冲击能对钙质砂砾颗粒级配影响试验研究

岛礁工程中常规模化采用冲击法加固钙质砂砾回填料地基。对钙质砂砾试样进行冲击试验,研究不同总冲击能对试样颗粒级配、变形、颗粒破碎的影响,结果表明:随着总冲击能的增大,试样孔隙比逐渐减小,并最终呈稳定趋势,二者满足指数函数关系;试样相对破碎率随总冲击能的增大而逐渐增大,但存在一临界总冲击能,超过该值后,相对破碎率增幅逐渐减弱;冲击能能改变试样粒径的构成,其不均匀系数、曲率系数随总冲击能增大而增大,并呈二项式函数关系;试样平均粒径随总冲击能的增大而减小,二者呈线性负相关;钙质砂砾经过冲击作用后,粒组百分含量变化较为明显,含砂量增大,砂砾比例发生变化,其中原粒组下一级粒径区间质量百分含量增量尤为明显。利用钙质砂砾进行规模化地基填筑时,应充分考虑冲击能对其颗粒级配、砂砾比变化的影响,选择合理的施工工艺。     

侧限条件下高压对钙质砂颗粒破碎影响研究

高应力环境中钙质砂的颗粒破碎会影响其工程稳定性,利用液压万能试验机在侧限条件下对0.25mm~0.5mm、1mm~2mm、2mm~5mm粒径和混合粒组钙质砂进行高压加载,研究终止压力、平均粒径、干密度等因素对其颗粒破碎影响。试验结果表明:终止压力P值对钙质砂颗粒相对破碎率Br值影响显著,用Slogistic函数拟合后相关性良好,根据拟合曲线可将破碎分为渐增和缓增阶段;同时相对破碎率Br与试样平均粒径d50呈线性负相关;控制变形速率和终止压力相同,干密度大的试样加荷速率更快,相对破碎率Br比干密度低的试样略大;同等终止压力水平下混合粒组钙质砂相对破碎率Br远小于单粒组;高压加载导致级配和粒组百分含量变化十分显著,工程中应充分考虑其对钙质砂工程性质的影响。试验结论对海相钙质砂区域工程建设具有参考意义。     

三轴排水剪切下钙质砂的颗粒破碎特性

颗粒破碎是影响粒状土的变形和强度机理的重要因素。为了研究钙质砂在剪切过程中的颗粒破碎特性及其对变形和强度性质的影响,对3种不同初始分布的钙质砂进行了不同围压下的三轴排水剪切试验。结果显示:初始分形的粒径分布在三轴剪切过程中始终保持着较为严格的分形特性,该现象与各粒组中的破碎颗粒主要向相邻的下一级粒组中迁移的机制有关。钙质砂的应力–应变特性与围压大小和初始粒径分布有关,围压越低,初始粒径分布越不均匀,钙质砂的剪胀效应越显著。随着围压的增大,钙质砂的剪胀倾向减少,并逐渐过渡到剪缩状态。钙质砂的破碎率随剪切过程中的应力和应变的增长而增大,其峰值内摩擦角随着破碎率的增大而降低,最后趋于定值。用非线性的指数函数来描述峰值内摩擦角与破碎率的相关关系,揭示了颗粒破碎对钙质砂抗剪强度的影响规律。     

基于动态图像技术的南海钙质土颗粒形态特征研究

颗粒形态是一项重要的细观指标，影响着粒状土的物理力学性质。钙质土因为其特殊的生物成因，有着复杂的颗粒形态。为了研究其颗粒形态特征，采用PartAn 3D颗粒动态图像分析仪对粒径0.5～20 mm的南海钙质土颗粒开展形状测试，采用延伸率、扁平度、球形度、圆度、棱角度和凸度指标定量描述颗粒形态特征。结果显示：南海钙质土颗粒的延伸率、扁平度、球形度和圆度符合正态分布，而棱角度和凸度符合幂律分布；颗粒形状以块状居多，随着粒径的减小，钙质土颗粒变得更扁平，形状更规则；通过研究样本数量对颗粒形状量化结果的影响，建议采用形状指标的算术平均值量化单一粒组钙质土颗粒形状特征时，颗粒数目不少于600。最后，运用统计学中的主成分分析方法，得到一个综合考虑钙质土颗粒形状信息的新指标，建立了该指标与钙质土最大、最小孔隙比的关系。     

三轴试验条件下钙质砂颗粒破碎影响因素研究

针对钙质砂颗粒易破碎的特点,对取自南海某礁的钙质砂进行大量三轴试验,研究钙质砂在不同围压、粒径、级配、密实度、排水条件和含水率条件下的颗粒破碎情况,分析以上因素对钙质砂颗粒破碎的影响。试验结果表明:钙质砂颗粒破碎受围压影响显著,与粒径并没有明显相关性;级配越好的钙质砂破碎越小;密实度增大会导致颗粒破碎加剧;不同排水条件下钙质砂颗粒破碎受有效围压影响;颗粒破碎随含水率增加而变大。     

珊瑚碎屑钙质砂的抗剪特性

珊瑚碎屑钙质砂是海洋沉积物中的一种,其强度低、形状不规则、内孔隙多和易破碎等性质和普通石英砂有很大差异。通过对取自南沙群岛永暑礁附近海域的钙质砂进行大型剪切试验,分析得到轴压、剪切速率和级配对钙质砂力学特性的影响。试验结果表明,钙质砂的抗剪强度在达到峰值之后都会出现应变软化现象,随后略微增长并且达到稳定。随着剪切速率的降低和轴压增大,试验土体的抗剪强度持续增加,出现的颗粒破碎增多,土体的剪胀量减小;随着级配粗颗粒的增加,土体的最大抗剪强度和颗粒破碎持续增加,并且在P5=65.14%情况下达到峰值,之后随着粗颗粒含量的增多而降低。     

钙质砂热传导性能试验

基于热针法测得了不同条件下南海钙质砂的热传导性能,探讨了含水率、干密度、温度、颗粒粒径等因素对钙质砂热导率的影响。研究结果表明：钙质砂热导率随含水率、干密度的增大而增大,且含水率越大,干密度对热导率的影响越明显;相对于随干密度的变化,钙质砂热导率随含水率的变化尤为显著;钙质砂热导率随温度升高而增大,但是热导率在不同温度下随含水率的增长趋势不同;颗粒粒径对钙质砂热导率的影响甚微;在钙质砂中掺入一定量石英砂有助于改善其导热性能。     

考虑剪胀性与状态相关的钙质砂双屈服面模型研究

根据重塑和原状钙质砂压缩特性的不同，提出颗粒破碎会引起原状钙质砂的附加孔隙比增量。在临界状态土力学的理论框架内，通过引入状态参数和帽盖屈服面，建立1个考虑颗粒破碎影响的钙质砂的弹塑性本构模型，在较大密度和应力水平范围内对1种土只需采用1组材料参数。通过与三轴试验结果比较，模型能较好地描述钙质砂的强度和剪胀性。     

钙质砂和石英砂压缩下的颗粒破碎与形状演化

颗粒破碎现象对土体力学性能具有显著影响。以往关于颗粒破碎的研究多关注于粒径的变化,忽视了颗粒形状的变化。为研究破碎过程中颗粒形状的演化规律,开展了钙质砂和石英砂的侧限压缩试验,对试验过程中颗粒形状参数的进行了量化研究。结果表明两种砂土的相对破碎率均随着单位体积塑性功的增加而增加,表现出明显的双曲线关系。钙质砂的长径比、球形度和圆度会随着破碎程度的增加而增加,凸度变化不明显;随破碎量的增加,石英砂颗粒的长径比和球形度先减少后增加,凸度持续减少至稳定,球形度持续增加。通过定义试样整体的形状值能很好的量化颗粒形状的变化规律。并且钙质砂整体形状值与相对破碎率满足双曲线关系,石英砂则为抛物线关系。     

矿渣细骨料混凝土孔隙结构对抗压强度的影响

通过试验分别测定了粒化高炉矿渣和天然砂分别作为细骨料时混凝土的抗压强度及孔隙分布和孔隙度,分析了混凝土孔隙结构对其抗压强度的影响。结果表明:与普通混凝土相比,粒化高炉矿渣代砂混凝土早期抗压强度略低于普通混凝土但其后期强度增长较快,且水胶比越大粒化高炉矿渣代砂混凝土后期强度增长越快;混凝土孔隙度随龄期的增大而减小。在水胶比相同的情况下,普通混凝土孔隙度小于粒化高炉矿渣代砂混凝土孔隙度。对于采用同种细骨料的混凝土,水胶比越大其孔隙度也越大;混凝土的抗压强度随孔隙度的增大而减小,且孔隙尺寸越大数量越多混凝土的抗压强度越小。     

One-dimensional dynamic compressive behavior of dry calcareous sand at high strain rates

Calcareous sand has distinct characteristics in comparison with silica sand, such as dynamic behavior at high strain rates (HSRs). This is closely related to pile driving, aircraft wheel loading and mining activities. To understand the response of calcareous sand at HSRs, a series of dynamic tests is performed using the split Hopkinson pressure bar (SHPB) with steel sleeve, including 6 validation tests of bar-against-bar and 16 comparative tests relevant to the relative density and strain rate of calcareous and silica sands. The apparent dynamic stiffness of calcareous sand is approximately 10% of that for silica sand due to different particle shapes and mineral compositions. The axial stress-strain response of silica sand is mainly governed by the deformation of individual grain and soil skeleton, and particle crushing. However, porous calcareous sand shows yielding and strain-hardening responses that are always followed by particle crushing. As the applied loading increases, the particle crushing of calcareous sand develops from local instability to whole breakage. Calcareous sand has lower viscous flow effects compared with silica sand at HSRs.     

Sand-geogrid interfacial shear response revisited through additive manufacturing

Though it is known that the geometric features of geogrids are crucial for deriving optimal interface shear strength, not much work is done to optimize the size and shape of the apertures relative to the particle size of the soils in contact. Most of the commercial geogrids have rectangular or square apertures, which are many times bigger than the soil particles. The present study explores the effects of aperture size and shape of geogrids relative to the size of the sand particles on their interface shear response through direct shear tests and digital image analysis. Geogrids of different aperture sizes and shapes were manufactured using a 3D printer. Shear tests were carried out on three sands of different grain sizes interfacing with geogrids of five different aperture sizes and three different aperture shapes. Through these tests, interface shear response with a wide range of aperture ratio and different shapes of geogrids is understood. Shear zone thickness of different sand-geogrid interfaces was computed through Particle image velocimetry (PIV). Based on the tests and analyses, triangular apertures are found to be more efficient compared to other apertures. The optimal range of aperture ratio is found to be 2–11.29.     

层状盐岩细观孔隙特性试验研究

孔隙特性是决定盐岩储库密闭性的关键因素。对层状盐岩中不同岩性的试样开展压汞法测试和电镜扫描试验,分析层状盐岩孔隙率与渗透率的关系,并对层状盐岩的孔隙结构、分布特性及其密封性能进行研究。压汞测试表明：盐岩、泥岩和泥质钙芒硝的孔隙率平均值分别为2.7%,6.0%和2.5%,与常规岩石对比发现层状盐岩的孔隙率处在非常低的水平。孔隙率与渗透率之间存在明显的关系：低孔隙率是低渗透率的充分非必要条件,而高渗透率是高孔隙率的充分非必要条件,高孔隙率的试样有时也会出现低渗透率的情况。对岩体渗透率影响最大的是相互连通的部分孔隙,一旦存在连通的孔隙,渗透率就会显著增加。电镜扫描结果显示,盐岩是典型的晶体构造,内部结构密实,无明显的孔隙存在。泥岩中黏土和有机质颗粒的差异会显著影响其孔隙性质,颗粒粒径大小不一且呈块状或卵石状的区域孔隙大量发育,颗粒均质、呈块状的区域结构密实,孔隙较不发育。泥岩和盐岩的交界面处并不是孔隙发育的区域,泥岩和盐岩相互咬合、嵌入及泥岩细小颗粒的填充使孔隙率降低。含泥盐岩(含盐泥岩)中局部孔隙较为发育,并且连通度较高,是渗流可能发生的区域,建议储气库设计建造时避免将关键部位设置在孔隙发育区域,以保障密闭性。本文的研究成果可为储气库的建造及密闭性分析提供指导。     

Macro-meso effects of gradation and particle morphology on the compressibility characteristics of calcareous sand

In order to fully develop the South China Sea, a large number of reclamation projects using calcareous sand have been carried out in this area recently. A deep understanding of the physical and mechanical properties of calcareous sand is of critical importance. Therefore, the calcareous sand near a certain reef of the South China Sea is used in this study to investigate the effect of three-dimensional (3-D) particle morphology and gradation on the compressibility characteristics of calcareous sand. This paper proposes a 3-D mesoscope observation method to obtain the average 3-D angularity parameter S _d and 3-D aspect ratio T _d of calcareous sand with different particle sizes. It is found that the morphology of coarse particles (diameter: 5 ~ 1 mm) is significantly multi-angular, while the morphologies of middle particles (1 ~ 0.25 mm) are mostly dendritic and schistic. Compared to the 3-D S _d of quartz sand, the calcareous sand’s particle morphology is much more irregular and multi-angular, which makes it easy for the calcareous sand to form large pores and, thus, be more compressible. In order to systematically study the effect of gradation on the calcareous sand’s compressibility characteristics, a number of compression tests on calcareous sand with different gradations are taken. The influential mechanism is then discussed by analyzing the test results from a mesoscopic viewpoint. It is found that changing the coarse fraction content is the most efficient way to reduce the compressibility of the calcareous sand. That is because of the coarse fraction’s high angularity, which makes the skeleton-bearing capacity of the calcareous sand sensitive to the change of coarse fraction content. An empirical formula is proposed to evaluate the compressibility of the calcareous sand with different coarse fraction contents.     

Impact of pore structure and morphology on flow and transport characteristics in randomly repacked grains with different angularities

The flow and transport behaviors in porous media are closely linked to the structure and morphology of the pore space. A fundamental objective of most studies of porous media is to link the pore structure to the hydraulic functions, such as the permeability, capillary pressure and diffusivity, which are necessary for engineering applications. In this paper, an attempt is made to build a direct link between the hydraulic functions and the morphological measures of diverse porous media. Porous columns with different structures and morphologies are generated by randomly packing grains with different shapes and sizes. The pore structure of the repacked porous media is visualized through X-ray computed tomography and quantified by a series of parameters, including the set of Minkowski functionals, diverse characteristic pore sizes, geometric tortuosity and fractal dimension. The intrinsic permeability, molecular diffusivity and apparent thermal conductivity of the repacked porous media are simulated numerically. The Minkowski functionals have the capacity to characterize the microscale complex pore domain of the porous media in a macroscale way. A good linear relationship is shown among the effective pore size, nominal opening dimension and critical pore neck size obtained from the morphological analysis regardless of the shapes and sizes of the grains. The three different pores may serve as the characteristic pore correlation to the intrinsic permeability. The Kozeny-Carman equation can be used to mimic the intrinsic permeability and to serve as a quality-control tool for porous media with different grain angularities. A topologically based model can generally provide a single relationship for porous media randomly repacked with grains of different angularities. The molecular diffusivity of angular grains is found to be larger than that of round ones. The molecular diffusivity is linearly related to the porosity and fractal dimension. Porous media repacked with round grains tend to attain denser packing, a higher number of contacts per unit volume and higher thermal conductivity than media packed with angular particles. The apparent thermal conductivity has a negative linear correlation to the porosity and fractal dimension of porous media with different grain morphologies.     

Dynamic properties of calcareous and siliceous sands under isotropic and anisotropic stress conditions

The strain-dependent dynamic properties of sand are generally described by their relative density and mean effective stress, while the contribution of other factors, like soil origin, mineralogy, grain morphology, and initial stress anisotropy, have not been fully recognized. This paper presents the results of an experimental study on the shear modulus and damping ratio of calcareous and siliceous sands of different origins and their identical grain size distribution and stress-density states. Resonant column and cyclic triaxial tests were conducted on reconstituted samples of these two sands obtained from coastal areas. The significance of the initial effective confining pressure and stress anisotropy on the dynamic properties of the sands is evaluated and compared. It is demonstrated that the small-strain shear modulus of the calcareous sand is more affected by an increase in mean effective confining pressure than the siliceous sand. However, the effect of the initial shear on the secant shear modulus of the sands is unique. Based on the test data, a rigorous correction factor is proposed to account for the influence of the initial stress anisotropy on the small-strain shear modulus of the sands. A comparison between the strain-dependent dynamic properties of the calcareous and siliceous sands reveals that the calcareous sand has a higher secant shear modulus, lower damping ratio, and higher linear and volumetric threshold strain. Since the stress-density states and grain size distribution of the two sands were identical in the experiments, the discrepancy in the dynamic properties can be attributed to other factors, including sand origin, grain angularity, mineralogy, and formation processes, which are not commonly taken into account in the current practice.     

Brief note on the influence of shape and percentage of gravel on the shear strength of sand and gravel mixtures

The paper records the influence of the shape and the percentage of gravel on the shear strength/frictional angle of sand and gravel mixtures using direct shear tests. The shear strength is mainly derived from the frictional forces developed due to sliding and interlock; they depend on the maximum particle size and shape, the uniformity coefficient, density and the effective normal stress. As the size of material in a mixture is variable, the shear strength also depends upon the ratio of the specimen diameter to the maximum particle size. In this study, two different shapes of limestone were used, angular and rounded, and the maximum gravel size was 6.3 mm in diameter. Air-dried samples were used in the tests. It is concluded that the shape and percentage of gravel have an important influence on the shear strength properties. Electronic Publication     

Relationships between Shape Characteristics and Shear Strength of Sands

In the Mohr-Coulomb criterion, the shear strength of sands is typically characterized by the internal friction angle, which depends on many factors such as grain size and distribution, the mineralogical origin of the particles, particle shape, unit weight, geological history, cementation, saturation, and overburden pressure. In this study, the empirical relationships among three particle shape indices, different fractal dimension definitions, and internal friction angles were investigated. Within this context, direct shear tests were conducted on 38 different types of sands from different origins and with various grain sizes. For each type of sand, image analyses were performed to find out the roundness, sphericity, regularity parameters belonging to individual grains. Additionally, several statistics of these parameters for different types of sands were determined. The results revealed that particle shape has a limited effect on the friction angle of sands in comparison to grain size distribution. Furthermore, it was found that decreasing regularity in particle shape caused an increase in the internal friction angle of uniform sands. These findings agree with the empirical relationship between the internal friction angle and particle shape suggested in the literature.     

Mechanical properties of rock materials with related to mineralogical characteristics and grain size through experimental investigation: a comprehensive review

Mechanical properties of rock materials are related to textural characteristics. The relationships between mechanical properties and textural characteristics have been extensively investigated for differently types of rocks through experimental tests. Based on the experimental test data, single- and multiple- variant regression analyses are conducted among mechanical properties and textural characteristics. Textural characteristics of rock materials are influenced by the following factors: mineral composition, size, shape, and spatial distribution of mineral grains, porosity, and inherent microcracks. This study focuses on the first two: mineral composition and grain size.This study comprehensively summarizes the regression equations between mechanical properties and mineral content and the regression equations between mechanical properties and grain size. Further research directions are suggested at the end of this study.     

Role of particle shape on the shear strength of sand-GCL interfaces under dry and wet conditions

Interface shear strength of geosynthetic clay liners (GCL) with the sand particles is predominantly influenced by the surface characteristics of the GCL, size and shape of the sand particles and their interaction mechanisms. This study brings out the quantitative effects of particle shape on the interaction mechanisms and shear strength of GCL-sand interfaces. Interface direct shear tests are conducted on GCL in contact with a natural sand and a manufactured sand of identical gradation, eliminating the particle size effects. Results showed that manufactured sand provides effective particle-fiber interlocking compared to river sand, due to the favorable shape of its grains. Further, the role of particle shape on the hydration of GCL is investigated through interface shear tests on GCL-sand interfaces at different water contents. Bentonite hydration is found to be less in tests with manufactured sand, leading to better interface shear strength. Grain shape parameters of sands, surface changes related to hydration and particle entrapment in GCL are quantified through image analysis on sands and tested GCL surfaces. It is observed that the manufactured sand provides higher interface shear strength and causes lesser hydration related damages to GCL, owing to its angular particles and low permeability.