【作者】 顾明；

【导师】 陈云敏； 陈仁朋； 孔令刚；

【作者基本信息】 浙江大学 ， 岩土工程， 2014， 博士

【摘要】 近海工程项目通常体型巨大、结构高耸,其下部基础除了要承担上部结构的自重荷载外,还长期受到风、波浪、水流等水平循环荷载的作用。群桩基础在该循环荷载作用下不仅容易产生累积变形,极端条件下甚至可能发生破坏。另一方面,大型结构在承受水平荷载时,由于荷载作用方向的随机性,传递至下部群桩基础的荷载在某些情况下还可能存在一定的偏心距。这意味着基础将同时承受水平和扭转荷载的作用,从而产生复杂的受力和变形响应。目前人们对群桩基础在水平循环及偏心荷载作用下承载性状的认识存在很多的不足,因此有必要结合近海工程的特点对上述问题展开系统深入的研究。本文通过1g大比尺模型试验以及离心机模型试验,分别研究了高承台群桩在水平循环及偏心荷载作用下的受荷性状。本文开展的主要工作及相应的研究成果如下：(1)采用钢管及混凝土分别设计并制作了大比尺的单桩及群桩模型,利用浙江大学大型土工物理模型试验系统开展了一系列加载试验,对水平循环及偏心等复杂荷载条件下的群桩受力和变形性状进行了系统的研究。全部模型试验包括：1×2钢管群桩水平偏心加载试验、2×2混凝土群桩水平偏心加载破坏试验、3×3钢管群桩水平循环加载试验,以及相关的单桩加载试验。(2)基于3×3钢管群桩水平循环加载试验,分析讨论了群桩水平刚度、基桩内力及荷载分配等响应随荷载循环的变化规律,并引入了循环效应系数以评估循环荷载对桩基水平承载性状的影响。研究结果表明,桩基的水平刚度随荷载循环次数的增加而减小,且先期循环加载历史会影响桩基后期的水平加载刚度；各级循环加载中,单桩及群桩的桩头峰值荷载随循环次数增加依次表现出稳定型、发展型和破坏型等三种不同的衰减形态,反映了桩周土体由弹性到塑性再到破坏的发展过程；群桩中各排基桩分担的荷载比例随着循环次数增加不断变化,其中前排桩分担的荷载逐渐增大；由于承台的约束作用,基桩轴向循环拉压受荷,导致群桩伴随水平循环加载过程产生严重沉降；荷载循环效应对群桩水平受荷性状的影响要比单桩大得多。(3)基于1×2钢管群桩水平偏心加载试验,分析了群桩的整体变形以及基桩内力等响应,并通过ABAQUS有限元模拟,讨论了不同偏心距大小及基桩布置形式对群桩水平偏心受荷性状的影响。试验结果表明,水平偏心荷载下群桩中各基桩的内力存在较大的区别,离加载点近的基桩承担的荷载更大；受水平与扭转荷载耦合效应影响,群桩中基桩的抗扭承载力较单桩显著提高；基桩桩头扭矩及水平剪力共同抵抗水平荷载偏心引起的附加扭矩,且基桩剪力对抵抗群桩扭转的贡献随荷载的增加不断增大。数值计算结果显示,水平荷载偏心距对群桩整体变形的影响相对较小,但对基桩的内力影响较大；基桩布置形式显著影响群桩的水平和抗倾覆刚度,但对群桩的整体扭转响应影响较小。(4)基于2×2混凝土群桩水平偏心加载破坏试验,揭示了群桩的渐进破坏过程,配合数值分析研究了各基桩的内力随加载的变化规律,并对混凝土群桩在水平偏心荷载下的破坏过程及机理进行了总结。研究结果表明,较高的加载高度、上部结构的自重以及桩身混凝土的开裂等因素加剧了群桩在水平偏心荷载下的倾覆破坏；群桩在加载初始阶段基本处于弹性状态,各基桩的水平刚度较大；当荷载超过一定的临界值后混凝土基桩先后出现开裂,群桩刚度随之降低,变形相应增大；上部结构自重荷载引发的P-△效应使前排基桩承受的轴向荷载迅速增加,最终前排角桩出现轴向偏压破坏,群桩及其上部结构随之发生整体倾覆。(5)利用浙江大学ZJU-400土工离心机,开展了砂土中群桩承受水平及偏心荷载的系列模型试验,着重对比研究了直桩群桩与斜桩群桩在不同荷载条件下的变形及承载特性。试验结果表明,斜桩群桩的水平及扭转承载力均显著大于直桩群桩；水平偏心荷载下斜桩群桩中各基桩的桩顶位移差异较直桩群桩更大；斜桩群桩通过基桩轴向受荷可以更有效地抵抗水平荷载。数值计算结果表明,基桩斜度对群桩水平承载性状的影响较大,群桩水平刚度随基桩斜度的增加而增大。(6)提出了考虑多向荷载共同作用的群桩分析模型；在刚性承台的假设下推导了群桩整体刚度矩阵的表达式；同时考虑不同方向的群桩效应以及基桩各向荷载之间的耦合效应影响,讨论归纳了基桩各向刚度的计算确定方法；总结了多向荷载共同作用下群桩分析的一般流程,并给出了相应的分析实例。更多还原

【Abstract】 The foundations of large-scale offshore structures are constantly subjected to cyclic lateral loads with considerable magnitude during their service lives, which induced by winds, waves, currents, etc. They are likely to produce accumulated deformation under such load conditions, or even be damaged in extreme cases. On the other hand, owing to the stochastic directions of loads or the asymmetric shape of superstructures, significant eccentric lateral loads would also be transferred to the foundations probably. Both the lateral cyclic and eccentric loads would give rise to the complicated responses of pile group foundations, which have not been understood very well at present. Hence, it is of great theoretical and practical significance to conduct systematic researches on this issue, combined with the specific features of offshore engineering.Some different methods, such as1g large-scale model test, centrifuge model test and finite element numerical simulation, are adopt in this thesis to investigate the behavior of pile group with elevated cap subjected to lateral cyclic and eccentric loads, respectively. The main research works and results are as follows:(1) A series of large-scale model tests on single piles and pile groups were performed in saturated silts to investigate their behaviors under lateral cyclic and eccentric loads. The designs, as well as the procedures of these tests were detailed, which included the eccentric lateral loading test on1×2steel-pipe pile group, the eccentric lateral loading destructive test on2×2reinforced concrete pile group, the cyclic lateral loading test on3×3steel-pipe pile group and some other relevant single pile tests.(2) Based on the3×3steel-pipe pile group test, the lateral stiffness of the pile group, the internal forces and the load distributions of individual piles were discussed. Besides, a new empirical coefficient was introduced to evaluate the cyclic loading effect on the pile responses. The test results reveal that the lateral stiffness of pile reduces with the increasing number of loading cycles, and would be significantly affected by the previous loading history. The peak lateral load at pile head decreases with loading cycles in stable, developing, and failure pattern, respectively, which reflects the developing process of the soil around piles from elastic stage to plastic and failure stages. The lateral loads carried by each pile row constantly vary with cyclic loading and the leading pile row will undertake more and more loads. The pile group settles significantly in the test due to the cyclic lateral loading, as well as the strong constraint between the individual piles and the cap. The cyclic loading effect has a far greater impact on the responses of pile group than single pile.(3) Based on the1×2steel-pipe pile group test, the overall deformation of the pile group under eccentric lateral loads and the internal forces of individual piles were discussed. ABAQUS finite element simulations were also conducted to investigate the influences of the load eccentricity, as well as the layout of individual piles on the behavior of pile group. It is found that the internal forces of individual piles are quite different with each other within the group, and the pile closer to the loading point generally undertakes more shear force and torque. The ultimate torsional resistances of individual piles are apparently larger than the torsionally loaded single pile due to the deflection-torsion coupling effect. The torque applied on the pile group is shared by the torsional resistances and the shear forces of individual piles. With the increase of applied load, the contribution provided by shear forces gradually increases. The results of the numerical calculation show that, the load eccentricity would affect the internal forces of individual piles, rather than the overall deformation of pile group. However, the layout of individual piles would significantly influence the lateral stiffness of pile group.(4) Based on the2×2reinforced concrete pile group test, the progressive failure process of the pile group was described. The failure mechanism of overturning was also summarized by conducting numerical simulation. It could be concluded that the high altitude of loading point, the remarkable self-weight of superstructure and the cracking of pile shaft concrete together aggravate the overturning failure of pile group. The concrete pile group basically remains elastic during the initial loading stage, when the stiffnesses of individual piles are relatively large. Once the applied load exceeds a certain threshold, the pile shaft concrete begins to crack successively, which simultaneously reduces the stiffnesses of piles and increases the lateral deformation. The P-△effect induced by the self-weight of superstructure then gives rise to the rapid increasement of axial forces within the individual piles in the leading row, and finally leads to its eccentric compression failure, as well as the overturning of the structure.(5) A series of centrifuge model tests were conducted in sand to investigate the behavior of pile group subjected to lateral and eccentric lateral loads. The different responses of plumb and battered pile group were compared in detail. The results reveal that the lateral and torsional bearing capacities of battered pile group are much higher than those of plumb pile group. Under the eccentric lateral load, the differences between the displacements of individual piles within battered pile group are larger than those in plumb pile group. The battered pile group could more effectively resist lateral load than plumb pile group by taking full advantage of the axial capacity of individual battered piles. Numerical analysis shows that the lateral capacity of battered pile group would obviously increase with the increment of pile inclination.(6) Under the assumption of rigid cap, an analysis model of elevated-cap pile group subjected to multidirectional loads was proposed. A variety of different pile group effects and load coupling effects were summarized, as well as the corresponding computing methods. These effects should be carefully considered to determine the stiffnesses of individual piles in different directions. The general analysis process of pile group under multidirectional loads was also briefly summarized.更多还原