【作者】 周杨；

【导师】 肖世国；

【作者基本信息】 西南交通大学 ， 地质工程， 2015， 硕士

【摘要】 变截面螺纹桩复合地基是一种新型地基处理方法,此种复合地基中的基桩为桩身呈圆台形的螺纹桩。以往针对这种桩型及由其构成的桩基复合地基竖向承载特性研究较少。本文对变截面螺纹桩竖向承载力计算方法进行理论分析,通过室内模型试验和三维有限元数值模拟对其进行验证,并对变截面螺纹单桩及复合地基竖向承载特性影响因素展开研究。本文的主要研究工作如下：1)根据太沙基(Terzaghi)关于地基极限承载力计算理论,推导了最优螺纹间距计算公式。基于汉森(Hanson)与魏锡克(Vesic)地基极限承载力计算理论,建立了大于最优螺纹间距变截面螺纹桩竖向承载力算法。考虑局部剪切破坏特征,确立了基于Vesic理论的计算方法更接近试验结果。当间距小于最优螺纹间距时,考虑土体抗剪强度与桩-土界面抗剪强度差异,建立了螺纹桩单桩极限承载力计算方法。所提出的变截面螺纹桩单桩极限承载力的理论计算方法与数值模拟和模型试验的结果具有较好的一致性。2)通过室内缩尺模型试验,对变截面螺纹桩、等截面螺纹桩和普通光面桩竖向承载性能进行了对比研究。结果表明,承担相同荷载变截面螺纹桩材料利用率是螺纹直桩的1.26倍,是光面直桩的2.97倍；通过轴力分析,变截面螺纹桩侧阻发挥程度要高于螺纹直桩及光面直桩,变截面螺纹桩端阻所占总承载力比值为25.8%-35.1%,光面直桩端阻则为47.7%～82.6%。同时,试验显示螺纹间距对单桩竖向承载力有显著影响,螺纹净间距为27mm的单桩竖向承载力能力要高于净间距为15mm和40mmm两种情况。3)依托西安至成都铁路某路基工程,通过三维数值模拟,分析了影响复合地基承载特性的各种因素及其影响规律。结果表明：桩径大小宜控制在400-600mm；合理桩间距宜控制在4倍桩径桩间距左右；有效桩长约为15m；螺纹间距和螺纹宽度对单桩竖向承载力能力影响较大,最优螺纹间距约为220mm,合理螺纹宽度应控制在100mm-200mm之间；螺纹厚度对单桩竖向承载能力影响较小,但宜与螺纹宽度相等以免影响成桩质量；桩间土体强度与复合地基承载力呈正相关关系,随着强度降低桩土应力比增大；宜使用刚度较大的垫层以增加桩土应力比,增加垫层厚度可以有效调节桩土应力分担比,但垫层厚度不宜超过400mmm。本文的研究工作可为变截面螺纹桩桩基复合地基的设计计算提供指导与参考,有助于深入认识此种复合地基的竖向承载特性。更多还原

【Abstract】 The tapered concrete screw pile composite foundation is a new kind of ground improvement. Piles in this composite foundation are screw piles which have frustum cone-like bodies. Nearly no studies have done on the bearing capacity of tapered concrete screw piles and the pile composite foundation composed of the new kind of piles before. This paper will discuss the calculation methods of the vertical bearing capacity of tapered concrete screw piles. Laboratory model tests and 3D numerical simulation have been carried out to verify the methods and to study on the influencing factors of variable cross-section screw pile and the pile composite foundation. The principal research results are as follow:1) On the basis of Terzaghi’s theory of ultimate bearing capacity of deep foundation, the derivation process of computing method of optimal screw-pitch is showed. Based on B.Hanson and Vesic’s theories of ultimate bearing capacity of soil foundation, this paper establishes calculation methods of the ultimate bearing capacity of tapered concrete screw piles when the screw-pitch is greater than the optimal screw-pitch. Considering the characteristics of local shear failure, it is sure that the results of the calculation method based on the theory of Terzaghi are closer to test ones. When the screw-pitch is smaller than the optimal screw-pitch, considering the differences of the shear strength of soil and pile-soil, this paper establishes the calculation method of the ultimate bearing capacity of variable cross-section screw pile. The theoretical calculation methods of the ultimate bearing capacity of variable cross-section screw pile proposed in this paper are in good accord with the test results and the numerical simulation results.2) Indoor scale model tests are carried out to study on the vertical bearing capacity of tapered concrete screw piles and to compare that with the conventional screw piles and the conventional straight piles. The results show that under the same conditions and the same loads, the stock utilization of the variable cross-section screw piles is about 1.26 times that of the conventional screw piles and about 2.97 times that of the conventional straight piles. It’s found from the analysis of the axial force diagram that the pile friction of tapered concrete screw piles is larger than that of the conventional screw piles and the conventional straight piles. The ratio of tip resistance of variable cross-section screw piles to the bearing capacity is 25.8%～35.1%, and the ratio is 47.7%～82.6% for the conventional straight piles. The results also show that the screw-pitch has a remarkable effect on the vertical bearing capacity of variable cross-section screw piles. The bearing capacity of screw piles which screw-pitch is 27mm is greater than the ones which screw-pitch is 15mm and 40mm.3) According to a subgrade improvement engineering in Xi’an-Chengdu railway project, 3D numerical simulation is carried out to analyze the influence factors and regularities to the bearing capacity of the pile composite foundation. The results show that the pile diameter should be controlled between 400-600mm. The rational space of piles should be four times of the pile diameter. The effective length of piles is about 15m. The pitch and the width of screw thread have a significant impact on the bearing capacity of piles, and the optimal pitch is about 220mm. And the rational width of screw thread should be controlled between 100-200mm. The thickness of screw thread has a relatively smaller impact on the bearing capacity of piles, and should make it equal to the width of screw thread to keep the quality of piles. Strength of soil among piles is positively correlated with bearing capacity of composite foundation, and negative correlated with pile-soil stress ratio. The cushion with larger rigid should be used to improve pile-soil stress ratio and the thicker one can have effective regulations on pile-soil stress ratio. But the thickness of cushion should not exceed 400mm.The research of this article can provide guidance and reference for the design calculations of the vertical bearing capacity of tapered concrete screw pile composite foundation, and can contribute to a comprehensive understanding of the characteristics of the new kind of composite foundation.更多还原