【作者】 黄占芳；

【导师】 白晓红；

【作者基本信息】 太原理工大学 ， 岩土工程， 2014， 博士

【摘要】 从历次发生地震的震害调查表明,可液化土层中桩基础容易产生破坏而导致上部结构损坏,引起严重的经济和生命财产的损失。研究液化土层中桩基础承载特性具有非常重要的意义,直接关系到国内外经济建设和工程的抗震减灾问题。虽然现行的相关规范和国内外相关学者在桩基础抗震设计方面已经提出了一些方法,但是这些方法对于存在液化土层时桩基础的设计还存有明显不足,其原因主要是现有的桩基抗震设计方法是基于静荷载条件下的理论与经验,对动荷载作用下低承台桩——液化土之间相互作用机理理解不够,使计算方法本身缺乏可靠的理论依据,存在许多不妥之处。在国内外相关研究的基础上,通过一系列单桩和群桩的小比例模型振动台试验和MIDAS GTS数值模拟,对复合桩基体系的竖向承载性能进行了深入的分析和系统的总结,取得以下研究成果：(1)基于模型试验动力相似性无量纲理论,通过对不同配比材料的密度和弹性模量的测定,首次研制了满足试验要求的混凝土桩的模型材料,保证了室内模型试验的可行性,为今后类似试验提供了经验。(2)室内模型试验表明：饱和砂土在水平周期荷载作用下,超静孔隙水压力随着振动时间逐渐产生和发展,土层中的孔压比随振动时间增长而逐渐增大,土层自上而下逐渐发生液化。振动停止以后,由于下部土层中的孔隙水压力大于上部土层的孔隙水压力,孔隙水产生向上渗流,因而导致下部土层孔隙水压力消散速度比上部快。(3)单桩试验中桩身应变测试分析表明：桩侧摩阻力随土层中的孔压比的提高而降低,当土层产生液化后,桩侧摩阻力降低显著,但是明显滞后于液化产生时间。随孔隙水压力的消散桩侧摩阻力又有所提高。(4)低承台3×3群桩体系的振动台试验表明：复合桩基体系对于土层液化产生有抑制作用,且桩间距越小,抑制作用越明显。这是因为间距越小,挤密效应越显著,桩—承台对土的夹持作用越大,因此桩间土产生液化需要的振动时间就越长。这一点通过对模型箱场地剪切波速的测定(本文发明的用于室内模型箱场地剪切波速测定装置)进一步予以了证明。(5)通过对群桩试验不同工况沉降时程的分析,引入沉降动力放大系数分析了不同桩间距条件下,SDAF随振动时间的变化规律,建立了线性统计表达式,为复合桩基动力设计的静力计算转化提供了基础。(6)有限元数值模拟分析结果进一步证明：复合桩基对桩间土的抑制作用随桩间距增大而减小,当桩距为6D(D为桩径)时,抑制作用几乎完全丧失。有限元分析还表明,承台刚度提高对桩间土液化产生有一定的延缓作用,且承台刚度越大,这种延缓作用越大。(7)对不同桩间距、承台刚度条件下,复合桩基体系竖向承载力随振动时间减小的变化规律进行了计算分析,通过对桩侧摩阻力和桩端阻力随振动时间变化的进一步深入研究,提出了动力荷载作用下的桩基竖向承载力计算公式：Q=β1η5Qsk/γs+β3ηpQpk/γp+β2ηcQck/γ,其中β1、β2、β3分别为考虑动力荷载作用的桩侧摩阻力、桩端阻力和承台下土抗力折减系数,与动荷载的作用和土层条件有关。在本次研究条件下,当桩周土体部分液化时,β1和β2可取0.7,当桩周土完全液化时,β1和β2可取(0～0.55),荷载作用时间越长,取小值。当土体未发生液化时,可取β3=1,当土体一旦发生液化取β3为0,即不计入承台下土的抗力。更多还原

【Abstract】 The damage of pile foundation is one of important disasters in liquefiable soil layer during earthquakes. Study on bearing capacity of pile foundation in liquefiable soil layer has generally become important subject in seismic design of soil and foundation engineering in the word. There are obvious deficiencies in seismic design method of pile foundation in liquefiable site used at present. However, the existing theory and experience for seismic design method of pile foundation depends on static condition. Except of that, the shortage or misunderstanding to mechanism of liquefied soil-pile-cap interaction are the other troubles.Based on the literature reviews of domestic and abroad scholars, the vertical bearing properties of composite pile foundation system are analyzed deeply and summarized systematically by shaking table tests of single and group of pile and numerical simulation. Following conclusions are drawn out:(1) Based on the theory about dimensionless similarity of dynamic model tests, a kind of simulated material is made to replace the concrete pile after several trial testing by measuring the density and elastic modulus of different proportions materials.the proportions is cement:sand:soil:water= 1:5.8:1.45:1.9, similar ratio is1.86. The test results prove that this material of model pile is feasible, and can be used in similar experimental study.(2) The model shaking table tests show that:the pore water pressure and the pore pressure ratio are increasing with vibrating time, the growth rate for the surface soil is faster than that at the bottom soil, so the liquefaction takes place from surface to bottom of the soil. After the vibration is stopped, the pore water pressure in the bottom soil layer is dissipated faster than the upper layer because the pore water pressure in the deeper soil layer is higher than that in the upper layer which makes the bottom pore water flow upward.(3) The measuring data of the strains installed in pile body show that pile side friction is reducing with pore water pressure increasing during vibration process, but there is a delay after pore pressure increase. Furthermore, the pile side friction is increasing again with pore water pressure dissipating after vibration stop.(4) The results of shaking table test with3×3pile groups system in which the pile cap is contacted with the soil closely show that the composite pile groups system posses an inhibiting effect for the soil between piles. Such inhibiting effect is more obvious with pile spacing decreasing, because the smaller pile spacing is, the denser the soil between piles, the more significant clamping effect, which inhibits the soil liquefaction. This phenomena is proved by the shear wave velocity of the soil measured during testing by using the method invented by authors. (5) Based on the settlement process analysis of pile groups shaking table tests in the different working conditions, a parameter, settlement dynamic amplification factor (SDAF) is proposed in this study. SDAF is defined as the ratio of the total dynamic settlement at giving time and total static settlement under the same loading action. The variation relationship of SDAF with vibration process is analyzed; a linear statistical expression is set up, which provides the theory foundation for the static calculation of composite pile foundation under the dynamic loading.(6) The results of finite element numerical simulation further prove the inhibiting effect, and the inhibiting effect almost disappears when the pile spacing is6D (D is pile diameter). The analysis results of finite element numerical simulation also show that soil liquefaction will be delayed with stiffness of the pile caps enhancing.(7) Based on the model shaking table testing and finite element analysis, a formula of vertical bearing capacity considering dynamic loading effect is finally established, ie. Q=β1ηsQsk/γs+β2ηpQpk/γp+β3ηcQck/γc. Hereβ1、β2β3are reduction coefficients of pile side friction, bottom resistance, and soil resistance under the cap, respectively. They are related with the characteristics of dynamic loading and soil conditions. In this study,β1、β2may take as0.7when the soil surrounding the pile is partially liquefied,β1、β2may take as (0～0.55) when the soil surrounding the pile is complete liquefied.β3may take as1when the soil under the cap is not liquefied,β3may take as0when the soil under the cap is complete liquefied.更多还原