Physical and Mechanical Properties of Coral Sand in the Nansha Islands

Coral sand is a unique material developed in the tropical ocean environment, which is mainly composed of coral and other marine organism debris, with the CaCO3 content up to 96 %. It has special physical and mechanical properties due to its composition, structure and sedimentary environment. In this contribution, we discuss its specific gravity, porosity ratio compressibility, crushing, shearing and intensity for coral sand samples from the Nansha islands based on laboratory mechanical tests. Our results show distinct high porosity ratio, high friction angle and low intensity as compared with the quartz sand. We believe that grain crushing is the main factor that influences the deformation and strength of coral sand. Comprehensive study on the physical and mechanical properties of coral sands is significant in providing reliable scientific parameters to construction on coral islet, and thus avoids accidents in construction.

Particle breakage evolution of coral sand using triaxial compression tests

Particle breakage continuously changes the grading of granular materials and has a significant effect on their mechanical behaviors.Revealing the evolution pattern of particle breakage is valuable for development and validation of constitutive models for crushable materials.A series of parallel triaxial compression tests along the same loading paths but stopped at different axial strains were conducted on two coral sands with different particle sizes under drained and undrained conditions.The tested specimens were carefully sieved to investigate the intermediate accumulation of particle breakage during the loading process.The test results showed that under both drained and undrained conditions,particle breakage increases continuously with increasing axial strain but exhibits different accumulating patterns,and higher confining pressures lead to greater particle breakage.Based on the test results,the correlations between particle breakage and the stress state as well as the input energy were examined.The results demonstrated that either the stress state or input energy alone is inadequate for describing the intermediate process of particle breakage evolution.Then,based on experimental observation,a path-dependent model was proposed for particle breakage evolution,which was formulated in an incremental form and reasonably considers the effects of the past breakage history and current stress state on the breakage rate.The path-dependent model successfully reproduced the development of particle breakage during undrained triaxial compression using the parameters calibrated from the drained tests,preliminarily demonstrating its effectiveness for different stress paths.

Experiment Study of the Evolution of Coral Sand Particle Clouds in Water

The motion of particle clouds(i.e.,sediment clouds)usually can be found in engineering applications such as wastewater discharge,land reclamation,and marine bed capping.In this paper,a series of laboratory tests are conducted on coral sand to investigate the shape feature of the single particle and the mixing processes of the coral sand particle clouds.The shape of coral sand particle is measured and quantified.The experimental results demonstrate that the shape of coral sand particles tends to be spherical as the particle size decreases,and empirical equations were established to explain the variation of D50 and fS,50 of coral sand.Compared with the silica sand,the evolution of the coral sand particle cloud still experiences three stages,but the threshold for the Reynolds number of particle clouds entering the next stage changes.Further,the normalized axial distance of the coral sand particle clouds is 58%smaller.The frontal velocity exhibits similar varying tendency for the coral sand particle cloud.Considering the difference in shape between coral sand particles and silica sand particles,a semi-empirical formula was proposed based on the original silica sand prediction formula by adding the shape factor and the experimental data of 122μm≤D_(50)≤842μm.It can predict the frontal velocity of the coral sand particle clouds.

Performance of X-Section Concrete Pile Group in Coral Sand Under Vertical Loading

To reveal the bearing capacity of the X-section pile group in coral sand, a series of model load tests are conducted.The testing results are presented as load-settlement curves, pile-soil stress ratios, distributions of side friction and axial force, and load-sharing ratio between side and tip resistances. The reliability and accuracy of the numerical simulation model are verified by comparing the results of the model test. Comparative analysis between X-section and circular section piles with the same cross-sectional area indicates that the bearing capacity of the X-section pile group is much larger than that of the circular pile group. The axial force of X-section piles is smaller while the peak skin friction is larger than that of circular piles at the same depth. The skin friction of the core pile is the largest,followed by the side pile and the corner pile is the smallest when the load is relatively small;however, it is converse when the load is larger than 10 k N. Compared with piles in silica sand, the pile in coral sand has a lower bearing capacity, and the sand breakage leads to the steep drop failure of pile foundation. Moreover, pile positions under the raft have less effect on the load-share differences among corner, side and core piles in coral sand. This study provides a reference for the construction of pile foundations in coral sand.

Seismic responses of slopes with different angles in coral sand

This paper investigates the seismic responses of slopes in coral sand taken from a reef island in the South China Sea.A series of shaking table model tests were conducted to explore the responses of soil acceleration,excess pore pressure,and slope displacement for three slope angles(5°,10°,and 15°).The results show that the excess pore pressure ratio of the slope decreases due to an increase in the initial shear stress when the slope angle increases.The acceleration response of the soil increases with the increase of the slope angle.The slope displacement presents substantial increments as the excess pore pressure ratio increases.In addition,the lateral movement,and slope settlement present substantial increments as the slope angle increases.No liquefaction is observed under a dynamic excitation of 0.2 g in the coral sand site.Under a dynamic excitation of 0.4 g,the site liquefies quickly,the acceleration amplification factor decreases,and the lateral movement and the settlement of the slope surface both increase compared with that under 0.2 g excitation.For the slope with an angle of 15°at 0.4 g,the flow distance of the sand strip increase by 289.47%compared with that in the 5°case.The lateral movement of the slope surface near the water level line is substantially larger than that away from the water line.The largest settlement is observed near the middle section of the slope(below the water level)under a dynamic excitation of 0.2 g.In contrast,the largest settlement under a dynamic excitation of 0.4 g occurs at the top of the slope.

Mechanical behaviors of interaction between coral sand and structure surface

Based on the interface shear tests,the macro-and meso-mechanical behaviors of interaction between coral sand and different structure surfaces are studied,in which CCD camera is used to capture digital images to analyze the evolution of the interaction band and a particle analysis apparatus is applied to studying the distribution characteristics of particle morphology.This study proposes four-stage evolution process based on the shear stress−strain curve.During the shear process,coral sand particles slide and rotate within the interaction band,causing the changes in shear stress and vertical displacement.In addition,the effects of structure surface roughness on shear strength,volume change and particle breakage are illustrated that the greater the roughness of slabs is,the larger the shear stress is,the more obvious the contraction effect is and the more the particles break.Furthermore,the change in particle’s 3D morphology during the breakage will change not only their size but also other morphological characteristics with convergence and self-organization.

Effects of biochar-amended alkali-activated slag on the stabilization of coral sand in coastal areas

Coral sand is widely encountered in coastal areas of tropical and subtropical regions.Compared with silica sand,it usually exhibits weaker performance from the perspective of engineering geology.To improve the geomechanical performance of coral sand and meet the requirement of foundation construction in coastal areas,a novel alkali activation-based sustainable binder was developed.The alkaliactivated slag(AAS)binder material was composed of ground granulated blast-furnace slag(GGBS)and hydrated lime with the amendment of biochar,an agricultural waste-derived material.The biocharamended AAS stabilized coral sand was subjected to a series of laboratory tests to determine its mechanical,physicochemical,and microstructural characteristics.Results show that adding a moderate amount of biochar in AAS could improve soil strength,elastic modulus,and water holding capacity by up to 20%,70%,and 30%,respectively.Moreover,the addition of biochar in AAS had a marginal effect on the sulfate resistance of the stabilized sand,especially at high biochar content.However,the resistance of the AAS stabilized sand to wet-dry cycles slightly deteriorated with the addition of biochar.Based on these observations,a conceptual model showing biochar-AAS-sand interactions was proposed,in which biochar served as an internal curing agent,micro-reinforcer,and mechanically weak point.

Dynamic Behavior of Gravity Retaining Walls with Coral Sand Backfill Under Earthquakes:Shaking Table Tests

The retaining walls in coral sand sites are inevitably threatened by earthquakes. A series of shaking table tests were carried out to study the seismic stability of gravity retaining walls with coral sand backfill. Parallel tests with quartz sand were performed to compare and discuss the special dynamic properties of coral sand sites. The results show that the acceleration difference between the retaining wall and the coral sand backfill is 76%-92% that of the quartz sand,which corresponds to the larger liquefaction resistance of coral sand compared with the quartz sand. However, the horizontal displacement of the retaining walls with coral sand backfill reaches 79% of its own width under 0.4g vibration intensity. The risk of instability and damage of the retaining walls with coral sand backfill under strong earthquakes needs attention.

Bearing Behavior of Cast-in-Place Expansive Concrete Pile in Coral Sand Under Vertical Loading

The low side friction of piles in coral sand results in the low bearing capacity of foundations.In this paper,expansive concrete pile is utilized to improve the bearing capacity of pile foundations in coral sand.Both model tests and numerical simulation are performed to reveal the bearing mechanism of expansive concrete pile in coral sand.Results showed that the lateral earth pressure near pile increases obviously and the side friction of piles is improved,after adding expansion agent to the concrete.The horizontal linear expansion is 1.11%and the bearing capacity increased 41%for the pile,when 25%expansion agent is added.Results in finite element numerical simulation also show that ultimate bearing capacity increases with the increase of the linear expansion ratio.Besides,the area for obvious increase in side friction is below the surface of soil about three times the pile diameter,and the expansion leads to a high side friction sharing of the pile.Therefore,the cast-in-place expansive concrete pile is effective in improving the bearing capacity of piles in coral sand.

Effect of coral sand on the mechanical properties and hydration mechanism of magnesium potassium phosphate cement mortar

Damaged structures on coral islands have been spalling and cracking due to the dual corrosion of tides and waves.To ensure easy access to aggregate materials,magnesium potassium phosphate cement(MKPC)and coral sand(CS)are mixed to repair damaged structures on coral islands.However,CS is significantly different from land-sourced sand in mineral composition,particle morphology,and strength.This has a substantial impact on the hydration characteristics and macroscopic properties of MKPC mortar.Therefore,in this study we investigated the compressive strength,interfacial mechanical properties,and corrosion resistance of MKPC CS mortar.Changes in the morphology,microstructure,and relative contents of hydration products were revealed by scanning electron microscope-energy dispersive spectrometer(SEM-EDS)and X-ray diffraction(XRD).The results indicated that the compressive strength increased linearly with the interfacial micro-hardness,and then stabilized after long-term immersion in pure water and Na2SO4 solution,showing excellent corrosion resistance.Compared with MKPC river sand(RS)mortar,the hydration products of CS mortar were an intermediate product 6KPO2·8H2O with a relative content of 3.9%at 1 h and 4.1%at 12 h.The hydration product MgKPO_(4)·6H_(2)O increased rapidly after 7-d curing,with an increased growth rate of 1100%.Our results showed that CS promoted the nucleation and formation of hydration products of MKPC,resulting in better crystallinity,tighter overlapping,and a denser interfacial transition zone.The results of this study provide technical support for applying MKPC mortar as a rapid repair material for damaged structures on coral islands.

A novel relationship between elastic modulus and void ratio associated with principal stress for coral calcareous sand

Elastic moduli,e.g.shear modulus G and bulk modulus K,are important parameters of geotechnical materials,which are not only the indices for the evaluation of the deformation ability of soils but also the important basic parameters for the development of the constitutive models of geotechnical materials.In this study,a series of triaxial loading-unloading-reloading shear tests and isotropic loading-unloadingreloading tests are conducted to study several typical mechanical properties of coral calcareous sand(CCS),and the void ratio evolution during loading,unloading and reloading.The test results show that the stress-strain curves during multiple unloading processes are almost parallel,and their slopes are much greater than the deformation modulus at the initial stage of loading.The relationship between the confining pressure and the volumetric strain can be defined approximately by a hyperbolic equation under the condition of monotonic loading of confining pressure.Under the condition of confining pressure unloading,the evolution of void ratio is linear in the e-lnp0 plane,and these lines are a series of almost parallel lines if there are multiple processes of unloading.Based on the experimental results,it is found that the modified Hardin formulae for the elastic modulus estimation have a significant deviation from the tested values for CCS.Based on the experimental results,it is proposed that the elastic modulus of soils should be determined by the intersection line of two spatial surfaces in the G/K-e-p’/pa space(pa:atmosphere pressure).“Ye formulation”is further proposed for the estimation of the elastic modulus of CCS.This new estimation formulation for soil elastic modulus would provide a new method to accurately describe the mechanical behavior of granular soils.

Numerical analysis of seismic response of rectangular underground structure in coral sand

The underground structure in coral sand is threatened by earthquake.The special dynamic characteristics of coral sand were realized by finite difference program.Specifically,the stress–strain loops,shear modulus attenuation and hysteresis behaviour of coral sand were simulated using hysteresis damping.On this basis,numerical models were established to study the seismic response of the rectangular underground structure in coral sand,and the fluid–solid coupling and soil-structure interaction were considered.The results illustrate that the increasing relative density of coral sand foundation reduces the excess pore water pressure(EPWP),but amplifies the horizontal dynamic soil pressure of the coral sand-underground structure system.The increase in the permeability coefficient of coral sand reduces the EPWP accumulation,which leads to an increase of the stiffness and a decrease of the acceleration amplification of coral sand sites.

Mechanical Properties and Microstructure of Portland Cement Concrete Prepared with Coral Reef Sand

The feasibility of using coral reef sand（CRS） in Portland cement concrete is investigated by testing the mechanical property and microstructure of concrete. The composition, structure and properties of the CRS are analyzed. Mechanical properties and microstructure of concrete with CRS are studied and compared to concrete with natural river sand. The relationship between the microstructure and performance of CRS concrete is established. The CRS has a porous surface with high water intake capacity, which contributes to the mechanical properties of concrete. The interfacial transition zone between the cement paste and CRS is densified compared to normal concrete with river sand. Hydration products form in the pore space of CRS and interlock with the matrix of cement paste, which increases the strength. The total porosity of concrete prepared with CRS is higher than that with natural sand. The main difference in pore size distribution is the fraction of fine pores in the range of 100 nm.

Mechanical Properties of Sea Water Sea Sand Coral Concrete Modified with Different Cement and Fiber Types

The mechanical properties of modified sea water sea sand coral concrete(SWSSCC)under axial compression were experimentally studied.Two different parameters were considered in this test:types of cement and fiber.An experimental campaign was developed involving uniaxial compression tests and the use of digital image correlation(DIC)method to analyze the strain distribution and crack propagation of specimen.Test results indicated that the compressive strength and elastic modulus of SWSSCC were improved by adding stainless steel fibers(SSF),while polypropylene fibers(PF)enhanced the SWSSCC peak deformation.It was found that the elastic modulus and strength of SWSSCC using ordinary Portland cement(OPC)were higher compared to specimen with low alkalinity sulphoaluminate cement(LAS).Typical strain distribution changed with the variation of fiber types.The propagation and characteristics of cracks in SWSSCC containing PF were similar to those of cracks in SWSSCC.However,the propagation of cracks and the development of plastic deformation in SWSSCC were effectively hindered by adopting SSF.Finally,an analytical stress-strain expression of specimen considering the influences of fibers was established.The obtained results would provide a basis for the application of SWSSCC.

Scanning Electron Microscopy (SEM) of the Bug Eye and Sand Coral

We present a Scanning Electron Microscopy (SEM) technique for the characterisation of biological and non-biological samples at nano-scale level. Scanning Electron Microscopy has been around for a long while especially in material science laboratories in developed countries. The SEM has enabled scientist to have a better understanding of microstructure by providing unsurpassed optical magnifications of samples. In this introductory paper, we introduce the techniques of using SEM to capture highly magnified microstructure of a fly found on an African soybean (Glycine max) seed. We are able to estimate the number of lenses in each eye and zoom into features that could describe its life characteristics. Hexagonal lenses are estimated to have sizes ranging from 14 um to 19 um. This paper also presents a finding of a sea coral “pie like structure” on a single grain of sand used for water filtration.

硫酸锌胶结珊瑚砂渗透特性试验及工程应用探讨

对松散珊瑚砂进行胶结处理以降低岛礁地层渗透性是促进生态岛淡水涵养的重要手段之一。为此,开展了硫酸锌胶结珊瑚砂的渗透试验,分析了不同浸泡时长、硫酸锌溶液浓度及初始干密度对胶结珊瑚砂渗透性的影响,采用干湿循环试验验证其耐久性,通过X射线衍射(X-ray diffraction,简称XRD)、扫描电镜(scanning electron microscope,简称SEM)、能量色散X射线光谱(energy dispersive spectrometer,简称EDS)及计算机断层扫描(computed tomography,简称CT)扫描研究微观胶结机制。结果表明:(1)硫酸锌胶结可将珊瑚砂渗透系数降低70.17%~95.79%;(2)干湿循环16次后,胶结处理36 h的珊瑚砂样品质量损失率不超过4%,渗透系数变化不超过1.0×10^(-3)cm/s,表明样品耐久性良好;(3)珊瑚砂和硫酸锌反应生成二水石膏与菱锌矿填充样品孔隙,使孔喉平均半径、配位数均减小,孔隙连通性显著下降。该技术可应用于岛礁吹填初期固砂防侵蚀、提高地基承载力、降低渗透性以促进地下淡水涵养和生态岛建设,具有重要的工程应用价值。

不同形状的珊瑚砂颗粒沉降试验研究

珊瑚砂在液体中的沉速是反映泥沙运动特性的一个重要物理量。由于珊瑚砂形状多样,套用现有的珊瑚砂沉速公式进行计算并不合适。针对珊瑚砂颗粒开展形状分析,将珊瑚砂颗粒分成枝棒状、片状和块状3种形状,然后开展单颗粒沉降试验。基于试验数据,分析不同形状珊瑚砂颗粒的沉降特性,与各家理论公式计算结果进行比较分析,选取经典的珊瑚砂颗粒沉速公式,引入颗粒形状系数进行公式修正,从而推求了枝棒状、片状和块状珊瑚砂颗粒的阻力系数和沉降速度公式。该公式充分考虑不规则珊瑚砂颗粒形状的影响,具有更高的计算精度和适用性。

珊瑚海岛包气带中氮磷的迁移、转化和累积规律

珊瑚海岛包气带中氮磷的迁移转化直接影响海岛地下水质量和土壤结构.通过XRD和SEM分析,明确了珊瑚砂特殊的CaCO_(3)物相成分和多孔形貌特征.采用静态吸附实验明确了珊瑚砂对N、P的吸附机理;通过短期连续降雨和长期间歇降雨模拟柱实验分别研究了N、P在珊瑚砂中的运移机制和不同深度剖面N的转化和P的累积规律.结果表明,珊瑚砂对NH_(4)^(+)和DIP的平均吸附量为192mg/kg和2051.75mg/kg,吸附模型分别符合可逆线性吸附和Freundlich等温吸附模型;NH_(4)^(+)的运移机制可由线性平衡CDE模型解释,低浓度DIP的运移过程符合含衰减项的双点位化学非平衡模型;而高浓度DIP的运移过程无法由模型拟合,结合pH值推测了其与CaCO_(3)发生的溶解沉淀平衡反应.长期降雨条件下,结合微生物分析得出土壤中存在N_(2)→NH_(4)^(+)→NO_(2)^(-)→NO_(3)^(-)→N_(2)/DON的氮转化路径,存在地下水氮污染的风险,而DIP则以非溶解态磷形式被固定,其长期累积可能对土壤结构不利.简言之,N、P在珊瑚海岛包气带中的迁移、转化、累积方面具有特殊性,明确其规律可为海岛生态建设中的环境风险控制提供科学依据.

珊瑚砂与土工格栅界面的剪切特性及本构模型

采用南海原位级配珊瑚砂和聚丙烯双向土工格栅,开展了不同法向应力下珊瑚砂和筋土界面大型直剪试验。试验结果表明,珊瑚砂和筋土界面剪应力—剪切位移曲线呈现明显的应变软化特征,峰值抗剪强度线呈现双折线形态,残余抗剪强度线则呈现较好的线性关系;各级法向应力下珊瑚砂与土工格栅界面摩擦比均值达到1.37左右,远高于石英砂与土工格栅界面摩擦比,说明在珊瑚砂中加筋能充分发挥加筋效果;珊瑚砂和筋土界面在剪切过程中总体上经历了相对剪缩—相对剪胀—相对剪缩的过程,对珊瑚砂加筋可显著减小其剪缩变形;珊瑚砂和筋土界面直剪试验试样的相对破碎率接近,均随法向应力的增大而增大,但增幅逐渐趋缓。采用邓肯—张模型和剪切软化模型分别描述南海原位级配珊瑚砂与聚丙烯双向土工格栅界面峰前和峰后剪应力—剪切位移曲线,所建立的筋土界面本构模型能够很好地反映其剪切特性。

《西沙群岛珊瑚岛(礁)地区岩土工程勘察标准》研究与编制

为适应和满足西沙群岛珊瑚岛(礁)工程建设的需要,使珊瑚岛(礁)岩土工程勘察有据可依,总结了西沙群岛岩土工程勘察项目实践经验和研究成果,通过资料收集、分析与研究,编制了《西沙群岛珊瑚岛(礁)地区岩土工程勘察标准》(DBJ46—060—2022)。介绍标准的编制背景、编制的工作流程、标准的主要内容和重点研究内容,探讨需要进一步研究的内容。