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1.
Many geotechnical problems involve undrained behavior of clay and the capacity in undrained loading. Most constitutive models used today are effective stress based and only indirectly obtain values for the undrained shear strength. To match the design profiles of undrained shear strengths, in active (A), direct simple shear (D) and passive (P) modes of loading are complicated. This paper presents the elastoplastic constitutive model NGI‐ADP which is based on the undrained shear strength approach with direct input of shear strengths. Consequently, exact match with design undrained shear strengths profiles is obtained and the well‐known anisotropy of undrained shear strength and stiffness is accounted for in the constitutive model. A non‐linear stress path‐dependent hardening relationship is used, defined from direct input of failure strains in the three directions of shearing represented by triaxial compression, direct simple shear and triaxial extension. With its clear input parameters the model has significant advantages for design analysis of undrained problems. The constitutive model is implemented, into finite element codes, with an implicit integration scheme. Its performance is demonstrated by a finite element analysis of a bearing capacity problem. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
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Circular opening is commonly encountered in wellbore drilling of petroleum engineering, boring for cast-in situ pile installation, and tunneling excavation. This paper presents a rigorous solution for the elastoplastic responses of the anisotropic soft soil mass around a circular opening excavated under undrained and drained conditions. Both the anisotropic elastoplastic behavior and the 3D strength of the soft clay are incorporated in the present solutions. The well-established anisotropic critical state elastoplastic model S-CLAY1, which can represent the initial fabric anisotropy and stress-induced anisotropy of soft soil, is further modified by the Spatially Mobilized Plane criterion to consider the 3D strength of geomaterials. Then, the investigated problems, excavation of a circular opening under both short-term (undrained) and long-term (drained) conditions, are formulated as a system of first-order differential equations and are solved as initial value problems. The distributions of stress components and anisotropy parameters around the opening, the stress trajectory of a soil particle at the opening wall, as well as the stress–displacement curve at the opening wall are presented to investigate the elastoplastic responses of the opening. Extensive parameters show that the overconsolidated ratio and coefficient of earth pressure at rest (K0) have remarkable effects on the elastoplastic responses around a circular opening.
相似文献3.
以静压沉桩后桩周土体的应力状态为初始条件,根据桩周土体孔隙比、渗透系数和有效应力之间的相关性,在考虑固结系数随固结时间变化的条件下改进了轴对称条件下的太沙基固结控制方程。随后,采用分离变量法和离散化分析推导得出了桩周超孔隙水压力消散的半解析半数值解,并将解答与实测数据进行对比验证。在此基础上,采用空间滑动面理论改进的修正剑桥模型（SMP-MCC）定义土体三维不排水抗剪强度,研究了静压桩周土体强度、剪切模量随固结时间的变化规律。研究结果表明：由于解答考虑了固结系数随固结时间的变化,因而与实测结果吻合良好;土体压缩指数与渗透指数之比对土体固结系数和孔压消散速率影响较大;当土体压缩指数与渗透指数之比为1时,土体固结系数保持不变,解答退化为经典的太沙基轴对称固结方程;土体强度和剪切模量随固结时间的增长而逐步增加,固结完成后其值超越了土体原位强度和原位剪切模量。 相似文献
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预制桩的沉桩过程产生了对桩周土体的挤密作用,桩周土体的力学特性发生了相应的改变,这种改变会直接影响预制桩桩基础的承载力。利用有限元计算分析方法对砂性土地基预制桩沉桩过程进行了数值求解,得到了桩周土体的位移和应力变化规律;在应力路径控制的三轴试验中,模拟土体在受到预制桩沉桩影响的应力状态,试验研究了标准砂在再受荷载作用时的力学特性;在荷载传递法中,运用桩周土体性质的变化试验结果,进行了有限差分数值分析,得到了考虑和不考虑挤土作用的单桩P-S曲线。研究结果表明：预制桩的沉桩过程对桩周土中的径向和轴向应力都有明显的影响,对径向应力的影响大于对轴向应力的影响;挤土效应提高了桩周土的剪切模量值和强度,影响范围大致相当;考虑沉桩侧向挤土影响单桩承载特性有明显的提高,现行的预制桩设计偏于安全。 相似文献
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泥岩地基中泥岩的桩侧摩阻力设计参数比较缺乏。通过对吉林省龙华松花江特大桥工程试桩的自平衡静载试验分析,并结合室内桩和泥岩接触中型剪切摩擦试验,揭示了该地区泥岩地基中大直径深长灌注桩泥岩侧摩阻力作用机制及承载特征,得出了一些对泥岩地基中钻孔灌注桩的设计和深入研究具有指导意义的结论。结果表明：大直径深长桩桩端阻力分担的荷载只占总承载力的一小部分,属摩擦型桩。自平衡测试中,全风化泥岩侧摩阻力达到173 kPa,强风化泥岩侧摩阻力达到279 kPa,中等风化泥岩侧摩阻力达到336 kPa,实测值较规范参考值大很多。建议在桩基设计时通过室内桩和泥岩接触中型剪切摩擦试验,基于莫尔库伦理论预估泥岩侧摩阻力,为桩基承载力的确定提供参考。 相似文献
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由于静压桩沉桩后桩周土重塑,静压桩承载力表现出随着休止期的延长而增长的特性。本文从静压桩沉桩后桩周土体内孔隙水消散固结的角度出发,对静压桩承载力时间效应的理论和试验分别进行归纳,结合孔隙水消散路径及固结模型,对桩周土体初始超静孔隙水压力大小及其分布特征进行总结,分析承载力各种测试方法的优缺点,对静压桩承载力的时效性进行深化研究,并探讨了不同地质条件、不同桩的类型对休止期内静压桩承载力的影响,进一步对基于实测数据得出的经验公式进行总结。讨论了基于不同本构关系模型的应力场及位移场解答和沉桩后孔隙水压力消散解答,在此基础上总结了桩基极限承载力理论公式;探讨了黏性土、砂土条件下,考虑超固结比、不排水抗剪强度和塑性指数比对桩基极限承载力系数A的影响,在此基础上归纳了桩基极限承载力经验公式。建议在经验公式基础上设置多重参数,以提升经验公式的精确度,并完善对不同桩、土类型的参数解答;利用BP神经网络,导入静压桩承载力相关参数,以得到针对不同地质条件、桩型、休止期的承载力最优解。 相似文献
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原状和重塑海洋粘土经历动载后的静强度衰减 总被引：5，自引：0，他引：5
根据海洋粉质粘土原状和重塑土样的动三轴试验结果, 比较和分析了两种土样应力、应变、孔隙水压力和不排水抗剪强度行为, 得到了土样的静不排水抗剪强度衰减与波浪荷载作用下土样产生的动应变以及平均累积孔压之间的关系。 并将波浪荷载作用使土样内孔压升高, 有效应力降低, 形成拟超固结现象的理论, 应用到土样不排水抗剪强度衰减与平均累积孔压之间关系的分析;同时与重塑土样的超固结静态剪切试验结果进行比较, 得到了土样在波浪荷载作用后的归一化不排水抗剪强度与拟超固结比之间的关系式。 建议以少量原状土样, 配合大量重塑土样的动三轴试验结果, 实现对实际海洋粘土地基在波浪荷载作用后的静不排水抗剪强度衰化规律的评估。 相似文献
8.
This study presents the response of a vertically loaded pile in undrained clay considering spatially distributed undrained
shear strength. The probabilistic study is performed considering undrained shear strength as random variable and the analysis
is conducted using random field theory. The inherent soil variability is considered as source of variability and the field
is modeled as two dimensional non-Gaussian homogeneous random field. Random field is simulated using Cholesky decomposition technique within the finite difference program and Monte Carlo simulation approach is considered for the probabilistic analysis.
The influence of variance and spatial correlation of undrained shear strength on the ultimate capacity as summation of ultimate
skin friction and end bearing resistance of pile are examined. It is observed that the coefficient of variation and spatial
correlation distance are the most important parameters that affect the pile ultimate capacity. 相似文献
9.
碱渣是氨碱法制纯碱时排出的固体废弃物,经适当的加固处理后作为工程填垫土可以有效解决占地和污染问题。碱渣土的力学性质不稳定,在循环荷载作用下强度会发生明显弱化,导致承载力大幅降低。以天津港地区回填碱渣土为研究对象,开展了系列动三轴试验,以固结围压、动应力比和偏压固结比为试验变量,研究了不同应力组合下碱渣土中循环累积孔压发展规律以及循环后碱渣土不排水强度演化规律。基于等效超固结模型,引入强度弱化系数β,建立了考虑偏压固结比和动应力比影响的碱渣土循环强度弱化模型;引入强度提升系数α,建立了考虑偏压固结过程和循环加载过程耦合影响的碱渣土强度演化模型,并通过试验数据验证了模型的正确性。 相似文献
10.
可液化地基在遇到地震作用或任何其他突变应力条件下,土体会迅速丧失强度和刚度,发生液化,从而失去承载能力。传统的抗液化措施主要集中在地基处理层面,而开孔管桩则兼具抗液化性能和承载能力。在假设开孔管桩孔洞内液体流动满足Poiseuille方程且等应变假设成立的前提下,结合开孔管桩复合地基的应力集中现象,推导出地基在时变荷载作用下的径竖向组合孔压解析解。考虑到循环荷载作用下的孔压积累现象,进一步利用孔压发展的经验模型改进该解析解。在此解析解的基础上,分析、对比开孔管桩、碎石桩、不排水桩以及天然地基的孔压发展情况,得出开孔管桩具有最好的孔压消散能力。桩-土模量比、井径比等因素对孔压的发展都有重要的影响,桩-土模量比越大、井径比越小,孔压消散速率越快。桩本身开孔系数对孔压发展的影响不明显。荷载频率只影响初始孔压峰值,并不影响孔压振荡衰减的速率。 相似文献
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针对浮式海洋结构采用的桩基础,考虑土的循环软化效应,将软土的循环强度与Mohr-Coulomb屈服准则相结合,基于拟静力弹塑性分析建立了循环波浪载荷作用下斜向抗拔桩循环承载性能的计算模型,确定了斜向上载荷作用下抗拔桩的循环承载力,并与单调加载作用下的斜向抗拔桩的极限承载力进行了对比,进一步探讨了桩长、桩径、桩体模量及载荷循环次数等因素对斜向抗拔桩循环承载力的影响。研究结果表明：循环波浪荷载的作用导致了斜向抗拔桩土体循环强度的分布不均匀,从而降低了地基的循环承载力。斜向抗拔桩的动态极限承载力随循环次数的增加而降低,随桩长、桩径及桩体模量的增大而增大。 相似文献
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注浆成型螺纹桩为一种利用施工工艺创新,结合钻孔灌注和二次注浆技术的新型螺纹抗拔桩型,目前已在软土地区开展应用。为了对其受力承载特性深入研究,使该桩型得到广泛推广,通过数值分析方法对其抗拔性能和承载机制进行了三维有限元数值模拟。首先,通过数值模拟桩-土界面室内大型直剪试验得到了有限元分析需要的桩-土接触面参数,而后将得到的参数带入注浆成型螺纹桩抗拔三维有限元数值模型,通过计算得到了不同距径比S/D（即螺距与桩径的比值）螺纹桩的抗拔荷载-位移曲线和轴力分布,并观察了抗拔过程中桩周土体塑性变形的发展。数值分析表明,螺纹桩与桩周土体的机械咬合作用增大了桩侧摩阻力,从而使桩体极限抗拔承载力较等截面圆桩提高约2～5倍;同时,其承载能力与桩体的S/D有关,当S/D取最优时,荷载-位移曲线的初始切向刚度最大,极限承载力最高,桩周土体形成的连续拱形破坏区域最大。 相似文献
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西南地区常见碎石土-基岩斜坡地基,在此类地基上的嵌岩桩基础,其上覆土层、嵌固段基岩多为倾斜。然而岩石试样中结构面倾角改变时,岩石试样的强度也随之发生变化。故当嵌固段基岩存在层面且层面具有倾角时,往往对桩基的水平承载特性影响很大,所以基岩层面是影响嵌岩桩水平承载性能的主要因素之一。本文采用物理模型试验,通过改变嵌固段基岩层面倾角,得出嵌固段基岩不同层面倾角对于桩顶位移,桩身内力的发展规律,进而研究其对水平受荷嵌岩桩承载性能的影响。试验结果表明:在碎石土-层状基岩斜坡地基场地中,嵌固段基岩存在层面会降低嵌岩桩水平承载性能。相对于完整基岩,嵌固段层状基岩存在水平层面时,临界荷载下降了17%、最大弯矩值下降了23%、最大剪力值下降了37.5%;而嵌固段基层层面为倾斜时,嵌岩桩水平承载性能下降的更多,且层面倾角为逆向30°时比顺向30°更加不利于嵌岩桩的水平受荷;桩身最大弯矩点与最大剪力点位置随嵌固段层状基岩倾角变化影响比较小,最大弯矩点位置几乎没有变化,最大剪力点位置在嵌固段基岩层面为顺向30°与逆向30°时下降了1倍桩径。该项研究可为在不同层面倾角下的层状岩体斜坡地基上受水平荷载的嵌岩桩设计作一定的指导。 相似文献
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能源桩传热与承载特性研究现状及展望 总被引：1，自引：0，他引：1
能源桩是一种在传递上部结构荷载的同时获取浅层地热能源的新技术,给桩基结构的设计及安全服役提出了新的挑战。从能源桩实际应用的角度,围绕能源桩传热性能和承载性能两个关键科学问题,从4个方面综述目前的研究现状：（1）能源桩的热传递研究;（2）能源桩的结构响应特征;（3）能源桩的承载变形特性;（4）能源桩的荷载传递机制。在此基础上,对当前能源桩传热模型的适用性、热交换作用下能源单桩及群桩的承载特性以及长期运行条件下的承载性能及结构安全性进行了讨论,最后对能源桩工程下一步研究进行了展望。分析成果对于保证结构安全,合理开发浅层地能具有重要意义。 相似文献
19.
Hanifi Missoum Mostefa Belkhatir Karim Bendani Mustapha Maliki 《Geotechnical and Geological Engineering》2013,31(1):279-296
In a number of recent case studies, the liquefaction of silty sands has been reported. To investigate the undrained shear and deformation behaviour of Chlef sand–silt mixtures, a series of monotonic and stress-controlled cyclic triaxial tests were conducted on sand encountered at the site. The aim of this laboratory investigation is to study the influence of silt contents, expressed by means of the equivalent void ratio on undrained residual shear strength of loose, medium dense and dense sand–silt mixtures under monotonic loading and liquefaction potential under cyclic loading. After an earthquake event, the prediction of the post-liquefaction strength is becoming a challenging task in order to ensure the stability of different types of earth structures. Thus, the choice of the appropriate undrained residual shear strength of silty sandy soils that are prone to liquefaction to be used in engineering practice design should be established. To achieve this, a series of undrained triaxial tests were conducted on reconstituted saturated silty sand samples with different fines contents ranging from 0 to 40 %. In all tests, the confining pressure was held constant at 100 kPa. From the experimental results obtained, it is clear that the global void ratio cannot be used as a state parameter and may not characterize the actual behaviour of the soil as well. The equivalent void ratio expressing the fine particles participation in soil strength is then introduced. A linear relationship between the undrained shear residual shear strength and the equivalent void ratio has been obtained for the studied range of the fines contents. Cyclic test results confirm that the increase in the equivalent void ratio and the fines content accelerates the liquefaction phenomenon for the studied stress ratio and the liquefaction resistance decreases with the increase in either the equivalent void ratio or the loading amplitude level. These cyclic tests results confirm the obtained monotonic tests results. 相似文献
20.
旋喷群桩复合地基承载特性的数值分析 总被引：2，自引：0，他引：2
旋喷桩加固软土地基在各种地基处理工程中得到了广泛应用。对旋喷桩的研究多数集中在其施工工艺的改进上,或者针对单桩的承载特性进行研究,而对旋喷群桩的承载特性则研究不多。根据工程实际情况,采用基于MIDAS-GTS的三维有限元分析技术,通过改变旋喷群桩的布置方式、桩弹性模量、桩长、桩径、桩距等设计参数及桩-土接触面等参数对旋喷群桩复合地基承载特性的影响进行了研究。研究表明：旋喷桩加固软土地基主要减小了地表至桩底深度范围内土体的竖向沉降,对桩底下方的土体沉降基本无影响;提高旋喷桩桩径及材料强度会提高复合地基承载能力;不同旋喷桩布置方式、桩-土之间是否设置Goodman接触面单元对地基承载能力基本无影响。 相似文献