Calcareous Sand Particles Research Articles

Comparative study on cyclic behavior of marine calcareous sand and terrigenous siliceous sand for transportation infrastructure applications

Calcareous sand has been used as an emerging unbound granular filler to replace siliceous sand in the construction of marine transportation infrastructure. However, the critical feasibility of and differences between using calcareous sand as a filler instead of siliceous sand have yet to be suitably assessed. In light of this, this paper presented a comparative study on the cyclic behavior of calcareous sand and siliceous sand through a series of high-cycle drained triaxial tests. To facilitate comparison, the same grain-size distribution and relative density were used for calcareous sand and siliceous sand to investigate any underlying cyclic behavior divergence caused by their respective distinctive particle shapes and breakage. Owing to the irregular and rough properties of calcareous sand particles, results showed that it exhibited stronger shear strength and resistance to permanent deformation than siliceous sand. However, during the early and middle stages of cyclic loading, the accumulation rate of permanent strain in calcareous sand was found to exceed that of siliceous sand due to its violent particle breakage habit, whereas a more severe dilatancy of calcareous sand induced by irregular particle shape was observed during the subsequent loading stage. Moreover, particle shape and particle breakage were found to have different competitive contributions to the increase of resilient modulus during different loading stages. In this work, the allowable cyclic strength ratio capable of better determining the suitable load level for a practical design was suggested. Furthermore, to capture the particle breakage extent under long-term cyclic loads, a refined hyperbolic equation was projected to model the relationship between relative particle breakage index and plastic work.

Laws of volumetric deformation and particle breaking of calcareous sand under cyclic loading

In order to study the change law of volume deformation and particle breakage of calcareous sand under cyclic loading, this paper carries out both conventional triaxial consolidated drained shear tests (the conventional tests) and triaxial cyclic loading tests on calcareous sand. The test results under these two stress paths were compared and showed that, compared with the stress path of the conventional tests, the stress path of cyclic loading aggravates the softening features of the calcareous sand. In each cycle, there is a hysteresis loop in the stress-strain curve, whose area is proportional to the deviator stress level at the initial stage of unloading. The stress path of cyclic loading suppresses the dilation trend of the sample under low cell pressure; the contraction amount is positively correlated with the deviator stress in the unloading phase. The two stress paths bring similar particle breaking law. Under the same cell pressure, the shear moduli of calcareous sand particles decrease with the growing cycle number. The manuscript clarified the law of deformation and particle breakage of calcareous sand under cyclic loading, which provided a theoretical basis for practical engineering.

Experimental and numerical study on the fragmentation mechanism of a single calcareous sand particle under normal compression

The rate-dependent mechanical properties of a specific geomaterial play a crucial role in engineering design and application. However, there have been very few studies involving rate-dependent mechanical properties of calcareous sand particles. This present investigation aims to study the rate-dependent breakage behavior of calcareous sand particles experimentally and numerically. Experimental tests were conducted under various loading rates. Moreover, the discrete element method was utilized in particle crushing tests to assess the breakage sub-processes. The experimental results reveal that the particle crushing strengths conform to the Weibull distribution. As the loading rates increase, the fragmentation mode changes from primary splitting and successive breaking to severe disintegration, corresponding to the three types of axial force-displacement curves, namely, the quasi-hardening, the slight quasi-softening, and the obvious quasi-softening. The simulation results show that the fractures initiate in the vicinity of contact point between the particle and loading walls, eventually penetrating through the particle along the vertical section of the sample. A higher loading rate may lead to a greater extent of particle breakage. The findings presented in this study may advance the understanding of the rate-dependent mechanical properties of calcareous sands.

DEM investigations of the effects of intermediate principal stress ratio and particle breakage on the critical state behaviors of granular soils

Three dimensional discrete element (DEM) analyses were performed in Particle Flow Code (PFC3D) to study the effects of intermediate principal stress ratio and particle breakage on the critical state behaviors of granular materials. The variation of intermediate principal stress ratio is achieved by increasing intermediate and major principal stresses at different ratios. The breakage of calcareous sand particles is simulated in DEM with clumped particles using the octahedral shear stress crushing criterion. The macroscopic behaviors of crushable and uncrushable particles during shearing and at the critical state are compared. It is found that intermediate principal stress ratio and particle breakage affect the dilation and peak strength but have different effects on the critical stress ratios of the assemblies. The evolutions of micromechanical parameters such as redundancy factor, fabric and force anisotropies are examined to explain the difference between the behaviors of crushable and uncrushable assemblies under different stress paths.

Revisiting the liquefaction resistance of calcareous sand using X-ray CT

The microstructure of calcareous sand sampled from the South China Sea Islands was investigated using X-ray microtomography (μCT). μCT images directly evidenced the amount of inner pores in the calcareous sand. The image analysis presented a map of the inner pore distribution and volume ratio compared with the volume of solid particles, showing that the inner pore distribution was affected by the particle size. Furthermore, owing to the special structure of calcareous sand particles, crushed particles were observed during cyclic loading, inducing obvious strain and releasing air from the inner pore of the particle, which subsequently desaturated the specimen and increased the liquefaction resistance. This evidence of inner pores in calcareous sand provides another way to examine the undrained response by considering the crushability of the calcareous sand.

Experimental and numerical investigation on size effect on crushing behaviors of single calcareous sand particles

Single particle crushing tests were conducted to investigate the size effect on the crushing behaviors of single calcareous sand particles experimentally and numerically. The particle crushing strengths and fragmentation modes of the calcareous sand particles with six different size groups (5–17 mm) were investigated. Moreover, the numerical simulation based on discrete element method was performed to assess the progressive fracture behavior. The particle crushing strength data are found to be satisfy the Weibull’s statistics model well. The Weibull modulus decreases with the increasing particle size. Meanwhile, the scaling law between the characteristic crushing strength and the particle size is smaller than the theoretical predication by the Weibull model. As the particle size increases, the fragmentation mode changes from simultaneous splitting and successive splitting to primary splitting. Furthermore, the numerical results reveal that the fractures occur firstly in the contact area between the loading walls and the top and bottom ends of the particles, eventually forming the penetrative fracture zones along the direction of loading.

Influence of Particle Shape on the Density and Compressive Performance of Calcareous Sand

Particle shape influence the bearing capacity of calcareous sand foundations in island reef engineering. An improved Computer Image Analysis method that combined with mathematical statistics was proposed to categorize and sort different shapes of calcareous sand particles within the size range of 5–10 mm. Three single-shape calcareous sand specimens (granular shape, dendritic shape, long strip shape) were obtained. A new self-developed vibration device was used measuring the maximum density of calcareous, the shape and content as the variables in the orthogonal test. On this basis, the lateral compression tests were carried out to assess the influence of particle shape on the density, compressive performance of calcareous sand. Test results showed that better discriminability was achieved by taking mean flatness (EL/B) as the quantification parameter of the particle shapes. The average flatness EL/B of specimens is approximately normally distributed in the statistical results. The shape and content of sand particles had an obvious influence on the density of calcareous sand. The compressive properties of calcareous sands with different particle shapes have obvious differences. After loaded, the particle size distribution of specimen improved, the average flatness EL/B of the granular-shaped specimens increased, while that of the dendritic-shaped and the long-strip-shaped specimens decreased, the average flatness EL/B of the specimens tended to be uniform. The relative fracture rate Br and the load value Pmax satisfied the exponential function fitting relationship. Therefore, it can be argued that particle shape should be taken into consideration when optimizing the foundation design in island reef engineering to achieve better compressive performance of the foundation.

Particle Strength of Calcareous Sand in Nansha Islands, South China Sea

Abstract Single particle strength is the main factor that influences particle breakage. In order to explore particle strength characteristics and the relationship between the strength and size of calcareous sand from the South China Sea, single particle crushing tests were conducted using self-made test equipment. Hundreds of particles (0.72–3.12 mm) divided into three groups of different sizes were used to perform the test. The results show that the strength characteristics of the particles were similar to those of a typical brittle material and were lower than that of quartz sand. Under flat unconfined compression, two types of uniaxial compressive force–displacement curves were defined according to the tests, related to the shape and structure of the calcareous sand particles. Statistical data analysis shows that the calcareous sand particles’ strength is in good agreement with the Weibull distribution. The particle crushing strength decreases as the particle diameter increases. Different shapes of calcareous sand particles show different strength characteristics. In addition, the mineral composition, microstructure, distribution of inner voids and fracture are related to the particle strength of calcareous sand.

Gradation affects basic mechanical characteristics of Chinese calcareous sand as airport subgrade of reefs

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.

Hydro-mechanical analysis of calcareous sand with a new shape-dependent fluid-particle drag model integrated into CFD-DEM coupling program

In this study, a new shape-dependent fluid-particle drag model for irregularly shaped particles is integrated into the fluid dynamic program CFD-DEM and used to analyse the behaviour of calcareous sand particles in liquids. The new drag model had been correlated by pervious settling experiments of calcareous sand particles in a static Newtonian Liquid. The new drag model defines the particle-fluid drag coefficient as a function of both the fluid regime and particle shape characteristics which can evaluate how the particle shape deviates from a regular sphere. This study is a follow-up work of the authors' previous experimental research, focusing on the numerical modelling of sand particle-fluid interaction. To validate the integration of the new drag model in program CFD-DEM, the code has been used to reproduce the calcareous sand particle settling and seepage experiments. The comparison with simulations using the existing drag models without considering particle shape effect demonstrates that, by means of the new drag model, a significant improvement in the capability of CFD-DEM program to predict the terminal falling velocity of irregularly shaped particles is obtained. With this implementation, CFD-DEM program is used to simulate seepage flows within calcareous sand sample in which particles are significantly different from regular spheres, as is usually the case of environmental and geological fluid-particle flows.

New simple correlation formula for the drag coefficient of calcareous sand particles of highly irregular shape

This paper presents a study of the terminal fall velocity and drag coefficient of natural particles of highly irregular shapes in a wide range of Reynolds number (0.01–3700). The study is based on experimental measurements of the terminal fall velocities of calcareous sand particles settling in liquids of different densities and viscosities. A total of 133 calcareous sand particles of highly irregular shapes were measured and 521 experimental data points were obtained. To characterize the particle shapes, a shape descriptor which can take into account both sphericity and inverse circularity, were introduced and it can be measured via image particle analysis techniques. This shape descriptor is particularly suitable for non-spherical and highly irregular particles, such as calcareous sand particles. The drag coefficient has been correlated to the particle Reynolds number and the shape descriptor, and a new model for predicting drag coefficient has been correlated; the correlation has the functional form of a power law. Its simple explicit form makes it easy to use in computer models and other applications. The performance of seven widely-used correlation laws has been examined; and the fitting of experimental measurements with the new correlation model shows that the new correlation model predicts the drag coefficients and the terminal fall velocities of irregularly-shaped particles, i.e., calcareous sand particles, more accurately than other shape-dependent drag laws. In addition, the influence of presence of neighboring particles on drag coefficient and terminal velocity was investigated under conditions where the test particle is surrounded by other equal-sized and similar-shaped particles arranged in different configurations. Furthermore, through proposing a new parameter which can consider the effects of the number of neighboring particle, particle separation distance and relative position of the particles, the determination of drag coefficient ratio for the settling of multiple particle was correlated based on the results of our settling tests. These findings provide a basis for further improvement for the general polydisperse particle system and analysis of flow through assemblage of numerous particles.

Experimental microscopic study of inner pores of calcareous sand

Microscopic experiments were performed on the inner pores of calcareous sand particles to ascertain the special mechanical properties of calcareous sand. A femtosecond laser was used to cut the calcareous sand particles to obtain cross-sections of particle. The micrographs of the inner pores of the sand particles were taken with a three-dimensional video microscopic visualiser. The parameters of the pores were analysed through the micrograph processing with MATLAB. The results show that calcareous sand has relatively low surface porosity. The surface porosities of particles with pore diameter >2 mm and those in the range of 1–2 mm are 0·66 and 1·07% respectively. Based upon the number of pores, the distribution characteristics of the pores show that there are fewer macropores but more micropores in the sand. In terms of the area of pores, micropores occupy a smaller proportion than macropores. It was found that 65% of the pores have area <1 μm2, and 55% or more of them are equiaxed pores.

Static and cyclic behavior of North Coast calcareous sand in Egypt

This research studies the behavior of calcareous sand located in the North Coast of Egypt, Dabaa area, under static and cyclic loading. The study is performed through a series of monotonic and cyclic undrained triaxial tests conducted on two relative densities and different effective confining pressures. The cyclic tests were carried out at different cyclic stress ratios. Failure under cyclic loading was found to be governed by the gradual development of excess pore water pressure until liquefaction is reached, rather than cumulative development of axial strain (cyclic mobility). The cyclic strength of the tested sand is compared with other calcareous and siliceous sands reported in the literature. The test results indicate that loose Dabaa calcareous sand has higher cyclic strength compared to other siliceous sands, probably due to the existence of different shapes of calcareous sand particles within the tested soil. Relationship between cyclic resistance ratio, effective confining pressure, and relative density was developed for the tested sand.