An effective stress analysis of partially embedded offshore pipelines: Vertical penetration and axial walking

A study on quantifying offshore pipeline resistance during vertical penetration and axial walking is presented, based on coupled pore pressure and displacement finite element analysis with the Modified Cam Clay model. Following the validation of the numerical method against published centrifuge test results and limit analysis solutions, we present the findings of a detailed parametric study on the response of partially-embedded pipelines under vertical and axial movements, employing 2-D plain strain and full 3-D soil–pipeline models. Emphasis is put on practical findings, and on proposing simplified expressions for the estimation of the contact enhancement factor and of the equivalent friction factor, that can be used at least for preliminary design purposes.     

砂土海床中海底管道贯入阻力演化特征及细观机制研究

理解海底管道竖向贯入过程是科学评估管道安装期间初始埋深及服役期间安全性和稳定性分析的关键。通过土工离心机模型试验,结合离散元数值分析研究了真实应力水平下不同密实度砂土中海底管道竖向贯入阻力演化特征及细观机制。研究结果显示,对于中密砂,管道贯入阻力曲线主要表现为硬化特征;对于密砂,贯入阻力曲线整体上呈现周期性软化特征,且埋深越大,软化程度越大。离散元计算分析表明,造成该现象的原因是不同密实度砂土中管道贯入的土体流动及破坏机制不同,且贯入阻力的演化与剪切带的形成与发展密切相关。利用现行海底管道设计规范评估密砂中管道埋深时应充分考虑其贯入阻力随深度的演化特征,当管道埋深初步评估大于0.1D(D为管径)时,应结合计算结果上下限值合理预测管道埋深。     

Experimental Study of Bearing Capacity of Granular Soils,Reinforced with Innovative Grid-Anchor System

The pull-out resistance of reinforcing elements is one of the most significant factors in increasing the bearing capacity of geosynthetic reinforced soils. In this research a new reinforcing element that includes elements (anchors) attached to ordinary geogrid for increasing the pull-out resistance of reinforcements is introduced. Reinforcement therefore consists of geogrid and anchors with cubic elements that attached to the geogrid, named (by the authors) Grid-Anchor. A total of 45 load tests were performed to investigate the bearing capacity of square footing on sand reinforced with this system. The effect of depth of the first reinforcement layer, the vertical spacing, the number and width of reinforcement layers, the distance that anchors are effective, effect of relative density, low strain stiffness and stiffness after local shear were investigated. Laboratory tests showed that when a single layer of reinforcement is used there is an optimum reinforcement embedment depth for which the bearing capacity is the greatest. There also appeared to be an optimum vertical spacing of reinforcing layers for multi-layer reinforced sand. The bearing capacity was also found to increase with increasing number of reinforcement layer, if the reinforcement were placed within a range of effective depth. The effect of soil density also is investigated. Finally the results were compared with the bearing capacity of footings on non-reinforced sand and sand reinforced with ordinary geogrid and the advantages of the Grid-Anchor were highlighted. Test results indicated that the use of Grid-Anchor to reinforce the sand increased the ultimate bearing capacity of shallow square footing by a factor of 3.0 and 1.8 times compared to that for un-reinforced soil and soil reinforced with ordinary geogrid, respectively.     

地基渐进破坏及极限承载力的Cosserat连续体有限元分析

利用Cosserat连续体理论和所发展的有限元数值方法,模拟了地基由应变软化引起的以应变局部化为特征的渐进破坏过程,并从等价塑性应变的发展变化,阐述了渐进破坏过程对所能发挥的极限承载能力的影响。结果表明,Cosserat连续体模型能有效地模拟由应变软化引起以应变局部化为特征的渐进破坏现象,对地基等土工结构物有必要进行渐进破坏分析。同时指出,在求解软化型土体地基的极限承载力时,如果仍按传统的极限平衡或极限分析理论进行分析,可能得出偏于危险的结果。     

珊瑚礁地层中PHC桩原位静载试验研究

珊瑚地层是钙质砂和礁灰岩的统称。在珊瑚地层中开展预应力高强度混凝土管桩（PHC桩）的原位堆载测试,分析了桩?珊瑚地质的相互作用规律。采用光纤光栅传感技术采集加载过程中的试桩桩身应变,由计算获取桩身轴力,分析加载过程中珊瑚地层中PHC桩基础的承载特性。试验结果表明,（1）桩?土响应处于线弹性阶段;（2）桩的承载发挥状态与入土深度显著相关,且入土深度超过15倍桩径时桩端阻力发挥峰值;（3）桩的安装方式几乎不影响其在珊瑚地层中的承载性能,但贯入能量的大小则显著影响;（4）入土深度和贯入能量的大小,影响珊瑚地质的破碎程度,颗粒破碎显著影响珊瑚地质中桩的承载发挥特性。通过原位测试可知在珊瑚地质中在贯入深度在15倍桩径以内,入土深度与桩端阻力正相关,与桩侧摩阻力反相关。     

寒区输油管道基于应变设计的极限状态研究

寒区输油管线沿线地质环境复杂,滑坡、冻胀、融沉等自然灾害会导致管道形成大差异变形量.而差异变形量所引起的应力应变行为直接影响管道的安全服役性能,严重时会使管道破坏失效.基于寒区输油管道在实际服役工况下的受力变形条件,充分考虑冻胀效应、油压效应和热应力效应对输油管道的影响,分析了不同长度、壁厚、油压条件下轴向拉伸应变的分布规律及其影响因素.基于轴向应变设计理论准则,建立了上述条件下输油管道的极限服役状态,得到了对应状态下输油管道的许应最大极限冻胀变形量.结果分析表明,采用基于应力的设计准则偏保守,采用基于轴向应变的设计准则能更多的利用管材的变形性能.可为管道的合理设计、安全评价、完整性管理提供一定的理论参考.     

软土中T型触探仪贯入阻力的数值模拟

T型触探仪近年来在国外的海洋软土地基原位测试中得到了广泛应用,但贯入速率、地基土强度的各向异性与渐进软化等因素对于贯入阻力的影响没有得到系统的研究。在大型有限元软件ABAQUS平台上进行二次开发,针对基于Tresca屈服准则的理想弹塑性模型进行了相应改进,使之可以比较合理地反映上述因素的影响,进而对软土中T型触探仪的贯入机制进行了比较系统的数值分析。计算结果表明,贯入速率、地基土强度的各向异性与强度软化效应对于T型触探仪的贯入阻力系数影响较大。通过与有关极限分析上限解的对比分析,在一定程度上验证了有限元分析结果的合理性。     

灌注桩套管高频振动贯入过程中挤土效应研究

随着全套管振动取土灌注桩施工工艺的发展,灌注桩在工程中的应用越来越广泛,但关于灌注桩套管高频振动贯入机制,特别是套管贯入引起的挤土效应研究还不全面。全面介绍了套管高频振动贯入全过程的有限元-无限元耦合模型的建立过程后,对地表隆起、土体侧移及超孔隙水压力等挤土效应的变化规律进行了详细研究。研究结果表明：水平向的挤土位移随套管贯入深度的增加而增大,竖向挤土位移随着套管贯入深度的增加,浅层土表现为隆起量增大,而深层土表现为下沉量增大;最大挤土位移与套管贯入深度存在滞后效应;浅层土体的隆起为水平向应力加载引起竖向应力增加所致,且隆起分界面深度随着动力荷载幅值增大而增大,随振动频率的增大而减小;超孔隙水压力随套管贯入深度增加而增大,随径向距离增大呈指数型衰减。     

青藏高原多年冻土区埋地输气管道周围温度场数值分析

通过建立管道与多年冻土热相互作用的计算模型,利用数值分析方法探究了不同管温（输运温度）工况下冷输天然气管道对管周土体冻融过程和多年冻土热稳定性的影响。研究表明：5℃正温输运天然气管道可造成下覆冻土上限下降约11倍管径,管周多年冻土退化严重;0℃输运会导致管底下部高温不稳定冻土范围扩大,管底土体强度及承载性能降低,不利于保持多年冻土和管道运营稳定性;-1℃和-5℃负温输运可有效提高冻土人为上限,保持管底冻土温度稳定,但-5℃时管道下部土体温度降低明显,可能导致冻胀病害发生。就管周冻土热稳定性而言,在青藏高原多年冻土区采用冷输（负温输送）工艺输运天然气有利于保护管周多年冻土,是可行的。     

砂土中隧道施工对相邻垂直连续管线位移影响的模型试验研究

隧道施工会对邻近管线造成危害,但目前对土体位移与管线位移两者之间的关系还没有清晰的认识。针对这一问题,设计砂土中考虑不同管线管径、埋深及抗弯刚度的3组隧道施工模型试验,分析垂直下穿隧道施工过程中砂土和管线位移规律。研究结果表明,Vorster修正高斯公式能较好地拟合砂土沉降分布,其控制参数?值在0.2～1.0之间变化,且与地层埋深成正比;土体沉降槽宽度系数i对管线变形有较大影响,埋深相同的条件下管线抗弯刚度与沉降值成反比;深埋管线的变形主要受上拱效应支配,且管径越大上拱效应越明显,而下拉效应主要支配着浅埋管线的位移;Smax /i为影响管土相对位移一个关键参数,在此基础上提出了修正的管土相对刚度计算公式。