【作者】 张立明；

【导师】 郑刚；

【作者基本信息】 天津大学 ， 岩土工程， 2012， 博士

【摘要】 高层、超高层建筑的建设使超长桩得到了广泛的应用,同时也促进了超深基坑的发展。超长桩的荷载传递机理与普通短桩存在较大差别,基坑开挖导致位于坑底的超长桩的工作性状更为复杂,研究超长桩以及开挖影响下的超长桩基础的承载性状和沉降计算理论非常必要。首先,基于超长桩静载试验进行实测分析,考察了超长桩的各方面性状。结果表明,超长桩侧阻发挥过程为由桩顶向下逐步发挥,工作状态下桩身侧阻呈现出单驼峰式的分布,且驼峰位于桩身的中部附近,长细比越大驼峰位置相对越靠近桩顶。超长桩在工作荷载下端阻比均较小,大多数不超过2%。进而,采用有限元法研究了不同长度的单桩荷载传递机理,根据其工作特性对传统桩基础沉降计算方法进行改进。结果表明,工作荷载下,短桩过渡到超长桩,轴力主要承担部分由桩身下部逐渐转为桩身中上部,侧阻由三角形分布转为单驼峰式分布。因此采用单驼峰式分布模式计算超长桩基础地基中附加应力及沉降,其计算沉降值较基于Geddes解的传统方法更接近于实测值。采用有限元法研究了基坑开挖条件下坑内单桩荷载传递机理,并对开挖影响下的超长桩基础地基竖向附加应力及沉降计算方法进行分析。结果表明,基坑开挖导致单桩刚度降低,并可导致桩身出现较大的拉力。且开挖条件下超长桩侧阻和端阻不同步发挥的现象更明显,不利于荷载向桩身下部传递及实际承载能力的发挥。短桩过渡到超长桩,工作荷载下桩身侧阻分布由三角形转变为“R”形,且超长桩侧阻主要发挥在桩身中上部,当桩长超过一定值时桩身下部较大范围甚至保持着负的摩阻力。考虑开挖条件下侧阻分布特点和土体回弹再压缩性质的沉降计算要较Geddes假设以及单驼峰式更合理且沉降值更接近于实测结果。通过有限元法进行了基坑开挖对坑内桩影响的参数分析。计算了不同基坑开挖尺寸及桩长的情况下,桩身由于受到基坑开挖影响产生的刚度降低、最大拉力及位置和侧阻分布随各因素的变化。分析结果表明,开挖导致基坑中不同位置处单桩刚度不同程度的降低,中心桩刚度降低程度最大,边桩次之,角桩最小;由于基坑开挖导致的不同位置处单桩刚度的不同可能加剧群桩基础的“碟形”沉降,导致筏板和上部结构开裂。开挖导致坑底的单桩几乎全长受拉,位于基坑中心处的桩拉力最大,边桩次之,角桩最小;当基坑开挖深度很大、桩长也较大时,基坑内工程桩的桩身配筋长度、配筋量、裂缝控制均要考虑桩身可能产生的最大拉应力及其位置。最后,给出了不同基坑开挖尺寸及桩长条件下基坑中不同位置处的单桩工作荷载下侧阻分布曲线。在以上研究基础上,以MATLAB为平台开发了考虑各种侧阻分布形式的沉降计算软件。更多还原

【Abstract】 The high-rise and super high-rise buildings promote the development of super-long piles and super-deep excavations. The load transfer mechanism of super-long piles is different from that of short piles. The working performance of the super-long piles beneath the excavation bottom is more complicated. It is quite necessary to research the bearing behavior and settlement calculation theory of super-long piles.Firstly, the characteristics of super-long piles are analyzed based on the measured static load test data. It is shown that the shaft resistance of super-long piles mobilizes from pile top to tip. The shaft resistance distributes in a single hump pattern under working load. The hump is usually located at the middle of the pile length, and its location moves upward as the slenderness ratio increases. Under working load, the super-long piles have small tip resistance ratios, most of which are not more than 2%.Furthermore, load transfer mechanism of different length of piles is analyzed by finite element method. According to the working performance of super-long piles, some improvements are made to the traditional settlement calculation method of pile foundation. It is shown that, under working load, the load of the short piles is mainly undertaken by the lower piles shaft, while that of the long piles is carried by the middle and upper piles shaft. The shaft resistance distributes in a triangle pattern and a single-hump pattern for the short and long piles respectively. Therefore, the settlement of super-long piles calculated by single hump distribution is closer to measured data than that of Geddes’solution.The load transfer mechanism of super-long piles beneath the excavation bottom is studied by finite element analysis. The additional stress in foundation soil and settlement calculation method of super-long piles under excavation are analyzed. It is shown that excavation could lead to tension in piles and a decrease of piles stiffness. The shaft resistance and tip resistance of piles beneath the excavation bottom are mobilized asynchronously due to excavation. This is adverse for transferring load to lower pile shaft and mobilizing bearing capacity. The shaft resistance distributes in a triangle pattern and an‘R’pattern for the short and long piles respectively. The shaft resistance is mainly mobilized along the upper part of the piles for super-long piles. The friction on the lower part is even negative for piles longer than certain value. The settlement considering the distribution and swelling-recompression of the soil under excavation is closer to measured data than that of Geddes’solution and single-hump solution.The parametric study on piles under the condition of excavation is conducted using finite element analysis. Models of different excavation geometries and different piles lengths are calculated. The reduction of stiffness, maximum tension and its position, and the distribution of shaft resistance are analyzed.It is shown that the reuduction of piles stiffness due to excavation varies with different positions. The maximum, medium and minimum reduction occurs at center, side, corner piles respectively. The different pile stiffness may intensify the disk-shape settlement, which even could cause the fracture of raft and superstructure. Excavation can cause tension in the full length of piles. The maximum, medium and minimum tension occurs at center, side, corner piles respectively. The maximum tension of piles should be taken into consideration in design for deep excavations and long piles.Finally, the distributions of shaft resistance under working load of different pile lengths and excavation geometries are presented. A software is designed based on MATLAB platform, which takes the different distributions of shaft resistance into account in calculating the settlement of pile foundation.更多还原