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地震引起的土体液化和地基失效对岩土工程师而言仍是一个热点问题。地震液化及地基变形可以采用多种地基加固方法防治,碎石桩技术是常用方法之一。碎石桩复合地基抗液化效用主要是增加桩周土体的密度、桩体的排水以及桩体分担地震水平剪应力作用(桩体减震作用)。目前,以抗液化为主的碎石桩复合地基的设计以及效果评价方法仍只考虑加密作用。首先通过3个模型(1个饱和砂土地基模型、2个碎石桩复合地基模型)的振动台试验研究抗液化碎石桩的减震作用。然后以试验记录的模型动力反应以及建立的理论模型为基础,分析碎石桩复合地基的桩体减震作用。试验及理论分析结果表明,复合地基中的碎石桩可以明显地降低作用在桩间可液化土上的地震剪应力。 相似文献
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《岩土力学》2020,(3)
为研究水平和竖向(双向)耦合地震作用下液化场地群桩基础的动力响应,设计了可液化地基-群桩基础-框筒结构动力相互作用体系振动台模型试验。选取不同类型模拟地震波作为振动台试验激励,通过对比水平地震作用和双向耦合地震作用下土体加速度、超孔隙水压力和群桩应变等试验结果,进而分析双向耦合地震作用对可液化地基和群桩基础动力响应的影响。研究结果表明:双向耦合地震作用下,液化场地土体竖向加速度峰值随土体埋深高度的减小而逐渐增大;饱和砂土的液化效应与双向耦合地震作用和输入地震波的类型有关;相比水平地震作用,不同种类波双向耦合地震作用下群桩基础桩身中部和底部的应变峰值增大,桩顶应变峰值变化略有不同;双向耦合地震作用加剧了建筑结构群桩体系的摇摆和倾斜。研究结果对可液化地基上群桩基础的抗震设计和防灾减灾具有十分重要的研究意义。 相似文献
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《岩土力学》2021,(3)
为研究水平和竖向（双向）耦合地震作用下液化场地群桩基础的动力响应,设计了可液化地基—群桩基础—框筒结构动力相互作用体系振动台模型试验。选取不同类型模拟地震波作为振动台试验激励,通过对比水平地震作用和双向耦合地震作用下土体加速度、超孔隙水压力和群桩应变等试验结果,进而分析双向耦合地震作用对可液化地基和群桩基础动力响应的影响。研究结果表明：双向耦合地震作用下,液化场地土体竖向加速度峰值随土体埋深高度的减小而逐渐增大;饱和砂土的液化效应与双向耦合地震作用和输入地震波的类型有关;相比水平地震作用,不同种类波双向耦合地震作用下群桩基础桩身中部和底部的应变峰值增大,桩顶应变峰值变化略有不同;双向耦合地震作用加剧了建筑结构群桩体系的摇摆和倾斜。研究结果对可液化地基上群桩基础的抗震设计和防灾减灾具有十分重要的研究意义。 相似文献
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砂土液化问题一直是土动力学与岩土地震工程研究领域的重要课题之一。基于南水北调中线某工程,通过现场和室内试验获取土体的物理力学参数,利用岩土数值分析软件FLAC3D对夯扩挤密碎石桩加固干渠液化砂土地基进行了动力数值分析。结果表明,由于夯扩挤密碎石桩的排水作用,干渠底部饱和砂土地基中的超静孔隙水压力和孔压比与加固前相比明显减小;干渠渠道底部饱和砂土中的监测曲线表明,随着地震荷载持续时间的增加,饱和砂土地基中超静孔隙水压力和孔压比峰值较加固前大幅值降低,且时程曲线达到峰值之后也由加固前的基本保持不变改为迅速消减降低;由于夯扩挤密碎石桩的排水和挤密作用,有效消除了干渠渠道底部以及渠堤坡面外侧平台至坡脚底部砂土层的液化现象,加固后干渠底部饱和砂土地基中没有液化现象产生。 相似文献
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砂土液化内部应力变化规律与工程液化判别 总被引：2，自引：0，他引：2
以往对砂土液化的研究主要侧重于水平场地、自由应力场条件下有关地基液化机理与判别等问题的研究。通常将Δu=σz=ΔUmax作为地基土液化的判据，而对工程结构物和场地条件的影响考虑不足。基于当前砂土液化问题的研究现状及工程特性，提出了将液化分为理论液化和工程液化。前者主要研究地基土液化的一般规律性问题；后者则针对具体工程结构物而言。其液化标准是以地基土在遭受地震液化时是否会导致工程结构物的破坏为依据。通过对砂土在震动液化过程中内部应力变化规律的理论分析，阐明了水平应力σx或σy对斜坡场地地基土发生侧向液化的作用机理，不能将斜坡场地的地基看作半无限空间体处理，提出了液化膨胀侧扩势Ψ的概念与计算式。指出：对斜坡场地，为避免这种侧向液化流动变形破坏，采取加强可液化土体的侧向约束、缩小偏应力差是必要的。根据工程结构物的承载力极限状态和正常使用极限状态，提出工程液化的判别准则：(1)可液化土体的地基强度τ降低到工程结构物所允许的强度值[τ，]；(2)可液化土体的膨胀侧扩势Ψ增加到其侧向约束强度[τh]；(3)可液化土体地基的变形s增大到工程结构物所允许的变形值[s]。 相似文献
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将饱和砂土视为土水两相介质,以Biot动力固结方程为基础,编制了完全耦合的三维排水有效应力动力反应分析程序。利用该程序对碎石桩复合地基进行了动力反应分析。结果表明:在地震荷载作用下,碎石桩具有明显的减震效应,碎石桩复合地基表层的最大水平振动加速度与天然地基相比明显减小,并且碎石桩对地基的沉降变形有明显的抑制作用;碎石桩的排水效应十分显著,随着输入地震加速度的减弱,在孔压达到峰值以后,由下到上出现了明显的孔压消散现象,碎石桩排水效应的影响范围为上窄下宽的圆台形;碎石桩的加密效应显著;考虑碎石桩各效应的耦合作用比仅考虑其单一效应,计算得到的孔压比要小,因此在进行碎石桩复合地基抗液化判别时,应该综合考虑碎石桩各效应的相互耦合作用。 相似文献
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针对大型炼厂工程地基处理的复杂性,开展了振冲碎石桩的现场试验。利用静力触探试验检测桩体密实度和判别饱和砂土液化。基于旁压试验、标准贯入试验和重型动力触探试验结果,分析了施工前后地基承载力和土体工程特性变化情况。以单桩和复合地基载荷试验结果验证了桩间土、单桩及复合地基的承载性能。研究结果表明,振冲碎石桩对桩长范围的砂土具有明显的挤密效应,工程特性和场地的均匀性在处理后有了明显改善和提高,有效地消除了桩长范围内砂土的液化可能性。静载荷试验结果表明,振冲碎石桩复合地基承载力能达到设计要求;振冲碎石桩对砂土层下卧黏性土层的加固作用不明显,部分深度范围内土体强度降低;当地面以下10 m内不存在厚度大于5 m的软土夹层时,较薄的软土夹层状对挤密加固其余深度的砂土未产生明显影响,对地基承载力影响亦较小。 相似文献
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针对已完成的液化水平自由场大型振动台试验,采用OpenSees有限元平台,建立了振动台试验的数值分析模型,并验证了该模型的可靠性。基于此,建立了地基整体倾斜的自由场数值模型,重点讨论了场地土体的非循环地震响应和液化侧向扩展机制。结果表明,建立的数值模型可以有效地模拟可液化场地的地震反应。倾斜场地中,可液化松砂与上覆非液化层界面处具有显著的相对位移,饱和砂土的应变累积从松砂层浅层开始,逐步向深层发展,超孔隙水压力增长和土体非循环应变累积未表现出耦合的现象,场地的中部土体控制非循环横向位移的发展。另一方面,土体液化过程中,当沿滑动面的剪切应力小于初始静剪应力时,液化侧向扩展启动。此时饱和松砂层的剪应力比在0.04～0.06范围内,略小于初始静剪应力比。此外,还发现液化诱导侧向扩展需要场地具有一定的倾斜度（大于0.5°）;土的侧向位移符合余弦分布模式;随着场地倾斜度的增大,可液化深层土对整体侧向位移的贡献更显著。 相似文献
10.
截断墙法降低地下结构地震液化上浮 总被引：4，自引：1，他引：3
位于饱和可液化土中的地下结构在地震激励下会由于土层液化而上浮,从而对结构造成严重破坏,设置截断墙是减少这种破坏的有效措施,在国外已经得到成功应用。但是,截断墙减小地下结构地震液化上浮的工作机理尚不明了,其设计参数的影响亦亟待进一步研究。采用以动力两相体有限元法,分析了钢板桩截断墙在不同地震强度激励下降低地下结构地震液化浮的效果,探讨了其工作机理。分析结果表明截断墙对降低地下结构地震液化上浮有明显效果。同时,通过对地层中超静孔隙水压、土体变形、应力路径、应力-应变关系的分析发现,截断墙的主要作用在于抑制可液化土的变形和流动。虽然,在较弱地震激励下截断墙可以抑制超静孔隙水压的上升,但在较强地震作用下超静孔隙水压不减反增,通过分析解释了这种矛盾。同时还分析了钢板桩的水平位置、嵌入下卧非液化土层深度与地下结构搭接长度等设计参数对降低地下结构上浮影响,为钢板桩截断墙的设计提供科学依据。 相似文献
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采用三维有限元程序建立了一长为6 m、直径为0.8 m的加筋碎石桩复合地基流固耦合数值模型,分析了其在堆载和孔压消散过程中的荷载传递和变形特性。较传统碎石桩,加筋碎石桩复合地基桩土应力比显著增大,超孔压、沉降和桩身侧向变形显著减小,且随筋材刚度的增大,其性能进一步改善。加筋碎石桩复合地基在桩间土固结过程中产生明显的桩土差异沉降,形成土拱效应,使得堆载结束后桩土应力比变化很小。筋材长度对加筋碎石桩复合地基桩土应力比和沉降影响显著,应对其全长加筋才能保证桩体刚度和有效减少沉降。 相似文献
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挤密碎石桩的材料是粒径较大的碎石,粒间粘结很小,其力学特性和其密度与侧压力有关,以往研究多作为弹性材料看待。利用大型三轴试验研究了碎石的变形特点,并根据试验结果总结了碎石的变形规律。在工程上常遇到的应变范围内,碎石三轴剪切试验的应力-应变关系可用双曲线拟合,体变曲线则接近于抛物线;单向压缩试验的轴向应力-应变符合幂函数曲线。徐－宿高速公路挤密碎石桩地基处理现场试验结果表明,经过碎石桩处理后的液化地基的液化势得到消除,该试验研究工作对类似工程有参考价值。 相似文献
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饱和可液化土中地下结构在强震作用下会由于土层液化而上浮,从而对地下结构造成破坏。可液化土在地震结束之后,将由于超静孔隙水压力的消散而产生固结变形,而地下结构在土层震后固结过程中的响应是一个值得研究的问题。以地下结构在土层震后固结过程中的垂直位移为重点,应用非线性动力两相体有限元方法研究饱和可液化土和地下结构相互作用体系在地震后土层固结过程中的响应。研究结果表明,地下结构的上浮趋势不会在地震结束时立即结束,而是在土层超静孔隙水压力经过一段时间的重分布及消散以后停止,之后地下结构有一定的沉降位移,但沉降位移远小于上浮位移,因此地下结构在震后会残留向上的垂直位移。文中还讨论了土层的应力路径响应、水压消散过程及截断墙与土层渗透系数对地下结构在震后固结过程中响应的影响,试图讨论饱和可液化土中地下结构在震后固结中响应的机理。 相似文献
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通过FLAC3D二次开发平台,在VC++环境下实现了基于砂土液化大变形机制的变动映射中心边界面弹塑性模型的二次开发。基于饱和砂土液化大变形本构理论,该模型符合三维应力空间边界面映射规则,并引入临界状态变量,可实现对不同密度和围压状态下砂土液化大变形分析。针对FLAC3D程序混合离散技术与数据调用模式和模型的体积相容性条件,测试分析了将模型采用不同植入方案的计算稳定性,开发了在FLAC3D混合离散技术下不同子区共享映射中心,进入与离开液化状态保持同步的开发方案,并给出模型开发的关键技术与实施方法。利用开发的模型,对饱和砂土开展了不排水/排水/循环三轴试验与不排水循环扭剪试验模拟,及三维地基的动力反应分析。计算结果表明,模型及所开发程序具有很好的模拟与分析砂土液化后大变形的能力。 相似文献
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Based on previous experimental findings and theoretical developments, this paper presents the formulation and numerical algorithms of a novel constitutive model for sand with special considerations for cyclic behaviour and accumulation of large post-liquefaction shear deformation. Appropriate formulation for three volumetric strain components enables the model to accurately predict loading and load reversal behaviour of sand, fully capturing the features of cyclic mobility. Compliance with the volumetric compatibility condition, along with reversible and irreversible dilatancy, allows for physically based simulation of the generation and accumulation of shear strain at zero effective stress after initial liquefaction. A state parameter was incorporated for compatibility with critical state soil mechanics, enabling the unified simulation of sand at various densities and confining pressures with a same set of parameters. The determination methods for the 14 model parameters are outlined in the paper. The model was implemented into the open source finite-element framework OpenSees using a cutting-plane stress integration scheme with substepping. The potentials of the model and its numerical implementation were explored via simulations of classical drained and undrained triaxial experiments, undrained cyclic torsional experiments, and a dynamic centrifuge experiment on a single pile in liquefiable soil. The results showed the model’s great capabilities in simulating small to large deformation in the pre- to post-liquefaction regime of sand. 相似文献
17.
《Computers and Geotechnics》2006,33(4-5):209-221
The uplift of large underground structures in saturated liquefiable soils under strong earthquake loadings may induce severe damages to the structures. Various mitigation procedures have been proposed to alleviate such damage, among which installation of cutoff walls next to underground structures was found to be effective. However, the working mechanism of cutoff walls in alleviating uplift of underground structures and the corresponding design parameters are still not clear. The liquefaction induced uplift behaviour of a subway tunnel in saturated sandy deposit over a layer of non-liquefiable soils and the working mechanism of cutoff walls for uplift mitigation purpose were investigated using the fully coupled dynamic finite element code DIANA Swandyne-II. A generalized plasticity model capable of simulating both cyclic liquefaction and pressure dependency of soils was used to model the sandy deposit. It is found that the small effective unit weight of underground structures, the development of excess pore pressure and the flow of liquefied soils were the sufficient and necessary conditions for underground structures to uplift during earthquakes. Cutoff walls could restrain the flow or deformation of liquefied soils and inhibit the uplift of underground structures but they could not necessarily prevent the liquefaction of the enclosed soils. After earthquake loadings, underground structures might settle due to the consolidation of soils and cutoff walls could also reduce the magnitude of settlement. The design parameters of cutoff walls, including the acting lateral pressure, the position, the stiffness and the permeability of cutoff walls, were also analyzed, the findings of which, together with the unveiled working mechanism, would be relevant for the design of cutoff walls for uplift mitigation purpose. 相似文献
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Triaxial tests on the fluidic behavior of post-liquefaction sand 总被引：2，自引：1，他引：1
Yu Huang Hu Zheng Wuwei Mao Maosong Huang Guanghui Li 《Environmental Earth Sciences》2012,67(8):2325-2330
Liquefaction-induced ground deformation is a major cause of structural damage during earthquakes. However, a better understanding of seismic liquefaction is needed to improve earthquake hazard analyses and mitigate structural damage. In this paper, a dynamic triaxial test apparatus was employed to investigate the fluidic characteristics of post-liquefaction sand. The specimens were vibrated to the point of liquefaction by dynamic loading, and then the liquefied sand was further sheared by triaxial compression in an undrained manner. It was found that a non-Newtonian fluid model can accurately describe the shear stress and the shear strain rate of post-liquefaction sand during undrained triaxial compression. The apparent viscosity, a major parameter in a constitutive model of a non-Newtonian fluid, decreases with an increase in the shear strain rate. 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