Holistic approach for the design of single piles and pile groups under cyclic loading

桩在循环轴向和水平荷载作用下的长期荷载-位移特性是桩基设计的基础。由于循环荷载的不同类型，以及与传统工程结构相比极高的荷载循环次数，这种行为的预测与巨大的不确定性有关。需要一种能够涵盖土体在循环和高周荷载作用下的特性以及桩结构的具体特性的设计方法。现有的设计方法对于单调加载已经显示出显著的不确定性。该研究项目的主要目标是进一步发展一种基于现有的地基反力方法SRMHYP的设计方法，该方法目前用于砂土中的横向循环荷载单桩。该模型计划在项目结束时涵盖以下情况：单桩和群桩、砂土和粘土、循环和高循环轴向和侧向加载、土体小变形和大变形时的刚度、砂土的速率无关特性和粘土的速率依赖特性、桩的安装和设置的影响。这一扩展模型将通过离心机试验、数值模拟和已有的现场试验结果进行验证。所有计划中的实验、模型和数值模拟都是针对两种不同的模型土设计的，即细小均匀的硅砂和代表无粘性土和粘性土的高岭土。砂土和高岭土在循环荷载作用下的室内试验结果被用作SRMHYP模型的输入参数，用于进一步发展高岭土的应力-应变模型，进一步发展砂土和粘土的高周累积模型，并用于校准砂土和粘土的亚塑性和粘性亚塑性模型。由于试验是在真实应力水平下进行的，因此本文采用了砂土和粘土中循环荷载桩在一定条件下的离心机试验结果来验证SRMHYP模型的有效性，并进行了数值模拟。基于连续体方法和有限元方法，对循环荷载和高周荷载作用下的桩基进行了数值模拟。因此，进一步发展了粘土的应力应变模型和砂土/粘土的堆积模型。对于接触面桩/土的接触模型以及用于有限元程序的用户子程序的工作是必要的。数值模拟的结果为更简单、更少计算机消耗的SRMHYP模型的抽象奠定了基础。利用离心机试验结果和现场试验结果对仿真模型进行了验证。

Long term load-displacement behaviour of piles under cyclic axial and lateral loading is essential for the design of pile foundations. The prediction of this behavior is linked to large uncertainties due to the distinct type of cyclic loading as well as the extremely high numbers of load cycles compared to conventional engineering structures. There is a need of a design approach being able to cover the soil behavior under cyclic and high-cyclic loading and the specific behavior of pile constructions. Existing design approaches show significant uncertainties already for monotonic loading. The major objective of the research project is the further development of a design approach based on an existing subgrade reaction method called SRMHYP, that is currently designed for lateral cyclic loaded single piles in sand. This model is planned to cover the following cases at the end of the project: single piles and pile groups, sand and clay, cyclic and high-cyclic axial and lateral lading, soil stiffness for small and large deformation, rate-independent behavior of sand and rate-dependent behavior of clay, effects due to pile installation and set-up. This extended model will be validated by means of centrifuge tests, numerical simulations, and existing results of field tests. All planned experiments, models and numerical simulations are designed for two different model soils, namely a fine uniform silica sand and a kaolin clay representing non-cohesive and cohesive soils. The results of the laboratory tests on sand and kaolin with cyclic loading are used as input parameters for the SRMHYP model, for further development of a stress-strain model for kaolin, for further development of high-cycle accumulation models for sand and clay, and for calibration of hypoplastic and visco-hypoplastic models for sand and clay. The results of the centrifuge tests on cyclic loaded piles in sand and clay under defined conditions are used to validate the SRMHYP model, and additionally the numerical simulations, because the tests are carried out under a realistic stress level. The numerical simulations of piles under cyclic and high-cyclic loading are based on the continuum approach and the FEM. Therefore, the further developed of the stress-strain model for clay and the accumulation models for sand/clay mentioned above are applied. Additional work on the contact model for the contact surface pile/soil as well as work on the user subroutines for the FE-program are necessary. The results of the numerical simulations serve as a basis for the abstraction of the simpler and less computer consuming SRMHYP model. The simulation models are validated by means of results of centrifuge tests and results from field tests.