In order to fully develop the South China Sea, a large number of reclamation projects using calcareous sand have been carried out in this area recently. A deep understanding of the physical and mechanical properties of calcareous sand is of critical importance. Therefore, the calcareous sand near a certain reef of the South China Sea is used in this study to investigate the effect of three-dimensional (3-D) particle morphology and gradation on the compressibility characteristics of calcareous sand. This paper proposes a 3-D mesoscope observation method to obtain the average 3-D angularity parameter S_d and 3-D aspect ratio T_d of calcareous sand with different particle sizes. It is found that the morphology of coarse particles (diameter: 5 ~ 1 mm) is significantly multi-angular, while the morphologies of middle particles (1 ~ 0.25 mm) are mostly dendritic and schistic. Compared to the 3-D S_d of quartz sand, the calcareous sand’s particle morphology is much more irregular and multi-angular, which makes it easy for the calcareous sand to form large pores and, thus, be more compressible. In order to systematically study the effect of gradation on the calcareous sand’s compressibility characteristics, a number of compression tests on calcareous sand with different gradations are taken. The influential mechanism is then discussed by analyzing the test results from a mesoscopic viewpoint. It is found that changing the coarse fraction content is the most efficient way to reduce the compressibility of the calcareous sand. That is because of the coarse fraction’s high angularity, which makes the skeleton-bearing capacity of the calcareous sand sensitive to the change of coarse fraction content. An empirical formula is proposed to evaluate the compressibility of the calcareous sand with different coarse fraction contents.

Traditional plasticity models of granular soils are usually established on different loading and plastic potential surfaces. This paper presents a nonassociated bounding-surface model by incorporating a fractional-order plastic-flow rule that is obtained by performing fractional-order derivatives on the bounding surface. It is found that the flow direction no longer necessarily coincides with the loading direction, even if the same potential and loading surfaces are used. The flexible ability of the model is then validated by simulating a series of experimental results of different granular soils, including ballast, rockfill, and sand, under both drained and undrained loading conditions. It is observed that the proposed model can well capture the key features (e.g., stress dilation, strain hardening/softening, and liquefaction) of various granular soils.

Construction of the reefs in the South China Sea is a significant foundation to the secure stability and economic development of China. The construction of an airport runway is necessary for this realization. The calcareous sand is the main primary material in the runway construction. A certain type of calcareous sand near a certain reef of the South China Sea was studied in this paper. To investigate this specific calcareous sand, quartz sand was used as a reference for comparison. Microscopic 3-D imaging, compression and triaxial tests were conducted to test the micro, squeezing and shear properties. The effect mechanism of gradation on the calcareous sand’s compressibility and shear characteristics are discussed from a mesoscopic viewpoint using 3-D morphology. Calcareous sand particles are multiangular and flatter in comparison with quartz sand. The larger the particle sizes are, the more different the two sands’ morphologies are. The compressibility of calcareous sand is greater, and the effect of the coarse fraction (5–1 mm) content in the gradation plays the most significant role in this feature. When the coarse particles’ content is less than 25% and the mass ratio of the middle and fine particles (M) is constant, there is the worst coarse fraction content causing the calcareous sand to be most likely compressed. The worst coarse fraction content decreases with the increase in M, and an empirical formula is proposed. When the gradation, relative density and confining pressure are the same, the peak shear stress and strain of calcareous sand are all at a high level. The effect of confining pressure is manifested in calcareous sand. The shear strength and dilation of calcareous sand are also most affected by the medium coarse fraction (5–0.25 mm) content.

Particle morphology is a fundamental inherent property that substantially affects the macroscopic behavior of granular materials. The division and separation of particle morphology at different scale levels contributes to the further multiscale morphology related orthogonal researches. In this context, the high-resolution X-ray micro-computed tomographic (X-CT) scanning and spherical harmonic (SH) analysis were combined to complete the precise and digitized reconstruction of sand particles. The 3D sphericity, roundness and roughness were introduced to define the particle morphology at three scale levels (the general shape, local angularity and surface textures). Two typical sand particles, Calcareous sand (CS) and Fujian sand (FS), were tested in this study. The results showed that the irregularity, angularity and roughness of CS is higher than that of FS, and the multiscale morphological features of the two types of natural sand were given and compared digitally.

Particle morphology is a basic inherent feature of granular materials that plays an important role on their macroscopic behaviors. A multiscale description of particle morphology is generally developed from three levels: large (general form), medium (local roundness), and small (surface roughness). The morphological features of two typical natural sands (Fujian sand and calcareous sand) were investigated in the present study based on spherical harmonic (SH) analysis. Three-dimensional (3D) printing was employed to develop the mass manufacture of grains with independent control of morphology and material properties. Full-scale morphologies of the sphere, Fujian sand (FS) as well as calcareous sand (CS), and single-scale morphologies of calcareous sand at large-(CS24), medium-(CS58), and small-(CS915) levels were printed. A series of triaxial tests were performed on these printed grains to explore their strength-dilation behaviors. The results suggested obvious strain-softening and volume-dilation behaviors were observed at a confining pressure varying from 20 to 100 kPa. A significant enhancement of both shear strength and bulk dilation was indicated from irregularly shaped particles compared with the volume-equivalent spheres. Also, the morphologically derived enhancement of shear strength is weakened from the general form to surface roughness while the enhancement of dilation is the strongest at the medium-scale level of local roundness, indicating different morphological origins between strength and dilatancy. In addition, Bolton’s dilatancy coefficient is a fixed value of 0.452 in this paper, independent of particle shape.