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%。     

含碳酸盐混合砂的三轴剪切试验研究

含碳酸盐混合砂作为一种特殊的岩土材料主要分布于热带沿海地区,独特的成因和组构使得其具有不同于陆源砂的岩土工程特性。针对取自我国南海的珊瑚砂与取自长江流域石英砂按不同质量百分比配制而成的混合砂,开展不同围压下的三轴固结排水剪切试验,研究不同碳酸钙含量混合砂的剪切特性。结果表明:(1)混合砂峰值强度随有效围压和碳酸钙含量的增加呈近似线性增长;(2)围压及碳酸含量是影响混合砂剪胀特性的主要参数,随着碳酸盐含量的增加,胀缩转换点及其对应的轴向应变量增大;(3)随着碳酸盐含量的增加,混合砂的黏聚力呈线性增加趋势,峰值内摩擦角?f缓慢增长;(4)混合砂的相对破碎率Br随碳酸钙含量及围压的增加而增加。     

Threshold of motion and settling velocities of mollusc shell debris: Influence of faunal composition

Bioclastic particles derived from mollusc shell debris can represent a significant fraction of sandy to gravelly sediments in temperate and cool‐water regions with high carbonate productivity. Their reworking and subsequent transport and deposition by waves and currents is highly dependent on the shape and density of the particles. In this study, the hydrodynamic behaviour of shell debris produced by eight mollusc species is investigated for several grain sizes in terms of settling velocity (measurements in a settling tube) and threshold of motion under unidirectional current (flume experiments using an acoustic profiler). Consistent interspecific differences in settling velocity and critical bed shear stress are found, related to differences in shell density, shell structure imaged by scanning electron microscopy and grain shape. Drag coefficients are proposed for each mollusc species, based on an interpolation of settling velocity data. Depending on the shell species, the critical bed shear stress values obtained for bioclastic particles fall within or slightly below empirical envelopes established for siliciclastic particles, despite very low settling velocity values. The results suggest that settling velocity, often used to describe the entrainment of sediment particles through the equivalent diameter, is not a suitable parameter to predict the initiation of motion of shell debris. The influence of the flat shape of bioclastic particles on the initiation of motion under oscillatory flows and during bedload and saltation transport is yet to be elucidated.     

Fibre reinforced sands: from experiments to modelling and beyond

Based on hypotheses derived directly from experimental observations of the triaxial behaviour, a constitutive model for fibre reinforced sands is built in this paper. Both the sand matrix and the fibres obey their own constitutive law, whereas their contributions are superimposed using a volumetric homogenization procedure. The Severn‐Trent sand model, which combines well‐known concepts such as critical state theory, Mohr‐Coulomb like strength criterion, bounding surface plasticity and kinematic hardening, is adopted for the sand matrix. Although the fibres are treated as discrete forces with defined orientation, an equivalent continuum stress for the fibre phase is derived to allow the superposition of effects of sand and fibres. The fibres are considered as purely tensile elements following a linear elastic constitutive rule. The strain in the fibres is expressed as a fraction of the strain in the reinforced sample so that imperfect bonding is assumed at the sand‐fibre interface. Only those fibres oriented within the tensile strain domain of the sample can mobilize tensile stress—the orientation of fibres is one of the key ingredients to capture the anisotropic behaviour of fibre reinforced soil that is observed for triaxial compression and extension loading. A further mechanism of partition of the volume of voids between the fibres and the sand matrix is introduced and shown to be fundamental for the simulation of the volumetric behaviour of fibre‐reinforced soils. Copyright © 2012 John Wiley & Sons, Ltd.     

Laboratory investigations into the threshold of movement of natural sand-sized sediments under unidirectional, oscillatory and combined flows

The threshold of movement of sediment obtained from sandbanks within the Bristol Channel (UK) is investigated under unidirectional, oscillatory and combined flows. The experiments were undertaken in a recirculating, unidirectional laboratory flume containing an oscillating plate to simulate wave action, with movement along the same axis as the unidirectional flows. The sand samples consisted of cohesionless quartz grains with median grain sizes between 0·315 and 0·513 mm. The experiments were performed under flow velocities (measured at 2 cm above the bed) ranging between 0 and 24 cm s^–1 and oscillatory currents (wave periods of 5, 12 and 15 s) ranging from 0 to 28 cm s^–1. The critical conditions for the initiation of sediment movement were assessed, by visual observation, using the Yalin criterion. The results show that, under unidirectional flow, there is a slight overestimation of the threshold of naturally graded sediments derived on the basis of empirically derived threshold curves for artificially prepared sediments under similar flow conditions. In the case of oscillatory flows, the threshold for the natural sands is found to be higher than that predicted by previously derived empirical curves. Under combined flows, wave period is shown to control threshold conditions, with the unidirectional and oscillatory flow components combining in a linear fashion for long-period (12 s and 15 s) waves. In contrast, in the presence of short-period (5 s) waves, the unidirectional and oscillatory components of the flow appear to 'decouple'. For high orbital velocities, in both cases, the effect of the wave period on threshold diminishes.     

袋装砂井爆夯处理软土地基的数值模拟方法及现场试验验证

袋装砂井爆夯法处理软土地基是利用炸药在设置有排水通道的软土中爆炸产生冲击和振动而使土体加固的方法。针对该法进行理论研究,提出了一种将袋装砂井爆夯处理软土地基的三维问题转化为二维平面应变问题的数值模拟方法：袋装砂井转化为等价砂墙;利用等效冲量原理,炮孔爆炸压力则转化为等效压力墙。数值模拟中考虑了土体骨架变形与孔隙水非达西渗流的耦合。对数值模拟的现场试验验证分析表明,沉降数值分析的结果与铁路宁启线软基处理现场测试结果具有很好的可比性。所提出的数值分析方法可模拟袋装砂井爆夯处理软土地基的超静孔隙水压产生和消散以及土体沉降变形的动态过程。     

Threshold of motion of bivalve and gastropod shells under oscillatory flow in flume experiments

The threshold of motion of non-fragmented mollusc shells was studied for the first time under oscillatory flow. In this regard, flume experiments were used to investigate the threshold of motion of three bivalve and three gastropod species, two typical mollusc classes of coastal coquina deposits. The sieve diameters ranged from 2·0 to 15·9 mm. These experiments were performed on a flat-bottom setup under regular non-breaking waves (swell) produced by a flap-type wave generator. The critical Shields values for each species of mollusc were plotted against the sieve and nominal diameter. Moreover, the dimensionless Corey shape factor of the shells was evaluated in order to investigate the effect of mollusc shell shapes on the threshold of motion. According to their critical Shields parameter, the mollusc threshold data under oscillatory flow present smaller values than the siliciclastic sediments when considering their sieve diameter. In addition, the mollusc datasets are below the empirical curves built from siliciclastic grain data under current and waves. When considering the nominal diameter, the critical Shields parameter increases and the mollusc data are closer to siliciclastic sediments. Bivalves, which have a flat-concave shape (form factor: 0·27 to 0·37), have a higher critical Shields parameter for smaller particles and more uniform datasets than the gastropod scattered data, which have a rounded shape (form factor: 0·58 to 0·62) and have varied morphologies (ellipsoidal, conical and cubic). The comparison between previous current-driven threshold data of bioclastic sediment motion and the data of mollusc whole shells under oscillatory flow shows a fair correlation on the Shields diagram, in which all datasets are below the mean empirical curves for siliciclastic sediments. These findings indicate that the shape effect on the transport initiation is predominant for smaller shells. The use of the nominal diameter is satisfactory to improve the bioclastic and siliciclastic data correlation.     

Gas flow in anisotropic claystone: modelling triaxial experiments

Selected gas pulse tests on initially saturated claystone samples under isotropic confinement pressure are simulated using a 3D thermo‐hydro‐mechanical code. The constitutive model considers the hydro‐mechanical anisotropy of argillaceous rocks. A cross‐anisotropic linear elastic law is adopted for the mechanical behaviour. Elements for a proper modelling of gas flow along preferential paths include an embedded fracture permeability model. Rock permeability and its retention curve depend on strains through a fracture aperture. The hydraulic and mechanical behaviours have a common anisotropic structure. Small‐scale heterogeneity is considered to enhance the initiation of flow through preferential paths, following the direction of the bedding planes. The numerical simulations were performed considering two different bedding orientations, parallel and normal to the imposed flow in the test. Simulations are in agreement with recorded upstream and downstream pressures in the tests. The evolution of fluid pressures, degree of saturation, element permeability and stress paths are presented for each case analysed. This information provides a good insight into the mechanisms of gas transport. Different flow patterns are obtained depending on bedding orientation, and the results provide an explanation for the results obtained in the tests. Copyright © 2012 John Wiley & Sons, Ltd.     

珊瑚砂三维孔隙微观特性研究

孔隙是多孔介质内渗流的发生场所,与介质渗透性存在必然的联系。珊瑚砂因其特殊的物质来源和形成过程,较陆源砂具有截然不同的孔隙特性。通过一系列微观研究手段,从本质上揭示了珊瑚砂特殊孔隙性质的原因。研究发现,从孔隙形状、孔喉尺寸和整体连通性3个角度描述孔隙性质较为合理。其中,孔隙形状用形状因子度量,孔喉尺寸包括孔隙半径和喉道半径,珊瑚砂多孔介质整体连通性利用配位数进行描述。而影响孔隙形状、孔喉尺寸和整体连通性的主导因素包括颗粒形状和颗粒表面粗糙度两方面。其中颗粒形状主要影响孔隙形状、喉道尺寸、孔喉尺寸离散性和介质内部连通性的均匀分布情况。颗粒表面粗糙度主要影响孔隙形状、孔隙形状离散性、孔隙尺寸和介质整体连通性。     

明渠均匀流Q结构分布及运动特性

为了解明渠均匀流湍流Q结构(Quadrant events)第一象限事件(Q1)、第二象限事件(Q2)、第三象限事件(Q3)和第四象限事件(Q4)的时均与瞬态运动特性,使用高频粒子成像测速系统对3种明渠湍流二维瞬时流速矢量场进行测量,统计分析明渠湍流脉动流速场中Q结构的雷诺应力、发生频率、占据测量区域面积率以及Q结构与涡结构运动关系。结果表明:Q1与Q3结构时均特性相似,具有发生频率低、面积占据量少、负瞬时脉动动量通量小等特点;而Q2与Q4结构时均特性与之相反,且Q4结构发生频率沿水深增加而逐渐增多至0.8H附近达到最大,Q2结构发生频率在壁面区域为最大,沿水深增加而减小。明渠瞬态运动特性主要表现为水面区造成Q4结构向下运动,挤压、推进、爬升床面区的Q2结构伴随其自身形变的瞬态发展过程。     

陆相湖盆深水重力流沉积特征、砂体结构研究及油气勘探意义—以鄂尔多斯盆地上三叠统长7油层组为例

结合深水重力流沉积理论、野外剖面、岩心、测井等资料的分析,对鄂尔多斯盆地合水地区延长组长7油层组重力流沉积相和砂体结构的类型、特征进行了深入分析。研究表明长7沉积期发育大面积的深水砂岩,主要发育砂质碎屑流、浊流等重力流砂体,砂质碎屑流沉积以块状层理为主,单砂体厚度大,浊流沉积以粒序层理为主,砂泥互层频繁;结合重力流沉积背景下各成因砂体测井曲线形态、垂向发育厚度、垂向连续性及组合特征,将砂体垂向叠置结构划分为多期砂叠置厚层型、厚砂-薄泥互层型、厚砂-薄砂-泥互层型及薄砂-厚泥互层型四种类型,并对研究区单井砂体结构进行了聚类。实践表明,砂质碎屑流沉积区虽是油气藏有利聚集分布带,但分布面积较广,结合砂体结构特征,可进一步细化有利区划分,为致密油藏有利区的优选提供了新的研究思路。     

Sand grain threshold, in relation to bed 'stress history': an experimental study

Besides particle size, density and shape, the erodibility of a sediment bed depends also upon the exposure to prethreshold velocities in the overlying flow. Such flow effectively rearranges the grains (at and below the bed surface), causing them to become more resistant to subsequent erosion. The effects of the ‘stress history’, leading up to the critical condition for sediment movement, are investigated for unidirectional flows generated in a recirculating laboratory flume. The sediment beds investigated consisted of cohesionless quartz sand grains, with mean grain diameters of 0·194 mm (fine sand), 0·387 mm (medium sand) and 0·774 mm (coarse sand), with narrow particle-size distributions. The critical (threshold) shear velocity (target value) for the three beds was established, within 2·5 min of increasing the flow from zero velocity. The subsequent experiments were performed under prethreshold velocities at 70% (for 5, 10, 20, 40 and 80 min exposure duration), 80% (for 5, 10, 20, 40 and 80 min exposure duration), 90 and 95% (for 5, 10, 20, 40, 80 and 120 min exposure duration) of the target value. Following exposure to these different prethreshold conditions, the flow was increased then to reach actual critical conditions, within a period of 2·5 min. The critical condition for the initiation of sediment movement was established using visual observation (supplemented by video recordings), according to the Yalin criterion. The results show that if the exposure duration to prethreshold velocities remains constant, then the critical shear velocity increases with increasing prethreshold velocity. Likewise, if the prethreshold velocity remains constant, then the critical shear velocity increases with increasing exposure duration. In some circumstances, the critical shear velocity was found to increase by as much as 27%. An empirical formula is proposed to account for the exposure correction to be applied to the critical shear velocities of sand-sized sediment beds; this is prior to their inclusion into bedload transport formulae, for an improved prediction of the magnitude and nature of transport.     

海南岛海砂资源的分类特征及成矿特点分析

海南岛海砂资源丰富,质量较好,储量位居全国前列,其中钛铁矿、锆英石和独居石是海南岛的优势滨海砂矿资源。根据其用途主要分为3类: 锆钛砂矿、石英砂矿和建筑用砂矿。通过资料收集和整理,分别研究了海南岛不同类型海砂资源的分布范围、勘查程度、资源储量、形成环境和资源潜力等。研究结果表明: 锆钛砂矿开发程度最高,主要分布于琼东沿海及水下阶地一带的全新统,规模较大,品位较高,开发条件较好; 琼西则以建筑用砂矿和石英砂矿为主,海砂质量好,杂质含量较低; 位于浅海区的潮流沙脊和河口三角洲,工作程度不高,但其海砂分布广、厚度大、砂质好,是今后海砂勘查的重点区域,具有良好的资源开发前景。总结海南岛海砂资源的主要成矿特点为: ①分布广泛,规模巨大,开发条件好; ②主要产于全新世地层; ③沉积厚度较大,纵向连续性好; ④海砂质量较好,并富含重矿物。     

Thresholds of aeolian sand transport: establishing suitable values

This paper assesses the practical use and applicability of the time fraction equivalence method (TFEM; Stout & Zobeck, 1996) of calculating a wind speed threshold for sand grain entrainment in field situations. A modification of the original method is used and is applied to 1 Hz measurements of wind speed and sand transport on a beach surface. Calculated grain entrainment thresholds are tested in terms of the percentage of sand transport events that they explain. It was found that the calculated thresholds offered a poor representation of the occurrence of saltation activity, explaining only about 50% of the measured transport events. Results are discussed in terms of system response time, wind speed measurement height, undetected events and sampling period. A shear velocity threshold for grain entrainment was also calculated, but this also failed to explain a high proportion of the sand transport events. The best results (67–91% of transport events explained) were found by calculating a threshold based on time‐averaged (≈ 40 s) wind velocity measurements. The applicability of a single threshold to a natural grain population is discussed. A natural surface is likely to possess a range of thresholds varying over short time scales in response to parameters such as grain rearrangement and changes in moisture conditions. The results show that calculated thresholds based on 40 s time‐averaged data consistently explain a high proportion of the recorded sand transport events. This is because such a time‐averaged approach accounts for higher frequency variability inherent in the sand transport system.     

福建浅海潮流沙脊分布及形态特征

福建浅海海砂资源丰富,沿海岸线分布广且资源量大,主要以潮流沙脊为主。对潮流沙脊的形态和分布特征进行研究,有利于海砂资源的勘查开发以及对资源量进行合理评价。基于福建海域多波束全覆盖资料,利用ArcGIS的ArcMap对多波束资料进行配准和数据化,对区内潮流沙脊的形态特征进行初步分析。结果显示,福建浅海潮流沙脊形态上长宽分别在2.8～31.3、1.9～12.3 km之间,面积在3.7～200.9 km2之间;分布上由北往南、自西向东,走向由东西向变为北东—南西向,沙脊发育规模逐渐增大且沙质变好,近岸以粉沙为主,远岸以沙为主。对比沙脊主体长与宽,将福建浅海潮流沙脊划分为A型（长度＞10 km,宽度＞5 km）、B型（长度＞10 km,宽度＜5 km）和C型（长度＜5 km,宽度＜5 km）3种类型。综合福建浅海潮流沙脊分布及形态特征,海砂资源勘查时可优选南部远岸区域。     

含浅层气砂土的赋存特征及其非饱和参数预测

从地质成因角度分析了杭州地区浅层气藏的形成过程,探讨了储层土体的原始赋存分带特征,认为规模气藏中位于浅层气富集带内砂土的吸力变化范围不大,初始赋存含水率大致与其残余含水率接近,一般处于残余含水状态。基于VG模型描述的储气砂土-水特征曲线,提出了间接获取储层砂土非饱和参数的工程预测方法。对比试验结果表明：借助VGM模型、Parker模型和乘幂形式的强度公式,能够有效地预测含浅层气砂层的渗水系数、渗气系数和抗剪强度等非饱和参数,可满足地铁工程超前采取防治措施、消除浅层气地质灾害的需要     

Settling velocities and entrainment thresholds of biogenic sands (shell fragments) under unidirectional flow

Settling velocities and entrainment thresholds of biogenic sedimentary particles, under unidirectional flow conditions, are derived on the basis of settling tower and laboratory flume experiments. Material consisting predominantly of equant blocks (shell fragments of Cerastoderma edule , density, ρ _s=2800 kg m⁻³) or of mica-like flakes and elongate rods ( Mytilus edulis fragments, ρ _s=2720 kg m⁻³) are used in separate series of experiments. Differences in the measured settling and threshold properties are related primarily to particle shape. The selection of a characteristic length scale for non-spherical grains is investigated by comparing two approaches used to define the grain size ( D ) of the sediment samples: grain settling and sieve analysis that are used to derive data for the threshold criteria, in terms of the Shields and Movability diagrams. The empirical curves effectively predict the threshold conditions for any grain shape, provided that grain size is defined in terms of grain settling velocity. However, a functional distinction is made between the characteristic `hydraulic' grain size, defined by grain settling for grain transport applications, and the actual (physical) grain size defined by sieve analysis.     

Large displacement FEM modelling of the cone penetration test (CPT) in normally consolidated sand

A new finite element model based on a large strain formulation has been developed to study cone penetration in normally consolidated sand. An auto‐adaptive remeshing technique was utilized for handling the very large distortion of sand surrounding the cone tip. A frictional contact interface utilizing Mohr–Coulomb's theory was chosen to represent interactions between the surface of the cone and sand. To model the sand behaviour, the non‐associated Drucker–Prager constitutive model was selected. ABAQUS, a commercial finite element software package, was used to implement the model. The explicit solution algorithm was chosen due to its effectiveness for complicated contact problems. Analysis results proved that the model successfully captured the cone penetration behavior in sand. In addition, a chart to predict internal friction angles based on cone tip resistance for different vertical effective stresses was provided. This paper also shows a typical distribution of sleeve resistance, tip resistance—penetration relationship, and typical contours of vertical, horizontal, and shear stresses in normally consolidated sand. Finally, a non‐uniform resistance was found along the length of the friction sleeve. Copyright © 2003 John Wiley & Sons, Ltd.     

粉细砂的真三轴试验与强度特性

采用柔性真三轴仪对上海粉细砂进行了一系列不同中主应力系数条件下的真三轴试验,针对中主应力对粉细砂强度特性的影响进行了系统分析。基于真三轴试验结果对Mohr-Coulomb强度准则的形状函数进行了改进,并采用试验结果对强度准则进行验证。结果表明,建立在真三轴试验基础上的强度准则能更准确地反映砂土三维应力状态下的强度特性。     

楚雄盆地洒芷油砂地质特征及成藏模式

楚雄盆地洒芷油砂矿位于元谋隆起西部边缘洒芷背斜上。洒芷油砂充填在上三叠统舍资组-干海资组砂岩孔隙和裂隙中，油砂多层分布，厚度变化大。通过薄片分析确定，储集层砂岩有效面孔率为9．13％～12．66％，结构成熟度和成分成熟度中等，储层次生孔隙和裂缝发育。湖相三角洲平原分流河道和三角洲前缘河口坝砂体为有利储层。洒芷油砂是古油藏经历燕山期和喜山期多次强烈构造活动改造而形成，成藏模式可以概括为：成藏-抬升氧化-埋深再成藏-热变质-抬升再氧化。