不同细颗粒含量甲烷水合物沉积物三轴剪切试验研究

海域试开采区域含水合物沉积物的粒度分析结果表明水合物沉积物骨架由粗、细颗粒混合构成,通过开展多组低温、高压三轴排水剪切试验研究细颗粒含量和密度对含甲烷水合物沉积物和无水合物沉积物的强度和变形特性的影响。试验结果表明,含水合物沉积物抗剪强度及剪胀性都随细粒含量提高而显著增强。这是由于细颗粒含量增加改变了颗粒间水合物的样貌和分布特征,形成了由水合物包裹着粗颗粒-细颗粒的团簇状集合体。然而,细颗粒含量对无水合物沉积物的强度和变形特性的影响却表现出相反趋势。另外,含水合物沉积物的剪胀关系可以使用修正剑桥模型中的剪胀关系式进行描述。结果表明,剪胀关系的拟合曲线依赖于水合物饱和度的大小。通过对比研究发现,天然水合物和实验室合成水合物试样在较高饱和度时的峰值摩擦角大小及其伴随水合物饱和度的增长趋势存在差异,这种差异主要来源于水合物在沉积物骨架颗粒孔隙中不同的赋存模式及分布特征。     

粗粒土强度和变形的级配影响试验研究

开展了不同细粒含量的无黏性和含黏粒粗粒土的共8组大型三轴排水剪切试验,研究了级配对粗粒土强度、变形、剪胀特性和颗粒破碎的影响。试验结果表明细颗粒含量的大小、是否含泥是粗粒土力学特性的重要影响因素;分析了无黏性粗粒土的颗粒破碎率随围压大小、级配的变化;研究了剪切峰值随围压、细颗粒含量的变化规律,讨论了不同围压、不同级配特征情况下粗粒土的剪胀特性。根据含黏粒粗颗粒土的试验结果,分析了含泥量对强度和变形特性的影响,并从机理上分析了细粒含量对无黏性和含黏粒粗粒土的力学特性影响的差异性。试验结果表明对于土石坝工程良好的坝体填筑料级配、严格控制小于0.075 mm颗粒含量,有利于提高坝体的稳定性和减小坝体沉降。     

含水合物沉积物强度特性的三轴试验研究

针对含水合物沉积物基础力学试验数据相对缺乏的现状，分别采用通气法和预冻结法制备了含甲烷水合物和含四氢呋喃(THF)水合物的砂性沉积物试样，并通过低温高压三轴试验系统研究了含水合物沉积物的强度特性及其影响因素，重点分析了试样强度在不同水合物饱和度情况下的发展规律，并证实了试验反压的重要影响。结果表明：试样黏聚强度随水合物饱和度增加呈指数型增长。当水合物饱和度不高时(<50%)，试样内摩擦角变化不大；当饱和度较高时（>80%)，由于土颗粒咬合作用不能充分发挥，试样内摩擦角降低。另外，反压有助于提高水合物强度，从而提高含水合物沉积物的强度。     

含四氢呋喃水合物沉积物无侧限压缩试验研究

参考南海神狐海域SH2站点含水合物地层沉积物颗粒级配分析结果,人工配置试验土骨架,制备不同水合物饱和度(0%,40%,50%,60%)的含四氢呋喃水合物沉积物试样,并在低温环境下进行无侧限压缩试验,获得了不同水合物饱和度试样的应力应变曲线、刚度和强度。结果表明,试样的应力应变曲线呈现应变软化现象,随着水合物饱和度的增加,软化现象更为明显;水合物能够显著增加试样的弹性模量,并随着水合物饱和度的增大而增大;随着水合物饱和度的增加,试样的无侧限抗压强度呈指数增加,与三轴和直剪试验所测得强度的趋势一致。     

水合物形成对含水合物砂土强度影响

采用非饱和成样法（A法）和饱和试样气体扩散制样法（B法）两种试验室方法,合成了含CO₂水合物的砂土试样,并采用改造过的三轴剪切试验仪完成了相应的三轴剪切试验。实验结果表明：A法制得试样强度和刚度随水合物饱和度增大而增大,且相当敏感;而B法制得试样在水合物饱和度为19.44%与纯砂土的力学特征差别很小,在较高饱和度（26.73%）时,含水合物砂土的强度和刚度就有了较为明显提高;由此可以得出含水合物砂土的强度特征是水合物含量和水合物于砂土中赋存状态联合决定的;同时也发现随着水合物饱和度的增大,试样的剪胀性越来越明显。最后,通过对A法制得试样的强度参数分析表明：含水合物砂土的黏聚力随饱和度的增大而提高,而摩擦角基本不变。     

考虑固相分解的含水合物沉积物体积应变分析模型

水合物开采通过打破固相水合物相平衡状态使其分解为水和气体,含水合物沉积物（gas hydrate-bearing sediment,GHBS）固相组分减少使孔隙体积增大,土骨架间胶结作用弱化,产生的水和气显著改变孔隙压力,造成沉积物软化和体积收缩。基于GHBS三轴压缩试验,考虑水合物降压分解过程对GHBS变形特性的影响,将固相骨架分为惰性土骨架和可分解的水合物固相,引入随水合物饱和度变化的压缩参数,建立了能够描述GHBS应力和水合物分解耦合作用、体积应变随时间变化的分析模型。该模型能够描述降压速率、降压幅值及水合物分解速率对GHBS变形特性的影响,结果表明：降压速率增大,降压阶段体积应变速率增大,达到相平衡时间缩短,降压开采时应综合考虑开采过程中储层变形速率和开采效率间的关系;不同粒径组成的沉积物水合物分解速率存在差异,分解速率对储层变形速率影响明显;降压开采稳定孔压影响储层最终沉降量,降低稳定孔压可以提高开采效率,但最终变形量增大。     

甲烷水合物三维离散元模拟参数反演初探

含填充型水合物的砂性能源土可视为特殊的散粒体材料（砂粒和水合物颗粒混合物）,具有明显的非连续特征。在离散元中若采用团粒（胶结成团的颗粒组）模拟填充水合物颗粒则需合理确定团粒结构内颗粒间胶结模型参数。为此,基于前人的室内纯水合物三轴试验资料进行离散元建模与参数反演。结果表明,宜采用松散且颗粒间摩擦系数较小的试样模拟水合物块体,当颗粒间摩擦系数小于等于0.0时,可确保无胶结试样的内摩擦角小于室内试验获得的纯水合物内摩擦角。胶结刚度只需在较小范围变化即可反映相同温度不同围压条件下的弹性特性,且微观刚度参数与胶结强度参数的相互作用较小,可以假定二者相互独立。通过选取不同的微观胶结强度值进行不同围压下的三轴压缩试验,建立微观胶结强度参数与宏观参数（内摩擦角和黏聚力）之间的关系,从而确定与室内试验强度特性相符合的微观胶结强度值,实现甲烷水合物三轴试验离散元模拟;由体变规律可知,甲烷水合物在发生剪胀前均存在一个初始的体积收缩阶段,且剪胀特性随着围压的减小而呈现增强趋势。通过微观变量颗粒接触方向组构的分布图可知,随着轴向应变增大,颗粒间接触主方向朝竖直方向偏转,表现出明显的各向异性特性。随着轴向应变的增大,颗粒间胶结残余率变小,表明试样逐步破坏。     

考虑损伤的含天然气水合物沉积物本构模型

对含天然气水合物沉积物施加荷载后,其内部固体颗粒间将会发生错动、滑移及滚动,当荷载较小时,沉积物表现为弹性变形;随着荷载的增大,内部将出现塑性流动,水合物晶体重新定向,此时沉积物表现为塑性变形;随着荷载的进一步增大,最终导致沉积物发生破坏,这一过程可以看作是沉积物内部损伤产生和逐渐发展的过程。从复合材料的细观力学机制出发研究含水合物沉积物的材料特性,得到等效弹性常数与水合物含量之间的关系;同时基于损伤力学理论,推导出损伤变量的表达式,建立考虑损伤的含天然气水合物沉积物的本构模型。通过比较不同水合物含量下的计算结果与试验结果,结果表明,所建模型能较好地反映含天然气水合物沉积物的应力–应变关系,验证了模型的可行性。     

水合物赋存模式对含水合物土力学特性的影响

基于非饱和制样法(US法)和饱和试样气体扩散制样(SD法)2种制样方法,利用三轴剪切试验研究不同水合物赋存模式对含CO2水合物土力学特性的影响规律。试验结果证实水合物的赋存模式及其含量对含水合物土的力学特性存在重要影响,微观机制分析表明：US法制得试样属于胶结赋存模式,该模式表现出典型的胶结土的结构特性,而 SD 法制得试样根据水合物含量不同,属于填充模式、接触模式和透镜体模式,显示出明显的颗粒摩擦材料的力学特性,但由于水合物的破碎损伤效应也会导致应变软化现象。最后,利用有效水合物饱和度的概念,提出能考虑2种水合物形成模式下抗剪强度预测的经验公式。     

细粒土不均匀分布对粗粒土力学特性的影响

为了研究冻融循环条件下细粒土的不均匀分布特性对粗粒土力学特性的影响,对不同冻融循环次数、冻结温度、围压条件下的含有不同细粒土分布的粗粒土进行常规的静三轴剪切试验,研究冻融循环后具有不均匀细粒土分布的试样的最大剪胀位置、应力–应变关系曲线、起始屈服强度、起始屈服应变、静强度、破坏应变变化规律。研究结果表明,在相同的细颗粒与粗颗粒不均匀分布的条件下,试验后试样的最大剪胀位置随着试样上两层中细粒土含量的增加而沿着试样高度上升,随着冻融循环次数的增加,最大剪胀位置的上升速率是不断增加的。细颗粒与粗颗粒不均匀分布的程度越高,尤其是粗颗粒的富集程度越高,试样的脆性越高,试样的应变软化程度越明显。冻融循环次数的增加,降低了粗、细颗粒较均匀分布的3种试样的应力–应变软化程度的差异性,即冻融循环次数的增加可以使粗颗粒和细颗粒的组合结构趋于稳定。在相同的围压条件下,冻融循环次数的增加削弱了因细粒土不均匀分布而导致的起始屈服强度的差异性和静强度的差异性。试样中的粗颗粒和细颗粒分布越不均匀,试样的起始屈服强度和静强度越大。当围压为100 k Pa时,静强度与起始屈服强度的比值随冻融循环次数波动较大,当围压增加到300 k Pa时,比值随冻融循环次数的波动较为平缓。细粒土不均匀分布是起始屈服应变和破坏应变的最显著性影响因素,但是围压对破坏应变的影响程度明显大于围压对起始屈服应变的影响程度。细粒土不均匀分布对破坏应变影响的显著性明显小于它对起始屈服应变影响的显著性。     

Shear behaviour of methane hydrate bearing sand with various particle characteristics and fines

It is recognized that methane hydrate (hereafter referred to as MH) is trapped in the sand sediments of alternating sand and mud layers in the turbidite of the Nankai Trough, Japan. The existence of fines within the marine sediment significantly affects its mechanical and physical properties. A series of plane strain compression tests at high pressures were performed in order to investigate the effect of the particle characteristics and fines content of the host sands on the shear behaviour of MH bearing sands. MH bearing sands were artificially produced using rounded glass beads and three other silica sands with different fines contents. A high-pressure low-temperature testing apparatus was equipped with a camera to observe deformation of the specimens during shearing and particle image velocimetry analysis was conducted on pictures taken during the experiments. The experimental results show that strength enhancement due to the bonding effect in MH bearing sand increases with the level of fines content. Values for both the cohesion and friction angle of MH bearing sand composed of Toyoura sand increased along with increasing MH saturation. However, in the case of MH bearing glass beads, only the value for cohesion increased when MH was formed. The maximum shear strain of MH bearing glass beads was mostly concentrated near the shear band. While the maximum shear strains of the three other MH bearing sands were concentrated within the shear band, some was widely distributed in the region outside of the shear band. A rise in the degree of MH saturation increased the angle and narrowed the width of the shear band, regardless of the fines content.     

Fines content for optimum stability within coastal dune sands within Northern KwaZulu-Natal,South Africa

This paper describes the results of geotechnical and geochemical laboratory tests, carried out on dune sands from northern KwaZulu-Natal, to which increasing percentages of fines (<63 µm) were added. The geotechnical tests detail the effects of the fines content on a number of properties of the dune sands, such as the permeability, maximum dry density and shear strength. The peak shear strength and dry density of the dune sands occurs when 20–30% fines were added, whilst the permeability was seen to decrease continuously with increasing fines. Compaction of the samples was seen to significantly increase the shear strength of the dune sands together with a corresponding decrease in the permeability. The results from these tests enabled slope stability analysis to be carried out. Typical fines used in the geotechnical tests were analyzed by XRD, and the clays were isolated and their morphology studied using a transmission electron microscope (TEM). Fines identified include kaolinite, quartz, montmorillonite, calcite, feldspars, iron-oxides and illite. The fines content of the dunes sands has been shown to profoundly effect the geotechnical properties of the dune sands.     

粉粒含量对砂土强度特性的影响

对含无塑性粉粒的砂土进行了三轴固结不排水试验,结果显示:粉粒含量通过颗粒组成和结构对粉砂强度和变形产生重要影响。在粉粒含量为6%,9%和12%时,松散试样在100 kPa围压下出现了静态液化现象,而粉粒含量增至15%时该现象消失,且随着围压的增大该现象也消失;在同一粉粒含量下,粉砂土残余内摩擦角高于峰值内摩擦角,但残余偏应力值很小,这是由于孔压增长很大,抵消了内摩擦角由?p′提高到?r′的有利影响;稳态线也受粉粒含量的影响,表现在随着粉粒含量的增加,稳态线逐渐向下移动;各粉细砂强度和变形对围压的敏感性强于纯净砂;粉粒含量对孔隙比的影响是含量超出一定值后才变得显著的,这说明粉粒加入到砂骨架中时,并未全部充填至砂粒孔隙中,而是以砂粒间接触点或面上为主,这样的接触结构导致了粉砂具有高体缩性,由此使得粉砂样表现出典型的应变软化特征。     

Effects of particle shape and size distribution on the shear strength behavior of composite soils

The effects of particle shape and size distribution on the constitutive behavior of composite soils with a wide range of particle size were investigated. Two comparable sets of specimens were prepared: (1) mixtures of fines (clay and silt) and an ideal coarse fraction (glass sand and beads), and (2) mixtures of fines and natural coarse fraction (river sand and crushed granite gravels). Direct shear box testing was undertaken on 34 samples and the structure of the shear surfaces, change in volume and water content and the particle shape coefficient of the sheared specimens were examined. The results indicate that the contraction/dilation a specimen exhibits is restrained within the shear zone while the outer zones remain unchanged during shearing. An increased coarse fraction leads to an increase in constant volume shear strength. In addition, increasing elongation or decreasing convexity of the coarse fraction increases the constant volume friction angle. The overall roughness of the shear surface at constant volume state is negatively related to particle smoothness (convexity) and positively related to the area of the shear surface occupied by particles with particular shapes. Two equations are proposed for the estimation of constant volume friction angle based on the proportion and shape coefficient of the coarse fraction. It is hoped this will assist in considering the shear strength of mixed soils when the size of the coarse fraction makes laboratory testing difficult.     

砂土颗粒级配对筋土界面抗剪特性的影响

为了研究砂土与土工合成材料相互作用时筋土界面的抗剪强度以及剪胀特性,采用3种不同级配的砂土分别与土工格栅和土工织物进行室内大型直剪试验,研究不同颗粒级配、密实度、筋材种类以及竖向应力对界面剪切特性的影响,并对界面剪胀系数进行分析。试验结果表明：粗砂和细砂与筋材的界面剪切强度要明显大于粗细混合砂;松砂剪切过程中只有剪缩效应的存在,但密实砂土呈现出明显的剪胀过程;当竖向应力较大时,筋土界面达到峰值剪切强度所需的剪切位移比低应力时大;粗砂与土工格栅作用时达到峰值剪切强度所需的剪切位移比与土工织物作用时大,而细砂则相反。     

Mechanical and dissociation properties of methane hydrate-bearing sand in deep seabed

A series of triaxial tests has been carried out on the mechanical properties and dissociation characteristics of sands containing methane hydrate using an innovative high pressure apparatus which has been developed to reproduce the in-situ conditions expected during proposed methane extraction methods. It was found that the strength of MH sand increased with MH saturation due to particle bonding. Dissociation by heating caused large axial strains for samples with an initial shear stress and total collapse for samples consolidated in the metastable zone. In the case of dissociation by de-pressurization, axial strains were generated by increasing effective stress until a stable equilibrium was reached. However, re-pressurization led to the collapse in the metastable zone.     

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.     

Effect of temperature cycle on mechanical properties of methane hydrate-bearing sediment

In this study, methane hydrate-bearing sand (MHBS) was created in the laboratory following two methods in order to obtain two types of gas hydrate morphology in sandy sediment. The hydrate morphology in the sediment was assessed by measuring the compressional wave velocity combined with models to predict the wave velocities of the sediment containing gas hydrates. The mechanical properties of the MHBS were investigated by triaxial compression tests. The results obtained by the compressional wave velocity show that after saturating the MHBS sediment (created by the excess gas method) with water, the methane hydrates are partly or completely converted from grain contacts to pore spaces depending on the hydrate saturation (ranging from 0 to 50%). A subsequent temperature cycle completes this conversion process for high hydrate saturation. The results obtained with the triaxial compression tests show higher shear strength, a higher secant Young’s modulus, and a higher dilation angle at higher hydrate saturation. In addition, the effects of hydrate saturation on the mechanical properties of the MHBS obtained by the two procedures (with and without the thermal cycle) are similar at low hydrate saturation. The effect of gas hydrate morphologies can only be detected in the case where the conversion (and/or redistribution) of gas hydrates from grain contacts to pore spaces is not complete (at high hydrate saturation).     

Compressive characteristics of methane hydrate-bearing sands under isotropic consolidation

Depressurization is an effective method to produce methane gas from methane hydrate reservoirs. However, during gas production, sediments consolidate due to increasing effective stress. Revealing the compressive characteristics of methane hydrate-bearing sands during consolidation is essential for an accurate understanding of sediment properties and for the development of a constitutive model. Therefore, a series of isotropic consolidation tests was performed on sand in which methane hydrate was artificially generated, and its compressibility characteristics were evaluated. Furthermore, to assess prolonged production, creep compressive behavior was investigated. The experimental results showed volumetric strain due to increasing confining stress decreased with increasing initial methane hydrate saturation. Particle crushing during consolidation was inhibited by the presence of methane hydrate. It was confirmed that the increase in the effects of methane hydrate on soil compressibility followed a power function with the increase in methane hydrate saturation. Creep deformation was observed during the stress holding period regardless of the presence of methane hydrate. Creep behavior during the stress holding period was related to the extent to which the creep component had dissipated before the stress holding period in the past. A theoretical concept for creep strain was proposed based on the experimental results.