Soil-pile interaction for bored cast-in-situ piles in stiff clays

Bored piles, Soil-pile interaction, Stiff clays, Distributed fibre optic sensing, Residual stresses, Influence of time, Alpha values, Hyperbolic non-linear loading model

Bored cast-in-situ piles are increasingly utilized as a foundation method in Denmark. Previously, driven precast concrete piles were the prevalent choice and are still widely applied. However, due to the construction of buildings in geotechnically challenging regions, the need to minimize noise and vibration in populated areas, and the construction of taller and more intricate structures with substantial loads, there is a gradual transition towards the use of bored cast-in-situ piles.

As the Danish construction industry has primarily relied on driven precast concrete piles, and the Danish National Annex to Eurocode 7 imposes restrictions on the shaft resistance for bored piles, it is necessary to expand the existing knowledge regarding the intricate interaction between the soil and the bored piles along their entire length.

Prior research conducted on bored piles in stiff clay has indicated that the interaction between the soil and the pile along the pile shaft is affected by the installation process, resulting in "locked-in" shear stresses on the pile shaft, also referred to as residual stresses. Furthermore, prior research indicates that the ultimate shaft resistance for bored piles can be predicted through a total stress analysis called the α method and that there is a linkage between the ultimate unit shaft resistance and the undrained shear strength.

The aim of this thesis is to examine the intricate soil-pile interaction along the entire length of bored piles in Danish Eocene clay, which is characterized as a stiff, over-consolidated, and highly plastic clay. The investigation takes place at a designated test site situated in Hinge, Jutland, Denmark. The research focuses on a field-test program involving eight bored piles, with a specific emphasis on understanding the effects of the pile installation process, particularly the residual stresses, and their impact on the mobilization of shaft resistance.

Additionally, the study explores the relationship between the ultimate shaft resistance and undrained shear strength, using the empirical adhesion coefficient known as α, and examines its compliance with the limitations presented in the Danish National Annex to Eurocode 7. The empirical adhesion coefficient, α, was found to be approximately 2.3 to 3.3 times higher than the value specified in the Danish National Annex to Eurocode 7. This indicates that the empirical adhesion coefficient provided in the Danish National Annex to Eurocode 7 may not accurately capture the behaviour of bored piles, emphasizing the need for further consideration and evaluation of this parameter in geotechnical engineering practices.

Lastly, the influence of time on the pile capacity is investigated to gain insight into the behaviour and performance of the piles over extended periods. The axial pile capacity was found to increase with approximately 12 % per log change in time. However, the trend is weak with an R² value of 0.48 indicating that additional data at various intervals after installation is required to improve the ability to predict the time evolution of pile capacity.

By analysing the results obtained from the static pile loading tests conducted during the study, an analytical model has been developed to determine the distribution of displacements and forces along the shaft of bored piles. This model can accommodate the consideration of residual stresses or exclude them, depending on the specific requirements of the analysis.

Furthermore, based on the soil properties observed in the Eocene clay at the designated test site in Hinge, a method for predicting the mobilised shaft resistance (known as load transfer functions) has been derived. These load transfer functions serve as a valuable tool in everyday practical geotechnical engineering, as they enable engineers to make initial estimations of the distribution of displacements and forces along the shaft of bored piles. This aids in the design and evaluation processes, providing crucial insights for geotechnical engineering projects.

The analysis of the test data obtained from the static pile loading tests revealed that residual stresses have an impact on the response of the piles to loading, specifically influencing the mobilised shaft resistance. Therefore, it is essential to consider these residual stresses when modifying the length, diameter, or other parameters of the piles.

It is worth noting that the residual stresses, although influential in the mobilization of the shaft resistance along the pile, do not impact the overall resistance of bored piles as demonstrated in a pile loading test. These stresses primarily affect the distribution and behaviour of the shaft resistance along the length of the pile.

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