钙质砂单颗粒破碎强度和模式的试验研究

通过自动单轴加载系统对不同粒径和形状的钙质砂颗粒进行了单颗粒压碎试验,同时借助于显微探头观测记录钙质砂颗粒的压碎过程和破碎模式,并利用统计方法分析粒径、形状和干湿条件等因素对钙质砂颗粒破碎强度和破碎模式的影响。此外,通过原位X射线μCT扫描支持下的微型单轴加载装置分别对两个钙质砂颗粒的压碎过程进行了高精度扫描,利用图像处理和分析技术对内孔隙结构的影响进行了深入研究。结果表明：形状类似时,钙质砂颗粒的粒径越大,特征强度越低;粒径相当时,块状颗粒的特征强度最大,片状颗粒次之,刃状颗粒再次之,条状颗粒最小;钙质砂颗粒的破碎过程可以归纳为3种裂纹形式和3种破碎模式,分别为单裂纹、交叉裂纹、拱裂纹和模式Ⅰ、Ⅱ、Ⅲ。μCT图像分析结果表明,颗粒破碎产生的裂纹主要集中于内孔隙较密集处。这说明内孔隙结构对于钙质砂颗粒的破碎行为具有重要的影响。     

珊瑚颗粒孔隙结构及渗流特性分析

珊瑚砂的孔隙结构对珊瑚砂的力学性质和渗透特性影响显著,采用高分辨率计算机断层扫描仪（computed tomography,简称CT）对珊瑚砂颗粒进行了扫描,建立了珊瑚砂颗粒的三维孔隙结构模型,并对孔隙的直径、体积、紧密度、球度进行了定量分析,采用数值模拟方法探究了松散堆积珊瑚砂及单颗粒珊瑚砂内孔隙结构的渗透特性。试验结果表明：孔隙数量随着孔隙直径的增加显著减少,且微孔数量较多,大孔数量较少;孔隙的直径与紧密度之间为幂函数关系,与球度之间则表现为线性关系。渗流分析结果显示,松散堆积珊瑚砂的渗透能力显示出各向异性,Z方向的绝对渗透率小于X、Y方向。珊瑚砂颗粒内的孔隙渗流速度较慢,不同颗粒之间渗流能力差距较大,部分颗粒不能发生渗流,且颗粒内孔隙的绝对渗透率仅占总绝对渗透率的1.26%。     

钙质砂颗粒的形状分析

钙质砂是一种含碳酸钙达50 %以上的海洋生物成因的粒状材料。颗粒棱角度高、形状不规则,影响钙质砂的压缩、填充等力学性能。通过光学显微镜获取钙质砂颗粒的几何投影图像,借助Matlab图像处理软件,分别运用常规统计方法和分形理论对钙质砂颗粒的几何形态进行了描述,并结合钙质砂特殊的生物成因对其表现出的特性进行了分析。结果表明：钙质砂颗粒的形状具有分形特性,且随着颗粒粒径的减小,分形特性越明显。     

钙质砂与硅质砂液化特性对比试验研究

钙质砂颗粒具有形状不规则、多孔隙、强度低、易破碎等特点,较硅质砂表现出更为复杂的液化变形特性.本文对相同级配的钙质砂和硅质砂进行了物性试验、不排水循环三轴试验、轻型动力触探试验,研究两种砂在物理性质、抗液化能力和贯入阻力三方面的差异,分析实验结果得到结论如下:(1)钙质砂比硅质砂具有更大的孔隙比和内摩擦角,这与钙质砂颗...     

南海岛礁吹填钙质砂渗透特性试验研究

南海吹填钙质砂的渗透性对于岛礁地下淡水的形成至关重要。通过室内常水头渗透试验研究了吹填钙质砂的级配、孔隙比和渗透性之间的相互关系,开展了相同级配钙质砂在不同孔隙比下的渗透性试验,研究了不均匀系数和曲率系数对钙质砂渗透系数的影响规律。试验结果表明,钙质砂的渗透系数和10~e(e为孔隙比)有很好的线性关系,并且与不均匀系数、曲率系数和颗粒粒径都有很好的相关性;通过对室内试验结果考虑多因素的分析,建立了钙质砂渗透系数计算模型,该模型公式可为岛礁吹填土层的渗透性评估和新填岛礁地下淡水的形成分析提供参考。     

内孔隙与各向异性对钙质砂液化特性的影响

针对循环荷载作用下钙质砂的力学特性，分析了各向异性与内孔隙对钙质砂液化特性的影响。提出围压增大时，内孔隙释放率增加，次生各向异性变小，二者均使钙质砂抗液化能力增强；固结应力比增加时，初始各向异性愈明显，钙质砂抗液化能力减弱。最后，分析了钙质砂的循环活动性机理。     

含橡胶纤维钙质砂的渗透和固结特性试验研究

珊瑚岛礁常年处于复杂的海洋动力环境中,岛上堤坝围堰、基坑等构筑物的地基渗透变形甚至破坏会导致地基承载力失效的可能性变高。为探究橡胶纤维固化钙质砂的渗透特性和固结特性,采用常水头渗透试验和固结试验研究不同纤维含量下钙质砂的渗透规律和固结变形规律,并设置含纤维玻璃珠对照组。钙质砂具有颗粒形状极不规则、多棱角、内孔隙多等特点,为进一步研究颗粒形状的影响,采用高速动态图像粒度分析仪对钙质砂和玻璃珠的颗粒形状和粒径进行分析。试验结果表明,纤维含量对钙质砂试样渗透特性几乎无影响,但是含纤维玻璃珠试样中,随着纤维含量的增加,渗透系数先增加后减小。由于形状不规则橡胶纤维的加入,一定程度上填补了钙质砂之间的孔隙;钙质砂试样存在 800 kPa 的压力阈值,当压力超过800 kPa后,其压缩模量增幅变缓;不同纤维含量试样的e-lg p曲线可以用Harris模型表示,钙质砂组的材料系数 C= 5,玻璃珠组材料系数C= 3,此外,材料参数a、b与纤维含量有较好的线性关系。提出了合理的预测模型指导地基加固,具有十分重要的理论价值与工程实际意义。     

基于南水模型的钙质砂应力−应变关系模拟

钙质砂是一种海洋生物成因的特殊岩土材料,具有颗粒性状不规则、易破碎的特点,表现出与石英砂不同的力学性质。为模拟钙质砂在不同应力水平下的应力?应变关系,首先分析南水模型的不足：即无法描述应力不变情况下应变无限增长的特点和不能描述钙质砂的剪胀性;然后通过在切线模量中引入应力比与峰值应力比的比值,合理考虑了颗粒破碎的影响;提出了切线模量和切线体积比与应力比间的函数关系式,使得改进后的模型能较好地描述应变?体变关系。同时,将孔隙压力系数引入到孔隙压力表达式中,较好地模拟了三轴不排水剪切试验结果。与现有的模型相比,改进后的模型表达式简单易懂,参数获取更为方便。     

剪切作用下钙质砂颗粒破碎试验研究

钙质砂是一种海洋沉积物,与陆源砂比起来,钙质砂受力后易产生颗粒破碎,从而使其力学性质发生变化。对取自南沙群岛永暑礁附近海域的钙质砂进行了不同围压、不同应变下的三轴剪切试验,对试验前后的试样进行了颗粒大小分析试验。试验结果表明,钙质砂在三轴剪切作用下颗粒破碎十分严重,同时用Hardin模型对其破碎进行了度量,并就围压、剪切应变与破碎之间的关系进行了分析。     

饱和钙质砂爆炸密实动力特性试验研究

开展饱和钙质砂爆炸密实动力特性试验研究,探索饱和钙质砂爆炸密实机制和密实效果,对钙质砂地层中进行的工程建设有重要的理论意义和工程实用价值。通过控制爆炸参数,测试不同参数作用时钙质砂爆炸前后声波特性和表面沉降规律,揭示饱和钙质砂爆炸密实动力特性。试验结果表明：钙质砂高孔隙比和颗粒破碎特性对爆炸密实效果有重要影响。爆炸密实作用后,在爆炸近区,钙质砂颗粒受到较强爆炸冲击作用,导致钙质砂颗粒破碎而形成破碎区和压缩区,压缩区随着时间的推移有松弛的趋势,钙质砂颗粒结构重新固结过程在爆炸后2 h内基本完成。     

钙质砂破碎过程及其微观机制试验研究

为了掌握南海钙质砂压缩变形特征及其微观机制,对3种不同粒组(S1:1.43~2mm、S2:0.5~1mm、S3:0.5~2mm)的钙质砂进行100~3200kPa压力范围的压缩试验,利用自制的砂土微观结构提取装置和图像处理软件(PCAS)获得并分析了钙质砂压缩过程中微观结构。结果表明:(1)钙质砂的大小、形状和级配对颗粒的破碎具有显著影响,当压力较低时(＜800kPa),粒径较大的S1组以砂颗粒棱角破碎为主;粒径较小的S2组没有明显破裂,相对规则的颗粒形态使S2粒组在该压力范围内主要因颗粒的滚动与重分布导致压缩;级配良好的S3组除部分低宽度断肢状颗粒外其余大小、形态颗粒无明显破裂。(2)当压力较大时(＞800kPa),S1组钙质砂逐渐转向以颗粒的整体破坏为主的破碎形式;S2、S3两组试样随着密实度的提高,砂颗粒的破坏以整体破碎为主。基于对破碎过程中试样微观结构变化的提取与分析,总结并提出了控制钙质砂颗粒破碎的4种接触模式:点-线接触、线-面接触、面-面接触和复合接触,可用于判断不同条件下的颗粒破碎形式。最后,讨论了钙质砂在破碎过程中颗粒几何参数的变化。     

Bio-mediated calcium carbonate precipitation and its effect on the shear behaviour of calcareous sand

Calcareous sands have abundant intraparticle pores and are prone to particle breakage. This often leads to poor engineering properties, which poses a challenge to coastal infrastructure construction. A study using bio-cementation to improve the engineering properties of calcareous sand is presented in this paper. The macro- and microscopic properties of bio-cemented calcareous sand were characterized by drained triaxial tests and scanning electron microscopy observations. Experimental results show that the precipitated calcium carbonate can effectively fill the intra- and interparticle pores and bond adjacent particles, thus enhancing the shear strength of calcareous sand. The special structures (e.g. abundant intraparticle pores and rough surface) and mineral components (i.e. calcium carbonate) of calcareous sand are beneficial for improving bacterial retention in soil, which leads to a relatively uniform and dense calcium carbonate distribution on the sand particle surface, exhibiting a layer-by-layer growth pattern. This growth pattern and the abundant interparticle pores would result in less effective calcium carbonate. The strength enhancement of bio-cemented calcareous sand is significantly lower than that of bio-cemented silica sand at the same calcium carbonate content, which may be caused by the differences in the following: (a) soil skeleton strength; (b) the amount of effective calcium carbonate; and (c) interparticle pore-filling of calcium carbonate.

     

THE CARBONATE FRACTION OF BEACH AND DUNE SANDS

The sand size carbonate fractions of beach and dune sands from the southeastern U.S. Atlantic coast were investigated from the standpoint of mineralogy, roundness and size distribution. The carbonate fractions of beach-dune sands used in this study range in abundance from less than 1% to over 40% and average about 10%. Calcium carbonate is least abundant in Georgia beaches and increases slightly to the north and greatly to the south. The size distribution of the carbonate fraction is similar to that of the non-carbonate fraction since both have been distributed by the same processes. The variable flat shape of calcareous fragments causes the carbonate fraction to be usually coarser and more poorly sorted than the acid insoluble residues of the samples. No regional relationship between roundness and wave energies was found in sand size materials although field observations indicate high energies strikingly round gravel sized calcareous fragments. There is some tendency for angularity to increase with decreasing grain size in the sand sizes as with quartz grains. The mineralogy of beach sand carbonate fractions is characterized by the almost total absence of high My calcite. Aragonite is the dominant mineral.     

Index test for the degradation potential of carbonate sands during hydraulic transportation

Carbonate sands undergo degradation when transported hydraulically from a dredging vessel to the reclamation site. Risks involved in the production of fines during hydraulic transport are a deterioration of the mechanical properties of the fill in certain areas of the reclamation area where these fines concentrate. In order to assess sand degradation in the concrete and road construction industry, the Micro-Deval test for fine aggregates was developed in France and Canada. During Micro-Deval testing, aggregates degrade when tumbled in a rotating steel drum with water and steel balls. In the Canadian standard, the steel charge is lower than in the French standard and balls have the diameter of the smallest balls used in the French test. During testing, the quantity of fines produced is measured. This makes the Micro-Deval test an option for assessing the degradation of sand during hydraulic transportation of slurries in the dredging industry. Three sands are tested: two shelly carbonate sands and one quartzic river sand for comparison. The carbonate sands were sampled on reclaimed land and had been subjected to hydraulic transportation in pipeline before artificial deposition. Due to differences in particle shape, the carbonate sands and the quartz sand have a different number of contacts with the steel charge in the drum of the Micro-Deval and are not exposed to the same degradation forces during testing. It was found that the quartzic sand suffers relatively much degradation as compared to the carbonate sands, despite its greater hardness and resistance to crushability. The French test appears to create turbulence that promotes floatation of the curved shells of the carbonate sands instead of shell grinding at the bottom of the drum by contact with the steel balls. Microscopic observations of sand samples before and after testing showed that the French Micro-Deval test is too destructive. The test leads to the fragmentation by impact of the quartz sands and the destruction of certain particle types that had survived hydraulic transportation. With respect to the Canadian test, microscopic examination of sand grains before and after testing shows there is still much fragmentation of quartz grains. A lower rotational speed combined with a limited steel charge made of uniform balls of a smaller diameter, reduces turbulence, shell floatability and last but not the least, impact forces. It produces the desired result: a high Micro-Deval loss for the carbonate sands in comparison to that for the quartz sands and a very limited fragmentation of the quartz grains. Once the slurry reaches its destination, the grain size distribution of the fines fraction will influence the mechanical behaviour of mixed sands. A procedure is proposed to measure the particle size distribution of the fines fraction produced during Micro-Deval testing.Field validation is needed to validate preliminary laboratory results.     

钙质砂抗剪强度特性的环剪试验

珊瑚礁沉积的钙质砂与石英砂的物理力学性质有较大差别。对取自南海岛礁的钙质砂进行了单次往返环剪试验以分析钙质砂的抗剪强度特性,试验中考虑了相对密实度和竖向应力对结果的影响,并与相同级配和试验条件下的石英砂进行对比分析。结果表明：钙质砂正向剪切时应力-位移曲线为软化型,具有明显的残余强度特性,而反向剪切时则表现为硬化型,正向和反向剪切强度基本一致;石英砂正向剪切和反向剪切均表现为软化型。钙质砂正向剪切和反向剪切残余强度与峰值强度的比值在0.75～0.93之间;石英砂正向剪切和反向剪切残余强度与对应峰值强度的比值在0.89～0.96之间。相同级配和试验条件下,钙质砂残余强度均大于石英砂,且强度比值基本保持在1.05～1.3之间。在100、200 kPa竖向荷载作用下,钙质砂0.5～2.0 mm的颗粒发生了破碎,破碎率分别为4%和6%。     

细颗粒组分含量对钙质砂抗剪强度的影响

由于吹填过程的水力分选作用钙质砂土地基颗粒分布不均匀,容易形成分布不均、形态各异的以不同细粒含量的粉细砂层为主的细粒汇集层,引起钙质砂土地基承载力和不均匀沉降问题。开展不同细粒含量的粉细砂三轴固结排水剪切试验,分析细粒含量对钙质砂土力学特性的影响。研究结果表明,（1）当细粒含量增加时相同围压下土样剪胀性逐渐减弱,且峰值偏应力也逐渐减小;（2）各组试样干密度为1.40 g/cm3状态下各含量粉细砂均表现出应变软化特征,当细粒含量为10%时粉细砂土体强度稳定性最差,之后稳定性随细粒含量增加而逐渐增大;（3）钙质细砂由于颗粒咬合作用,具有表观黏聚力,当细粒含量小于40%时,随着细粒含量增加,颗粒之间的咬合作用显著降低,表观黏聚力线性减小。研究成果可为粉细砂层的地基处理及边坡稳定分析提供参考。     

Single-particle crushing behaviour of carbonate sands studied by X-ray microtomography and a combined finite–discrete element method

This paper investigates the particle breakage behaviour of a carbonate sand based on single-particle compression experiments with in situ X-ray microtomography scanning (μCT) and a combined finite–discrete element method (FDEM). Specifically, X-ray μCT is applied to extract the information on grain morphology and intra-particle pores of carbonate sand particles to establish an FDEM model. The model is first calibrated by comparing the simulation results of two carbonate sand grains with the corresponding single-particle compression experiment results and then applied to model the stress evolution, cracking propagation and failure of other carbonate sand particles under single-particle compression. To study the influence of intra-particle pores, FDEM modelling of carbonate sands with completely filled intra-particle pores is also performed. The particle strength of carbonate sands both with and without pore filling is found to follow a Weibull distribution, with that of the sand with pore filling being considerably higher. This behaviour is associated with lower stress concentration, resulting in later crack development in the pore-filled sand than in the sand without pore filling. The cracks are found to usually pass through the intra-particle pores. Consequently, a larger proportion of particles fail in the fragmentation mode in the sand without pore filling.

     

钙质砂一维冲击响应及吸能特性试验

钙质砂为海相沉积的多孔介质,是防护工程分配层的理想填充材料。利用改进的铝制分离式霍普金森压杆（split Hopkinson pressure bar,简称SHPB）,分别对颗粒级配以及相对密实度相同的南海钙质砂和福建石英砂进行了83组一维冲击试验,得到了应力波透射率、冲击响应和吸能效率曲线,分析了应变率、相对密实度和含水率对两种砂冲击特性的影响。结果表明,相同的荷载和边界条件下,钙质砂的刚度仅为石英砂的1/10。由于内孔隙的存在,钙质砂对冲击波的衰减作用大于石英砂,在承载性能允许的情况下,适当减小相对密实度、增加含水率可有效提高分配层消能效果。     

颗粒级配与形状对钙质砂渗透性的影响

钙质砂是富存于热带海洋环境（包括中国南海海域）中的一种特殊岩土介质,具有不同于陆源砂的水理性质。采用传统常水头渗流试验,首先探究了不同不均匀系数和曲率系数条件下级配对钙质砂渗透性的影响。针对钙质砂的颗粒形状特性,采用扫描电镜（SEM）与图像处理技术,引入球度 和圆度X的比值 从三维空间角度上对颗粒形状进行了定量描述。并在粒径区间、相对密实度相同的条件下,通过与福建标准砂、玻璃珠进行对比试验来考察钙质砂渗透特性,从而进一步探究颗粒形状对钙质砂渗透性的影响。试验结果表明,钙质砂渗透性随着颗粒粒径不均匀系数和曲率系数的增大而增大,符合级配对一般砂土渗透性的影响规律;钙质砂颗粒形状具有较强的结构性和不均匀性,同等密实度下钙质砂的渗透性小于陆源的石英砂。本研究获得的钙质砂渗透规律对今后南海岛礁填筑和海上平台基础工程设计具有一定的指导意义。     

Effects of internal pores on the mechanical properties of marine calcareous sand particles

Calcareous sand is a typical problematic marine sediment because of its angular and porous particles. The effects of internal pores on the mechanical properties of calcareous sand particles have rarely been investigated. In this paper, the apparent morphology and internal structure of calcareous sand particles are determined by scanning electron microscopy and computed tomography tests, finding that the superficial pores connect inside and outside of the particles, forming a well-developed network of cavities and an internal porosity of up to 40%. The effects of particle morphology and internal porosity on the mechanical responses of particle were investigated by conducting photo-related compression test and 3D numerical simulations. Two failure modes are observed for the porous calcareous sand, i.e., compressive failure indicates that the particle skeleton is continually compressed and fragmented into small detritus without obvious splitting, and tensile failure indicates that the particles are broken into several fragments when the axial force clearly peaks. Calcareous sand particles with a high internal porosity or with small and dense pores often exhibit compressive failure, and vice versa. The particle strength is considerably reduced by increasing the internal porosity, but affected by pore size in nonlinear correlation. The crushing stress–strain points can be well fitted by an exponential curve, which is supplied for discussion.