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Micro-and macro-mechanical behavior of crushable calcareous sand in South China Sea

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Abstract

To develop offshore facilities in South China Sea, it is vital to understand the characteristics of particle shape and breakage properties of the calcareous sand. As a result, micro- and macro-mechanical behaviors of crushable calcareous sand sampled from South China Sea are addressed in this study. Firstly, calcareous sand particles are scanned by electron microscope and analyzed by an image processing software (i.e., ImageJ). Based on the processed image, two shape parameters, namely circularity and solidity, are defined and quantified. Secondly, drained triaxial tests on calcareous sand at various confining stresses are carried out, so as to investigate the effects of particle breakage on deformation, shear strength and energy dissipation of the calcareous sand. Results show that the shape of the calcareous sand with relatively large particle size (diameter greater than 2 mm) and relatively small particle size (diameter less than 0.5 mm) tend to be circular and the particle surface is relatively smooth. Comparatively, shape of calcareous sand with moderate-size diameter (grain diameter ranged between 0.5 and 2 mm) is more irregular, and sand particles have more surface edges. Particle breakage is identified to occur in triaxial tests, leading to a better graded sand packing. With the increasing initial confining pressure, the degree of particle breakage and the energy dissipation due to the breakage both increase. In the meantime, dilation of the sand is partially suppressed by the breakage. Under two conditions only considering friction dissipation and considering both friction and volume dissipation, the dissipated energy resulting from breakage in the triaxial tests with relatively high (600 kPa) initial confining stresses could account for 25% and 18% of the total plastic energy input, respectively.

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... e energy consumed by particle breakage is a function of B r or crushing strength, and the influence of particle breakage on strength of calcareous sand first increases and then decreases with increasing confining pressure and finally tends to be stable. e effect of dilatancy on its strength is significantly stronger than that of particle breakage at low confining pressure and gradually weaker than that of particle breakage with increasing confining pressure [19,[56][57][58]. Some studies also show that the particle shape plays a key role in the shear contraction of sands, and particle breakage has a significant influence on the shear response of calcareous sand during dilatancy. ...
... e distribution law of particle size range is similar under different loading modes, but the distribution of new particles produced during particle breakage is different [70]. In addition, existing studies primarily focus on the size range of fine-grained (calcareous sand) in coral reef sediments and rarely consider the related research on the mechanical properties of calcareous gravel particles [19,71]. ...
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In order to better understand the particle breakage mechanics and characterization methods of calcareous sand, the particle breakage characteristics of calcareous sand in one-dimensional compression tests, isotropic compression tests, triaxial shear tests, and ring shear tests are reviewed and analyzed. The results show that the mechanism of shear crushing is essentially different from compression crushing. Compared with one-dimensional compression, it is easier to break calcareous sand under triaxial shear. In the shearing process, the particle breakage of calcareous sand will not continue indefinitely. The gradation tends to be stable and controlled by confining pressure and shear strain. The characteristic particle size measurement method for particle breakage is simple and has limitations. The description method using the particle size distribution curve is more comprehensive, but it is impossible to compare the crushing degree of particle with different particle sizes, and a suitable measurement method needs to be proposed.
... It is of great significance to study its mechanical properties. Recent research on the calcareous sand in the South China Sea found that its particle breakage is far less than that in previous research under the same load condition [14]. This may be related to its generation environment, hydraulic fill method, and so on [5,15]. ...
... This shows that the particle breakage of this type of calcareous sand is insignificant and can be ignored. This conclusion is similar to that of the aforementioned research [14]. Therefore, this paper focuses on exploring the influence of the fractal distribution of particle size on the critical state characteristics of calcareous sand, without considering the impact of particle breakage. ...
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To study the influence of the fractal distribution of particle size on the critical state characteristics of calcareous sand, a type of calcareous sand from a certain reef of the South China Sea was used in this study. For comparison, standard quartz sand was also used. A series of drained shear tests on the two sands were then conducted to investigate their critical state characteristics. It was demonstrated that the fractal dimension is suitable for characterizing the particle size distribution (PSD) of calcareous sand with different fine sand content. The critical state equation of sand proposed by Li and Wang (1998) is suitable for fitting the critical state line of calcareous sand. In the plane of deviatoric stress versus the effective confining pressure (q–p′ plane) and the plane of void ratio versus (p′/pa)α, the critical state lines of calcareous sand are always above those of quartz sand. The critical state lines of calcareous sand with different fractal dimensions in the q–p′ plane are unique. However, in the e–(p′/pa)α plane, the critical state lines appear to rotate anticlockwise as the fractal dimension increases. In addition, there is an “intersection” in the e–(p′/pa)α plane. Considering the influence of the fractal distribution of particle size, an expression for the critical state line of calcareous sand in the e–(p′/pa)α plane was proposed. The related constitutive model was also revised, where a complete set of model parameters suitable for modeling calcareous sand was provided.
... Because of the differences in coral species and sedimentary environments, the shear characteristics of calcareous soils from different regions are not precisely consistent. Calcareous soil from the South China Sea (Hu 2008;Zhang et al. 2008;Mo 2015;Weng 2017;Wang et al. 2018), Port Wakefield in southern Australia (Desrosiers and Silva 2002), Tonbak (Hasanlourad, Salehzadeh, and Shahnazari 2008) and Hormuz Island (Dehnavi et al. 2010) in the Persian Gulf all change from dilatancy to contractive (Table 3). Although the phase transition point values of different regions are similar, they are not consistent. ...
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Calcareous soil is widely distributed throughout tropical areas. Due to its unique sedimentary environment and soil behaviors, severe engineering problems and disasters have occurred in many regions. To understand the differences in the mechanical properties of calcareous soils in different regions, material composition, physical and mechanical properties of calcareous soils, including specific gravity and void ratio, compression properties, shear properties, and strength properties, were analyzed. The relationship between the sedimentary environment, material composition, and mechanical properties of calcareous soil was examined based upon the results. Results show that the calcareous soil in the South China Sea has a high calcium carbonate content, large particle size, high compressibility, and low bearing capacity, while sand from the Kish Island in the Persian Gulf and Agami sand from Egypt have low compressibility and high bearing capacity. In the South China Sea, Calcareous soil is more difficult to crush than those from the Hormuz Island and Bushehr Port in the Persian Gulf, Dog’s Bay in Ireland, and Quiou in France. Overall, calcareous soils from various locations exhibit significant differences in properties based upon their physical traits. It is suggested that the influence of sedimentary environment and geological conditions should be considered in engineering constructions.
... ere have been numerous studies on the engineering characteristics of calcareous sand at the macroscopic level. However, the related research on the mechanical behavior of calcareous sand at the mesoscopic level is quite limited [2][3][4][5][6]. Although the mesostructural parameters including particle geometry and internal porosity of calcareous sand have been studied by many researchers, the relationship between these parameters and the macromechanical behavior is unclear. ...
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The study of the mesostructure of soil under loading is the basis for understanding its macromechanical properties and for establishing its constitutive model. In this study, a series of shear tests was performed on dry calcareous sand under constant normal stress by a modified direct shear apparatus. Digital images of the sample at different shear stages are obtained. The mesostructural parameters of the sample are then extracted and analyzed using an image analysis technique. The results show that the shear-band is located at the junction of the upper and lower shear boxes with a thickness of 0.79-1.59 mm. During shearing, the position of the maximum shear strain incremently shifted to the junctions between the two shear boxes. The azimuths of the particles prior to the test distribute symmetrically on both sides of 90°. After the test, the azimuths of the particles are mainly obtuse angles (150-180°) and the long axis of the particles generally points in the opposite direction from the shear-band. The sand particles undergo four stages: random arrangement during initial sample preparation, compaction under normal stress, particle rotation during shearing, and ordered alignment after shearing. The test results help to reveal the movement mechanism of calcareous sand at the mesoscopic level during the direct shear process. © 2020 Jianhua Shen et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
... 173 account for a huge percentage in samples to function as the granular skeleton for bearing load. Another 174 fact is that the grains was relatively multi-angular(Wang 2018). Except that the largest with the initial mass percentage of grains in the sample.178New ...
Article
In previous experimental studies, the detailed amount of particle breakage under impact loading cannot be obtained due to the coexistence of fragments and unbroken grains in multi-sized sands. In this study, a series of impact loading tests were carried out to three kinds of coral sands to explore the detailed particle breakage in multi-sized sand samples. Before impact loading tests, coral sands in different size groups were dyed in different colors, then mixed as multi-sized sand samples. After impact loading, particle image segmentation method was used to particle images to obtain the mass ratio of different color grains. Results show that the actual amount of particle breakage was found to be larger than the amount of relative particle breakage in most size ranges, and the “absolute particle breakage” wi satisfied logarithmic relationships with the input work. The breakage index Bw of multi-sized sand samples, excluding the interference effect of the coexistence of fragments and unbroken grains, was a little larger than the breakage index Bg and satisfied logarithmic relationships with the input work. This study could be useful in analyzing the particle breakage model and the constitutive model of granular materials.
... 173 account for a huge percentage in samples to function as the granular skeleton for bearing load. Another 174 fact is that the grains was relatively multi-angular(Wang 2018). Except that the largest with the initial mass percentage of grains in the sample.178New ...
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针对混合粒径钙质砂中不同粒径颗粒绝对破碎量无法获得和现有破碎率难以考虑破碎重叠掩盖破碎量这两个问题开展研究。设计了粗砂、中砂、细砂分别占主体的3种级配钙质砂试样,进行侧限压缩试验。对不同粒径区间钙质砂分别染成不同颜色,拍照获取各粒径区间钙质砂破碎信息;采用Image J软件进行彩色图像颗粒分割、二值化处理、统计各颜色颗粒面积,换算得各颜色颗粒破碎后含量;并提出考虑破碎重叠掩盖的试样累积破碎率指标Ba。结果表明,随压力增大及颗粒分布集中,试样的重叠掩盖破碎量增大。混合粒径钙质砂中的中间粒径(0.25-1mm)颗粒易于破碎,各粒径颗粒破坏模式以颗粒边角破碎为主;累积破碎率Ba值较相对破碎率较大,与垂向压力对数值间满足线性关系。本研究为颗粒破碎研究提供了新的思路。This paper aims at acquiring the absolute breakage amount of particles in a certain size range and considering the hidden breakage amount of particle breakage rate in a sample. Three kinds of calcareous sands, rich in coarse, medium and fine sands respectively, are used in the confined compression test. Particles in different size are dyed in different colors, and pictures of particle in all certain particle size range are segmented and binarization by Image J. Mass percentages of calcareous sand in different color are acquired by calculating the area ratio of calcareous sand in different color. Accumulated particle breakage Ba, which considered the absolute breakage amount of particles in a certain size range, is proposed. Test results showed that, with the increase in pressure or the concentration of particle distribution, overlapping breakage of samples is increased. In calcareous sand samples consisted of particles in different sizes, the sand in intermediate particle sizes (0.25-1mm) is breakable, and edge broken is the main breakage way of particles in samples. The accumulated particle breakage Ba is larger than the relative breakage Br, and has a linear relationship with the logarithmic value of vertical pressure. This paper provides a new method for the research of particle breakage.
... account for a huge percentage in samples to function as the granular skeleton for bearing 179 load. Another fact is that the grains was relatively multi-angular (Wang, 2018). Except that the largest d New breakage indexes of non-uniformly graded sand samples 184 ...
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The coexist of broken and unbroken grains in each size range disturbed the judgement of actual amount of particle breakage. In this study, a series of one-dimensional compression tests were carried out to three kinds of coral sands and a silica sand to clarify the detailed amount of particle breakage for non-uniformly graded sands. Before the compression tests, both coral and silica sand assemblies were divided into different grain-size groups and dyed in different colors, then mixed as non-uniformly graded packings. After the compression, grains of different color in each size range were discerned quantitatively by implementing particle images segmentation on images of grains. Results shown that the extent of particle breakage was found to be larger than the change in relative mass percentage in most size ranges; and the new “absolute particle breakage” for each size range satisfied linear relationships with the logarithmic value of compression pressure. Compared with silica sand, coral sand has weaker abrasion under high pressure due to the strong interlock among grains. New breakage indexes of sand samples, based on detailed particle breakage, were proposed. The detailed particle breakage could be useful for proposing breakage-dependent constitutive model of crushable granular soils.
... At present, a great deal of research work has been carried out on its macro and micro structural characteristics [4][5][6], static properties [7,8], cyclic load [9,10] and mechanical behavior under explosion impact load [11,12], and fruitful research results have been obtained. According to the relationship between the change rule of mechanical properties of coral sand and the degree of particle breakage found in shear or compression tests of coral sand by Sun Jizhu [13], Zhang Jiaming [14], Wang Yiqun [15], He Jianqiao and others [16], it can be seen that the change of particle-size distribution (PSD) caused by particle breakage under external force is the main reason why the mechanical properties of coral sand are different from those of continental sand [17,18]. In view of this, many scholars [19][20][21] have also proposed different quantitative indicators of fragmentation to explain the change law of macro-mechanical properties of sand through PSD changes before and after the test. ...
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The time-dependent deformation property of backfill coral sand is of great important to the long-term stability of engineer facilities bulit on reefs and reclaimed land. In order to investigate the long-term deformation behavior, one-dimensional compression creep tests under different constant stresses were carried out for coral sand taken from a reef in the South China Sea by WG type high-pressure consolidation instrument. The test results show that under the action of constant stress, coral sand has a strong deformation timeliness and shows remarkable nonlinear attenuation creep characteristics. The creep of coral sand has obvious stages and has gone through three stages of instantaneous deformation, accelerated deformation and slow deformation phase tending to stability. The relationship of strain-time can be fitted with power function in mathematic. The particle breakage state of any single particle size group of coral sand after creep can be well described by using the two-parameter Weibull distribution function, Weibull parameters a and b have a good exponential relationship with stress, and have a negative linear relation with quantitative index Br of particle breakage, and have a negatively correlated with final total strain. Under the action of low stress level, the main cause of creep deformation is the movement and recombination of particles. At low stress level, the movement and recombination of particles are the main reason of creep deformation, while at high stress level, the slippage and filling pores of broken coral sand particles are the main reason of creep deformation.
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Yang, J.; Wang, J.; Dong, L., and Fan, P., 2022. Axial deformation behavior of precompressed coral sand under repeated impacts. Journal of Coastal Research, 38(3), 592602. Coconut Creek (Florida), ISSN 0749-0208. To investigate the subsidence behavior of backfill foundation under the disturbance of repeated landing of planes, a series of one-dimensional tests under repeated impacts were conducted on precompressed crushed coral sand with a variety of grading and relative densities. For each sample, the impacts (amplitude 600 kPa, frequency 5 Hz) were loaded three times in cluster form (lasting 6 s, one hour apart), while the static stress (200 kPa) was maintained continuously for 4 h. The axial deformations of the samples were recorded, and a comparative study was conducted by referencing the results of silica sand samples under the same conditions and coral sand samples under static loadings. The test results indicate that compared with silica sand, coral sand has a much larger total deformation, which is mainly influenced by the relative density, gradation, and order of impacts. By analyzing the results, it can be concluded that the deformation of coral sand has a greater plastic component and decreases faster than that of silica sand under repeated disturbances, and the total deformation and proportion of irreversible deformation decrease with increasing relative density. The influence of initial grading on the particle breakage index is more significant than the relative density. Improving the grading of coral sand can reduce the total deformation and minimize the proportion of unrecoverable deformation.
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Calcareous sand is a special geomaterial susceptible to breakage. In this study, calcareous gravelly sand (CGS) was subjected to multiple consolidated-drained triaxial shear tests under different relative densities (Dr) and effective confining pressures (σ3′). The purpose was to investigate the effects of particle breakage on the shear characteristics of calcareous sand. According to the results, the strain-softening and dilatancy characteristics of CGS gradually weakened with increasing σ3′ and decreasing Dr. Within σ3′ ≤ 400 kPa, the peak-state friction angle (φp) of calcareous sand collected from the island-reefs in the South China Sea had a range of 37.8–62.9°. Moreover, φp decreased with increasing σ3′ and decreasing Dr. A physical model was built to predict the secant modulus (E50) of CGS based on its Dr and σ3′. Particles 1–2 mm in size had the largest breakage extent, and mainly broke into particles of 0.5–1 mm and 0.075–0.25 mm. The applicability of inverse proportional function for describing the relationship between plastic work and relative breakage ratio for CGS was validated. Neither Dr nor σ3′ greatly affected their relationship. Increasing particle breakage weakened the strain-softening and dilatancy characteristics of CGS and reduced its φp but enhanced its deformation resistance. The findings of this study provide a theoretical basis for infrastructure construction on the island-reefs in the South China Sea.
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Calcareous sand is widely used as backfill material for land reclamation, but due to its special mechanical characteristics such as easily broken particles, calcareous sand foundation is facing more complex engineering problems. In this study, drained and undrained shear tests were carried out on calcareous sand samples from the South China Sea using K0 consolidation and isotropic consolidation conditions. It was found that the particle breakage of calcareous sand has obvious dependence on the initial consolidation stress path and drainage condition, thus showing different shear strength behavior. The particle breakage under drained shear is greater than that under undrained shear, and the particle breakage under isotropic consolidated shear is greater than that under K0 consolidated shear. The larger the particle breakage, the smaller the dilatancy and internal friction angle of calcareous sand. The dilatancy and internal friction angle of K0 consolidated specimen are larger than those of isotropic consolidated specimen. It is suggested that the actual stress path of calcareous sand should be considered in the engineering.
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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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Understanding the time-dependent deformation behavior of backfill coral sand is important to the long-term stability of engineering facilities built on reefs and reclaimed land. A series of one-dimensional compression tests (with no lateral strain) were carried out on crushed coral sand with a variety of grading and relative densities (50%, 70%, and 90%) sampled from the South China Sea. Axial pressure was applied in stepped loading form: 100, 200, 400, 800, and 1600 kPa. Each level loading was applied for 3 days and then completely unloaded until the deformation was stable, after which the next loading level was applied. The test results indicate: (i) the deformation of coral sand is much larger than silica sand and involves a larger proportion of time-dependent and plastic deformation; (ii) the total deformation of coral sand and proportion of irreversible deformation decreases as the relative density increases; (iii) coral sands of better grading tend to deform less in total and have larger proportions of elastic and time-dependent deformation; and (iv) the grading of the coral sand changes during the deformation process due to particle breakage. Based on the test results, the relationships between particle breakage and pressure, relative density, and grading, as well as the grain-scale mechanism of the deformation, are discussed.
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