可液化砂土中复合桩基体系的动力响应研究

发布时间：2018-08-29 10:48

【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。
[Abstract]:The investigation of earthquake damage shows that the pile foundation in liquefiable soil layer is liable to damage the superstructure and cause serious economic and property losses. Although the current relevant codes and domestic and foreign scholars have put forward some methods in pile foundation seismic design, but these methods for the existence of liquefied soil layer pile foundation design still has obvious shortcomings, the main reason is that the existing pile foundation seismic design method is based on the static load theory and experience, dynamic. The interaction mechanism between pile cap and liquefied soil under load is not well understood, which makes the calculation method itself lack reliable theoretical basis and has many defects.
Based on a series of shaking table tests and MIDAS GTS numerical simulations of single pile and group piles, the vertical bearing capacity of composite pile system is analyzed and systematically summarized.
(1) Based on the non-dimensional theory of dynamic similarity in model test, by measuring the density and elastic modulus of materials with different proportions, the model material of concrete pile was developed for the first time, which ensured the feasibility of indoor model test and provided experience for similar test in the future.
(2) Laboratory model tests show that the excess pore water pressure of saturated sands under horizontal cyclic loading is gradually generated and developed with the vibration time, the pore water pressure ratio in the soil layer increases with the vibration time, and the soil layer liquefies gradually from top to bottom. The pore water pressure in the soil layer causes the pore water to flow upward, which results in the pore water pressure dissipating faster in the lower layer than in the upper layer.
(3) The strain analysis of pile body in single pile test shows that the side friction of pile decreases with the increase of pore pressure ratio in soil layer. After liquefaction, the side friction of pile decreases significantly, but lags behind the time of liquefaction. The side friction of pile increases with the dissipation of pore water pressure.
(4) The shaking table test of 3 *3 pile group with low cap shows that the composite pile system has an inhibitory effect on soil liquefaction, and the smaller the pile spacing, the more obvious the inhibitory effect. This is further proved by the measurement of the shear wave velocity at the model box site (the apparatus for measuring the shear wave velocity at the model box site invented in this paper).
(5) Based on the analysis of settlement time history of pile group under different working conditions, the variation of SDAF with vibration time under different pile spacing is analyzed by introducing settlement dynamic amplification coefficient, and the linear statistical expression is established, which provides the foundation for the static calculation and transformation of composite pile foundation dynamic design.
(6) The results of finite element numerical simulation further prove that the restraining effect of composite pile foundation on soil between piles decreases with the increase of pile spacing, and the restraining effect is almost completely lost when the pile spacing is 6D (pile diameter). The finite element analysis also shows that the increase of pile cap stiffness has a certain retarding effect on soil liquefaction between piles, and the greater the pile cap stiffness, this is the case. The greater the delay is.
(7) Under the condition of different pile spacing and pile cap stiffness, the variation law of vertical bearing capacity of composite pile system with the decrease of vibration time is calculated and analyzed. K / gamma S + beta 3_pQpk / gamma P + beta 2_cQck / gamma, in which beta 1, beta 2 and beta 3 are related to lateral friction, pile tip resistance and soil resistance reduction factor under cap, dynamic load and soil condition respectively. under this study condition, beta 1 and beta 2 can be taken up to 0.7 when the soil around the pile is partially liquefied, and beta 1 and beta 2 can be taken into account when the soil around the pile is completely liquefied And beta-2 is preferable (0-0.55), the longer the loading time is, the smaller the value is. When the soil is not liquefied, beta-3=1 is preferable. When the soil is liquefied, beta-3 is 0, that is, the soil resistance under the cap is not included.
【学位授予单位】：太原理工大学
【学位级别】：博士
【学位授予年份】：2014
【分类号】：TU473.1

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