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Abstract
Particle breakage at low stress level leads to a special stress-strain relation for calcareous sand. By assuming that the strengths of particles follow a certain probability, damage parameter is obtained, and then a damage bounding surface model is constructed. The relationship among particle breakage, shear expansion and confining pressure are discussed, and theory and experiment results are in good agreement.
... Coral sand is a type of geotechnical material that is formed by combining the skeleton remains of various marine calcareous organisms, such as algae, shells, corals, etc. Thus, the main chemical component of coral sand is high-content calcium carbonate and magnesium carbonate, and it is mainly composed of calcite and aragonite [2].Coral sands are mainly distributed on the surface layer of the coral reef flat, and the lagoon in the tropical marine environments such as the South China Sea Islands, Western Pacific coral reefs, Waters of the west of India, Waters of North America, and many other regions, as shown in Fig. 1. However, because of the particular formation process of coral sand, that it usually underwent little disturbance and long-distance transportation like river sand, the coral sand usually retained the characteristics of irregular shape, porous-and breakable micro-structures [3]. ...
... Establishing a local coordinate system at the center of particle to obtain the local coordinates of each particle vertexes, and storing these coordinate data in a linked list Gr-link. (2) Generating origin tetrahedron grids (seen in Fig. 8b) in the initial auxiliary cube, and the grid data is stored in another linked list Gr-ori. (3) According to the Delaunay method [44,45], all vertex coordinates of the particle were plugged into the Gr-ori to remesh the original origin grid. ...
This paper develops a novel three-dimensional (3D) mesoscale modelling approach to study the mechanical responses of coral sands under quasi-static and dynamic compressive loading. A series of algorithms were developed to generate the 3D random particle models with controllable shape and size configurations to simulate the coral sands with random shape characteristics. Subsequently, a new compaction algorithm combining the gravity action and mechanical motion of particle models was proposed to create the 3D sand particulate system with various particle gradations. Using the 3D unstructured meshing algorithm, the finite element model of particulate system was obtained. Finally, the quasi-static and dynamic compressive behaviors of coral sands were numerically investigated in terms of the axial/lateral stress–strain curves, the mesoscopic deformation processes, and the particle breakage patterns. The results indicate that there are obvious differences in the stress–strain curves and deformation modes of coral sands under quasi-static and dynamic compression loads. Through the explicit simulation and quantitative analysis on the deformation process and particle breakage of coral sands, the grain-level responses and failure mechanisms of coral sands under different loading conditions were fully understood. Above all, it was demonstrated that the developed 3D mesoscale model has a significant feasibility in simulating and analyzing the mechanical properties of sand particulate system.
Graphical abstract
... Calcareous sands differ from those sourced from land in terms of the particle shapes, internal porous structure, and the mechanical properties [1,2]. Owing to such variations, the particles are easy to crumble even under the small stresses [3][4][5][6]. In the field, the engineering structures constructed on the calcareous sand are prone to undergo large differential settlement. ...
Many studies have demonstrated the fragility of calcareous sands even under small stresses. This bears an adverse influence on their engineering properties. A series of laboratory tests were carried out on poor-graded calcareous sands to investigate the crushability mechanism. Einav’s relative breakage and fractal dimension were used as the particle breakage indices. The results show that the particles broke into smaller fragments at the low-stress level by attrition which was caused by friction and slip between particles. In contrast, particles broke in the form of crushing at the relatively higher stresses. The evolution of the particle size was reflected by the variation in Einav’s relative breakage and fractal dimension. As testing commenced, the breakage index rapidly increased. When the stress was increased to 400 kPa, the rate of increase in the breakage index was retarded. As the stress was further increased beyond 800 kPa, the rate of increase in the fractal index became much smaller. This elucidated that the well-graded calcareous sands could resist crushing depending on the range of applied stresses. Based on the test findings, a new breakage law is proposed.
... 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. ...
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.
... It is a common phenomenon for particle breakage to occur in granular soils, exerting a significant influence on their behavior (e.g., Hardin 1985;Mcdowell et al. 1996;Konrad 1998;Liu and Wang 1999;Ueng and Chen 2000;Nakata et al. 2001;Hyodo et al. 2002;Coop et al. 2004;Salim and Indraratna 2004;Sun and Wang 2004;Yao et al. 2008;Carraro et al. 2009;Donohue et al. 2009;Kikumoto et al. 2010;Olson 2010, 2011;Carrera et al. 2011;Xiao et al. 2014Xiao et al. , 2016aXiao et al. , b, 2017aAlikarami et al. 2015;Ovalle et al. 2015;Wang and Coop 2016;Xiao and Liu 2017;Yu and Su 2016;Yu 2017aYu , b, c, d, 2018a. As a characteristic of the flow behavior of granular soils, stress dilatancy has been widely investigated, taking into consideration particle breakage or fines content (e.g., Ueng and Chen 2000;Salgado et al. 2000;Miura et al. 2003;Yu and Su 2016;Yu 2017bYu , c, d, 2018a. ...
This paper presents a laboratory experimental investigation of the behavior of coral sands. The investigation incorporated particle breakage to interpret its influence on consolidation and shear behavior, friction and dilatancy behavior, excess friction behavior, and state behavior of precrushed sands as measured by triaxial tests on a coral sand. Particle breakage impaired dilatancy, leading to a more contractive soil. It reduced friction and dilatancy angles, but increased basic friction angles. It also showed a complex influence on excess friction angles, revealing underestimation of the peak-state dilatancy angle by the excess friction angles. Particle breakage resulted in movement of the phase transformation and peak states toward reduction in deviator stress, mean effective stress, and void ratio in the e-logp′ and q-p′ planes. In the e-logp′ plane, it resulted in translation, rotation, or translation and rotation of the critical-state line. In the q-p′ plane, particle breakage resulted in rotation of the critical-state line, incorporating lower-left movement of the critical states.
... However, most of these studies have focused on the dilatancy of terrestrial soil such as siliceous sand and coarse-grained soil. Relatively few studies have been done on the dilatancy of calcareous soil (Sun and Wang 2004;Sun and Luo 2006;Hassanlourad, Salehzadeh, and Shahnazari 2008;Zhang, Jiang, and Wang 2009;Shahnazari et al. 2014;Shahnazari, Rezvani, and Tutunchian 2015;Xiao et al. 2017b). ...
In this article, the dilatancy of calcareous soil is studied systematically based on triaxial consolidation drainage shear tests, and the difference in dilatancy between calcareous soil and siliceous soil is also investigated. It was found that: ① Calcareous soil experience obvious dilated deformation. Dilatancy tendency increases with increasing related density and decreases with increasing confining pressure. ② The volumetric strain rate initially increases from negative to positive. After it reaches a maximum, there is a small decrease in the volumetric strain rate, but it is still greater than zero, and the stress-strain curves are of softening type. ③ For the same condition, the dilatancy deformation of calcareous sand begins later than that of siliceous sand, and the volume compression before dilatancy is also larger for calcareous sand. ④ The critical state alone cannot accurately describe the entire deformation process of soil, and it is proposed that the phase transformation state be added to the standard method used to assess soil dilatation and contraction. ⑤ Based on the statistical analysis of experimental data, mathematical relationships were established between void ratio, relative density, and effective confining pressure of phase transformation state and critical state, respectively.
• Highlights
• Reports results from a well-designed experiment that includes a good amount of samples and data.
• Effects of relative density and effective confining pressure on deformation mode and mechanical properties of calcareous sand are evaluated.
• The difference in dilatancy between calcareous sand and siliceous sand was compared
• The phase-transformation state and critical state were compared with the axial strain, volumetric strain and deviatoric stress.
• Using phase-transformation void ratio and critical void ratio to describe the whole deformation process of calcareous sand is proposed.
• The mathematical expressions of phase-transformation void ratio and critical void ration were given, respectively.
The grain crushing of rockfill materials with high stress level has significant influence on its stress-strain properties upon monotonic loading or axially cyclic loading. Based on the theory of generalized plasticity, and by introducing state parameter, a new dynamic constitutive model for rockfill materials was formulated in complex stress paths, and the model parameters was determined. Finally, the constitutive model was used to model the test results of rockfill materials under both monotonic loading at 400, 1200, 2000 and 3500 kPa confining pressures and axially cyclic loading at 800 kPa and 2200 kPa confining pressures. The result showed that the proposed constitutive model could simulate well the effects of grain crushing on dynamic responses of rockfill materials.
The grain crushing of rockfill materials with high stress level has significant influence on its strength and deformation mechanisms. The soil mechanics theory based on critical state concept can describe the stress-strain relationship of reconstituted soils successfully, but currently for rockfill materials, few studies are conducted on influence of grain crushing on critical state and its mathematical description. The consolidation drained and undrained triaxial compression tests of 18 sets with consolidation stress ranging from 0.4 MPa to 4 MPa and isotropic compression tests of 6 sets were conducted on rockfill materials. The experimental results show that: for all the consolidation confining pressures, the samples tended to critical state under drained and undrained conditions; and the critical states of rockfill materials in q-p' plane and e-lgp' plane all behave nonlinear. Based on these experimental results and by introducing state parameter, a new constitutive model for rockfill materials is formulated under the framework of generalized plasticity; and the determination of model parameters is provided as well. Finally, the constitutive model is used to simulate the test results of rockfill materials from Changhe Dam; and it is verified that the proposed constitutive model could simulate well the stress-strain features of rockfill materials with the consolidated confining pressures ranging from 0.4 MPa to 3.5 MPa.
The geotechnical properties of the coral reef materials widely distributed in all islands in South China Sea have been extensively studied since 1980s in China. However, the majority of these researches are focused on the uncemented calcareous sandy soils and limited studies are available on the engineering properties of the naturally cemented calcareous soils such as calcarenites, which are the prevailing materials at the coral reefs in South China Sea. The state-of-the-art engineering practices in the calcareous soils worldwide indicate that the geotechnical properties of the cemented calcareous sandy soils are significantly different from those of uncemented coral reef soils. Considering the spatial heterogeneity of the naturally cemented calcareous soils, this paper presents factors that affect the strength properties from the microscopic studies. The correlations between the strength of the cemented calcareous sandy soil sample and various parameters, such as unit weight, cementation degree, void ratio, shape of grain as well as grain size, are established from the test results. The results show that good correlations exist between the strength with the unit weight and the degree of cementation of the calcareous sandy soils.
Based on two-dimensional discrete element method, this paper simulates the calcareous sand particle of easily-crushed feature as a cluster cell. It uses the clump cells with the same shape and deformation characteristics to simulate the particles without crushing. The jacking processes for different types of pipe piles are simulated with the model to analyze the meso-mechanical behaviors and breakages of the calcareous sands. The results show that the particle breakage can cause the gradation readjustment, resulting in a better contact between the particle and the pile. The driving speed in calcareous sands is relatively fast. There is a smaller disturbance to the foundation soil. The stress field around the pile has the same tendency. The stresses near the pile tip increase dramatically and result in the formation of the stress nucleus. Under the same surrounding conditions, the horizontal and vertical stresses of crushable calcareous sands are obviously greater than those of the unbroken soil.
The multifunctional large-scale triaxial experimental system invented by the author and his research team in Nanjing Hydraulic Research Institute (NHRI) is the earliest one in China. It can dynamically capture the volumetric strain during cyclic loadings. Using this system, rockfill materials in more than 70 worldwide earth-rock dams are tested, and a fruitful database is established. On this basis, the particle breakage during static and dynamic loadings as well as the influencing factors, the nonlinear variation of the strength and dilatancy indexes, the evolution of residual strains during dynamic loadings, the creep behaviors of rockfill materials and the liquefaction properties of sand-gravel materials are extensively explored. For the investigation of dynamic responses of high earth-rock dams, experimental techniques for centrifugal shaking table tests and the corresponding method for result interpretation are proposed and successfully applied in studying the failure mechanism of earth-core rockfill dams (ECRD) and concrete face rockfill dams (CFRD), and in verifying the effectiveness of reinforcement measures. The acceleration response and the residual deformation as well as the earthquake failure mechanism of earth-rock dams obtained by centrifugal shaking table tests are found in good agreement with the in-situ observations from Zipingpu CFRD and Bikou ECRD after great Wenchuan Earthquake. The use of a centrifuge effectively solves the problem that the stress within a conventional (ground) shaking table differs too much from that within the prototype, and the proposed experimental and analytical method makes the conditions that the stresses within the model and the prototype are approximately the same unnecessary. Therefore, the centrifugal shaking table can be used to investigate the behaviors of vast masses like high earth-rock dams in spite of the limitation of capacities of the involved equipments. For the breaching mechanism of earth-rock dams, experimental techniques and corresponding analysis method are established. A flow discharge control equipment is devised for the centrifugal model test system so as to maintain a stable transition between the normal and high gravity fields for water flows. Meanwhile, a specific model container for dam breaching experiments that integrates the measurement system with the model arrangement system organically is fabricated, and such an arrangement effectively eliminates the deficiency of using pipeline flowmeters outside the model container under large variation of debris flow discharge. Therefore, an accurate measurement of flood discharge in the whole breaching process is attained by utilizing this measurement system. The above achievements and their engineering applications provide a technological support for improving the level of both the disaster consequence assessment and the safety control of earth-rock dams. ©, 2014, Yantu Gongcheng Xuebao/Chinese Journal of Geotechnical Engineering. All right reserved.
Geomaterials are frictional materials, and their strength indexes play a key role in depicting strength characteristics. Illegible knowledge on dilatancy angle has existed for a long time, which caused large errors in theoretical analysis and numerical simulation. It was proved with the generalized geotechnical plastic mechanics principle that the angle of dilatancy should be /2 and the corresponding volumetric strain is zero for geomaterials under non-associated flow rule, based on traditional slip line theory. The classic foundation bearing capacity topic of Prandtl's solution, which has accurate theoretical interpretation, was analyzed systemetically with step loading finite elements under the following three conditions: 1) associated flow rule, 2) non-associated flow rule with the dilatancy of /2 and 3) non-associated flow rule with the dilatancy of 0. Results showed that the errors of ultimate capacity obtained under the above three conditions were within 2%, but the slip line fields were quite different. The slip line field obtained under conditon 2) was identical with that by Prandtl's solution, but a large deviation existed between condition 3) and Prandtl's solution. Thus a correct ultimate load and a slip line field with a great error can be acquired under condition 3), but both are precise under condition 2). Therefore, the angle of dilatancy should be /2, not 0 widely used at present when non-associated flow rule is applied.
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