Investigations of vertical bearing capacity of open-ended piles in cohesive soils with respect to soil-plug formation

目前，对开口桩堵土问题的研究主要集中在散体土中。本研究旨在探讨粘性土中开口桩施工过程中土塞的形成机理，为此，拟对管桩在粘性土中的施工进行现场试验和离心机试验。一方面，在现场试验中，最多只能考虑真实的应力状态。另一方面，离心机试验用于可重复条件下的试验，以及明确定义的土壤机械边界条件下的试验。尤其是离心机试验非常适合于岩土工程中的模型试验，因为施加的应力状态真实地再现了现场条件。此外，在离心机测试中，可以调查与土塞倾向有关的各种影响参数(例如固结状态、安装方法、桩径)。根据开口桩内部的内应力发展作为拱形和跟随土塞形成的指标对测试进行评估。通过室内试验和室内试验，可以更好地了解粘性土，特别是桩-土-土相互作用的情况。此外，对粘性土中开口桩的堵土过程进行了数值模拟的可行性研究。在这个框架中，使用离心机和现场测试的结果来研究不同的数值模拟方法(CEL、DEM、SPH)在粘性土中打桩过程中土体堵塞的可能性和局限性。该程序的主要目的是验证所建立的数值模型，使其能够应用于今后的研究。最后，综合所有的结果，建立了粘性土中开口桩竖向抗力预测的分析方法及其主要影响参数。因此，将开发一种方法来基于可能的土塞区域内的内部有效土压力系数来预测粘性土中的塞的垂直承载力，因为该系数是由申请人在之前的DFG研究项目中成功地开发的，该研究项目关于在颗粒材料中安装开口桩。

Soil plugging in open-ended piles until now was mainly investigated regarding installation in granular soils. In the present research project, the aim is to investigate the mechanisms of soil plug formation during the installation of open-ended piles in cohesive soils.Thus, it is planned to carry out in-site as well as centrifuge tests regarding the installation of tubular piles into cohesive soils. On the one hand, during the in-site tests a realistic stress state can be considered at the best. On the other hand, centrifuge tests are used for tests under reproducible conditions as well as well defined soil mechanical boundary conditions. Especially, centrifuge tests are well suited for model testing in geotechnical engineering as the applied stress-state realistically reproduces in-site conditions. Furthermore, during centrifuge testing it is possible to investigate a wide range of influencing parameters (e.g. consolidation state, installation method, pile diameter) regarding soil plugging tendency.The tests are evaluated regarding the internal stress development inside the open-ended piles as indicator for arching and following a soil plug formation. Out of these results, general conclusions regarding the mechanisms of soil plugging in cohesive soils with respect to the main influencing parameters are drawn.Accompanying laboratory and element tests lead to better undestanding of the investigated cohesive soils especially in context with pile shaft-soil-interaction.Besides, a feasibility study regarding the possibilities to numerically simulating the soil plugging process during installation of open-ended piles into cohesive soils is carried out. In this framework, the results of centrifuge as well as in-site testing are used to investigate possibilities and limitations of different numerical modelling approaches (CEL, DEM, SPH) regarding soil plugging during pile installation in cohesive soils. Main aim of this work package is to validate the numerical models such that they can be used in future research.Finally, all results are consolidated to develop an analytical approach for prognosis of vertical pile resistance of open-ended piles installed in cohesive soils with respect to sthe oil plugging and its main influencing parameters. Thus, an approach will be developed to predict vertical capacity of a plug in cohesive soils based on an internal effective earth pressure coefficient in the area of a possible soil plug as it was succesfully developed by the applicant during a preceding DFG-research project regarding the installation of open-ended piles in granular material.