【作者】 戚春香；

【导师】 王建华；

【作者基本信息】 天津大学 ， 岩土工程， 2008， 博士

【摘要】 评价地震荷载作用下桩的水平承载特性是进行桩基础设计的一项重要内容。虽然国内外对饱和砂土液化后的桩基水平承载力已有一些研究,但是仍然没有得到一个被广泛接受的结论。注意到地震荷载作用下,尽管饱和砂土没有液化,但是由于震动残余孔压的产生会导致土层发生弱化,以往的研究很少探讨弱化土层中桩的水平承载特性。为此,本文通过水平荷载作用下的桩土相互作用模型试验,研究不同弱化状态以及液化后的饱和砂土中单桩的水平承载特性及相应的p-y关系曲线,具体内容如下:首先,应用有效应力原理,提出了一种模拟震动导致的具有残余孔压饱和砂土与桩相互作用的模型试验方法。该方法通过在饱和土层中施加反压,使饱和砂土中维持某一特定的孔压比,以此模拟震动荷载作用下饱和砂土中由于残余孔压的产生而导致土强度降低的弱化状态。进而针对不同弱化状态的饱和土层进行了水平荷载作用下的单桩承载特性模型试验,研究了残余孔压对桩基水平承载力的影响。结果表明,随土层残余孔压的增加,桩的水平承载力逐渐降低,土层液化后,其降低的程度与已有的振动台及离心模型试验结果一致。进一步,分析了弱化饱和砂土层桩土相互作用的p、y关系,研究了确定弱化土层p-y曲线必要参数的变化规律。通过分析土层在液化过程中的破坏模式,提出了依据弱化土强度参数确定其水平极限抗力的方法。通过利用3-D有限元的方法研究三段结构p-y曲线上的特征桩侧位移yu、ym与土反力模量系数的变化,分析了它们随弱化土参数以及桩径的变化关系,建立了依据桩、土参数确定yu、ym及土反力模量系数的方法。最后,参照Reese及API规范建议的方法,综合本文对弱化p-y曲线及相应参数的研究结果,建立了构造饱和砂土三段结构形式与双曲形式弱化p-y曲线的方法。采用所建立的两种方法确定的饱和砂土弱化p-y曲线与模型试验得到的p-y曲线基本吻合。进一步,依据弱化p-y曲线计算的桩身弯矩与模型试验实测结果也基本一致。这说明本文建立的两种方法均可用于分析具有残余孔压弱化土层中的桩基水平承载特性。相对三段结构形式的弱化p-y曲线,双曲形式的弱化p-y曲线更为简便,更加便于工程应用。更多还原

【Abstract】 It is very important for the design of foundations to evaluate lateral bearing capacity of piles under seismic loads. Although some researches were done on the lateral bearing capacity of piles in liquefied sand, the conclusion which was widely accepted has not obtained. It is observed that the saturated sand would be degraded because of the residual pore water pressure despite it was not liquefied under seismic loads. There are few researches on lateral bearing capacity of piles in degraded saturated sand so far. Therefore, Model tests of the soil-pile interaction were conducted to research lateral bearing capacity of a single pile in degraded stratum and p-y curves of the stratum in this dissertation. Main achievements are described as follows:Firstly, a model test method was developed to simulate the interaction of pile and saturated sands with the excess pore water pressure from seismic loads based on the principle of effective stress, in which the residual pore water pressure was simulated by applying the back pressure to the saturated sand stratum. Model tests of the soil-pile interaction corresponding to different degradation state strata were conducted to research the effect of residual pore water pressure on the lateral bearing capacity of the single pile. The results showed that lateral bearing capacity of piles gradually decreased with an increase of residual pore water pressures and model test results were consistent with shaking table and centrifugal model test results for liquefied sands.Further, the relationship between p and y of degraded saturated strata was analyzed to research variations of parameters determining p-y curves of degraded saturated strata. The method to determine ultimate lateral resistances based on soil strength parameters was established by analyzing the failure mode of liquefying saturated strata. Variations of characteristic lateral displacements, yu and ym, and subgrade reaction modulus coefficient were analyzed to determine degradation p-y curves of three segments frame using the three-dimensional finite element method. Relationships of these parameters with the pile diameter and the degradated strata strengths were established. Methods to determine characteristic lateral displacements and the subgrade reaction modulus coefficient of degraded strata were developed using characteristic parameters of soils and piles. Finally, methods determining the three segments frame p-y curve and the hyperbolic p-y curve of degraded saturated strata were analyzed based on research results and referencing to methods suggested by Reese and API Code. P-y curves determined by those methods were basically consistent with model test results, and responses of the pile determined using calculated p-y curves were also consistent with model test results, which showed that the lateral capacity of the pile in saturated degradation sand strata could be evaluated using those methods. Comparing to the method to determine degradation p-y curves of three segments frame, the method to determine hyperbolic degradation p-y curves is more convenient and will be easy accepted in engineering practices.更多还原