【作者】 秋仁东；

【导师】 刘金砺；

【作者基本信息】 中国建筑科学研究院 ， 岩土工程， 2011， 博士

【摘要】 随着我国建设规模的扩大,桩基础领域发展迅速,长桩基础被越来越多的工程所采用。工程实测表明,桩侧土模量较高的非软土地区的长桩静力试桩的荷载传递和桩身压缩性状与软土地区超长桩性状相似,长桩的桩身压缩量相当可观,计算中应予以考虑。目前的桩身压缩测试结果大都基于工程试桩所得,为了能够探讨研究考虑桩身压缩的群桩基础沉降性状,通过现场大型单、群桩模型试验,深入研究了群桩基础承载力、桩身压缩和土体压缩的沉降性状,本文主要完成了以下工作：1.通过系统的模型试验研究了单、群桩基础的承载力性状。群桩中基桩桩身轴力、侧阻力的分布形式因位置的不同而不同。群桩端阻表现出明显的沉降硬化效应,即端阻力随着群桩基础沉降的增大端阻力都会得到不同程度的提高,端阻力在较小桩距条件下提高程度较大,在较大桩距条件下提高程度较小。2.工程实测表明,桩身压缩产生的沉降始于加载初期,并伴随于加载全过程。桩侧土模量较高的非软土地区的长桩静力试桩的荷载传递和桩身压缩性状与软土地区超长桩性状相似,长桩的桩身压缩量相当可观,计算中应予以考虑。3.群桩试验表明,桩间土的竖向变形在桩顶处为最大,随着深度的增加桩间土的竖向变形而逐渐变小。桩间土除了产生剪切变形(桩、土相对位移)外,还出现压缩变形。4.在同一荷载水平下,桩端以下土层的沉降值随深度的增加而减小,整体压缩主要产生在距桩端下一定厚度的土层范围内,即土层距桩端越近,单位厚度土层的整体压缩量越大；同一深度处,土体整体压缩沉降值随荷载的增大而增大。5.当复合基桩分担荷载值一定时,较大桩距群桩桩端整体压缩沉降值较小桩距群桩为小；多桩数群桩桩端整体压缩沉降值较少桩数群桩为大,这是由于群桩效应增强所致,即桩距一定时,随着桩数的增多,桩与桩之间的增沉效应增强。6.群桩桩端平面以下地基土整体压缩变形及压缩层深度因桩距大小差异很大,即在P=Pu/2荷载条件下,大桩距群桩基础地基土整体压缩变形及压缩层深度较小桩距为小。本次结论与粉土、软土中群桩试验的结果一致。7.探讨研究了桩身压缩沉降与桩侧阻力分布模式相关的机理,给出了考虑不同侧阻分布模式确定桩身压缩系数的三个公式：ξe正三角=0.33α+0.67ξe矩形=0.50α+0.50ξe倒三角=0.67α+0.338.给出了小桩距群桩基础的沉降计算公式,该公式可考虑桩身的压缩、桩数、群桩的几何特征、侧阻力分布模式、端阻比例等因素对桩基沉降的影响：9.基于目前规范的压缩层厚度确定方法,探讨研究了群桩基础的压缩层厚度确定方案,最终确定了压缩层厚度的计算公式。10.在已有的考虑桩径影响Mindlin解竖向应力系数的研究成果上,给出了小桩距群桩基础的平均竖向应力系数的数值解,制作了相应的数据表格,该表格可供相关的工程设计人员手算沉降量时使用。更多还原

【Abstract】 In recent years, the pile foundation field has a rapid development due to the scale enlargement of engineering construction in China. More and more types of long pile foundations, meanwhile, are adopted in many construction projects. Static testing results of long piles in different strata figure out the essential behaviors of long piles under vertical loading, e.g. load transfer mechanism and compression behavior. Generally, the loading behaviors of long piles in hard soil ground are similar to super-long piles in soft soil and the total compression settlement of pile body is so great that it should not to be ignored for the compression calculation of long piles in practice. At the present stage, in-situ static testing is still one popular and authentic method to acquire compression behaviors of long piles. Consequently, in order to investigate the elastic compression behaviors of pile group foundation, the field tests are designed and carried out. Based on a series of large scale model tests of single pile and pile group, the settlement behaviors of pile group foundation are observed and the load transfer mechanisms are analyzed by the corresponding test data. Furthermore, the pile group effects of shaft resistance, point resistance, soil resistance beneath cap, and load-bearing capacity under vertical load are revealed. The principal conclusions may be summarized as follows.1. The load transfer mechanism of single pile and group piles under vertical loads are analyzed by large scale model tests. The distribution modes of axial force and lateral resistance along pile body differ with the location within the pile group. Analysis results show that the point resistance of pile group has an obvious hardening effect of settlement, which means that the point resistance increases distinctly with pile group settlement. On the other hand, the increase magnitudes of point resistance are different in the cases of small and large intervals between two adjacent piles. Popularly, the point resistance has a greater magnitude in a smaller interval condition than that one in a larger condition.2. The engineering static load testing results show that pile elastic compression settlement appear at the initial stage of loading process and then keep existing and developing in the whole process. Generally, the settlement behaviors of long piles in hard soil ground are similar to the super-long piles in soft soil and the total compression settlement of pile body is so great that it should not to be ignored for the compression calculation of long piles in practice.3. By the analysis of soil settlement behavior within group piles, the largest settlement appears at the top position of pile closed to the cap, and the settlement decreases with the burial depth. Meanwhile, the settlement covers two kinds of deformations, i.e. shear and compression.4. Under the same loading conditions, it is disclosed that the soil settlement below pile tip decreases with the burial depth by the analysis results of soil settlement at different depths within pile groups. Furthermore, one conclusion may be obtained that the compression settlement of soil mass mostly occurs in a certain range of soil closed to the pile tip. Actually, the compression settlement of per unit thickness of soil mass closed to the pile tip is greater than that one with a larger distance away the tip; the settlement value of soil mass increases with the load magnitude at the same burial depth.5. For a certain average loading magnitude on each pile within pile groups, the total compression settlement at the tip is greater in a smaller interval condition between two adjacent piles than that one with a larger interval. On the other hand, the settlement is smaller in the case of less numbers of piles than the case of more piles. This observation result may be connected with the pile group effect, which means that at a certain interval between two adjacent piles within pile groups, the settlement-increase effect of pile group becomes sharper with the increase of pile number.6. The thickness of compressed layer and total settlement behavior of soil ground below the tip of pile group both differ with the intervals between two adjacent piles within pile group. The field test results show that at the loading condition of P=Pu/2, the total settlement and thickness of compression soil layer both are smaller in a case of lager intervals between two adjacent piles within pile group than that one with an opposite case. These results agree with the previous field tests in silt and soft soil.7. Based on the analysis results of the pile elastic compression settlement and the lateral resistance distribution pattern, three linear equation of compression coefficient, which considers the difference of lateral resistance distribution pattern, are proposed as follows.ξe正三角=0.33α+0.67ξe矩形=0.50α+0.50ξe倒三角=0.67α+0.338. By the analysis of settlement mechanism of pile group foundation, one equation to calculate the foundation settlement of pile groups in the less interval cases is obtained. In the equation, the critical factors influencing pile foundation settlement, such as compression behavior of pile body, pile number, geometrical characteristics of pile group, lateral resistance distribution pattern, and resistance ratio of tip to lateral, etc., are considered completely and the detailed format, which is more reasonable than current methods, is showed as the follow.9. Based on the assessment method of compression soil thickness in current code and the observation and analysis results in this dissertation, the author take more efforts to optimize assessment scheme for compression soil thickness in the cases of pile group and one more reasonable calculation formula is acquired finally.10. According to the research achievement of vertical stress coefficient solved by Mindlin method considering the influence of pile diameter, numerical solutions for the mean vertical stress coefficient of pile group with small pile interval are proposed and listed in one table. These results in the table are convenient for engineer to calculate the compression settlement of pile group foundation by hand.更多还原