【作者】 刘俊伟；

【导师】 张忠苗；

【作者基本信息】 浙江大学 ， 岩土工程， 2012， 博士

【摘要】 静压开口混凝土管桩的施工效应包括土塞效应、挤土效应、承载力时间效应和残余应力四部分。它们彼此相互影响共同制约桩的承载力性状。本文通过现场足尺试验、室内物理力学试验,统计分析和理论建模解析计算,系统揭示了施工效应中各个方面对静压开口混凝土管桩受力特性的影响规律。本文的主要工作及创新成果如下：土塞效应是指挤入桩孔内的土柱对桩-土体系的影响。本文通过现场试验和室内土工试验,获得了不同土层中土塞的物理力学特性、分层特征及发展规律,建立了土塞端阻与土塞增长率的线性表达式；发现桩端以上4-5倍桩径范围内土塞的物理力学指标优于原状土,桩端处土塞的静力触探锥尖阻力高于原状土67%。基于土塞效应,首次建立并解答了开口混凝土管桩“桩中桩”荷载传递解析模型。研究表明土塞摩擦力的发挥主要集中在桩端以上2倍桩内径范围内,桩端处的土塞摩阻力为桩壁外侧摩阻力的3.4倍。同时,本文提出了更适用于开口混凝土管桩的基于静力触探试验的承载力设计方法-ZJU设计法。挤土效应是指沉桩挤土对桩-土体系的影响。本文在粉土地基中进行了开口管桩的挤土效应试验,揭示了沉桩过程及静置期内桩周土体的应力、孔隙水压力和位移的变化规律,发现单桩压入对周围土体的影响范围约为15倍桩径。基于试验结果,建立了开口管桩挤土效应解析计算模型,将桩体的压入过程模拟为半无限体中一系列球孔的扩张。现场实测表明黏性土中管桩的挤土效应导致群桩中单桩的承载力降低35%-75%,并讨论了挤土效应的防治措施。通过自制的恒刚度剪切试验,揭示了桩侧摩阻力随剪切循环的指数型退化规律。时间效应是指管桩沉桩后承载力随休止时间的提高。本文提出了承载力三阶段增长理论模型,并建立了基于固结理论的承载力时效解析计算模型。计算表明,完全非闭塞的摩擦型开口管桩承载力随时间的相对增长速度高出闭口桩约10%。采用隔时复压试验和静载荷试验揭示了开口管桩的时效性规律,发现基桩承载力随时间呈对数型增长,每时间对数循环的增幅处于15%-29%范围。利用时效性的有益影响,提出了基于隔时复压试验的静压桩承载力优化方法。残余应力是指沉桩后由于桩身压缩不能完全恢复而残留于桩内的应力。本文利用光纤传感技术对开口管桩的残余应力展开足尺试验研究,建立了残余应力与沉桩过程的关系。根据试验结果,实现了基于能量守恒的残余应力模拟计算解答,揭示了桩土参数和沉桩方式对残余应力的影响规律。研究表明,忽略残余应力将高估中性点以上摩阻力,而低估中性点以下摩阻力及桩端阻力。开口管桩施工效应的系统研究表明,土塞效应、挤土效应、承载力时间效应和残余应力共同制约管桩的承载力性状。统计近2000根管桩静载荷试验发现,管桩的极限承载力与压桩终止瞬间的压桩力存在相关性：短桩的极限承载力往往小于终压力,而随着桩长的增加承载力将超过终压力；单桩极限承载力与终压力的比值随桩长径比呈双曲线型增长,持力层为黏性土且桩侧为粉土时增幅最显著。更多还原

【Abstract】 The construction effects of the jacked open-ended concrete pipe pile consist of plugging effect, compaction effect, time effect on bearing capacity, and residual load. All of these effects interact with each other, controlling the characteristics on pile bearing capacity. A comprehensive research program, involving in-situ and laboratory test, numerical solution and statistical analysis, was conducted to reveal the influences of construction effects on mechanical characteristics for open-ended concrete pipe pile. This study includes the topics as follows:Plugging effect means the influences of the soil column squeezed into pile on the. A series of in-situ and laboratory tests reveal the physical, mechanical characteristics and the layer sequence of soil plug, as well as the changes on plug length during pile penetration in different soils. A linear relationship between the plug resistance and the plug incremental fill ratio is established. The test results indicate that the engineering properties of soil plug in the range of 4-5 pile diameters above the pile tip is better than the corresponding in-situ deposit. The CPT cone tip resistance for the soil plug near the pile tip is 67% larger than that for the in-situ deposit. Considering the plugging effect, a pile-in-pile transfer model is proposed. The research indicates that the mobilized plug friction is concentrated within the range of 2 diameters above the pile tip. The plug friction on the pile tip is 3.5 times as large as the pile shaft resistance at same level. A CPT-based design method for jacked open-ended concrete pipe pile, named ZJU method, is proposed in this study.Compaction effect means the influences of displacement behavior on pile-soil system. Full-scale tests in silty deposits reveal the developments of the total radial stress, pore pressure, movements in surrounding soils during and after pile installation. The test results indicate that the affecting range of single pile jacking on surrounding soil is approximately 15 pile diameters from pile wall. Based on test results, a computation model is proposed for researching the compaction effects of open-ended pipe piles. This model simulates the pile penetration as the expansions of a series of spherical cavities in semi-infinite space. Moreover, a in-situ research indicate that the compaction effects of pile group reduce the single pile capacity by 35%-75% in clayey soil, and several suggestions that could reduce the influence of compaction effects are proposed in this part. A series of tests using an invented constant normal stiffness direct shear system indicate an exponential decay trend of unit shaft resistance with increasing shear cycles. Time effect is defined as an increase in pile capacity over time after installation. A three-phase conceptual model is suggested in this paper. In addition, a computation model is proposed based on consolidation theory for evaluating the soil set-up on pile capacity. The research indicates that the relative setup rate for friction-type open-ended pile under full coring condition is 10% larger than that for corresponding closed-ended pile. A series of researches using re-pressed tests and static load tests reveal an approximately logarithmic set-up on bearing capacity with time after installation. The average increase rate for total capacity is in the range of 15%-29% per log cycle. In addition, an optimum design method of jacked pile based on re-pressed test is proposed taking advantage of the setup effect.Residual forces are compressive forces in a pile caused during pile penetration due to elastic compression of the pile material. A series of full-scale tests using optical fiber sensors was conducted on jacked open-ended concrete pipe pile to reflect the post-installation residual loads. Based on the test results, an expression of residual stress for jacked pile is resolved according to energy balance formulation. Then the influences of characters for pile and soil as well as installation methods on post-installation residual stress are researched. The researches indicate that the toe resistance and shaft resistance under neutral point would be under-interpreted and the shaft resistance above neutral point would be over-interpreted in pile load tests if the residual force was ignored.All the above mentioned studies on construction effects reveal the significant influences of plugging effect, compaction effect, time effect on bearing capacity and residual load on the characteristics on pile bearing capacity. A database including about 2000 static load tests for open-ended concrete piles is established to reveal the relationship between the ultimate capacity for single pile and terminative. The statistical analysis reveals that the ultimate capacity is always less than the terminative jacking force for short pile. With the increase of pile length, the ultimate capacity will exceed the terminative jacking force for some extent.The ratio between the ultimate capacity and terminative jacking force increases hyperbolically with the increase in the ratio between pile length and diameter, and a largerst ratio is corresponding to the bearing stratum of cohesive soil and surrounding soil of silty soil.更多还原