its special marine biogenesis and porosity structure, coral sand particles are easily crushed at normal level of engineering stress. In this study, drained triaxial shear tests were carried out on coral sand retrieved from a reclamation reef in the South China Sea to study the evolution law of the strength, deformation and particle breakage properties of coral sand with different levels of density and confining pressure. The comparative analysis of shear strength index is made between current study and previous investigations. The results show that strain softening and dilatation tendency of coral sand gradually weaken with an increase in confining pressure and decrease in compactness. Within the normal confining pressure range, the values of the peak and critical state frictional angles of coral sand were 33º-58º and 28º-47º, respectively, both of which decreased with an increase in the confining pressure. The relationship among the secant modulus E50, relative density and effective confining pressure for coral sand was established. The relationship between the peak friction angle of coral sand and the modified relative breakage index (Br*) can be fitted by a power function equation with a negative index. When the extent of particle breakage is large, the decreasing trend of peak friction angle with an increase in the modified relative breakage index slows down. The correlation between the modified relative breakage index and plastic work for coral sand could be simulated using hyperbolic curve, and it is hardly affected by density. The research results can provide useful reference and technical support for island reclamation and infrastructure construction in the South China Sea.
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