【作者】 徐春华；

【导师】 徐学燕；

【作者基本信息】 哈尔滨工业大学 ， 岩土工程， 2009， 博士

【摘要】 目前多年冻土区混凝土灌注桩使用数量与规模在我国急剧增长,由此而带来的施工过程中单桩承载力形成时间计算、夏季融沉时承载力的变化规律、使用阶段桩周冻土蠕变沉降对承载力影响等一系列问题已成为国内外普遍关注的热点问题,对其承载性能研究一直是冻土工程中的前沿课题。本文在对国内外相关研究进行系统总结与深入分析基础上,采用桩基冻害调查,试验与数值模拟相对照的方法。以冻土温度为主线,完成了三个方面的研究工作:1.研发相关试验设备,完成冻土导热系数、冻结强度、不同外加剂成分的水泥水化热、不同含水量冻土融沉时桩侧负摩阻力等四个系列(27个土样)试验工作,为有限元计算参数的确定创造了条件。2.基于不同的热传导模型、冻土非线性模型、冻土静三轴试验资料编制了桩周冻土温度场、桩承载沉降变形的计算程序。通过15种工况计算,得到冻结力沿桩分布规律与各个因素对其影响规律。3.根据冻土流变试验资料,构建了一个全新的非线性粘弹流变模型,采用编制的相关程序对实际工程中单桩极限承载力进行了修正,为工程冻害治理提供了依据。通过以上工作,本文在以下方面取得了一些有意义的研究成果:1.给出桩周温度场计算程序,通过不同热源温度(砼灌注温度)、不同砼外加剂成分的桩周冻土温度场数值计算对比,证明砼外加剂选用粉煤灰10%+硅粉5%+早强剂3%+减水剂0.8%配比、砼浇筑温度为5℃时,承载力形成时间可缩短至33天,提前了30天左右。2.给出多年冻土区单桩荷载—变形有限元计算方法与相关程序,发现增大桩径有利于加大桩侧冻结力数值,可有效减小沉降量30%。通过冻结力在桩侧分布随冻土温度变化规律研究发现存在“温度中性点”处,在该处桩侧冻结力发挥不受冻土温度影响,在其上桩侧冻结力随温度降低而递增,以下反之减小。3.根据融沉负摩阻力试验结果,运用Mindlin解,推导出融沉产生的附加力及其分布规律的理论计算公式。夏季冻土融化时,正融土中桩的承载能力衰减了8%左右。4.提出考虑冻土蠕变的单桩沉降有限元计算方法,对比某工程桩的单桩承载力数值计算、静载荷试验结果发现,单桩极限承载力在冻土流变影响下,其数值将降低10%左右。更多还原

【Abstract】 Currently, the application of cast-in-place concrete pile rapidly rises in quantity and scale in domesticTT TTpermafrost region, and there are such a series of problems as calculation of bearing capacity forming time of single pile during construction, change law of bearing capacity with thawing settlement in summer and influence of creep settlement of frozen soil around pile on bearing capacity in service stage. All the problems have been generally concerned focal problem at home and abroad and the research on bearing capacity has been one of the front subjects in frozen soil engineering.Based on systematic summary and deep analysis of relevant research at home and abroad, the following research in three aspects was carried out on the principle of temperature of frozen soil through the investigation of freezing damage and comparison between test and numerical simulation.1.The relevant experimental devices were developed, four series of tests were conducted such as thermal conductivity test on frozen soil, freezing strength test, hydration heat test on cement with different ingredients of admixture and thaw-settlement negative friction test on pile with different water content frozen soil, so all the above research provided important parameters for FEM calculation. 2. Grounded on different heat transfer model, nonlinear model of frozen soil and test data of static triaxial experiment on frozen soil, the calculation program was designed to compute temperature field of frozen soil surrounding pile and settlement of pile under load. Through plenty of calculation work, the distribution rule of adfreezing force of pile was obtained, as well as the affecting law of every parameter on it. 3. According to the data of rheology experiment on frozen soil, a whole new nonlinear viscoelastic rheological model was founded. The bearing capacity of single pile in correlative engineering was modified through the developed program, which gave theoretical basis to control the freezing damage of projects in permafrost.The main research results as follows: 1. The calculation program of temperature field of frozen soil around pile was designed and provided. In terms of numerical calculation and comparison of temperature field of frozen soil around pile with different temperature of heat source (temperature of cast-in-place concrete) and different ingredients of concrete admixture, it was proved that the forming time of bearing capacity was shorten to 33 days, about 30 in advance, when the concrete admixture consisted of 10% fly ash, 5% silica fume, 3% early strength admixture and 0.8% water reducing agent.2. This paper proposed FEM calculation method and program of load-deformation of single pile in permafrost. It was founded that adfreezing force increased with the increment of pile diameter, which can effectively reduce amount of settlement by 30%.“Neutral point of temperature”was founded through investigation into the change law of adfreezing force along pile length with variation of temperature of frozen soil. At the point, the forming of adfreezing force was free of the influence of temperature of frozen soil; above the neutral point side, adfreezing force increased with the reduction of temperature of frozen soil, but, below the neutral point side, it decreased with the reduction.3. According to experimental results of thaw-settlement negative friction, by Mindlin’s solution, the theoretical calculation formula was proposed to compute the additional force owing to thawing collapse and its distribution law. In summer, bearing capacity of pile decreased by 8% or so in permafrost active layer thawing.4. FEM calculation method of settlement of single pile was proposed considering creep deformation of frozen soil. The calculation result of bearing capacity of single pile was compared with the experimental data of static load test in certain engineering, and it was concluded that ultimate bearing capacity of single pile reduced by about 10% under the influence of rheology of frozen soil.更多还原