考虑砂土渗透性变化的吸力锚沉贯及土塞特性研究

吸力锚负压沉贯过程中锚内部常常产生土塞现象,这将阻碍锚体的进一步沉贯,影响吸力锚在海床土体中的安装深度,最终导致吸力锚承载力的降低,本研究通过1g条件下室内模型试验研究土塞的形成机理。通过试验模拟吸力锚水下砂土中吸力沉贯过程,借助高分辨率相机记录锚体整个沉贯过程,同时利用微型孔压传感器测量锚内外负压值随沉贯深度变化情况。试验过程中发现吸力锚内部土柱在向上渗流作用下发生细颗粒迁移现象,且随着沉贯深度的增大,土塞隆起的高度逐渐增加。提出土塞的形成是由于吸力锚内部土体膨胀引起的,进而引起渗透系数的增加。本研究分析了渗透系数在锚体沉贯过程中的变化情况,并对Houlsby和Byrne提出的吸力锚砂土中沉贯吸力计算模型进行了改进,从而获得更加准确的沉贯吸力随沉贯深度变化关系理论计算方法,为吸力锚的工程设计和施工提供理论依据。     

粉土中吸力式桶形基础沉贯及抗拔特性试验研究

吸力式桶形基础的负压沉贯控制和抗拔承载力确定是海洋钻井平台和海上风电机组多桶基础以及深海吸力锚基础设计和施工中的关键问题。通过室内大比例模型试验研究了吸力式桶形基础在饱和粉土中的负压沉贯及抗拔特性。负压沉贯试验结果表明负压并不能明显减小吸力式桶形基础在粉土中的沉贯阻力,基于CPT试验结果可较为准确地预估沉贯施工所需负压,从而确保沉贯的顺利实施和防止沉贯过程中地基发生管涌破坏。不同加载速率下的上拔试验结果表明存在一临界加载速率,当实际加载速率超过该临界值后,加载速率对吸力式桶形基础的抗拔特性影响较小。结合桶体及桶外土体的变形,提出了粉土地基中不同受力状况下吸力式桶形基础的抗拔承载力计算方法。     

基于CFD-DEM流固耦合方法的吸力锚基础负压沉贯数值模拟

吸力锚基础作为一种新型高效的海洋结构物基础形式在海洋工程中得到了广泛应用。对于砂土中的吸力锚沉贯研究，传统试验或有限元数值模拟存在一定局限性。采用基于离散单元法与计算流体动力学理论（CFD-DEM）流固耦合方法对吸力锚在砂土中的吸力贯入过程进行数值模拟分析；通过与室内物理模型试验、沉贯阻力理论解析计算结果进行对比分析，验证CFD-DEM流固耦合方法的有效性和准确性。进一步地，从土颗粒细观尺度上深入分析吸力锚在砂土中沉贯特性。数值模拟捕捉到贯入过程中锚内土塞和砂层变化现象，发现砂层呈现出中间向上凸起的弧状分布，说明贯入产生挤土效应，其造成的土体位移和膨胀也是土塞产生的原因。同时模拟得到贯入过程中的超孔隙水压力等势线分布变化情况和锚内砂层水力梯度的变化情况，证实了砂土中吸力贯入的“安全机制”。最后将数值所得的锚外负压比变化与Houlsby和Byrne理论模型计算结果进行对比，得到锚内土体渗透系数随沉贯的变化规律。通过本研究验证，采用离散元法结合计算流体动力学理论有效地模拟和捕捉吸力锚沉贯过程中的细观土水相互作用、宏观土体变形及渗流场分布等现象，该方法可作为吸力锚沉贯特性研究的一种有效手段。     

粉砂中筒型基础沉贯过程筒–土作用机理试验研究

筒型基础沉贯安装是一项关键施工过程,目前相关研究多集中在沉贯阻力与吸力的计算方法上,而缺乏对沉贯过程筒–土作用机理研究。针对这一问题,开展了粉砂中筒型基础沉贯试验,得到了静压和吸力沉贯工况下筒壁内、外侧所受土压力发展变化规律。静压试验结果表明,筒壁内侧所受土压力远大于外侧,内侧土体挤压程度随下沉深度增加而增大,外侧相反。吸力试验结果表明,施加吸力时内侧土压力减小,外侧土压力先增大后迅速减小至稳定值,吸力产生的渗流可以大大减小端阻力与内侧摩阻力。试验结果还表明现有临界吸力计算公式是过于保守的,并根据试验结果对现有计算所需吸力的方法进行了修正,得到了一种更加准确的计算方法。     

分层土中吸力基础沉贯吸力变化及土体变形规律试验研究

吸力基础近年来在海上风电工程中逐渐得到推广,我国海上风场地基广泛分布分层土,研究吸力基础在分层土中的沉贯特性有助于推动其在我国海上风电工程领域的应用。开展模型试验,探讨了土层分布形式(砂土、黏性土、上层砂土下层黏性土(简称上砂下黏)、上层黏性土下层砂土(简称上黏下砂))对吸力基础沉贯吸力值、沉贯阻力、土塞高度和土体变形的影响。研究发现在上砂下黏土层中,当基础贯入至土层分界处时,基础吸力值陡然增加;当吸力基础贯入至上黏下砂土层分界处时,基础内部吸力值陡然降低,通过有机玻璃吸力基础模型试验阐述了吸力变化的原因。不同土层条件下,基础内部最终土塞高度规律由高到低为：上黏下砂土层、单一砂土层、上砂下黏土层、单一黏性土层,阐明了不同土层分布情况下基础内部土塞形成机理。对于单一砂土层和上砂下黏地基,沉贯结束后,基础周围土体出现环形沉降区域,单一砂土地基中沉降区域范围大于上砂下黏地基情况。对于单一黏性土和上黏下砂土层中,沉贯结束后基础周围黏性土中产生裂缝,且吸力沉贯下土体裂缝数量及裂缝扩展范围大于压贯条件,土体最大裂缝范围约为1.9倍基础直径。     

新型组合动力锚安装性能现场试验研究

动力锚是近年来海洋工程领域发展的一种锚固基础,依靠自身重力势能贯入海床土中,用来锚固浮式结构。本研究针对一种自主研制的新型组合动力锚(板形锚+助推器),在已有室内模型试验和数值模拟的基础上,开展现场大比尺试验(海试)来验证考虑真实环境荷载作用下锚的安装性能,并分别探究了板形锚形状、助推器重量以及组合锚在水中的释放高度对最终沉贯深度的影响。试验结果表明:在风浪流等环境荷载作用下,组合动力锚仍然能成功贯入海床土中,安装结束后锚轴线相对铅垂方向的偏角在8°以内。根据海上试验结果建立了基于锚的总能量的沉贯深度预测方法,可快速预测锚在海床中的最终沉贯深度,以指导工程设计和施工。     

黏性土中裙式吸力基础沉贯与注水拔出试验研究

开展模型试验研究海洋黏性土中裙式吸力基础沉贯和注水上拔特性,并研究了土体强度、基础尺寸和安装方式等影响因素。研究表明:主桶长径比为1.0和2.0的裙式吸力基础最终沉贯深度较相同高度传统吸力基础仅降低2%和6%,证实了裙式吸力基础在黏性土中具有良好沉贯性。主桶和裙结构在吸力沉贯时对土体造成扰动,导致吸力沉贯阻力小于压力贯入时的阻力。基于极限平衡方法,提出了裙式吸力基础在黏性土中的沉贯阻力与所需吸力的计算公式,并验证其准确性。得到传统和裙式吸力基础在注水拔出过程中基础内部水压力、上拔阻力与基础上拔位移之间的关系,发现基础内部水压力随上拔位移先迅速增加至最大值,然后逐渐降低,裙式吸力基础最终上拔位移小于相同基础高度的传统吸力基础。得到了裙式吸力基础注水拔出阻力计算公式。     

桶基在粉土中负压沉贯过程的试验研究

主要通过模型试验对粉土中负压沉贯过程的失稳机制进行了研究,并探讨了不同比尺桶基的沉贯效应,根据模型试验结果对不同比尺桶基的沉贯行为进行了论述,并初步分析了沉贯行为效应产生的原因.     

砂土中静压挤土桩周压力的监测与研究

在城市中,传统锤击桩等动力沉桩产生了包括噪音、振动等环境问题。而静压桩由于其对环境影响比较小等特点得到了越来越广泛的应用。通过模型试验研究了静力压桩过程中桩周砂土的侧向土压力的发展规律。文章监测了整个沉桩过程中几组测点的土压力的发展,当桩尖向下刺入接近测点时,土压力急剧增长;当桩尖继续下沉到测点以下,测点处的土压力又急剧减小;当桩体尚未刺入到测点深度的时候,测点处的土压力就会提前达到峰值。由沉桩引起的侧向土压力最大值沿径向呈指数函数衰减,而随深度呈近似的线性增长。模型试验对比了不同沉桩速度所需的沉桩贯入力,发现沉桩速度越快,需要的沉桩贯入力越大。沉桩速度的快慢对桩周土体压力的产生没有明显影响。     

吸力锚土塞在粉质粘土中形成的模型试验研究

介绍在室内粉质粘土土槽中进行吸力锚土塞形成试验 ,给出了部分试验结果。通过模拟海上吸力锚沉放过程 ,研究土塞形成和发展的规律及模型沉放过程中压力差、沉放速度等因素对土塞形成产生的影响。并对一些试验现象和结果进行了初步分析 ,得出了一些结论。     

饱和细砂中裙式吸力基础水平单调加载模型试验——承载力及变形分析

海上风电资源的开发和利用是当今世界关注的热点问题,作为其塔架的基础,主控荷载是水平荷载。裙式吸力基础具有更高的水平承载能力和控制水平位移的能力,故非常适合作为海上风电塔架的基础。通过饱和细海砂中裙式吸力基础的水平单调加载模型试验,探究基础水平承载力的影响因素及转动点位置的变化规律,并分析了地基土的变形影响范围及规律。研究发现：与传统吸力基础相比,裙式吸力基础的水平承载力提高显著,且能有效控制水平位移;水平承载力随基础的裙高、裙宽的增加而增大,随加载高度的增加而减小;在水平荷载作用下基础主要是绕某一点（即转动点）发生转动,转动点位于主桶长度的0.45～0.7倍之间;达到极限荷载时,地表隆起范围远远大于沉降范围,沿加载轴线方向,隆起范围约为2.5倍主桶直径。     

砂土中螺旋锚上拔承载特性模型试验研究

螺旋锚基础因其能够利用深层土体抗力且具有快速安装和承载的优势而广泛应用于各类岩土工程问题中。多锚片螺旋锚上拔承载特性受埋深、锚片间距、数量、土质条件等因素影响。相邻锚片相互影响导致土体破坏区域重叠,从而影响破坏模式和极限承载力,然而多锚片螺旋锚承载特性的理论及试验研究有限。针对砂土中螺旋锚锚片间距及数量对上拔承载特性及极限上拔承载力影响进行室内1g模型试验研究。结果表明,在中密砂及密砂中,单锚埋深比分别超过6.0和10.5时可认为是深埋锚。中密砂中深埋多片螺旋锚锚片间距在3.0D~4.5D时,各锚片承载能力能够独立发挥,承载量破坏模式发生;密砂中浅埋多片螺旋锚保证承载量破坏模式的锚片间距超过6.0D,但间距为6.0D时,螺旋锚发挥效率超过90%。增加锚片数量可适当提高上拔承载力,但当锚片数量增加使得锚片间距小于某一临界值时,柱状破坏模式发生,螺旋锚承载力不再增加;中密砂中此临界间距约为1.5D,密砂中临界间距约为2.0D。     

Tragstruktur und Installation der Offshore‐Windenergieanlage Repower 5M

Design, fabrication and installation of the offshore wind turbine REpower5M. In summer 2006 the first 5 MW offshore wind turbine has been installed in 45 m water depth in Scotland. The large water depth and other difficult site conditions, like harsh wave climate and unfavourable soil conditions, made numerous innovations for the support structures and installation necessary. This paper describes design, fabrication and installation of the jacket structures which have been developed. Furthermore the installation methodology for the turbine for which a floating crane instead of a fixed platform was used, is described. The knowledge gained from this project will enable realization of offshore windfarms in water depth beyond 20 m.     

Seasonal influence on cone penetration test: An unsaturated soil site example

Interpretation of electric cone penetration test (CPT) based pore water pressure measurement (CPTu) is well established for soils with behavior that follows classical soil mechanics. The literature on the interpretation of these tests performed on unsaturated tropical soils is limited, and little is known about the influence of soil suction on in situ test data. In this context, the CPT data are presented and discussed to illustrate the seasonal variability in an unsaturated tropical soil site. The test data show that soil suction significantly influenced CPT data up to a depth of 4 m at the study site. It shows the importance of considering seasonal variability in unsaturated soil sites caused by soil suction, which was related to water content through a soil-water retention curve (SWRC). It is also important to consider this aspect in the interpretation of CPT data from these soils.     

Winkler springs for axial response of suction bucket foundations in cohesionless soil

Jacket structures mounted on suction buckets carry potential as cost-efficient foundations for next-generation 10 MW+ offshore wind turbines located in transitional water depths. Given this foundation method, resistance to overturning moment relies mainly on the axial response of the buckets. In practice, suction bucket foundations can be modelled as “Beams on Non-Linear Winkler Foundation” where soil is idealised as non-linear springs. This paper describes the derivation process of static load-transfer or t–z curves for suction buckets installed in cohesionless soil. The mathematical formulation of the curves is based on regression analysis of data obtained from 100 axisymmetric numerical models in a medium characterised by the Hardening Soil model for representing the stress–strain relationships for Frederikshavn sand, which is a typical offshore sand. Various bucket dimensions, soil properties and drainage conditions were simulated considering tension and compression, in order to describe frictional behaviour at the skirt–soil interface. The non-linear springs' properties are therefore linked to foundation diameter, friction angle and vertical overburden pressure. By superimposing the effects of all springs, load–displacement curves are generated and compared with results from available experimental and numerical studies on suction buckets, revealing reasonable agreement. It is shown that the existing t–z formulations for piles are inapplicable to large-diameter suction buckets.     

Mechanical and durability properties of concrete made with dredged marine sand

The process of depletion of sources of natural aggregates challenges the production of technically and environmentally adequate concrete. Alternative material from marine sources, especially for concrete maritime structures, could represent an acceptable solution for this problem and it might also be of great interest for port management. This research work describes the study of the mechanical and durability properties of concretes fabricated with dredged marine sand (DMS) as a fine granular corrector in partial substitution of raw sand (from 15% to up to 50% by raw sand mass) designed for harbor pavements. Three DMS samples were extracted from the Port of Barcelona. The material was stockpiled in the open air and no washing, drying or decontamination process was carried out. Mineralogical, physical and chemical properties of DMS material were determined. Eight different mixtures were produced incorporating three types of DMS material as granular corrector in partial substitution of crushed limestone raw sand. The concretes were submitted to compressive strength tests after 7 and 28 days of moist curing, as well as density, absorption, accessible pores, elastic modulus, tensile and splitting tensile strength, abrasion, capillary suction, water penetration under pressure and ultrasonic pulse velocity (UPV) tests, all of them after 28 days of moist curing. This study shows that dredged marine sand can be successfully used as a fine aggregate for concrete production. This is justified by the similar physical and mechanical properties of concrete made with DMS comparing to reference concrete. It was verified that the use of DMS in substitution of raw sand maintained or reduced the accessible pores, the sorptivity and the water penetration depth under pressure.