自升式钻井平台桩靴贯入饱和砂土的机制研究

为研究桩靴贯入饱和砂土的承载机制以及桩靴周围砂土变形机理，开展桩靴的抗压承载土工模型试验以及桩靴-土体相互作用的透明土试验，测得了桩靴荷载-沉降变化规律、桩靴周围砂土的位移向量场和等值线图，初步探讨了桩靴贯入饱和砂土时的承载机制与桩靴周围砂土变形机理。基于圆孔扩张理论及分段位移迭代算法，推导出静载作用下桩靴荷载-沉降变化规律；与试验结果的对比发现：计算误差约为11.7%。通过浅应变路径法(SSPM)计算得到桩靴周围土体位移理论值；与试验结果对比发现：计算误差在16.7%～26.3%。     

基于PIV技术的沉桩过程土体位移场模型试验研究

基于粒子图像测速(PIV)技术,利用自行设计的静压桩自动沉桩模型试验系统,对沉桩过程中桩周土体位移场进行测量。在沉桩过程中,利用CCD(charge coupled device)高速工业相机连续拍摄试验观测面上桩周土体变形的灰度图像,然后对初始点和峰值点进行分析,得到位移场分布。研究了不同沉桩深度、桩-土摩擦系数和桩尖形式情况下沉桩对周围土体位移场的影响规律;相应试验结果与圆孔扩张理论解进行对比分析,其径向位移变形的规律基本一致,验证了模型试验的可靠性。相对常规试验方法,该试验操作简便,对环境要求不高,可以进行非插入式全场测量。研究结果表明:在该文试验条件下,对于平底圆桩,沉桩过程对桩侧土体位移影响范围在8R附近,对桩端底部土体影响范围在4R附近;桩身摩擦的存在使得桩侧向土体位移影响范围增大2R左右,对桩端底部土体位移基本没有影响。当桩尖角度由0°变化为45°时,桩侧的水平和竖向位移均增大,桩端底部的竖向位移明显减小;而桩身摩擦对其水平位移和竖向位移影响不大。     

无砟轨道端刺结构纵向位移的解析算法及应用

考虑高速铁路纵连板式无砟轨道底座板、摩擦板、端刺结构及路基土体的相互作用关系,深入分析了端刺顶部水平力的传递机理,借鉴水平荷载及弯矩共同作用下单桩承载力及变形的计算公式,推导出适用于计算端刺结构纵向受力与变形的解析算法。该方法中,底座板与摩擦板间“两布”滑动层的摩擦力假定为均匀分布,摩擦板和端刺间传递纵向力和弯矩且满足变形协调条件,路基土体对端刺侧面均匀支撑。解析算法的计算结果和基于有限元软件ANSYS 所建立的空间耦合模型具有较好的一致性,说明该文提出的解析算法正确可靠,可实现纵连板式无砟轨道端刺结构纵向位移的快速准确计算。端刺结构计算结果表明:随着大端刺深度、宽度及小端刺数量的不断增加,端刺纵向位移会不断减小,有利于减小底座板的纵向位移及土体的压缩变形。     

复杂地层中自升式平台插桩的数值模拟

以胜利9号平台在老174号井位插桩过程为例,基于RITSS大变形数值计算方法,建立了自升式平台桩靴贯入多层地基的有限元模型,研究该平台插桩过程中的土体变形以及地基承载力随深度的变化规律。与SNAME（美国造船与轮机工程师学会）规范推荐的桩靴插桩过程地基承载力公式的计算结果进行对比,规范结果与数值结果的规律较为一致,数值上相差10%,验证了RITSS方法研究桩靴在软硬相间地基上插桩的有效性。结果表明:RITSS数模方法可以有效地模拟自升式平台插桩过程中各个土层的变形、土体流动,并得到合理的地基承载力结果;对于砂土层下卧软土层地基,可以得到承载力峰值。     

饱和粘土中挤土桩球形孔扩张的弹塑性分析

将桩体的贯入模拟为球形孔扩张过程,基于球形孔扩张理论,在大变形假定情况下,结合修正剑桥模型及水力压裂理论,由最小耗能原理从能量的角度推导出了饱和粘土中挤土按球形孔扩张后,土体整个范围内,包括弹性区、塑性区、破坏区内的应力、位移及超孔隙水压力分布的解析解,并分析了土的超固结比对结果的影响,和其他文献的比较结果证明了理论解析解的准确性。其结论可为饱和土地区的静压桩、打桩等岩土工程问题提供理论依据。     

防挤槽在沉桩挤土中效果的有限元模拟

本文基于有限元软件,建立了考虑桩土相互作用、几何大变形、材料非线性的沉桩挤土有限单元法模型,得到了设置防挤槽对沉桩后桩周土体位移和应力分布规律的影响。防挤槽槽前土体水平向失去约束,在桩的挤压作用下,土体运动主要表现为水平位移,竖向则表现为轻微下沉;防挤槽的存在能有效地减少槽后的土体位移,但在槽深范围内效果明显,而槽底以下的挤土位移场和无槽时差别不大。     

轴向荷载作用下单桩沉降的非线性分析

将单桩简化为受侧摩阻力的弹性杆单元,土体简化为均质弹性半空间无限体。以桩顶荷载作用下桩单元的变形、结点位移及结点力作为中间变量,依据弹性半空间无限体的Mindlin解以及桩-土相对滑动的性质,建立了土体的位移方程、接触面的平衡方程,并以此构造了以结点位移为基本未知量的非线性方程组。求解包含桩顶位移在内的各结点位移后,可进一步计算单桩及接触面的内力、土体的位移等。考虑了桩-土相对滑动及桩底沉降的非线性性质,所建立的桩-土体系分析模型力学机理简单、明确,便于应用,分析结果与试验结果较为符合。     

An elastic mechanics model and computation method for geotechnical material

岩土材料内摩擦性质是岩土的基本力学性质之一,无论岩土处于何种受力状态,都应考虑岩土体的内摩擦力。然而,至今只有岩土极限分析与塑性力学中考虑岩土体的内摩擦力,而在弹性理论与能量理论等诸方面均未体现。认为岩土体无论是处于塑性状态还是弹性状态,都存在着内摩擦力,为此建立岩土材料弹性力学的摩擦体力学单元。基于土体试验提出黏聚力先发挥,摩擦力随变形逐渐发挥,并假设摩擦因数与应变成正比,由此确定摩擦力的计算,最后仿效线弹性力学计算方法,但此时摩擦体的剪切模量G已非常数,从而形成摩擦体的非线性弹性力学计算方法。算例表明,按该方法计算出的弹性地基上的位移和剪应力小于传统方法计算出的位移和应力值,这比较符合实际情况,表明采用摩擦体力学单元对岩土材料是合适的。     

海洋环境下桩-土相互作用问题的数值解

考虑波压力对海床土体应力状态的影响,基于Biot固结理论及虚拟桩技术对海洋桩-土相互作用体系进行了数学建模,并应用FlexPDE对所建立的数学模型进行了数值求解,得到了作用在桩体上的侧向土压力、桩体周围土体有效正应力以及自由场土体相对水平位移和相对垂直位移的分布,这些结果可为海洋桩基的设计与监测提供依据。     

基桩承载力动态测试的最小击振能量研究

经长期对桩的承载力的现场测试研究发现，动力打桩公式在测试贯入度时无法考虑土体对桩的影响，而“低应变”法的击振能量太小，同时要计算“参振系统”的参振土体的质量，特别是，该计算方法又没有足够的理论依据。另一方面，“高应变”法对一般单桩的击振能量：赶大导致桩士系统进入较大的塑性破坏。因此，采用一维单自由度模式分析系统最小击振力，利用一维杆件振动模式导出桩土系统的0阶与1阶振动频率（准静态），得出桩顶振动位移与桩土系统最小击振力的函数关系，能够克服“低应变”法和“高应变”法存在的缺陷。实践检验表明，所提出的测试方法能较好地解决桩基础承载力测试的工程应用问题。     

Influence of pile shape and pile interaction on the crushable behavior of granular materials around driven piles: DEM analyses

This study presents an analysis and a visualization of the effect that the pile shape has on the penetration resistance of driven piles in crushable granular materials. Discrete Element Method (DEM) simulations of single piles with different shapes (flat tip, open pile, triangular tip) being driven into a previously compacted uniform crushable soil are presented. The results from the DEM simulations showed that the shape of the driven piles has a significant influence on the development of penetration resistance and particle crushing. This study also presents the penetration resistance and particle crushing results when a second flat tip pile was driven in a region near a pre-existing single flat tip pile. It was found that considerable high crushing was induced by the second pile. The second pile induced crushing not only on the particles surrounding itself but also on the particles surrounding the pre-existing pile, showing that particle crushing around a driven pile not only takes place when the pre-existing pile is being driven, but it also occurs during the installation of a second pile, in a region closely located to the first one.     

PSO优化的SVM旋压触探岩土识别方法

针对现有的旋压触探测量精确度不高,没有准确的土力学模型的问题,该文提出基于支持向量分类机的岩土识别方法,并且用粒子群寻优算法来确定支持向量机的惩罚因子和核函数参数。首先,根据旋压触探勘测的实际情况,选取锥尖阻力、侧壁阻力、摩阻比、探杆倾斜角、岩土温度及湿度等参数构建旋压触探分析的指标决策体系;其次,基于粒子群寻优算法和支持向量分类机,建立岩土测试指标和土力学指标映射体系;最后在天津某地进行5次实地勘测,通过得到的静力触探和旋压触探的711组对比数据,进行该方法的验证,其分类结果准确率大于96%,符合实际工程的需要。     

考虑温度效应的软土固结蠕变耦合模型

软土的固结蠕变是软土地基及构筑物长期变形和稳定性的重要影响因素。在涉热岩土工程兴起和逐渐发展的背景下,研究软土在热力耦合作用下的固结蠕变特性并建立力学模型,对于评估涉热工程软土场地长期变形及稳定性具有重要意义。对此,针对典型滨海软土开展热固结蠕变试验,得到热固结蠕变特性及相关参数取值,建立考虑温度效应的固结蠕变经验公式。结合弹黏塑性理论和热固结理论,推导考虑温度效应的固结蠕变耦合力学模型。在合理性验证基础之上开展算例分析,结果表明:新型热固结蠕变耦合模型能反映土体变形的弹性、塑性、黏性和热胀冷缩属性,能描述土体的热孔压、热回弹和热沉降现象,还能分析软土长期变形的温度效应和时间效应;该成果为软土场地涉热岩土工程问题分析提供了可供参考的理论基础。     

超前微桩复合土钉支护稳定及变形简化计算方法

超前微桩复合土钉支护是一种经济有效的基坑支护形式, 但对其设计分析方法的研究远落后于工程实践。该文在对此种复合土钉支护的构造及工作性能进行简要分析的基础上, 对其整体稳定验算方法和变形计算方法进行讨论, 借鉴已有的一些相关研究提出了相应的简化计算方法。针对其整体稳定验算, 提出考虑微桩-土相对刚度采用不大于2.5倍特征长度上土体被动抗力来考虑微桩作用的建议, 从而给出一种较为简便的整体稳定验算方法, 就一些实际工程的计算对比表明所建议的方法合理可行。针对其坑壁位移计算, 在仔细分析此种支护体系受力变形机理的基础上, 基于基坑支护计算的增量法及弹性地基梁求解的链杆法, 提出一种思路清晰的简化计算方法, 给出了具体的计算模型、参数确定方法及问题求解方法, 并通过与有限元计算及工程实测的对比对所提出的方法进行了检验。     

Effect of particle size distribution on pile tip resistance in calcareous sand in the geotechnical centrifuge

Until recently, the micro mechanical origins of soil behaviour have remained illusive, but it is now known that that the constitutive behaviour of a soil is largely determined by its particle size distribution. This paper examines the specific boundary problem associated with the penetration of a model pile into two different gradings of dry calcareous sand in a geotechnical centrifuge, in order to establish the effect of the inclusion of fine particles on the pile end bearing resistance. The first grading of sand comprised particles smaller than 0.5 mm; the second grading contained particles of nominal size d such that 0.15 mm < d < 0.5 mm. Each test was performed on each of two samples of each grading. Tip resistance was observed to rise to a peak at shallow depths, and then fall; a micro mechanical explanation is presented for this instability. Following the centrifuge tests, particles were retrieved from the centres of the soil samples, where the pile had previously been driven, for subsequent particle size analysis. It was found that insignificant crushing had occurred in the sand retrieved from depths less than the depth of peak resistance, but that significant crushing had occurred in the sand retrieved from greater depths. The peak in tip resistance was a factor of two larger for the well-graded sand, but the ultimate tip resistance at greater depths was found to be approximately independent of the initial particle size distribution for all four tests. A micro mechanical explanation is also proposed for this observation. Received: 11 November 1999     

Classical and non-classical kinematic fields of two-dimensional penetration tests on granular ground by discrete element method analyses

The penetration test is a widely used in-situ test in geotechnical engineering which mechanism is very important to geomechanics. This paper presents a numerical study on both classic and non-classic kinematic fields in penetration tests on granular ground. A two-dimensional Discrete Element Method (DEM) has been used to simulate penetration tests on a full-size granular ground that is under an amplified gravity and under a K ₀ lateral stress boundary. In addition to classical kinematic variables, i.e. displacement and velocity, a non-classical kinematic variable called the average pure rotation rate (APR), which represents particle sizes and particle rotations (M. J. Jiang et al. Kinematic models for non-coaxial granular materials: Part I: theories. Int J Numer Anal Methods Geomech 2005; 29(7): 643–661), is investigated in the penetration test. The DEM numerical results show that the penetration leads to significant changes in displacement, velocity and APR fields, making the soil near the penetrometer move in complex displacement and APR paths. In comparison to velocity field, APR field is very ‘localized’ in the area close to the penetrometer shoulder during penetration. Based on the normalized tip resistance, the penetration process can be described by three phases of penetration, in which the granular ground undergoes three types of failure mechanism, respectively.     

层状粘性土及砂土地基中静力压桩连续贯入的数值模拟

总结分析了软硬交互层状土中静压桩沉桩阻力的变化规律。为了使ABAQUS模拟层状土地基,特别是既有粘性土层又有砂土层的层状地基中静压桩贯入,分段采用不同本构关系模型,即在粘性土层中采用修正剑桥模型,砂土层中采用扩展D-P模型。同时,采用多项措施对层状土地基土体初始应力进行平衡。结合工程实例,模拟计算了不同土层性质差异明显、软土中有硬土夹层的层状土中静压桩连续贯入过程,得出了贯入过程中沉桩阻力随深度变化曲线,反映了通过硬土层时桩周应力和沉桩阻力的突变现象,与实测资料较吻合。研究结果有助于静压桩沉桩可能性分析以及挤土效应分析。     

Plastic notch stress intensity factors for pointed V-notches under antiplane shear loading

The paper deals with a work-hardening, elastic–plastic, stress analysis of pointed V-notches under antiplane shear deformation loading both under small and large scale yielding. Stress and strain field intensities are expressed in terms of plastic Notch Stress Intensity Factors, which are analytically linked to the corresponding linear elastic ones under small scale yielding. The near tip stress and strain fields are then used to give closed-form expressions for the Strain Energy Density over a circular sector surrounding the notch tip, and for the J-integral parameter, both as a function of the relevant plastic NSIFs, these expressions being valid both under small and large scale yielding.