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

• Applicant: Professor Dr.-Ing. Sascha Henke
• Subject Area: Geotechnics, Hydraulic Engineering
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 495328022

The axial capacity of open-ended pipe piles in both cohesive and non-cohesive soils is strongly influenced by whether soil plugging develops during installation, leading to significant soil displacement and a corresponding increase in stresses in the vicinity of the pile. This process is generally associated with an increase in pile base resistance. In non-cohesive soils, the mechanisms governing soil plugging are sufficiently well understood to be incorporated into pile design practice. In contrast, soil plugging in cohesive soils remains comparatively poorly investigated. As a result, reliable empirical data are scarce and existing design approaches for open-ended piles in clay are still incomplete. The objective of the proposed project is therefore to improve the understanding of soil plugging and axial capacity of open-ended pipe piles in cohesive soils. During the first project phase, substantial new insights into soil plugging in cohesive soils were obtained, primarily focusing on monotonically jacked piles. The second project phase places emphasis on the pile base resistance of open-ended piles after installation. Different installation methods—monotonic jacking, vibratory driving, and impact driving—are considered, as they significantly influence both soil plug formation during installation and the structure of the surrounding soil. In addition, the dissipation of excess pore water pressures following installation is examined in order to derive reliable estimates of plug resistance and, consequently, pile base resistance. To investigate stress development within the soil plug, laboratory model tests using a specially developed pipe shear apparatus are planned. This test setup allows the investigation of inner shaft friction as a function of boundary conditions such as stress state and shear rate. A key feature of the apparatus is the realistic orientation of soil particles relative to the pile wall and the ability to study silo effects by varying the H/D ratio (specimen height to specimen diameter). In addition to pipe shear tests, interface ring shear tests are planned to characterise pile–soil interaction, along with element tests to determine constitutive model parameters for numerical simulations. Numerical analyses will simulate pile installation by jacking, impact driving, and vibratory driving, followed by consolidation and static load testing. These simulations provide detailed insights into stress and pore pressure evolution that are difficult to obtain experimentally. To validate the laboratory and numerical results, a field test will be conducted in which stress development along the pile and soil plug height are measured. Based on the combined findings, an analytical design model for open-ended pipe piles in cohesive soils will be developed.

• DFG Programme: Research Grants